Mechanistic Investigation of Iridium-Catalyzed Hydrovinylation of Olefins

Abstract

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−CHCH2 complex, which then isomerizes to an Ir(η3-allyl) complex through a series of β-hydride transfer reactions. The η3-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 η3-allyl complex leads to an Ir−(CH2−CHCH−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 ΔH⧧ = 31.6 kcal/mol and ΔG⧧ = 32.1 kcal/mol. Other pathways were found to have reasonable barriers, but are not competitive due to very facile barriers leading to the η3-allyl complex. From the allyl 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.