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首页> 外文期刊>Chemistry: A European journal >Racemization of Alcohols Catalyzed by [RuCl(CO)_2(η~5-pentaphenylcyclopentadienyl)]—Mechanistic Insights from Theoretical Modeling
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Racemization of Alcohols Catalyzed by [RuCl(CO)_2(η~5-pentaphenylcyclopentadienyl)]—Mechanistic Insights from Theoretical Modeling

机译：[RuCl（CO）_2（η〜5-五苯基苯基戊二烯基）]催化的醇的外消旋化-从理论建模的机理研究

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Two possible pathways of inner-sphere racemization of sec-alcohols by using the [RuCl(CO)_2(η~5-pentaphenylcyclopentadienyl)] catalyst (1) have been thoroughly investigated by means of density function calculations. To be able to racemize alcohols, catalyst 1 needs to have a free coordination site on the metal. This can be achieved either by a η5→η3 ring slippage or by dissociation of a carbon monoxide (CO) ligand. The η5→η3 ring-slip pathway was found to have a high potential energy barrier, 42 kcalmol~(-1), which can be explained by steric congestion in the transition state. On the other hand, CO dissociation to give a 16-electron complex has a barrier of only 22.6 kcalmol~(-1). We have computationally discovered a mechanism involving CO participation that does not require h5!h3 ring slippage. The key features of this mechanism are 1) CO-assisted exchange of chloride for alkoxide, 2) alcohol–alkoxide exchange, and 3) generation of an active 16-electron complex through CO dissociation with subsequent b-hydride elimination as the racemization step. We have found a low-energy pathway for reaction of 1 with potassium tert-butoxide and a pathway for fast alkoxide exchange with interaction between the incoming/ leaving alcohol and one of the two CO ligands. We predict that dissociation of a Ru-bound CO ligand does not occur in these exchange reactions. Dissociation of one of the two Ru-bound CO ligands has been found necessary only at a later stage of the reaction. Though this barrier is still quite high, our results indicate that it is not necessary to cross the CO dissociation barrier for the racemization of each new alcohol. Thus, the dissociation of a CO ligand is interpreted as a rate-limiting reaction step in order to create a catalytically active 16-electron complex.
机译：利用[RuCl（CO）_2（η〜5-5-五苯基苯基戊二烯基）]催化剂（1），对仲醇内球消旋的两个可能途径进行了深入研究。为了能够消旋醇，催化剂1需要在金属上具有自由的配位点。这可以通过η5→η3环滑移或一氧化碳（CO）配体的离解来实现。发现η5→η3环滑途径具有较高的势能垒，为42 kcalmol〜（-1），这可以用过渡态的空间拥塞来解释。另一方面，CO离解生成16电子配合物的势垒仅为22.6 kcalmol〜（-1）。我们通过计算发现了一种涉及CO参与的机制，该机制不需要h5！h3环滑移。该机制的关键特征是：1）氯化物辅助氯化物交换为烷氧化物; 2）醇-烷氧化物交换; 3）通过CO分解生成活性的16电子配合物，随后将消旋的b-氢化物消消为外消旋步骤。我们发现了1与叔丁醇钾反应的低能途径，以及通过进/出醇与两个CO配体之一相互作用而快速交换烷氧化物的途径。我们预测在这些交换反应中不会发生Ru结合的CO配体的解离。发现仅在反应的后期才需要两个Ru-键合的CO配体之一的解离。尽管该障碍仍然很高，但我们的结果表明，对于每种新醇的外消旋化，都没有必要跨越CO解离障碍。因此，将CO配体的解离解释为限速反应步骤，以产生催化活性的16电子络合物。

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