Three positional isomers of substituted triphenylmethanes from reactions of trityl anion with 1-acyl-2,2-dimethylaziridines

Abstract

Ring opening of aziridines 4a–d in reactions with trityl anion Tr^– proceeds exclusively by cleavage of the NCMe₂ bond. Substitution of the benzylic carbon of Tr^– leads to ‘central’ products 10a–d in yields of 0-5%. This is ascribed to an S_N2 reaction with borderline character, as is well known from reactions of aziridines 4a–d with other nucleophiles. All remaining ring-opening reactions result from single-electron transfer (SET). This is direct SET from Tr^– to aziridines 4a–c. For compound 4d (acyl = cinnamoyl), the SET reaction is of the innersphere type and proceeds via Michael addition, at least in part. Homolytic ring opening of the generated aziridino ketyls 5 forms the tertiary amidatoalkyl radicals 6. Main reaction of radicals 6a–c is transfer of a hydrogen atom from one of its two methyl groups to die generated trityl radical Tr˙. Methallylamides 7 and enamides 8 are the final products. ortho-Substituted triphenylmethanes 12 and/or its olefinic precursors 13 arise in ∼ 20% yield. A mechanism for the formation of these unique products is proposed that first converts the radicals 6 into the corresponding carbanions 16 which undergo an S_N2′ reaction with one allylic system TrCHCHCH* of the dimer 14 of Tr˙. The leaving group Tr^– is eliminated from this partial structure when carbanions 16 attack the marked carbon converting it finally into the substituted ortho carbon of compounds 12. Addition of radicals 6 to Tr^– is probably the way to the para-substituted triphenylmethanes 11, which arise in yields of only 0–1% from aziridines 4a,b (acyl = 4-benzoyl, pivaloyl). Higher yields of para-substituted compounds 11 are obtained from aziridines 4c (acyl = 4-phenylbenzoyl) and 4d. This is ascribed, at least for substrate 4c, to a chain reaction because ketyl 5c must be formed more rapidly than ketyls 5a,b. A substantial part of radical 6d cyclizes, ending up as the triphenylmethane compound 26 that carries a pyrrolidone ring in the para position.