CHAPTER - II

ELECTRON TRANSFER REACTIONS OF ORGANIC CATIONS AND

RELATED SPECIES

I I ,1 Oxidation o f electron rich organic compounds to

stable rad ica l cations by DDQ in acid media*

11*2 Carbocations as electron transfer oxidants*

11*3 Electron Transfer reactions o f arene-diazonium

ions*

145

INTRODUCTION

Interaction o f two organic reagents, one

containing electron withdrawing groups such as C-0,

C«N, N»N# halogen, N02 or ca tion ic centres, and the other

electron rep e llin g ones e . g . , a lk y l, amino, alkoxy, and

anionic centres can lead to an in i t ia l step o f s in g le electron

transfer to form rad ica l cations- rad ica l anion pairs orkeep

rad ica l ion s-free rad ica ls in solution • Reports/pouring in

which suggest electron transfer steps in many o f these

reactions as a v iab le a ltern a tive to the conventional

mechanisms*

Highly cyanated unsaturated compounds such as

tetracyanoethylene (TCNE) and tetracyano-quinodimethane (TCMQ)

with weakly oxid is ing properties can undergo electron transfer

with eas ily ox id izab le compounds. For instance reaction o f

TCNE with metal hydrides proceeds by a rad ica l cation -rad ica l

anion pair from an in i t ia l charge-transfer complex1. Radical

processes have been invoked in the reaction o f TCMQ with some

anions such as sodium benzoate, sodium s a lic y la te and2

sodium acetate .

Peroxides reduce a host o f organic substrates by the

ET pathway. An ea r lie r example o f such a reaction is the

in teraction o f dibenzoylperoxide with M,N-dimethylaniline to3

generate the rad ica l cation o f the amine . A number o f other4 5compounds such as aromatic hydrocarbons , dimethoxybenzenes,

c 7 8anisplnacolone', diphenylhydroxyl amine and su lfid es have

been oxidised by the peroxides to the in i t ia l rad ica l cation.

Radical cations iso la ted in the so lid s ta te e f fe c t

oxidation o f a wide va rie ty o f compounds by the electron, . Q-14

transfer (ET) pathway . These reagents o f fe r a convenient

means o f generating rad ica l cations from organic substrates

fre e o f complications,such as ligand transfer, normally

encountered in oxidations with metal ions. The rad ica l cations

generated by th is method undergo a series o f bond scission

and bond formation analogous to the reactions encountered in

anodic processes.

Stable carbocations form charge transfer complexes

with donors e .g . , aromatic hydrocarbons and under certain

conditions complete ET from donor to acceptor can occur.

Certain o f the carbonium ions such as t r i t y l and i t s

deriva tives and tropylium ions are oxidants o f moderate

poten tia ls and many o f th e ir reactions involve electron

transfers. In many cases the rad ica l cations thus generated

have been id en tified by EPR spectroscopy^. Thus when t r i t y l

cation is mixed with perylene and tetracene in methylene

ch loride, the corresponding rad ica l cation is formed as

indicated by i t s EPR spectra (Eq,1 ). Tris-p-chlorophenyl

carbenium ion oxid ises pyrene and anthracene to the cation 15rad ica ls J•

146

(1 )Triphenyl~methyl cation accepts an e lectron from

ferrocene in nitromethane forming ferricin ium ions and the

triphenyl methyl rad ica l which reacts with oxygen to form

the peroxide1 (Scheme 11*13.

147

Ph3C

Fe4-

* Ph3C*

Ph3 COOCPhj \KU/

Scheme I I . 1

9-Methyl-9-fluorenyl trimethyl t in reacts with17 18triphenyl methyl sa lts by an ET process ’ (Scheme I I . 2)

+ Ph,CT +Ph,C*+Me,Sn+ 0 5

Scheme I I . 2

The fa te o f fre e rad ica ls formed on one electron

reduction o f the carbocations depends on the reaction1 Qconditions. In some cases dimer formation is observed (Eq.2).

(2 )Ph3C* Ph3C - CPh3

In the reaction o f tropylium sa lts with carbazole

evidence has been presented towards the formations o f20carbazole rad ica l cation . The ET reaction o f carbocations

21with o le fin s leads to polymerization • Several other reports

ex is t which describe the ro le o f stable carbocations as22-24electron acceptors •

148

Aryl diazonium ions have a high e lectron a f f in it y

and many o f i t s reactions with anions involve electron

transfer. For example Singh and coworkers have demonstrated25 26

that iodide and n it r i t e ions reduce various aryldiazonium

cations by a s ing le ET process to produce a ry l radicals which

partic ipa te in a chain reaction (Scheme I I . 3 ) .

r^ n. n-

A « l7 MC>2

Scheme I I . 5

The formation o f azo compound in the reaction o f

diazonium cation with sodium phenoxide has been shown to

involve the in i t ia l generation o f rad ica ls followed by a27coupling reaction to form products •

In favourable circumstances, reactions o f diazonium

cations with certain neutral organic molecules lead to the

formation o f cation rad ical o f the substrates* Thus reactions

o f phenothiazlne and substituted phenothiazines with

4-methoxybenzenediazonium ion leads to the formation o fOQ

phenothiazlne radical cation (Scheme 11*4),,

149

Scheme I I . 4

N,M ,N 3N -tetraraethyl-p-phenylene diamine(TMPD)is oxidized by diazonium ion to the rad ica l cation,

29Wurstura blue • K inetic study o f the reaction o f

R BF (R * MOg, Brf H, Me, MeO and Et^N) with dibenzo

tetra th ia fu lva lene and tetra th ia fu lva lene in MeCN indicates

the involvement o f s ingle ET in these reactions^0,

ET characteristics are exhibited by the aryla tion

reaction o f thiophene with 2 ,4-dinitrobenzenejiiazonium te tra -

flu orobora te^ and that o f pyridine (Py) with p-nitrobenzene-32diazonium ion (Scheme I I . 5 ) . An analogous case where the

aromatic species appears to in it ia t e ary la tion is the in teraction

o f ferrocene with benzene diazonium c h lo r id e^ .

ArM+ BF ♦ nPy ---------[ A rN ^ P y jJ * BF^

Our quest fo r id en tify in g reagents that would

e f fe c t oxidation by the ET pathway led us to examine the

reactions o f some high poten tia l quinones such as

2 ,3-d ich loro-5 » 6-dicyano-p-benzoquinone (DDQ) with aromatic

hydrocarbons o f moderate poten tia ls that are known to y ie ld

stab le rad ica l cations chemically or a t an anode. In the

present in vestiga tion i t was found that in the presence o f

acid, aromatic and heterocyclic compounds read ily g ive up

an electron to DDQ generating the corresponding rad ica l

cations, the DDQ being converted to the dihydroquinone (DDQH .

Thus oxidations with DDQ provide an Instance o f acid induced

electron transfers taking place, and o ffe r newer avenues fo r

the generation o f stable rad ica l cations which can even be

iso la ted provided a judicious se lection o f solvent acid mixtures

i s made.

A number o f reports e x is t demonstrating the

involvement o f ET in the reaction o f stable carbocations with

many ea s ily ox id is ing molecules* Having a general in teres t in

ET reactions and in the use o f organic ET oxidants, I t was

considered worthwhile to in vestiga te the scope o f stable

carbocations such as t r i t y l hexachloroantimorate, tropylium

hexachloroantimonate and xanthylium perchlorate as electron

acceptors* In c iden ta lly , there are no reports on the ET

reactions o f xanthylium cation.

Stable solutions o f rad ica l cations were obtained

on treating the above mentioned carbocations with aromatic

and heterocyclic compounds o f moderate to low oxidation

poten tia ls such as phenothlazine, , N*,N*-tetramethyl-p-

150

151

phenylene diamine, tetra th ia fu lva lene e tc . Moreover

tropylium and t r i t y l hexachloroantimonate were found to e f fe c t

oxidation o f some nitrogen containing compounds such as

d iazoalkanes and hydrazones v ia the intermediacy o f rad ical

cations* A ll these resu lts lend credence to the view that ET

must be more prevelant than is recognised at present in the

reactions o f stable carbocations.

Reports presented in recent years regarding the

involvement o f ET in oxidation reactions o f diazonium sa lts

in cited the author to try unprecedented reactions that would

serve to exemplify novel ET processes. The substrates chosen

were diaryldiazomethanes, hydrazones o f aromatic ketones,

phosphazines and phosphoranes and oxidations were brought

about by p-nitrobenzenediazoniura tetra fluoroborate.

I I .1 Oxidation o f e lectron r ich organic compounds to

stable rad ica l cations by DPQ in acid media.

