Chiral triaryl phosphite-based palladacycles and platinacycles: synthesis and application to asymmetric Lewis acid catalysis

Abstract

The optically pure monophosphites P(OAr)(BINOLate) (7, where Ar = 2,4-di-tert-butylphenyl) have been prepared by treatment of PCl₂(OAr) with R- or S-BINOL. Treatment of [PdCl₂(NCMe)₂] with 7 gave [PdCl₂(7)₂] (9) or the binuclear orthometallated complex [Pd₂Cl₂(7-H)₂] (8) depending on the reaction conditions. Bridge cleavage reactions of 8 gave [PdCl(7-H)(L)] with L trans to carbon when L = PPh₃ or 7 and cis to carbon when L = N-heterocyclic carbene. Treatment of [PtCl₂(NC^tBu)₂] with 7 gave [PtCl₂(7)₂] (18) which upon further reaction with PtCl₂ furnished a mixture of binuclear [Pt₂Cl₂(7-H)₂] (17) and cis-[PtCl(7-H)(7)] (19). The palladium complexes containing cyclometallated 7 were screened for catalysis of 1,4-conjugate addition of phenylboronic acid to cyclohexen-2-one and the allylation of benzaldehyde with allyltributyltin. Conversions were generally high in each case but enantioselectivities were low (15% e.e. at best). The X-ray crystal structures of 8, 17 and [PdCl(7-H)(NHC)] (10a, where NHC = 1,3-(dimesityl)imidazolidin-2-ylidene) have been determined.