ELECTRONIC CONTROL OF FACE SELECTION IN THE CAPTURE OF RADICALS

A simple procedure is described which leads to the pure epimeric 2-(5-phenyl)adamantanecarboxylic acids E-1 and Z-1. Both acids upon treatment with bromine and mercuric oxide in carbon tetrachloride undergo the Hunsdiecker reaction to give the same mixture of E- and Z- 2-bromo-5-phenyladamantanes 4. 5-Phenyl-2-methyleneadamantane 6 undergoes reaction with bromotrichloromethane to give two diastereomeric adducts. In both instances, the major isomer results from the abstraction of a bromine atom by the zu face of the intermediate 5-phenyl-2-adamantyl radicals. The results mesh with other examples of face selection which we have previously ascribed to transition-state hyperconjugation. An additional case (hydride shift in a carbocation) was encountered in this work, as well as one apparent exception: the oxirane formation from adamantanone and sulfonium ylids. That result is attributed to thermodynamic control of the initial addition step.