Cyclopropylcarbinyl free radicals, hexacyclopropylethane

T&abdron

Letters

No.46, pp. 4629-4634,1967. Pergamon press

Ltd.

Rinted in Great Britain.

CTCLCPROPTLCARBIRTLFREE RADICALS, BBXACTCLOPROPTLBTRANB J.

C. Martin,

John E. Schultz,

Department

of Chemistry, Urbana,

(Received Several

cases

the

introduction

not

clear,

have of

reaction

from relief

of

polarization semblance

strain

of

to carbonium

the

large

ions

to cyclopropylcarbinyl composition

state

reactions

in which

related

to cleavage

effects

seem to be unimportant. Overherger

of

the

(AIBN, In). yield

Since

in solution

observed

acceleration

*A three-bond propane ring,

to the

only

decomposition could

of

could

produce

reports

paper

associated

with

with

times

the

evidence

faster rings

possibility

to a concerted

cleavage (similar to that illustrated by V) would yield one radical with the cyclopropa:e

4629

than

intact remains cleavage

force state

toluene)

of

small. leading

leading

a series

charge

charge

a re-

A very

substituents

(80°, that

a driving

reactions

for

state

transition

bond

in reactions

for

is

multiple

ion.

seen

by

It

reflecting

carbonium

cyclopropyl

center.

Transition

available

decomposition

cyclopropyl

Ib (A),

be attributed

the

ring.

substituent

kinetic

and in which

reflect

an acceleration

data

accelerated

radical

might

cyclopropyl

rate

are

A concerted

of a three-membered

This

report

incipient

system

the

rings

reactions

explained.

to explain

to be twenty

product

best

of s cyclopropyl

acceleration

(1 g,h)

(Ib) the

(3).

1967)

on the

corresponding

effect

cyclopropyl

and Berenbaum

2-cyclopropylpropionitrile

opening reaction

be sufficient radicals

directly

allylcsrbinyl

the

Illinois

radical-forming

to the

accelerating

would

in which

is

accompanying

of

61801

acceleration

directly

transition

Illinois

substituent

how the

University

in USA 10 July

(I)

in a radical-forming

of the

fraction

just leading

(2)

reported

a cyclopropyl

however,

cleavage

been

and Jack W. Timberlake

of deis

not

polarization

2-2’-azobis-

of azobisisobutyronitrile (III) that

is

formed

all*

in only

or most

of

of C-N and C-C bonds.

involving only ring intact.

a single

cyclo-

19% the In

46X

Co.46

certain

other cases acceleration

thought

to be the result

of

in the rate

“manifestations

of

formation

of

cyclopropylcarbinyl

of the ring strain

only”

radicals

is

(5).

Ib

TABLE 1 Decomposition

of Azo Compounds

f‘l

r1

-N=B-C-R

R2 - f:

I

R3

2

R3

In Toluene at 80.2'

Compound

Ia

Rl

R2

R3

k,Se?

relative

x lo4

rates

1.4sa

CN

Ib

cH3 rn3 cH3 cycle-C3H5

CN

Ic

_eyclo-C3H5 cycle-C3H5

CN

AH*

1.0

37.sb

30.7 ;tO.la 10.4 + 0.2'

25.8 240

347c

AG*

As*

27 .o

27.7 2 0.4b 8.4 + L2b

24.7

23.7 ;f0.7' 1.5 f 2.5'

23.2

In Diphenyletherat 135.0' IIa fib

CH3 cycle-C3H5

CH3

lx3

0.0094

m3

cH3

0.252

CH3

3.41

IIC cyclo-c3115cp&-C3H5

XII cycle-C3H5 cycle-C3H5c&lo-C3H5 23.9 IIe ~J~&-C~H~

cycLo-C3H5

1.0

42.2 t_0.3' 16.2 + O.fid 35.6

26.8

37.8 2 0,3e 12.4 + 0,6e

32.7

362

35.6 i 0.2f 12.1 + 0,6f

30.6

2,540

34.3 + 0.38 L2.8 + 0.88

29.1

286

38.0 + 0-P 17.6 + 0.6h

30.8

A-C3H7 2.70

aCalculated

from data

by Y. P. Van Hook and A. V. Tobolsky,

bCalculated

from data

in reference

between 2000.

15 and 45o. Values

dCalcuLated

of 42.3

2 0.8

using

the gas phase rate

K.,

84,

and 175”. gcalculated

2922 (1962). f Calculated

‘Calculated

AH* and 17.5 + 1.5

constants %lculated

determined

determined

determined for

determined rate

rate

80,

determined constants

rate

determined rate

constants

between

rate

constants between

120 and 147”.

