CYCLOHEXYL METAL COMPOUNDS .3. STEREOCHEMISTRY OF SOME SUBSTITUTION REACTIONS AT CARBON-LITHIUM BOND

Several reactions of menthyl- and 4-t-butylcyclohexyllithium which result in substitution at the carbon-metal center have been studied. Configurations of products (axial/equatorial ratios) indicate that most common substitution reactions such as carbonation, protonolysis, and coupling proceed with predominant retention of configuration. A four-center SEi transition state is postulated for some reactions, although a six-center mechanism involving the dimer of the lithium reagent represents a reasonable alternative in other cases. Halogenations with Cl2, Br2, and I2 result in significant, and often predominant, inversion of configuration. A backside SE2 attack at the carbon atom, perhaps involving halogen radicals, is postulated. Bromination with 1,2-dibromoethane, bromobenzene, and pyridine-bromine complex results in predominant retention of configuration, and apparently proceeds via a concerted, multicenter process. © 1969, American Chemical Society. All rights reserved.