Mechanistic investigation of iridium-catalyzed hydrovinylation of olefins

C2H3-Ir(III)(acac-O,O)(2)(Py) dimerizes olefins through a C-H activation mechanism. The starting catalyst first isomerizes to the cis conformer through a dissociative process, where pyridine is lost, and then adds the olefin substrate to the cis conformer. [1.2]-Insertion of the vinyl moiety into the coordinating olefin generates a Ir-CH2-CH2-CH=CH2 complex, which then isomerizes to an Ir-3(eta-allyl) complex through a series of 8-hydride transfer reactions. The eta(3)-allyl complex is significantly more stable than any other part of the surface and is expected to be the resting state of the catalyst. Addition of a second olefin to the eta(3)-allyl complex leads to an Ir-(CH2-CH=CH-CH3) complex with a coordinating olefin, which can transfer a hydrogen to the product, 2-butene, via a C-H activation transition state. This transition step is the rate-determining step, with a calculated Delta H-double dagger = 3 1.6 kcal/mol and Delta G(double dagger) = 32.1 kcal/mol. Other pathways were found to have reasonable barriers, but are not competitive due to very facile barriers leading to the eta(3)-allyl complex. From the alkyl complex, neither 1-butene or butadiene are feasible products. The presence of 1-butene in the product mixture is attributed to isomerization of 2-butene; that is, it is not a kinetic product.