Effective and Selective Mild Catalytic Hydrodehalogenation of Halocyclopropanes: Preparative Capabilities and Mechanistic Aspects

This review surveys both data obtained by the authors and published data on the partial or full hydrodehalogenation of di- and polyhalocyclopropanes (chlorides and bromides) with Grignard reagents catalyzed by titanium or zirconium compounds. The factors affecting the efficiency and selectivity of the hydrodebromination of bromocyclopropanes are considered: the nature of Grignard reagents (including isotopically labeled reagents), their transformations and effects in catalyzed and uncatalyzed reactions, the participation of solvents, catalytic and stoichiometric amounts of the catalyst, etc. A scheme is proposed in which the key steps of the mechanism of hydrodebromination of bromocyclopropanes includes three blocks of reactions: (a) the generation of a catalytically active Ti(II) species; (b) the hydrodehalogenation of bromocyclopropanes involving electron transfer from a low-valent catalyst species, formation of the cyclopropyl radical, and stabilization of this radical as a result of hydrogen atom transfer from the solvent molecule; and (c) transformations of previously formed radical species, such as dimerization and disproportionation (for example, of radical species generated from Grignard reagents or ether molecules) or the linking of alkyl radicals to radical species produced from solvent molecules.