THE REGIOSELECTIVE HYDROFORMYLATION OF VINYLSILANES - A REMARKABLE DIFFERENCE IN THE SELECTIVITY AND REACTIVITY OF COBALT, RHODIUM, AND IRIDIUM CATALYSTS

The zwitterionic rhodium complex, Rh(COD)BPh(4), produces 2-(trimethylsilyl)propanal selectively in the hydroformylation of trimethylvinylsilane under quite mild conditions [100 degrees C, 200 psi (CO:H-2 = 1:2), 1.5-3 h]. Excess hydrogen is crucial for obtaining the branched aldehyde as the major product (B:L = 70:30). This is the first reported example of an alpha-selective hydroformylation of vinylsilanes. The potentially useful alpha-silyl aldehydes are difficult to prepare and isolate by other means, but using this procedure they can be obtained in one step from commercially available vinyltrimethylsilane, albeit in low isolated yields (20-30%). The addition of as little as 2 equiv of PPh(3) causes a complete shift in the selectivity yielding the linear isomer as the major product in the hydroformylation of vinyltriethylsilane (B:L = 7:93). The isolated yield increases from 30% to 91% after only 1.5 h. Iridium catalysts yielded the linear aldehydes (3-(trialkylsilyl)propanal) with excellent regioselectivities (90-100%) without the addition of any phosphine. A competing reaction is hydrogenation of the starting material. This can be suppressed by increasing the amount of CO in the gas mixture (CO:H-2 = 7:1). Of those complexes examined, hydrated IrCl3 (after preactivation at 160 degrees C) gave the best selectivity for the linear isomer (98-100%). The cationic complex, [Ir(COD)(2)](BF4-)-B-+, also gave the linear aldehyde predominantly (95-97%) in 75-80% yields. Although most of the cobalt complexes tested were unreactive, the cationic cobalt cluster [Co-3(eta(6)-C6H6)(3)(mu(3)-CO)(2)]BPh(4) was the most active catalyst for the hydroformylation of vinyltriethylsilane. At 100 degrees C, this complex completed six turnovers per minute, yielding the linear silyl aldehyde as the major product (B:L = 24:76).