aIRON CATALYSIS OF GRIGNARD REDUCTIONS. MECHANISM OF 1,3-REDUCTIVE ELIMINATIONS FROM γ-PROPYL HALIDES

Article abstract of DOI:10.1016/S0022-328X(00)86830-8

The iron-catalyzed reduction of various 3-substituted propyl bromides by Grignard reagents affords propylene and cyclopropane.The reduction to propylene is particularly noteworthy since it formally represents a 1,2-hydrogen shift.Two key intermediates have been identified in propylene formation, in which 3-methoxypropyl bromide is first catalytically reduced to the magnesium derivative by Grignard reagent.The iron-catalyzed β-elimination of the 3-methoxypropylmagnesium intemediate affords allyl methyl ether, which is then reductively cleaved to propylene.Extensive studies of deuterium labeling in the reactants, as well as in both intermediates, allow the course of the hydrogen shift to be followed unequivocally.The mechanism of iron catalysis is proposed in Schemes 2 and 3, representing the first and second stages of the reduction to propylene.