Calix[4]arene bisphosphite ligands bearing two distal 2,2'-biphenyldioxy or 2,2'-binaphthyldioxy moieties: conformational flexibility and allyl-palladium complexes

Abstract

Achiral and chiral calix[4]arene bisphosphite ligands (2 and 3) bearing two distal 2,2'-biphenyldioxyphosphinoxy and 2,2'-binaphthyldioxyphosphinoxy moieties, respectively, have been synthesized. Each of these ligands exists in two pairs of interconverting conformations in solution. The partial cone conformer (A) of the (bis)biphenyldioxyphosphinoxy ligand 2 has been separated by fractional crystallization and its structure established by X-ray crystallography. The mechanism of interconversion of the pairs of conformers (A/B and C/D) has been probed by two-dimensional NMR spectroscopy. The ¹H and ³¹P NMR evidence strongly supports a similar kind of exchange mechanism for ligand 3. Freezing of the cone conformer from the interconverting C/D pair of conformers of ligand 2 has been achieved by complexation with (allyl)palladium moieties. The methyl-allyl complex (2d) is moderately effective for catalytic regioselective allylic alkylation of crotyl acetate.