Mechanism of Alkane Transfer-Dehydrogenation Catalyzed by a Pincer-Ligated Iridium Complex

Abstract

The mechanism of (PCP)Ir-catalyzed transfer-dehydrogenation has been elucidated for the prototypical substrate/acceptor couple, COA/TBE, at 55 degrees 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 beta-H elimination).

Redox and Lewis Acid Reactivity of Unsaturated OsII

“…Even when this involves cleavage of an H-C(sp 3 ) bond, the favored oxidation state is Os II , and this is possible by formal addition of the C-H bond across the Os-amide bond, a process best described as C-H bond heterolysis. This is not the favored reaction for pincer ligands with sp 2 or sp 3 C atom at the central anionic site, [23][24][25][26][27] but it is the reaction between Ru-NR 2 and the H 2 σ bond, in the Noyori hydrogenation catalyst, forming H-Ru-NHR 2 . [5][6][7][8][9] This difference is attributed to the lone pair on the central pincer donor for anionic PNP pincers.…”

Pincer‐Type Iridium Complexes for Organic Transformations

“…In this light it is not surprising that detailed kinetic investigations revealed catalyst deceleration also due to olefi n coordination. Optimal catalyst operation therefore relies on a delicate balance of product olefi n and hydrogen acceptor concentrations [11] . On a practical basis, large product : substrate ratios are readily avoided by using the substrate alkane as solvent.…”

Activation of C–O and C–F Bonds by Pincer–Iridium Complexes

“…The first pincer-iridium-based alkane dehydrogenation catalyst, (PCP)IrH 2 (PCP = κ 3 -C 6 H 3 -2, 6-[CH 2 P(t-Bu) 2 ] 2 ) (1 -H 2 ), was reported by Kaska and Jensen [11] in the mid-1990s, and is active for the transfer dehydrogenation of nalkanes and cycloalkanes, a reaction that we have developed and studied mechanistically [12][13][14][15][16][17]. The initial success of (PCP)Ir in transfer dehydrogenation was followed by the development of a series of (pincer)Ir catalysts with varied steric and electronic properties [18][19][20].…”

“…These complexes have been found to effect the oxidative addition of a variety of C-H bonds including those with sp 2 -and sp-hybridized carbon [8][9][10]. Most notable, however, has been the activation of C(sp 3 )-H bonds, leading to alkane dehydrogenation [6,7].The first pincer-iridium-based alkane dehydrogenation catalyst, (PCP)IrH 2 (PCP = κ 3 -C 6 H 3 -2, 6-[CH 2 P(t-Bu) 2 ] 2 ) (1 -H 2 ), was reported by Kaska and Jensen [11] in the mid-1990s, and is active for the transfer dehydrogenation of nalkanes and cycloalkanes, a reaction that we have developed and studied mechanistically [12][13][14][15][16][17]. The initial success of (PCP)Ir in transfer dehydrogenation was followed by the development of a series of (pincer)Ir catalysts with varied steric and electronic properties [18][19][20].…”

“…15 Mechanism for the reaction of (PCP)Ir(NBE) and CHF 3 to yield the difluorocarbene complex, (PCP)IrCF 2 .…”