J. appl. Chem. Biotechnol. 1977, 27, 291-295

Thermolysis of Mono-, Di- and Tri-benzylamines, IV

M. Zarif A. Badr, Morsy M. Aly, Hassan A. H. El-Sherief and Abdo E. Abdel Rahaman

Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt, A.R.E.

(Paper received 29 November 1976 and accepted 11 February 1977)

Thermolysis of mono-, di- or tri-benzylamine for some days in the absence of promoters gave the same products: ammonia, toluene, dibenzyl, stilbene and benz- aldehyde. Thermolysis of dibenzylamine in isoquinoline as a solvent gave 1 -benzyl- isoquinoline together with the normal pyrolysis products. When naphthalene and 2-naphthol are used as solvents, benzylation of the solvent nuclei was observed to give 1- and 2-benzylnaphthalenes and 1-benzyl-2-naphthol respectively. In conclusion, thermolysis of tri-, di- or mono-benzylamines proceeds through a free radical mechanism with successive conversions of the tertiary, through secondary to primary alkylamines resulting in the same pyrolysis products.

1. Introduction

The base induced rearrangement of tetra-alkyl quaternary ammonium salts, to give tertiary amines has long been known as Stevens’ rearrangement.l Rearrangement of benzyl trialkyl quaternary ammonium salts in basic medium into o-alkyl substituted benzyl dialkyl tertiary amines was known as the Sommelet-Hauser rearrangemerL2

These types of rearrangements were assumed to be i n t r a m ~ l e c u l a r ~ ~ ~ and proceed through car- banion intermediates in which the alkyl groups migrate from the nitrogen atom to the negatively charged carbon atom. Retention of configuration of the migrating alkyl group was ~bse rved .~ Heating aqueous or alcoholic solutions of tetra-alkyl quaternary ammonium hydroxide above 100°C had resulted in the formation of tertiary amine and olefin6 through a mechanism of Ez character. As far as our observation goes, decomposition of benzylamine by electron bombard- ment,? into benzyl and amino radicals is the only available information concerning the homolytic fission of alkylamines in the absence of promoters.

2. Experimental

2.1. Analysis The i.r. spectroscopic analysis was carried out on a Model SP 200 G Pye-Unicam, i.r. spectro- photometer. Gas-liquid chromatographic analysis was carried out on a Pye-Unicam gas chromato- graph, “Series 104”, with a dual flame ionisation detector, and temperature programming, Model 24. Column used was packed with 20% SE 30 on Chromosorb W (35-80 mesh).

2.2. Pyrolysis of dibenzylamine Dibenzylamine was refluxed either alone or with isoquinoline, 2-naphthol or naphthalene for about 5 days. The temperature fell progressively from 300 to 210°C. The evolved ammonia was trapped as its hydrochloride. The products of reflux were distilled up to about 200°C under normal pressure whereby toluene and benzaldehyde were collected. The higher boiling products were separated into

29 1

M. Z. A. Badr et a[. 292

neutral and basic components (in isoquinoline) by HCI, or into neutral and phenolic components (in 2-naphthol) by Claisen's solution. Each type of component was subjected to fractional distillation under reduced pressure. The results are summarised in Table 1.

2.3. Pyrolysis of benzylamine

Treatment of benzylamine (20g) under the same conditions gave rise to ammonia [as NH4CI (4.3 g)], benzaldehyde (2.1 g), toluene (1.3 g), dibenzyl (3.2 g), stilbene (6.7 g) and non-distillable residue (4 g).

2.4. Pyrolysis of tribenzylamine

Treatment of tribenzylamine (10 g) under the same conditions gave rise to ammonia [as NH4CI (1.5 g)], benzaldehyde (0.3 g), dibenzyl (1.5 g), stilbene (2.25 g) and non-distillable residue (2.5 g).

