Novel Pd(PN,S)-complexes: Highly active catalysts designed for asymmetric allylic etherification

Abstract

• A highly modular approach has been developed for the synthesis of chiral PN,S-ligands. • The ligands were used in Pd-catalyzed asymmetric allylic etherification. • High yields (up to 95%) could be obtained at unprecedentedly low catalyst loadings (0.2 mol%). • The excess of the reagents were significantly decreased compared to literature examples. • The main factors affecting activity and enantioselectivity have been investigated. Six novel thioether-aminophosphine type ligands with a general formula (Ar 1 ) 2 PN(R 1 )CHR 2 (CH 2 ) n CH(R 3 )SAr 2 has been synthesized. The modular structure of the ligands and the new methodologies developed for their preparation enabled the systematic variation of their bridge length (n = 0 or 1), the substitution pattern of the backbone (R 2 , R 3 = H or Me) as well as the P-, N- and S-substituents (Ar 1 = Ph or 3,5-Me 2 C 6 H 3 , R 1 = Et or i Pr and Ar 2 = Ph, 4-MeC 6 H 4 , or 4-MeOC 6 H 4 , respectively). The ligands proved to be effective in Pd-catalyzed asymmetric allylic etherification reactions providing the products in high yields (up to 95%) and with good enantioselectivities (up to 86%) using unprecedentedly low (0.2 mol%) loadings of the chiral Pd-catalyst. Based on these findings, a new scalable protocol has been developed for the preparation of chiral allylic ethers. Furthermore, the Pd(II) coordination chemistry of the ligands was thoroughly investigated by 1D and 2D NMR methods as well as by X-ray crystallography with special attention to the conformation of the chelate ring and the stereoselectivity of the sulfur coordination. Based on these studies, the main factors determining activity and selectivity of the catalytic system have been identified.