Theoretical study of the regiochemistry-determining step of the Pauson-Khand reaction
Theoretical study of the regiochemistry-determining step of the Pauson-Khand reaction
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DOI：
10.1021/ja015781y
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发表时间：
2001-07-25
影响因子：
15
通讯作者：
Greene, AE
中科院分区：
文献类型：
--
作者：
de Bruin, TJM;Milet, A;Greene, AE
Pauson-Khand反应（PKR），一种钴介导的炔、烯烃和一氧化碳的连接以产生环戊烯酮（方案1），在过去30年中一直是大量合成研究的主题。[1]相比之下，迄今为止，对这一重要但尚未深入理解的反应的高水平计算研究非常缺乏。2，3在此，我们提出了详细的B3 LYP研究4的区域化学决定事件的PKR，即，形成的钴配合物III从烯烃配位的二钴配合物II，5与乙烯作为烯烃和丙炔作为一个典型的非对称炔。如在丙炔-二钴六羰基络合物1（图1）中所见，轴向（ax）和两个赤道位置，相对于甲基的反式（eqtr）和顺式（eqcis），潜在地可用于平行或垂直烯烃配位。6在乙烯的情况下，垂直配位模式有利于eqtr位置（3.3 kcal·mol-1，eqcis）的3.4 kcal·mol-1，旋转势垒（TStr//，ε）计算为4.7 kcal·mol-1（图2）。CO和乙烯的假旋转的障碍可以在室温下容易地克服，因为TSeq，ax分别比eqtr//和ax配位的最小值高2.3和2.9 kcal·mol-1。假设其他两个伪旋转（ax h eqcis和eqtr h eqcis）的势垒是相似的，可以得出结论，乙烯在室温（或高温）下不占据唯一的配位位点，但容易从一个位置移动到另一个位置。如将看到的，在钴形成的能量势垒范围内，从11.0到16.8 kcal·mol-1，8很明显，乙烯的初始配位位置（以及当伪旋转相对容易时的其它烯烃）并不决定PKR中的区域化学。由三种最稳定的反应性络合物2 eqtr，2 eqcis，和2ax与两个钴环3a和3b的配位关系已经被表征（图3）。[9]两个赤道异构体2 eqcis和2 eqtr分别比2ax稳定2.2和2.8 kcal·mol-1。10配合物2ax具有CC键形成的最短距离，r（CCh））2.816 π;在2 eqtr中的距离为r（CCh））2.935 π，在2 eqcis中的距离为r（CCCH 3））2.955 π。对于从2ax形成CC键的过渡态，观察到基本上相反的情况：TS 1具有最长的距离，r（CCCH））1.953 π，其次是TS 3，r（CCCH 3））1.980 π。由于反应2ax f TS 1 f 3a比
The Pauson-Khand reaction (PKR), a cobalt-mediated joining of an alkyne, an olefin, and carbon monoxide to yield a cyclopentenone (Scheme 1), has been the subject of a multitude of synthetic studies over the past 30 years. 1 In contrast, there has to date been a remarkable dearth of high-level computational studies of this important, yet still not intimately understood, reaction. 2, 3 Herein we present a detailed B3LYP study4 of the regiochemistry-determining event in the PKR, ie, the formation of the cobaltacycle III from the olefin-coordinated dicobalt complex II, 5 with ethylene as the olefin and propyne as a prototypical nonsymmetric alkyne. As seen in the propyne-dicobalt hexacarbonyl complex 1 (Figure 1), an axial (ax) and two equatorial positions, trans (eqtr) and cis (eqcis) with respect to the methyl group, are potentially available for parallel or perpendicular olefin coordination. 6 In the case of ethylene, the perpendicular coordination mode is favored by 3.4 kcal ‚mol-1 for the eqtr position (3.3 kcal ‚mol-1, eqcis), with a barrier for the rotation (TStr//,⊥) calculated to be 4.7 kcal ‚mol-1 (Figure 2). The barrier for the pseudorotation of a CO and ethylene can easily be overcome at room temperature, since TSeq, ax lies only 2.3 and 2.9 kcal ‚mol-1 higher than the minima for eqtr//and ax coordination, respectively. Assuming that the barriers for the two other pseudorotations (ax h eqcis and eqtr h eqcis) are similar, it can be concluded that ethylene does not occupy a unique coordination site at room (or elevated) temperature, 7 but moves easily from one position to another. In that the energy barriers for cobaltacycle formation range, as will be seen, from 11.0 to16.8 kcal ‚mol-1, 8 it is apparent that the initial coordination position of ethylene (and other olefins when pseudorotation is relatiVely facile) does not determine the regiochemistry in the PKR.Four transition states leading from the three most stable reactive complexes 2eqtr⊥, 2eqcis⊥, and 2ax to the two cobaltacycles 3a and 3b have been characterized (Figure 3). 9 The two equatorial isomers 2eqcis⊥ and 2eqtr⊥ are more stable than 2ax by 2.2 and 2.8 kcal ‚mol-1, respectively. 10 Complex 2ax has the shortest distance for the CC bond formation, r (CCh)) 2.816 Å; the distance in 2eqtr⊥ is r (CCh)) 2.935 Å and in 2eqcis⊥ r (CCCH3)) 2.955 Å. Essentially the opposite is observed for the transition states for CC bond formation from 2ax: TS1 has the longest distance, r (CCh)) 1.953 Å, followed by TS3, r (CCCH3)) 1.980 Å. Since the reaction 2ax f TS1 f 3a is more