Olefin Isomerization by Iridium Pincer Catalysts. Experimental Evidence for an η3-Allyl Pathway and an Unconventional Mechanism Predicted by DFT Calculations

The isomerization of olefins by complexes of the pincer-ligated iridium species ((PCP)-P-tBu)Ir ((PCP)-P-tBu = kappa(3)-C6H3-2,6-((CH2PBu2)-Bu-t)(2)) and ((POCOP)-P-tBu)Ir ((POCOP)-P-tBu = kappa(3)-C6H3-2,6-((OPBu2)-Bu-t)(2)) has been investigated by computational and experimental methods. The corresponding dihydrides, (pincer)IrH2, are known to hydrogenate olefins via initial Ir-H addition across the double bond. Such an addition is also the initial step in the mechanism most widely proposed for olefin isomerization (the hydride addition pathway); however, the results of kinetics experiments and DFT calculations (using both M06 and PBE functionals) indicate that this is not the operative pathway for isomerization in this case. Instead, (pincer)Ir(eta(2)-olefin) species undergo isomerization via the formation of (pincer)Ir(eta(3)-allyl)(H) intermediates; one example of such a species, ((POCOP)-P-tBu)Ir(eta(3)-propenyl)(H), was independently generated, spectroscopically characterized, and observed to convert to ((POCOP)-P-tBu)Ir(eta(2)-propene). Surprisingly, the DFT calculations indicate that the conversion of the eta(2)-olefin complex to the eta(3)-allyl hydride takes place via initial dissociation of the Ir-olefin pi-bond to give a sigma-complex of the allylic C-H bond; this intermediate then undergoes C-H bond oxidative cleavage to give an iridium eta(1)-allyl hydride which closes to give the eta(3)-allyl hydride. Subsequently, the eta(3)-allyl group opens in the opposite sense to give a new eta(1)-allyl (thus completing what is formally a 1,3 shift of Ir), which undergoes C-H elimination and pi-coordination to give a coordinated olefin that has undergone double-bond migration.