ii. 1. oxidation of alcohols with different oxidizing...
TRANSCRIPT
23
Oxidation is an important process and has played an important role in
organic synthesis1. Oxidation of various compounds such as alkanes, alkenes,
alkynes, arenes, ethers, alcohols, aldehydes, ketones amines etc. are oxidized by
several oxidizing agents such as KMnO4 , Osmium tetroxide, Ruthenium
tetroxide, Cr(VI) compounds, Palladium (II) and Platinum (II)compounds, Lead
(IV), Hg(II), Thallium (II) compounds, copper (II), Vanadium pentoxide,
Manganese dioxide, Cobalt (III), selenium dioxide, glycol-cleavage agents in the
oxidative scission of carbon-carbon double bonds, peracid, peroxide.
Majority of the oxidation processes employed industrially involve catalysis
by metal and their compounds.
A brief discussion on different reactions related to oxidation of alcohols with
different oxidizing agents has been given below.
II. 1. Oxidation of Alcohols with Different Oxidizing Agents
II. 1. A. Oxidation of alcohols with Potassium permanganate:
Primary and secondary alcohols are readily oxidized by permanganate ion
both in acidic and basic medium.2-5 Permanganate ion is a strong oxidizing agent
and the product obtained from the oxidation of alcohol is mainly corresponding
carboxylic acid3. (Scheme-1)
CH3CH2CH(CH2)5OH
CH3
KMnO4/1.5 MH2SO4
250CCH3CH2CH(CH2)4CO2H
CH3
1 2
24
CHOHCO2HKMnO4/ 0.9MNaOH
COCO2H
-200C, 2hr
3 4 (50-67%)
Scheme-1
Under neutral or mild acidic conditions the rate of reaction is very slow;
e.g. the relative rates of oxidation of 2-butanol under alkaline, acid and neutral
conditions are 21:25:1; however, addition of peroxide is known to accelerate the
rate of oxidation in dilute acid.6
Stewart & Co-Workers 7-11 reported that the rate acceleration in basic
solution is caused by ionization of the alcohols to give alkoxide ion which readily
transfers electrons to the oxidants. (Scheme-2)
R2CHOH OH
-
R2CHO- MnO4
-
R2CO
5 6 7
Scheme-2
In general good yields are obtained from alkaline permanganate oxidation
except when the initial product is an enolizable aldehyde or ketone. In such cases
oxidation of the enolic C-C double bond may become a serious side reaction.
Alkaline permanganate has been successfully used in the oxidation of
carbohydrates to monocarboxylic acid derivatives.12, 13 In this procedure all the
hydroxyl except the one undergoing oxidation are first protected by acetoneation
or similar means. For example Link and Sell12 have been describing the
25
conversion of diacetone D-galactose (8) to diacetone D-galacturonic acid (9).
(Scheme-3)
HC
OH
HO
HO
H
CH2OH
O
O KMnO4/ 1.7M KOH
250C, 24 hr
HC
OH
HO
HO
H
CO2H
O
O
8 9 (49-65%)
Scheme-3
The base catalysis of these reactions is due to ionization of the alcohols.
However, the observation of acid catalysis is probably due to a change in nature of
the oxidant rather than the substrate, i.e., in acidic solutions manganese (VII)
exists partially in the form of permanganate acid, which is presumably a better
oxidant than the anion.10, 14
MnO4- H+ HMnO4
Since permanganate is a fairly vigorous and non selective oxidant, it does
not readily lend itself to use in organic solvents. However acid permanganate
oxidation can be conducted in glacial acetic acid; e.g. Neidig and co-workers 15
obtained good yields of alkyl phenyl ketones (71-96%) by the oxidation of
alkylphenylcarbinol. A solution of potassium permanganate dissolved in 175ml
water and 400ml of glacial acetic acid was added dropwise to a stirred solution of
26
the alcohol over a period of 3 hr during which time the temperature was
maintained between 25-300C .The products then isolated by steam distillation and
purified by conventional methods. Yields were greatly decreased with t-
alkylphenylcarbinols because these compounds readily undergo cleavage to give
benzaldehyde. (Scheme-4)
CHC(CH3)3 CC(CH3)3
KMnO4
250C, 3hr
OHO
CH
O
10 11 (34%) 12 (46%)
Scheme-4
Under neutral aqueous conditions 1, 4-diols undergo an oxidative
cyclization to give γ-lactones when treated with permanganate ion.16 (Scheme-5)
OH
OHKMnO4/ H2O
20C, 8hrO
O
13 14 (71%)
Scheme-5
II. 1.B. Oxidation of alcohols with Manganese Dioxide:
Manganese dioxide occupies an important place as oxidants because of its
considerable selectivity. MnO2 exhibits considerable selectivity and under mild
conditions it is a specific oxidant for α, β-unsaturated alcohols. However in the
27
absence of unsaturation. It will also slowly attack primary and secondary alcohols
to give aldehydes and ketones in good yields. It can be used as a suspension in a
variety of solvents (water, acetic acid, carbon tetrachloride, chloroform,
methylene chloride, hexane, benzene, tetrahydrofuran, isopropyl alcohol,
acetonitrile, ether ethyl acetate, pyridine, DMSO etc. ) to selectively convert α, β-
unsaturated alcohols to α, β- unsaturated carbonyl compounds but other easily
oxidizable functional groups such as carbon-carbon double bonds, saturated
alcohols and vicinal glycols are much less readily attacked by this reagent at room
temperature. These may be understood from following reactions.17-19 (Scheme-6)
HC CC
CH3
CHCH CH CHCH3
OH
HC CC
CH3
CHCH CH CCH3
O
MnO2/ pet ether
250 C, 30 min
15 16
OHOH
OH
O
OHOH
MnO2/ CHCl3
250 C, 6.25 hr
17 18
OH
C CCHCH3
OH
MnO2
OH
C CCCH3
O
19 20
Scheme-6
28
It is apparent that it makes little difference if the unsaturation is part of an
olefinic, acetylenic, aromatic, and allylic or carbonyl system. α-hydroxy ethers are
also oxidized by MnO2, suggesting that the reaction may be initiated with any
structure that provides an electron-rich source in the position adjacent to the
hydroxyl20. Much evidence has accumulated suggesting that the activity of this
reagent depends on the procedure used in its preparation.17, 21-23
II.1.C. Oxidation of alcohols with Lead Tetraacetate:
Lead Tetraacetate can be used as an alcohol oxidant in a variety of
solvents including acetic acid, benzene, heptane, pyridine, chloroform and
mixtures of these. Isolation of the crystalline ester
(AcO)2Pb
OCH3
OCH3 from a wet
methanol solution of lead tetra acetate indicates that the reaction probably
proceeds by way of an esterification mechanism reminiscent of the one previously
discussed for chromic acid oxidations24. In nonpolar solvents these esters can then
undergo decomposition to give carbonyl compounds, cyclic ethers or olefins. All
these products can be formally accounted for by assuming a cyclic transfer of
electrons with concomitant hydrogen abstraction from either the α-, β-, δ- or ε-
carbons.
If hydrogen is abstraction from α-position, the products are carbonyl
compounds.(Scheme-7)
29
O
HPb(OAc)2
O
Ac
RCOR' + Pb(OAc)2 + HOAc
R
R'
21 22
Scheme-7
With short chain primary aliphatic alcohols almost equal quantities of
ether and aldehyde are formed 25, but with longer chain primary and secondary
alcohols the yield of carbonyl product decreases.26, 27 (Scheme-8)
CH3CH2CH2CH2CH2OHPb(OAc)2/heptane
reflux, 18 hr
O CH3
C4H9CHO C5H11OAc C5H11OH
23 24(25%) 25(20%) 26(30%) 27(25%)
Scheme-8
Mihailovic and co-workers28 investigated optimum conditions for the
formation of cyclic ethers from aliphatic alcohols and found it best to dissolve
both the alcohol and the oxidant in benzene before starting the reaction by
applications of heat.
The products obtained from oxidations of δ,ε- andγ, δ-unsaturated alcohols
can be satisfactorily accounted for by similar cyclic electron transfers.29, 30
In such cyclic mechanism it is difficult to assign the direction of electron
flow and thus determine if the hydrogen is being transferred as a proton, as a
hydride ion, or as an unchanged atom. Hence the arrows used in the above
30
formulation are illustrative of only one of three possibilities. Current evidence, in
fact, indicates that an ionic mechanism predominates in acetic acid 31and
pyridine26, 28 solutions, with single-electron shifts being more likely in nonpolar
solvents such as benzene and heptanes.32 (Scheme-9)
O
O
Pb(OAc)2
O
OAcPb(OAc)2
CH2O
28 29
OAc
OPb(OAc)2 O
OAc+ Pb(OAc)2
30 31
Scheme-9
For example:
Nonpolar solvents would favor the hemolytic process, while polar and
basic solvents such as pyridine would promote carbonyl formation. Thus when
primary and secondary alcohols are oxidized in pyridine, the yield of carbonyl
CH2=CHCH2 CH 2OHPb(OAc)2/C6H6
refluxCH2=CHCH2CH2OCH2 C H =CH2+C H2=CHCH2O A c
(30%) (13%)
+ CH2=CHC H 2CH2OAc +
CH 2CH=C H2
(20%) (32%)
31
product is generally increased (50-90%) and no cyclic ether is formed. Hence use
of lead tetraacetate in this solvent provides an easy route from primary alcohols to
aldehydes. (Scheme-10)
CH3CH2CH2CH2OHPb(OAc)4/ Pyridine
ref lux, 18 hrCH3CH2CH2CHO
32 33 (75%)
Scheme-10
II.1.D. Oxidation of alcohols with Copper and Cupric ion:
Primary and secondary alcohols may be dehydrogenated to aldehydes and
ketones, respectively, by passing the vapors at an elevated temperature over
activated copper, copper-chromium or copper-nickel catalysts in the presence of
oxygen.33 Fair yields (46-72%) of aldehydes are obtained by passing primary
alcohol vapors over copper gauze at 5000 C and 80-150 mm of Hg pressure33.
Copper catalysts have also been employed in the commercial preparation of α, β-
unsaturated carbonyl compounds from the corresponding alcohols. Thus
methacrolein can be prepared in good yield (95%) from methallyl alcohol by
passing the vapor along with air 1 atm and about 3500 C over copper gauze.34
(Scheme-11)
CH=C-CH2OH
CH3
CH2=CH-CHO
CH3Cu, O2
3500C
34 35 (95%)
Scheme-11
32
Cupric ion can also be used in aqueous acetic acid or pyridine solution to
convert α-ketols into the corresponding α-diketones.35-36 The typical application
of this reactions are found in the conversion of benzoin (36) to benzil35,36 ; the
oxidation of sebacoin (38) to sebacil.35(Scheme-12)
C CH
O OH
Cu(OAc)2, NH4NO3/HOAc
reflux, 1.5 hrC C
O O
36 37
(CH2)8
C=O
CHOH
(CH2)8
C=O
C=O
Cu(OAc)2/ AcOH
reflux, 1 min.
