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Lecture 1 Base Catalyzed Reactions I
1.1 Principles
The base catalyzed carbon-carbon bond formation is closely related to the carbon-carbon
bond formation from organometallic reagents. In both methods, the negatively polarized
carbon reacts with electrophilic carbon of carbonyl groups and related compounds.
The scope of the base-catalyzed reactions depends on three facts: (i) a wide range of
organic compounds is able to form carbanions, (ii) these carbanions undergo reaction
with electrophilic carbon in a variety of environments, and (iii) the basicity of the reagent
used to abstract the proton may be widely varied.
1.2 Reactions of Enolates with Carbonyl Compounds
1.2.1 Reactions with Aldehydes and Ketones
1.2.1.1 Aldol Condensation
The reaction has become one of the most important methods for carbon-carbon bond
formation. It consists of the reaction between two molecules of aldehydes or ketones that
may be same or different. One of the reactants is converted into a nucleophile by forming
its enolate in the presence of base and the second acts as an electrophile (Scheme 1).
R''R'
O
R'
+ '"RR''
O
R
Base
AcidR"
O
R R"'
HO R'
R'
R'' If R = HR"
O
R"'
R'
R'
R''
Scheme 1
The geometry ((Z)- or (E)) of the enolate depends on the reaction conditions and the
nature of the substituents. Strong base (e.g. LDA), low temperature and short reaction
time lead to kinetic enolate, while weak base (e.g. hydroxide ion), high temperature and
longer reaction time favour the formation of thermodynamic enolate (Scheme 2)
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Me
OBaseR
CH2
R
O
"Kinetic enolate"
CH2
R
O
"Kinetic enolate"
Me
O
R
H
Me
O
R+ Me
O
R
"Thermodynamic enolate"
Scheme 2
If the reactants are not the same, they can lead to the formation of diastereoisomers and
their distribution depends on the reaction conditions and the nature of the substituents
(Scheme 3).
R'
O
R + R"CHOBase
R' R" R' R"
O
R
OH O
R
OH
+
R' R" R' R"
O
R
OH O
R
OH
+
syn
anti
Scheme 3
Under thermodynamic control, the (Z)- and (E)-forms of the enolates are in rapid
equilibrium, and the product distribution is determined by the relative stabilities of the
six-membered chair-shaped cyclic transition states that includes the metal counter-ion
(Scheme 4). Transition sate that leads to the syn product has R in the less stable axial
position, whereas in that leading to anti product both R and R’ are in the more stable
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equatorial position. The latter is therefore of lower energy, leading to a major anti
product.
RR'
O
BaseR
R'
O
Base
M
R
R'
O M
R"CHO
R"CHO
O
MO
R"R
R'
O
MO
R
R'
R"O
O
O
O
R'
R
R'
R
R"
R"
M
M
R' R"
OO
anti /threo
R' R"
O
R
O
syn / erythro
R
M
M
E
Z
Scheme 4
In contrast, under kinetic control, the (Z) and (E) enolates are formed rapidly and
irreversibly, and their relative amounts determine the product distribution (Scheme 5).
For examples, for ketone CH3CH3COt-Bu, the (Z)-enolate is normally formed to afford
the syn diasteroisomer as a
Me
OLDA
-78 oC
Me
OLiPhCHO
-78 oCPh
O
Me
OLi
> 98%
Me
OLDA
-78 oC
Me
OLiPhCHO
-78 oCPh
O
Me
OLi
4
Ph
O
Me
OLi
+
1
Scheme 5
major product. But, the selectivity falls to 4:1 (syn:anti) when the size of the R is reduced
from t-Bu to isopropyl. This is presumably because of the difference in steric repulsion of
the methyl group with t-butyl and isopropyl groups.
However, there is a general technique to increase the degree of diastereoselectivity. The
enolate can be converted into silyl enol ethers that can be separated by distillation. The
separated silyl enol ethers can then be converted into pure (Z)- or (E)-enoate by treatment
with fluoride ion (Scheme 6).
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MeR
O
Base
Base
MeR
Me
R
O
O
M
M
TMSCl
TMSCl
MeR
OTMS
Me
R
OTMS
Me
R
OTMS
FMe
R
O
+ TMSF
Scheme 6
Asymmetric version of this reaction has also been well explored. For examples, chiral
auxiliaries and chiral catalysts have been used as chiral source for asymmetric aldol
reactions (Scheme 7).