779 (1958). rate

between

constants 165 and

from calculations

by A. U. Blaekham and N. L. Eatough,

determined

constants

&I. _-C&em. SQC.,

AS* were obtained

from experimentally

from experimentally

2.

from experimentally

from experimentally

for

from experimentally

from experimentally

lh.

constants

between

2.

&.

between

120 and 150°.

105 and 135O.

hCafculated

Chew 145

4631

No.46 Results

of our

result

from the

If

driving

the

studies

substitution force

maximum additional of two for involved,

on the

for

of the

rate

a second

involving

4,

seem unlikely.

_

and makes

between

between

of

of cyclopropyl

9.3,

cleavage

F’N-N--~~ *-- 6-

than

greater sole

= ---

able

cyanomethyl

for

of Ic in

the

78% of the material

radical

in which

both

presence

mp 78-79’),

were

cyclopropyl

LN

sembling

identified

from

their

mp 139-140’)

extent

radical

that

decompositions

products,

of replacing

each

and Ic is

roughly

AG* by 2.3 The nature

which the

uould

highly

of

the

elemental

pair

these

of a type

energies

with

adjacent

of methyl

additive

in

and the

density

transition expected

groups free

analyses

of hexaphenylethane of

the

we are

dicyclopropyl-

The products,

intact.

and infrared

2,2-

and nmr spectra.

state.

with

through

a pair

transition

1 reflect

centers.

the It

pair

states

stabilizing

can be seen

of cyclopropyl

The first

(25%)

groups

of cyclopropyl

reinter-

that

the

in the

series

substituents

by 1.5 kcal/mole.

might, from

in Table radical

VIII

(53%)

proceed

energy.

second

interactions

electron

polarizable

of azonitriles

activation

substftuents

kcal/mole

remove

IX and IL, is

force

and tetracyclopropylsuccinonitrile

VII

effect

lowers

driving

hexaphenylethaneO

of cyclopropyl

Ib,

of the

LN

action

Ia,

simultaneously

the maximum statistical

products

are maintained

Ic To the

(a factor

%

excess

as reaction

rings (VII,

V

&Y-N=N-~4

of a slight

decomposed

dicyclopropyl-3,3,3_triphenylpropionitrile (VIII,

were

the

VI

decomposition

to account

N---C__ ‘4

then

in origin

two rings

r!N

V From the

cleavage,

provider

LN

CN

c!N

is

to

the AIBN molecule.

be statistical if

--C--N

acceleration

ring

V, or four

as the

J

into

from concerted

of one ring,

ring

rate

groups

Ib and Xc would

Ib and Ic,

concerted

Ic show a similar

originates

cleavage

The difference

consideration,

pair

decomposition

acceleration

a mechanism VI) .

decomposition

the

however, central

The first

when a bond

joining

involve carbon type,

charge atom of

represented

two atoms

polarization the

incipient

by resonance

of different

of two types radical

in

structures

electronegativity

No.46

4632 is

stretched

of charge

or when an electronegative

polarization

is

atom abstracts

a possible

factor

have been drawn relative

to the stability

cases

might be expected

where polarization

attributable able

to interactions

(5).

of both

unlikely

that

Contributions

of

C-N bonds

transition

(6).

type

II

presents

state

of

from a C-H bond

the kinetic

studies

cyclopropylcarbinyl

not

to be large,

groups

a case

thought

in which

is much greater

IX and X place

negative

suggest ion

type

+RN-N-R

+--a

charge

polarization, is

resemblance

In order

contributor

of

aZObi8nitrile8,

a possible

(3,7).

series

of

formation

or essentially

on adjacent

undetect-

by simultaneous

considerations

make it

seem

polarization.

nitrogen

atoms and

-+ +----)

R-N==N

IX

decomposition

in creveral

radical

than ground state

charge

8Ort

delimiting.

[R-N-N-R

The second

of

to proceed

sysssetry

lhi6

In fact,

accelerations (la),

(2).

from which argument8

radicals.

are very minor

azo compounds,

polarieation

from structures

would be mutually

of

of cyclopropyl

The decomposition8

cleavage

in all

hydrogen

of

to accelerations

of aso compounds of

X

which might be expected

illustrated

by resonance

the transition

state

the

importance

of

seen for

Ib and Ic,

we studied

to determine

type

II,

lacking

R]*

to

this

the cyan0

to be nresent

structures

the very type

XIII

stable

and XIV.

cyclopropyl

of polarization

the rates

function.

in These carbonium

as a possible

of decomposition

Compounds IIa

(8);

for

a

IIb,

+

NXI bp 43-45’

