Stereospecific synthesis of cis -azobenzenes

Tetrahedron Letters No. 2, pp 141 - 14.2,1977.

Pergamon Press. Printed in Great Britain.

STERROSPECIFIC SYNTHESIS OF -cls-AZOBRt'IZRWRS* John A. Wyatt Research Laboratories, Tennessee Eastman Company, Division of Eastman Kodak Company, Ringsport, Tennessee 37662 (Received in USA 29 September 1976; received in DK for publication 30 November 1976) To date the only method for preparing -cis-asobenseneshas been via photochemical equilibration of trans-asobensenesand chromatographicseparation of the -cis component.'-'We report that room-temperatureoxidation by active manganese dioxide of substituted anilines or, more conveniently, hydrasobensenes,produces the corresponding-cis-asobenxenesin excellent yield and purity. The oxidation of c-chloroaniline with Mn0~ at 25" afforded a 20% yield of e-2,2'dichloroazobenzene (2b). Thermal isomerisation to the trans isomer would account for the 'v\r published results.&"

R. Rs or ArN&

,N=N, bOZ Ar CWCls? Ar 25"

2 %

3 ?J

> 70" A.

/N,\,,

Ar

Ar

1 %

It was also found that 2,2'-dichlorohydrazobensene3b affords -cis-isomer 2b in good yield and % 'v\, purity upon MnOa oxidation. The isomeric purity of the 2b was increased to > 95% by argon'VI, purging the MnOl slurry prior to the reaction and by excluding light. These measures prevent air oxidation of 3b and photoisomerisationof 2b. % 'v\, Pratt and McGovern' have reported trans-asobensenesfrom the MnOa oxidation of hydrasobensenes carried out at > 80'. As in the case of aniline oxidations under these conditions, thermal isomerisation of initially formed -cis-azobenzenesaccounts for the results reported. In view of the novelty of these results, the MnOa oxidation of a series of hydrasobenzenes, 3a-31, was carried out. As shown in Table 1, high yields of -cis-asobenzeneswere obtained 'Il\rQ% except in those systems bearing strongly conjugating E-substituents. * Dedicated to Prof. R. B. Woodward on the occasion of his sixtieth birthday. 141

w

No. 2

Table 1. MnOl Oxidation of Hydrazobenzenesa

Compound

Starting Material && I% ?L

R,

&

Compound &

&

Product !& !&

!Lz

5

!.!R Yield, %

a

H

H

H

H

H

H

&

H

H

H

H

H

H

65-67"

89

a

Cl

H

H

Cl

H

H

j$$

Cl

H

H

Cl

H

H

91-92"

86

k

H

BrH

H

Br

H

&

H

Br

H

H

Br

H

45-47"

98

&j

H

H

Cl

H

H

Cl

,@

H

H

Cl

H

H

Cl

66-68"

98

&

OCHs H

H

OCHS H

H

k

OCH,H

H

OCHs H

H

96-98"

96

x

H

H

CHs H

H

CHs

a

H

CHs

96-98"

97

&

H

H

H

H

H

OAc

_Trans-isomer produced

a

H

H

H

H

H

OCH,

_Trana-isomer produced

a

H

HH

H

HNHAc

H

CHs H

H

Trans-isomer produced _

aCompounds a-,$$ were prepared by zinc-sodium hydroxide reduction of corresponding azo compounds'. Oxidations were carried out by adding the hydraxobenzene to an Ar-purged, darkened slurry of 5 to 10 parts by weight of activated MnOae in 10 parts by weight chloroform. Work-up by filtration, evaporation of solvent below 40" in the dark, and recrystallization afforded ,&-a.

Products were characterizedby IR, NMR, LTV-VIS,and elemental analysis and by

thermal isomerization (refluxing toluene) to the known trans-isomers.

References and Notes

1.

G. S. Hartley, Nature, I&,

281 (1937).

2.

A. H. Cook, J. Chem. Sot., 876 (1938).

3. A. H. Cook and D. C. Jones, J. Chem. Sot., 1309 (1939). 4. M. 2. Barakat et&.,

J. Chem. Sot., 4685 (1956).

5.

0. H. Wheeler and D. Gonzalez, Tetrahedron,20, 189 (1964).

6.

E. F. Pratt and T. McGovern, J. Org. Chem.. 29, 1540 (1964).

7. H. Snyder etc.,

J. Am. Chem. Sot., 7l, 290 (1949).

8.

Either commercial (Alfa Inorganics) or Attenburrow' MnOz may be used.

9.

J. Attenburrowet&..

J. Chem. Sot., 1094 (1952).