Photoreactions of Nitroso Compounds in Solution. XXI. Stereochemical Course of Photoaddition of Nitrosamines to some Cyclic Olefins and Conformational Isomers Generated by A1,3-Interaction

The results of the photoadditions of N-nitrosopiperidine and N-nitrosodimethylamine to three substituted cyclohexenes are discussed in terms of an "electrophilic" free radical attack of the aminium radicals generated from the photolysis of the nitrosamines. The photoaddition to 4-t-butylcyclohexene gave all four possible oximes; the axial approach of the aminium radical was preferred over the equatorial approach. The photoadditions to 3-methylcyclohexene are highly stereoselective giving oxime pairs 1–2 and 4–5. Both oximes in each pair have 2-amino and 6-methyl substituents in trans-relation; but locked in the alternative conformations through A1.3 interaction of the oximino group. In this addition the aminium radical almost exclusively approaches the C-1 carbon of the most stable conformer from the axial side. The steric hindrance to all possible axial and equatorial approaches to conformers of olefins have been compared. Piperidinium radicals, however, abstract an allylic hydrogen from Δ9,10-octaline since the steric hindrance in the addition process is too great to overcome. The hydrogen abstraction process leads to an intermolecular hydrogen–nitroso group exchange reaction which is similar to that in the photolysis of nitrosamides. A pathway is suggested to account for the generation, insitu, of the heteroannular diene 28 from which oxime 17 could be derived by photoaddition in 1,4-mode.