J Am Chem Soc. 2018 Mar 21;140(11):4144-4149. doi: 10.1021/jacs.8b01017. Epub 2018 Mar 9.
Attachment of N-heterocyclic carbenes (NHCs) on the surface of metal nanoparticle (NP) catalysts permits fine-tuning of catalytic activity and product selectivity. Yet, NHC-coated Au NPs have been seldom used in catalysis beyond hydrogenation chemistry. One challenge in this field has been to develop a platform that permits arbitrary ligand modification without having to compromise NP stability toward aggregation or leaching. Herein, we exploit the strategy of supported dendrimer-encapsulated metal clusters (DEMCs) to achieve aggregation-stable yet active heterogeneous Au NP catalysts with NHC ligands. Dendrimers function as aggregation-inhibitors during the NP synthesis, and NHCs, well-known for their strong attachment to the gold surface, provide a handle to modify the stereochemistry, stereoelectronics, and chemical functionality of the NP surface. Indeed, compared to "ligandless" Au NPs which are virtually inactive below 80 °C, the NHC-ligated Au NP catalysts enable a model lactonization reaction to proceed at 20 °C on the same time scale (hours). Based on Eyring analysis, proto-deauration is the turnover-limiting step accelerated by the NHC ligands. Furthermore, the use of chiral NHCs led to asymmetric induction (up to 16% enantiomeric excess) in the lactonization transformations, which demonstrates the potential of supported DEMCs with ancillary ligands in enantioselective catalysis.
将 N-杂环卡宾 (NHCs) 附着在金属纳米粒子 (NP) 催化剂的表面上，可以精细调节催化活性和产物选择性。然而，除了加氢化学之外，NHC 涂层的 Au NPs 在催化中很少被使用。该领域的一个挑战是开发一种允许任意配体修饰的平台，而不必牺牲 NP 对聚集或浸出的稳定性。在此，我们利用受支持的树状大分子封装金属簇 (DEMCs) 的策略，实现了具有 NHC 配体的聚集稳定但仍具有活性的非均相 Au NP 催化剂。树状大分子在 NP 合成过程中充当聚集抑制剂，而 NHC 因其与金表面的强附着而闻名，为修饰 NP 表面的立体化学、立体电子和化学功能提供了一种手段。事实上，与在 80°C 以下几乎没有活性的“无配体”Au NPs 相比，NHC 键合的 Au NP 催化剂使模型内酯化反应能够在相同的时间尺度（小时）内在 20°C 下进行。基于 Eyring 分析，原脱金是由 NHC 配体加速的周转率限制步骤。此外，手性 NHC 的使用导致内酯化转化中的不对称诱导（最高 16%对映体过量），这证明了辅助配体的受支持的 DEMCs 在对映选择性催化中的潜力。
