C₂-symmetric bisphosphinobioxazoline as a chiral ligand. Highly enantioselective palladium-catalyzed allylic substitutions and formation of P,N,N,P tetradentate palladium (II) complexes

The C₂-symmetric bisphosphinobioxazoline [(S,S)-Phos-Biox] 4 was found to be a highly efficient chiral ligand for Pd-catalyzed enantioselective allylic substitution of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate and afforded enantioselectivities of up to 97% ee. Moderate enantioselectivities have been observed in Pd-catalyzed desymmetrizations of cis-1,4-bis(benzoyloxy)-cyclopent-2-ene (12) with dimethyl malonate (51-78% yield, 38-58% ee) and N-benzyl-N-methylamine (87% yield, 33% ee) nucleophiles and of biscarbamate 15 of cis-1,4-dihydroxycyclopent-2-ene (90% yield, 50% ee). A 1:1 molar mixture of (S,S)-Phos-Biox 4 with Pd(CH₃CN)₂Cl₂ and (η³-C₃H₅)PdCl]₂ afforded the P,N,N,P-tetradentate Pd(II) complexes 19a and 19b, respectively. The structures of the complexes 19a, b were determined by X-ray analysis. The complex 19a also exhibited high enantioselectivity (86% yield, 92% ee) in allylic substitution of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate.