Mechanism of alkane transfer-dehydrogenation catalyzed by a pincer-ligated iridium complex

The mechanism of (PCP)Ir-catalyzed transfer-dehydrogenation has been elucidated for the prototypical substrate/acceptor couple, COA/TBE, at 55 °C (COA = cyclooctane; TBE = tert-butylethylene). The catalytic cycle may be viewed as the sum of two reactions: (i) hydrogenation of TBE by (PCP)IrH₂ and C-H addition of a second mole of TBE to give (PCP)IrH(tert-butylvinyl), and (ii) dehydrogenation of COA by (PCP)IrH(tert-butylvinyl) to give (PCP)IrH₂, COE, and TBE. These two stoichiometric reactions have been observed independently and their kinetics determined. The overall catalysis has also been monitored in situ, and (PCP)IrH₂ and (PCP)IrH(tert-butylvinyl) have been observed as the resting states; the ratio of these two complexes is found to be proportional to [TBE]². Based upon the proportionality constant thus obtained and the catalytic rate as a function of [TBE] (which reaches a maximum at ca. 0.3 M), the respective rate constants for the hydrogenation and dehydrogenation segments can be obtained. Good agreement is found between the rates independently obtained from stoichiometric and catalytic runs. Within the overall TBE-hydrogenation reaction, labeling experiments indicate that the rate-determining step is the reductive elimination of TBA (2,2-dimethylbutane) from (PCP)IrH(tert-butylethyl) (which is formed via insertion of TBE into an Ir-H bond of (PCP)IrH₂). Based upon considerations of microscopic reversibility, it can be further inferred that the rate-determining step for the alkane dehydrogenations is C-H addition (and not β-H elimination).