N S
S
Me
OTiCl4, TMEDA
PhCHO
N S
SO
+ N S
SO
PhPh
OH
Me
OH
Me
97:3
M. T. Crimmins, K. Chaudhary, Org. Lett. 2000, 2, 775.
OHC+
OHCL-Proline (catalyst)
TBS-OTf
OHC
Me
OH
P. M. Pihko, A. Erkkila, Tetrahedron Lett. 2003, 44, 7607.
Scheme 7
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Examples:
MeCHO
NaOH
H2O
MeCHO
Me86%
M. Haussermann, Helvetica Chimica Acta 1951, 34, 1482.
Me Me
O
O
2% NaOH
EtOH
O
Me
Me+
O
Me
94 6
P. M. McCurry, Jr., R. K. Singh, J. Org. Chem. 1974, 39, 2316.
1.2.1.2 The Reformatsky Reaction
The enolate generated from an-bromo ester with zinc reacts with an aldehyde or ketone
to give an aldol-type product in diethyl ether (Scheme 8).
ROBr
O
Me Me
Me Me
O
ZnRO
O
Me MeOH
Me Me
Mechanism
ROBr
O
Me Me
Me Me
O
Zn
RO
O
Me MeOH
Me Me
ROZnBr
O
Me MeRO
O
Me
Me
Zn
Br
RO OZnBr
O
Me Me
Me Me
H
Scheme 8
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Examples:
CHO
OMe
MeO
Br
O
OEt
Mn2+, THF OMe
MeO
CO2Et
OH
(Other metals can also be used)
Y. S. Suh, R. D. Rieke, D. Reuben, Tetrahedron Lett. 2004, 45, 180.
O Br
O
OEt
Zn Powder, THF
Me Me
79%
OO
PhPh
Me
Me
+ HOCO2Et
Ph Ph
Me Me
49% 9%
H. Schick, R. Ludwig, K.-H. Schwarz, K. Kleiner, A. Kunath, J. Org. Chem. 1994, 59, 3161.
1.2.1.3 The Perkin Reaction
This process consists of the condensation of an acid anhydride with an aromatic aldehyde
using carboxylate ion (Scheme 9).
Me O Me
O O
+ ArCHO
i. NaOAcii. NaOH
iii. HAr
COOH
Mechanism
Me O
O O
HOAc
Me O
O OAr H
O
Me O Ar
O O O Me O Me
O O
-OAc
Me O Ar
O O O
O
Me
H
OHMe O Ar
O O O
O
Me-OAc
Me O Ar
O OOH
-OAcHO Ar
O
Scheme 9
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1.2.1.4 The Stobbe Condensation
The Stobbe condensation leads to attachment of three carbon chain to a ketonic carbon
atom (Scheme 10).
RO2C OR
O
R R
O
+Base
Acid R R
RO2CCO2H
Mechanism
OR
OBase
RO
O
H
OR
O
RO
O
R R
OO
OR
RO2C
ORR
-OR
O
O
RR
RO2C
H
Base
RR
RO2C
O
OH
RR
RO2C
OH
O
Scheme 10
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Examples:
OH
MeO
CHO MeO2CCO2Me
MeO, MeOH OH
MeO
CO2Me
CO2H
68%
J. D. White, P. Hrnelar, F. Stappenbeck, J. Org. Chem. 1999, 64, 7871.
CHO
Me Me
EtO2CCO2Et
t-BuO, t-BuOHMe Me
CO2Et
COEt68%
R. Baker, B. H. Briner, D. A. Evans, Chem. Commun. 1978, 410.
1.2.1.5 The Darzen Reaction
The base-catalyzed condensation between an-halo ester and an aldehyde or ketone
affords glycidic ester (Scheme 11).
RO
O
R'
HX
+ R" R"'
OBase RO
"R R"'
R'
O
O
RO
O
R'
HX
R" R"'
O
Base RO
"R R"'
R'
O
ORO
O
R'
XRO
O
R'"R"
O
X R'
-X
Scheme 11
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Examples:
O
KOtBu, tBuOH
O CO2Et
95%
R. Hunt, L. Chinn, W. Johnson, Org. Synth. CV4, 459.
EtOBr
O
Me
O
LDA, THF, HMPA, hexane
EtO
O
O
70%F. E. Anderson, H. Luna, T. Hudlicky, L. Radesca, J. Org. Chem. 1986, 51, 4746.
