Tricyclic Sulfoxide–Alkene Hybrid Ligands for Chiral Rh(I) Complexes: The “Matched” Diastereomer Catalyzes Asymmetric C–C Bond Formations

Deprotonation of phenyldibenzo­[b,f]­tropylidene (8) with LDA/t-BuOK followed by quenching with either diastereomer of inexpensive glucose-based t-Bu-sulfinate (R)- or (S)-11 affords a sulfoxide–alkene hybrid ligand as the diastereomeric pairs (SS,SC)-9/(SS,RC)-10 and (RS,RC)-9/(RS,SC)-10, respectively, which via chromatographic/recrystallization may be separated into the four isomers. The optically pure diastereomeric ligands (SS,SC)-9 and (SS,RC)-10 react with [RhCl­(coe)2]2 to form the dinuclear complexes (RS,SC)-11 and (RS,RC)-12, respectively, in which the bidentate ligands coordinate the metal centers through the sulfur and alkene donor functions. These complexes catalyze the conjugate addition of arylboronic acids to cyclic Michael acceptors with enantioselectivities of up to 99% ee. DFT calculations show the preponderant influence of planar chirality of the ligand alkene function. The enantioselectivity switch observed between (RS,SC)-11 and (RS,RC)-12 is explained by the inverted cis–trans coordinations of the substrate molecules in catalytic steps.

将苯基二苯并[b,f] tropylidene（8）用二异丙基氨基锂（Lithium Diisopropylamide, LDA）/叔丁醇钾（potassium tert-butoxide, t-BuOK）进行去质子化，随后分别以廉价葡萄糖基叔丁基亚磺酸盐的两种非对映异构体（(R)-或(S)-11）进行淬灭反应，可分别得到亚砜-烯烃杂合配体的非对映异构体对：(SS,SC)-9/(SS,RC)-10与(RS,RC)-9/(RS,SC)-10；通过色谱分离与重结晶可将该混合物分离为四种单异构体。光学纯的非对映异构配体(SS,SC)-9与(SS,RC)-10分别与[RhCl(coe)₂]₂反应，生成双核配合物(RS,SC)-11与(RS,RC)-12；在此类配合物中，双齿配体通过硫原子与烯烃给体官能团与金属中心配位。该类配合物可催化芳基硼酸与环状迈克尔受体的共轭加成反应，最高可获得99%对映体过量值（ee）的对映选择性。密度泛函理论（DFT）计算结果表明，配体烯烃官能团的平面手性对反应具有主导性影响。实验中观察到的(RS,SC)-11与(RS,RC)-12之间的对映选择性反转现象，可通过催化过程中底物分子的顺式-反式配位模式反转得到合理解释。