Diastereomeric diphosphites (L∩L ) 3-8) have been synthesized from enantiomerically pure pentane-2,4-diol and axially chiral 3,3′-bis(trialkylsilyl)-2,2′-bisphenol phosphorochloridites and 3,3′-bis(trialkylsilyl)-2,2′-bisnaphthol phosphorochloridites. These diphosphites have been used to test the influence of chiral cooperativity in the rhodium-catalyzed asymmetric hydroformylation of styrene. Systematic variation in chirality at both the chiral ligand bridge and the axially chiral biphenyl and binaphthyl substituents revealed a remarkable effect on the selectivity of the hydroformylation catalysts. For the atropisomeric bisnaphthol-based diphosphites, cooperative effects were observed in the asymmetric hydroformylation of styrene. High enantioselectivities (87%) and regioselectivities up to 95% for 2-phenylpropanal were found under mild reaction conditions (15-50°C, 20 bar of syn gas CO-H 2 [1:1]) for the ligand derived from (2R,4R)-pentane-2,4-diol and (S)-bisnaphthol. The same high enantiomeric excess was observed for the free-rotating bisphenol-substituted ligands. The highest selectivity was obtained with trimethylsilyl substituents at the ortho position. The solution structures of the active catalysts [HRhL∩L-(CO) 2 complexes (L∩L ) 3-8)], have been studied by 31 P and 1 H NMR spectroscopy at variable temperature (313-213 K). Spectroscopic data, in combination with the obtained results in catalysis, suggest that diphosphite ligands (L∩L) containing the conformationally flexible axially chiral biphenyl moieties predominantly exist as single atropisomers in the HRhL∩L-(CO) 2 complexes. Comparison of the bisphenol and bisnaphthol substituents suggests that the high enantiomeric excesses obtained with the former are caused by the preferential formation of the most selective diastereomer.

The solution structures of the dominant species present during the styrene hydroformylation using rhodium precursors [Rh(µ-OMe)(cod)] 2 (1) and the dithiolate-bridged complex [Rh 2 (µ-BCOS)(cod) 2 ] 2 (9) in the presence of BDPP [(2S,4S)-bis(diphenylphosphine)pentane] have been determined. The high-pressure spectroscopic studies have revealed that the mononuclear complex [HRh(BDPP)(CO) 2 ] ( 7) is the predominant species during the hydroformylation process, but the dimeric species [Rh(BDPP)(CO) 2 ] 2 ( 8) is also present and in equilibrium with 7. Equilibrium constants were measured by 31 P NMR spectroscopy (at [Rh] ) 0.0138-0.0157 grat‚L -1 , 5-50 bar of H 2 , 5 bar of CO, 298-343 K, the molar ratio of 7:8 ranges from 1 to 20). Kinetic measurements have shown that the half-life of the species at ambient temperature is on the order of magnitude of 1 min. Variable-temperature 31 P and 1 H NMR spectra (293-193 K) revealed fluxional behavior in complex 7 where the bidentate diphosphine BDPP adopts equatorial-axial coordination to rhodium in the trigonalbipyramidal complex [HRh(BDPP)(CO) 2 ] (bite angle ≈90°). A low free energy of activation (∆G q 193K ) 44 kJ‚mol -1 and ∆G q 293K ) 44 kJ‚mol -1 ) for phosphorus exchange has been measured for the fluxional process observed in complex 7.

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A series of new and known Rh(1) complexes [ Rh(R2P(CH2),PR2),)Y and [ Rh(diene)(R2P(CH2),PR,)1Y (R = Me, Et, Ph, C,F,, C,H,CF,; Y = C1, CIO,, PF,, BF,, OSO,CF,; diene = norbornadiene, cyclooctadiene) were prepared and their lo3Rh NMR spectra were recorded on a 100-MHz spectrometer by using inverse two-dimensional "P, 'oBRh{*H} NMR. This technique provides quick access to "' Rh NMR data (2-8 h for cu. 0.1 M samples) when dealing with Rh-phosphine complexes. The Io3Rh NMR data are discussed. All G(103Rh) of the cationic Rh(1) complexes appear at the lower frequency end of the Rh(1) range, i.e. between -1350 and +ZOO ppm, which is in agreement with the fact that the Rh shift is determined by up and with a large AE in square-planar d8 complexes. The relative change in AE between complexes is not large and the changes in paramagnetic shift for these complexes are essentially determined by their relative adonor capacities. G(lo3Rh) shows an inverse correlation with S("P), which was rationalized by invoking the chelate ring size containing the Rh centre and the bidentate phosphine ligands.