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

Reaction of the dimers [RuCl2(eta6-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.