The amine assisted photo-dehalogenation of halo-aromatic hydrocarbons

Tetrahedron Letters,Vo1.24,No.50,pp Printed in Great Britain

THE AkINE ASSISTED Richard

Summary:

004O-4039/83 $3 00 + @ 1983 Pergamon Press

5673-5676,1983

PHOTO-DEHALOGENATION

OF HALO-AROMATIC

HYDROCARBONS

A. Beecroft, R. Stephen Davidson* and Dean Goodwin Department of Chemistry, The City University, Northampton Square, London EClV OHB.

The rate constants for quenching the triplet states of 4-chlorobiphenyl, l-chloronaphthalene and l-methylnaphthalene have been determined by use of the technique of laser flash photolysis. Evidence in favour of the quenching occurring by a charge transfer process, is presented.

The irradiation

of many halo-aromatic

leads to homolysis subsequently

compounds

of the aryl-halogen

abstract

hydrogen

Ar-Hal

hv

donating

in hydrogen

solvents

The aryl radicals

bond [l].

from the solvent to give the related

so formed, aromatic

hydrocarbon. +Ak

+

H-donating

Hal

+ H-Hal

,ArH

solvent

The rate of dechlorination polar solvents ethylamine

of many chloroaromatics

containing

A suggested

[2,3].

chloroaromatic

a tertiary

mechanism

acting as an electron ArCl*

+

Et3N

for this process

include

presented

--b

the solvent polarity fluorescence

1

[3,4] and (b) the finding

the intermediacy

[3,5].

Although,

of radical

ted by the finding

it had been tacitly

that the excited

constant

However,

Thus the case for quenching only been established

singlet

for fluorescence

the triplet

containing [4].

deuterium

oxide

Until recently

state of the chloroaroma-

and triplet quenching,

by plotting

[6,7] that

state on the being less than

the reciprocal

of the

of the amine concentration.

state of chloroaromatics

means. 5673

for

appears to have been valida-

singlet

versus the reciprocal

by indirect

quench the

fluorescence

it has now been claimed

the value for the slope of the line obtained for reaction

amines

no one has any direct evidence

into the products

can involve both the excited

basis of the Stern-Volmer

quantum yield

that tertiary

ions, their formation

state.

process

caused by increasing

and in some cases exciplex as yet,

of deuterium

assumed

tics is the only reactive the reactions

of a charge transfer

that the use of solvent mixtures

leads to the incorporation

tri-

the excited

Ar' + Cl- + Et3N+'

in the rate of dechlorination

of many chloroaromatics

can be observed

involves

[(ArC1)'(Et3N)+']*

in support of the occurrence

(a) the enhancement

by the use of containing

acceptor.

ArH + HCl + CH2 =CHNE$f--Arguments

is enhanced

amine, e.g. acetonitrile

by amines has

.oo Ltd.

We now report the rate constants chloronaphthalene

by several amines

with those for quenching obtained

by utilising

Measurements

triplet

the technique

of amine.

and these are shown in the Table,

of nanosecond

Rate constants

and ltogether

The rate constants

laser flash photolysis

of the various hydrocarbons

=-

were evaluated

were

[8].

in the pre-

using the

1

+ ki [Amine]

TO

where T = triplet

lifetime

in the presence

= triplet

lifetime

in the absence

and ki

4-chlorobiphenyl

: 1 -r

TO

triplet

l-methylnaphthalene.

were made of the lifetimes

sence of known amounts eauation

for quenching

= bimolecular

rate constant

of amine at a concentration

[Amine].

of amine.

for triplet

quenching.

TABLE Rate Constants Amines

for Quenching

in Acetonitrile

for Quenching

the Fluorescence Acetonitrile

4-Chlorobiphenyl -__---~ kT (M-l s-l) :4eNA 0

1.4

+ 0.20

Triplet

Solution

9.46? 0.48

2.18 1 0.20

24.3Ot1.16

I-Chloronaphthalene

+

2.75 f 2.0

/ 15.65? 0.47

x lo6 24.682 0.41

0.40

1.08 1 0.14 x lo8

x lo7 9.13 + 0.80

3 MeN

9.84 + 0.90

17.66? 0.82

8.6

30.3710.16

+ 0.70 x lo7

x lo7 29.87i 0.91

1.59 ? 1.20 x lo8

x lo7

for quenching

in

x lo7

3 MeN

(a) KS,

9.4

x lo7 5.45 t 0.55

Et3N

15.54+ 0.82

by Amines

kT (M-l s-l) M

x lo7

2.62 + O.lU

by

Constants

(Ksv)(a)

l-Methylnaphthalene

1 KSv W1) I

Hydrocarbons

of Chloroaromatics solution

x lo7

YeN3

Aromatic

(ki) and Stern-Volmer

of singlet

l-methylnaphthalene

not determined.

5675

For the cyclic

amines,

the order of reactivity

N-methylazacycloheptane

The reactivity

N-methyl-4-oxa-azacyclohexane.

of quenching

and whether

amines.

>

energies

of triethylamine

Quenching

to be

gives any clue as

via energy transfer

of the hydrocarbons

appears

arises as to the

this order of reactivity

to the nature of the mechanism. since the triplet

is

> N-methylazacyclohexane

The question

similar to that of N-methylazacyclopentane. mechanism

for the three hydrocarbons

> N-methylazacyclopentane

seems unlikely

should lie below those of the

It was found that the quenching of the triplet states by the amines

is much greater suggests

in acetonitrile

the occurrence

verify the production Further evidence

than in cyclohexane.

of a charge transfer

of radical

in support

excited

singlet states.

