Chiral half-sandwich ruthenium(II) complexes as catalysts in 1,3-dipolar cycloaddition reactions of nitrones with methacrolein

Abstract

Ruthenium complexes of formula [(6-arene)Ru(LL*)(H2O)][SbF6]2 (arene = C6H6, p-MeC6H4iPr, C6Me6; LL* = bidentate chelate chiral ligand with PN, PP or NN donor atoms) have been tested as catalyst precursors for the asymmetric 1,3-dipolar cycloaddition of nitrones to methacrolein. The reaction occurs quantitatively with perfect endo selectivity and moderate enantioselectivity (up to 74 % ee). The ruthenium aqua complexes can be prepared from the corresponding chlorides, [(6-arene)RuCl(LL*)][SbF6]. Dipolarophile intermediates [(6-arene)Ru(PNiPr)(methacrolein)][SbF6]2 {PNiPr = (4S)-2-(2-diphenylphosphanylphenyl)-4-isopropyl-1,3-oxazoline} as well as nitrone-containing complexes [(p-MeC6H4iPr)Ru(PNiPr)(nitrone)][SbF6]2 (nitrone = N-benzylidenephenylamine N-oxide, N-benzylidenemethylamine N-oxide, 3,4-dihydroisoquinoline N-oxide) have been also isolated and characterised. The crystal structures of the chlorides (RRu)-[(6-C6Me6)RuCl(PNiPr)][SbF6], (RRu)-[(6-C6H6)RuCl(PNInd)][SbF6] {PNInd = (3aR,8aS)-2-[2-(diphenylphosphanyl)phenyl]-3a,8a-dihydroindane[1,2-d]oxazole} and those of the aqua solvates (RRu)-[(6-arene)Ru(PNiPr)(H2O)][SbF6]2 (arene = C6H6, C6Me6) were determined by X-ray diffraction methods.