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-(trimethyl silyl)propanal selectively in the hydroformylation of trimethylvinylsi lane 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 ald ehyde as the major product (B:L = 70:30). This is the first reported e xample of an alpha-selective hydroformylation of vinylsilanes. The pot entially useful alpha-silyl aldehydes are difficult to prepare and iso late by other means, but using this procedure they can be obtained in one step from commercially available vinyltrimethylsilane, albeit in l ow isolated yields (20-30%). The addition of as little as 2 equiv of P Ph(3) causes a complete shift in the selectivity yielding the linear i somer as the major product in the hydroformylation of vinyltriethylsil ane (B:L = 7:93). The isolated yield increases from 30% to 91% after o nly 1.5 h. Iridium catalysts yielded the linear aldehydes (3-(trialkyl silyl)propanal) with excellent regioselectivities (90-100%) without th e 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 s electivity 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)-C O)(2)]BPh(4) was the most active catalyst for the hydroformylation of vinyltriethylsilane. At 100 degrees C, this complex completed six turn overs per minute, yielding the linear silyl aldehyde as the major prod uct (B:L = 24:76).