The problem of reductive coupling of cyclopropenium cations. Bicyclopropenylsulfones - synthesis and properties

Tetrahedron

Letters

THE

No.

48,

PROBLEM

pp

OF

4827 - 4830.

REDUCTIVE

COUPLING

BICYCLOPROPENYLSULFONES R. Institut (Received Because

of

system2-5) system.

We

of

study.

this In

vious tive

became

the

general

on

many

Chemie

der

some

to

according

R,

in

to

unexpected

-%

/

cations the

we

cyclopropenipm

made

in

salts

bicyclopropenyl

of the

the

system

this course

most

is

by

ob-

reduc-

(1):

ZnX2

+

Mg

is

too

low:

R,=H,

identifiable

R2=R9=,Ph) product

is

cases

in

order

metals

less

of

Zn

&

to or

kind. rather

Mg

about

than

bicyclopropenyl OS27

2)):

(1)

was &

where

reasons

strongly of

of

1

metals

strongly

these

. 2) For

however

cyclopropenium

triphenylcyclopropenium

studied

(z:R,=H, system

of

potential

required;

6)

case

1) for

reduction

are

the

single

a number

sufficient-but

This than

are

cit.

2 formed

are

the

reduction

bring

stable

and

lot.

the

of

to

there

amalgames

amount

or

2) . The

limited

(cf.

R,=R2=R,,=NR2)

a different where

far

observations

destraythe

equally

of

our

alkali

potentials are

so

other

(e.g.

like

are

in

1

irreversibly that

to

fail

agents

reduction

reaction

According

and

encountered (1:

bicyclopropenyl

synthesis

e.g.

this

species

reducing to

the

‘R3

reactions

stabilized

tend

of

2.

R,=R2=R3=Ph2).

En

Miinchen

efficient

X8

Unfortunately

like

KlSlbl

UniversitPt

and

1

the

1)

,,PQfR2

R3

why

H.

CATIONS.

PROPERTIES

observations

accessible

eq.

AND

properties

the

in Great Britain.

in UK forpublication 18 October 1973)

a simple

synthesize

Printed

CYCLOPROPENIUM

and

rearrangement

readily

strategy

coupling

Schlierf

C.

interested

report

of

OF

in Germany 29 August 1973; received

here

view

WeiD,

remarkable

Press.

- SYNTHESIS

fiir Organische

the

we

Pergamon

now in

the

some

R2=Rg=Ph) is

known

to

ion

difficulties

detail

for

-la

is

the

only

be

particularly

No. 48

4828

prone

to

rearrangement

independently fluence tion

of

they

starting

in

the

felt

a different tion

be

ment.

ditropyl

2

0

must

able

to

be

to

ZnX2

and

rearrange

- with

do

not

an

isomeric

e

allow

with

the

surprisingly

both

under

being

one

to

conclude

one

synthesis sources

-

the

of

the

inreac-

whether system

bicyclopropenyl

bicyclopropenyl cope

- not

on

the

2)

basis

of

self-destruction

be

overcome

of

system.

reducing

we

into found

according

part

the

agent

an

difficulties

which

effective

that to

of

the

eq.

would

catalyst

tropylium

cation

could

not

in

for

bicyclopropenyl

is

the

course

of

by

using

the

reac-

rearrangereduced

quantitatively

(2):

2 so,

+ Na2S20’

2NaX

(2’

X0 , C@ one

choice

tested

for

action

proceeded

for

would

expect

reductive

triphenyl-

and

according

then

that

coupling

dithionite

of

to

eq.

(3)

ion

ought

cyclopropenium

diphenylcyclopropenium

R + Na2S20k Ph’

shown

or

any

of

analogy of

&

that

type

X0 = Bre agent

be

results

yields

least

+

By

these

at

0

agents3-5)

Lewis-acids

that

Earlier

could

a warning

transformed

to

and

of B

coupling

inherent

g

Although

clearly

reductive

We

2

coupling

are

electrophilic

synthesized

products.

reductive

by

salts

to

be

the

re-

salts.