38 39
Wendler and Coworkers have reported the conversion of α-ketols (40) and
(42) to the corresponding disophenol37 (43) or the glyoxal (41).38 (Scheme-13)
C=O
CH2OH
Cu(OAc)2, O2 / CH3OH
C=O
CH(OH)2
250C, 15 min
40 41
Scheme-12
33
O
OH
Cu(OAc)2/ CH3OH
O
OH
reflux, 1 hr
42 43
Scheme-13
In addition , both liquid and vapor phase dehydrogenation of 1,4-,1,5- and
1,6- diols to the corresponding lactones is accomplished by use of a copper
chromite catalyst.39-41 (Scheme-14)
HOCH2CH2CH2CH2CH2OHCopper Chromite
2100C, 30 min
O
O
44 45
Scheme- 14
II.1.E. Oxidation of alcohols with Platinum and Oxygen:
Primary and secondary alcohols are readily oxidized by oxygen in the
presence of platinum.42 The reaction is of particular importance because of its
unique selectively. Primary alcohols are usually oxidized more than secondary
alcohols and with polyhydroxy compounds such as carbohydrates, oxidation of
only the primary hydroxyl can be achived.43-46 Under alkaline conditions (water or
aqueous acetone) primary alcohols are converted, often in quantitative yields, to
the corresponding carboxylic acids. However when the reactions is carried out
under neutral conditions in an organic solvent (heptane, dioxane, ethyl acetate,
34
glacial acetic acid, chloroform, butanone or benzene), the products are
aldehydes.47
Steroids containing two or more hydroxyls, with one being in the 3-
position, undergo selective oxidation at that position.48-52 (Scheme-15)
OH
HO
OH
CO2CH3
Pt, O2/ EtOAc
250C, 16 hr
OH
OH
CO2CH3
O
46 47 (70%)
Scheme-15
In an investigation of the oxidation of several cyclitols it was observed that
axial hydroxyl were attacked in preference to equatorial hydroxyl groups; i.e.,
myo-inositol (48) underwent oxidation only at the lone axial hydroxyl to give the
corresponding ketone (49) as the product, while scyllo-inositol (50) lacking axial
hydroxyls, was resistant to oxidation42 When two or more axial hydroxyls are
present, only one is oxidized, no diketones have been found in the products.
(Scheme-16)
HO
HO
OH
OH
HO
HO
HO
OHHO
HO
Pt, O2
O
OH
48 49
35
HO
OH
OH
OH
HO
HO
Pt, O2
50
Scheme-16
The reagent can also be used in the oxidation of unsaturated alcohols since
it does not readily attack carbon- carbon double bonds.53
II.1.F. Oxidation of alcohols with Ruthenium Tetroxide:
Ruthenium tetroxide is a vigorous oxidant that can be used for the
oxidation of secondary alcohols to ketones. Following reactions indicate the
successful use of RuO4 as an oxidant for different types of alcohols.54-56 (Scheme-
17)
OH O
RuO4/CCl410-150C
51 52 (93%)
OH
RuO4/CFCl3
10C, 1-5 min
O
53 54 (62%)
Scheme-17
36
Although benzyl alcohol has been reported to give benzaldehyde in good
yield (90%) when oxidized by RuO4, the yield from other primary alcohols is
much lower.56
II.1.G. Oxidation of alcohols with Bismuth Trioxide:
Bismuth trioxide has been used in the oxidation of acyloins and it appears
to be a specific reagent for this type of compound. Acyloins are easily oxidized to
α-diketones by heating in acetic acid.57 For example, octane-4, 5-dione is obtained
in good yield (84%) from the oxidation of butyroin with bismuth trioxide in acetic
acid for 15 min at 1000C. (Scheme-18)
RCOCHOHRBi2O3/AcOH
RCOCOR
55 56
Scheme-18
Application of this reaction to steroids has been made in the conversion of
α-ketols into diosphenols.58 (Scheme-19)
O
HO
Bi2O3/AcOH
OH
O
57 58
Scheme-19
37
II.1.H.Oxidation of alcohols with Ceric ammonium Nitrate (Ceric
ion):
Ceric ion is a strong oxidant which readily attacks alcohols. However
carbonyl compounds bearing α-hydrogen are also very rapidly degraded by this
reagent thus unless a large excess of alcohol is used 59 its synthetic usefulness is
limited to the preparation of aldehydes and ketones which do not bear α-
hydrogen.60-61
For example, benzaldehyde and several substituted benzaldehyde were
prepared in over 90% yield by oxidation of the corresponding benzyl alcohols
with 0.5 M ceric ammonium nitrate in 50% aqueous acetic acid. 62 (Scheme-20)
CH3O CH2OHCAN
750CCH3O CHO
59 60 (80%)
CH2OHCAN
750CCHO
61 62 (64%)
Scheme-20
This is most useful and widespread oxidation, as it encompasses the
alcohols to aldehydes to carboxylic acids sequence that is used so often in
synthesis.
38
II.2. Oxidation with Cr(VI) Compounds:
A great variety of compounds containing Cr (VI) have been proved to be
versatile reagents capable of oxidizing almost every oxidizable organic functional
group.63(a-g) In spite of this wide spectrum of action the reagents may be controlled
to give largely just one single product.64 Chromic acid is the most popular reagent
of this type used in organic chemistry for well over a century.64 In the last two
decades, however, a number of new chromium (VI) containing reagents together
with special reaction conditions have been developed to improve the selectivity of
oxidation and to deal with complex, highly sensitive compounds.64
II.2. Oxidation of Different Substrates with Cr(VI) Compounds:
II.2. A. Oxidation of alkanes:
Chromium (VI) oxidation of saturated hydrocarbons does not represent a
useful synthetic procedure since the reaction lacks selectivity and owing to the
vigorous reaction conditions and considerable second stage oxidation occur. As a
result complex mixtures of products are obtained, e.g. it is difficult to oxidize a
methylene group to secondary alcohols without further oxidation of the alcohol to
ketone and in some cases it is necessary to use condition sufficiently vigorous to
cause carbon-carbon bond cleavage as well. Usually it is possible to convert
tertiary C-H bonds to tertiary alcohols and isolated methylene to ketones.
However, many tertiary alcohols undergo a facile dehydration, making these
compounds difficult to obtain in good yield with reagents such as chromic acid
which require acidic conditions.
39
In principle it might seem that compound containing Cr(VI) (chromic acid,
chromyl acetate, chromyl chloride, t-butyl-chromate, or dichromate ion ) should
be excellent oxidants for alkanes since they are soluble in a number of non-
aqueous solvents such as acetic acid , acetone and pyridine. However it has been
found difficult to avoid further oxidation of the initial products when chromium
(VI) oxidants are used due to the presence of tertiary C-H bonds, it is usually
possible to oxidize tertiary hydrogens preferentially in the presence of secondary
hydrogens. For example cis-decalin gives as the major products cis 9- decalol
when oxidized by dichromate ions in aqueous acetic acid.65 However, the tertiary
alcohols so produced often dehydrate readily and undergo a facile second stage
oxidation.
The Sager and Bradley 66reported that triethylmethane was easily oxidized
to triethylcarbinol, but unfortunately the product underwent dehydration and
further oxidation to diethyl ketone. The alcohol was obtained in 41% yield only if
the reaction was quenched after an appropriate time interval. Clearly maximum
possible yield for particular compounds will be determined by the relative
oxidation rate for the hydrocarbon and the corresponding alcohols reaction shown
below (Scheme-21):
H
Na2Cr2O7
AcOH/H2O
OH
+
O
63 64(41%) 65 (36%)
Scheme-21
40
Wiberg and Foster studied the oxidation of tertiary C-H bonds with
chromic acid predominantly to retention of configuration about the carbon atoms.
Oxidation of (+)-3- methylheptane by dichromate ion in 91% acetic acid gives
(+)-3- methyl -3-heptanol in only about 10% yield, but 70-85% retention of
configuration.67 (Scheme-22)
CrO3
AcOH
H OH
66 67 (7%)
Scheme-22
II.2.B. Oxidation of alkenes:
Chromium (VI) oxidation of C-C double bonds can lead to formation of a
variety of compounds, including epoxides, α-ketol and cleavage products. Since
mixtures of products are usually obtained, the synthetic value of this reaction is
minimized. However, it is possible to obtain respectable yields of some products if
experimental conditions are carefully controlled and if structure of the oxidant is
such possible number of oxidation path is limited.68
II.2.B. (i) oxidation of allylic C-H bonds
Good yields of α, β unsaturated ketones have been obtained from certain
compounds (usually steroidal alkenes) when they are subjected to oxidation by Cr
(VI) in acetic acid solution.
41
Marshall et. al.69 has reported the introduction of a carbonyl at the C-7
position of pregneolone acetate by use of sodium chromate in an acetic acid –
acetic anhydride solution. (Scheme23)
AcO
C=O
CH3
CrO4-2
/AcOH
CH3
C=O
AcO O
68 69 (79%)
Scheme-23
With ∆8–pregnene derivatives, oxidation of allylic C-H bond is
complicated by the possibility of carbonyl formation at both the C-7 and C-11
position. For example 3β, 21- diacetoxy- 4,4,14 α-trimethyl ∆8-5α-pregnene-20-
one (70) gives the triketone (71) when oxidized by chromium trioxide in acetic
acid.70 (Scheme-24)
C=O
OAc
CH2OAc
C=O
OAc
CH2OAc
O
O
CrO3/ AcOH
70 71
Scheme24
42
From other compounds which have two unsubstituted allylic position,
mixture of products has been obtained. Thus both carvotanacetone (73) and
piperitone (74) obtained from carvomenthene (72).71, 72 (Scheme-25)
Cr(VI)
O
O
72 73 (25%) 74 (26%)
Scheme-25
The products of these reactions are sometimes contaminated by side
products arising from epoxide formation or double bond migration. For example
Wintersteiner and Moore73 obtained fine products (76-80) in varying yields when
they oxidized α (∆8-14) –cholesteryl acetate (75) with chromium trioxide in acetic
acid. (Scheme-26)
AcO
R
CrO 3
AcOH
AcO
R
O
O
+
AcO
R
O
75 76(16%) 77 (6%)
AcO
R
O
OH
+
AcO
R
O
AcO
R
+
OO
O OH
OH
O
+
78 (2%) 79 (1%) 80 (1%)
Scheme-26
43
II.2.B. (ii) Formation of epoxide and cleavage products:
Phenyl substituted C-C double bonds often yield epoxides and /or cleavage
products when oxidized by chromium (VI). These reactions are considerable
interest from a mechanistic point of view74 but because of the variety of products
usually obtained and because the epoxides and Cleavage products can be more
easily produced using other above reagents, they have little synthetic value. For
example A synthetically useful cleavage of a double bond by means of chromium
trioxide in acetic acid has been devised by Barbier75 which has first applied for
degradation of bile acid side chains by Wieland.76 The degradation involves
oxidation of trisubstituted olefins obtained by reaction of a carboxylic ester with
Grignard reagent (usually phenyl magnesium bromide or, less frequently methyl
magnesium iodide). Followed by dehydration of the resulting tertiary alcohol.
(Scheme-27)
R'
R-CH-COOCH3C6H5MgBr R-CH-C-OH
R' C6H5
C6H5
C C
C6H5
C6H5
R'
R
CrO3 AcOH
R-COOH R-C-R'
O
ab
81 82
Scheme-27
44
II.2.C. Oxidation of arenes:
Chromium (VI) is the most versatile reagents for the oxidation of arenas
used in its various forms [ HCrO4-, Cr2O7
2- ,CrO2Cl2, CrO2(OAC)2] . Under proper
condition it will selectively convert side chain to carboxylic acids, aldehydes,
ketones, and diacetates. Under other conditions it attacks polynuclear arenes and
leaves the side chains intact.
Chromic acid oxidizes the side chain of phenylalkanes to give benzoic acid
derivatives77, 78 but benzene ring is itself attacked only under very vigorous
condition.79 Polynuclear aromatic hydrocarbons, however readily undergo ring
oxidation to yield the corresponding quinine.80 This chemoselectivity is shown in
the Cr (VI) oxide in anhydrous acetic acid (Fieser reagent).
