Petros Assithianakis

Nucleophilic cleavage of 2,2-dimethylaziridines: competition between SN2 and postulated “SET” mechanism.

Abstract Regioselectivity of nucleophilic attack on 2,2-dimethylaziridines depends on the degree of leaving group activation: in highly activated aziridines it occurs at the methylene carbon and in less activated at the tertiary carbon. This latter abnormal ring opening is explained by an SET mechanism.

Electron attachment to N-benzoylaziridines followed by C−N homolysis of the aziridine ring

Reactions of N-benzoylaziridines with strong electron sources provided direct evidence for the formation of ketyls (2) and radicals (3), both of which are postulated intermediates in the single electron transfer mechanism of nucleophilic ring opening of activated aziridines.

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

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 SN2 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 SN2′ 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.

Radical combination in the ortho position of trityl radical observed in single-electron transfer reactions of trityl anion

Single-electron transfer reactions between trityl anion and 1-acyl-2,2-dimethylaziridines provide, among other products, the methallyl amides 7 and the triphenylmethanes 8 carrying an amidoethyl chain attached with the tertiary carbon ortho to the triphenylmethane.