Chemistry of the bicyclopropenyl system

155 - 158,1974. Per@lKm Prosa. Printed in Great Britain.

Tetrahedron Lettera No. 2, pp

CHEMISTRY I.

OF THE

FRAGMENTATION

vs. R.

Institut

REARRANGEMENT

WeiS

and

fiir Organische

was

not

until

to

complete

recognized 2-

contrast its

resp.

to

mechanism

of

cyclopropenyl

set

they

scribed

in

of

Kempcke der

that

the

benzene

4).

of z

'lJ - 12

of

jb,

was

exclusively

ing

results:

with

In

and

Universitlt

Miinchen

29 November 1975)

bicyclopropenyl

valence

isomers

this 2

us

be

formed

attack

to

no

we

system of

the

1 was

(CH)6

type

same

of

same

Recently

the we

chemistry have

the

interaction so

as

as

to

of

the

type

of

the

bi-

find

principles

a model

case

of 1

established

rearrangement

mechanistic

describe

Br2

product

reaction

bromine

of

electrophiles

to &

together

(cf.loc.cit.

of

much

the

out as

de-

course

of

temperatures.

table

other

set

investigate

the

ambient

the

this

explored.

other

amounts

in

the

to

paper

at

of

bicyclopropenyl

various

equimolar

under

to

according

material

Electrophilic

remains

stimulated

contained

polymeric

members

Ag' -catalized

proceeded

Addition

With

the

work

ref.

bromination

other

3)

system

whether

some

the

derivatives

This

and

2)

1)

BY HALOGENATION

accepted for publication

recently

the

SYSTEM

4:

In

the

very

H.P.

Chemie

(Reoeived in BK 12 KovemberI97<, It

BICYCLOPROPENYL

in

with

could

their

be

conditions, 3)).

Scheme

CHCl., yielded relevant

data.

cpds.

cyclopropenium

I accounts

the

for

these

intermediate

-10

Besides

detected. the

on 2 generates

155

abs.

salt

2

diverg-

5 which

Bo. 2

156

is

faced

tion.

with

With

the

R=Ph

alternatives

the

latter

of

electrocyclic

possibility

is

ring

exclusively

opening

or

realized

fragmentaas

fragmenta-

Scheme I Br2

abs.CHCL,

a)

R= H

bj

R=

R=H R= Ph

Disrotation

Ph

Fragmentation

I

ph)$-f$-Ph 95% Ph

Ph

I!

1.

12

I

-Br,

tion a)

of 5

the

into

well-known

1

is

strongly

favoured

hi.gh stability

of

by the

two

complementary

triphenylcyclopropenim

factors: cation

and

No.

b)

2

157

steric

these

hindrance

two

of

factors

disrotation)

disrotatory

missing

followed

(R=H)

- or

ring 5

collapses

accompanied

propenylmethyl

cation

-_) cyclobutenyl

mediate

Bre

l-position

2

hindered

side

When

an

with main

of

Br

The

2'

subsequent

product

of

(cf.

by

solid

ring

most

E

plausible

most

enlargement

type 4)

state

(model).

(sterically

from

the

the

With favourable

of

. Finally

vinyl-cyclobutene

of

Br

the key

cyclointer-

sterically

least

2.

rearranges

to -11

mechanism

for

wi+.iA con-

this

transfocma-

2'

bromination,

crystallization

-

in 5

vinylcyclobutene-hexatriene-cyclohexadiene

loss

13

to g

cation

the

the

electrocyclic

-dibromo-hexatrienes

tional

in

modes

stereospecifically

of

150'

elimination

sequence

c,o

formation

beyond

involves

The

in

with

heated

comitant tion

adds

opening

- u

however, which

is a mixture

were

obtained

in

of

stereoisomeric

pure

form

by

frac-

table).

Table NMR

WV (Dioxane)

145O (dec.)

206~

-

v-CH=CH(trans) cm

980 162O

Topologically tion

of

under

1) There

cpds.

intermediate

the

against

225O

reaction

this

n

303

17800

336

31000

336

34500

38600

stem

conrotatory

conditions

E

339

-13 - 15. may

2 and

xmax

CDC~,)

JRR=

1.1

r'=

4.02

(2H,s)

JRR=

15.5

Hz

I=

3.08

(211,s)

Hz

-1

"

1

10%

15

(TMS,

from ring

5) . However,

addition opening

various

of of

Br * to

the

vinylcyclobutene

arguments

can

be

3-posi12 raised

interpretation:

is no

convincing

reason

for

a significantly

higher

stability

of -10

158

lo. 2

(isolated) 2)Control

as

experiments

to yield the

cpds.

observed

dencies trast

to

an

the

view

of

z-

this

to

for

the as

come

extent the

BX-additions

Support

of

it

of

the

it

is

Ag'-catalyzed

this

must

of

quite

- probably of

type

This

2

cpds.

ten-

is

should

in

con-

exhibit

a

4) .

likely

that

there

radical

in nature

- 12.

Experiments

12

shift

Therefore,

relative

in l-position

therefore

most

the

respectively.

addition is

1,3-bromine

conditions.

reflect

intermediate

by

conclusion

that

ionic

associated

- is

bicyclopropenyl

certain

work

reaction

a

exists

- which concerned

progress.

to

that

and

an

nucleophilic

in

experience

or a-position

that

formation

not

the

table)

of bromination

are

we

does

under

(cf.

in l-

observed).

2

lC)

arguments

the

problem

that

(via

Bre

pathway

summary

pattern for

add

these

for

(not

10/13-z

preference

responsible

In

- a

expectation

alternative

with

13

to

to 12

showed

ratio

of 2

strong In

compared

types

the

of

Br2 -induced

the

same

4) . The

rearrangement

cycloaddition

Deutsche

with

rearrangement

of

topological same

holds

6) .

Forschungsgemeinschaft

is

gratefully

acknowledged.

LITERATURE

1) Reactions St.Andrae,

of coupled 3-membered .8ngew,Chem. in press T.Scott

Chem.Rev.

VI. z

Part

R.WeiS

181

(1972)

and

Lawrence The parent H.W.Chang

4)

R.WeiD

5)

In this context, no mechanistic conclusions can be drawn from the stereo-chemistry of compounds g - fi because Br -induced cis-transisomerizationa cannot be excluded under the reac 5 ion conditions.

6)

R.WeiD

compound is unknown. Derivatives: and L.J.Altmann, J.Amer.Chem.Soc., St.Andrae,

H.P.Kempcke,

Angew.Chem.,

to

be

Int.Ed.Engl.

published.

(2),

V:

2)

and

M.Jones,Jr.,

part

3)

and

and

rings,

R.Breslow, 3, 5139 -12

P.Gal, (1965)

(2),

150,153

(1973)

is