A Tamed Intermediate: The Pivotal Role of Cp* in Hypercoordinate Boron Cation Catalysis

Compared to borenium ions, the synthetic application of its dicoordinate variant, borinium ion, remains much less explored due to the facile decomposition of borinium ions. Previously, we showed that [Cp*B-Mes]+ ([1]+) can be viewed as a masked borinium ion with great potential in catalysis. In this study, we further demonstrate that [1]+ serves as a precatalyst in catalytic cyanosilylation of ketones. When treated with TMSCN, [1]+ transforms into a TMSCN coordinated boronium ion [Cp*BMes(TMSCN)2]+ ([4]+), which is highly efficient in catalyzing cyanosilylation. However, such reagent-coordinated boronium ion cannot be accomplished in the absence of Cp* ligand. Experimental and computational investigations revealed that the Cp* ligand could mitigate the Lewis acidity of the putative borenium ion, [Cp*BMes(TMSCN)]+ ([5]+), via the coordination of Cp* to boron in a η2-coordination fashion. Such a bonding interaction significantly reduces the positive charge on the boron atom [5]+, preventing the decomposition of the critical intermediate.

与硼鎓离子（borenium ions）相比，其二配位变体——硼宾离子（borinium ion）——的合成应用因后者极易发生分解而远未得到充分探索。此前本团队的研究表明，[Cp*B-Mes]+（即阳离子[1]+）可被视为一种掩蔽型硼宾离子，在催化领域具备极大应用潜力。本研究进一步证实，阳离子[1]+可作为酮类催化氰基硅烷化反应的预催化剂。当与三甲基硅基氰（TMSCN）反应时，阳离子[1]+会转化为配位有TMSCN的硼鎓离子[Cp*BMes(TMSCN)₂]+（即阳离子[4]+），该物种在催化氰基硅烷化反应中展现出极高的催化效率。然而，若缺乏五甲基环戊二烯基（Cp*）配体，则无法合成此类试剂配位型硼鎓离子。实验与计算研究共同揭示：Cp*配体可通过以η²配位模式与硼原子结合，缓解推定中间体[Cp*BMes(TMSCN)]+（即阳离子[5]+）——一种硼鎓离子——的路易斯酸性。此类成键相互作用可显著降低阳离子[5]+中硼原子的正电荷，从而避免关键中间体发生分解。