SINGLE-ELECTRON TRANSFER VERSUS NUCLEOPHILIC RING-OPENING IN REACTIONS OF CIS-TRANS PAIRS OF ACTIVATED 2-PHENYLAZIRIDINES - STRONG INFLUENCE OF NITROGEN PYRAMID FOR N-BENZOYLAZIRIDINES

Activated 2-phenylaziridines with a second substituent R in position 3 were made to react with xanthyl anion X-. Nucleophilic ring opening ia the only reaction that occurs with sulfonyl activation. The analogous N-benzoylaziridines 1 undergo this type of ring opening when the two substituents Ph and R are trans. The cis isomers (cis-1, Ph and R cis) react in this manner to a negligible extent if any. The (nearly) exclusive ring cleavage reaction of cis-1 is C-N homolysis of an intermediate ketyl formed by single electron transfer (SET) from X-. This cis-trans phenomenon is in accordance with the hypothesis that the two competing reactions depend in an opposite manner on the steepness of the nitrogen pyramid. A predominant or exclusive final result of SET is reductive aziridine opening and dimerization of the xanthyl radical X.. Formation of both diastereomers of the [GRAPHICS] amidoethylated xanthene in one case (R = Me) is evidence for a cross combination of X. with the radical formed by homolytic ring opening. Cross combination is also a likely path for R = H (no cis-trans isomerism), in addition to reductive ring opening. cis-Aziridines with dimethylcarbamoyl on nitrogen do not react via SET since the ketyl is not stabilized and therefore difficult to generate. Carbonyl attack on both types of acylaziridines competes more or less successfully with the two ring cleavage reactions.