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

Abstract

The isomerization of olefins by complexes of the pincer-ligated iridium species ((tBu)PCP)Ir ((tBu)PCP = κ(3)-C(6)H(3)-2,6-(CH(2)P(t)Bu(2))(2)) and ((tBu)POCOP)Ir ((tBu)POCOP = κ(3)-C(6)H(3)-2,6-(OP(t)Bu(2))(2)) has been investigated by computational and experimental methods. The corresponding dihydrides, (pincer)IrH(2), 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(η(2)-olefin) species undergo isomerization via the formation of (pincer)Ir(η(3)-allyl)(H) intermediates; one example of such a species, ((tBu)POCOP)Ir(η(3)-propenyl)(H), was independently generated, spectroscopically characterized, and observed to convert to ((tBu)POCOP)Ir(η(2)-propene). Surprisingly, the DFT calculations indicate that the conversion of the η(2)-olefin complex to the η(3)-allyl hydride takes place via initial dissociation of the Ir-olefin π-bond to give a σ-complex of the allylic C-H bond; this intermediate then undergoes C-H bond oxidative cleavage to give an iridium η(1)-allyl hydride which "closes" to give the η(3)-allyl hydride. Subsequently, the η(3)-allyl group "opens" in the opposite sense to give a new η(1)-allyl (thus completing what is formally a 1,3 shift of Ir), which undergoes C-H elimination and π-coordination to give a coordinated olefin that has undergone double-bond migration.

Computational Studies of Synthetically Relevant Homogeneous Organometallic Catalysis Involving Ni, Pd, Ir, and Rh: An Overview of Commonly Employed DFT Methods and Mechanistic Insights

“…538 Surprisingly, a direct C−H activation pathway via an allylic σ-complex (η 1 -allyl) was found to be favored. The mechanistic proposal includes η 1 -allyl formation from the η 2 -olefin, C−H activation at the η 1 -allyl to afford an η 1 -allyl hydride, followed by closing to yield an η 3 -allyl hydride (Scheme 57).…”

Recent Advances in Alkane Dehydrogenation Catalyzed by Pincer Complexes

“…This isomerization can occur via either via a "hydride addition pathway" or via an "η 3 -allyl pathway" (Scheme 1). Brookhart, Goldman, and coworkers have performed a computational and experimental study to elucidate the actual mechanism that is operative in this isomerization reaction catalyzed by ( tBu4 PCP)Ir (11) and by ( tBu4 POCOP)Ir (21) [83]. The hydride addition pathway requires the Ir-H bond of 43 to undergo 2,1-addition across the double bond of α-olefin to generate the intermediate 47 that can undergo a β-hydride elimination to give internal olefin and regenerate dihydride 43.…”

Scope and Mechanism of Homogeneous Tantalum/Iridium Tandem Catalytic Alkane/Alkene Upgrading using Sacrificial Hydrogen Acceptors

“…Second order rate constants for alkene isomerization were determined by fitting according to the equation: Cp*TaCl2(1--alkene) + 1--alkene → Cp*TaCl2(1--alkene) + 2--alkene (not an elementary step) These values are 3.6(2) x 10 --3 M --1 s --1 and 1.8(2) x 10 --3 M --1 s --1 for the formation of cis--and trans--2--hexene respectively. Presumably this isomerization occurs via a π--allylic--type mechanism, 6 since the major catalyst resting state is the [Ta](1--alkene) complex, rather than an insertion/β--hydride elimination mechanism; 6 however, due to the low degree of isomerization observed, the mechanism of this side--reaction was not experimentally tested. .04 M The singlet corresponding to the methyl protons of the Cp* ligand are diagnostic for Cp*TaCl2(alkene) (~1.6--1.7 ppm) and Cp*TaCl2(metallacycle) (~1.8--1.9 ppm) based on reference spectra for precatalyst 1 (alkene complex, 1.58 ppm, 22 °C) and 1 treated with a large excess of 1--hexene (metallacycle, 1.81 ppm, 22 °C).…”