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 the 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 TrCHCH=CH* 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.
|Number of pages||8|
|Journal||Journal of the Chemical Society, Perkin Transactions 1|
|Publication status||Published - 1995|
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