A dichromate ion in neutral aqueous solution does not attack aromatic
systems but instead oxidizes aryl alkanes to the corresponding carboxylic acids or
ketones.81-82 Chromyl chloride converts toluene derivatives to substituted
benzaldehydes and other aryl alkanes primarily to β-phenyl carbonyl products.83
Chromyl acetate reacts with toluene derivatives to give fair yields of the
corresponding benzal diacetates.84
The selectivity that can be achieved with these reagents is indicated by the
reactions on the following pages.78, 82-84 (Scheme-28)
45
O2N CH3
Cr2O7-2/H2SO4
1000C, 30min
Cr2O2O7-2/H2O
2500C, 18hr
Cr2O2Cl2/CCL4
770C, 10hr
Cr2O7(OAc)2O/ (OAc)2O
5-100C, 2 hr
O2N CO2H
O2N CO2H
84 (82-86%)
85 (94%)
O2N CH
O2N CH(OAc)2
O
86(26%)
87 (66-67%)
83
Scheme-28
Chromic acid probably attacks arylalkanes at the C-H bond adjacent to the
ring. Four piece of evidence support this view. (Scheme-29)
(1) Chromic acid will not oxidize side chain such as t-butyl which lack α-
hydrogens.
(2) The oxidation of secondary alkyl chain gives phenyl ketones and a carboxylic
acid from CrO3
CH
CH3
CH2RC
CH3
CH2R
OH
CrO3 -H2OC
CH3
CHR
88 89 90
46
CrO3
C
O
CH3+RCOOH
91
Scheme-29
(3) oxidation of n-alkyl benzene gives benzoic acid and an aliphatic carboxylic
acid with one carbon atom less than the original side chain.85 (Scheme-30)
CH2CH2RCrO3 CO2H + RCO2H
92 93
Scheme-30
(4) The products obtained from the oxidation of equilenin acetate (94) and similar
compounds obviously result from α attack.86 (Scheme-31)
CrO3
O
OAc
OO
OAc
OH
O
OAc
94 95
Scheme-31
47
II.2.D.Oxidation of ethers:
Inorganic oxidants have seldom been used to convert ether to more highly
oxidized compounds. However several reagents are capable of reacting with
methylene groups activated by adjacent oxygen.
Chromic acid converts ethers into esters or lactones by oxidation of α-
methylene groups to carbonyl functions.
Henbest and Nicholls 87 obtained a high yield of the (97) from the
corresponding cyclic ether (96). Use of acetic acid as the solvent in a similar
reaction gave a lower yield (54%) of lactone.88 (Scheme-32)
O
CrO3/acetone-H2SO4
200C, 12hr
O
O
96 97 (82%)
Scheme-32
Anhydrous chromium trioxide in acetic acid can also be used to convert
methyl ethers (98) into the corresponding (99).89 (Scheme-33)
CH3ORHCOR
O
CrO3/AcOH
98 99 (50%)
Scheme-33
48
II.2.E. Oxidation of aldehydes:
Dichromate ions readily oxidize aldehydes to the corresponding carboxylic
acids in aqueous solutions. With both reagents however, and particularly under
strongly acidic or basic conditions, enolizable aldehydes suffer C-C bond
cleavage. Hence because of its greater reactivity under neutral conditions,
permanganate ion is probably the more useful oxidant for these compounds.
Furoic (101) for example, has been prepared in good yield from furfural by
use of potassium dichromate in an aqueous sulphuric acid solution.90 (Scheme-34)
O
CH
O
K2Cr2O7/9H2SO4
1000C, 45 min.O
CO2H
100 101 (75%)
Scheme-34
Aldehydes hydrates form chromate esters which undergo an acid catalyzed
decomposition with cleavage of the α C-H bond in the rate determining step.
(Scheme-35)
RCHO + H2O RCH(OH)2
RCH(OH)2 + HCrO4- RCH(OH)OCrO3
- + H2O
RCH(OH)OCrO3- + H+ RCOOH + H2O + Cr(IV)
Scheme-35
49
In contrast to alcohols oxidation however the rate of reaction is not greatly
increased by replacing water with acetic acid as solvent.91 Such a change
suppresses the hydration equilibrium and this reagent is a C-C bond cleavage
initiated by attack at the α position of enolizable aldehydes. Thus oxidation of
normal and isobutylaldehyde results in formation of approximately 30% CO2 by
the following reactions.92 (Scheme-36)
(CH3)2CHCHO (CH3)2C=CHOHCr(VI)
CH3COCH3 + CO2
CH3CH2CH2CHO CH3CH2CH=CHOHCr(VI)
CH3CH2CO2H + CO2
Scheme-36
II.2.E. (i) Oxidation of aldehydes with heterocyclic halochromate:
Pyridinium Chlorochromate (PCC) is a mild acidic oxidizing agent which
can promote oxidative cationic cyclization of certain unsaturated aldehydes.93
This 2 isobutenylcyclohexan carboxaldehyde is converted to 3-methyl Δ2-
1-octalone in 78% yield in methylene chloride with 3 equivalent of PCC at room
temperature, followed by acidic treatment of the crude cyclization product with p-
toluenesulphonic acid. Formations of intermediate bicyclic alcohols which further
oxidize to the corresponding ketones have also been observed. (Scheme-37)
CHO
PCC
CH2Cl2
OH
+
OH
OH
102 103 104
50
O
+
O
OH
H+
CH2Cl2
O
107 105 106
Scheme-37
Jameel94 have investigated the oxidation of disubstituted benzaldehyde by
PCC in binary solvent mixture of 50 % (v/v) aqua-acetic acid. The reaction is first
order with respect to [aldehyde], [PCC] and [H+].
Pyridinium dichromate (PDC) is found as an useful oxidant of aldehydes
in aprotic media. Unconjugated aldehydes are readily oxidized to the
corresponding carboxylic acids by PDC in dimethylforamide at 250C95.
Cyclohexene-4-carboxyaldehyde and 4-methyl cyclohexene-4- carboxaldehyde
have been converted under these conditions into the corresponding carboxylic acid
in 90% yield. (Scheme-38)
RCHO
PDC
DMF, 250C
RCOOH
108 109
R=H, CH3
Scheme-38
51
Oxidation of both aliphatic and aromatic aldehydes by PFC has been
reported.96, 97 The kinetics is similar to those observed in the oxidation of
alcohols. Oxidation of deuteriated acetaldehyde and benzaldehyde indicated the
presence of substantial primary kinetic isotope effects. The following
mechanism has been proposed to account for the observed data
(Scheme-39).
R C O
H
+ Cr
O
O O-PyH
+
F
R C O
H
+Cr
O
O O-PyH
+
F
R C+
O + (HOCrOPyH)-
RCOOH + CrOFOPyH
Scheme-39
The oxidation of aliphatic aldehyde by BPCC in DMSO has been reported
by Banerji et.al.98. The reaction was found first order with respect to aldehyde and
BPCC. The reaction is catalyzed by hydrogen ion, the hydrogen ion dependence
taking the form kobs = a + b [H+]. The oxidation of deuterated acetaldehyde
indicated the presence of primary isotope effect. A mechanism involving transfer
via a chromate ester has been proposed.
52
Pandurangan et. al.99 observed that the oxidation of substituted
benzaldehydes by QFC is first order with respect to each of the reductant, QFC
and hydrogen ions. They obtained a reaction constant of 1.16 for the oxidation of
m- and p-substituted benzaldehydes. The authors have suggested a mechanism
involving formation of a chromate ester in a rapid pre-equilibrium step. The
decomposition of the chromate ester, in the rate-determining step, is base-
catalyzed.
Jameel100 studied the kinetics of oxidation of benzaldehyde and several
para and meta substituted benzaldehydes to corresponding carboxylic acid. The
reaction proceeds with the formation of QCC-ester as the intermediate.
II.2.F. Oxidation of ketones:
Oxidation of ketones with dichromate ion usually results in formation of a
carboxylic acid by cleavage of an adjacent C-C bond.
2-acetylfluorene (110) is converted to fluorenone -2-carboxylic acid (111)
in fair yield by treatment with sodium dichromate in acetic acid101. Oxidation of
cyclic ketone gives a dicarboxylic acid as principle product containing the same
number of carbons as the parent compound.102 (Scheme-40)
C CH3
O Na2Cr2O7/CH3COOH
reflux, 10 hrCO2H
110 111 (67-74%)
Scheme-40
53
Sometimes when glacial acetic acid is used as the solvent, it is found
helpful to add acetic anhydride in the midway of the reaction to remove water
which is produced during the oxidation. At the completion of the reaction excess
chromium (VI) is reduced by addition of a few milliliters of ethanol.
Rocek & Riehl 102, 103 have shown from a kinetic study of the oxidation of
isobutyrophenone and 2-chlorocyclohexanone that the reaction proceeds via an
end intermediate. At high oxidant concentrations the rate of reaction becomes
approximately equal to rate of enolization and independent of the hexavalent
chromium concentration. Oxidation of the enol produce a diketone which can in
some cases be isolated 104, 105 but which usually undergo a facile cleavage to give
final products. For example the oxidation of cyclohexanone has been studied in
some details. It has been shown that the first step of the reaction in aqueous
perchloric acid is an α-oxidation leading to 2-hydroxycyclohexanone. Less than
3% of the reaction proceeds by dehydrogenation 2-cyclohexen-l-one, 2-
hydroxycyclohexanone is then oxidized further to 1, 2- cyclohexandione and
cleaved to adipic acid.102 (Scheme-41)
O
aq. perchloric acid
O
OH
O
O
+
COOHCOOH
112 113 114 115
Scheme-41
54
II.2.G. Oxidation of amines:
Cr(VI) compounds react with amines by abstraction of a hydrogen atom
from the α-position to give either a compound with an oxygen function in that
position or one containing a carbon nitrogen double bond. In the latter case the double
bond is often cleaved to give carbonyl compounds. An example is provided by
reaction of an indole derivative, deoxygajmaline O-acetate, with chromium
trioxide.106 (Scheme-42)
N
CH3
CrO3/Pyridine
250C, 16 hrN
CH3
OH
+
N
116 117 (19%) 118 (15%)
Scheme-42
The oxidation is affected simply by adding the amine in pyridine to an ice-
cold solution of CrO3 in 30 ml of pyridine and stirring at room temperature for
several hours.
Bottini and Olsen107 have developed a procedure whereby primary and secondary
amines can readily be converted to the corresponding carbonyl compounds. In this
procedure the amine is first reacted with 2, 4-dinitroaniline (119) which can then
be oxidized with hexavalent chromium. (Scheme-43)
NO2RR'CHNH
O2N
CrO3/12MH2SO4
1250C, 15 minNO2H2N
O2N
+RCOR'
119 120 (25-95%)
Scheme-43
55
The greater difficulty with this reaction is the fact that although 2,4-
dinitroaniline is relatively resistant to further oxidation under these conditions, the
carbonyl compounds are not , and lowest yields were observed for those
aldehydes and ketones that are most susceptible to further reaction with chromic
acid.
II.2.H. Oxidation of Alkyl Halides:
The direct conversion of alkyl halide into the corresponding carbonyl
compounds, despite its potential interest for organic synthesis, is seldom referred
to in the literature. Only few examples of the oxidation of benzylic halides with
aqueous acidic chromic acid or with aqueous sodium dichromate and alkali
hydroxides under rather drastic conditions have been reported.108 A preparatively
useful example is the oxidation of α-chlorohydrindene to α- chlorohydrindanone
in 50-60% yield by means of chromic acid in aqueous acetic acid at 400 C.109
(Scheme-44)
Cl
CrO3
H2O/AcOH
Cl
O
121 122
Scheme-44
Oxidation of steroidal vicinal dibromide to the α-bromoketone by
means of chromic acid in aqueous acetone in the presence of silver chromate at
room temperature forms a key step in an early procedure for the synthesis of
corticoids from bile acids.110 (Scheme-45)
56
Br COOMe
O
CrO3
Ag2CrO4
Acetone - H2O
BrBr
O
COOMeO
123 124
Scheme-45
The reaction is performed in hexamethylphosphoramide using a slight
excess of potassium chromate in the presence of one equivalent of dicyclohexyl-
18-crown-6 at1000c. The yields are in the range of 80%.
