While the reaction of the dibromocarbene adduct of trimethylethylene with methyllithium gives only trimethylallene, the dibromocarbene adduct of tetramethylethylene reacts with methyllithium to give a nearly quantitative yield of 1,2,2-trimethylbicyclobutane.' We have examined a number of other systems to determine to what extent this pattern of reactivity for tetrasubstituted gemdibromocyclopropanes is general and report on two herein. Treatment of I, the dibromocarbene adduct of tetraethylethylene, with methyllithium in ether at O" gave a single product (>96% yield based on glc) to which we assign structure 2, exo-4-methyl-l,2,2-triethylbicyclobutane.f The -37c Br Br 1 +r w 2 exo assignment of the C-4 methyl group is based on the nmr spectrum' as well as on steric considerations and results obtained from a related system (below). An endo methyl group at C-4 would result in extreme crowding with the endo ethyl at C-2. This severe interaction (absent in 2, must be developed sufficiently in the transition state leading to endo-to preclude C-H insertion in this sense. The behavior of the dibromocarbene adauct of 2-ethyl-3-methylpentene-2 presents a direct comparison of the reactivities of methyl and methylene groups. Dibromide 2 reacts with methyllithium in ether at 0 o to give a mixture (ca. 95% yield) of three bicyclobutanes 4, 1, 5 in ratios of 96 : 1 : 3.* No detectable