Conformational Memory in Enantioselective Radical Reductions and a New Radical Clock Reaction

Abstract

The radical decarboxylation of acid 1 led to tetrahydropyran 3 with significant optical activity. This transfer of chirality is an example of a conformational memory effect and derives from the slow ring inversion of the atropisomer 2 and its enantiomer 2‘. A similar conformational memory effect was observed in the reductive decyanation and reductive lithiation of cyanohydrin 4. We propose that the racemization of radicals with chiral conformations such as 2 could be used as new radical clocks. The rate of racemization of 2 was evaluated and used to determine the rate of H atom transfer to 2 by different H atom donors. The rate of racemization of 2 is 5.7 × 108 s-1 at 22 °C, which is about five times faster than ring opening of a cyclopropylmethyl radical.