A remarkable improvement of this reaction which avoids the use of
hexmethylphosphramide and the crown ether has been obtained by using polymer
supported chromic acid as oxidant.111 (Scheme-46)
P CH2N+
Cr
O
O
OH-O
+ RCH=CHCH2-HalC6H6 RCH=CHCHO
125 126
Scheme-46
II.2.I. Oxidation of Acetals:
Cyclic methylene or benzylidene acetals of alditols cleaved by chromium
trioxide in glacial acetic acid to give derivatives of ketoses, provided that the
hydroxyl groups are suitably protected, e.g. by acylation. For example, 1, 3, 5-tri-
O-benzylidene-DL-xylitol (127) is oxidized smoothly by the reagent in 1 hr at
57
room temperature to 1, 3, 5-tri-O-acetyl-4-O-benzoyl-keto-DL-thero-pentulose
(128) in 84% yield.112 (Scheme-47)
CH2OH
CH2OAc
O
O
AcO PhCrO3
AcOH, 250C
CH2OH
CH2OAc
OCOPh
O
AcO
127 128 (84%)
Scheme-47
Unsymmetrical acetals, however, can open at either position of attachment
to yield two products. Thus , 1,3,5,6-tetra-O-acetyl-2,4-O-benzylidene-D-glucitol
(129) is oxidized to yield 1,3,5,6- tetra-O-acetyl-2,4-O-benzoyl-keto-D-fructose
(130) , as the main product, together with small amounts of 5-O-benzoyl-oxyhex-
3-ulose (131).112 (Scheme-48)
CH2OAc
CH2OAc
OAc
AcOH
O
CH2OAc
CH2OAc
OAc
O
AcOH
OCOPh
+
OCOPh
CH2OAc
CH2OAc
OAc
O
AcO
O
CrO3
250C/AcOHPh
129 130 131
Scheme-48
Methylene acetals behave similarly. For example1 3, 4, 6-tetra-O-acetyl-
2,5-O-methylene-D-mannitol (132) under similar conditions affords 1,3,4,6- tetra-
O-acetyl-5-O-formyl-keto-D-fructose (133) in 87% yield.112 (Scheme-49)
58
CH2OAc
AcO
O
OAc
CH2OAc
CH2
O
CrO3
AcOH, 250C
CH2OAc
AcO
OAc
CH2OAc
OCHO
O
132 133
Scheme-49
The oxidation of aromatic acetals by acidic Pyridinium chlorochromate
(PCC) has been studied by P.S. Ramakrishnan 113. The reaction is first order each
in [PCC] and [Acetal]. The reaction does not induce polymerization of
acrylonitrile. The presence of electron withdrawing substituents in the benzene
ring enhances the rate of oxidation. The rate of oxidation depends on the nature of
alkyl group.
II.2. J. Oxidation of sulphides:
Oxidation of Sulphides is oxidized to sulphones by chromic acid under
acidic conditions although the conversion is under acidic medium affected more
efficiently using other oxidants such as potassium permanganate. For example, 2,
2, 4, 4’, 6, 6’-hexanitro-3-methyldiphenyl sulphide (134) is converted to the
corresponding sulphone (135) in 95%yield on treatment with chromium trioxide
in concentrate nitric acid at room temperature.114(Scheme-50)
O2N
NO2
S
NO2
O2N
NO2
O2N
CrO3
HNO3 conc.200C
O2N
NO2
NO2
O2N
NO2
O2N
SO2
134 135 (95%)
Scheme-50
59
Meenakshisundaram et.al.115 studied the oxidation of organic sulphides by
PDC in acetonitrile medium. They observed a first order dependence on PDC and
sulphide but second order in TsOH. In case of aryl methyl sulphides, the order
with respect to TsOH is >1 and < 2, while Michaelis–Menten type kinetics were
observed with respect to the sulphide. A nonlinear Hammett plot was obtained
with both electron donating and electron withdrawing groups slowing down the
rate of reaction. The reaction was retarded by acrylonitrile, thereby indicating a
one electron oxidation giving rise to free radicals.
The oxidation of a number of organic sulphides116 showed by PFC that the
reaction is second order, first in each reactant. The substituent effect was analyzed
using techniques of correlation analysis. Solvent effect showed that the transition
state is more polar than the reactants.
Meenakshisundaram et.al.117 studied the oxidation of sulfides by QFC in
non-aqueous medium. The oxidation kinetics of sulfides was studied under pseudo
first order condition with QFC and second order in catalyst, MeCN α-
MeC6H4SO3H. Sulfide concentration has no effect on the reaction rate. CH2
=CHCN retard the oxidation rate significantly in di-Ph sulfides, indicating the
involvement of free radical cation intermediate in rate determining step. The
Michaelis - Menten dependence each on [sulfide] [α-MeC6H4SO3H] was well
explained by two-pathway mechanism in sulfide oxidation.
The rate of oxidation of dialkyl and alkyl phenyl sulphides by QCC has been
studied in aqueous acetic acid.118 The proposed mechanism envisages oxygen
transfer from QCC, which is in agreement with the earlier observations.
60
II.2.K.Oxidation of Oximes:
Chromic acid in 50% aqueous acetic acid, Jones’ reagent and chromium
trioxide-(pyridine)2 in methylene chloride have been shown to bring about
oxidative deoximation of ketoximes to the corresponding carbonyl compounds
with varying degrees of success.119 (Scheme-51)
R'-C-R"
N
Cr(VI)
OH
R'-C-R"
O
136 137
Scheme-51
Palaniappan et.al.120 studied the oxidation of oxime by PDC in 60 % acetic
acid-water (v/v) medium. The order of reaction is first with respect to [oxidant],
second with respect to the [substrate] and zero with respect to [H+].
The use of 2 equivalent of pyridiniunm chlorochromate was in methylene
chloride at room temperature for 12 hrs. It is also effective in converting
ketoximes to the corresponding ketones in yields of 50-85% while benzaldoxime
affords benzaldehyde without further oxidation & oxime ethers are resistant to the
reagent.121 (Scheme-52)
O
PCC
CH2Cl2
N
OH
138 139
Scheme-52
61
The oxidative deoximination of several aldo and ketooximes by
Quinolinium fluorochromate [QFC] in DMSO exhibited a first order dependence
on both the oxime & QFC.122 The oxidation of ketoximes is slower than that of
aldoximes. The oxidation of aldoximes correlated well in term of PAVELICH-
TAFT dual substituent–parameter equation .The low positive value of polar
reaction constant indicated a nucleophillic attack by a chromate-oxygen on the
carbon. The reaction is subject to steric hindrance by alkyl groups. A mechanism
involving the formation of a cyclic intermediate in the rate determining step has
been proposed.
II.3. Oxidation of Alcohols with Cr(VI) Compounds:
The first review pertaining to the use of novel chromium (VI) reagents for
the oxidation of primary and secondary alcohols was published in 1982, which
summarized the salient features of several hexavalent chromium compounds123.
The oxidation of alcohols constitutes the synthetically most important application
of Cr (VI) compounds. It has been largely divided into four sections.
1. Acid catalyzed oxidation with chromium trioxide and chromic acid in
acetone(Jones’s reagent), DMSO and dimethylforamide (DMF)
2. Oxidation promoted by a family of Cr (VI) reagents obtained from chromium
trioxide, chromic acid and halochromic acids heterocyclic bases.
3. Oxidation carried out under neutral or almost neutral conditions
4. Chromium (VI) reagents supported on various insoluble organic and
inorganic matrices.
62
II.3.A. Oxidation of alcohols by Cr(VI) under acid conditions:
Much important chromium (VI) oxidation of alcohols are carried out under
acid condition. Aqueous acidic solution of chromium trioxide as well as chromate
and dichromate salts in the presence of co-solvents like acetic acid or acetone
have been widely used as oxidizing reagents. Oxidations have also been
performed in a two phase system consisting of an aqueous acidic solution of
chromic acid and an organic immiscible solvent like benzene, DCM, or ether or
without a phase transfer catalyst.
In 1859 G.Stadler124 reported that the first description of oxidation of
primary alcohols with potassium dichromate in aqueous sulfuric acid.
3 R-CH2OH 2 CrO3 3 H2SO4 3 R-CHO Cr2(SO4)36 H2O
Beckmann 125and Kilani126 introduced two standard chromic acid solutions
in aqueous sulfuric acid .The first is made with Na2Cr2O7/K2Cr2O7, and the second
with chromium trioxide. This method seems to be particularly useful for the
preparation of aromatic aldehydes127 even if they are sterically hindered.128
(Scheme-53)
CH2OH
Na2Cr2O7
H2SO4, H2O
CHO
140 141 (42%)
Scheme-53
63
Oxidations with aqueous chromic acid in a heterogeneous system have
proved to be useful the direct conversion of low molecular weight primary
alcohols to the corresponding acid. The absence of a co-solvent in this case greatly
facilities the isolation of oxidation products. In respect to reaction conditions best
results were obtained by adding the alcohol to an excess of the oxidizing
mixture.129, 130 (Scheme-54)
CH2FCH2CH2OH K2Cr2O7
H2SO4, H2OCH2FCH2COOH
142 143 (80%)
Scheme-54
Unlike primary alcohols; the secondary ones which are sufficiently in
water are generally oxidized in good yield when other oxidation sensitive groups
absent.131 (Scheme-55)
OH
Na2Cr2O7
H2SO4, H2O
O
144 145 (90%)
Scheme-55
A common co-solvent with water in chromic acid oxidation of alcohols is
acetic acid. The use of acetic acid not only improves the solubility of the organic
substrates in the oxidizing mixture but also increase the rate of the reaction. The
oxidation rate can be further increased by adding sulfuric acid.
64
Primary alcohols have been oxidized to aldehydes under these conditions in
moderate yield.132 (Scheme-56)
CH2OH
CrO3/AcOH
H2O, 00CCHO
146 147 (33-35%)
Scheme-56
Less hindered primary alcohols may be converted to the corresponding
carboxylic acid under milder conditions. Thus, for example, a variety of
halogenated primary alcohols were oxidized to carboxylic acids in moderate
yields upon overnight-treatment with chromium trioxide in acetic acid at room
temperature.133 (Scheme-57)
CH2OHF
CrO3/ AcOH
H2O, 2hr, 50CCOOHF
148 149 (93%)
Scheme-57
Chromic acid oxidation of secondary alcohols in aqueous acetic acid yields
cleavage products (route A) in addition to the expected ketones (route B).
(Scheme-58)
R'R
OHA
RCHO
R'R
O
+ R'OH
B
Scheme-58
65
The cleavage products may then be further oxidized to corresponding acids
and carbonyl compounds or may combine, via the formation and oxidation of the
hemiacetal to the ester. The cleavage reaction becomes more pronounced for
secondary alcohols bearing one or more alkyl group in α position. For example,
the oxidation of n-propyl-t-butylcarbinol with chromium trioxide in aqueous
acetic acid yields 41% of the corresponding ketone, 4% tert-butylcarboxaldehyde
and a corresponding amount of tert-butyl cabinol.
