Thermal and photochemical α-carbon–carbon cleavages in imines

Abstract

Autoxidation produced cyclohexyl isocyanide and benzoic acid from N-cyclohexyl-1,3-diphenylpropan-2-imine (16b), and 6-cyclohexyaminodibenzo[a,c]cyclohepten-5-one (24) from N-cyclohexyl-5,7-dihydrodibenzo[a,c]cyclohepten-6-imine (17b). The α,α-cleavage required for the formation of the isocyanide was attributed to a ready fragmentation of the intermediate 4-cyclohexylimino-3,5-diphenyl-1,2-dioxolan-3-ol (22).

Photolysis of 3,4-dihydro-3-imino-1,9-di-t-butyl-1,4-ethenonaphthalen-2(1H)-ones (14) and of 3-imino-2,2,4,4-tetramethylcyclobutanones (15) resulted in extrusion of isocyanides and carbon monoxide. Other products included 1,3-di-t-butylnaphthalene from (14), tetramethylethylene from (15), an adduct (38) of tetra-methylcyclopropanone and furan (solvent) from (15b and c) and 2-isopropylidene-4,4-dimethyl-3-phenyliminooxetan (39) from (15c). Irradiation at 254 nm transformed the imine (16b) into 1,2-diphenylethane and N-cyclohexyl-2-phenylvinylideneamine (43) but had no effect on the N-cyclohexylthujan-3-imine (18) in cyclohexane or isopropyl alcohol. The N-unsubstituted five- and seven-membered ring α-cyanocycloalkanimines were inert to irradiation at 254 nm.

Reactions in the mass spectrometer correlated with thermal and photolytic extrusion of the C₂O₂ bridge from 1,4-dihydro-1,9-di-t-butyl-1,4-ethenonaphthalene-2,3-dione (9), with photolytic extrusion of the RNC₂O bridge from the corresponding monoimines (14), and with the photolysis of the imine (16b) to give the vinylideneamine (43). An intense M⁺– CO peak was also obtained for the diketone (9). Isocyanides were detected as RNC⁺ from the imines (14a—e), (15b), (17b), and (18b).