The Stern-Volmer

is more reactive

difference

in the conformation

quenching

for the complex

of excited

singlet process

for fluorescence

of ionisation

and oxidation

ever, the ionisation

which,

quenching.

process.

the are

for fluorescence

This may reflect

a

Since it is well established hydrocarbons

by amines

[9], it seems from the similarity

potentials

potential

comes

which gives rise to fluorescence

states of aromatic

and triplet

also occurs via a charge transfer

to

failed.

mechanism

the fluorescence

than N-methylazacyclopentane.

occurs via a charge transfer of reactivity

for quenching

is triethylamine

to that which causes triplet

that the quenching

quenching

is the same as that towards

constants

The only anomaly

shown in the Table.

all attempts

of the cyclic amines towards

states of the chloroaromatics

quenching,

However,

ions using laser flash photolysis

of the charge transfer

from the finding that the order of reactivity

triplet

This solvent effect

process.

quenching,

that triplet

Unfortunately

for the amines

in order

quenching

a comprehensive

is not available.

range How-

for N-methyl-4-oxa-azacyclohexane is considera10 and not surprisingly it is

bly higher than that of N-methylazacyclohexane the less efficient one electron reactive

quencher.

process

Chlorine

than N-methylazacyclohexane

azacyclopentane

is a better quencher

This work shows that the triplet amines.

unequivocal

proof that dechlorination

tion.

tertiary

amines

that -50%

At this latter concentration

via a

is more

The Table shows that the N-methyl-

than the N-methylazacyclohexane.

hydrocarbons

for quenching

of the triplets quenched

are quenched

this does not provide

occurs from the triplet

of the rate constant

of 2 x 10v4 M and -90%

Since dechlorination

[ll].

In the case of the chloroaromatics

nyl it can be calculated concentration

oxidises

states of aromatic

by tertiary

from a consideration

dioxide

and it was found that N-t-butylazacyclopentane

state.

triplet

are quenched

However,

4-chlorobipheat an amine

at a 10-S M amine concentra-

very little fluorescence

occurs at amine concentrations

quenching

occurs.

in the range of 10 -3 to

5676

1O-4 M it appears

reasonable

state. To get 50% quenching

to invoke reaction of the excited

tion of 4.5 x 10-2 M is required. excited

singlet

state quenching

Thus at high amine concentrations

plots of the reciprocal

versus the reciprocal

of the amine concentration

presented triplet

of reaction

are linear over a wide range

occur via the excited

data

singlet

and

[7].

Acknowledgements: financial

of the quantum yield

some quenching.

it must be, in the light of the rate constant

in this paper, that the reactions

states

via the triplet

states an amine concentra-

will occur and nearly total triplet

Since the Stern-Volmer

of amine concentrations

occurring

singlet

We thank the SERC for funds to purchase

support

(to R.A.B.)

and The City University

the laser and for

for a Fellowship

(to

D.G.).

References 1.

P.G. Sammes in "Chemistry of the Carbon-Halogen Bond", Ed. S. Patai, Part II, Chap. II, J. Wiley and Sons, New York (1973). R.K. Sharma and N. Kharasch, Angew. Chem. Int. Engl., 1968, L, 36. J. Grimshaw and A.P. de Silva, Chem. Sot. Rev., 1981, @, 181. R.S. Davidson, J.W. Goodin and G. Kemp, Adv. Phys. Org. Chem., 1984, 3. In the press.

2.

N.J. Bunce, Y. Kumar, L. Ravanal and S. Safe, J. Chem. Sot. Perkin Trans. g, 1978, 880. L.O. Ruzo, S. Safe and M.J. Zabuk, J. Agric. Food Chem., 1975, 23, 594. L.U. Ruzo, N.J. Bunce and S. Safe, Can. J. Chem., 1975,53, 688.

3.

N.J. Bunce, P. Pilon, L.O. Ruzo and D.J. Sturch, J. Org. Chem., 1976, 2, 3023.

4.

R.S. Davidson

and J.W. Goodin,

5.

M. Ohashi,

6.

N.J. Bunce, J. Org. Chem.,

7.

M. Ohashi

8.

The experimental set-up has been previously described:R.A. Beecroft, R.S. Davidson, D. Goodwin, J.E. Pratt, L.A. Chewter D. Phillips, Chem. Phys. Letters, 1982, 93, 468.

K. Tsujimoto

Tetrahedron

and K. Seki, J.C.S. 1982, 9,

and K. Tsujimoto,

Letters,

1981, 163.

Chem. Conun., 1973, 384.

1948.

Chem. Letters,

1983, 423.

and

9.

This subject has been comprehensively reviewed:R.S. Davidson in Molecular Association, Ed. R. Foster, Vol. I. Academic Press, London, 1975; N. Mataga and M. Ottolenghi in Molecular Association, Ed. R. Foster, Vol. II, Academic Press, London, 1979; R.S. Davidson Adv. Phys. Org. Chem., 1983, 19, 1.

10.

A.M. Halpern

11.

L.A. Hall, G.T. Davis, D.H. Rosenblatt 73, 2142.

and T. Gartman,

J. Amer. Chem. Sot., 1974, 96, 1393.

(Received in UK 30 September 1983)

and C-K. Mann, J. Phys. Chem.,

1969,