The stevens rearrangement in aminimides

Tetrahedron

Letters

No.

37,

pp.

3479

-

Pergamon Press.

3482,1971.

Printed

in Great

Britain.

* THE

STEVENS Herman

REARRANGEMENT P.

Department

benzyl

chemically

region

induced

of the Stevens

benzylamine-2-acetimide reaction

pathway.

mechanism

must

We have

1

However,

PNOZ

further

A -

dynamic

product

(1).

or only

has

31%

yield

(4).

investigated

Aminimide

when

Wikel

University

(CIDNP)

thermolysis

of 1, l-dimethyl-l-p-nitro-

a diradical recently

pathway

criticized

the

when

of the Stevens

inner

salt yield

Kinetic

(2) and

?zY

t

rN- CH,9

A

author.

Current PA

t -

address:

Department

3479

yHs9-PNQ

utilizing

1, 4, 4-

1, I-dimethyl-I-benzylamine-2-

whereas

aminimide

data

shown

are

2 produced

in Table

1.

CHs

g FH” CHs-C-N-N(CH,), 5

18703.

type

CHs-C-N-N(CH,),

rearrangement

4

Primary

that this 2 is observed.

CHs

A

in the

to be the only

I

9 -+FJW

Wilkes-Barre,

assumed

0

CH,-C-N-N(CH,),

t

reported

assumption

CIDNP

0

CH

was

was

0 lCH,9-pN0, II . . . CH,-C-N-N(CH,),

2 in 100%

in nitrobenzene.

mechanism

operative

the mechanism

4. produced

decomposed

CH,

from

14 August 1971)

polarization

0 lCH,9-pN0, II . + CHs-C-N-N(CH,), -

trimethyl-I-benzyl-3-oxapyrazolidinium acetimide

H.

Marshall

nuclear

resulting

Therefore,

Jacobus

be the major

0 CH,9II +/ CH,-C-N-N(CH,), 1

and James

of Chemistry,

Huntington, West Virginia 25701 in uSA 11 June 1971; received in UK for publication

(Received Recently,

Benecke

IN AMINIMIDES

of Chemistry,

Wilkes

College,

2 in only

3480

No. 37

TABLE Rate

Data

Aminimide

Decomposed

(a)

benzene Rates

(b)

The

with

An X-ray

zimide

4

0.007

of formation

associated z_

1.7

(6) is

increase

the change

study

has

A-60-A

nmr

products

extrapolated

with 2 is probably

in hybridization that the

3

However,

caused

of the a-nitrogen imide nmr

nitrogen evidence

atom

atom

- t

the radical

pair

concomitant would

rehybridization

was

C% CH,

partially

bonded

of z would

radical

2 would

from

described The

to relieve

atom)

between

(I)

2 and

p-bromobenatom

is sp3 hybridized.

c-

of 4

y-CWW

assumed

bonding atom,

of the benzyl then no rate

to the u-nitrogen

be markedly

also

be more

ring

strain.

state atom

enhanced. tightly This

from

radical

and this

only formation

explanation

atom,

due to relief

the product

atom

in trapping

radicals

(BTCM)

% benzyl

from

forming

radicals

2 and 4 were toluene

trapped

trapped

and benzyl

with varying

with

bromide

concentrations

benzyl

thiol

that the migrating radical

from

experiments

(BT)

respectively. of scavenger

no

the benzyl

rehybridized,

than the migrating

is realized

with

of

of strain

in which

had partially indicates

of a

significantly

involved

to the a-nitrogen

acceleration

to substrate

expectation

state

resemblc,d

This

bound

the angles

sp2 to sp3 would

If the transition

if the transition

Since

to be sp3 hybridized.

in hybridization

system.

I-

N(CH,), t

below. migrating

chloromethane the mole

also

change

of that

However,

then the rate

4 in order

this

of the ring

and no germinant

be observed.

radical

IOS”,

strain

(imide

that the a-nitrogen

7

of 2 and 2 are

are

the angle

nitro-

of strain

of trimethylamine

indicates

a pentagon

by the relief

iodide

pBr-cp-!-N-N(CH,)3

regular

in purified

0

0

decrease

spectrometer

to 170”.

1, 1,4,4-tetramethyl-2-~-methylbenzyl-3-oxapyrazolidinium

atoms

Constants

1

in Varian

associated

indicated

sp2 hybridized.

The u-nitrogen

Rate

(degassed).

of Stevens

rate

Relative

240

temperatures

vacuum

-2 and -4 in Nitrobenzenea b

(mm-i)

k Stevens

at three

fold

Aminimides

-2

under

240

from

I

and bromotri-

Figure from

1 indicates z and _* 4

3481

No. 37

FIGURE Mole ‘% Benzyl Radicals

1

Trapped

With Scavenger

s--BT 2 -0

Mole % Trapped

4 - 6M BT (98%) 6M BTCM (84%)

= -BTCM

Molar

Concentration

of Scavenger

. Rearrangements were The initial concentration of aminimide is 0.05 M in chloroform. Each performed in sealed tubes and analyzed by vpc techniques using an internal standard. An increase aminimide was decomposed for at least 5 half lives (150” for z and 185 o for 2). in temperature had no effect on the 70 radicals trapped at constant scavenger concentration. Products were stable at the reaction conditions.

