The Regioselectivite Hydroformylation of Vinylsilanes. A Remarkable Difference in the Selectivity and Reactivity of Cobalt, Rhodium, and Iridium Catalysts

Article abstract of DOI:10.1021/jo00090a029

The zwitterionic rhodium complex, Rh(COD)BPh4, produces 2-(trimethylsilyl)propanal selectively in the hydroformylation of trimethylvinylsilane under quite mild conditions <100 deg C, 200 psi (CO:H2 = 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 α-selective hydroformylation of vinylsilanes.The potentially useful α-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 (20-30percent).The addition of as little as 2 equiv of PPh3 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 30percent to 91percent after only 1.5 h.Iridium catalysts yielded the linear aldehydes (3-(trialkylsilyl)propanal) with excellent regioselectivities (90-100percent) 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:H2 = 7:1).Of those complexes examined, hydrated IrCl3 (after preactivation at 160 deg C) gave the best selectivity for the linear isomer (98-100percent).The cationic complex, (+)BF4, also gave the linear aldehyde predominantly (95-97percent) in 75-80percent yields.Although most of the cobalt complexes tested were unreactive, the cationic cobalt cluster 6-C6H6)3(μ₃-CO)2>BPh4 was the most active catalyst for the hydroformylation of vinyltriethylsilane.At 100 deg C, this complex completed six turnovers per minute, yielding the linear silyl aldehyde as the major product (B:L = 24:76).