NIOBIUM(III)-INDUCED REDUCTIVE COUPLING OF BENCHROTRENIC ALDEHYDES AND KETONES

The (arene)tricarbonylchromium(benchrotrenic) oxo compounds BctCOR (Bc t = Cr(CO)(3)C6H5; R = H, alkyl or aryl) undergo a reductive coupling reaction with NbCl3(DME), leading to the mono- and bis-metallated alke nes, Bct(R)C=C(R)Ar and Bct(R)C=C(R)Bct, respectively. Such a mixture of mono- and bis-metallated products is obtained when the benchrotreni c ketones are used as substrates. Starting from benchrotrenic aldehyde 4, however, the exclusive formation of the mono-tricarbonylchromium t rans-stilbene 5 takes place. This has been shown to occur by a two-ste p process. The Nb(III)-induced partial Cr(CO)(3) decomplexation of the substrate is followed by the reductive coupling, preferentially betwe en the free aldehyde formed and the benchrotrenic aldehyde. The differ ences in the reactivity observed are explained in terms of the favored sigma (in plane) geometry for the NB(III)-Lewis acid activation of th e carbonyl toward metalloxirane nucleophilic attack.