Reactivity of the Chiral Metallic Brønsted Acid [(η<sup>6</sup>‑<i>p</i>‑MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)Ru(κ<sup>3</sup><i>P</i>,<i>O,O</i>′-<b>POH</b>)][SbF<sub>6</sub>]<sub>2</sub> (<b>POH</b> = (<i>S</i><sub>C1</sub>,<i>R</i><sub>C2</sub>)‑Ph<sub>2</sub>PC(Ph)HC(OH)HCH<sub>2</sub>OMe) toward Aldimines
2014-12-08T00:00:00Z (GMT) by
Dichloromethane solutions of complex <b>1</b> 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 <i>N</i>-(4-methylbenzylidene)aniline (<b>I</b>) and <b>1</b>, in a 20/1 molar ratio, reveal the formation of the deprotonated compound <b>2</b> and the corresponding iminium cation <b>HI</b><sup><b>+</b></sup> (H–N<sup>+</sup>). This cation is responsible for the achiral catalysis. Below 273 K, in 1/1 molar ratio mixtures of <b>1</b> and <b>I</b>, the chiral species <b>1·I</b> (O–H···N) and <b>2·HI</b><sup><b>+</b></sup> (O<sup>–</sup>···H–N<sup>+</sup>) were formed by intermolecular hydrogen bonding and proton transfer, respectively. The latter solutions catalyze stoichiometrically the ADA reaction between aldimine <b>I</b> 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 <b>1</b> and aldimine <b>I</b> irreversibly evolve to the aniline complex <b>3</b>, which was isolated as a racemic mixture. The molecular structure of <b>3</b> has been determined by X-ray diffractometric methods. Hydride <b>5</b>, 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 <b>3</b> from <b>1</b> is proposed.