EXPERIMENTAL

Pu rifica tion o f reagents. The aromatic compounds were obtained

commercially and were pu rified by re c ry s ta llis a t io n . DDQ (BDH, Poole England) was pu rified by rec ry s ta llis a t io n from

chloroform (m.p. 217-219°). Dichloromethane and tr iflu o ro a ce tic

acid were purified as described previously (Chapter I ) .

EPR spectra were recorded on a Varian E-109

spectrometer in solutions deoxygenated by freeze-thaw technique

with in term ittant introduction o f n itrogen. UV-Vis spectra

were recorded on a P\.je lin'icom -SP 8100 scanning

sp ectrophotometer•

152

Experimental procedures; The oxidations were e ffec ted by

d isso lv ing the aromatic hydrocarbon ( ~ 10~* M) in CH2C12/TFA

(app. 9s1 v/v) and then adding so lid DDQ ( — lO ^So. Most o f

the reactions were carried out in a ir . Where EPR signals had

to be recorded, solutions were scrupulously degassed before

spectral measurement.

Iso la tion o f the stable t r la - (4-bromophenyl) amlnlum perchlorate

by_oxldatioo o f the amine w ith DDQ in ad d media.

To a solution o f tris(4-bromophenyl) amine

(1*3g, 2.7 mmol) in CH2C12(15 ml) was added O'OQ (0.6g, 2.6 mmol)

in MeCM-HClO (8s2 v/v) solvent mixtures (15 ml) while purging

with nitrogen. The reaction mixture developed a deep blue colour

almost instantaneously and on pouring the mixture into

scrupulously dried ether (100 ml) cooled previously to -10°C,

a blue p rec ip ita te o f the perchlorate sa lts o f the rad ica l

cation se tt led down which was f i l t e r e d and dried in a vacuum

desiccatdr ( m.p. 128°, l i t . m.p.34. 129°C, y ie ld 70%). A

major portion o f the f i l t r a t e was found to contain the

dihydroquinone, DDQHg.

RESULTS AND DISCUSSION;

We have observed that deep coloured solutions are

formed when electron donor molecules and high poten tia l

quinones, pa rticu la r ly DDQ are allowed to react in d ich loro-

methane or 1,2-dlchloroethane containing small amounts o f

tr if lu o ro a c e t ic acid (TFA). For example on adding a few drops

153

o f TFA to a mixture o f thianthrene and DDQ in CH2CI2 a t room

temperature (28°C), the solution developed a purple colour

almost instantaneously whose e lectron ic absorption spectrum

( X___ 545 nm) len t a convincing proof o f the thianthrenemaxrad ica l cation having formed.

S im ilarly , other electron r ich molecules o f a wide

range o f oxidation poten tia ls y ielded the corresponding rad ica l

cations whose e lectron ic absorption spectra matched very well

with those reported in lite ra tu re . Addition o f small amounts o f

triphenylphosphine, a p lausib le rad ica l cation scavenger ,

to each o f these solutions provided a more searching tes t in

that the rad ica l cations were reverted to parent molecules asTL-

marked by sudden discharge o f colours. In F i g u r e a r e shown

the EPR spectra o f 9,10-dimethylanthracene and phenothiazinen.

cation rad ica ls while F igu re^ presents the v is ib le absorption

spectra o f the rad ica l cations o f 9,10-dimethylanthracene,

ferrocene, biphenyl, te tra th ia fu lva lene and rubrene. Listed in

Table I I . 1 are the substrates oxid isable by DDQ in C^C l^/TFA

solu tion . The concentration o f TFA required to bring about

complete conversion o f the substrate to i t s rad ica l cation was

only 7-10^ (v/v) o f the solvent CH2C12.

1,2-Dichloroethane/TFA, dichloromethane/trichloroacetic

acid , nitromethane/HC10^, sulfolane/HClQ^ and MeCft/TFA systems

also worked but best resu lts were obtained with CH2C12/TFA

solvent mixtures.

The recogn ition o f ET having occurred in oxidations12with DDQ can be traced to 1#ed-with e t a l , who reported that

Figure I I . 1 Elec&on spin resonance spectra o f the radical cations o f (a ) dimethylanthracene and (b ) phenothiazine obtained by oxidation o f the parent compound with DDQ in CH2C12/TFA(9:1)

400 500 600 700

WAVE L E N G T H ( D m )

Figure I I . 2 V is ib le absorption spectra o f t h e radical cations o f (a ; te tra th ia fu lva lene,(b ) rubrene, (c ) ferrocene, (d ) biphenyl and (e ) dimethylanthracene obtained by oxidation o f the parent‘ compount with ODQ in CH2C12/TFA (9 :1 )

carbazole reacts with DDQ in acid medium to generate

carbazole rad ica l cation. The f i r s t in-depth in vestiga tion36has been carried out recently by Verhoeven et a l , who

proposed that protonation o f the oxidant, quinone (Q ), occurs

and i t is the protonated quinone (QH+, ) that in i t i a l l y accepts

an electron from the substrate (Eq«3)

Q + H+ --------------QH+V " 1 " 1 ...........

ArH ♦QH* -------------- * ArH+* + QH* . . .t--------------

ArH +QH* -------------- * ArH+* ♦ QH2 . . .T................

2ArH +Q+2H*------------- 2ArH** ♦ QH2 . . . (3 )

154

Our studies, wherein stoichiometry and products

o f the reaction were used to probe the mechanism, do as w ell

account fo r the «^uilibrium which is reminiscent o f the

electrochemical h a lf reaction (Eq.4)

Q ♦ 2H* +2e~--------- QH2 . . . (4 )

One may question, however, whether or not the ro le

o f acid is to form the protonated quinone (Eq.3a), that in

turn acts as one electron oxidant. We are o f the opinion

that the active species is not the protonated quinone, but

the quinone i t s e l f , whose ox id iz ing capacity is grea tly

enhanced in the presence o f strong acid. Reasons fo r th is

are as fo llo w s :(1 ) the e lectronegative ly substituted quinones,* » q

particu la r ly DDQ, possess low b a s ic ity * to be susceptible

to protonation by acids such as TFA. (2 ) I t was found that when

(3a)

(3b)

(3c)

an e x p e r im e n t p a r a l l e l t o t h a t o f E q ,3 was c a r r i e d o u t r e p l a c i n g

DDQ w i t h Pb02 » MnQg, o r ev en t h e weak o x i d a n t p - n i t r o b e n z e n e

d iazo n iu m t e t r a f l u o r o b o r a t e , i n w h ich t h e q u e s t i o n o f p r o t o n a t i o n

d o e s n o t a r i s e , an i d e n t i c a l se q u e n c e o f r e a c t i o n s o c c u r e d and

r a d i c a l c a t i o n s o f a l l t h e compounds l i s t e d i n T a b le I I . 1 w ere

fo rm ed .

T h is f a c t , th e n , s e r v e s t o e x c lu d e t h e p o s s i b i l i t y o f

t h e o x id a n t b e i n g p r o to n a t e d b u t a s s i g n s a d i f f e r e n t r o l e t o

t h e a c i d i n t h e o x i d a t i o n . A lth o u g h an e x p l i c i t answ er to

t h i s problem i s n o t known, i t i s r e a s o n a b le t o assume t h a t

t h e f u n c t i o n o f t h e a c i d i s t o prom ote t h e o x i d i z i n g power

o f t h e o x id a n t and th u s in d u c e e l e c t r o n t r a n s f e r s . I t i s

p e r t i n e n t t o m en tion h e re t h a t a c t i v a t i o n o f m e ta l o x id a n t s

i n TFA o r s t r o n g a c i d s h as b e e n o b s e r v e d w it h C o ( I I I ) , C e ( l V )

i i n ( l l l ) , C u ( I l i e t c ^ . Thus w h eth e r t h e o x i d a n t i s DDQ or

Pb02 , r a d i c a l c a t i o n s a r e form ed i n th e p r e s e n c e o f a c i d t h a t

prom otes t h e o x i d i s i n g power o f t h e o x id a n t ( E q . 5 ) .

ArH 4- DDQ + H* ---------- -*• ArH+* ♦ DDQH* . . . (5a)■I111"'...... ....

ArH ♦ DDQH* +H* ---------------- ArH+* + DDQHg . . . (5b)T.. .........

1 ArH + DDQ ♦ 2H*---------- *2ArH+* + DDQH ••• }t ...mmmmm.. .

Under a n h yd ro u s c o n d itio n s , s o l u t i o n s o f r a d i c a l c a t i o n s

formed b y DDQ o x i d a t i o n s ( T a b le I 1 . 1 ) were fou n d t o be s t a b l e

f o r l o n g p e r i o d s o f t im e (some o f them i n d e f i n i t e l y ) . I t was

v i s u a l i s e d t h a t r e a c t i o n s o f t h i s t y p e s h o u ld open up s im p le

p r e p a r a t i v e r o u t e s t o d i f f i c u l t y a c c e s s i b l e r a d i c a l c a t i o n s and

155

156

Table 11*1 A___ o f cation rad ica ls obtained by oxidation.............. . max *o f the parent compound with DDQ in acid media

at room temperature (28°C)

S. 'Jo Substrate* ObservedW ”

Reported\iax,nm

Ref.

