Aj. Buckmelter et al., Conformational memory in enantioselective radical reductions and a new radical clock reaction, J AM CHEM S, 122(39), 2000, pp. 9386-9390
The radical decarboxylation of acid 1 led to tetrahydropyran 3 with signifi
cant optical activity. This transfer of chirality is an example of a confor
mational memory effect and derives from the slow ring inversion of the atro
pisomer 2 and its enantiomer 2'. A similar conformational memory effect was
observed in the reductive decyanation and reductive lithiation of cyanohyd
rin 4. We propose that the racemization of radicals with chiral conformatio
ns 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 b
y different H atom donors. The rate of racemization of 2 is 5.7 x 10(8) s(-
1) at 22 degrees C, which is about five times faster than ring opening of a
cyclopropylmethyl radical.