Theoretical study of the reaction of alkyllithium with pyridylphosphines

The reaction of pyridylphosphines with alkyllithium reagents has been investigated using ab initio methods. In earlier experimental studies, it was found that reaction of RLi with R'PPy2 produced a mixture of RPy, R'Py, (R)(Py)PLi, and (R')(Py)PLi; the reaction was assumed to involve "reductive elimination" from an (R)(R')(Py)(2)P- Li+ intermediate. Our calculations show that the R for R' exchange does indeed involve such a species, although it is a transition state rather than an intermediate. Formation of phosphide, however, proceeds by direct attack of the alkyllithium on carbon C2 of a pyridyl group and is more properly designated as a nucleophilic substitution at carbon. Coordination of the pyridyl groups to lithium appears to be important in both reactions.