Chloroacetylenes as Michael acceptors. I. mechanism of enolate dichlorovinylation.

Tetrahedron Letters,Vo1.23,No.Z3,pp 2369-2372,1982 Printed in Great Britain

CHLOROACETYLENES MECHANISM

AS MICHAEL

OF ENOLATE

Andrew

S.

Kende

0040-4039/82/232369-04$03.00/O 01982 Pergamon Press Ltd.

ACCEPTORS.

I.

DICHLOROVINYLATION.

and

Pawel

Fludzinski

Department of Chemistry University of Rochester Rochester, New York 14627

Summary ketones

The

condensation

has been

In a recent CHCI

to yield

shown

of certain

to proceed

communication

enolates

we described

cl-dichlorovinyl

with

trichloroethylene

by way of dichloroacetylene

ketones,

a novel

illustrated

condensation

in eqn

1.

LDA IHMPA

2.

C12C=CHCI

to yield

a-dichlorovinyl

as an obliqatory of certain

intermediate.

enolates

with

CI2C=

1.’

(1) 1 Although

some limited

mechanism

2 -

synthetic

was understood.

applications

At least

the initial data, namely (1) transfer 4 tion, and (4) elimination-addition. conditions)

argued

had effect tween

against

on product

yield,

addition-elimination

SCHEME

transfer makinq

four

of this discrete

alkylation, 5

The

2(2)

complete

alkylation. a radical

reaction

mechanisms s NR1 absence

Neither chain

and elimination-addition

were

could

radical

mechanism

Elimination-Addition

C12C=CHCI

Clcrccl

s 2369

addition-elimina-

1.1 was less obvious.

addition

its

to rationalize

(stable

to reaction

nor AIBN, nor t-Bu2N0 3a untenable. Differentiation be-

C12C=CHCI

addition

3 (3)

its scope nor

light,

Addition-Elimination

CICZccl

be proposed

chain,

I

elimination

neither

of cl-chloroketones

0 2, nor

(Scheme

shown,

The

observed

2310

formation

of 1,2-dichlorovinyl

addition-elimination shown

that

through

the

since the reaction

initial

formation

ethylmalonate,

would

CIC:CCI

free

with

predict

intermediate

rather

than

the possible

would

products

of the

our

based

limited

the observed

dichlorovinyl in our

of Ott

with

substitu-

sodio diethyl

from a dichloro-

we undertook

enolates.

proceeds

of the with

derivatives 9 products.

dichlorovinylations

its reactions

nucleophiles

knowledqe

on the work

a-( 2-chloroethynyl)

role of CIC:CCI and to examine

or phenolate*

hand,

nucleophiles,

seemed to argue against 6 In addition, it has been

be anticipated.

and arylthiolate’

On the other

carbon

formation

of C12C=CHCI

2,2-dichlorovinyl

C12C=CHCI

of CIC:CCI.

of CIC-CCI

To establish

than

2,2_regiochemistry

between

tion chemistry

acetylene

rather

to generate

None of the

several

pub-

lished

procedures for preparation of CIC?CCI were suitable in our hands for this purpose. That 10 11 of Ott gave unacceptable mixtures, and that of Kloster-Jenson qave unreproducible results. We ultimately

found

LiN(SiMe3)2 through (glc)

that

in hexanes,

ca 50% CIC:CCI,

THF

with

freshly

ketone

excess

by warming

LiN(SiMe3)2 4.

column

distilled

instead

of LDA,

qave

a distillate 12 10% hexanes.

and

prepared

result.

(-78OC

the enolate

How can the choice

was 2 (40%) accompanied

12.

we considered

CCI

reaction

be operative,

reaction

can serve

mixture

HN(i

anion

generated

gave

only

A reliable

reactants.

served pared

isotope

2.

toward

Cl-

2, whereas

by partial

1.

and

5 with

between

would

in

no dichloroClCrCCl

or

using

by a small amount

of the

deuteration

(cf

6) formed proton further, the

serves

5,

MeLi,

CIC:CCI.

