Cp*rhodium complexes with salicyloxazolines: Diastereoselective synthesis, configurational stability and use as asymmetric catalysts for a Diels–Alder reaction

Reaction of [RhCl2Cp*]2 (Cp* = η-C5Me5) with salicyloxazolines in the presence of NaOMe gives complexes [RhCl(R-saloxaz)Cp*] (1–4) which have been fully characterised. The diastereoselectivity of complexation depends on the substituents and the absolute configuration at the metal centre is unstable in solution. Treatment of 2 with 4-methylpyridine and NaSbF6 in methanol at reflux gave [Rh(4-Mepy){(S)-iPr-saloxaz}Cp*][SbF6] (5) whilst [Rh(OH2)(Me2-saloxaz)Cp*][SbF6] (6) was prepared by reaction of 1 with AgSbF6. Three complexes, [RhCl(Me2-saloxaz)Cp*] (1), [RhCl{(S)-iPr-saloxaz}Cp*] (2), and [Rh(OH2)(Me2-saloxaz)Cp*][SbF6] (6) have been characterised by X-ray crystallography. Some of the complexes, after treatment with AgSbF6, have been tested as enantioselective catalysts for the Diels–Alder reaction of methacrolein with cyclopentadiene.

Complexes [RhCl(R-saloxaz)Cp*] have been synthesised, the nature of R determines which diastereomer (A or B) is favoured. The absolute configuration at the metal is unstable, epimerisation occurring at a rate comparable with the NMR timescale.Figure optionsDownload as PowerPoint slide