Palladium-catalyzed asymmetric intermolecular arylation of cyclic or acyclic alkenes using phosphinite-oxazoline ligands derived from d-glucosamine

Abstract

Chiral phosphinite-oxazolines, 2-alkyl- or 2-aryl-4,5-(4,6- O-benzylidene-3- O-(diphenylphosphino)-1,2-di-deoxy-α- d-glucopyranosyl)-[2,1- d]-2-oxazolines 1a– f derived from d-glucosamine hydrochloride, are revealed to work as effective P, N-bidentate ligands in the palladium-catalyzed enantioselective arylation of 2,3-dihydrofuran to give 2-aryl-2,5-dihydrofuran selectively in high yield with up to 96% ee. The first asymmetric phenylation reaction of trans- and cis-crotyl alcohols as acyclic alkenes with iodobenzene is also carried out to afford 3-phenylbutanal in moderate chemical yield with up to 17% ee. The complex [PdCl 2( 1b)] is newly prepared and its structure is characterized by X-ray crystallography. Structures of the new complex [( p-MeO 2CC 6H 4)PdI( 1a)] ( 8) and its cationic complex [( p-MeO 2CC 6H 4)Pd( 1a)] +OTf − ( 9) are also elucidated on the basis of 1H-, 13C-, and 31P-NMR spectra, p-MeO 2CC 6H 4 moiety on the palladium being located trans to the nitrogen of 1a. This configuration might be responsible for an enantiofacial discrimination of 2,3-dihydrofuran to produce ( R) isomer predominantly. The stoichiometric reaction of [PhPd( 1f)] +OTf − ( 11) with 2,3-dihydrofuran has provided the mechanistic aspect for the arylation using P, N-ligands, in which the base-promoted deprotonation at β-position leading to an alkene(2-aryl-2,5-dihydrofuran)–palladium(0) complex has been shown to be an important step for the selective formation of the product.