Reactivity of the Chiral Metallic Brønsted Acid [(η6‑p‑MeC6H4iPr)Ru(κ3P,O,O′-POH)][SbF6]2 (POH = (SC1,RC2)‑Ph2PC(Ph)HC(OH)HCH2OMe) toward Aldimines
2014-12-08T00:00:00Z (GMT) by
Dichloromethane solutions of complex 1 catalyze both the aza-Diels–Alder (ADA) reaction of aldimines with cyclopentadiene and the aza-Friedel–Crafts (AFC) reaction between aldimines and indoles. Racemic adducts were obtained in both cases. NMR measurements of mixtures of the aldimine N-(4-methylbenzylidene)aniline (I) and 1, in a 20/1 molar ratio, reveal the formation of the deprotonated compound 2 and the corresponding iminium cation HI+ (H–N+). This cation is responsible for the achiral catalysis. Below 273 K, in 1/1 molar ratio mixtures of 1 and I, the chiral species 1·I (O–H···N) and 2·HI+ (O–···H–N+) were formed by intermolecular hydrogen bonding and proton transfer, respectively. The latter solutions catalyze stoichiometrically the ADA reaction between aldimine I and HCp with 16% ee. With 5 mol % of catalyst loading, a 6% ee can be obtained if the aldimine is slowly added to the solution (20 equiv in 60 h). Above 273 K, solutions containing 1 and aldimine I irreversibly evolve to the aniline complex 3, which was isolated as a racemic mixture. The molecular structure of 3 has been determined by X-ray diffractometric methods. Hydride 5, an intermediate in this reaction, has been isolated and spectroscopically characterized. On the basis of synthetic and spectroscopic studies, a plausible pathway for the formation of 3 from 1 is proposed.