Cyclohexene apparent trans-addition

The addition of nitrosyl chloride to cyclohexene in sulfur dioxide at low temperature21 -25, or the reaction of cyclohexene with 3-methylbutyl nitrite and hydrochloric acid (producing nitrosyl chloride in situ)25 gave only the tram-adduct 1, isolated in high yield as a crystalline nitroso dimer (azodioxy compound) with E configuration at the N—N double bond. The trans configuration of the chloro and nitroso groups (mixture of diaxial and diequatorial conform-ers) was deduced by IR and H-NMR analyses (a mixture of conformers with both axial and both equatorial substituents is apparent)23-25 and by reduction with lithium aluminum hydride in tetrahydrofuran to yield tram-2-chloro-l-cyclohexanamine (trans-2)23. 

Sensitized irradiation of cyclohexenes and cycloheptenes in protic media results in protonation. This phenomenon, which is not shared by other acyclic or cyclic olefins, has been attributed to ground-state protonation of a highly strained tran -cycloalkene intermediate. In aprotic media, either direct or triplet-sensitized irradiation of cyclohexene produces a stereoisomeric mixture of (2 -I- 2] dimers 49-51 as the primary products, with 50 predominating. The reaction apparently involves an initial cis-trans photoisomerization of cyclohexene followed by a nonstereospecific nonconcerted ground-state cycloaddition, promoted by the high degree of strain involved. In contrast, cycloheptene undergoes only a slow addition to the p-xylene used as sensitizer. 

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