TetrahedroB Letten Me. 3, pp 191 - 194, 1975. rer#uon ?re~r. Printed in Groat Britain.

ON THE STRUCTURE OF ACYLAMINYL RADICALS

Charles Brown and Alexander J. Lawson*

Chemical Laboratory, University of Kent, Canterbury, Kent. CT2 7RB

(Reoeired in DK 13 Ilovember 1974; aooepted for publioatian 5 Deoember 1974)

The detailed structure of acylaminyl radicals (such as I) has been the subject of much

debate.L The ESR measurements of Danen and Cellert', however , resolved much of the con-

troversy surrounding the proposed o and v structures, in that the unpaired electron was

deduced to be localised primarily in a nitrogen

clear, however, whether the orbital in question

la, or is located in the nodal plane as in lb2.

orbital of mainly p-character. It is not

conjugates with the carbonyl n-lobes as in

lb

191

192 Ilo. 3

That the latter merits consideration is shown in particular by the work of Bedaya et. at.

who concluded3 that the nitrogen non-bonding electrons may preferentially conjugate with the

carbonyl function(s) in acylaminyl 1 and succinimidyl radicals 2.

From consideration of interactions4 between the nitrogen 2p orbital and the C-1102

sigma bond, lb should have lal,CI values for C-l ard C-2 comparable to one another as in 3,

in which the unpaired electron can with confidence be assigned 4-6 to the nitrogen 2p

orbital in the C-N nodal plane. On the other hand, structure la does not permit extensive

interaction between the unpaired electron and the C-l/C-2 bond, and so this structure should

lead (on the basis of a simple spin-polarisation model) to a substantial value of ler,CI

for 01, but not for G2.7

We wish to report the results of some "C-CIIMP studies (Figure) on the thermal

rearrangement of N-benzoyl-N-methyl-O-thiocarbamoylhydroxylamines which not only confirm

our earlier proposal' that this reaction proceeds via acylaminyl radicals, but which also

allow us to distinguish between structures la and lb for such radicals.

Ihe magnitudes of CIDNP effects depend', inter at+ on the values of the appropriate

hyperfine splitting constants lal. We have therefore compared the polarisations of

nuclei in the products 4 and 5 derived from radical 1 with those 10

in the product6

from the iminyl radical 3 (equations 1 and 2).

l’C

derived

4MeCoH6CONHMe

4

P i CDCl, .O

4-MeC.H.CNOCNMes 1100

l 1 +

S--/L-NMes + (1)

Ae

0 0 4MeC.H~CNStXUea

Ae 5

CDCla

80' e 3 + s/i--NMe,

e -(C.H.)aC=NSCWMea (2)

6

lo. 3 193

10 As reported, both C-l and C-2 Yn 6 exhibit substantial polarisation. In4and 5,

however, although marked polarisatlon of a number of nuclei including C-l is observed, no

polarisation of the C-2 resonance can be detected (Figure). Preliminary measurements

indicate that this is not due to a small Tr value for C-2 in 4 and 5, and the difference in

chemical shift for C-2 in authentic samples of 4 and 5 measured at the reaction temperature

rules out the possibility of fortuitous cancellation of E and A effects in the two products.

These results clearly favour structure la for acylaadnyl radicals, with the unpaired

electron in conjugation with the carbonyl group.

We have also carried out INDD calculations on radicals 1 and 3 to compare with the

results of the CIINP experiments. These calculations confirm that while in 3, \ax.Cl for

C-l and C-2 are of comparable magnitude (cu. -1.7 mT and +1.6 mT respectively) the calcu-

lated values of azsc for the structure la are co. -1.7 mT for C-l and only oa. 0.4 mT for

C-2. For conformr lb the lal.CI values are more comparable (cu. -1.2 and 0.9 respect-

ively).

1. W.C.Danen and R.W.Cellert, J.Amer.Chem.Soc., 2, 6853 (1972) and references therein.

2. The orbital assigned to the lone pair may be regarded as sp2 or pure p (inwhich case

as sp-hybrldised) without appreciably affecting the argment.

3.

the nitrogen is taken

E.Redaya, R.L.Hlnman,

4875 (1967).

V.Schomalcer, S.lbeodoropulos and L.M.Kyle, J.Amer.Chem.Soc., z,

4.

5.

6.

7.

.J.A.Brivati, K.D.J.Root, M.C.R.Sywns and D.J.A.!Hnling, J.Chem.Soc.(A), 1942 (1969).

D.E.Wood, R.V.Lloyd and D.W.Pratt, J.AmeP.Chem.Soc., 92, 4115 (1970).

8.

9.

10.

D.Banhs and

There is m

acylamlnyls

W.B.Anhers,

W.Gordy, IcbZ.Phgs., 2, 1555 (1973).

couqelling evidence for n-delocalisatlon of the unpaired electron in

- see Y.L.Chow and T.C.Joseph, Chsm.Contn., 490 (1969).

C.Brown, R.F.Hudson and A.J.Lawson, J.C.S.Chem.Camn., 935 (1972).

See for example, M.Lehnig and H.Flscher, Z.hkztu.rforsch., 2, 1963 (1970) and

J.I.Morris, R.C.Morrison, D.W.Srdth and J.F.Garst, J.Amer.Chem.sOc., 94, 2406 (1972).

C.Broun, R.F.Rudson and A.J.Lavson, J.Amer.Chem.Soc.. 95, 6500 (1973).

NOTES AND REFERENCES

C-l i,

%

. w