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Until the early 1970s, chemical access to enantiomerically pure substances from prochiral precursors remained extremely difficult. Recent dramatic advances in man-made catalysts, however, are converting the chemists' dream into reality. Particularly, homogeneous asymmetric catalysis using chiral metal complexes provides an ideal way to multiply chirality, beginning a new era where production of large amounts of chiral compounds of either absolute configuration is possible with a very small quantity of chiral source. This methodology based on accumulated chemical knowledge is highly rational and flexible, and the opportunities it affords are practically unlimited. Reported examples of the highly enantioselective catalyses include hydrogenation, hydrosilylation, and hydroboration of unsaturated compounds, epoxidation of allylic alcohols, vicinal hydroxylation, hydrovinylation, hydroformylation, cyclopropanation, and isomerization of olefins, propylene polymerization, organometallic addition to aldehydes, allylic alkylation, organic halide-organo-

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A new chiral phosphine−phosphite ligand, (R)-2-(diphenylphosphino)-1,1‘-binaphthalen-2‘-yl (S)-1,1‘-binaphthalene-2,2‘-diyl phosphite [(R,S)-BINAPHOS, (R,S)-2a], was synthesized. Its Rh(I) complex was prepared, and its structure has been characterized by 1H and 31P NMR spectroscopy. Using Rh(I) complexes of (R,S)-2a and its enantiomer, highly enantioselective hydroformylation of styrene has been performed (94% ee, iso/normal = 88/12). The catalyst system was also effective for a variety of other olefins. Some other phosphine−phosphite ligands bearing 1,1‘-binaphthyl and biphenyl backbones, such as (S)-3,3‘-dichloro-6-(diphenylphosphino)-2,2‘,4,4‘-tetramethylbiphenyl-6‘-yl (R)-1,1‘-binaphthalene-2,2‘-diyl phosphite [(S,R)-BIPHEMPHOS, (S,R)-5a], (R,R)-2a, (R,S)-2b, (R)-2c, and (R)-5b, were tested for asymmetric hydroformylation. The results indicate that the sense of enantioface selection for the prochiral olefins is mainly determined by the absolute configuration of the phosphine site, for example, the (R)-2-(diphenylphosphino)-1,1‘-binaphthalen-2‘-yl group in (R,S)-2a. The relative configurations of the two biaryl groups in the phosphine−phosphites play crucial roles in the degree of the enantioselectivities, that is, the (R*,S*)-isomer generally gives products in high ee's and the (R*,R*)-isomer does in low ee's. Treatment of Rh(acac)[(R,S)-2a] with a 1:1 mixture of carbon monoxide and hydrogen gave a hydridorhodium complex, RhH(CO)2[(R,S)-2a], as a single species. Trigonal bipyramidal structure is suggested for this complex, in which the hydride and the phosphite moiety are located at the apical positions and the phosphine and the two carbonyls occupy the equatorial positions. The interchange of the phosphine and the phosphite sites with each other through rapid pseudorotations has not been observed in RhH(CO)2[(R,S)-2a]. The structural deviations of the monohydride complexes from an ideal trigonal bipyramid seem to be larger in (R*,R*)-isomers than in the corresponding (R*,S*)-isomers. The existence of only one active species involved in the Rh(I)−(R,S)-2a-catalyzed hydroformylation has been manifested by the plot of ln([R]/[S]) of the hydroformylation product vs the reciprocals of the reaction temperatures. The higher thermodynamic stability of Rh(acac)[(R,S)-2a] than its diastereomer Rh(acac)[(R,R)-2a] is demonstrated in relation to the restricted configuration of (R)-2c to (R,S)-2c in its complex formation with Rh(I).