ClCH2-CO2Et
1.2.1.6 The Knoevenagel Reaction
The condensation of methylene group bonded with two electron withdrawing groups with
aldehydes or ketones using weak base is known as the Knoevenagel reaction (Scheme
12). The reaction is more useful with aromatic than with aliphatic aldehydes.
H
HEWG
EWG+
R
HO
RNH2R
H
EWG
EWG
H
HEWG
EWG
RNH2
H
EWG
EWG
R
HO
EWG
EWG H
R O
H
RNH3EWG
EWG H
R OH
H RNH2
EWG
EWG
R
Scheme 12
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Examples:
KOH, bmim PF6
96%
P. Formentin, H. Garcia, A. Leyva, J. Mol. Catal. A: Chem. 2004, 214, 137.
P. J. Jessup, C. B. Petty, J. Roos, L. E. Overman, Org. Synth. CV6, 95.
CHOCH2(CN)2
CN
CN
O
H Pyridine46%
CH2(CO2H)2CO2H
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Problems:
What products would you expect from the following reactions? Provide mechanism.
OHCMe
CH2(CO2Me)2
piperidine1.
2.
CHOCH2(CO2H)2
pyridine
3.
OMe
MeO CHO
CO2Me
CO2Me
t-BuOH
4.
CO2Et
CO2Et
CHO
NaH, Ph-HHydrolysiis
5.
Me
OHC
+ HMe
O
Me
NH
CO2Hcat.
TBS-OTf
6.
CHOO
O O
EtCO2Na
7.
F3C
CF3
O O
O O
AcO2Na
t-BuO
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Text Books:
R.O.C. Norman and C. M. Coxon, Principles of Organic Synthesis, CRC Press, New
York, 2009.
B. P. Mundy, M. G. Ellerd, F. G. Favaloro Jr, Name Reactions and Reagents in Organic
Synthesis, Wiley Interscience, New Jersey, 2005.
J. March, Advanced Organic Chemistry, 4th
ed, Wiley Interscience, Yew York, 1992.
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Lecture 2 Base Catalyzed Reactions II
1.2.2 Reactions with Esters and Analogues
1.2.2.1 The Claisen Condensation
The condensation of between esters is known as the Claisen condensation. It is important
to note that an equivalent amount of base must be employed for this reaction (Scheme 1).
ROR'
O
H
R" OR"'
Oi. Base
ii.
iii. H
OR'
R"
O
O
R R" = a group that can not form an enolate
Mechanism
ROR'
O
H
R" OR"'
O
Base OR'
R"
O
RR
OR'
O
HH H
R'"O O
OR'
R"
O
R
H
O
+OR"'
-R"'OH
OR'
R"
O
R
O
workupOR'
R"
O
R
O
H
*The reaction with a mixtue of esters each of which contains-hdyrogen may yield four products
Scheme 1
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Examples:
EtO2CCO2Et +
CO2Et
CO2Et
NaOEt
Toluene, Et2O
CO2Et
CO2Et
OCO2Et
L. Friedman, E. Kosower, Org. Synth. CV3, 510.91%
MeO Me
O
MeOMe
O O
MeO OMeNaOMe
MeOMe
O
MeO OMe Conc. HCl MeOMe
O O
O
A. G. Cameron, A. T. Hewson, M. I. Osammur, Tetrahedron Lett. 1984, 25, 2267.
1.2.2.2 Dieckmann Condensation
Condensation of the diesters of having C6 and C7 can be accomplished to afford five and
six membered cyclic -ketoesters (Scheme 2). The diesters of short-chain do not show
cyclization, while diesters with C8 and C9 provide the cyclized products in fewer yields.
OOR
ORO
H
H
Base
OO
RO
Mechanism
OOR
ORO
H
HBase
OO
RO
OORRO
OH
OO
RO
OR+H
OO
RO
H
Scheme 2
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Examples:
Me
Me
EtO2C
EtO2C
NaH, cat. EtOHbenzene
Hydrolysis
OMe
Me77%
D. P. Provencal, J. W. Leahy, J. Org. Chem. 1994, 59, 5496.
EtO2CCO2Et
Na, Cat. EtOHToluene
H
OCO2Et
81%P. S. Pinkney, Org. Synth. CV2, 116.
1.2.2.2 The Thorpe-Ziegler Reaction
The cyclization of dinitriles using base can be accomplished (Scheme 3). Although it is
similar to the Dieckmann reaction, the former is often better compared to the latter.