Detailed investigations 134 (a) revealed that Cr (VI) oxidized cyclobutanol
to the “normal” product of cyclobutanone in a C-H bond cleavage reaction,
whereas the intermediate chromium (IV) reacts by carbon-carbon cleavage to give
a free radical CH2CH2CH3CHO which, upon further oxidation, gives rise to the
hyroxyaldehyde. (Scheme-59)
OH
CrO3/AcOH
O
+
CH2OH
CHO
H2O
150 (48%) 151 (31%) 152
Scheme-59
Tertiary alcohols are generally inert toward oxidation by chromic acid.
Expectations have, however been reported.
Several tertiary cyclopropanols 134( a, b) 124,125 bicyclic tertiary alcohols react
quickly135 with chromic acid yielding ketones arising by cleavage products may
be considered to arise from breakage of the C1-C7 or C1-C2 bond. The relative
yields depend on the substituents at C2-C7. (Scheme-60)
66
HO
H2CrO4
AcOH
O
COOH
153 154 (16%)
Scheme-60
A notable improvement in the oxidation of alcohols was achieved by using
a solution of chromic acid/sulphuric acid in water for which oxygen 8 N (the so
called Jones reagent) using acetone as co-solvent. The procedure involves titration
of alcohol in acetone with the standard chromium trioxide-sulphuric acid solution.
The reaction mixture separates into a green layer of chromium salts with an upper
layer of an acetone solution of the oxidation product
A limitation of the method is the low solubility power of acetone and strong
acidic conditions of the reaction. Primary alcohols are rapidly oxidized by this
reagent to yield good amounts of carboxylic acids, even in the presence of double
136 or triple137 carbon-carbon bonds.
Primary α, β unsaturated alcohols may be converted into high yields of the
corresponding aldehydes under nitrogen at 00C. As a matter of fact α, β
unsaturated and aromatic aldehydes seems to be relatively inert towards the
normal conditions of the Jones’s oxidation 138 However, a small amount of
isomerization of the double bond occurs in certain cases. (Scheme-61)
67
OHCHO
Jones reagent
155 156 (84%)
Scheme-61
Chromium trioxide dissolves in DMF to give solution with very little
oxidation power toward alcohols. Thus cholesterol is recovered unchanged after
three weeks of exposure to the solution and only a 50 % conversion of 5 α-
cholestan-3-ones is observed after heating 5α-cholestan-3 β- ol for 45 hours at
500C. However, the oxidation is strongly catalyzed by adding a few drops of
con.H2SO4 acid. Under these conditions a number of steroidal alcohols have been
converted to ketones at room temperature with 80% yield.139
DMSO acts as an excellent solvent for sodium dichromate dehydrate. The
solution at 700C cause only slight oxidation of benzyl alcohol to benzaldehyde,
but the addition of small amount of H2SO4 strongly accelerates the reaction.
Primary and secondary saturated, allylic and benzylic alcohols are oxidized by this
procedure to aldehydes and ketones in 80-90% yield.140
Many modified procedures have been developed to simplify the isolation
process, to achieve certain selectively, and to improve the yield as well as the
purity of products. A simple useful procedure consists in stirring a solution of the
compound to be oxidized in an immiscible solvent stable to chromic acid
(benzene, DCM, ether) with an acidic aqueous solution of the oxidant. The
organic phase protects the carbonyl compound from undesirable side reactions,
68
such as further oxidation or epimerization. In this way sensitive ketones may be
isolated in high yield without appreciable epimerization of adjacent chiral centers.
Benzene and DCM have been widely used as an organic phase. As an
example, 2, 5, 5, - trimethylcyclohex-2-en-1-one has been obtained in a 91% yield
from the corresponding alcohol in a benzene- water two phase system.141
(Scheme-62)
HO
Na2Cr2O7/H2SO4
C6H6
O
157 158 (90%)
Scheme-62
An advantage of the use of water immiscible solvents like hydrocarbon
and chlorinated hydrocarbon lies in the fact that they easily from emulsion which
hamper the isolation of the product. Ethyl ether has proved 142(a, b) to be superior,
oxidation proceeds smoothly as no emulsions are formed. Under these conditions
oxidation of (-) menthol give to (-) menthone in 97% yield along with mere traces
of (+) –isomenthone.
The two phase procedure has recently been modified by adding a phase
transfer catalyst 143,144 Under these conditions primary alcohols which were
insoluble in the aqueous phase were oxidized to aldehydes at a higher than 90%
yield. The temperature and concentration of acid in the aqueous phase and that of
the substrate are crucial factors for the reaction.
69
II.4. Oxidation of Alcohols with Compounds of Cr(VI) and
Heterocyclic Bases
Chromium trioxide is capable of reacting as Lewis acid with many
heterocyclic bases leading to a number of addition compounds. Pyridine,
dipyridine, quinolone, 3, 5, dimethyl pyrazole etc. are among the bases which
gives complexes by simple additions of the compounds at room temperature.145
Moreover, chromic acid and chlorochromic acid was found to give a
variety of heterocyclic bases.
Such useful reagents are dipyridine-chromium (VI) oxide complex 146, 1,
8-naphthpyridinium chlorochromate and pyrazinium chlorochromate 147,
tetrakis(pyridine) silver dichromate 148 , nicotinium dichromate and isonicotinium
dichromate149 quinolinium dichromate150, imidazolinium Fluorochromate151
,
imidazolinium dichromate152, tetraalkylammonium chlorochromate153
, 2,4-
bipyridinium chlorochromate154, pyridinium dichromate 95
, trimetylsilyl
chlorochromate 155, 2,2'-bipyridinium chlorochromate 156
, pyridinium
Fluorochromate157, pyridinium bromo-chromate158 quinolinium chlorochromate
159, quinolinium Fluorochromate 160
, quinolinium bromochromate161, 4-(Dimethyl-
amino) pyridinium chlorochromate162, bis[benzyltrimethyl
ammonium]dichromate163, 3,5-dimethylpyrazole chromium(VI) complexes164
,
benzotriazole trioxide complexes 165, halosilanes chromium trioxide 166
,
butylammonium chlorochromate & teramethyl-ammomium fluorochromium
trioxide 167, chromium peroxide etheratem (CPE) and pyridine chromium peroxide
(PCP) and 2,2’–bipyridyl chromium peroxide (BPCP) 168, biphosphonium
70
dichromate 169, butyltriphenyl phosphonium dichromate (BTPPD 170
, ferric
dichromate 171, zinc dichromate trihydrate (ZnCr2O7)
172, 1-methylimidazolium
chlorochromate (MCC), imidazolium chlorochromate (ICC) and isoquinolinium
chlorochromate (IQC) 173 benzyl trimethyl ammonium dichloroiodate 174 have
been reported as mild and selective oxidizing reagent.
Some heteocyclic base and halochromate Cr(VI) reagents were
described in this section.
II.4. A. Oxidation of alcohols with Chromium Trioxide-(Pyridine)2
Complex.
Chromium trioxide in pyridine was introduced as a unique, nonacidic
reagent for alcohol oxidations and has been used extensively to prepare ketones
175, but has been applied with only limited success to the preparation of aldehydes.
2-Methoxybenzaldehyde was obtained in 89% yield, 4-nitrobenzaldehyde and
heptanal were obtained in 28 % and 10 % yields, respectively176.
Using the performed dipyridine chromium (VI) oxide in DCM, the rate of
chromium ester formation and decay to the aldehyde 177 is enhanced at least
twentyfold over the rate observed in pyridine solution.146 Isolation of products is
facile, and aldehydes appear to be relatively stable to excess reagent. The reagent
has been used extensively to prepare acid-sensitive aldehydes, particularly
intermediates in the total synthesis of prostaglandins 178 and steroids.179 An 85%
yield was reported for the conversion of 2-vinylcyclopropylcarbinol to the
aldehydes.180 Although excess reagent is required for the oxidations (usually
71
sixfold), the reaction conditions are so mild and isolation of products so easy that
the complex will undoubtedly find broad use as a specialty reagent. Isolation of
complex can be avoided by in situ preparation of the chromium oxide/pyridine
complex.181
II.4.B. Pyridinium Dichromate (PDC)
PDC reagent was prepared from pyridine, CrO3 in water. It is bright
orange solid. It is very soluble in water, DMF, DMSO, dimethylacetamide (DMA)
and it is soluble in DCM, ethanol-free chloroform or acetone and not soluble in
hexane, toluene, ether or ethyl acetate. Although PDC dissolve in acetonitrile, the
solutions are not stable.
This reagent was first used for oxidation of alcohols by J.W.Conforth182
for the oxidation of 2, 4 -dimethyl-4-(3-hyroxy-4-methyl pentyl)-1, 3-dioxane to
the corresponding ketone with a yield of 88%.
The wide applicability of PDC for oxidation of alcohols to aldehydes,
ketones and carboxylic acid has been shown by E.J. Corey.95 The reagent was
used in most cases in the form of DMF solution or a DCM suspension. In DMF
primary and secondary allylic alcohols and secondary saturated ones are rapidly
oxidized to the corresponding carbonyl compounds. Thus, 2-cyclohexen-1-ol,
cinnamyl alcohol and geraniol are converted to high yields to α, β- unsaturated
carbonyl derivatives at 00C in 4-5 hours using only 1.25 equivalent of the reagent.
Oxidation occurs without overoxidation in the case of aldehydes and no E to Z
isomerization for geraniol.95 Saturated primary alcohols are readily converted to
72
carboxylic acids in good yield with 3.5 equivalents of PDC in DMF at 250C in 7-9
hours via the corresponding aldehydes which have been isolated.95, 183
B. L. Hiran and J. Choudhary 184 developed a new reagent 4-Methyl
pyridinium dichromate (m. p. 109° C), and they studied oxidation of some
substituted benzhydrols (4-methyl, 2-methyl, 4-chloro, 4, 4-dichlorobenzhydrol
etc.) in acetic acid–water medium and observed first order with respect to
[substrate], [oxidant] and [H+] each. The rate of reaction increases with increase
in hydrogen ion concentration and percentage of acetic acid. Thermodynamic
activation parameters have been computed. A plausible mechanism consistent
with the observed experimental results and suitable rate law has been deduced. In
rate determining step water acts as proton abstracting agent and C-H bond
breaking takes place.
Hiran & Kailash Chand185 studied oxidation of aliphatic alcohols by PDC
in aquo-dioxane medium and observed first order with respect to [substrate],
[oxidant] and [H+] each. The rate of reaction increases with increase in hydrogen
ion concentration. Thermodynamic activation parameters have been computed. A
plausible mechanism consistent with the observed experimental results and
suitable rate law has been deduced. In rate determining step water acts as proton
abstracting agent and C-H bond breaking takes place.
II.4.C. Pyridinium Chlorochromate (PCC)
Corey and Suggs introduced PCC for the oxidation of both primary and
secondary alcohols to carbonyl compounds186 PCC was prepared from pyridine
73
and CrO3 in HCl. This reagent which appears particularly suitable for moderate to
large scale oxidations is normally used in a 1:1.5 molar ratio as a suspension in
DCM at room temperature. More polar solvents, like acetone and acetonitrile in
which the reagent was soluble greatly reduce the reaction times.
The stochiometry of the oxidation of 1-octanol with PCC in DCM has
been studied by H.C. Brown 187 by using varying amounts of reagents. The use of
both 16.5 and 22.5 mmole of reagent for 15 mmole of alcohol affords a
quantitative yield of octanol. It, therefore, seems likely that a substrate: reagent
molar ratio of 1:1.1 should be to perform quantitative oxidation.
