Chiral
propargylsilanes and chiral allenylsilanes have emerged
as versatile building blocks for organic synthesis. However, efficient
methods for preparing these organosilicon compounds are lacking. We
herein report a highly enantioselective method for synthesis of chiral
propargylsilanes and chiral allenylsilanes from readily available
alkynyl sulfonylhydrazones. Specifically, chiral spiro phosphate dirhodium
complexes were used to catalyze asymmetric insertion of alkynyl carbenes
into the Si–H bonds of silanes to afford a variety of chiral
propargylsilanes with excellent enantioselectivity. Subsequently,
a platinum catalyst was used for stereospecific isomerization of the
chiral propargylsilanes to the corresponding chiral allenylsilanes.
Catalytic
enantioselection usually depends on differences in steric
interactions between prochiral substrates and a chiral catalyst. We
have discovered a carbene Si–H insertion in which the enantioselectivity
depends primarily on the electronic characteristics of the carbene
substrate, and the log(er) values are linearly related to Hammett
parameters. A new class of chiral tetraphosphate dirhodium catalysts
was developed; it shows excellent activity and enantioselectivity
for the insertion of diarylcarbenes into the Si–H bond of silanes.
Computational and mechanistic studies show how the electronic differences
between the two aryls of the carbene lead to differences in energies
of the diastereomeric transition states. This study provides a new
strategy for asymmetric catalysis exploiting the electronic properties
of the substrates.
A new graphdiyne-based carbon material was synthesized as an ideal substrate for electroless deposition and stabilization of sub-nanometric Pd catalysts, which showed extremely high catalytic activities for the reduction of nitroarenes and Suzuki coupling reactions.
The Ir-catalyzed asymmetric hydrogenation of olefins is widely used for production of value-added bulk and fine chemicals. The iridium catalysts with chiral spiro phosphine-oxazoline ligands developed in our group show high activity and high enantioselectivity in the hydrogenation of olefins bearing a coordinative carboxyl group, such as α,β-unsaturated carboxylic acids, β,γ-unsaturated carboxylic acids, and γ,δ-unsaturated carboxylic acids. Here we conducted detailed mechanistic studies on these Ir-catalyzed asymmetric hydrogenation reactions by using (E)-2-methyl-3-phenylacrylic acid as a model substrate. We isolated and characterized several key intermediates having Ir-H bonds under the real hydrogenation conditions. Particularly, an Ir(III) migratory insertion intermediate was first isolated in an asymmetric hydrogenation reaction promoted by chiral Ir catalysts. That this intermediate cannot undergo reductive elimination in the absence of hydrogen strongly supports the involvement of an Ir(III)/Ir(V) cycle in the hydrogenation. On the basis of the structure of the Ir(III) intermediate, variable-temperature NMR spectroscopy, and density functional theory calculations, we elucidated the mechanistic details of the Ir-catalyzed hydrogenation of unsaturated carboxylic acids and explained the enantioselectivity of the reactions. These findings experimentally and computationally elucidate the mechanism of Ir-catalyzed asymmetric hydrogenation of olefins with a strong coordinative carboxyl group and will likely inspire further catalyst design.
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