The intermolecular cyclopropanation of simple alkyl-substituted alkenes with Fischer methoxycarbene complexes under thermal conditions is reported. The scope and limitations of this [2 + 1] cycloaddition with respect to the nature of the carbene complex, substitution pattern of the electron neutral olefin, and functional group tolerance in the electron neutral olefin are described in detail. This methodology provides functionalized cyclopropanes in good to excellent yields and generally with high diastereoselectvity. A mechanism involving the initial formation of a chelated tetracarbonyl complex intermediate is proposed to account for the observed results.