Hydrodehalogenation of 1,1-dibromocyclopropanes by Grignard reagents promoted by titanium compounds

1,1-Dibromocyclopropanes are converted into the corresponding monobromocycl opropanes (as mixtures of stereoisomers where appropriate) by reaction with 1.0-1.3 mol equiv. of ethylmagnesium bromide and 2-10 mol% titanium isopro poxide for < 1 h in ether at ambient temperature; in most cases examined, t he yields were ca. 95%. With an excess of the Grignard reagent, the product is the non-halogenated cyclopropane (> 90%). With ethylmagnesium bromide, the reaction occurs very slowly in the absence of catalyst; with methylmagn esium bromide, the reaction does occur in the absence of catalyst, but is o nly slightly promoted in the presence of titanium isopropoxide. Reactions w ith a number of other Grignard reagents are also discussed. In the case of phenethylmagnesium bromide, the major product containing the phenethyl-grou p is ethylbenzene, together with small amounts of styrene and ethyl 4-pheny l-2-butyl ether, a product of trapping of the solvent, ether. In other case s, relatively large amounts of a diether, formally derived by hydrogen abst raction adjacent to the ether oxygen followed by dimerisation, are isolated . No products were identified incorporating the cyclopropane and either the Grignard alkyl group or the solvent. Labelling studies indicate that the h ydrogen introduced into the cyclopropane is not derived from either the alp ha- or beta-positions of the Grignard reagent. When the reduction is carrie d out with phenethylmagnesium bromide in d(8)-tetrahydrofuran both monobrom ides contain deuterium.