1. Pyrene 505 505 402. Rubrene 525 b3. Fluorene 532 b4. Perylene 543 540 415. Acenaphthene 512 b6. Anthracene 715 715 407. 9,10-Dimethylanthracene 669.5 b8. 9,1O-Diphenylanthracene 727,652,593 724,653,596 369. Naphthalene 535 535 4010. Thianthrene 545 546 4211. Phenothiazine 513,439 515,437 4312. Tetrath iafu lvalene 578,434 580,435 4413. Biphenyl 658 660 4514. T r is - (4 -bromophenyl)amine 726 725 3415. Tetraphenylethylene 495 494 4616. M,N,M*, M1-Tetramethyl-p-

phenylene diamine 615,565 618,567 4717. Ferrocene 619.5 619 4818. 1 ,4-Dimethoxybenzene 472,437 475,440 49

b * not located in lite ra tu re .

also fa c i l i t a t e iso la tion o f stable rad ica l cation ?;alts.

Attempts in th is d irection proved rewarding whereby we could

is o la te perchlorate s a lt o f tris~(4~bromophenyl)amine

(.oee experimental) •

Thus oxidation with DDQ o ffe r newer avenues to the

formation o f rad ica l cations and as more work is done particulariy

in making a Judicious se lection o f solvent-acid mixtures,

oxidations o f th is type may rece ive wide-spread preference in

rad ica l cation formation and in reactions where rad ica l cations

intervene.

n .2 Carbocatlons as Electron.Transfer Oxidants

EXPERIMENTAL

The aromatic compounds were obtained commercially

and used as such. Hydrazones were prepared as described

previously (Chapter I ) . A ce ton itr ile was fra c t io n a lly d is t i l le d

from phosphorous pentoxide through a ten inch h e lix packed

column u n til the d is t i l la t e was free from hydrogen cyanide as

Indicated by the absence o f a p rec ip ita te with aqueous s i lv e r

n itra te . I t had a bo ilin g point o f 81°C and water content o f

0.02%.

.o£..,jyJB&P S2ffl.P.9¥Rfl,§,

a ) Diazodiphenvlmethanet To a mixture o f benzophenone hydrazone

(5.0. g ) and yellow mercuric oxide (10.0 g ) in petroleum ether

40-60° (25 m l), a few drops o f concentrated a lcoholic potassium

157

hycroxide were added. The mixture was covered with a s i lv e r f o i l

and l e f t s t ir r in g overnight. The resu lting purple solu tion was

f i l t e r e d through a sintered funnel and the solution concentrated

by using a rotary vacuum evaporator and then again f i l t e r e d .

C rysta llisa tion was then brought about by placing the solution

in a re fr ig e ra to r u n til needle-shaped crysta ls o f diazodiphenyl-

methane were obtained. These were dried in a vacuum desiccator

and stored in a brown b o ttle in the re fr ig e ra to r ,

\nax " 526 nm; 29-30°C, l i t m,p5° 29°

b) 9- D ia z o f lu o r e n e : F lu oren on e h yd razon e (4,7 g), y e llo w

m e rcu ric o x id e (1 , 5g ) a n d anhydrous sodium s u l f a t e (4 , 5g ) were

ground to g e th e r and suspended in d ry e th e r (50 ml ) , A s a tu r a te d

s o lu t io n o f KQH in m ethanol (1 m l) was th en added and th e

s o lu t io n s t i r r e d m a g n e t ic a lly f o r 1 h r . The s o lu t io n was

f i l t e r e d and th e r e s id u e washed w ith e th e r u n t i l th e w ashings

were no lo n g e r c o lo u r e d . The e th e r was removed to le a v e a red

s o l i d , w hich was r e c r y s t a l l i s e d from 40 - 60° p etro leu m

e th e r, m .p. 94- 95° •

Preparation o f stable carbocatlons

a) T r ity l hexachloroantlmonate: Antimony pentachloride

(BDH Poole, England) ( I 5 . 5g ) in carbon tetrach loride (75 ml)

wasaadded to a solution o f triphenylmethyl chloride (14 ,5g) in

carbon tetrach loride (100 m l), A yellow p rec ip ita te which formed

immediately was f i l t e r e d o f f , washed several times with carbon

te trach loride , and dissolved in a minimum quantity o f hot

dichloromethane. Pure triphenylmethyl hexachloroantlmonate

separated as orange crys ta ls . The product was stored under vacuuo.

158

m.p 230° | l i t m.p51 230°.

159

b) Tropyllum hexachloroantimonatet A solution o f cycloheptatrim e

(Fluka, Swiss) (13 ml) in GHgClg (50 ml) was added dropwise

during 30 min, to a solution o f triphenylmethyl hexachloroanti­

monate (32g) in CH2C12 (250 ml) under nitrogen* The resultant

s lu rry was s tirred fo r a further 30 min a fte r which 100 ml o f

carbon tetrach loride was added* The mixture was ch illed in

ic e , f i l t e r e d and washed with cold carbon te trach loride (25 ml)*

The product was redissolved in CH2C12(150 m l), p recip ita ted with

carbon tetrach loride (150 ml) and r e f i lt e r e d to g ive white

tropyllum hexachloroantimonate*

m.p 283°; l i t m.p51 284°

c ) Xanthyllum perch lorate: Xanthene (1 ,82g), triphenylmethyl

perchlorate (3.43g) and acetic acid (100ml) were heated fo r

2 min. Xanthylium perchlorate began to separate out from the

b o ilin g solution as yellow p la tes . The product, f i l t e r e d from

the cooled solution was rec rys ta llis ed from acetic acid.

m.p 225°* l i t m.p52 225-226°C

II.2B Oxidation o f Dlazocompounds

a) Reaction ofdiazodlphenylmethane with t r i t y l hexachloroanti­

monate

A solution o f dlazodiphenylmethane (1*5g, 7.7m mol)

in dry a c e to n itr ile (25 ffll) was taken in a 100 ml fla sk . To th is

was added an MeCN solu tion o f t r i t y l hexachloroantimonate

(2 .2g, 3.8 mmol) with constant s t ir r in g at room temperature. A

brisk evolution o f nitrogen ensued and the purple colour o f

the solution gradually disappeared. The reaction was over in

ten minutes• On allowing the reaction mixture to stand, a

mass o f shining white crysta ls se ttled down. These were f i l t e r e d ,

washed and id en tified as tetraphenylethylene (m.p 224°, l i t m .p^

224°) (67.5%). The f i l t r a t e was chromatographed on a s i l ic a

column. Thus benzhydrol (13»5%) and t r i t y l peroxide (traces )

were obtained. The compounds were characterised by mixed

melting points.

* ) Beastion o f dlazodljphenyliB ethane with tropyllum hexachloroanti-

On adding slowly a solution o f tropylium hexachloroanti-

monate (1.5g, 8 mmol) in MeCN to a solution o f diazodiphenylaiethan«

0 *5 g , 7*7 mmol) in 25 ml o f MeCN with constant s t ir r in g while

maintaining the temperature at 25°C, rapid expulsion o f nitrogen

gas occurred and the purple colour o f the diazoalkane disappeared

gradually. The solution on concentration yielded colourless

prisms, m.p 224° id en tified as tetraphenylethylene (70$), The

f i l t r a t e was found to contain benzhydrol (15%) and benzophenone

(4.5%).

c ) Reaction o f 9-diazofluorene with t r i t y l hexachloroantlmonatet

A solution o f t r i t y l hexachloroantlmonate (2.2g, 3.8rmnol

was added slow ly with s t ir r in g to a solution o f 9-diazofluorene

0 »5 g , 7.7 mmol) in dry a c e to n itr ile (25 ml) a t room temperature.

Brisk evolution o f nitrogen gas took place and the red colour o f

the solution turned yellow , the in tens ity o f which increased

with time. The reaction was over in 25 min. On completion, the

160

reaction mixture was allowed to stand whereupon orange crysta ls

deposited. These were f i l t e r e d , washed, rec ry s ta llized from

benzene-petrol and id en tified as b ifluorenylidene, m.p 191-192°,

l i t m.p^4 189-190° (66.5%)by comparisons (mixed melting point

and superimpossible IR spectrum) with authentic sample. The

f i l t r a t e on coluamc chromatography yielded fluorenone azine

(5*5%)» fluorenone (2.0%) and fluorenol (14.8%) also characterised

by comparisons with authentic samples.

d ) Reaction o£_fe jlazo flaargpg^vfith tropvllum hexachloroantlmonatg

To a solution o f 9-diazofluorene (1*0g, 5 mmol) in

dry a ce to n itr ile was added tropylium hexachloroantimonate

(1. Og, 2.5 mmol) in MeCM (10 m l). A brisk evolution o f nitrogen

ensued and the reaction was over in 15 minutes. Upon concentration

orange crysta ls o f b ifluorenylidene (68%) separated out and the

f i l t r a t e yielded fluorenol (15%)» fluorenone azine (6.2%) and

fluorenone (tra c e s ).

