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

Abstract

1,1-Dibromocyclopropanes are converted into the corresponding monobromocyclopropanes (as mixtures of stereoisomers where appropriate) by reaction with 1.0–1.3 mol equiv. of ethylmagnesium bromide and 2–10 mol% titanium isopropoxide for <1 h in ether at ambient temperature; in most cases examined, the 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 methylmagnesium bromide, the reaction does occur in the absence of catalyst, but is only slightly promoted in the presence of titanium isopropoxide. Reactions with a number of other Grignard reagents are also discussed. In the case of phenethylmagnesium bromide, the major product containing the phenethyl-group is ethylbenzene, together with small amounts of styrene and ethyl 4-phenyl-2-butyl ether, a product of trapping of the solvent, ether. In other cases, relatively large amounts of a diether, formally derived by hydrogen abstraction 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 hydrogen introduced into the cyclopropane is not derived from either the α- or β-positions of the Grignard reagent. When the reduction is carried out with phenethylmagnesium bromide in d₈-tetrahydrofuran both monobromides contain deuterium.