The oxidation of secondary hydroxyl groups in sugars has been found to
be very slow with PCC in DCM, even in the presence of a large excess of oxidant
under reflux conditions. Instead, benzene was found to be a good alternative
solvent. Oxidation of sugars, which failed with DMSO- dicyclohexylcarbodimide
and other reagent related to DMSO, was readily accomplished in good yield, also
on a molar scale, simply by heating a benzene solution of the sugar under reflux in
the presence of 1.7 equivalent of PCC.
The reactivity of PCC towards slow reacting alcohols like nucleosides or
sugars may be increased by adding 3A molecular sieve powder to a suspension of
PCC in DCM.188 The attempts to catalyze the reaction by celite, alumina and silica
gel were unsuccessful. Thus, for instance, the oxidation of benzhydrol to
benzophenone with two equivalent of PCC and 0.5 g of molecular sieve powder
leads to a quick and complete in 15 min. The acidic character of PCC (pH of a
74
0.01 M solution=1.75) 170 may be convenient to bring about a one-step conversion
(70%) of citronellol essentially to pulegone.189
Efficient cyclization with PCC is only observed with substrates which
produce tertiary cations as initial cyclic intermediate which then lead to six
membered enones. This process represents a milder and more efficient alternative
in organic synthesis to the two-step cationic cyclization of preparing α, β-
unsaturated enones190. The oxidation of cyclic tertiary allylic alcohols to α, β-
unsaturated cyclic ketones with 2 equivalent of PCC in DCM affords transposed
3-alkyl - α, β- unsaturated in good yields.191 Various tertiary vinylic alcohols of
natural origin have also been transformed into the corresponding unsaturated
aldehydes following the same procedure .A cyclic allylic tertiary alcohols192 also
afford transposed α, β- unsaturated ketones albeit in lower yield. However,
byproducts are frequently observed. For exp. Oxidation of 2-phenyl-3-buten-2-ol
gives a 35 % yield of acetophenone in addition to the transposed aldehyde.
The first report about the kinetics and mechanism of the oxidation of
alcohol was published in 1978. Banerjee 193,194 and Venkatsubrumaian et.al195
reported the oxidation of alcohols by PCC in 1:1 (v/v) DCM and nitrobenzene
solution. The reaction was first order with respect to PCC and the alcohol.
Banerjee193, 194 reported the reactions catalyzed by toluene-p-sulphonic acid
(TsOH). The reactions exhibited negative reaction constants. The oxidation of
deuteriated benzyl alcohol 193 and ethanol 194 showed the presence of substantial
kinetic isotope effects. This confirmed the cleavage of an α-C-H bond in the rate
determining step. The oxidation of cycloalkanols by PCC 196 in chlorobenzene-
75
nitobenzene solvent presented similar kinetics. The author suggested, on the basis
of the reactivity of the cycloalkanol and effect of solvent, that probably a
chromate ester mechanism is operative. Banerjee studied the temperature
dependence of kinetic isotope effect in the oxidation of substituted benzhydrol
197by PCC in DMSO. An analysis of the temperature- dependence of the kinetic
isotope effect by the method of Kwart and Nickle 198, in the oxidation of
benzhydrol, showed that the hydride-transfer take place via a chromate ester
through a symmetrical cyclic transition state. Agarawal et.al 199 studied the
oxidation of 2-pentanol by PCC in chloroform solution. They observed that
kinetically the oxidation is comprised of two reactions. The first one consisted of
the formation of a chromate ester and the second one a decomposition of ester.
They obtained a kinetic isotope effect, kO-H/kO-D =1.30, for the first reaction. In
essence, their data support a hydride-ion transfer via a chromate ester.
II.4.D. Pyridinium Fluorochromate (PFC)
Development of PFC was essential to overcome the practical difficulties
associated with PCC, which was found unfortunately quite unstable. Mihir Kanti
et.al. reported a noble and environmentally cleans process for the preparation of
Pyridinium salt of fluorotrioxochromate (VI) having a chemical formula
C5H5NH[CrO3]200. The compound was prepared by direct reaction of HF acid,
chromium trioxide and pyridine. This complex is soluble in benzene, CCl4,
chloroform and hexane. Unlike PDC, the reagent does not react with acetonitrile
which also may be used as a solvent. The acidity of PFC is less pronounced than
that of PCC which may have advantage of preventing side reactions.
76
The first preliminary report on the oxidation of alcohols by PFC was
published by Bhattacharjee et.al.201 In this reaction the molar stoichiometry of the
oxidation of n-butanol, isopropanol, benzyl alcohol and cyclohexanol involving
PFC in DCM has been found to be 1:1.5. They obtained Michaelis-Menten type
kinetics with respect to the alcohols and observed that the reactions were
catalyzed by TsOH but could not determine the order with respect to the acid, as
the reactions were very fast. A systematic study of the oxidation of substituted
benzyl alcohol and of aliphatic alcohols by PFC in DMSO was reported by
Banerjee.202, 203 He also observed Michaelis-Menten type kinetics with respect to
alcohols, suggesting the formation of an intermediate in a pre-equilibrium and its
subsequent disproportionation in the rate-determining step. The oxidation of
deuteriated ethanol 202 and benzyl alcohol203 exhibited a substantial primary
kinetics isotope effect. This confirmed the α-C-H bond in the rate-determining
step. The oxidation of aliphatic alcohols is susceptible to both the polar and steric
effects of the alkyl group. The polar reaction constants have negative values. The
oxidation of benzyl alcohol 203was studied in nineteen organic solvents. An
analysis of the solvent effect, in term of swains 204 equation, indicated the greater
importance of the cation-solvating power of solvents. In both the cases, the in
analysis of the temperature-dependence of the kinetic isotope effect indicated the
presence of a symmetrical cyclic transition state in the rate –determining step of
the oxidation. The oxidation of vicinal and non-vicinal diols by various
halochromates viz. PFC 205 in DMSO has been studied. The oxidation of vicinal
diols correlated well with the polar and steric substituent constant of the alkyl
group.205-208 Kinetics and mechanism of the oxidation of p-chlorophenol, p-
77
nirophenol and p-cresol in glacial acetic acid as a solvent at 25-450C using PFC as
have been studied and reported by Patil and Coworkers.209 The oxidation of D-
glucose and some other monosaccharaides by PFC210 in aqueous perchloric acid
solution has been reported. The reactions are first order with respect to each of the
oxidant, the sugar and hydrogen ions. The oxidation by PFC210 on the other hand,
yields only formic acid and D-arabinose. In the PFC oxidation, a solvent isotope
effect, k (D2O)/ k (H2O) = 3.1 has been obtained. The author interpreted this in
term of two acid-catalyzed reactions
(i) The acid-catalyzed anomerization of α-D- glucose to β-D-glucose
(ii) A pre-equilibrium protonation of PFC
The author suggested a bimolecular hydride ion transfer in the rate
determining step.
II.4.E. Bypyridinium Chlorochromate (BPCC)
Frank S. Guziec reported the successful use of 2, 2- Bypyridinium
Chlorochromate (BPCC) for the oxidation of 2-isopropyl cyclohexanol in DCM at
room temperature211 2-isopropylcyclohexanone was the main oxidized product.
Percentage yield was as high as 86 %. Kabilan et.al reported the kinetics and
mechanism of the oxidation of cyclopentanol, cyclohexanol, cycloheptanol and
cyclooctanol by BPCC in the presence of H+ ions.212 The oxidation of benzyl
alcohol 213 by BPCC was examined in various solvents and greater yield of
benzaldehyde was observed in the solvent with higher dielectric constant such as
chloroform, dichloromethane etc. The oxidation of benzyl alcohol derives by
78
BPCC in DMF was examined and similar product was observed regardless of
substituent. The oxidation rate was dependent linearly on the concentration of acid
and Arrhenius equation indicated that there was a charge transition state.
Kinetics of the oxidation of substituted benzyl alcohol with 4, 4’ BPCC has
also been reported in the literature.212 Treatment of primary and secondary alcohol
in DCM or acetone with 2:1 to 4:1 excess of bipy H+ CrO3Cl- affords aldehydes or
ketone in high yields. The chromium containing byproducts from these reactions
are water soluble crystalline materials which are easily removed by filtration
through a 1 cm celite pad. The bipy H+ CrO3Cl- complex may prove to be
especially useful in oxidation of compounds with acid-sensitive protecting groups,
due to the internal buffering of the 2, 2’- bipyridyl system.
II.4.F. Quinolinium Chlorochromate (QCC)
Quinolinium chlorochromate was first reported in 1986 by Kalsi et. al.159
as effective and mild oxidant for organic substrates under mild condition.
R. Srinivasan, and Coworkers have reported the selective oxidation of 10
alcohols in presence of 20 alcohols by mild, stable and efficient QCC. 213, 159QCC
is prepared by treating a solution of CrO3 in 6M HCl with quinoline at 00C. The
orange red solid compound is stable to ordinary exposure to air, moisture and
light. It efficiently oxidizes 10 and 20 alcohols to the corresponding carbonyl
compounds. QCC is soluble in water, DMF and DMSO, sparingly soluble in
DCM and chloroform and insoluble in ether, ethyl acetate, toluene, and heptane.
79
QCC (1 molar equivalent) oxidizes 10 alcohols to the corresponding
aldehydes in refluxing DCM and reasonably oxidizes 20 alcohols to ketones.
Solution of QCC in DMF oxidizes benzoin and benzhydrol to the corresponding
ketones in excellent yields. Mechanism and comparative study with IQCC has
also been reported.214 a
The oxidation of aliphatic primary alcohol215 by QCC has been studied in
aqueous acetic acid medium. The rate shows first order dependence each in
[QCC] and [alcohol]. The linear increase in the oxidation rate of alcohol with
acidity suggest the involvement of protonated Cr(VI) species in the rate
determining step which is confirmed by decrease in the rate of reaction with an
increase in dielectric constant with the change in ionic strength. The reaction does
not induce the polymerization of acrylonitrile. As a result, hydrogen abstraction
mechanism is unlikely. The oxidation of [1, 1-2H2]- ethanol shows the presence of
primary kinetic isotope effect. The observed results suggest a hydride transfer in
the rate determining step. In a concerted sigmatropic reaction a cyclic hydride
transfer takes place.
The oxidation of substituted benzyl alcohols by QCC involves the
decomposition of an initially formed chromate ester via a concerted symmetrical
transition state. 214b It has been found as a more efficient reagent for the oxidation
of aromatic alcohols as compared to the oxidation of aliphatic alcohols.
80
II.4.G. Quinolinium Fluorochromate (QFC)
Chaudhary and coworkers reported the synthesis of quinolinium
fluorochromate (QFC) in 1991Though the details were published in 1994216 in
the meanwhile, Murugesan and Pandurangan also reported the synthesis of
QFC.160 The preparations, reported by the two groups of workers are similar.
Chaudhury et.a.l216observed that QFC was univalent electrolyte and corresponds
to the formula C9H7NH+[FCrO3]-. They characterized the compound by IR and
EPR spectroscopy and SEM techniques. QFC is less acidic as compared to PCC
and PFC.
QFC was prepared from quinoline, 40% HF acid and chromium trioxide in
molar ratio of 1:1.5:1. QFC is soluble in in water, DMF, DMSO and acetone. It is
sparingly soluble in DCM and chloform and insoluble in benzene, 1, 4 dioxane,
heptane, ethyl acetate, toluene, ether.
