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-un saturated ketones via hydrogen-transfer from secondary alcohols[PP3=P( CH2CH2PPh2)3]. Alpha,beta-Unsaturated ketones bearing bulky substituen ts at the double bond (i.e. benzylideneacetone) are chemoselectively r educed to allylic alcohols by using either the iron or the ruthenium c atalyst. In contrast, the osmium system catalyzes the reduction of alp ha,beta-unsaturated ketones to saturated ketones via isomerization of the initially produced allylic alcohols. A number of reducible substra tes including various unsaturated and saturated ketones, aldehydes, al kenes, and alkynes have been studied in order to get information on th e steric and electronic factors which may affect the interaction of th e substrate with the metal center and, thus, control the selectivity o f the hydrogen-transfer reductions. Evidence is provided for the forma tion of an eta1-O-benzylideneacetone complex of the formula [(PP3)Os(H )eta1-OCMe-(CH=CHPh)}]Bph4 which has been characterized by multinuclea r 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 r esult, valuable information has been obtained which allows one to prop ose 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.