Preparation of the PdCl₂ Complex of TADDOP, the Bis(diphenylphosphinite) of TADDOL: Use in enantioselective 1,3‐diphenylallylations of nucleophiles and discussion of the mechanism

Abstract

Abstract Reaction of the tartrate‐derived diol ( R , R )‐α,α,α′,α′‐tetraphenyl‐2,2‐dimethyl‐1,3‐dioxolane‐4,5‐dimethanol (TADDOL) with chlorodiphenylphosphane gives a new bis(diphenylphosphanyl) ligand (TADDOP). The complex 4 formed with PdCl 2 has been crystallized and its structure determined by X‐ray diffraction ( Fig.1 ). The complex is used for Pd‐catalyzed enantioselective 1,3‐diphenylallylations of various nucleophiles which give products with enantiomer ratios of up to 88:12 ( Scheme 2 ). Crystallization procedures lead to the enantiomerically pure (> 99:1) product 11 derived from dimethyl malonate. The structure of the TADDOP complex 4 is compared with those of other transition‐metal complexes containing chelating bis(diphenylphosphanyl) ligands ( Fig.2 ). A crystallographic data base search reveals that the structures of transition‐metal complexes containing two Ph 2 P groups (superpositions in Fig.3 ) fall into one of two categories: one with approximate C 2 symmetry and the other with C 1 symmetry (20 and 19 examples, resp.). A mechanistic model is proposed which correlates the conformational chirality (δ or λ) of the four Ph groups' arrangement in such complexes with the topicity of nucleophile approach on Pd‐bound trans , trans ‐1,3‐diphenylallyl groups ( Scheme 3 and Table ).