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 (PhC⋮CH, PhCH2C⋮CH, and p-OMeC6H4C⋮CH), double
insertion occurs stepwise, each alkyne undergoing rearrangement to a vinylidene intermediate independently to afford an iridium(III) η2-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-CF3C6H4C⋮CH and p-NO2C6H4C⋮CH) also undergo
double insertion into 1, but deuterium labeling experiments using p-CF3C6H4C⋮CD indicate
reversible C(sp)−H oxidative addition. Insertion of highly polarized alkynes [R1C⋮CC(O)R2] to 1 occurs only once and involves no vinylidene intermediates even when R1 = H. The
regio- and stereochemistry in this case are mainly controlled by the steric effects of R1. In
this series, rare cis-(PPh3)2 intermediates were isolated for HC⋮CC(O)R (R = Me or OMe).
X-ray crystal structures of representative products are reported.