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

Reaction of the tartrate-derived diol (R,R)-alpha, alpha, alpha', alpha'-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. I). 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 (delta or lambda) 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).