Chiral Half‐Sandwich Ruthenium(II) Complexes as Catalysts in 1,3‐Dipolar Cycloaddition Reactions of Nitrones with Methacrolein

Abstract

Ruthenium complexes of formula [(η6‐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, [(η6‐arene)RuCl(LL*)][SbF6]. Dipolarophile intermediates [(η6‐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)‐[(η6‐C6Me6)RuCl(PNiPr)][SbF6], (RRu)‐[(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 (RRu)‐[(η6‐arene)Ru(PNiPr)(H2O)][SbF6]2 (arene = C6H6, C6Me6) were determined by X‐ray diffraction methods. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Metal complexes versus organocatalysts in asymmetric 1,3-dipolar cycloadditions

“…51 As it was reported, chiral complexes 74b and 74c gave the best 3,4:3,5 regioisomeric ratios, and compounds 97-99 were obtained as unique 3,5-regioisomers. In general, the enantioselectivities were very low, whilst chemical yields were very high or quantitative.…”

Asymmetric 1,3‐Dipolar Cycloadditions of N‐Benzyl and N‐Diphenylmethyl Nitrones and α,β‐Unsaturated Aldehydes Catalyzed by Bis‐Titanium Chiral Lewis Acids

“…The asymmetric 1,3‐dipolar cycloadditions of nitrones and methacrolein reported to date have been limited to the preferential formation of the 5‐CHO‐ endo isomer4a,c,d,e or the generation of both 5‐CHO‐ endo and 4‐CHO‐ endo isomers in the moderate preference to the 4‐CHO‐ endo isomer (Scheme ) 4f–h. Contrary to these results, our research revealed that the regiochemical outcome of the bis‐titanium chiral Lewis acid catalyzed reaction of N ‐benzyl nitrone and methacrolein was found to be exclusively 4‐CHO‐ endo ‐selective, albeit in quite low yield 9.…”

Iridium-Catalyzed 1,3-Dipolar Cycloadditions

“…Typically, quantitative conversions are achieved at À25 C, in a few hours. Perfect diastereoselectivity for the endo adducts is observed, but only low to moderate enantioselectivity is obtained [37].…”