Palladium-catalyzed Asymmetric Hydrosilylation Of Styrene Research Articles

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.

ChemInform Abstract: Asymmetric Hydrosilylation of Styrene by Palladium Catalysts Coordinated with Chiral Phosphoramidite Ligands from 3,3′‐Disubstituted 1,1′‐Binaphthols.

Owing to its high catalytic activity, perfect regioselectivity, and an easy-manipulation without producing any byproduct, palladium-catalyzed asymmetric hydrosilylation of styrene has become a potent methodology to show their efficiencies of newly-developed chiral monodentate phosphorus ligands recently. Ever since a significant improvement for catalytic activity and enantioselectivity in the hydrosilylation of styrenes was achieved by chiral monophosphine (MOP) ligands based on the 2-diphenylphosphino-1,1'-binaphthyl skeleton, several monodentate phosphorus ligands, such as phosphoramidites, planar chiral monophosphines, and helically chiral phosphines have been applied to the hydrosilylation of styrenes to afford moderate to excellent enantioselectivities. Of those monodentate ligands, H-MOP derivative (R)-L1 lacking a substituent at the 2'-position of the binaphthyl and a phosphoramidite ligand (SA,RC,RC)-L2 3a derived from (S)-1,1'-binaphthol and bis((R)-1-phenylethyl)amine have been reported to be the most efficient ligands in the hydrosilylation of styrenes (Figure 1). In particular, chiral phosphoramidites have been paid much attention mainly due to their excellent catalytic activities and stereoselectivities in various catalytic reactions and easy accessibility from readily available diols and amines nowadays. On continuous efforts to expand the substrate scope of the hydrosilylation with various ligands, we are interested in the modifications of phosphoramidite ligands based on the 1,1'-binaphthol to evaluate substituent effects on enantioselectivity in the hydrosilylation. Here we wish to report a palladium-catalyzed asymmetric hydrosilylation of styrene (1) with trichlorosilane using chiral phosphoramidites L3 derived from (S)-3,3'-disubstituted 1,1'-binaphthols and secondary amines. In the hydrosilylation of styrene, 1-phenyl-1-(trichlorosilyl)ethane (2) was obtained in high yield and the benzylic silane 2 was transformed to optically active 1-phenylethanol (3) by a stereospecific oxidative cleavage of the C-Si bond with retention of configuration (Scheme 1). Chiral ligands L3 were prepared from (S)-1,1'-binaphthols having diphenyl or dimethyl groups at 3,3'-positions of the binaphthyl according to the known procedure. The hydrosilylation was carried out without solvent in the presence of 1.0 mol % of palladium catalysts generated in situ by mixing [PdCl(πC3H5)]2 and L3 (Pd/P = 1/2). The reactions were completed 20 C within 20 h to give 2, whose enantioselectivities were determined after the stereospecific oxidation into 3. The catalytic activity with the ligands L3 was high enough with all the ligands except for L3a and L3d, however, the enantioselectivities varied according to the ligands as shown in Table 1. The enantioselectivities were strongly dependent on the substituents at 3,3'-positions of the binaphthyl in the ligands. It was found that the ligands having diphenyl groups induced the higher enantioselectivities. Thus, the phosphoramidites from 3,3'-diphenylbinaphthol showed the higher

Asymmetric Hydrosilylation of Styrene by Palladium Catalysts Coordinated with Chiral Phosphoramidite Ligands from 3,3'-Disubstituted 1,1'-Binaphthols

