Isotope effects and mechanism in the base-promoted dehydrochlorination of 1,1-dichloro-2,2-diarylethanes-1- 14C

Tetrahedron Letters,Vo1.27,No.42,pp Printed in Great Britain

ISDTDPE EFFECTS AND MCHANISM

5055-5058,1986

IN THE DASE-PIWWTED

0040-4039/86 $3.00 + .OO Pergamon Journals Ltd.

~H~DRDCMl?INATION

OF I.I-DICHLORO-2,2_DIARYLETHA)(ES-I-

Agnes Pulay

Department

of

Chemistry

and Arthur

:

and

D-nitro

analog,

support

an.Elcb-like

results

In the

Isotope have

El,

base-promoted

effect

the

E2,

(extension

and activated and

Elcb

of the

dehydrochlorinT;ion

isotope

of

Arkansas,

U.S.A.

of

for

in elimination the

complex

mechanisms arguments

effort

the to

to

structures)

base-promoted have

concluded

Elcb

character,

elimination that

the

depending

of

on

especially

details

of

isotope

C

respectively.

carbon

mechanisms

isotope

These

effect

studies

(reactfon

In particular.

for

co2nsequences

4.5

coordinate irreversible

are

as

follows

is straightforward):

,4cx”c(

Effect

Expectations 0 C IE

aC IE yes

E2

yes

yes

no

yes

HCI from

the

1.024,

reactions.

El

has carried

compounds

and

cX

Elcb

group

2.3

- H-X

Isotope

research

1.045

reactions.

carbon

Mechanism

Mclennan’s

are

elimination

these

cases

0’: a - ;-

both

establish

expected

reversible

k,

reactions,

of

I.i-dichloro-2,2-diphenyiethane-l-

of

k/

effects,

E2 mechanism

studies

become a mainstay

motions

University

AR 72701,

14

Abstract its

Fry*

and Biochemistry,

Fayetteville.

C

out

extensive

DDD-type

react nature

studies

compounds

mainly of

no

the

5055

by the ring

6-10

of

the

mechanisms

(I,]-dichloro-2,2-diarylethanes), E2 mechanism,

substituent,

with with

varying the

D-nitro

of and

degrees

of

compound

5056

being

at

the

Elcb

Intr~ieSljiar NO

;

k/

dafa.

In isotope

1.0038,

1.0035,

k =

and

limit.

chlorine

the

lack

oflla

Elcb

for

R-nitro

isotope

effect

case might

expected

for

measured

the

7th;”

’

data:

1.0023,

and

isotope

an Eicb-like

last

conclusion

is

based

for ring substituents. 1.0000

(ail

effect

for

f

the

E2 mechanism

about

(at

the

their

Cl-,

The

and

trend

compound are

over

on

H-.

0.0002).

R-nitro

changing

primarily

CH30-,

in

claimed

pnitro

the

to

be

compound)

mechanism.

We have been searching carbon-14

effect

chlorine

what would be expected to a limiting

particular,

for

be it.

the

some time4

studies

elimination

One notes

from the

E2 mechanism,

aC isotope

for

of

but

effects

for

not the

a

convenient reaction

table

for

above

the

Elcb

unsubstituted

“Elcb

calibration

mechanisms.

and

that

isotope

an aC

mechanism.

and R-nitro

it

case” appeared

our

that

effect

this

is

Accordingly. DOD-type

for

to

be

we have

now

corrpounds:

base )2CH-aCHCi 2

kjak

The

carbon-14

means from one,

CH ONa/CH OH at fr%ction

30 (Z = W2)

I in Table

unsubstituted

Note changes,

that or

data

for

radiochemical reaction agreed

and closely

there

as the the

. The results Similar

1 below.

and data

are

workup with

relatively

acid

followed

a

of

the

reactions amOUnt

limited

and product

conventional

chloride,

acetophen-

by treatment

by nitration

of

aikene

with

benz-

unsubstituted

were

carried

base

to

were

comout

COntrOi

separated

Radioactivities

radioactivity.

in the

for

activities

starting are

the

critical

5 fractions

compounds are

the

data

in

calculation This

gives

materials

adequate. of

the

(from

compound. the

procedures the

both

by which

unsubstituted of

data for

no trends

equation

purities

reactant

by

the

The elimination

constant

Z = NO 2

in the

by column

of

the

start-

reactant, R , and the product aikene. R were measured by k/aktPwere calculated by the , and the kiietic isotope effects,

equations

compound,

through

60 (Z = H) OC using

to

counti(;l;

Tong and Yankwich

later.

for

was synthesized

compound was prepared

The recovered

, the recover$

R

scintiii~tion

f 0.002

2,2-dichioro-I-phenyiethanol,

and recrystallized

ing material.

detaf

or

1.024

reactant

I be published

f.