2.5. Preparation of reference compounds

Dibenzylamine:8 colourless oil, b.p. 300°C, nD22 1.5743. Triben~ylamine:~ prisms from ether, mp. 92"C, b.p. 228"C/12 mm Hg ; hydrochloride: prisms from ethanol, m.p. 227-8°C. trans-Stilbene:10 cryst. from ethanol m.p. 1234°C. Dibenzy1:ll cryst. from ethanol m.p. 52°C; 4,4'-dinitro derivative m.p. 180°C. l-Ben~yl-2-naphthol:~~ cryst. from benzene/petroleum ether (60-80°C) m.p. 110°C; benzoate from ethanol m.p. 97°C. 9-Phenyl-l,2-7,8-dibenzoxanthene:13 cryst. from glacial acetic acid, m.p. 188-9°C. 1-Benzylisoquinoline:14 cryst. from chloroform/petroleum ether (40-60°C) mixture, m.p. 56°C; picrate from ethanol m.p. 182°C. l,l '-Bi-is~quinolyl:~~ cryst. from benzene m.p. 162-3°C; picrate from benzene m.p. 207°C. l-Benzylnaphthalene:16 prisms from ethanol m.p. 59-60°C ; picrate from ethanol m.p. 104-5°C. 2-Benzylnaphthalene:17 platelets from ethanol m.p. 55-56°C; picrate from ethanol m.p. 95°C. 2,2'-Dinaphthol :18 colourless crystals from toluene m.p. 216-218°C.

3. Results and discussion

A novel pyrolysis of mono-, di- and tri-alkylamines was observed when they were heated alone, in the absence of any promoter, or in the presence of aromatic solvents, when their nuclear alkylation occurred. Thus, refluxing each of mono-, di- or tribenzylamine for several days in air resulted in the production of ammonia, toluene, dibenzyl, trans-stilbene and benzaldehyde.

The similarity of the products obtained from these pyrolyses in the case of mono-, di- or tri- benzylamine confirms the identity of their pyrolytic behaviour. The presence of toluene, dibenzyl and ammonia among the products suggests a free radical mechanism. This suggestion was further supported through a study of the thermolysis of dibenzylamine in free radical scavengers. In naphthalene as a solvent, beside the previously mentioned products of pyrolysis, isomeric, 1- and 2-benzylnaphthalenes (yield, 52%) were produced in the ratio of 72.3 % 1- to 27.7% 2-isomer.

In isoquinoline, the normal pyrolytic products of the benzylamines were obtained together with 1-benzylisoquinoline (yield, 30.6 %) and 1 ,l'-bi-isoquinolyl. With 2-naphthol, the normal pyrolytic products were accompanied by 1-benzyl-2-naphthol (yield, 35.7 %) and 9-phenyl-1,7,8-dibenzo- xanthene together with 2,2'-dinaphthol as summarised in Table 1.

A general feature of the pyrolysis of mono-, di- and tri-benzylamines was the similarity of their products. On the other hand, dibenzylamine pyrolysis products were not changed by its heating in presence of solvents. Consequently, it is possible to provide a general satisfactory picture of the benzylamine thermolysis and formation of the products, through their initial homolytic fission to

Tab

le 1

. Pr

oduc

ts o

f py

roly

sis

of d

iben

zyla

min

e

Exp

. C

ondi

tions

N

o.

solv

ent/g

, tem

p.

E.P

./ m

m.

__

_~

Hg 1

0

-

Ref

lux

2~

Isoq

uino

ltne

(20)

,,

3e

2-N

apht

hol

(20)

,,

4 N

apht

hale

ne (

20)

,,

Am

ine

in g

Tim

e da

ys

Use

d C

onsu

med

a

5 20

20

(100

%)

5 20

20

(100

%)

2 20

20

(100

%)

7 20

20

(100

%)

Prod

ucts

in g

(%)

Ben

zyl d

eriv

. R

esid

ues

Am

mon

ia

Tol

uene

D

iben

zyl

Stilb

ene

of s

olve

nt

(g) .. -

__

__

__

__

__

_~

~~

110"

14

0-1

5O"j

lO

160-

1 65

"/ 10

6 1.

15

4.0(

20%

) 2.

3(12

.50/

,)

2. o

q10

%)

6. 7d

(30.

6 %)

6

1.1

4.

0(20

%)

4.02

(10%

) 2.

0( 10

%)

11.5

2'(5

2.1%

) 5

1.22

1.

2(6%

) 3.

0(15

%)

5.0(

25 %

) -

1.15

6.

