High-Molecular-Weight Polyquinoxaline-Based Helically Chiral Phosphine (PQXphos) as Chirality-Switchable, Reusable, and Highly Enantioselective Monodentate Ligand in Catalytic Asymmetric Hydrosilylation of Styrenes
A polyquinoxaline-basedhelical polymer ligand bearing both helical-sense-determining chiral side chains and coordinating diarylphosphino side chains exhibits solvent-dependent formation of P- or M-helical structures, with which either the S- or R-hydrosilylation product was obtained with high (>93% enantiomeric excess) enantioselectivities.
A highlyefficient majority‐rules effect of poly(quinoxaline‐2,3‐diyl)s (PQXs) bearing 2‐butoxymethyl chiral side chains at the 6‐ and 7‐positions was established and attributed to large ΔGh values (0.22–0.41 kJ mol−1), which are defined as the energy difference between P‐ and M‐helical conformations per chiral unit. A PQX copolymer prepared from a monomer derived from (R)‐2‐octanol (23 % ee) and a
Palladium-catalysed asymmetric hydrosilylation of styrenes with a new chiral monodentate phosphine ligand
作者:Kenji Kitayama、Yasuhiro Uozumi、Tamio Hayashi
DOI:10.1039/c39950001533
日期:——
hydrosilylation of styrenes (ArCHCHR) with trichlorosilane in the presence of a palladium catalyst (0.1 mol%) bearing a new chiralmonodentatephosphineligand, (S)-2-diphenylphosphino-1,1′-binaphthyl [(S)-H-MOP], followed by oxidation of the resulting 1-aryl-1-silylalkanes, gives opticallyactive benzylic alcohols of up to 96% enantiomeric excess (e.e.).
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-l,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 P-hydrogen elimination from l-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.
Asymmetric palladium-catalyzed hydrosilylation of styrenes using efficient chiral spiro phosphoramidite ligands
Asymmetric hydrosilylation of styrene derivatives with trichlorosilane in the presence of palladium complexes of chiral spiro phosphoramidites provided 1-aryl-1-silylalkanes as single regioisomers in high yields, which have been oxidized with hydrogen peroxide to give the corresponding chiral alcohols in up to 99.1% ee. (C) 2004 Elsevier Ltd. All rights reserved.