CHEMOSELECTIVE HYDROGEN-TRANSFER REDUCTION OF ALPHA,BETA-UNSATURATED KETONES CATALYZED BY ISOSTRUCTURAL IRON(II), RUTHENIUM(II), AND OSMIUM(II) CIS HYDRIDE ETA(2)-DIHYDROGEN COMPLEXES

The nonclassical trihydrides [(PP3)M(H)(eta2-H-2)]BPh4 (M = Fe, Ru, Os) are efficient catalyst precursors for the reduction of alpha,beta-unsaturated ketones via hydrogen-transfer from secondary alcohols[PP3=P(CH2CH2PPh2)3]. Alpha,beta-Unsaturated ketones bearing bulky substituents at the double bond (i.e. benzylideneacetone) are chemoselectively reduced to allylic alcohols by using either the iron or the ruthenium catalyst. In contrast, the osmium system catalyzes the reduction of alpha,beta-unsaturated ketones to saturated ketones via isomerization of the initially produced allylic alcohols. A number of reducible substrates including various unsaturated and saturated ketones, aldehydes, alkenes, and alkynes have been studied in order to get information on the steric and electronic factors which may affect the interaction of the substrate with the metal center and, thus, control the selectivity of the hydrogen-transfer reductions. Evidence is provided for the formation of an eta1-O-benzylideneacetone complex of the formula [(PP3)Os(H)eta1-OCMe-(CH=CHPh)}]Bph4 which has been characterized by multinuclear NMR spectroscopy. The latter compound and the related complex [(PP3)Os(H)(eta1-OCMe2)]BPh4 have been used in a number of reactions. As a result, valuable information has been obtained which allows one to propose catalytic cycles for the hydrogen-transfer reduction of alpha,beta-unsaturated ketones to unsaturated alcohols assisted by the Fe and Ru complexes, and for the isomerization of allylic alcohols to saturated ketones catalyzed by the Os complex.