Owing to its high catalytic activity, perfect regioselectivity, and an easy-manipulation without producing any byproduct, palladium-catalyzed asymmetric hydrosilylation of styrene has become a potent methodology to show their efficiencies of newly-developed chiral monodentate phosphorus ligands recently. Ever since a significant improvement for catalytic activity and enantioselectivity in the hydrosilylation of styrenes was achieved by chiral monophosphine (MOP) ligands based on the 2-diphenylphosphino-1,1'-binaphthyl skeleton, several monodentate phosphorus ligands, such as phosphoramidites, planar chiral monophosphines, and helically chiral phosphines have been applied to the hydrosilylation of styrenes to afford moderate to excellent enantioselectivities. Of those monodentate ligands, H-MOP derivative (R)-L1 lacking a substituent at the 2'-position of the binaphthyl and a phosphoramidite ligand (SA,RC,RC)-L2 3a derived from (S)-1,1'-binaphthol and bis((R)-1-phenylethyl)amine have been reported to be the most efficient ligands in the hydrosilylation of styrenes (Figure 1). In particular, chiral phosphoramidites have been paid much attention mainly due to their excellent catalytic activities and stereoselectivities in various catalytic reactions and easy accessibility from readily available diols and amines nowadays. On continuous efforts to expand the substrate scope of the hydrosilylation with various ligands, we are interested in the modifications of phosphoramidite ligands based on the 1,1'-binaphthol to evaluate substituent effects on enantioselectivity in the hydrosilylation. Here we wish to report a palladium-catalyzed asymmetric hydrosilylation of styrene (1) with trichlorosilane using chiral phosphoramidites L3 derived from (S)-3,3'-disubstituted 1,1'-binaphthols and secondary amines. In the hydrosilylation of styrene, 1-phenyl-1-(trichlorosilyl)ethane (2) was obtained in high yield and the benzylic silane 2 was transformed to optically active 1-phenylethanol (3) by a stereospecific oxidative cleavage of the C-Si bond with retention of configuration (Scheme 1). Chiral ligands L3 were prepared from (S)-1,1'-binaphthols having diphenyl or dimethyl groups at 3,3'-positions of the binaphthyl according to the known procedure. The hydrosilylation was carried out without solvent in the presence of 1.0 mol % of palladium catalysts generated in situ by mixing [PdCl(πC3H5)]2 and L3 (Pd/P = 1/2). The reactions were completed 20 C within 20 h to give 2, whose enantioselectivities were determined after the stereospecific oxidation into 3. The catalytic activity with the ligands L3 was high enough with all the ligands except for L3a and L3d, however, the enantioselectivities varied according to the ligands as shown in Table 1. The enantioselectivities were strongly dependent on the substituents at 3,3'-positions of the binaphthyl in the ligands. It was found that the ligands having diphenyl groups induced the higher enantioselectivities. Thus, the phosphoramidites from 3,3'-diphenylbinaphthol showed the higher

ChemInform Abstract: Catalytic Asymmetric Synthesis Using Chirality‐Switchable Helical Polymer as a Chiral Ligand

AbstractReview: 43 refs.

Catalytic asymmetric synthesis using chirality-switchable helical polymer as a chiral ligand

Single-handed PQXphos, i.e., helical poly(quinoxaline-2,3-diyl)s bearing diarylphosphino pendant groups, served as remarkable chiral ligands in palladium-catalyzed asymmetric hydrosilylation of styrenes and asymmetric biaryl synthesis by Suzuki–Miyaura coupling, affording up to 98 % enantiomeric excess (e.e.) in both reactions. A palladium complex of high-molecular-weight variant (1000mer) of PQXphos could be reused eight times by virtue of the formation of an insoluble polymer complex. PQXphos underwent solvent-dependent inversion of the helical sense, enabling production of either of two enantiomeric products using a single PQXphos.

A Versatile Approach to Planar Chiral Monophosphane Ligands

The enantioselective reaction of a chiral bisamide and an electrophile with benzyl ether complexes of arene tricarbonyl chromium(0) was used to introduce the asymmetry into a synthesis of novel enantiopure planar chiral ligands. Elaboration of the resulting ether complexes [η 6 -4-t-BuC 6 H 4 CHE 1 (OR)]Cr(CO)3 via a diastereoselective ortholithiation/chlorophosphane quench protocol led to the synthesis of planar chiral monophosphane complexes with high enantiomeric purity (typically 96-98% ee) for assessment in the palladium-catalyzed asymmetric hydrosilylation of styrene. The identity of benzylic substituent E 1 (installed using the chiral base reaction) was found to have a significant effect on the enantioselectivity of resulting catalysts. Removal of the tricarbonylchromium(O) unit from the structurally optimized phosphane ligand was accomplished using a high-yielding borane protection-aerial oxidation-deprotection strategy.

Non- C2-symmetrical antimony–phosphorus ligand, ( R/ S)-2-diphenylphosphano-2′-di( p-tolyl)stibano-1,1′-binaphthyl (BINAPSb): preparation and its use for asymmetric reactions as a chiral auxiliary

A new non- C 2-symmetrical antimony–phosphorous ligand, (±)-2-diphenyl-phosphano-2′-di( p-tolyl)stibano-1,1′-binaphthyl (BINAPSb) 3 , has been prepared from 2-bromo-2′-diphenylphosphano-1,1′-naphthyl 4 via its borane complex 6 , and could be resolved by the separation of a mixture of the diastereomeric palladium complexes 8A and 8B derived from the reaction of (±)- 3 with optically active palladium reagent ( S)- 7 . The enantiomerically pure BINAPSb 3 has proved to be highly effective in the palladium-catalyzed asymmetric hydrosilylation of styrene as a chiral auxiliary.

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.

Modification of Chiral Monodentate Phosphine (MOP) Ligands for Palladium-Catalyzed Asymmetric Hydrosilylation of Styrenes

Abstract In the palladium-catalyzed asymmetric hydrosilylation of styrene with trichlorosilane, several chiral monophosphine ligands, (R)-2-diarylphosphino-1,1′-binaphthyls (2), 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 of 98% ee after oxidation of the hydrosilylation product.