Ab initio theoretical study of metallo-dehydrogenation and nucleophilic addition of methyllithium and methylpotassium with crotonaldehyde dimethyl acetal

The metal-hydrogen exchange reactions of lithium and potassium methide with the dimethyl acetal of crotonaldehyde are compared with the competing additions to the double bond. In the case of lithium the nucleophilic addition reaction is clearly preferred; in contrast, potassium methide shows two barriers of comparable height, with a slight preference for addition, and free methide is more inclined to act as a base. The results appear to be consistent with available experimental evidence and suggest that (i) the alkyl carbon must be in an almost carbanion-like state to exhibit the reactivity actually observed when a superbase is used; (ii) RK is a poor model for a RLi/R'OK superbase. Therefore, potassium alkoxides appear to operate not only by encouraging a deaggregation of alkyllithium oligomers, but also by promoting a weakening of the C--Li+ interaction. (C) 1997 Elsevier Science Ltd.