Renkema Kenton B, Kissin Yury V, Goldman Alan S
Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903, USA.
J Am Chem Soc. 2003 Jul 2;125(26):7770-1. doi: 10.1021/ja0289200.
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).
在55摄氏度下，针对典型的底物/受体对COA/TBE（COA = 环辛烷；TBE = 叔丁基乙烯），已阐明了(PCP)Ir催化的转移脱氢机制。催化循环可视为两个反应的总和：(i) (PCP)IrH2对TBE进行氢化，以及第二摩尔TBE进行C-H加成，生成(PCP)IrH(叔丁基乙烯基)；(ii) (PCP)IrH(叔丁基乙烯基)对COA进行脱氢，生成(PCP)IrH2、环辛烯（COE）和TBE。这两个化学计量反应已被独立观察到并测定了其动力学。整体催化也已通过原位监测，并且观察到(PCP)IrH2和(PCP)IrH(叔丁基乙烯基)为静止状态；发现这两种配合物的比例与[TBE]2成正比。基于由此获得的比例常数以及作为[TBE]函数的催化速率（在约0.3 M时达到最大值），可以获得氢化和脱氢阶段各自的速率常数。从化学计量和催化运行中独立获得的速率之间发现了良好的一致性。在整个TBE氢化反应中，标记实验表明，速率决定步骤是从(PCP)IrH(叔丁基乙基)（通过TBE插入(PCP)IrH2的Ir-H键形成）中还原消除2,2-二甲基丁烷（TBA）。基于微观可逆性的考虑，可以进一步推断，烷烃脱氢的速率决定步骤是C-H加成（而不是β-H消除）。
