A new class of bis‐cyclometalated rhodium(III) catalysts containing two inert cyclometalated 6‐tert‐butyl‐2‐phenyl‐2H‐indazole ligands and two labile acetonitriles is introduced. Single enantiomers (>99 % ee) were obtained through a chiral‐auxiliary‐mediated approach using a monofluorinated salicyloxazoline. The new chiral‐at‐metal complex is capable of catalyzing the visible‐light‐induced enantioselective α‐cyanoalkylation of 2‐acyl imidazoles in which it serves a dual function as the chiral Lewis acid catalyst for the asymmetric radical chemistry and at the same time as the photoredox catalyst for the visible‐light‐induced redox chemistry (up to 80 % yield, 4:1 d.r., and 95 % ee, 12 examples).
Chiral transition metal catalysts represent a powerful
and economic
tool for implementing stereocenters in organic synthesis, with the
metal center providing a strong chemical activation upon its interaction
with substrates or reagents, while the overall chirality of the metal
complex achieves the desired stereoselectivity. Often, the overall
chiral topology of the metal complex implements a stereogenic metal
center, which is then involved in the origin of the asymmetric induction.
This review provides a comprehensive survey of reported chiral transition
metal catalysts in which the metal formally constitutes a stereocenter.
A stereogenic metal center goes along with an overall chiral topology
of the metal complex, regardless of whether the ligands are chiral
or achiral. Implications for the catalyst design and mechanism of
asymmetric induction are discussed for half-sandwich, tetracoordinated,
pentacoordinated, and hexacoordinated chiral transition metal complexes
containing a stereogenic metal center. The review distinguishes between
chiral metal catalysts originating from the coordination to chiral
ligands and those which are solely composed of optically inactive
ligands (achiral or rapidly interconverting enantiomers) prior to
complexation (dubbed “chiral-at-metal” catalysts).
The mechanism of [2 + 2] cycloadditions activated by visible light and catalyzed by bis-cyclometalated Rh(iii) and Ir(iii) photocatalysts was investigated, combining density functional theory calculations and spectroscopic techniques.
A class of stereogenic-at-iron
catalysts is introduced and applied
to asymmetric 3d-transition metal catalysis. The pentadentate coordination
of a tripodal ligand creates a stereogenic iron center, while an isopropyl
group in the ligand backbone serves as a chiral lever to thermodynamically
favor one diastereomer (>20:1 dr) of the obtained iron complexes.
The coordination sphere is completed with a labile acetonitrile ligand.
Ligand coordination and dynamics were studied by NMR spectroscopy.
A modular design of the tripodal ligand enables the convenient implementation
of different modifications, which have been incorporated and investigated
in this work. The catalytic performance of the obtained complexes
was evaluated in the ring contraction of isoxazoles to chiral 2H-azirines in up to 99% yield and with up to 93% ee and
features a remarkably high catalytic activity with up to 10,000 turnover
numbers (TON).
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