2(31

%)

2.0(

10%

) 3.

2(16

%)

8.4f

(35.

7%)

4.5h

a C

alc.

on

basi

s o

f N

H4C

l co

llect

ed.

b B

enza

ldeh

yde

colle

cted

at

b.p.

70-

75"C

/lO m

m H

g; 0

.35

g.

c0.2

5 g

; D

.N.P

. de

riv.

m.p

. an

d m

ixed

m.p

. 23

5°C

. I-

Ben

zyl

isoq

uino

line

colle

cted

at

b.p.

220

-225

"C/1

2 m

m H

g, t

oget

her

with

I,l'

-bi-

isoq

uino

lyl

(2.4

g)

at 2

40-2

52"C

/12

mm

Hg,

mix

ed m

.p.

162-

163°

C;

picr

ate

mix

ed

Wat

er c

olle

cted

(0.

3 g)

. m

.p.

205-

206°

C.

f I-

Ben

zyl-2

-nap

htho

l co

llect

ed a

t 14

0-14

7"C

/10

mm

, to

geth

er w

ith 9-phenyI-1,2-7,8-dibenzoxanthene (8 g

) at

260

-265

"jlO

mm

Hg,

mix

ed m

p. 1

86-1

88"C

, 2-

dina

phth

ol

(0.3

g) a

t 14

5-15

2"C

/10

mm

Hg,

mix

ed 2

11-2

13°C

. N

eutr

al a

nd a

min

e pr

oduc

ts.

it T

oget

her

with

phe

nolic

pro

duct

s.

j B

enzy

lnap

htha

lene

s co

llect

ed a

t 24

5-25

5"C

/10

mm

Hg,

ana

lyse

d by

g.1

.c. i

nto

ratio

of

72.3

%1-

to

27.7

% 2

-iso

mer

.

294 M. Z. A. Badr et al.

benzyl (I) and dibenzylamino (11) or mono-benzylamino (Il l) free radicals according to the following scheme :

The formation of benzaldehyde may be due to air oxidation of benzyl free radical.19 Dimerisation of benzyl radicals results in the formation of dibenzyl, while abstraction of hydrogen by benzyl and benzylamino radicals produces both toluene and either di- or mono-benzylamines, which are assumed to be formed as intermediate products.

On the other hand, ammonia was also produced together with the same neutral products of pyrolysis of tri-, di- or mono-benzylamines. Consequently, ammonia was assumed to be formed through the homolytic fission of the primarily produced benzylamine into benzyl (I) and amino (IV) free radicals (route c) that subsequently form the pyrolysis products.

Coupling by-products produced during the free radical substitution of solvent nuclei by benzyl free radicals, confirmed the assumption that the substitution mechanisms proceeded partially through a process of initial hydrogen abstraction from the solvent nuclei forming the intermediate aryl radicals, namely : isoquinolyl and B-oxynaphthyl radicals. Their subsequent combination with benzyl free radicals, produced the substitution products. However, subsequent dimerisation of each of the two aryl radicals produced 1 ,l'-bi-isoquinolyl or 2,2'-dinaphthol respectively, according to scheme (2).

11-benzyl-2-naphthol

f CgHsN -+ (C9HeN)z 1 ,l'-bi-isoquinolyl

C9H7N CgHsN+ C&IsCH2 - C&IsCHz-CgHsN

1-benzyl isoquinoline

On the other hand, absence of binaphthyl from the pyrolysis mixture in the presence of naph- thalene confirms the assumption that a radical substitution process takes place to give benzylation products.

The most satisfactory explanation for the formation of 9-phenyl-l,2-7,8-dibenzoxanthene in considerable yield during the amine pyrolysis in 2-naphthol is assumed to be the same as that suggested previously.20 Formation of stilbene may be attributed to hydrogen abstraction from dibenzyl by free radicals present in the reaction medium (route d) as shown in previous work.21 The observed high yield of benzylation products of aromatic solvents suggests that benzylamines may be considered, in general, as valuable thermal benzylating agents for aromatic systems.

Thermolysis of benzylamines 295

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2. 3. 4.

5 . 6 .

7. 8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

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