Highly Enantioselective Hydroformylation of Olefins Catalyzed by Rhodium(I) Complexes of New Chiral Phosphine−Phosphite Ligands

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).