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

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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 PdCl2 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 Ph2P groups (superpositions in Fig.3) fall into one of two categories: one with approximate C2 symmetry and the other with C1 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).

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