Further QFC does not react with acetonitrile and nitrobenzene which is a
suitable medium for showing oxidation kinetics and mechanism.10 alcohols are
oxidized to aldehydes by QFC in DCM at room temperature. The diverse types of
organic substrates that have been oxidized by it have highlighted the versatile
nature of QFC. Recently Chandrasekhar et al217 reported that QFC in DCM was
able to deprotect and oxidize the 10 alcoholic group, while leaving the protect
secondary 10 alcoholic group intact
Most of the workers have explained the hydrogen ion dependence by
assuming the formation of a protonated chromium (VI) species in a pre-
81
equilibrium, either both the unprotonated and protonated or only the protonated
form being the reactive species. In the oxidation by QFC218 the rates were
obtained in different organic solvents and the analysis of the solvent effect
indicated the greater importance of the cation-solvating power of the solvents. The
temperature-dependence of reactions indicated a symmetrical transition state in
the rate-determining step of this oxidation. The oxidation of vicinal and non-
vicinal diols by various halochromates, viz QFC in DMSO has been studied.
Kinetics and mechanism of the oxidation of substituted benzyl alcohols by
QFC have been reported. Itishri Dave et.al. 219 reported the correlation analysis of
reactivity in the oxidation of substituted benzyl alcohols by QFC. Oxidation of
benzyl alcohol and some ortho, meta, and para-monosubstituted ones by
Quinolinium Fluorochromate in DMSO leads to the formation of corresponding
benzaldehydes. The reaction is first order each in both QFC and alcohol. The
effect of hydrogen ions was studied in the oxidation by QFC. In the oxidation by
QFC, the hydrogen-ion dependence has the form
kobs = a+b [H+]
Oxidation of α, α- dideuteriobenzyl alcohol has exhibited a sustainable
primary kinetic isotope effect. The reaction has been studied in nineteen organic
solvents and the effect of solvent analyzed using Taft’s and swain’s muti-
parametric equations. The rates of oxidation of para and meta substituted benzyl
alcohols have been correlated in terms of Charton’s triparametric LDR equation.
Whereas the oxidation of ortho-substituted benzyl alcohols with tetraparametric
LDRS equation. The oxidation of para-substituted benzyl alcohols is more
82
susceptible to the delocalization effect than that of ortho- and meta substituted
compounds, which display a greater dependence on the field effect. The positive
value of η suggests the presence of an electron-deficient reaction center in the rate
determining step. The reaction is subjected to steric acceleration by the ortho-
substituents.
II.5. Oxidation of Alcohols with Cr(VI) Under Neutral Conditions
The oxidations of particularly sensitive alcohols may require complete
neutral conditions to attain good yields. For this purpose some neutral Cr (VI)
containing reagent and special reactions conditions have been developed.
Chromium trioxide dissolves in tert-butyl alcohol by depolymerization to
give di-tert-butyl chromate which acts an oxidation in a variety of apolar solvents.
di-tert-butyl chromate (DTBC) has been especially used for oxidation of allylic
oxidation of double bonds, it may also be used for oxidation of alcohols. The
reaction is performed in non-polar media such as pet.ether, benzene, or carbon
tetrachloride. The oxidation of primary alcohols in pet.ether or benzene at room
temperature with 1 equivalent of TBC does not appear to be particularly
promising; however, a mixture of the aldehyde, acid and the ester of the acid is
invariably obtained.220 (Scheme-63)
CH2 OH CHO COOH COOCH2
DTBC
C6H6
159 160(40%) 161(28%) 162 (19%)
Scheme-63
83
In each case a small percentage of starting material was recovered. On the
other hand, oxidation of benzylic primary alcohols gave the aldehyde in good
yields together with small amounts of the corresponding acid, but not the ester.221
(Scheme-64)
CH2OH
DTBC
CHO COOH
163 164 (90%) 165(7%)
Scheme-64
Better results were obtained by using TBC in situ from chromyl chloride.
This procedure avoids the formation of water which may cause side reaction.222
When chromyl chloride is added to a solution of pyridine/ tert-butyl alcohol in
DCM in a dry ice/ acetone bath and the reaction mixture allowed warming at room
temperature, the solution becomes orange-red and a white precipitate of pyridine
hydrochloride appears. (Scheme-65)
OH
N
2CrO2Cl2
- 780 C, then r.t.
Cr
O
O
O
O
2N
2
H
Cl
Scheme-65
This solution is capable of oxidizing aliphatic, allylic and benzylic
alcohols to aldehydes and ketones in high yield when treated at room temperature
for 2 hours with a ratio of chromyl chloride to substrate of 1.1:1.
84
Solutions of chromium trioxide in hexamethylphosphoramide (HMPA) are
also capable of converting alcohols to carbonyl compounds under mild conditions.
Chromium trioxide readily dissolves in HMPA at room temperature to give stable,
deep red solutions which have been used on steroids under neutral conditions for
the selective oxidation of certain allylic and benzylic hydroxyl group in the
presence of other unprotected saturated groups.223 ( Scheme-66)
OH
OH
O
OH
CrO3
HMPA
166 167 (90%)
Scheme-66
Saturated alcohols are oxidized more slowly while aldehydes may,
nevertheless, be obtained in good yields from saturated primary alcohols under
appropriate conditions. The reagent was found useful for performing selective
oxidations of primary alcohols in the presence of unprotected secondary ones for
example; strophantidol is slowly converted to strophantidine in about 35 % yield
without ketonic products being formed.224
Chromate salt of tetraalkylammonium ions is soluble in aprotic solvents,
as e.g. benzene (orange benzene), resulting in solutions which show interesting
oxidizing properties. The use tetraalkylammonium salts to facilitate the solubility
of inorganic ions in organic solvents of low solvating ability has recently called
increasing attention.225 It was eventually found that a liquid commercial mixture
of methyl trialkyl ammonium chlorides, facilitates the solubilization of potassium
85
dichromate in several apolar organic solvents as DCM, CHCl3, tetachloromethane,
and benzene; a 2:1 adogen to dichromate molar ratio is used. The organic solution
of the dichromate can be formed without priorly dissolving the salt in water,
simply by vigorously stirring the crushed salt in the desired solvent containing two
equivalents of adogen.226 The resulting orange solutions are fairly stable at room
temperature but slowly darken after several days. Dichromate anions in orange
benzene reveal to be a very mild oxidizing reagent which discriminately oxidizes
alcohols to the corresponding carbonyl compounds. In fact, α, β unsaturated
alcohols are oxidized to corresponding carbonyl compounds at 550C in good yield
while saturated alcohols only give poor yields under acidic conditions. This
procedure may, therefore be useful for neutral oxidations of activated alcohols
where acidic or basic conditions must be avoided.
II.6. Oxidation of Alcohols with Oxochromium(VI) Supported on
Polymers.
Literature survey 227 reveals that through a good number of oxochromium
(VI) reagents have been developed for the oxidation of alcohols but their
commercial uses are still restrict mainly due to long reaction time, low selectivity,
non-catalyzed nature and above all the difficulty in product isolation.
The concept of supporting Cr (VI) reagents onto inert inorganic and
organic polymeric matrices may be circumvented some of the problems associated
with the soluble Cr (VI) reagents.228 It has been found that Cr (VI) derivatives
supported on an insoluble organic or inorganic matrix are particularly convenient
in organic synthesis. The insoluble support provides a particular environment
86
capable of enhancing and modifying the reactivity of the bound reagent.229The use
of polymeric reagents facilitates the purification of final product. In order to
increase the industrial applicability of soluble Cr (VI) compound significant
attempts have been made to anchor the soluble reagent to both organic 230-231
inorganic 232-235polymers. The polymer supported reagent often exhibits higher
thermal stability and better selectivity.
Reported polymer supported Cr (VI) reagents may be broadly classified as follow:
(i) Polymeric analogue of heterocyclic halochromate. 230, 231, 236(a, b)
(ii) Chromium trioxide supported on inert inorganic supports. 237(a-i)
(iii) Heterocyclic halochromate supported on inert inorganic supports. 238 ( a-d)
Poly (vinylpyridine) was used as an organic polymer support for PCC 230and
PDC231. Both the reagents were found moderately effective towards the oxidation
of different types of allylic, benzylic primary and secondary alcohols. (Scheme-
67)
OHPVPCC
CHO
168 169 (94%)
CH2OH
PVPDC
CHO
170 171 (99%)
Scheme-67
87
The main disadvantage associated with these organic polymers were
their very low accessibility requiring a two to five fold excess of the reagent.
Literature survey reveals that in many occasions chromium trioxide
were supported on inert inorganic polymers such as silica gel, α-alumina, γ-
alumina, ZSM-5 etc233-235, 239.
Chromium acid adsorbed on silica gel was found suitable for the oxidation
of alcohols to the corresponding carbonyl compounds 237( b) This reagent prepared
by addition of silica gel to an aqueous solution of chromic anhydride subsequently
evaporated to dryness. The yellow resultant solid was then kept overnight at
1000C and stored under vacuum in the dark.
Pyridinium chromate on silica gel was obtained by treating chromic acid
adsorbed on silica gel with pyridine and was used for the oxidation of alcohols
containing acid-labile functions 237 c Chromyl chloride was supported onto silica –
alumina 240 and the reagent was used for the oxidation of primary and secondary
alcohols to respective aldehydes and ketones. (Scheme-68 )
OH
CrO2Cl2
silica-alumina
onCHO
172 173
OH O
CrO2Cl2 on
silica-alumina
174 175
Scheme-68
88
B. Khadikar and coworkers 237(g) prepared silica gel supported chromium
trioxide by co-grinding anhydrous CrO3 with silica gel. This reagent was used in
equimolar quantity to oxide alcohols to carbonyl compounds with good yields. γ-
Alumina, which is frequently used as a solid support consists of very high area.
Dispersion of CrO3 and wet alumina in hexane afforded the alumina-supported
chromium reagents may be used for the chemo selective oxidation of a wide range
of alcohols. Thus 2-cyclohexen-1-ol in hexane afforded 94% 2-cyclohexenone on
treatment with alumina-supported chromium (VI) reagents237(h). Masao Hirano
et.al. 241 reported the oxidation of alcohols with chromium trioxide in the presence
of wet alumina oxide in aprotic solvents.
In some cases the bulk of a high surface area material serves as catalyst.
Such material is called a uniform catalyst. One such example of uniform catalyst
is ZSM-5. It has three dimensional, network built from AlO4 and SiO4
tetrahedral.242 Zeolites are used as catalyst in various synthetic organic
transformations much more effectively and selectively than the Lewis acid
catalysts. Thus the use of heterogeneous catalysis in different areas of organic
synthesis has now reached to significant level not only because it enables
environmentally begin synthesis, but also due to the good yields, accompanied by
excellent selectivities. These properties of ZSM-5 may allow it to act as a highly
effective solid support for oxochromium (VI) reagents and may lead the hybrid
reagents highly effective and catalytic towards the oxidation of primary and
secondary alcohols.
89
A variety of alcohols were oxidized to the corresponding carbonyl
compounds in excellent yields by chromium trioxide / ZSM-5 in DCM at room
temperature.243 The oxidation of alcohols to the corresponding carbonyl
compounds using zeolites has been reported by a number of researchers.243-245
Attempts were also made to have solution of some the problems associated
with the soluble heterocyclic halochromate by immobilizing the same with inert
inorganic supports.246, 247, 237(a-d )
Frederick A. Luzzio et.al. reported a facile oxidation of cis, trans-4 – tert-
butyl cyclohexanol to 4- tert-butyl cyclohexanone and d,l menthol to dl menthone
using PCC on silica gel. It was used in conjunction with silica gel to facilitate
removal of reduced chromium byproduct and to provide anhydrous conditions that
would otherwise lead to unwanted side reaction with decreased yields.246
M. M. Heravi and coworkers247 were reported a simple and efficient
method for oxidation of alcohols to carbonyl compounds under non-aqueous
conditions employing 3-carboxypyridinium Chlorochromate (CPCC) adsorbed on
alumina. CPCC adsorbed on alumina was prepared by adding alumina to a
solution of CPCC in DCM and evaporating to dryness. The yellow orange solid
was then kept in vacuum at room temperature and stored in the dark before use.
