Catalytic Mechanisms of Direct Pyrrole Synthesis via Dehydrogenative Coupling Mediated by PNP-lr or PNN-Ru Pincer Complexes: Crucial Role of Proton-Transfer Shuttles in the PNP-lr System
Catalytic Mechanisms of Direct Pyrrole Synthesis via Dehydrogenative Coupling Mediated by PNP-lr or PNN-Ru Pincer Complexes: Crucial Role of Proton-Transfer Shuttles in the PNP-lr System
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PNP-lr 或 PNN-Ru 钳配合物介导的脱氢偶联直接合成吡咯的催化机制：质子转移梭在 PNP-lr 系统中的关键作用
DOI：
10.1021/ja411568a
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发表时间：
2014
影响因子：
15
通讯作者：
Wang Zhi-Xiang
中科院分区：
文献类型：
--
作者：
Qu Shuanglin;Dang Yanfeng;Song Chunyu;Wen Mingwei;Huang Kuo-Wei;Wang Zhi-Xiang
肯佩等人。和米尔斯坦等人。最近，他们提出了脱氢偶联方法，分别使用 PNP-Ir (1) 和 PNN-Ru (2) 钳形配合物，从仲醇（例如 3）和 β-氨基醇（例如 4）合成吡咯。我们在此提出了一项 DFT 研究来表征这些反应的催化机制。经过预催化剂活化得到活性 1A/2A 后，转化经过四个阶段进行：1A/2A 催化醇 (3) 脱氢得到酮 (11)，碱促进 11 和 4 的 C-N 偶联形成亚胺-醇中间体 (18)，碱促进 18 环化，以及通过 H2 释放进行催化剂再生 从1R/2R。对于醇脱氢，双功能双氢转移途径比β-氢消除途径更有利。一般来说，质子转移（H 转移）穿梭机促进两个系统中的各种 H 转移过程。尽管如此，氢转移梭在 PNP-Ir 系统中比在 PNN-Ru 系统中发挥着更重要的作用。如果没有氢转移梭，PNP-Ir 系统中高达 45.9 kcal/mol 的关键势垒太高而难以达到，而 PNN-Ru 系统中相应的势垒 (<32.0 kcal/mol) 也并非无法达到。两个系统之间的另一个显着差异是，向 1 中添加醇得到烷氧基复合物是吸能的，为 8.1 kcal/mol，而向 2 A 中添加醇时为放能的，为 8.9 kcal/mol。热力学差异可能是 PNP-Ir 系统需要比 PNN-Ru 系统更低的催化剂负载量的主要原因。我们讨论了催化剂的电子和几何结构方面的差异是如何产生的，以及如何在催化剂开发中使用这些特征。
Kempe et al. and Milstein et al. have recently advanced the dehydrogenative coupling methodology to synthesize pyrroles from secondary alcohols (e.g.,3) and β-amino alcohols (e.g.,4), using PNP-Ir (1) and PNN-Ru (2) pincer complexes, respectively. We herein present a DFT study to characterize the catalytic mechanism of these reactions. After precatalyst activation to give active1A/2A, the transformation proceeds via four stages:1A/2A-catalyzed alcohol (3) dehydrogenation to give ketone (11), base-facilitated C–N coupling of11and4to form an imine-alcohol intermediate (18), base-promoted cyclization of18, and catalyst regeneration via H2release from1R/2R. For alcohol dehydrogenations, the bifunctional double hydrogen-transfer pathway is more favorable than that via β-hydride elimination. Generally, proton-transfer (H-transfer) shuttles facilitate various H-transfer processes in both systems. Notwithstanding, H-transfer shuttles play a much more crucial role in the PNP-Ir system than in the PNN-Ru system. Without H-transfer shuttles, the key barriers up to 45.9 kcal/mol in PNP-Ir system are too high to be accessible, while the corresponding barriers (<32.0 kcal/mol) in PNN-Ru system are not unreachable. Another significant difference between the two systems is that the addition of alcohol to1Agiving an alkoxo complex is endergonic by 8.1 kcal/mol, whereas the addition to2Ais exergonic by 8.9 kcal/mol. The thermodynamic difference could be the main reason for PNP-Ir system requiring lower catalyst loading than the PNN-Ru system. We discuss how the differences are resulted in terms of electronic and geometric structures of the catalysts and how to use the features in catalyst development.