When

this

found

that

the

we

was re-

instead:

~ph)j$02~ph;;~x

13)

2h,abs. 3

X8

Ph

Ph 3

X8

After

= Bre , ctop

appropriate

a) R= Ph, b) R= H

work-up

the

first

representatives

of

the

hitherto

un-

No. 48

4829

known

bicyclopropenylsulfones

moderately basis

of

vC=C1820 1810

stable

and

2 1120

vso

cm-',

isolated can

spectroscopic 1380

100,

vso

are

Structures

solids.

analytical cm-',

2

1300

cm-';

cm-';

in

be

(2:

mp.

nmr(CDC13,

yield

unequivocally

data 2:

60-80s

mp.

190°C TMS):

crystalline on

assigned

204OC

(dec.);

(dec.); r=

as

ir(KBr):

ir(KDr):vC

6.33

the

(2H,S);

5

C

uv(CHC13):

2 A

max

= 314

nm

(log

The

mass

spectra

the

corresponding Attempts

penyl

systems

rearranged products. &

6)

of. 2

to use

cpds.

failed

so

completely Photolysis

results

recently

and

are

J&

indicate

cyclopropenium

beyond

and easily

proposed

by

2

far:

of 2

(quantitatively)

These

e = 4.62)).

ions as

(base

(70$,

to

for

the

an

and

formation

corresponding

of

basis

(5:

X

cleanly

scheme

= -C-S,

cpds.

mixture

material) of

bicycopro-

solvents

however,

photochemical

(1,2,3-triphenylcyclopropenyl)-thioketone

SO2

various

polymeric the

of

unidentified

C6H6)

besides on

the in

give

(32Onm,

understood Trost 7)

for

thermolysis

-lOO°C -3b

loss peak).

precursors

upon

and

-6b

facile

2

of

yielded

respectively. I

(pathway

rearrangement R=Ph)

to

542-b of

bis

hexaphenyl-

benzene. Scheme

mixture

I

isomeric

of tetra-

phenylbenzenes

R= H,Ph Ph

Ph

There

is

a reasonable

alternative

Ph

to

this

mechanism

not

discussed

by

4830

No. 48

Trost:

photochemical

mediately

formed

quantitatively of

7)

ref.

reason

R=Ph)

SG2,

R=H)

natives:

rearranges

results are

benxenes8).

gives This

chemical

mode

fully

of

the

of

However,

2

are

this

is

where

by

the

inter-

212. exclusively

in

favour

of

as

intermediate.

of

the

2

a mechanism

(2:

the

two

&

upon

isomeric

X z X =

alter-

tetraphenyl-

with

well

same

(2:

of 2

yields

as

conditions

For

between

bioyclopropenylsulfones

the

taken.

rearrangement

to decide

investigated

reaction

rearrangement

(non-interconverting)

(X=SO2)

being

with

introduced

of

of

work

to which

photochemical

is uniquely

formation

properties

Support

is

as

of

hexaphenylbioyclopropenyl

route

a mixture

result

As

possible

conditions

quadrioyclane-S.S-dioxide The

systems.

rearrangement

the

labeling

reaction

subsequent

under

inconclusive.

under &

is

X with

to hexaphenylbenzene

concerning

sufficient

photolysis

of

bicyolopropenyl

no decision

our

SO2,

extrusion

as

a thia-

their

further.

Deutsche

Forsohungsgemeinschaft

is

grate-

acknowledged.

LITERATURE

1) Reactions 2)

of

R.Breslow,

coupled

P.

Gal,

3-membered H.W.

rings,

Chang

and

Part

L.J.

IV.

Altmann,

Part

R.WeiO

and

C!. Schlierf,

4)

R.WeiD

and

St,

Andrae,

Angew.Chem.

f3,

145

(1973)

5)

R.WeiD

and

St.

Andrae,

AnPew.Chem.

85, =_

147

(1973)

6)

R.WeiB

and

G.

7)

B.M.Trost,

R.C.Atkins

8)

R.WeiD

H.KLilbl,

and

Heinz,

Angew.Chem.

unpublished and

L.

unpublished

22,

887

lit. 5)

J.Amer.Chem.Soc. 5139

3)

III:

g2,

(1965)

(1971)

results

Hofmann,

J.Amer.Chem.Soc.

results;

see

also

lit.

21, _2)

1285

(1973)