(hum);

IIc,

prepared

by oxidative

Complete

synthetic

factory

elemental

were determined added isoquinoline of each pair

XIII

XII bp 80-81’ coupling procedures analyses

(0.25~~s); of will

mp 17-18.5’;

the corresponding be reported

as a stabilizer

range against

substituents

105-200’

spectra

iodine

pentafluoride In all

were obtained. ether

with

cases

(8). satis-

The decomposition

rates

a small

amount of

Again

the addition

decomposition.

an approximately

were

mp 45.5-46.5’,

publication.

in diphenyl

acid-catalyzed produces

and IIe,

amines with

in a later

and nmr and infrared

in the temperature

of cyclopropyl

IId,

XIV

additive

change

in the free

4633

No.P6 energy of activation.

The AAG’ for progression

2.1 kcal/mole;

to IId,

be the result

and IIc

of a saturation

ance as the number of cyclopropyl and free

energy of activation

that the increase in steric

repulsion

Consideration acceleration

in compounds IIc

in the two series

of Table

cyclopropyl

1 indicates

1 provides

from strict

The close

similarity

provides IIb.

and IId

that the acceleration

at the central

a stabilizing

interaction

carbon in the transition

between the cyclopropyl

more than 80% of l,l-dicyclopropyl-1-butene The latter

l thane .

in 402 yield

compoundwas identified

from photolysis

lexacyclopropylethane -3 -1 1.31 x 10 set .

state,

as in XIII

substituent

important and (b) radicals

is small,

(A8

for III,

out ring

The estimate while

larger

estimated consistent although

cleavage for

of a cyclopropane

concerted with central

the bond dissociation

than that of hexaphenylethane,

value of 67.5 kcal/moIe for with the postulate the differences

ring

(9).

effects

are un-

of two tricyclopropylmethyl the bond dissociation

suggests It

is,

that decomposition

however, impossible

C-C bond cleavage.

energy of hexscyclopropytethane, 15 kcal/mole,

is considerably

2,2,3,3_tetramethylbutane

that tricyclopropylmethyl

in bond dissociation

Using

into two tricyclo-

solvation

of 45 kcal/mole for

The low value of this estimate

rate constant of

of 41.5 kcal/mole.

for recombination

at a rough estimate

does not proceed by simple cleavage

sample prepared

with a first-order

and assuming that (a)

that the energy of activation we arrive

center.

at 0'.

of hexacyclopropylethane

l),

radical

in 1,4-cyclohexadiene

to a free energy of-activation

Table

carbonium

or XIV, but reflects

by comparison with an authentic

the dissociation

energy of hexacyclopropylethane.

to rule

that

seen in &W* values

and a developing (IId)

decomposes at 295’ in decalin

This corresponds

radicals

reasoning

and a small amount (2%) of hexacyclopropyl-

of a sample of III

a value of 13 eu for MS for propylmethyl

state.

of decomposition rate seen on

Decomposition of l,l,l,l’.l’,l’-hexacyclopropylazomethsne yields

a contention

for methyl in .an azornethane does not depend on developing

ion character

rates

is due to an increase

and the similarity

of ping strain,

of reson-

in relative

in the transition

an argument against

to IIc,

in AG* could

inhibition

evidence agafnst

IIc,

III

linearity

steric

that is relieved

of the data in Table for by relief

IIa,

is 2.9 kcsl/mole;

an increasing

and IIe

in the series

in the ground state

is provided

substituting

or, more likely,

groups is increased.

in rate observed

to III

The deviation

1.5 kcal/mole. effect

from IIa

radicals

(10).

45 kcal/mole,

less

than the

Ihese results

are

are resonance stabilized,

energy may, in part,

be attributed

to differences

4634

No.46

in steric interactions. Acknowledgement. -

The work was supported in part by a grant from the U. S. Public

Health Service (GM-12296) and in part by a grant from the U. S. Army Research Office (Durham). References 1.

(a)

D. C. Neckers, A. P. Schaap, and J. Hardy, 2. &.

(b)

E. S. Huyser and D. T. Wang, 2. w.

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76, 2722 (1954);

3.

88, 1265 (1966);

(dF

z)

84, 3697 (1962); (hr

E. S. Huyser

C. Walling and P. S. Fredricks, 2.

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H. Hart and D. P. Wjman, u.,

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C. G. Ovzberger

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Chem. E.,

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C. G. Overberger, M. Tobkes, and A. Zweig, J. 9.

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6.

S. Seltzer, 2. &.

a.

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T. E. Stevens, 2. a.

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