II.2C Oxidation o f hydrazones

a) ReactlOP-Of henaophenone hy dr azo ne with trlphenylaethyl

To a solution o f benzophenone hydrazone (1.0g, 5 mmol)

in dry a c e to n itr ile (20 ml) was added an MeCN solution (15 ml)

o f triphenylmethyl hexachloroantimonate (2.7g, 6 mmol). A brisk

evolution o f nitrogen resu lted . The reaction was carried out a t

room temperature and took about 20 minutes fo r completion. On

allowing the reaction mixture to stand fo r 4 hrs, colourless

needle shaped crysta ls appeared. These were f i l t e r e d and

161

162o 5*5 oId en tified as benzophenone azine, m.p, 162 , l i t ®.p 163

by mixed melting poin t. The mother liquor gave more o f the

azine (to ta l y ie ld 65%), tetraphenylethylene (5%) and t r i t y l

peroxide (62%) m.p.185°, l i t m.p56 185-186°.

b) Reaction o f benzophenone hydrazone with troplium

hexachloroantimonate

Benzophenone hydrazone O.Og, 5 mmol) was dissolved

in dry a c e to n itr ile (25 ml) and tropylium hexachloroantimonate

(2 .0g, 5 mmol) in MeCN (15 ml) was added a t once. A brisk

evolution o f nitrogen ensued which continued throughout the

reaction . Towards completion, colourless needle shaped crysta ls

deposited. These were f i l t e r e d and id en tified as benzophenone

azine (63.6%). The f i l t r a t e was found to contain more o f the

azine, trace amount o f tetraphenylethylene and b itrop y l (61.3%)

(m.p 61°, l i t m.p'5 61°).

c ) Beas-tlon o f fluorenone hydrazone with t riphenylmethyl

hexachloroant imonate

The addition o f an MeCN solution (15 ml) o f

triphenylmethyl hexachloroantimonate (2,7g , 5 mmol) to fluorenone

hydrazone (1.0g, 5 mmol) in MeCN (25 ml) was marked by a slow

evolution o f n itrogen. The reaction carried out at 25°,

completed within 30 minutes by which time a red c ry s ta llin e

p rec ip ita te se ttled down. This was separated and id en tified as

fluorenone azine fup 268°, l i t m.p^8 270° by mixed melting

point and superimposible IR spectrum with that o f the authentic

sample. The f i l t r a t e indicated the presence o f more azine

( to ta l y ie ld 68.2%), b ifluorenylidene (traces ) and t r i t y l

163

peroxide.

d ) Reaction o f fluorenone hvdrazone with tropyllum

hexachloroantlmonate

When a solution o f tropyllum hexachloroantlmonate

(2 .0g, 5 mmol) was added to fluorenone hydrazone (1.0g, 5 mmol)

in a c e to n itr ile (25 ml) a slow evolution o f nitrogen could be

observed which continued throughout the reaction carried out

a t room temperature. A fter 30 minutes a red crop o f fluorenone

azine settled down. TLC examination o f the mother liquor showed

the presence o f b ifluorenylidene (tra ces ) and b itropy l

(not determined).

RESULTS AND DISCUSSIONi

IX*2A Formation o f stable rad ica l cations by oxidation with

carbo cations

On adding t r i t y l hexachloroantlmonate, tropyllum

hexachloroantlmonate and xanthyllum perchlorate (10" "S i) to a

solution o f te tra th ia fu lva lene (1 ), phenothiazine (2 ),

ft,M,N*, M*-tetramethyl-p-phenylene diamine (3 ) and ferrocene (4 )

(10“ %I) in a c e to n itr ile , deep coloured solutions were formed

(red in the case o f 1, reddish yellow with 2., blue with 2. and

green with 4 ). These deep coloured solutions were found to be

EPFi active . The ESR spectra recorded at room temperature in

degassed solutions were w ell resolved and matched reasonably59w ell with those reported in lite ra tu re . UV-Vis scanning o f

the solutions gave peaks sim ilar to those reported fo r the

165

ma^or product o f the reaction alongwith benzhydrol and traces

o f t r i t y l peroxide.

Likewise the addition o f tropylium hexachloroantimonate

to a solution o f PhgC^ in MeCN resu lts in a rapid decomposition

o f the diazoalkane as seen in the change o f colour o f the

solution from dark pink to colourless. Nitrogen is evolved

throughout the reaction which ends up in the formation o f

tetraphenylethylene as the p rinc ipa l product.

In an analogous manner, 9-diazofluorene reacts with

t r i t y l and tropylium hexachloroantimonate to e f fe c t a rapid

decomposition o f the diazoalkane to b ifluorenylidene and small

amounts o f fluorenone azine, fluorenone and flu oren o l.

The iso la tion o f t r i t y l peroxide in trace amounts in

the reaction o f PhgCN with t r i t y l hexachloroantimonate is

suggestive,o f the reaction to invo lve ET process. The in i t ia l

step o f the reaction presumably involves the Ph2CN2 or F1N2

cation rad ica l (5 ) (Scheme I I . 6) as a resu lt o f s in g le ET

from the parent diazoalkane molecule. Such intermediates havegO 4

been proposed in the electrochemical and C u ( I I )J induced

decomposition o f PhgCM .

In the absence o f any nucleophile, the rad ica l

cation (5 ) reacts with the parent molecule in a ra te lim itin g

step as shown in Scheme I I . 6. Such a coupling would e ither

occur at the diazocarbon atom or a t the terminal nitrogen o f

the reactant with the subsequent loss o f nitrogen to y ie ld

the rad ica l cations o f the o le f in (6 ) and the azine (7 ).

The emergence o f dimeric o le f in (8 ) as the principal

product o f the reaction suggests that the major portion o f the

reaction proceeds v ia pathway (a ) that involves the rad ica l

cation ( 5 ) as chain ca rr ie r .

Traces o f water present in the solvent MeCN are

expected to attack the principa l intermediate ( 5 ) , in it ia t in g

a pathway that Involves rad ica l species such as 10 and V\_ as

formulated in Scheme I I . 6. This explains the formation o f

products obtained in small amounts, such as alcohol ( 12 ) and

ketone (14 ). The explanation receives strong support from the

fa c t that when water is d e lib era te ly added to the system, products

such as T2 and 14 particu la rly the alcohol, emerge in good

y ie ld s at the expense o f the o le f in .

166

(a )-2 N ,

R2C*”C+R2(6)

R2™2

r2c- cr2» r2c. n2( 8 ) (5 )

■* R2C- f,2 ‘ - ... t *2 C0H2 r(5 )

R2C-N2

(10),-H

T b T-Mr

R~C - OH (11 )

R2C#-N-N-C'>R2(7 ) F^CHOH

R2C»^2 R2C-0H

r 0c«n-m«cr0 2 (9 ) +•

r2c« n+-(5 )

R2C-0H (13)

Disproportion* -ation

R-CHOH R5C-O (12) (14)

R-j - t r i t y l or tropyllum cation .

R2 » diphenyl or fluorenyl residue

Scheme I I .6

1G7

II.2C Oxidation o f hydrazones

An examination o f products formed in the reaction o f

benzophenone and fluorenone hydrazone with carbocations ( t r i t y l

hexachloroantimonate and tropylium hexachloroantimonate) revealed

that the corresponding azines were formed in appreciable y ie ld s .

The emergence o f t r i t y l peroxide in the oxidations with t r i t y l

cation and that o f b itropy l in reactions with tropylium cation

indicates that the reaction proceeds by an ET mechanism,

A single ET from the hydrazone to the organic cation

should in p rin c ip le generate the cation rad ica l o f the hydrazone.

A ready loss o f proton from the rad ica l cation (15) would

generate the rad ica l (16 ), as shown In Scheme I I . 7, The rad ica l

(16) can partake in two competing reactions, one involving

dimerisation with a subsequent oxidation o f the dimer ( 1 7 ) ,

coupled with nitrogen loss, to y ie ld the product azine (18 ). The

second pathway involves a further oxidation o f the rad ica l (16 ),

followed by a loss o f proton, to form the dlazoalkane (20 ). The

diazoalkan* is subject to oxidation by the carbenium ion to

y ie ld the o le f in (21) by a mechanism as suggested in the

preceeding section .

