Synthetic Communications A Facile Reductive Cleavage of Allylic and

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Synthetic Communications

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A Facile Reductive Cleavage of Allylic and Benzylic Esters with Low Valent Titanium Reagents K. Satyanarayanaa; N. Chidambarama; S. Chandrasekarana a Department of Chemistry, Indian Institute of Technology, Kanpur, India

To cite this Article Satyanarayana, K. , Chidambaram, N. and Chandrasekaran, S.(1989) 'A Facile Reductive Cleavage of

Allylic and Benzylic Esters with Low Valent Titanium Reagents', Synthetic Communications, 19: 11, 2159 — 2166 To link to this Article: DOI: 10.1080/00397918908052611 URL: http://dx.doi.org/10.1080/00397918908052611

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SYNTHETIC COMMUNICATIONS, 19(11&12), 2159-2166 (1989)

A FACILE REDUCTIVE CLEAVAGE OF ALLYLIC AND BENZYLIC

ESTERS WITH LOW VALENT TITANIUM REAGENTS

K. Satyanarayana,

*

N. Chidambaram and S. Chandrasekaran

Downloaded At: 08:50 17 January 2011

Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India

-

The reductive cleavage of allylic and benzylic esters titanium(I1) reagent derived from Mg/Hg-TiC14 yielded the corresponding acids a and dimeric hydrounder mild reaction conditions. carbons

a with

Allylic and benzylic esters are often used as protective groups for carboxylic acids. methods

There are a

few

available for the non-saponicative hydrolysis

of these esters. Reported methods include the specific deblocking

of cinnamyl esters by Corey and Tius

using

successive methoxymercuration and demercuriocarboxylation procedure,' and cleavage of allylic lithium have

dimethylcuprate.

esters with

Cleavage of benzylic

esters

been achieved with nitrosonium hexafluorophosph-

ate,3 palladium on carbonI4 sodium in liquid

-

* To whom

-

-

correspondence should be addressed. 2159

Copyright 0 1989 by Marcel Dekker, Inc.

ammonia5

2160

SATYANARAYANA, CHIDAMBARAM, AND CHANDRASEKARAN

and

chromium

Barton’

has

trioxide

chlorotrimethylsilane.6

and

reported the deoxygenation of

sterically

hindered esters with lithium/ethylamine. Earlier work from our laboratories has shown

that

the low valent titanium reagent generated by the reduction

of titanium tetrachloride with amalgamated magne-

sium is very useful in the reductive coupling of carbonyl

compounds’


nitro compounds. benzylic

in

the

selective reduction of

We anticipated that the allylic

and

esters of carboxylic acids on treatment with

low valent anionic

and

titanium reagents would form

intermediate

A

which would

the

radical

readily

undergo

cleavage of the C-0 bond with the formation of carboxylate anion and a stable allylfienzyl radical (Scheme 1):

B R’ = -CH2Ph or -CH2-CH=CH2

Hence the

it appeared reasonable to study this method

deblockine of allylic and benzylic

esters

for

under

mild reaction conditions. Herein cleavage of

we

report

our results on

the

allylic and benzylic esters

reductive

with

Ti(I1)

CLEAVAGE OF ALLYLIC AND BENZYLIC ESTERS

2161

reagents generated by the reduction of TiC14 with amalgamated magnesium and they are summarised in the Table. Esters at lic

bd (Table) on treatment with Ti(I1)

for 2 h yielded the corresponding carboxy-

0-20 OC acids

and the dimeric hydrocarbons

acetophenone

5 (14%) was also formed.

Formation

earlier

is possible that the dimeric hydrocarbons 2a-f

formed

of

like 5 from benzylbenxoates have been reported 10 in the reaction with lithium naphthalenide.

ketones


U . In

apart from the above .products, 2-phenyl-

the case of

It

reagents

from

reaction

diols 4~

conditions

alicyclic ester

by

hydrogenolysis

under

whereas the diol 4g derived

are the from

undergoes deoxygenation to yield the

olefin 2g. l 1 (Scheme 2) :

R-CH2-CH2-R

R-CHzCH-R

2le.f

2g

Another pathway for the formation of the dimeric hydrocarbons lic acids is

based

2ra;f could be by further reduction of carboxy-

with low valent titanium reagents. on the observation that benzoic acid

This

on

2162


TABLE

Reductive Cleavage of Allylic and Beneylic Esters with TiC14/Mg-Hg

:

Entry

Substrate

Dimeric hydrocarbona

Acida

(Yield %)b

(Yield

%Ib

0

II


1

Ph-C-0-CH$h lA

PhCH 2- CH $h 2.a (18%)

Ph-COOH 3 2 (64%)

+ PhCH2-C-Ph 8

2

d

s

Ph-C-OCH2CH=CH2

(14%)

Za ( 2 3 % )

& I (57%)

Ih 3

4

0 II 4-tBu-C6H4 -COCH2CH=CH,

8

4-Me-C 6H4-COCH 2CH=CH2

hi 5

0 I1 4-C1-C6H4-COCH2CH=CH2

La

4-tBu-C6H4-COOH

4-Me-C # 4-CH 2-

)

29 (18%)

4-C1-CgHq-CH2-)

2a ( 2 0 % )

4-tBu-C6H4COOH

4-Me-C 6H4-COOH

& (67%) i

4-C1-C6H4-C00H 2 3 2 (43%)

CLEAVAGE OF A L L Y L I C AND BENZYLIC ESTERS

2163

Table (contd.)

