A convenient synthesis of new chiral phosphine-phosphites (P-OP) has been described. The versatility of the synthetic protocol developed has allowed the preparation of ligands with different phosphine fragments and the choice of the stereogenic element location. Analyses of the values of 1 J PSe of the corresponding diselenides are in accord with the expected lower σ-donor ability of the phosphite fragment, with respect to the phosphine group, and with an increase of phosphine basicity after substitution of phenyl substituents by methyl groups. Inspection of υ(CO) values on a series of complexes RhCl(CO)(P-OP) demonstrated a variable π-aceptor ability of the phosphite group, compensating for the change of basicity of the phosphine functionality, as well as having a rather reduced electron density at the metal center compared with diphosphine analogues. The distinct nature of the phosphorus functionalities has also been evidenced in rhodium-catalyzed enantioselective hydrogenation of methyl Z-R-acetamido-cinnamate (MAC). Thus, the coordination mode of the substrate is governed by the chiral ligand, directing the olefinic bond to a cis position with respect to the phosphite group, as demonstrated by NMR studies performed with [Rh(P-OP)(MAC)] + complexes. In consequence, the phosphite group has a greater impact on the enantioselectivity of the product. However, the optical purity of the process also depends on the nature of the phosphine group, and hence, an appropriate election of both phosphorus functionalities is required for the attainment of excellent enantioselectivities (99% ee).
The asymmetric hydrogenation of α-(acetyloxy)- and
α-(benzoyloxy)acrylates 4 catalyzed by
cationic
rhodium−DuPHOS complexes has been examined. A wide range of
substrates (4) were prepared via a
convenient Horner−Emmons condensation protocol, and subsequently
hydrogenated under mild conditions
(60 psi of H2) at substrate-to-catalyst ratios (S/C) of
500. Overall, enol ester substrates 4 were reduced by
the
cationic Et−DuPHOS−Rh catalysts with very high levels of
enantioselectivity (93−99% ee). Importantly,
substrates 4 bearing β-substituents could be employed as
E/Z isomeric mixtures with no detrimental effect
on
the selectivity. Labeling studies indicated that no significant
E/Z isomerization of the substrates occurs
during
the course of these reactions. Details concerning optimization of
the reaction, interesting solvent effects, and
deprotection procedures for the synthesis of highly enantioenriched
α-hydroxy esters and 1,2-diols also are
provided.
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