Thus a close s im ila r ity ex is ts in the lead dioxide/TFA

(Section I-2A ) and carbocation in it ia te d decomposition o f the

hydrazones. The reaction is yet another demonstration o f the

ET attribu tes o f the stable carbocations. The work suggests that

the involvement o f rad ica ls in the reactions o f carbocations

must be more wide-spread than is generally recognised.

168

R2c « N-NH2 ♦ R

R

r2c- n- mh- nh- n« cr2(17)

-2H+ -N„

R2C«M-M«CR2

(18)

•[r2C-M-NH2]+* + R* (15)

- Hi R*.—R*r2c- m- n*h (2 3 ) '

(16)

R-OH-N H2 (19)

-H*

R2C- » 2 ( 20)

"1R2<5.CR2

(21)

R ^ » P h j C

R1-0-0-R1(22)

* t r i t y l or tropylium cation

R2 « diphenyl or fluorenyl residue

Scheme I I . 7

11*3 Electron Transfer,reactions o f arene d iazonluia long

EXPERIMENTAL

Starting M aterials

Diazodiphenylmethane, 9-diazofluorene and the

hydrazones were prepared as described e a r l ie r . The preparation

o f phosphazines is described in Chapter I I I while fluorenyl

triphenyl phosphonium y lid e was prepared by trea ting

9-brdmofluorene with trlphenylphosphine and treating the

resu lting colourless phosphoriim sa lt (m.p 303° ) with ammonium

hydroxide, when yellow le a f l ik e crysta ls o f the y lid e separated

out. R ecrysta llisa tion from MeCN yielded pure fluorenyl triphenyl

phosphonium y lid e , m.p 253 ( l i t m.p 253 • p-Nitrobenzene-

diazonium tetrafluoroborate^3 and 4 ,4~dini trobiphenyl^4 were

prepared according to la id down procedures.

II.3A RgfteMon_ofL arene diazonium ion with aliphatic dlazocompounds

a) Baasiifln af.Prnltrobeaaenedlazonium tetrafluoroborate withdlazodiphenvlmethane.

A solution o f p-nitrobenzened lazonium tetra fluoroborate

(1 .Og, 45 mmol) was added with constant s t ir r in g , to a solution

o f diazodiphenylmethane (3.88g, 20 mmol) in a c e to n itr ile placed

In a 100 ml round bottomed fla sk while maintaining the temperature

a t 15°C. The solution assumed a transient green colouration

followed by a brisk evolution o f nitrogen and the reaction was

complete in 5 min. On concentration, colourless crystals

separated out. R ecrysta llisa tion from benzene-absolute efchanol

gaye colourless prisms id en tified as tetraphenylethylene,

m.p 224° ( l i t m.p'*3 224°) in Ca. 68% y ie ld . A fter the removal o f

tetraphenylethylene, the remaining solution was chromatographed

over an alumina column. Elution with petroleum ether gave a

liqu id which on d is t i l la t io n in a bulb gave nitrobenzene at a

bath temperature o f 90° ( l i t b.p 85°) (8% y ie ld ) . F'urther

e lu tion with pet-ether/benzene (1*1 ) gave 4 ,4*-d in itrob iphenylf

m.p 238°, l i t m.p64 238-239° (14%).

In a separate experiment, allowing equimolar quantities

o f p-nitrobenzenediazonium tetrafluoroborate (4.38g, 20 mmol)

169

170

in MeCN and diazodiphenylmethane in a c e to n itr ile (3#88g,20 mmol)

to react* a transient green colour again appeared followed by a

vigorous evolution o f n itrogen. The yellow solution on standing

overnight deposited yellow needle-Shaped crys ta ls , id en tified

as p-nitrophenylazodlphenylmethanol. The f i l t r a t e was found to

contain nitrobenzene and 4 ,4-d in itrobiphenyl. When the same

reaction was carried out In the presence o f water (0.5M), an

almost instantaneous expulsion o f nitrogen occurred and a

thick yellow p rec ip ita te o f p-nitrophenylazodiphenylmethanol

separated out (80%), The compound, m.p 150° was characterised

by IR, NMR & mass spectrometry.

b ) RgflC-t.iPtv, o f-^ltrobenzjtn.edlazonlum. te tra flu o rob ora tew ith

9-dlazofluorene

The addition o f an MeCN solution o f p-nitrobenzenedia-

zonium tetra fluoroborate (1.06g, 4.5 mmol) to 9-diazofluorene

(3*88g, 20 mmol) in MeCN was marked by a brisk evolution o f

n itrogen. The reaction was over in two minutes. On completion,

the reaction mixture was allowed to stand whereupon reddish-

orange c rys ta ls , id en tified as b ifluorenylidene (m.p 190°, l i t

m.p'*** 189-190°) were obtained in Ca 60% y ie ld . The f i l t r a t e

revealed the presence o f nitrobenzene and 4,4*-d in itrob iphenyl.

In a d iffe ren t experiment, equimolar amounts o f

p-nitrobenzenediazonium tetra fluoroborate (3*43g# 14.5 mmol)

and 9-dlazofluorene (2.98g, 15»5 mmol) in MeCM were allowed to

react a t 15°C. Nitrogen was evolved and a dark yellow precipitali

se ttled down. This was rec rys ta llis ed from CHCl and id en tified

by IR, NMR and mass spectra as fluorenone-p-nitrophenylhydrazone

(m.p 269°, l i t m.p65 269°) (70#). The f i l t r a t e was

chromatographed over an alumina column to g ive nitrobenzene

k , 4-dinitrobiphenyl and traces o f b ifluorenylidene•

R eac tion o f arenedlazonlum ions with hydrazones

a ) Reaction ofp-nitrobenzenedlazonlum tetra fluoroborate

with benzophenone hydrazone

In a round bottomed fla sk was placed an MeCN solution

o f tbenzophenone hydrazone (2 .0gt 10 mmol). To th is was added

p-nltrobenzenediazonium tetra fluoroborate (2.32g, 10 mmol) in

25 ml o f MeCN with constant s t ir r in g . A brisk evolu tion o f

n itrogen ensued and the solution turned yellow . A fte r allowing

to stand fo r sometime, colourless crysta ls deposited. These

were separated and washed with MeCN. R ecrysta llisa tion from

ethanol gave f in e needle shaped crysta ls (m.p 162°) id en tified

as benzophenone azine ( l i t m .p^ 163°), The f i l t r a t e was

chromatographed on an alumina column. Elution with petroleum

ether (40-60°) gave tetraphenylethylene m.p 224°, l i t ra.p^

224°) (12%), nitrobenzene (a t bath temperature 90° and 10mm

pressure) (18%), p-nitrophenylazodiphenylmethanol (m.p 150°,8%)

while use was made o f pet.ether/benzene mixtures to elu te

4 ,4-din itrobiphenyl (m.p 237°, l i t m.p64 238-239°, 10%).

Fluorenone hydrazone (2 .0g, 10 mmol) in MeCN (25ml)

was allowed to react with p-nitrobenzenediazonium te tra flu o ro ­

borate {2,32g, 10 mmol) a t 25°C. The reaction was marked by a

slow evolution o f nitrogen which continued throughout the

171

172

reaction . The yellow solution turned red and a dark red mass

se ttled down. This was separated and rec rys ta llis ed to g iv e

brigh t red crysta ls o f fluorenone azine (m.p 269°, l i t m.p^8

269°) (80% y ie ld ) . The f i l t r a t e was concentrated and

chromatographed on an alumina column. Thus nitrobenzene and

4 ,4 , -d in itrobiphenyl were obtained.

Reaction with benzaldehyde hvdrazAne

To a solution o f benzaldehyde hydrazone (1*5g»

7.5 mmol) in 25®1 o f MeCN was added a solu tion o f p-nitrobenzene

diazonium tetra fluoroborate (2.9g, 7.5 mmol) with constant s

s t ir r in g a t room temperature. A brisk evolution o f nitrogen

was fo llow ed by turning o f the solution to bright ye llow . A fter

the usual work-up and chromatographic separation o f the product

mixture on an alumina column benzaldehyde azine (m.p 92°, l i t

m .p^ 93°) (55%), nitrobenzene and 4,4-din itrobiphenyl were

obtained.