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The reaction steps responsible for the highly enantioselective asymmetric copolymerization of propene with carbon monoxide catalyzed by a cationic Pd(II) complex bearing an unsymmetrical chiral bidentate phosphine−phosphite, (R,S)-BINAPHOS [(R,S)-2-(diphenylphosphino)-1,1‘-binaphthalen-2‘-yl 1,1‘-binaphthalene-2,2‘-diyl phosphite = L1], have been studied. Stepwise identification and characterization were carried out for catalyst precursors (SP-4-2)- and (SP-4-3)-Pd(CH3)Cl(L1) (1a and 1b) and (SP-4-3)-[Pd(CH3)(CH3CN)(L1)]·X1 (X1 = B{3,5-(CF3)2C6H3}4) (2), and complexes related to the reaction steps, (SP-4-3)-[Pd(COCH3)(CH3CN)(L1)]·X1 (3), (SP-4-3)- and (SP-4-4)-[Pd{CH2CH(CH3)COCH3}(L1)]·X1 (4a and 4b), (SP-4-3)-[Pd{COCH2CH(CH3)COCH3}(CH3CN)(L1)]·X1 (5), and (SP-4-3)-[Pd{CH2CH(CH3)COCH2CH(CH3)COCH3}(L1)]·X1 (6). An X-ray structure of alkyl complex 4a has been obtained. Studies on [Pt(CH3)2(L1)] (8) reveal that the methyl group is more stabilized at a position trans to the phosphine than at the cis position. This is consistent with the structures of 1−6 in which all carbon substituents are trans to the phosphine moiety in their major forms. On the basis of analogous studies using platinum complexes, an isomerization from (SP-4-3)-[Pd(CH3)(CO)(L1)]·X1 (13a) to the (SP-4-4) isomer (13b) is suggested to occur for the CO-insertion process 2 → 3, which results in the activation of the methyl group for the migration to the coordinated CO. Rapid equilibrium was observed between the two isomers 4a and 4b during the CO insertion process to give 5. Theoretical studies have been carried out on the transformation of 3 to 4a and 4b. The B3LYP and MPn calculations indicated that the alkene insertion into the Pd−acyl bond trans to a phosphine is more favorable than that into the Pd−acyl bond trans to a phosphite. The MM3 calculations demonstrated that one specific transition structure is more favorable than the other possible transition structures for the transformation of (SP-4-4)-[Pd(COCH3)(propene)(L1)]·X1 (14b) to 4b. The difference originates from the steric effects of the BINAPHOS ligand, and the results account for high enantio- and regioselectivities experimentally observed. The two key steps, propene insertion into 3 and CO insertion into 4, were monitored by 1H NMR spectroscopy. The activation energies for these two steps were estimated to be 19.0−19.6 kcal/mol at −20 to 0 °C, their difference being insignificant. The living nature of the copolymerization was proved. Some related chiral ligands were examined for the copolymerization. Copolymerization of other olefins with CO was also investigated.

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Hydroformylation is one of the most versatile methods for the functionalization of C=C bonds. Despite recent extensive investigations, however, highly enantioselective hydroformylation catalyzed by chiral metal complexes has rarely been attained. [1][2][3] We now report that the Rh(I) complexes of new chiral phosphinephosphite ligands, (7?)-(2-(diphenylphosphino) -1,1 '-binaphthalen-2'-yl)-((S)-l,l'-binaphthalen-2,2'-yl)phosphite [(7?,S)-la] (hereafter abbreviated (/?,S)-BINAPHOS) and its enantiomer (S,7t)-la, are highly efficient catalysts for asymmetric hydroformylation of both arylethenes and functionalized olefins such as vinyl acetate and JV-vinylphthalimide.1 23456The enantiomerically pure ligand (R^-la7 was readily obtained in 98% yield from (7?)-2a by the reaction with (S)-3 in ether in the presence of triethylamine (eq 1). Similarly, (S,7?)la, (7?,7?)-la, (7?,S)-lb, and (7?)-(2-(diphenylphosphino)-1, 1'binaphthalen-2'-yl)-diphenylphosphite [(7?)-4] have also been prepared in high yields.7 Since the starting compounds 28 are easily accessible from enantiomerically pure 1,1 '-binaphthalene-(1) A recent review: