Regioselective Formation of Alkylidenecyclopropanes from 2-substituted Cyclobutylidenes Generated from Geminal Dibromocyclobutanes and Methyllithium

Four different 2-substituted geminal dibromocyclobutanes were reacted with methyllithium at -78 degrees C. In contrast to previous studies using diazocyclobutanes as carbene precursors at temperatures above 200 degrees C via reaction of the corresponding tosylhydrazone sodium salts, the organometallic route in each case produces only an alkylidenecyclopropane that could be isolated in good yields. B3LYP calculations were employed to rationalize the observed regioselective ring contraction of the generated cyclobutyliden(oid)s. [reaction: see text]