I I . 3C Reaction of^arenedlazonlum ion with phosphazlnes and

phosphoranes.

a) RaagJtlQa o f p-Mtrobenzenediazonlum tetra fluoroborate with

dlRheaylmethvlene^trlphenvl phosphazlne

On adding a solu tion o f p-nitrobenzenediazonium

tetra fluoroborate (0 .8g, 3.3 mmol) in MeCN (10 ml) to

diphenylmethylene triphenylphosphazine (1.5g» 3.3 mmol) in

CH2Cl2 (20 ml) a brisk evolution o f nitrogen commenced and the

solution darkened to bright yellow . Upon completion, the

reaction mixture was concentrated and chromatographed on an

alumina column. Elution with petroleum ether followed by

evaporation o f the solvent gave f i r s t a colourless so lid (m.p224°

id en tified as tetraphenylethylene ( l i t m*p^ 224°) (35%)

followed by a liqu id product which on d is t i l la t io n in a bulb

gave nitrobenzene at a bath temperature o f 90° (18%). Further

e lu tion w ith the same solvent yielded a yellow compound

(m.p 150° ) (30%) which was id en tified as p-nitrophenylazodiphenyl

methanol. Use o f pet.ether/chloroform mixtures fa c i l i ta te d the

iso la tion o f 4 ,4-dinitrobiphenyl (m.p 239°) (10.5%) fo llowed

by triphenylphosphine oxide, m.p 150°, l i t m .p^ 150-151°.

b ) R eaction w ith flu oren v lld en e trlphenvlphosphazlne

p-Mitrobenzenediazonium tetra fluoroborate (1 .0g,

4.2 mmol) in a c e to n itr ile (10ml) was added to fluorenylidene

triphenylphosphazine (2 .0g, 4.4 mmol) in CHgCl,, (20 ml) a t

room temperature. A brisk evolution o f nitrogen ensued, the

solution turning to dark yellow . Upon completion, the reaction

mixture was concentrated and chromatographed on an alumina

column. Bifluorrenylidene (m.p 190°, l i t m .p^ 189-190°) was

iso la ted f i r s t (38*») fo llowed by nitrobenzene (20%), fluorenone-

p-nitrophenyl hydrazone (35%), 4 ,4 '-d in itrobiphenyl (12%) and

triphenylphosphene oxide (75%)* A ll the compounds were

characterised by a comparison (mixed melting point and

superimposible 3R spectra) with authentic samples.

c ) Reaction with fluorenvl triphenvlphosphonlum v lid e

A solution o f p-nitrobenzenediazonium tetra fluoroborate

(0.5g, 2.1 mmol) in 10ml o f MeCN was added to fluorenyl

173

174

triphenylphosphonlum y lid e (1.0g, 2.3 mmol) in CH^Cl^ (15ml)

a t room temperature with constant s t ir r in g . A v io le t coloured

solution was formed which decomposed a fte r sometime to y ie ld

a yellow solu tion . The reaction mixture was allowed to

concentrate and on standing a dark yellow p rec ip ita te

(m.p 260°) separated out. This was f i l t e r e d , washed with MeCN

and rec rys ta llis ed from CHCl and id en tified as fluorenone-p-

nitrophenyl hydrazone (46^). The f i l t r a t e yielded

triphenylphosphine oxide (48%) and nitrobenzene (6%).

RESULTS AND DISCUSSION

II.3 A iAlBfraUfl

dlazocompounds

On adding p-nitrobenzenedlazonium tetra fluoroborate

to Ph2CN2 at 15°C, a transient green colour was formed followed

by brisk evolitton o f nitrogen gas. The reaction ended up in

the formation o f tetraphenylethylene as the major product

(6Q% y ie ld ) .

Likewise, the reaction o f 9-diazofluorene with

p-nitrobenzenediazonium tetra fluoroborate produced a brisk

evolu tion o f nitrogen and yielded b ifluorenylidene in Ca.60%

y ie ld .

In a separate experiment, u t i l iz in g equlmolar amounts

o f PhgCNg and P"^°2C6H4N2 * reac^ on was again marked by

the appearance o f transient green colour and a rapid evolution

o f n itrogen . TLC examination o f the reaction mixture revealed

175

the presence o f tetraphenylethylene in only trace amounts* The

reaction mixture was allowed to stand overnight when a yellow

c ry s ta llin e p rec ip ita te appeared, which was separated and

id en tif ie d as p-nitrophenylazodiphenylmethanol.

On repeating the above experiment in MeCN to which

preadditions o f water had been made (0.5M), the reaction was

again marked by a brisk evolution o f nitrogen and a fte r

10 minutes a thick yellow p rec ip ita te separated out. This was

id en tif ie d as p-nitrophenylazodiphenylmethanol (80%). Besides

th is , small amounts o f tetraphenylethylene, nitrobenzene and

4 ,4*-din itrobiphenyl were also obtained.

On the other hand, 9-diazofluorene reacted with

p-nitrobenzenediazonium tetra fluoroborate to y ie ld a dark yellow

p rec ip ita te id en tified as fluoren©ne-p-nitrophenylhydrazone.

A mechanistic in terpreta tion , formulated in Scheme I I .

8, Is proposed on the basis o f the aforementioned resu lts . The

f i r s t step o f the reaction may be considered to involve electron

tran sfer to generate the rad ical cation o f the diazo compound.

I t is known that diazoalkanes l ik e PhgCNg and Fl?^ are in i t ia l l yfiooxidised to the corresponding cation rad ica ls at an anode or

in chemical oxidation with C u (ll) perchlorate^1.

I f we are r ig h t in assuming that the key step in the

reaction o f arenediazonium ion with diazoalkane molecule en ta ils

diazoalkane cation rad ica l formation, i t implies that the

diazonium ion must possess the a b i l i t y to tr ig g e r an ET reaction .

Although not many examples are known, there are some references

in the lite ra tu re that subscribe to the ro le o f diazonium cations

178

as weak ET oxidants* For example, reaction o f phenothiazine and

i t s methylated d eriva tive toy arened iazonium ions has been shown2Bto y ie ld the corresponding phenothiazine cation rad ica l •

Likewise, reaction o f tetra th ia fu lva lene with several p-substitu­

ted arenediazonium ions are reported to involve s ingle electron

transfers^0* Arylation o f pyridine^2 and thiophene^1 proceeds

by the Intermediacy o f rad ica l cations* Other references are

ava ilab le in lite ra tu re which demonstrate the ET character o f29 32arenediazonium ions • Moreover, i t was observed by us that in

a c id ic medium, i f e* in the presence o f TFA, the arenediazonium

ion atta ins s u ff ic ie n t poten tia l to oxid ise aromatic and

heterocyclic compounds o f moderately high oxidation poten tia ls

such as thianthrene, t r i s - (-4 bromophenyl) amine,

9,10-diphenylanthracene, perylene, tetraphenylethylene e tc . to

the corresponding rad ica l cations.

Returning to our discussion o f the oxidation o f

diazoalkanes with N02 18 obv*ous that once the

diazoalkane cation rad ical is formed, i t should partic ipa te in

the reaction co-ordinate in very much the same way as discussed

in the preceeding section and outlined in pathway (a ) o f

Scheme I I .8.

With the use o f equimolar amounts o f the

diazocompound and p-^O,, the reaction assumes new

dimensions. In such a s ituation the d iazorad ical cations are

qu an tita tive ly generated in the in i t ia l fa s t ET step o f the

reaction . There are p ra c tica lly no diazoalkane molecules l e f t

fo r the cation rad ica l to couple w ith. Hie probable fa te o f

cation rad ica ls in th is situation is a coupling reaction with

the diazenyl rad ica ls to y ie ld the carbocation (28) a fte r the

loss o f n itrogen from the diazonium intermediate (27) as shown

in pathway (b ) o f Scheme I I . 8. The carbonium ion (28) is

bound to undergo a fa s t s o lv o ly t ic reaction to y ie ld the

product (29) provided water is present in the solvent system.

Thus when delibera te additions o f water are made 29 emerges as

the sole product o f the reaction . Even in dried MeCN presence

o f trace amounts o f water cannot be r ll led out. Further proof

o f the intermediacy o f the carbonium ion was obtained when the

reaction o f Ph2CM2 with p-tf02 was carried out in the

presence o f MeOH wherein the methoxy d er iva tive (31) emerged as

the principa l product.

The formation o f fluorenone-p-nitrophenyl hydrazone

(30) In the reaction o f 9-diazofluorene with arenediazonlum

Ion probably resu lts from a further redox reaction o f68p-nitrophenylazo-9-fluorenol formed in i t ia l ly .

Small amounts o f 4 ,4 , -d ln itrobiphenyl and nitrobenzene

which were formed are considered to be the products o f side

reaction o f the a ry l rad ica l derived from the diazenyl rad ica l.

These are the usual reactions involv ing dim erisation to the

biphenyl and hydrogen atom abstraction to y ie ld nitrobenzene.

