Observation of unusual kinetics in Heck reactions of aryl halides: The role of non-steady-state catalyst concentration
Observation of unusual kinetics in Heck reactions of aryl halides: The role of non-steady-state catalyst concentration
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DOI：
10.1021/ja005872f
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发表时间：
2001-05-16
影响因子：
15
通讯作者：
Blackmond, DG
中科院分区：
文献类型：
--
作者：
Rosner, T;Pfaltz, A;Blackmond, DG
在均相催化反应开始时的瞬态行为是一种常见的现象，因为活性催化物种通常在反应条件下形成。如果该过程是缓慢的，则在反应网络内的催化物质达到“稳态”之前，可能发生催化循环的显著次数的周转。1诱导期对从实验数据中提取有意义的动力学信息有影响，特别是当采用初始速率方法时。监测反应进程的原位技术可以有助于识别瞬态行为的情况，并且可以为评估反应的复杂性提供有价值的工具。2-4在过去的几年中，许多报道已经注意到二聚Pd（II）络合物如4在芳基卤化物的Heck烯化中的高活性。3b，4，5在常规接受的反应机理中，6通过在烯烃加成之前将芳基卤化物氧化加成到Pd 0络合物上形成四配位芳基-Pd（II）中间体。这表明二聚体前体络合物的裂解、Pd 2+的还原和配体解离结合联合收割机以给出可行的催化物质。如果这些过程发生在与催化反应相当的时间尺度上，则在活性催化剂形成的同时可能发生非稳态催化。
Transient behavior at the beginning of homogeneous catalytic reactions is a common phenomenon, since the active catalytic species is often formed under reaction conditions. If this process is slow, a significant number of turnovers of the catalytic cycle may occur before the catalytic species within the reaction network attain “steady-state”. 1 Induction periods have implications for the extraction of meaningful kinetic information from experimental data, especially when initial rate methods are employed. In situ techniques to monitor reaction progress can be helpful in identifying cases of transient behavior and can provide a valuable tool for assessing reaction complexity in general. 2-4 In the past few years, a number of reports have noted the high activity of dimeric Pd (II) complexes such as 4 in Heck olefinations of aryl halides. 3b, 4, 5 In the conventionally accepted reaction mechanism, 6 a four-coordinate aryl-Pd (II) intermediate is formed by oxidative addition of the aryl halide to a Pd0 complex prior to olefin addition. This suggests that cleavage of the dimeric precursor complex, reduction of Pd2+, and ligand dissociation combine to give a viable catalytic species. If these processes occur on a time scale comparable to that of the catalytic reaction, nonsteady-state catalysis could occur while the active catalyst is forming.