Gold Catalysis: Hydrolysis of Di(alkoxy)carbenium Ion Intermediates as a Sensor for the Electronic Properties of Gold(I) Complexes

Six different cationic gold(I) complexes LAu+ were converted to the corresponding di(alkoxy)carbenium ions by reaction with ethyl 2,5-dimethylhexa-2,3-dienoate. These conversions were monitored by in situ IR spectroscopy; at room temperature they proceeded in only a few seconds. The ligands L are based on the most popular ligand types in gold catalysis: phosphanes, phosphites, carbenes, and isonitriles. The di(alkoxy)carbenium ions were stable, not short-lived intermediates, and could be characterized. This allowed the kinetic study of the next step, the hydrolytic cleavage to the Hammond-type vinylgold species. Depending on the ligand on gold, large rate differences were detected. Computational chemistry revealed a correlation of the experimental reaction rates with the LUMO energies of the di(alkoxy)carbenium species and the direct influence of the ligand on gold on these LUMO energies. Thus, the di(alkoxy)carbenium ion could be utilized as an easy to use benchmark system for the electronic characterization of LAu+ catalysts by theory, spectroscopy, and kinetic experiments.

六种通式为LAu+的阳离子金(I)配合物，与2,5-二甲基己-2,3-二烯酸乙酯反应后可转化为对应的二(烷氧基)碳正离子。通过原位红外光谱(in situ IR spectroscopy)对该转化过程进行监测，结果显示室温下反应仅需数秒即可完成。所用配体L均涵盖金催化领域最主流的配体类型：膦配体(phosphanes)、亚磷酸酯配体(phosphites)、卡宾配体(carbenes)与异腈配体(isonitriles)。所得二(烷氧基)碳正离子为稳定而非瞬态的中间体，可完成结构表征。这一特性使得我们得以开展后续反应的动力学研究：该中间体经水解裂解生成哈蒙德型乙烯基金物种(Hammond-type vinylgold species)。研究发现，金中心所连配体的差异会导致反应速率出现显著差异。计算化学(computational chemistry)研究表明，实验测得的反应速率与二(烷氧基)碳正离子的最低未占据分子轨道(LUMO)能量存在显著关联，且金中心配体可直接对该LUMO能量产生影响。因此，二(烷氧基)碳正离子可作为便捷易用的基准体系，通过理论计算、光谱表征与动力学实验，实现对LAu+催化剂的电子性质表征。