Electronic and steric effects in the insertion of alkynes into an iridium(III) hydride

Insertion of a variety of alkynes into the Ir-H bond of trans-[IrH(PPh3)(2)(C(Ph)=CHC-(O)Me)(acetone)](+) (1) follows three different routes depending on the alkyne structures. For relatively electron-rich alkynes (PhCdropCH, PhCH(2)CdropCH, and p-OMeC(6)H(4)CdropCH), double insertion occurs stepwise, each alkyne undergoing rearrangement to a vinylidene intermediate independently to afford an iridium(III) eta(2)-butadienyl. In the first alkyne insertion, deuterium labeling and crossover experiments confirm that the alkyne to vinylidene rearrangement is intraligand. Both a vinyl and a vinylidene intermediate were trapped and isolated during this first insertion. In the second alkyne insertion, a C-H agostic intermediate was isolated. Electron-poor alkynes (p-CF(3)C(6)H(4)CdropCH and p-NO(2)C(6)H(4)CdropCH) also undergo double insertion into 1, but deuterium labeling experiments using p-CF(3)C(6)H(4)CdropCD indicate reversible C(sp)-H oxidative addition. Insertion of highly polarized alkynes [R(1)CdropCC(O)-R-2] to 1 occurs only once and involves no vinylidene intermediates even when R-1 = H. The regio- and stereochemistry in this case are mainly controlled by the steric effects of R-1. In this series, rare cis-(PPh3)(2) intermediates were isolated for HCdropCC(O)R (R = Me or OMe). X-ray crystal structures of representative products are reported.