Towards Asymmetric Catalysis in the Major Groove of 1,1′-Binaphthalenes

Abstract

Four new diphosphane ligands, (R)-4, (R)-5, (S)-6, and (R)-7 (Schemes 3, 4, 6, and 7), featuring metalcoordination sites located in the major groove of chiral 1,1′-binaphthalene clefts, were prepared in enantiomerically pure form. The performance of this new class of ligands was tested in enantioselective, Pd-catalyzed allylic alkylation reactions with acyclic and cyclic methyl carbonates 28 – 30 as substrates under various reaction conditions (Schemes 8 and 9). Using sodium phenyl sulfinate as a nucleophile, the reactivity of the catalysts formed with the new ligands and suitable palladium precursors was found satisfactory (>90%); however, the ee values were in all cases poor (<4%). Slightly better results were obtained using anions of dimethyl malonate as nucleophiles, but, also in these cases, the ee values never exceeded 17% (Table). 31P-NMR-Spectroscopic investigations revealed the formation of multiple-catalyst species in solution (Fig. 2), and molecular modeling suggested a lack of embedding of the coordinated substrate in a `chiral pocket' (Fig. 3), which probably accounts for the observed low level of enantioselectvity.