C1-Symmetric Diphosphite Ligands Derived from Carbohydrates: Influence of Structural Modifications on the Rhodium-Catalyzed Asymmetric Hydroformylation of Styrene

New 3,5-diphosphite-substituted xylofuranoside (1b, 25a,b, and 26a,b) and glucofuranoside (3a, 7a, 8a,b) ligands with C-1 symmetry have been prepared and used in the Rh-catalyzed asymmetric hydroformylation of styrene. The main structural features of these ligands are a) the presence of a 6-O-isopropyl group in ligands with a gluco configuration, b) the absence of 1,2-O-isopropylidene, a common group in many ligands with a furanoside skeleton, c) the presence of an alkyl chain bound to the 2-OH, and d) modification of the diol in the phosphite moiety. Modification of the carbohydrate backbone and diphosphite bridge affects the activity and selectivity of the reaction. Catalytic systems with ligands 1b and 8b were not active at 40 degrees C, although the formation of the expected hydride species [RhH(CO)(2)(1b)] was demonstrated by NMR spectroscopy. The highest enantioselectivity (83%) was obtained with the catalytic system Rh/8a. The complex [RhH(CO)(2)(8a)] was characterized by NMR spectroscopy using high-pressure techniques and was shown to exist in solution as two isomers in equilibrium; the two isomers adopt an equatorial-equatorial (eq-eq) configuration. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)