177

178

Ar2£ - M* ~ Nf ♦ m 2 C6H4N+ = N

Ar2 C* - + N02 C6h4 ^ N*_n

(2 4 )

L§1

Ar2CN2

Ar2C*-C+A»2

mo2 c 6h;

I si4iau

'SH ^NO,C,H. -M02C6H5

5 c6h 4 - mo2

c o u p li n g

A r2 C-N* - M

(2 5 )

Ar2C»J2

Ar2C» CAr2 ♦ (26)

Ar2 CM*’ (24)

( 2 7 )

^ 2 °iU m- c 6ha - no2

Aj~ C-OMe 21 A r oC-0H 2 1N-M-C6H4N02

( 3 1 )

N«N-C6H4 i\T02

(2 9)

N-MH C6H4M02(30)

Scheme 11*8

179

^ 3B Reaction with hydrazones

The reaction of aldehyde hydrazones with diazonium

(Scheme II.9)

To our knowledge no report exists on the reaction of benzophenone or fluorenone hydrazone with diazoniuin salts.

On adding an MeC'J solution of p-nitrobenzenediazonium

evolution of nitrogen took place and the solution turned yellow. After allowing to stand colourless crystals identified as benzophenone azine appeared in Ca. 55% yield together with tetraphenyl ethylene (12%), p-nitrophenylazodiphenylmethanol (8%). Products of dediazonation of N O ^ - s u c h as nitrobenzene (18^) and 4,4*-dinitroblphenyl (v^10%) wfere also obtained.

In a similar manner fluorenone hydrazone reacted with yield fluorenone azine (Ca.80%) as the main

69cations is known to yield forraazans (Scheme II.9) via the intermediate formation of bis-arylazocompound (32).

Phft.2 N* NPh Ph* Ph C - M-M*"""\ \Nf - N... H

IPh

PhCH« 4-MHPh * PhCHNPh

(32)

tetrafluoroborate to benzophenone hydrazone at 25°, a brisk

product.

An analogous reaction occurred when under identical

benzaldehyde azine emerged as the principal product of the reaction.

180

On the basis of pattern of products obtained in the reaction of these hydrazones with p-nitrobenzenediazonium tetrafluoroborate, a mechanistic interpretation is outlined inScheme 11.10.

The diazonium cation believably acts as an ET oxidant, generating the cation radicals of the hydrazones in an initial ET step. The radical cation (33) once formed would most readily lose a proton to form a radical (34). Dimerisation of the radical would yield the dimer (35), which being unstable can easily undergo oxidation to yield the azine as th« principal product of the reaction.

An alternative pathway of consumption of the radical(34) involves further oxidation followed by deprotonation to form the diazoalkane (38), the follow-up reaction of which involves a chain conducive to the formation of the olefin (43)*

The formation of p-nitrophenylazodiphenylmethanol in the reaction of Ph2O N M H 2 with f l O ^ - s u g g e s t s the presence of diazenyl radicals and Ph2CN2 radical cations in the system which partake in a coupling reaction followed by loss of ^ and hydrolysis of the resulting carbonium ion (40) to form the azo product (41) (as detailed in section II.2B)

The diazenyl radical on nitrogen loss would yield the p-nitrophenyl radical (44) which is expected to abstract hydrogen atom from the solvent to form nitrobenzene (45) or dimerise to4 ,4-dinitrobiphenyl (46).

jj»N-<g>-N02(41)

r2c-oh (43)

Scheme 11.10

182

B ftftgttoaof arertedlazonluro Ions with phosphazlnes and phosphorane.

To our knowledge no report exists on the reaction of diazonium salts with fluorenylldene triphenylphosphazlne and diphenyliaethylene triphenylphosphazlne, although coupling products have been reported in the reaction of phosphoranes with diazonium ion70.

The r e s u l t s o b t a i n e d i n t h e p r e s e n t s t u d y c o n c e r n i n g

the r e a c t i o n o f U02 w i t h Ph2 C«^-N«PPh3 and F l - N - N - P P h y

and F l - P P h j , a r e suggestive o f an e l e c t r o n t r a n s f e r mechanism

b e i n g o p e r a t i v e h e r e a l s o .

On adding MeCM solution of m 2 to Ph2C«N-N»PPh3i n CH2 C 12 at 25° , a brisk evolution of nitrogen gas occurredand the solution turned bright yellow. On work-up for products, tetraphenylethylene (40&), p-nitrophenylazodiphenylmethanol (25%), triphenylphosphine oxide (69^), together with products of dediazonation of X02 nitrobenzene (18%) and4,4*-dinitrobiphenyl (10.5%) were obtained.

Likewise, Fl«?J-M«PPh3 reacted with M02 to yieldMfluorenylldene (38&) fluorenone-p-nitrophenlhydrazone (33*5%), rh^P-0 (75#), nitrobenzene (12#) and 4,4*-dinitrobiphenyl (8$).

The reaction of Fl-PPhj with NX, CgH^N2 proceeded through the formation of a violet coloured intermediate which soon decomposed to yield fluorenone-p-nitrophenylhydrazone (46%), Ph^P-0 (48%) and nitrobenzene (6%).

183

On the basis o f our resu lts obtained in the reaction

o f Pi^ON-N-PPhj and Fl-M-N-PPh^ with NOg a mechanistic

in terpretation outlined in Scheme 11,11 is proposed*

The oxidation o f Fl*N-N-PPh, and Ph0C»N-N«PPh, withD Z 3SbCl,j and tris(-4-bromophenyl) aminium s a lt proceeds by the

in i t ia l generation o f the rad ica l cation and y ie ld s the dimeric

o le fin s and Ph^P-0 in good y ie ld s* The intermediacy o f rad ica l

cations in these reactions has been confirmed by EPR

spectroscopy and cy c lic voltammetric studies (see Chapter I I I ) .

Considering the a b i l i t y o f diazonium ion to act as

an ET oxidant and keeping in mind the moderate oxidation

poten tia l o f the phosphazines (see Chapter I I I ) and also the

fa c t that the dimeric o le fin s are produced in substantial

amounts, we propose that the reactions under discussion proceed

by an in i t ia l ET step to generate the cation rad ica l o f the

phosphazine (47) and the d iazenyl rad ica l.

The rad ica l cation once formed su ffers a homolytic

cleavage o f the C-M and M-P bonds to form the carbene (48 ),

nitrogen and the cation rad ica l o f PPhj(49) which on in teraction71with oxygen y ie ld s the oxide by a known mechanism .

Dimerisation o f the carbene forms the o le f in (50 ).

An a lte rn a tive pathway also seems to be operative.

In the solution are present the rad ica l cations (47) and the

diazenyl rad ica ls . A coupling o f the two followed by expulsion

o f PPh and nitrogen would lead to the carbonium ion (53) which

on so lv o ly t ic reaction with water, generally present in MeCN

despite scrupulous drying, y ie ld s the azo product (55 ). PPh on

reaction with excess would form the oxide, PPh^-0, as

184

R20 N-N«PPh3 ♦ N02 CgH^NSN

R2C-N«N-PPh3 N02 CgH^N-N(47)

combinationI

R0Cs + N- ♦ PPhI* R0C-M«ft-PPh,2 2 ? 2n=n^ o, 3(48)

R2C-CR2

(*9 )

1(52)

(50)PPh3-0 R2C

(51) /♦ Nf2 * PPh-j

A t

N-N-<O>-N0(54)

\N°2 c6H4*2eOH

R-C-OMe 2 i

h20' PPh3-0

N-M-©>N0p RpC-OHn« n- (Q )no,(54) (5 5 r fR2»Fluorenyl

d> * IN-N;J i

\C6H4*°2(56)

Scheme 11.11

185

was found to be the case when a sample o f PPh^ was separately

treated with N02

In the case o f Fl«N-M»PPhj, the formation o f the

hydrazone ( 56 ) may be due to the further redox reaction o f the

in i t ia l l y formed p-nitrophenylazo-9-fluorenol (see section II.2 B )

Evidence fo r the intermediacy o f carbo cation (53) was

obtained when the reaction o f Ph^ON-N-PPh^ with MC CgH N was

carried out in the presence o f methanol wherein the methoxy

product, p-nitrophenylazodiphenylmethoxymethane (54) m.p 135° ,

could be iso la ted in 20% y ie ld .

Fluorenyl triphenylphosphorane has a very low

oxidation poten tia l (0.05V) and an ET reaction with ArM is- + 72

quite conceivable. Fl-PPh^ has been shown to be oxidised by

chemical and electrochemical means to i t s rad ica l cation . The

transient v io le t colour that develops in these reactions may be

due to the formation o f these paramagnetic species. A coupling . +

o f Fl-PPhj (57) with the diazenyl rad ica l followed by expulsion

o f PPhj would generate the carbocation (59 ). This on hydrolysis68and a further redox reaction leads to the formation o f

fluorenone-p-nitrophenylhydrazone (61) Scheme 11.12 .

