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

Asymmetric Hydrosilylation of β-Silyl Styrenes Catalyzed by a Chiral Palladium Complex

A palladium complex coordinated with a chiral SIPHOS ligand was evaluated as an efficient catalyst for asymmetric hydrosilylation of β-silyl styrenes with trichlorosilane and 23 1,2-bis(silyl) chiral compounds were produced. Good to excellent enantioselectivities were observed with 1-aryl-2-silyl ethanols, where the trichlorosilyl groups of the hydrosilylation products were selectively converted into a hydroxyl group in the presence of pre-installed trialkylsilyl groups. Asymmetric hydrosilylation of β-silyl styrenes followed by methylation of the trichlorosilyl group gave stable 1,2-bis(silyl) chiral compounds 4 with excellent yields. DFT calculations of hydridopalladium B coordinated with a SIPHOS ligand, an intermediate of the hydrosilylation reaction, established the optical structures to be energy minima, and the structural information could well illustrate the enantioselectivity for the hydrosilylation reaction.

Palladium(0)-catalyzed C(sp3)-Si bond formation via formal carbene insertion into a Si-H bond

Pd(0)-Catalyzed formal carbene insertion into Si-H bonds has been achieved as an efficient method for C(sp3)-Si bond formation. The reaction, which uses readily available N-tosylhydrazones as the diazo precursors, is highly efficient and shows a wide substrate scope. Rh(ii) and Cu(i) salts, which are the widely used catalysts for carbene insertion reactions, have been proved to be ineffective for the current reaction. A Pd(ii) carbene migratory insertion/reductive elimination mechanism is proposed.

Diastereodivergent Reductive Cross Coupling of Alkynes through Tandem Catalysis: Z- and E-Selective Hydroarylation of Terminal Alkynes

A diastereodivergent hydroarylation of terminal alkynes is accomplished using tandem catalysis. The hydroarylation allows highly selective synthesis of both E and Z diastereoisomers of aryl alkenes, from the same set of starting materials, using the same combination of palladium and copper catalysts. The selectivity is controlled by simple changes in the stoichiometry of the alcohol additive. The hydroarylation has excellent substrate scope and can be accomplished in the presence of various classes of compounds, including esters, nitriles, alkyl halides, epoxides, carbamates, acetals, ethers, silyl ethers, and thioethers. The Z-selective hydroarylation is accomplished using a new approach based on tandem Sonogashira coupling and catalytic semireduction. The E-selective hydroarylation involves an additional catalytic isomerization of the Z-alkene. Our explorations of the reaction mechanism explain the role of individual reaction components and how the subtle changes in the reaction conditions influence the rates of specific steps of the hydroarylation. Our studies also show that, although the Z- and E-selective hydroarylation reactions are mechanistically closely related, the roles of the palladium and copper catalysts in the two reactions are different.

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 Copper Hydride-Catalyzed Markovnikov Hydrosilylation of Vinylarenes and Vinyl Heterocycles

We report a highly enantioselective CuH-catalyzed Markovnikov hydrosilylation of vinylarenes and vinyl heterocycles. This method has a broad scope and enables both the synthesis of isolable silanes and the conversion of crude products to chiral alcohols. Density functional theory calculations support a mechanism proceeding by hydrocupration followed by σ-bond metathesis with a hydrosilane.

Highly efficient regioselective hydrosilylation of allenes using a [(3IP)Pd(allyl)]OTf catalyst; first example of allene hydrosilylation with phenyl- and diphenylsilane

A [(3IP)Pd(allyl)]OTf complex was shown to have high activity and regioselectively for allene hydrosilylation employing a wide range of silanes.

Asymmetric reduction of ketones by biocatalysis using clementine mandarin (Citrus reticulata) fruit grown in Annaba or by ruthenium catalysis for access to both enantiomers

Biocatalytic reduction of prochiral ketones using freshly ripened clementine mandarin (Citrus reticulata) in aqueous medium is reported. High enantioselectivities were observed, especially for the bioreduction of indanone , tetralone , and thiochromanone with respectively 95%, 99%, and 86% enantiomeric excess (ee). Enantioselective bio- and metal-catalyzed reactions were compared. Chiral ruthenium catalysts afforded good asymmetric inductions (>75% ee) in most cases, enantiomeric excesses depending on the nature of substrate and ligand. N-aminoindanol prolinamide was revealed as the best ligand for most ketones. Interestingly, for several substrates both enantiomers could be obtained using either Citrus reticulata or ruthenium complex.

Complementary Induction of Right- and Left-Handed Helical Structures by the Positioning of Chiral Groups on the Monomer Units: Introduction of (-)-Menthol as Side Chains of Poly(quinoxaline-2,3-diyl)s

Poly(quinoxaline-2,3-diyl) bearing menthyloxymethyl side chains derived from (-)-menthol at the 6- and 7-positions of the quinoxaline ring showed a single, right-handed helical structure in chloroform. Upon introduction of the same (-)-menthol-derived side chains into the 5- and 8-positions, a single, left-handed helical structure was formed in chloroform. The former poly(quinoxaline-2,3-diyl)s showed solvent-dependent inversion of the helical sense in 1,1,2-trichloroethane to form a left-handed helical structure with high screw sense purity. Copolymers bearing both menthyloxymethyl and o-(diphenylphosphino)phenyl groups in their side chains served as highly enantioselective chiral ligands in the palladium-catalyzed hydrosilylation of styrene.

Enantioselective catalysis with a chiral, phosphane-containing PMO material

A novel bistriethoxysilyl-BINAP monomer was prepared and co-condensed with a biphenylene-bridged siloxane precursor in the presence of surfactant templates to give periodic mesoporous organosilicas (PMOs) functionalized with BINAP. Complexation of ruthenium followed by asymmetric catalytic hydrogenation and asymmetric transfer hydrogenation were carried out, and demonstrated that high levels of activity and selectivity are achievable with the chiral material.

Asymmetric Silaboration of Terminal Allenes Bearing α-Stereogenic Centers: Stereoselection Based on “Reagent Control”

A highly enantioface-selective silaboration of allenes having stereogenic centers at the alpha-positions of the double bonds has been achieved using a combination of a chiral silylborane (-)-2 and a chiral Pd/(R)-3 catalyst. The chiral reagent system efficiently controlled the stereochemistry of the new stereogenic centers even in the reactions of mismatched combinations. [reaction: see text]