Z = H. and

)2C=aCHCi

sodium acetate-2-14C

The nitro wii

3of reaction,

chromatography

liquid

and

acid.

I details

Fui

for

unsubstituted

available

dichioroacetophenone,

pound.

f 0.003

labeled

cotnnerciaiiy

ene and sulfuric

the

= 1.045

>

- HCI

Also.

products

at

R-nitro of

compound are

reaction)

sumnarized

the

table

is

made is

us high

were

under

as

the

the

confidence

and

products

and

the

activities

of

100 X reaction,

obtained equation

fraction

changed.

presented

Of

for

in the

above.

reaCtiOn

The same applies in

to

chemical

and

demonstrates

that

our

the

materials

as they

the

starting should.

5057

Table the

1.

Fractions

of

CH30Na-Promoted

Methanol

at

Reaction.

Molar

a

Activities,

Dehydrochiorination

of

and

Carbon-14

Kinetic

Isotope

Effects 14 C in

1.1~Dichioro-2,2-di(4-nitrophenyi)ethane-l-

in

30 ‘C.

Fraction of

R r

k/=k

R P

Reaction f

from:’

R t 0

Roe Rr. f

R 9R r

p

, R , f

R

Roe R , f P

r

P

0.30

0.28353

0.27597

1.0239

1.0188

1.0248

1.0230

0.50

0.28690

0.27729

1.0240

1.0273

I .0228

1.0250

0.70 a R

The lower the

expected

isotope

effect

bond rupture isotope

the

at

effect

the

R-nitro

which

group

result

isotopic would

have

(and RC)

is slower

than

for

“washed tial

out”

becomes

the

on the is

effect.

isotope

Reversibility

probably

lead

a semantics

to

concerned.

effect

mechanism

it

question

Elcb to

To our

is

is

thus

return)

same isotope in the

effect

of

indirect

the

too

effect

absence

out

have

to

consequences

of appropriate

of

result

of

Such a mechanism

on the

unsubstituted any

exchange

deuterium would

possible.

reason show

for

experimental

isotope have been Substan-

there

up in

as the

as far

experiments

that a

been

the

both

the

deuterium

as the fast

and

E2 mechanism

evidere

the

in

Clearly

effect.

no exchange out

The

observed4

aikene

being

to measure

rapid. would

ruled

the

aC and

complex.

average.

isotope

know1 edge,

was possible had been

than

mechanism.

those

of the

carbanion-like,

have been carried

there

measurement

(internal the

=C isotope

mpound-

bond formation

to those

both more

, and thus a lower

similar

exchange

both

smaller

is an “C

magnitude

in an E2 activated

and

a reversible

since if

one shows

that there

the

in magnitude

stabilizing

observed

However,

and

Elcb

by

compound (they

reaction,

compound,

no deuterium

largely

are

g-nitro

the of

unsubstituted

since

involved

reactant-like

results

no exchange).

by a reversible

would

step

deviation).

fact

for

considerably

path

more

of

second

effects

be

bond rupture

quai itative

elimination

pnitro

and

isotope

experiments This

the

the

reaction

an =C

out

and there

reaction

complex

the the

fashion,

is similar

compound is

possibility

have

isotope

been carried

compound’,

effect

the in

to

the

for but

The expectations

compound

pnitro

(standard

has not been reached at the p-nib-o

in a qualitative

carbon-chlorine

consider

Elcb-like,

1.0237

1.0280

0.0024

compound than

must necessarily

activated

fractionation

‘C

the

influences the

be expected

as the

carbon

for

in reduced

One should

R-nitro

E2 mechanism.

unsubstituted

that

making

the

f

becomes more

to analyze

alpha the

E2 systems:

carbanion,

El&-like

difficult

for

for

reaction

1.0240

I shows that the Elcb limit

by an are

=

k/=k

effect

as the

at al

reacts

isotope

mean:

=C isotope

trend

effect

still

other

$veraii

1.0218

I. 0254

0.27765

0.28899 = 0.28167;

0

it

mCi/moie

mCi/moie

obtained

------------------_-~~-~-~~-~~~~~~~~~---~----~-~---~~

being

exchange

E2 mechanism. evidence.