CNNC
H
H
Base
CNH2N
CNNC
H
H
Base
CNCC
H
NNCNN
H
CNHNH
H2N
Scheme 3
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Examples:
CNCN
NaH
CN
NH2
J. J. Bloomfield, P. V. Fennessey, Tetrahedron Lett. 1964, 5, 2273.
S
Me
Me
CNCN DMSO
S
Me
Me
CN
NH2
G. Seitz, H. Monnighoff, Tetrahedron Lett. 1971, 12, 4889.
EtO
1.2.2.3 Enamines
The reaction of secondary amine with aldehyde or ketone that contains an -hydrogen
atom affords enamine (Scheme 4). The process is driven to right by removing the water
as it is formed, either by azotropic distillation or with molecular sieves.
R'
H
R
R
O
+ R"2NH R'
H
R
R
HO NR"2
R'R
R
NR"2
-H2O
Scheme 4
Similar to the enolates derived from ketones, enamines react with acid chlorides to give
imine derivative that could be hydrolyzed to -diketones (Scheme 5).
R2N:
Cl
O R2N O Cl
-Cl
R2N O
H2O, H+
O O
-R2NH
Scheme 5
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In case of unsymmetrical ketones, less substituted enanime forms as a major product
(Scheme 6).
O
NH
+-H2O
N N
+
90% 10%
Scheme 6
1.3 The Alkylation of Enolates
Enolates, like other nucleophiles, also undergo reaction with alkyl halides and sulfonates
with the formation of carbon-carbon bonds. Depending on the reaction conditions and
nature of the substrates, the reaction can occur either at oxygen atom or carbon atom of
enolate (Scheme 7).
O
CH3 XO
CH3
-X
COEt
O
COEt
O
COEt
O
Base EtICO2Et
O
CO2Et
OEt
+
Et
71% 13%
Scheme 7
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1.3.1 Alkylation of Monofunctional Compounds
Depends on the reaction conditions (kinetic vs thermodynamic control), enolate can be
selectively alkylated (Scheme 8).
O
MetBuOLDA
OMe
MeMeIMeI
O
MeMe
Scheme 8
Kinetic control with LDA: Proton abstraction takes place at less hindered -CH position
and the reaction is faster and essentially irreversible.
Thermodynamic control with tBuOK: equilibration takes place between the two enolates
and the methyl-substituted one, being the more stable, is present in high concentration.
However, when the highly substituted position is strongly sterically hindered, alkylation
with tBuOK occurs at the less sterically substituted carbon (Scheme 9).
O
tBuOK
MeI
O
Me
Scheme 9
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1.3.2 Alkylation of Bifunctional Compounds
A C-H bond adjacent to two electron withdrawing groups is more acidic than that
adjacent to one electron withdrawing group and the alkylation could be carried out in
milder conditions (Scheme 10).
BrCO2Et
CO2Et -EtOH
CO2Et
CO2Et
CO2Et
CO2Et
KOH, H
CO2H
CO2H180 oC
-CO2CO2H
EtO
Scheme 10
1.4 Addition of Enolates to Activated Alkenes
Enolates undergo addition to alkenes that are activated by conjugation to carbonyl, ester,
nitro and nitril groups (Scheme 11). These reactions are usually referred to as Michael
addition.
+OEtPh
O
OEt
Ph O
EtOCH3NO2 O2N
CO2Et
CO2Et+
OEtPh
O
OEt
Ph O
CO2Et
EtOC
EtO
Scheme 11
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Examples:
O
O
O
NaH
DMF
O
O
O
55%
D. M. Gordon, S. J. Danishefsky, G. K. Schulte, J. Org. Chem. 1992, 57, 7052.
Me
H2C
CO2Et
O
NaH
EtOHO
CO2Et
Me
S. Nara, t. Toshima, A. Ichihara, Tetrahedron 1997, 53, 9509.
76%
1.5.1 Reactions Involving Alkynes
Acetylene and its monosubstituted derivatives are more acidic than alkenes and alkanes
and take part in reactions with both carbonyl-containing compounds and alkyl halides in
the presence of base (Scheme 12).
H HNa-NH3
H
Br
-Br
H HNa-NH3
HMe Me
O H
O
H
H
OH
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Application
H
+Cl
I
NaNH2
-NaI Cl
KCN
HCO2H
CO2H
Oleic Acid
reduction
Scheme 12
1.5.2 Reactions of Cyanides with Alkyl Halides and Sulfonates
HCN, like acetylene, is a weak acid whose anion may be generated by base and is
reactive towards primary and secondary alkyl halides and sulfonates to give the
corresponding nitriles. It is more convenient to introduce the cyanide as cyanide ion (e.g.
