Ab initio theoretical study of the reactivity as bases or nucleophiles of potassium and lithium methides

Using propene as a model reference system, the reactivity of potassium and lithium methides in metal-hydrogen exchange and nucleophilic addition to the double carbon-carbon bond was investigated, These extreme models, which imply a tight interaction of the organic anion with its counterion, were compared with the corresponding free anion reactions, which provide the opposite extreme situation, The lithium system appears to be more inclined toward nucleophilic addition, whereas the potassium system would more readily undergo a metallo-dehydrogenation reaction, The results appear to be consistent with available experimental evidence and allow a qualitative rationalization of the different basic strengths and nucleophilicities of Li and K alkyls. As an ancillary but interesting result, the hydrogen transfer reactions see a significant amount of electron density located on the transferred hydrogen in the transition structure; this process can hardly be defined as a proton abstraction operated by the strong base. (C) 1997 John Wiley & Sons, Ltd.