Stereodynamics and Asymmetric Hydrosilylation with Chiral Rhodium Complexes Containing a Monodentate N-Heterocyclic Carbene

Imidazolinium salts derived from (1S,2S)-diphenylethylenediamine were used to prepare chiral
N-heterocyclic carbene (NHC) complexes of rhodium of the form (cod)Rh(NHC)Cl via silver
transmetalation. The neutral complexes were readily converted to cationic amine analogues [(cod)Rh(NHC)(L)]SbF6, where L = isoquinoline and 3,5-lutidine. The NHC ligands contain unsymmetrically
substituted wingtips that give rise to chiral axes; the conformations of the substituents, as well as other
ligands, are influenced by the fixed chirality in the backbone of the ligand. The stereodynamics of both
the neutral and cationic complexes were studied, and it was observed that strong conformational preferences
exist both in solution and in the solid state. The interplay of the various chiral elements provides the
environment that eventually controls the enantioselectivity in the catalytically active species. These
complexes are active catalysts for the hydrosilylation of acetophenone with diphenylsilane and furnished
(S)-1-phenylethanol in up to 58% enantiomeric excess.

以(1S,2S)-二苯基乙二胺衍生得到的咪唑鎓盐为起始原料，通过银转金属化反应制备了结构为(cod)Rh(NHC)Cl的铑手性氮杂环卡宾（N-heterocyclic carbene, NHC）配合物。将上述中性配合物进一步转化为阳离子胺类衍生物[(cod)Rh(NHC)(L)]SbF6，其中L分别为异喹啉与3,5-二甲基吡啶。该类NHC配体带有不对称取代的翼状取代基，从而形成手性轴；配体骨架上的固定手性会对取代基及其他配体的构象产生影响。研究了中性与阳离子配合物的立体动力学行为，结果显示其在溶液与固态中均存在显著的构象偏好。多种手性因素的协同作用构建了可最终调控催化活性物种对映选择性的微环境。此类配合物可作为苯乙酮与二苯基硅烷氢化硅烷化反应的活性催化剂，最高可得到对映体过量值为58%的(S)-1-苯乙醇产物。