Arene–ruthenium complexes with salicyloxazolines: diastereoselective synthesis, configurational stability and applications as asymmetric catalysts for Diels–Alder reactions

Abstract

Reaction of the dimers [RuCl₂(η⁶-arene)]₂ (arene = benzene, p-cymene, mesitylene) with salicyloxazolines in the presence of NaOMe gives complexes [RuCl(R-saloxaz)(arene)] (1–5) which have been fully characterised. Complexes [RuL(ⁱPr-saloxaz)(mes)]Y (L = py, 2-Mepy, 4-Mepy; PPh₃; Y⁻ = SbF₆ or BPh₄) 6–9 were prepared by treating the chloride 2a with ligand L and NaY (Y⁻ = SbF₆ or BPh₄) in methanol at reflux. Halide complexes [RuX(ⁱPr-saloxaz)(mes)] (X = Br, 10; X = I, 11) were synthesised by treating 2a with AgSbF₆ then with 1.2 equivalents of KBr or NaI, the methyl complex [RuMe(ⁱPr-saloxaz)(mes)] 12 was synthesised from 2a by reaction with MeLi. Five complexes, [RuCl(ⁱPr-saloxaz)(mes)] 2a, [RuCl(^tBu-saloxaz)(p-cymene)] 3b, [RuCl(Ph-saloxaz)(mes)] 5a, [Ru(4-Mepy)(ⁱPr-saloxaz)(mes)][SbF₆] 7, and [Ru(PPh₃)(ⁱPr-saloxaz)(mes)][SbF₆] 9, have been characterised by X-ray crystallography. Treatment of complexes 1–5 with AgSbF₆ gives cationic species which are enantioselective catalysts for the Diels–Alder reaction of acroleins with cyclopentadiene, the effect of substituents on enantioselectivity has been examined.