C12C=CHCI

formation predict

excess

of dry

of C12=CHCI

a mixture

a small,

dichlorovinylation

(deuteron)

transfer

only

Ca(OD)2

secondary

(path

with

depend

donor

kinetically

this

hypothesis

MeLi,

followed

2 (Scheme

isotope

in

less

II)

was available

from

and C12C=CDCI

as

effect,

whereas large

the pri-

to form the ob-

50+2% C12C=CDCI,

was reacted

were by

II).

lead to a ootentially a, Scheme

containinq

in D20,13

would

the trichlorovinyl

of C12C=CHCI

could

step

trichloroethylene

using

source,

one equiv.

qave

of CIC:

or HN(SiMe3)2

or the

and elimination-addition

using only

reactions

from a proton

C12C=CHCI

Consistent with

As a workinq

elimination-addition,

ketone

as the proton

upon treatment then

(4),

that

p)

the intermediacy

in these

startinq

I

4

types?

with

abstraction

suqqested,

w \

product

mechanism

2, whereas

reqenerate

for enolate

at the proton A ten-fold

anion

addition-elimination

in product

mechanism

effect

data anion

different

words,

When C12C=CHCI

Addition-elimination

product

3 h),

+

be consistent

form 3 vs.

to

Our

could

In other

the TMS enol ether

competition

elimination-addition mary

to yield sources

distinction

intermolecular

data

fate of the vinyl

-Pr12 cannot.

that

1.

elimination

can eliminate

the observations CIC!CCI

Cl-

all of our

in eqn

and the

as proton

amine

that

depicted

between

acidic

contained

was reacted

3; excess

-

)2

CICEccl

of base lead to such distinctly

hypothesis,

the

that

of 1 was prepared

-3

could

+ rt, ketone

of

at 30-36OC

each of LDA and HMPA)

1. LiN(SiMe3)2/HMPA

2. CIC_ccl

on competition

to a suspension

distillation

2).

1. LDA /HMPA

for the

at -78’C

of C12C=CHCI

cl-chloroethynyl

when

product

free

as above,

64% of the

However,

the major

in Et20

3 h and direct

1. and one equiv

We obtained

a similar

(Eqn

ether

of 1 (from

ClCtCCI’Et20,

gave

of C12C=CHCI to rt over

reproducibly

40% diethyl enolate

2 was formed.

ketone

isomer

of a solution

followed

a 12 cm Vigreux

When the kinetic

vinyl

addition

with

pre-

the kinetic

2371.

1. MeLi 2. ClCrCCl

enolate

in the usual

manner.

The

resultinq

spectrometry

and by 400 MHz nmr.

of the enone

(duplicate

two separate

reaction

protons,

THF, the

then usual

after

with

reaction

integration

with

formation,

LDA/HMPA.

incorporation

unlabeled No exchange

2. C12C=CHCI

of vinyl

a primary

tween

molecular addition anion

with

enolate

using

data

throuqh

that

of Cl-

recent

synthetic

systems.

the

reaction

and

such as vinyl

B-proton using

out of 1.0 vinyl that

loss of label

of 100% C12C=CDCI

( < 1%) could

be detected

in

under

I ”

0

( 3)

I

synthetic

intermediate.

anion

5,

proton

C12C=CHCI

observations

reported

of the enolate

in which

CIC:CCI

of Melander

for enolate

bimolecular

in a propagative

studies

and Saunders

for

E2 eliminations

abstraction.

of 1 with The

chemistry are outlined

trichloroethylene

fate of the initial by competition

Hence,

acts as proton fashion.

l4 qave kH/kD= 15 and clearly

dichlorovinylations.

is determined

in the hexachlorobutadiene

scope of haloacetylene

These

to those

mechanism

a dichloroacetylene

is operating,

regenerates our

protons

I

the method

comparable

and an enolate,

elimination

to the

effect

effect

from our

mechanism

5, and

sistent tions

isotope

proceed

CIC:CCI

5 equiv.

by mass

the

calibration

.78+.03

To establish

with

-*

(50% D)

the elimination-addition

We conclude z does indeed

after

showed 3).

against

22+3% D incorporation

of the isotope

with

(Eqn

2 was mixed

-1

3.7kO.5,

products,

100%-H C12C=CHCI)

1. LDA /HMPA

I

consistent

reaction

2 was analyzed

proton

conditions.

\Q/ Calculations

product

of the vinyl

on two separation

obtained

to 22+3% deuterium

product

treated

dichlori5vinylation

Careful

determination products

correspondinq

did not occur

or

to yield adduct

between

be-

uni-

for eqn l_, an elimination-

source

in quenching

vinyl

These

conclusions, which are con16 series, have important implica-

for the ethynylation

and vinylation

in the accompanyinq

communication.

of 17

2312

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