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.