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

Ruthenium complexes of formula [(η⁶-arene)Ru(LL*)- (H₂O)][SbF₆]₂ (arene = C₆H ₆, 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, [(η⁶-arene)RuCl(LL*)][SbF₆]. Dipolarophile intermediates [(η⁶-arene)Ru(PNiPr)(methacrolein)][SbF ₆]₂ (PNiPr = (4S)-2-(2-diphenylphosphanylphenyl)-4- isopropyl-1,3-oxazoline) as well as nitrone-containing complexes [(p-Me-C ₆H₄iPr)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 (R_Ru)-[(η⁶-C ₆Me₆)RuCl(PNiPr)][SbF₆], (R_Ru)- [η⁶-C₆H₆)-RuCl(PNInd)][SbF₆] {PNInd = (3aR,8aS)-2-[2-(diphenylphosphanyl)phenyl]-3a,8a-dihydroindane[1,2-d] oxazole} and those of the aqua solvates (R_Ru)-[(η⁶- arene)Ru(PNiPr)-(H₂O)][SbF₆]₂ (arene = C ₆H₆, C₆Me₆) were determined by X-ray diffraction methods.