Exploring the utility of neutral Rh(I) and Ir(I) κ2-(P,O)MCOD catalyst complexes for the addition of triethylsilane to styrene

The ability of neutral Rh and Ir phosphinoenolate complexes of the type (COD)M(κ2-3-PiPr2-2-O-indene) (M = Rh, 2a; M = Ir, 2b; COD = η4-1,5-cyclooctadiene) to mediate the addition of triethylsilane to styrene was explored under various reaction conditions in which the solvent, temperature and substrate ratio were altered. Throughout the course of these catalytic studies, head-to-head performance comparisons were made with Wilkinson’s catalyst ((PPh3)3RhCl; 3) and Crabtree’s catalyst ( [ ( COD ) Ir ( PCy 3 ) ( Py ) ] + PF 6 - ; Cy = cyclohexyl; Py = pyridine; 4). While 2a proved to be an active catalyst for dehydrogenative silylation, exhibiting selectivity for E-1-triethylsilyl-2-phenylethene (5a) comparable to that of 3 under appropriate conditions, the Ir analogue 2b displayed rather poor catalytic productivity. In contrast, 4 exhibited good catalytic activity, generating 5a as well as 1-triethylsilyl-1-phenylethane (5c) as major products. Crystallographic data for 2a · 0.25CH2Cl2 are also reported.