Chiral Cooperativity in Diastereomeric Diphosphite Ligands:  Effects on the Rhodium-Catalyzed Enantioselective Hydroformylation of Styrene

Abstract

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.

Bulky Monodentate Phosphoramidites in Palladium‐Catalyzed Allylic Alkylation Reactions: Aspects of Regioselectivity and Enantioselectivity

“…[65] The TADDOL-type backbones employed were synthesised according to literature procedures. [66,67] (R)-3,3'-Trimethylsilyl-1,1'-binaphtyl-2,2'-diol was synthesised as reported by Buisman et al [68] N-Benzyl-(1R,2S)-norephedrine was synthesised by a literature procedure. [69] [{Pd(…”

Strategies for the development of enantioselective catalysts

“…Thus, the P/O heterocycles play a crucial role in determining the direction and degree of enantioselectivity. Such an effect has not been previously observed in other systems involving diphenol ligands [26,27], It is known from a different study concerning hydroxylation [30] that diastereomeric catalysts can lead to opposite enantiomeric products, and that a type of nonlinear effect may pertain (although its origin remains unclear). In the present case a detailed kinetic study unveiled this type of nonlinear effect [31].…”

New C2‐Symmetric Diphosphite Ligands Derived from Carbohydrates: Effect of the Remote Stereocenters on Asymmetric Catalysis

“…These broad signals suggest a fluxional process on the NMR time scale. [25] The doublets appeared between 152.7 and 158.1 ppm with 1 J Rh-P coupling constants between 230.0 and 234.1 Hz. These large 1 J Rh-P coupling constants indicate that the ligands coordinate in an equatorial-equatorial fashion in the trigonal-bipyramidal hydridorhodiumcarbonyl species.…”

“…[8,[25][26][27] It is generally assumed that they have a trigonal-bipyramidal structure. Two isomeric structures of these complexes can be formed with a coordinated bidentate ligand ( Figure 12).…”

“…It is known that the sterically hindered biphenyl moieties of chiral diphosphites preferentially adopt one configuration, that can change by the interaction with chains in positions 2 and 5. [25,28] Finally, the diphosphite ligand 12a was used in the rhodium-catalysed hydroformylation of various substituted vinylarenes. The results are summarised in Table 5.…”