Identification of an elusive catalyst: IrH(eta(2)-(C6H4PBU2)-B-t)(Cl)(P(t)Bu(2)Ph) as a precursor for C=C bond migration

The catalytic isomerization of allylbenzene to cis/trans-beta-Me styrene in solutions containing Ir(III) hydride halide, hydride hydroxide and hydride alkoxide Ir(H)(2)X(P(t)Bu(2)Ph)(2)(X=F, Cl, Br, I, OH, OCH2CF3) is described. Large variations in isomerization rate (k(obs)) are found to correlate with the electron donating ability (pi + sigma) of the X ligand: the rate is enhanced by more effective donation by the X Ligands. A number of kinetic studies and phosphine exchange experiments utilizing a deuterium labeled phosphine, P(t)Bu(2)(d(5))Ph, indicate that the active isomerization catalyst is a 14-valence-electron Ir(III) species derived from the ortha-metallated complex, IrH(eta(2)-C(6)H(4)P(t)Bu(2))(X) (P(t)Bu(2)Ph), by phosphine dissociation. Using IrH(eta(2)-C(6)H(4)P(t)Bu(2)) (Cl)(P(t)Bu(2)Ph) as a catalyst precursor, the catalytic isomerization of alpha,alpha-d(2)-allylbenzene in the presence of an excess of 4-allyl anisole has yielded primarily d(1)-cis/trans-beta-Me styrene, which confirms that the isomerization proceeds via an insertion-elimination mechanism of a metal hydride bond.