PHOTOOXYGENATION OF 1,4-DIAZA-1,3-BUTADIENES (ALPHA-DIIMINES) - FORMATION OF ISONITRILES

Dye-sensitized photooxygenations of cyclic and acyclic 1,4-diaza-1,3-butadienes (alpha-diimines), carrying a methyl or an ethyl group in the 2-position, yield isonitriles and amides, as well as formaldehyde or acetaldehyde. Thus, cyclic alpha-diimines such as 5,6-dimethyl- (1a), 5-methyl-6-phenyl- (1b), 5-methyl-6-(p-tolyl)- (1c), and 5,6-diethyl-2,3-dihydropyrazine (1d) yield the respective 1-isocyano-2-(acylamino)ethanes 2a-d, whereas 5-isopropyl-6-phenyl- (1e) and 5,6-diphenyl-2,3-dihydropyrazine (1f) are inert toward photooxygenation. Acyclic alpha-diimines such as 1,8-bis(acetylamino)-3,6-diaza-4,5-dimethyl-3,5-octadiene (4a), 2,5-diaza-3,4-dimethyl-2,4-hexadiene (4b), 1,4-diaza-2,3-dimethyl-1,4-diphenyl-1,3-butadiene (4c), and N,N'-bis[(1S,2S,3S,5R)-pinan-3-ylmethyl]-2,3-butanediimine (4d) afford the respective isonitriles 5a-d and acetamides 6a-d. With retention of configuration, chiral alpha-diimines such as (-)-N,N'-bis[(S)-1-phenylethyl]-2,3-butanediimine [(S,S)-4e] and its R,R enantiomer [(R,R,)-4e], and (3S,8S)-(-)-3,8-dicarbomethoxy-4,7-diaza-2,5,6,9-tetramethyl-4,6-decadiene [(S,S)-4f] yield the corresponding enantiomerically pure isonitriles (S)-5e, (R)-5e, and (S)-5f and acetamides (S)-6e, (R)-6e, and (S)-6f. Evidence for a singlet oxygen reaction is presented. A working hypothesis is proposed which includes the formation of a transient hydroperoxide 7 that fragments into an isonitrile molecule, an aldehyde, and an N-alkyl- or N-arylnitrilium cation and a hydroxide ion; the latter combine to yield the amide groups in 2a-d and the amide compounds 6a-f.