Mohammed M. Hasmi and coworkers248 were introduced silica gel
supported 4-aminopyridinium Chlorochromate. It was prepared from reaction of
4-aminopyridine with activated silica gel which was then reacted with a solution
of chromium trioxide in hydrochloric acid. The oxidation of alcohols is performed
in DCM as solvent and work up was simply by separation of the heterogeneous
90
supported oxidant by filtration and purification by column chromatography led to
aldehydes and ketones in high yields. Mild reaction conditions, easy work up,
high yield and reusability of the supported reagent are the most significant aspect
of this method. Quinolinium fluochromate (QFC) supported on alumina 238 d
prepared by a new method oxidizes selectively aliphatic 10 and 20 alcohols,
substituted benzylic alcohols and allylic alcohols to their corresponding carbonyl
compounds in good yields. The effect of solvent in the oxidation reaction was
evaluated by carrying out the oxidation in a series of solvents with varying
polarity. The period for maximum conversion was found to be minimum in
hexane. Although the oxidation reactions proceed smoothly in DCM, hexane was
chosen as the solvent for the entire oxidation process keeping in view of the
environmental hazards associated with chlorinated hydrocarbon. A comparative
study of the oxidation by this reagent and the one already reported 237c (prepared
by a different method) was carried out with a series of aliphatic 10 alcohols, benzyl
alcohol, furfurol, cyclohexanol and benzoin. Studies revealed that the present
reagent is a better one in terms of reaction period and percentage yield.
A new selective method of QFC-supported silica gel for the oxidation of a
wide variety of alcohols to the corresponding carbonyl compounds in good yield
has been reported.249 The maximum conversion of benzyl alcohol to
benzaldehyde occurred in a shorter reaction period in hexane and DCM. The
oxidation of a series of aliphatic 10 alcohols with QFC-silica gel revealed that the
lower homologues up to 1-octanol are readily converted to the corresponding
aldehydes at room temperature while the higher homologues such as 1-nonanol
and 1-decanol require reflux conditions. It was interesting to note that the
91
oxidation of 10 alcohols with alkyl group at C-2 position such as 2-ethyl 1-hexanol
and neopentyl alcohols proceeds smoothly at a faster rate than 1-hexanol and 1-
pentanol. It was observed that the oxidation of 20 alcohols with QFC-silica gel
proceeds smoothly only under reflux conditions resulting in high conversion of
alcohols to the corresponding ketones. The reagent selectively oxidizes 10
hydroxyl group in the presence of secondary hydroxyl group. The selectivity is
also exhibited in the oxidation of axial hydroxy group of the cis-t-butyl
cyclohexanol in preference to equatorial hydroxyl group. Oxidation of steroidal
homoallyilc alcohols is a crucial step in the synthesis of most of the steroidal
hormones; oxidation of 3-β-chloestanol with QFC-silica gel gave chloestanone
with 60% yield. These reactions clearly indicate the potential utility of QFC-silica
gel in biosynthesis. QFC-silica gel is quite effective for the oxidation of
heterocyclic alcohols also. On the other hand the oxidation 3-pyridyl methanol
afforded 3-pyridyl carboxaldehyde with 65 % along with 20 % quinoline. ESR
spectral analysis of the reduced product of the QFC-silica gel showed a single
band with a g value of 1.982 and this value shows the presence of d2 Cr (IV)
species.
Synthesis organic chemist has wide variety of technique and method in
this repertoire to tailor an elegant synthesis but today any new development in
synthesis has to take into consideration of ecological aspect, toxic waste
generation, and formation of unwanted byproducts and cost of manufacture, no
matter how innovative chemistry is employed in the process. Therefore, the
demand today is for a synthesis which (a) is environmentally begin (b) minimizes
or eliminates waste generation (c) uses chemicals which do not need for special
92
methods of preparation (d) avoids the use of environmentally unfriendly solvents
(e) is stereoselective or regioselective for the desired enantiomers or isomers and
(f) a number of steps into a single operation. Microwave assisted organic synthesis
is the outcome of these progressive thoughts.
II.7. Microwave Technology
Since the appearance of the first article on the uses of microwave for
chemical reactions 250, the approach has blossomed into a useful technique for a
variety of applications in organic synthesis and functional group
transformations.251 Microwave technique has a great potential and can be utilized
as a highly innovative synthetic tool for the generation of highly functionalized
products.
The practical utility of MW assisted green protocol has been realized in
several synthetic operations such as protection/deprotection, condensation,
oxidation, reduction, rearrangement reaction and in the synthesis of various
heterocyclic systems. The reviews on this subject illustrate clearly the ingenuity
and imaginative breath in this area by presenting a showcase of research over the
past ten year.252-256
With easy availability of ultrasound and microwaves, their use in
chemistry has gained momentum recently.257 Microwave heating emerged as a
powerful technique to promote a variety of chemical reactions.
Microwave was first reported by the group of Gedye 250 and Giguere
Majetich 258 in 1986, but the use of microwave in organic synthesis was initially
93
hampered by a lack of understanding of basic principles of MW dielectric
microwave ovens. Today over 2000 articles have been published in the area of
MW assisted organic synthesis, including several reviews.259-265
II.7.A. Application of Microwave for the oxidation of alcohols
Balogh and Coworkers266 have introduced a facile method for the
oxidation of alcohols to carbonyl compounds with yields wherein clayfen [ Iron
(III) nitrate] was used under microwave irradiation in solvent free condition
(Scheme-69)
CH OH
R2
R1
Clayfen
MW, 15-60 Sec.C
R2
R1
O
176 177 (87-96%)
R1=Ph, p-MeC6H4, p-MeOC6H4 ; R2=H
R1=Ph; R2=Et, PhCO
R1= p-MeOC6H4 R2= p-MeOC6H4CO
Scheme-69
R.S.Varma and Coworkers267have developed a new reagent manganese
dioxide-silica for the oxidation of alcohols to carbonyl compounds under
microwave irradiation. Benzyl alcohol were selectively oxidized to carbonyl
compounds using 35% MnO2 ‘doped’ silica under MW irradiations (Scheme-70)
94
CH OH
R2
R1
MW, 20-60 Sec.C
R2
R1
OMnO2 - Silica
178 179(67-96%)
R1=Ph, p-MeC6H4, p-MeOC6H4, PhCH=CH ; R2=H
R1=Ph; R2=Et, PhCO
R1= p-MeOC6H4 ; R2= p-MeOC6H4CO
Scheme-70
R.S. Varma and coworkers268have studied the facile oxidation of alcohols
to carbonyl compounds with alumina supported iodobenzene diacetate (IBD)
under solvent free condition in microwave irradiation with quantitative yields. The
advantage of using alumina as a support was apparent in marked improvement in
yields obtained with the alumina-IBD system as compared to neat IBD. (Scheme-
71)
CH OH
R
R1
CR
R1
O
MW, 1-3 min
IBD/ Neutral alumina
180 181
Scheme-71
R.S. Varma and coworkers269 were used chromium trioxide impregnated
pre-moistened alumina for the oxidation of benzyl alcohol at room temperature.
The reactions were observed clean with no tar formation. No over oxidation to
carboxylic acid was observed (Scheme-72).
95
CH OH
R
R1
C
R
R1
O
Wet CrO3-Al2O3
MW, 40 Sec.
182 183 (68-90%)
R=Ph, p-MeC6H4, p-MeOC6H4, p-NO2C6H4; R1=H
R=Ph; R1=Et, PhCO
Scheme-72
R.S.Varma and coworkers270 have studied the oxidation of symmetrical
and unsymmetrical benzoin to benzil by copper (II) sulfate-alumina or oxone-wet
alumina under microwave heating (Scheme-73).
OH
R1
O
O
R1
O
CuSO4-Al2O3 or Oxone -Al2O3
RR
MW, 2-3.5 min
184 185
R=R1=C6H5, p-MeC6H4, p-MeOC6H4, p-ClC6H4
R=C6H5; R1= p-MeC6H4, p-MeOC6H4
R= Me; R1=C6H5
Scheme-73
M. M. Heravi and coworkers271 reported a facile and selective oxidation of
alcohols to carbonyl compounds using ammonium chlorochromate on
96
montmorillonite k-10 under solvent free conditions that was accelerated in most
cases by exposure to microwaves. Primary alcohols were oxid oxidized to the
corresponding aldehydes and the oxidation of aldehydes to the corresponding acid
derivative was not observed even after prolonged irradiation and with excess of
supported chromium reagent.
M. M. Heravi and coworkers272reported oxidation of various alcohols to the
corresponding carbonyl compounds in excellent yields by chromium trioxide –
HZSM-5 zeolite under microwave irradiation in a solventless system. Two
equivalent of CrO3 and a weight equivalent of HZSM-5 zeolite per mol of benzyl
alcohol was used and exposed to microwave irradiation for 30 sec. which led to
the formation of benzaldehyde almost quantitatively, no trace of benzoic acid was
observed showing that no overoxidation occurs. Cinnamyl alcohol (186) was
converted to cinnamylaldehyde (187) in 90 % yield. (Scheme-74).
HZSM-5 Zeolite, CrO3
MWOH O
186 187
Scheme-74
M. M. Heravi and coworkers273 reported the selective oxidation of alcohols
by silica-gel supported ammonium Chlorochromate in solvent less system. The
oxidation reaction is conducted by mixing finely ground reagent with alcohols.
This procedure was found a rapid, economic, manipulatively, simple, selective
and environmentally benign protocol when compared to the conventional solution
phase reaction.
97
M.M. Heravi and coworkers 274 used CrO3/wet silica gel for oxidation of
alcohols to corresponding carbonyl compounds under microwave irradiation in
solvent free conditions. The oxidation of cinnamyl alcohol (188) with this method
gave a moderate yield of cinnamaldehyde (189) (70%) and benzaldehyde (190)
(30 %) showing the carbon-carbon bonds are prone to cleavage by this procedure.
(Scheme-75).
CrO3- Silica gel
MWOH O
O
+
188 189 190
Scheme-75
N. Noroozi –Pesyan and Coworkers275have used the alumina or silica gel as
catalysts for a solvent-free oxidation of benzoins to the corresponding benzils.
These catalysts were easily recovered after completion of the reactions.
Comparison of the results obtained with both catalysts indicates that all the
reactants were oxidized faster on alumina than on silica under these conditions.
Scheme-76)
O
Ar' Ar
Alumina or Silica gel
Microwave or Heat
OH
Ar' Ar
191 192
Ar' = C6H5 , Ar =C6H5-CO
Ar' = C6H5 , Ar = 4-Me2NC6H4-CO
Ar' = 4-MeOC6H4 , Ar = 4-MeOC6H4-CO
Ar' = C6H5 , Ar =4-MeOC6H4-CO
Ar' = Ar = C6H5
Ar' = C6H5 , Ar = H
Scheme-76
98
Jitender M. Khurana and Coworkers276 have reported the simple and
efficient protocol for microwave-assisted solvent-free oxidation of hydrobenzoins
to benzoins or benzils, benzoins to benzils, and alcohols to the corresponding
aldehydes or ketones, using N-bromosuccinimide – neutral alumina.
Amit Sahu and coworkers277have been studied chromium based oxidants
like PCC, BIFC, PFC ACC etc. supported on alumina for oxidation of benzoin
under MW irradiation. Alumina was acted both as a catalyst as well as a solid
support in the reaction and increases the yield of product (Scheme-77).
OH
Acidic Alumina
I. PCCII. BIFCIII. ACCIV. PFC
O
OO
193 194
Scheme-77
99
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