-

0

6

II

3-C1-C 6H4-COCH 2CH=CH2

be 7

3-C1-CgH4-CH2-)

3-C1-C 6H4-COOH

3€ ( 4 2 % )

2 s (17%)

C gH lCH=CHC@

CgHll-COOCH2CH=CH2

CSHll-COOH

Eis

u


8

2 a (38%)

-

4-Me-CI;H4-COOMe

Lh 9

Ph-COOH

&L

(70%)

2.Q a) All the products gave satisfactory spectral data and were compared with authentic samples. b) Yield refers to pure isolated products.

treatment with

TiCll/Mg-Hg under

conditions yielded

the

same

the dimeric hydrocarbon

reaction

(15%)

along with recovered starting material (70%). In

order

to show that only allylic and

benzylic

esters undergo this reductive cleavage with facility to yield

the

toluate

corresponding carboxrlic

acids,

methyl

fi waB subjected to the same reaction condi-

tions with TiC14/Mg-Hg. The only products that could be isolated

in the reaction were the dimeric

hydrocarbon

2164

a


(30%) and benail (12%) along with recovered starting

material

(41%).

Titanium tetrachloride was Habn,

Germany

determined apparatus


recorded

with

and

was

obtained

used

a hi-melt

such.

Riedel-de

M.ps.

were

capillary melting

and are uncorrected. on

as

from

point

Infrared spectra were

Perkin-Elmer model 1320 and 580

spectro-

meters. Proton magnetic resonance spectra were recorded at

90 MHz on a Varian EM-390 instrument and at 80 MHz

on a Brucker WP-80 instrument.

To a solution of mercuric chloride (0.243 g, mmol)

in anhydrous tetrahydrofuran (10 ml),

0.9

magnesium

powder (0.72 g, 30 mg atom) was added and the resulting mixture was stirred at room temp. under nitrogen atmosphere,

for 0.25 h.

withdrawn portions

The turbid supernatent liquid

and the amalgam was washed with of THF.

three-5 ml

20 ml THF was added to the amalgam,

cooled to 0 OC and titanium tetrachloride (2.267 g, mmol, tion

was

12

1.31 ml) was added slowly, followed by the addiof beneylbeneoate

(1.272 g,

6 mmol) in

5

ml

THF. The reaction mixture was stirred for 2 h by which time the temperature was allowed to raise to 20 O C . 10%

CLEAVAGE OF A L L Y L I C AND BENZYLIC E S T E R S

2165

aq. K2C03 solution (10 ml) was added and stirred for 10 min.

(25 ml) was added and

Chloroform

mixture The

was

filtered over a pad of Celite

filtrate

aqueous

was

thoroughly

and

with

sand.

saturated

The bicarbonate layer was neutralised

10% HC1 solution at 0 The

with

0

C and was extracted with chloro-

chloroform layer was washed with brine

dried over anhyd. Downloaded At: 08:50 17 January 2011

washed

reaction

bicarbonate solution and the organic layer was

separated.

form.

the

sodium sulphate.

and

Evaporation of the

solvent afforded benzoic acid 23 (0.472 g,

64%).

M.p.

by

flash

120-122 OC (lit. l2 M.p. 120-122 OC). The

neutral

chromatography g,

18%),

on

purification

on silica gel yielded dibenzyl

m.p. 50-52 OC (lit. l2 m.p. 52

3060, 2920, 1600 (s, 10 H)

7.19

portion

14%),

m.p.

3080,

3060,

ern-'. and

IR (KBr):

H' NMR (CDC13): 6 2.83 ( s , 4 H), 2-phenylacetophenone

58-59 OC (lit.l 3 m.p. 1690,

OC).

(0.2

1610 cm-l.

60 OC).

2

(0.165 g,

IR

(KBr):

NMR (CDC13): 6 4.1 (s,

2 H), 7.2 (s, 5 H), 7.4 ( m , 3 H), 7.9 (m, 2 H). Acknoulednemm.

The

authors thank the Department of

Science and Technology, New Delhi for financial support.

Ref e rences

2.

Corey, E. J. and Tius, M. A., Tetrahedron Lett., 1977, 2081. Ho, T. L., SYnth. C o m m u ., ~ 1978, &, 15.

3.

Ho, T. L. and Olah, G. A., &nthe&,

1.

1977, 418.

2166

4.

5. 6. 7.

8. 9.


10.

11.

12. 13.


Anantharamaiah, G. M. and Sivanandaiah, K.M., J . Chem. S Q E . , .I, 1977, 490. Roberts, C. W., J . Am. Chem . S Q S . , 1954, 76, 6203. Aizpurua, J. M., Juaristi, M., Lecea, B. and Palomo, C., Tetrahechn, 1985, U , 2903. Boar, R. G., Joukhader, L., McGhie, J. F., Misra, S. C., Barrett, A. G. M., Barton, D. H. R. and Prokopiar, P. A., J . Chem . S s . , Chem . co- 8 1978, 68. Bhushan, V. and Chandrasekaran, S., LZhaglistrv let