186

FI -PPhj ♦ N02 C6Ha N*

• +Fl-PPhj + N02 C6H4N

(57)

Fl-PPh,

N

n- n- c6h4no2

(58)

PPh, + F l+ 3 IN - M « C6H ft02

N32C6H4M2

PPh3«0

(59)

h2o

FI-OHN ■ N -C6H/fN02

(60)

(61)

Scheme 11,12

REFERENCES187

1. K linger R J, Mochida K and Kochi J K, J Araer Chem Soc,101 (1979) 6636.

2. Farcasiu M and Russell C S, J Org Chem, 41_ (1976) 571.

3. Horner L and Schwenk E, Justus L ieb igs Ann Chem, 566 (1950) 69.

4a. Zupancic J J, Horn K A and Schuster G B, J Amer Chem Soc,102 (1980) 5279.

b. Dixon B G and Schuster G B, J Amer Chera Soc, 103 (1981) 3068.

5a. Walling C and Zhao C, Tetrahedron, £8 (1982) 1105.

b. Zhaq C, E l'Ta liw a i G M and Walling C, J Org Chem, 48 (1983) 4908.

6. Kateuchi K, Murai 0, Matsui S, Inoue T, Kitagawa T and Okamato K, J Chem Soc Perkin I I , (1983) 1301.

7. Chalfond C R and Perkin M J, J Chem Soc Bt (1971) 245.

8. Pryor W A & Henrickson W H Jr, J Amer Chem Soc, 105 (1983) 7114.

9. Beresford P, Lambert M C and Ledwith A, J Chem Soc C,(1970) 2508.

10. Ledwith A, ACC Chem Res, £ (1972) 133.

11. Kricka L J & Ledwith A, J Chem Soc Perkin I , (1973) 294.

12. Beresford P, I le a D H, Kricka L J and Ledwith A, J Chem Soc Perkin I , (1974) 276.

13* B ell F A, Beresford P, Kricka L J and Ledwith A, j Chem Soc Perkin I , (1974) 1788.

14. Nelson S F, Weissman " R, Hintz P J, Olp D and Fahey M J,J Amer Chem Soc, £6 (1974) 2916.

15* Dauben H J and Wilson J D, J Chem Soc Chem Comaun, (1968) 1629.

16. Hawthrone M F, J Org Chem, 21, (1956) 363#

17. Kashin A N, Bumagin N A. Beletskaya I P and Reutov 0 A, JOrg Chem USSR, 1*[ (1979) 204.

18. Kashin A M, Bumagin N A, Beletskaya I P and Reutov 0 A,Dokl Akad Nauk SSSR, 244 (1979) 98.

19. Murray R W and Kalpan M L, J Org Chem £1 (1968) 962.

20. Ledwith A & Sambi M, «J Chem Soc Chem Commun, (1965) 64.

21. Bawn CEH, Fitzsimmons C and Led with A. Proc Chem Soc, (1964) 391.

22. B ilev itch K A, Bubnov N N and Okhlobystin 0 Yu, Tetrahedron Lett, (1968) 3465.

23. Singh H and Tedder J M, J Chem Soc Chem Commun, (1981)70.

24. Okamoto K, Komatsu K, Mural 0 and Sakaguchl 0,Tetrahedron L e tt, (1972) 4989.

25. Singh P R and Kumar R, Aust J Chem, 2£ (1972) 2133.

26. Singh P R and Kumar R, Tetrahedron Lett, (1982) 5191.

27. Bubnov M N, B ilev itch K A, Polyakova L A and Okhlobystin0 Yu, J Chem Soc Chem Commun, (1972) 1058.

28. Bisson J M, Hanson P and Salocum D, J Chem Soc Perkin I I ,(1978) 1331.

29». B ilev itch K A, Bubnov N M e t a l . Dokl Akad Nauk SSSR, (1970) 583.

b. B ilev itch K A et a l . , Teor-i-k KSP Khia, 8 (1972) 256.

30. Kampars, V, Bumbure G, Kokars V and Neilands 0, Zh Obshch Khim £0 (1980) 2057.

31. Bartie MG, Machie R K and Tedder J M, J Chem Soc Chem Commun, (1974) 271.

32. Elofson R M, Gadallah F F and Schulz K F, J Org Chem,26 (1971) 1526.

33. Broadhead G D and Panson P L, J Chem Soc, (1955) 367.

34. B e ll F A, Ledvrith A and Sherrington D C, J Chem Soc C, (1969) 2719.

35* Handoo K L and Gadru K, Indian J Chem, (1985)* In press.

36. Sep W J, Verhoeven J W and DeBoer Th J, Tetrahedron, 35(1979) 2161.

37. Paul M A and Long F A, Chem Rev, £Z (1957) 1.

38. Nepras M, Kratochvil V, T itz M, Novak A and S lavik V, C o llect Chech Chem Commun, (1973) 1003.

39a. Kochi J K, Tang R T and Bernath T, J Amer Chem Soc,(1973) 7114.

b. Bard A J, Ledwith A and Shine H J, A d v , Phys Org Chem,11 (1976) 155.

188

c ) Heiba E I , Dessan R M and Koehl Jr W J, J Amer Chem Soc,21 (1969) 138, 6830;

Helba E I and Dessau R M, J Amer Chem Soc, 22 (1971) 995.

40. Das M and Basu 3, Spectrodhira Acta, (1961) 897.

41. Hoijtink G J and We land W P, Rec Trav Chitn, JS (1957)836.

42. Pava A, Sago P B and Calvin M, J Amer Chem Soc, 79 (1957)1078.

43* Dwivedi P C, Rao K G, Bhatt 3 M and Rao CMR, SpectrochimActa, 21A (1975) 129.

44. Coffen D L, Chambers J Q, Williams DR, Garrett P E andCanfield N D, J Amer Cheai Soc, 92. (1971) 2258.

45. Sehested K and Hart H J, J Phys Chera, 22 ( 1975) 1639.

46. Suzuki H, Koyano K. Shida T and Kira A, Bull Chem Soc Jpn, 22 (1979) 2794.

47. Foster R and Thomson T J, Trans Faraday Soc, *>8 (1962)560.

48. Wilkinson G, Rosenblum M, Whiting M C and Woodward R B,J Amer Chem Soc, £4 (1952) 2125.

49. Zarubin M Ya, Kutnevich A M and Rudenko A P, Zh Org Khim, 11 (1975) 1284.

50. Roberts J D and Wantabe W, J Amer Chera Soc, 72(1950)4869.

51. Cowell C W, Ledwith A, White A C and Woods H J, J Chem Soc B, (1970) 227.

52. Bonthrone w and Reid D H, J Chem Soc, (1959) 2773.

53. Schlenk and Bergmann, Anaalen, 463 (1928) 15*

54. Ingold C K and Jessop J A, J Chem Soc, (1929) 2357.

55. Szmant H H and McGinnis C, J Amer Chem Soc, (1950)2890.

56. Gomberg M, J Amer Chem Soc, 22 (1900) 757.

57. Doering W E and Knox L H, J Amer Chem Soc, 22 ( 1957) 352.

189

190

58. Pinck L A and H ilbert G Ef J Amer Chem Soc, 68 (1946)867.

59a. B illon J P, Cauquis G and Combrisson J, J China Phys, (1964) 374.

b. Wudl F, Smith G M and Hufnagel E J, J Chem Soc Chem Commun, (1970) 1453.

c. Bolton J R» Carrington A and Santos-Veiga J D, Mol Phys,2(1962) 615.

60. Jugelt W and Pragst F, Tetrahedron 24 (1968) 5123IJugelt W and Pragst F, Angew Chem Int Ed Engl,2 (1968) 290.

61. Bethell D, Handoo K L , Fairhurst S A and S u tc lif fe L H,J Chem Soc Perkin 11,(1979) 707.

62. Pinck L A and Guido H E, J Amer Chem Soc, 69 (1947) 723.

63. Starkley E B, Org. Synth, Coll Vol I I , (1943) 225.

64. B e ll F and Kenyon J, J Chem Soc, (1926) 2767.

65. Vogel A I , A Text Book o f P ractica l Organic Chemistry, Longmans, 4th ELBS Ed, page 1196.

66. Pearson D E, Carter K N and Greer C M, J Amer Chem Soc, 75 (1953) 5905.

67. Blake C A, Brown K B and Coleman C F, U S Atomic Energy Coma, 0RNL-1964 (1955)106pj CA 50* 15320.

68. Huisgen V R and Koch H J, Ann Chem, $9± (1955) 200.

69. Organic reactions, Ed B la tt AH , Boekethelde V, Cope C A, David D Y, McGraw F C and Niemann C,John Wiley & Sons,New York, (1959), Vo l.10, Chap 1.

70. Mareki G, Tetrahedron Lett, (1961) 807*

71. P litsyna 0 A, Pudeeva M E and Reutov 0 Af Dokl Akad Nauk SSSR, 2§S, (1965) 838.

72. Koul A, M.Phil Thesis, University o f Kashmir (1985).