5058

The chlorine whether

or

expect note since

there

the

to

11). the

one

for

there

to

the

basic

effect

mechanism

be

for

they the

do not

the

R-nitro

little

to

the

for

on

it,

nil.

bond rupture

these

in the

isotope

question

limit--one

Elcb

effects

are

way for

this

E2 activated

is taking

A solvation

chlorine

Elcb

E2 and

obvious

and

withdrawing.

of

the

critical

the

al I the

The most

Elcb-like.

quite

on the

reached

both

carbon-chlorine

low values

directly

since

is always

becomes more electron

be contributing

bear

effects

indirectly

compound is

E2 mechanism

really

compound has

isotope

do bear

R-nitro

be very

substituent

data

for

intramolecular

However,

that the

isotope

not

mechanisms

(see

so small,

and

to

Elcb

meaning

character

effect

leveling

is

be true

complex,

on may

of

would

may

as also

effects.

References and Motes 1.

The financial

2.

Fry.

A.

Rev.

1972.

3.

W. H..

Fry,

leading effects

Jr.

H.,

7.

Grout,

A.

J.

8.

A.:

8.

tlclennan,

9.

Grout,

L.;

Saunders,

R.

is gratefully

acknowledged.

Elinination

Reactions;

Chem. Sot.

Studies

Chem. 1985, 0.

D. J.;

J.

J.

0.

the

Jr.

Kanski,

Reactions: &action

1983.

John Wiley

8

Rates

of

Isotopic

F.

A.;

Crook,

W. P.:

Susan.

research

by the

see Wu, S-L.:

50.

2392-2394.

J.

Chem. Sot.

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H. J.

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Pettigrew.

l?eac!tions.in

A.

8..

Eds..

Proc. Elsevier

133-138.

extensive

I.

R.:

of Sane Elimination

Duncan,

reactions,

Perkin

J.;

T.;

of Hechanism

Spa&man,

Chem. Sot.

KcLennan,

W. H.,

Hasan,

Compounds.

to

of Elinimtion

19%.

I.;

in elimination

Helennan,

D. J.;

J.

reference

HcLennan,

A.:

llechanislrr

Company: Amsterdam,

Oro.

N.;

F.

New York,

Labeled

Publishing

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Gray,

tlelander,

Effect

ARIJ~.

For a recent

W.

A.

Eubanks.

Isotope

atom

6.

CHE-8314767

of the Mechanisms of

Cockerill,

8.;

Svmp. Svn.

Scientific 5.

NSF Grant

Studies

1973:

L.

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Int.

U.S.

Wiley-Interscience:

A; Sims,

S .t

Jr.:

New York,

Ibiecules:

of

Effect

163-210.

I.

Saunders, Sons:

4.

support

Isotope

Trans.

Chem. Sot.

2 1977.

Saunders

Hargreaves,

2 1974,

group R.

T.;

on heavy Saunders,

1377-1380.

Chem. Comnun.

1976.

775-776.

1753-1758.

Spackman,

1. H.

J.

Chem. Sot.

llclennan.

D.

J.

Chem.

Perkin

Trans.

2

1977,

1758-

1977,

1763-

1763. IO.

Burton.

G.

W.;

Sims,

L.

8.;

J.

Sot.

Perkin

Trans.

2

1770. 11.

It

is our

;v5en

if

Clif

and

that

view the 37

that

an

is an activated

discrimination

should

(not

and

measured)

12.

Hasan.

13.

Tong,

Y; Yankwich.

14.

Hill,

J.

W.;

(Received

chlorine

Fry,

P. E. A.

J.

J.

Phvs.

Chem.

1986)

effect

choice

from

an

the

1957, 1962,

a 84,

be

normally

between

irreversibly

formed

mechanistic

E2 mechanism.

effects

would

would

must be by the

Am. Chem. Sot.

Am. Chem. Sot.

in USA 23 June

it

isotope

A. J.

isotope the

molecule

And for

result.

intramolecular Fry.

Elcb;

(and

process,

: Sims, L. 8.;

T.

intr~leculer

mechanism were limiting Cl from within the sacs

of

course,

would

be expected.

1983.

jD5.

540-543. 2763-2769.

3967-3975.

be expected

elimination

of

carbanion.

and

definition) both

isotopic

intermolecular