Asymmetric hydrosilylation of dihydrofurans by use of palladium-MOP catalyst

Amide-Directed, Rhodium-Catalyzed Enantioselective Hydrosilylation of Unactivated Internal Alkenes

Despite the recent advances made in the area of asymmetric hydrosilylation, metal-catalyzed enantioselective hydrosilylation of unactivated internal alkenes remains a challenge. Here, we report a rhodium-catalyzed enantioselective hydrosilylation of unactivated internal alkenes bearing a polar group. The coordination assistance by an amide group enables the hydrosilylation to occur with high regio- and enantioselectivity.

Catalytic asymmetric transformations of oxa- and azabicyclic alkenes

This review provides an overview of the fundamental concepts and recent developments in a wide range of enantioselective transformations involving oxa- and azabicyclic alkenes.

Copper-Catalyzed Enantioselective and Exo-Selective Addition of Silicon Nucleophiles to 7-Oxa- and 7-Azabenzonorbornadiene Derivatives

An enantioselective formal hydrosilylation of 7-oxa- and 7-azabenzonorbornadiene derivatives is reported. The exo-selective addition of the silicon moiety across these strained alkenes is achieved under copper catalysis, employing Si-B reagents as silicon pronucleophiles in the presence of an alkoxide base. No ring opening is observed. While successful for those substrates, the same procedure cannot be applied to benzonorbornadiene, norbornadiene, or norbornene.

Synthesis and Catalytic Applications of a Triptycene-Based Monophosphine Ligand for Palladium-Mediated Organic Transformations

1-Methoxy-8-(diphenylphosphino)triptycene (1), featuring high structural rigidity and steric bulkiness around the phosphine functionality, was synthesized as a new chiral monophosphine ligand for transition metal-catalyzed reactions. In the presence of 5-10 mol ppm (i.e., 0.0005-0.001 mol %) Pd(OAc)₂ and 1 (2 equiv for Pd), Suzuki-Miyaura cross-coupling reactions of aryl bromides and arylboronic acids proceeded effectively under mild atmospheric conditions to give the corresponding biaryl compounds in a high yield. The single-crystal X-ray analysis of a Pd(II) complex of 1 revealed its coordination structure, in which two homochiral molecules form a dimer, suggesting that triptycene could provide a chiral environment for asymmetric organic transformations. In fact, optically active 1 obtained by optical resolution showed good enantioselectivity in the palladium-catalyzed asymmetric hydrosilylation of styrene, which represents, for the first time, the asymmetric catalytic activity of triptycene-based monophosphine ligands.

Asymmetric reductive ring-opening of bicyclic olefins catalyzed by palladium and nickel complexes

[reaction: see text] Asymmetric reductive ring opening of oxa- and azabenzonorbornadienes with organic acids and zinc powder under mild conditions catalyzed by Ni(binap)Cl(2) or Pd(binap)I(2) produces the corresponding 1,2-dihydronaphth-1-ols in good to excellent yields with high enantioselectivity.

Highly enantioselective hydrosilylation of aromatic alkenes

Currently, the most effective and economic way to convert an alkene into an optically active alcohol is the two-step sequence: hydrosilylation/oxidation. Much work has been devoted to elucidating effective catalysts for this process, but hitherto only one effective and highly stereoselective process has been available. Herein we present a novel catalytic system for the asymmetric hydrosilylation of aromatic alkenes, giving the products in high yields and with the highest enantioselectivity (up to 99% ee) ever observed for this reaction. The reaction works efficiently for a variety of substituted aromatic alkenes, giving access after Tamao oxidation to almost optically pure benzylic alcohols in high yields.

Asymmetric hydrosilylation of styrenes catalyzed by palladium-MOP complexes: ligand modification and mechanistic studies

In the palladium-catalyzed asymmetric hydrosilylation of styrene (3a) with trichlorosilane, several chiral monophosphine ligands, (R)-2-diarylphosphino-1,1'-binaphthyls (2a-g), were examined for their enantioselectivity. The highest enantioselectivity was observed in the reaction with (R)-2-bis[3,5-bis(trifluoromethyl)phenyl]phosphino-1,1'-binaphthyl (2g), which gave (S)-1-phenylethanol (5a) of 98% ee after oxidation of the hydrosilylation product, 1-phenyl-1-(trichlorosilyl)ethane (4a). The palladium complex of 2g also efficiently catalyzed the asymmetric hydrosilylation of substituted styrenes on the phenyl ring or at the beta position to give the corresponding chiral benzylic alcohols of over 96% ee. Deuterium-labeling studies on the hydrosilylation of regiospecifically deuterated styrene revealed that beta-hydrogen elimination from 1-phenylethyl(silyl)palladium intermediate is very fast compared with reductive elimination giving hydrosilylation product when ligand 2g is used. The reaction of o-allylstyrene (9) with trichlorosilane catalyzed by (R)-2g/Pd gave (1S,2R)-1-methyl-2-(trichlorosilylmethyl)indan (10) (91% ee) and (S)-1-(2-(propenyl)phenyl)-1-trichlorosilylethanes (11a and 11b) (95% ee). On the basis of their opposite configurations at the benzylic position, a rationale for the high enantioselectivity of ligand 2g is proposed.

Chiral monodentate phosphine ligand MOP for transition-metal-catalyzed asymmetric reactions

Chiral monophosphines, whose chirality is due to biaryl axial chirality, have been prepared from enantiomerically pure 2, 2'-dihydroxy-1,1'-binaphthyl and demonstrated to be highly efficient chiral ligands for transition-metal-catalyzed organic transformations, especially for reactions where chelating bisphosphine ligands cannot be used. The high efficiency is observed in palladium-catalyzed asymmetric hydrosilylation of a wide variety of olefins such as alkyl-substituted terminal olefins and in asymmetric reactions via pi-allylpalladium intermediates represented by asymmetric reduction of allylic esters with formic acid.

Cyclization of o-Allylstyrene via Hydrosilylation: Mechanistic Aspects of Hydrosilylation of Styrenes Catalyzed by Palladium-Phosphine Complexes

Hydrosilylation of o-allylstyrene with trichlorosilane in the presence of 0.3 mol % of a palladium catalyst bearing triphenylphosphine gave trans-1-methyl-2-(trichlorosilylmethyl)indan, 1-(2-(2-propenyl)phenyl)-1-trichlorosilylethane, and 1-(2-((E)-1-propenyl)phenyl)-1-trichlorosilylethane. The reaction of styrene with trichlorosilane gave a quantitative yield of 1-phenyl-1-(trichlorosilyl)ethane while allylbenzene did not give silylation products under the same reaction conditions. These results show that the hydropalladation process is operative in the hydrosilylation of styrene derivatives with trichlorosilane catalyzed by palladium-phosphine complexes.