NaCN, TMSCN) rather than as HCN. These reactions provide a way of extending
aliphatic carbon chains by one carbon atom. Scheme 13 summarizes some of the useful
transformations.
Cl
CO2H
OHCl
CO2
CN OHCO2
NO2C CO2
Ca2+
O2C CO2 Ca2+
2HCl
HO2C CO2H
-CaCl2BrCuCN/Pyridine
Heat
CNSnCl2-HCl
H2O
CHO
Reduction
NH2
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Mechanism for Stephen Aldehyde Synthesis (Stephen Reduction)
NR :HCl
NR H + Cl N
Cl
R H
.. HClN
Cl
R H
HCl
SnCl2 N
R H
HH
2SnCl4
H2O
R OH
NHH ..
H
H
proton
transferR OH
NH
H
H
H
RCHO
Scheme 13
1.2.2.5.2.2 Reactions of Cyanides with Carbonyl Compounds
Cyanide ion adds to aldehydes and ketones to give cyanohydrins that can be hydrolyzed
to-hydroxy acids (Scheme 14).
R R
OCN
R CN
OR H
RCN
OH
RH
RCOOH
OH
R
Scheme 14
Asymmetric version of the process is also well explored. For example, chiral main chain
polymer having Ti(VI) has been found to be an effective recyclable catalyst to obtain the
cyanohydrin with up to 88% ee (Scheme 15).
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1 mol % Ti*
CHCl3, 0 °C, 48 h
R2 equiv TMSCN CN
OH
R = alkyl, aryl Yield 90-97%
N N
O O
R'
Ti
O
O
Ti* R' = -(CH2)4- n = 15
R'
n
17HO8C
OC8H17
*R H
O
CN
OH
OCH3
CN
OH
CN
OH
CN
OH
OC2H5
3HC
CN
OH
3HCO
CN
OHOH
CN3HC
3HC
95%, ee 88% 91%, ee 78%
Br
90%, ee 75%
97%, ee 78% 97%, ee 68% 90, ee 74%
CN
OH
OC3H5
90%, ee 75%
OH
CN
90%, ee 55% 93%, ee 66%
CN
OH
Cl
95%, ee 60%
ee 55-88%
S. Sakthivel, T. Punniyamurthy, Tetrahedron: Asymmetry 2010, 21, 2834
Scheme 15
Imine that can be prepared from amine and aldehyde readily undergoes reaction with
cyanide ion to give -amino nitrile which can be hydrolyzed to amino acid (Strecker
synthesis) (Scheme 16).
R H
O
+ R'NH2 + HCNAcOH HN
OH
R'
R
O
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Mechanism
R H
O: :H
R H
OHR"NH2..
"RH2N
OH
RH
"RHN
OH2
RH.. -H2O
N
"R
H
H
R
CN
HN
R"
R
N:
H
HN
R"
R
NH
HN
R"
R
N
H
H2OH
N
R"
R
N
H
OH2
HN
R"
R
N
H
OH
HH
N
R"
R
O
HNH2
H H2O
HN
R"
R
O
HNH2
HN
R"
R
OH
H
NH2
OH2H
N
R"
R
OH
H
NH3
OHH
N
R"
R
O
OH-NH3
Scheme 16
Example:
N
OMe
HN
NH
O
O
Ph
HN
N
NH
NH2
2 mol%
HCN, MeOH
HN
OMeCN
M. S. Iyer, K. M. Gigstad, N. D. Namdev, M. Lipton, J. Am. Chem. Soc. 1996, 118, 4910.
Problems:
A. How would you use base-catalyzed reactions in the synthesis of the following compounds?
OCN
1
2
3
O
O
CO2H
4
O
CO2Et
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MeOEt
Oi. NaNH2, C6H6
MeO
CO2Et
ii.
CO2Et
S
EtO2CNaOEt
C6H6
1.
2.
CN
MeO3. +
CO2Me
NaH/THF
iii. Hydrolysis
MeO
OMe
S
OEtO2C
ONC
OMe
B. Rationalize the following reactions.
Text Books:
R.O.C. Norman and C. M. Coxon, Principles of Organic Synthesis, CRC Press, New
York, 2009.
B. P. Mundy, M. G. Ellerd, F. G. Favaloro Jr, Name Reactions and Reagents in Organic
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