Helical Poly(quinoxaline-2,3-diyl)s Bearing Metal-Binding Sites as Polymer-Based Chiral Ligands for Asymmetric Catalysis
作者:Takeshi Yamamoto、Michinori Suginome
DOI:10.1002/anie.200803719
日期:2009.1.5
Living it up: Helical polyquinoxalines with single and multiple metal‐binding sites, prepared by living polymerization of o‐diisocyanobenzenes, are used in the asymmetric hydrosilylation of styrenes, resulting in comparable enantioselectivities to those obtained by low‐molecular‐weight catalyst systems (up to 87 % ee, stereochemistry was determined by a chiral initiator) and a turnover number of almost
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.
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.