(ETA(3)-OXAALLYL)RHODIUM(I) COMPLEXES AS CATALYST PRECURSORS FOR THE DISPROPORTIONATION OF ALDEHYDES

Addition of 50 equiv of benzaldehyde to a benzene solution of (Ph(3)P) (2)Rh(eta(3)-CH2C(Ph)O) (1) resulted in rapid disproportionation of th e aldehyde, yielding benzyl benzoate in near quantitative yield. Simil ar disproportionation reactions occurred with isobutyraldehyde, n-hept anal, and furfural; however, the yields in these reactions were lower. The catalytic efficiency of the (eta(3)-oxaallyl)rhodium(I) complex d epended on the alpha-substitution pattern of the aldehyde, the concent ration of the aldehyde, and the polarity of the aldehyde. The labeled complex (Ph(3)P)(2)Rh(eta(3)-(CH2C)-C-13(Ph)O) (C-13-1) transformed du ring catalysis into two carbon-labeled products, demonstrating that th e eta(3)-oxaallyl complex served as a precursor to the actual-catalyst . The (eta(3)-allyl)rhodium(I) complex (Ph(3)P)(2)Rh(eta(3)-CH2CHCH2) (5) failed to react with benzaldehyde, but upon addition of 4 equiv of hydrogen gas, catalysis ensued with near quantitative disproportionat ion of the aldehyde. Rapid disproportionation of benzaldehyde also occ urred when (DIPHOS)Rh(C6H6)(ClO4-)-Cl-+ and 18-crown-6-solubilized PhC H(2)O(-)K(+) were mixed, establishing the intermediacy of a rhodium al koxide. A complete mechanistic scheme for oxaallyl modification and di sproportionation catalysis is presented.