The mole

% benzyl

radicals

BT at low concentrations steadily

increases

the reactivity trations

radius

of each scavenger,

ability

of the hydrogen

currently

atom compared

70 benzyl

(after correction

germinant terminal

BTCM

radicals

factors:

cage.

to 2,

atom.

out at this value. this value more

6

but

The maximum of scavenger

Crossover

in trapping from 2 forms

than in 2.

Van der Waals

% of radicals

is approximately

experiments

are

is 98% and 84% respectively

efficiency

values from

can be attributed

a tighter

radical

2 are

to two

pair due to increased

(2) Since the ring strain. 7 the solvation by chloro-

about the C-N bond,

Due to the decreased

would encounter

of

accurately.

atom in order to relieve rotating

at all concen-

in terms

of the smaller

The corresponding

rearrangement).

radical

atom of 4 is rapidly

the scavenger

can be explained

from 4 by 6M BT and BTCM

bonding with the u-nitrogen nitrogen

form at this point should be less compared

to the bromide

This increase

(1) The benzyl

out for BTCM

should be higher for BTCM cross

cage of g in the absence

levels

for the 69% Curtius

84% and 43% respectively. possible

% trapped

into the solvent cage as a result

being conducted to determine

The mole

The curve then levels

of BTCM is 54 k. cal. /mole whereas the 5 If bond strengths alone reflected 75 k. cal. /mole.

the mole

from the solvent

43% since the curve from

is higher than the amount trapped by

bond strength

The fact that the two curves

of BT to intrude

which would escape

BTCM

(up to 0. 6M).

The C-Br

of BT is approximately

of scavenger.

increased

of scavenger

with BT.

S-H bond strength

trapped from &by

less

resistance

ordering

of solvent

in intruding into the radical

3482

No. 37

The trapping is entirely

radical

experiments in nature.

since hexachloroethane radicals

was detected

trapped from -2 increased

instead of chloroform. radical

strongly

Further

information

solvent

from

causing

concerning

CIDNP was detected protons

emission

benzyl and N-methyl

methylene times

protons

larger

an increased

protons

nitrogen

on that atom can preferentially

potential

coupling

states,

i.e.,

the benzyl

radical

consistent

with this explanation.

at 130”,

Wadsworth

of the benzyl 8

was obtained from

and 150”. at 150”.

intensity

that the asymmetric

protons

the study of

The duration However,

of

the ring

environment so that the lone

with the longest

held rigidly

and

was three

after initial bond dissociation

couple with those protons

those methylene

The %

of 2 but also in the N-methyl

140”,

above the plane of the ring on the same

species

was used in nitrobenzene

lifetime.

and the relative

This indicates

of alkyl groups also involves

Acknowledgments:

radical

was three minutes

atom is maintained

in aminimides

was used.

of a n-complex

rearrangement

for four minutes

electron

rearrangement

to the formation

the Stevens

than the other two signals.

about the quaternary

yields when BTCM

35% to 62% when IM BTCM

when g was decomposed

showed emission

rearrangement

than anions are the abstracting

not only in the benzyl protons1

ring methylene from

that the Stevens rather

in significant

This is attributed

with the aromatic

CIDNP.

suggest

Benzyl radicals

lifetime

by the ring.

Migration

side from which it dissociated

has shown that the intramolecular attack from the side of dissociation.

in of

is

Stevens 9

The authors thank Professor Arthur Lepley for helpful correspondence The assistance of an NSF grant administered by concerning CIDNP. Marshall University is greatly appreciated.

References:

* 1. 2. 3. 4. 5. 6.

7. 8. 9.

The Stevens rearrangement is sometimes referred to as the V.awzonek rearrangement. R. W. Jemison and D. G. Morris, Chem. Commun. 1969, 1226. J. Jacobus, Chem. Commun. 1970, 709. Ph.D. Thesis, University of Cincinnati (1969). L. Michaels, All methyl groups are magnetically nonequivalent at room temperature. Absorption8 are of equal intensity which implies slow inversion of the sp” hybridized u-nitrogen. The S-H bond of benzyl thiol should resemble that of phenyl thiol which is 75 k. Cal. /mole. J. A. Kerr, Chem. Rev. 66, 465 (1966). The Van der Waals radii for the hydrogen and bromine atoms are 1.2A” and 1.95A” L. C. Pauling, “Nature of the Chemical Bond, ” 2nd ed., Cornell Univerrespectively. sity Press, New York, 1948, p. 189. University of Cincinnati (1969). H. P. Benecke, Ph.D. Thesis, G. A. Russell, J. Am. Chem. Sot. so, 4987 (1958). W. S. Wadsworth, Jr. and W. Bruxvoort, Chem. Commun. 1968, 542.