Chiral brønsted acid catalysts. Activation of methyl 3,3,3-trifluoropyruvate by hydroxymethylpyridine-containing half-sandwich complexes

摘要

The coordinated OH group in cationic complexes [(ηn-ring) M(NOH)(Solv)][SbF6] and [(ηn-ring)M(NOH){(R)-P1}] [SbF6]2 ((ηn-ring)M = (η5- C5Me5)Rh, (η5-C5Me 5)Ir, (η6-p-MeC6H4iPr)Ru; NOH = hydroxypyridine ligand; (R)-P1 = (R)-monophos) is deprotonated by Na 2CO3, rendering bi- or mononuclear compounds of formulas [{(ηn-ring)M(κ2N,O-μ-O-NO} 2][SbF6]2 and [(ηn-ring)M(NO) {(R)-P1}][SbF6], respectively. The complexes have been characterized by analytical and spectroscopic means, including the determination of the crystal structures of [{(ηn-ring)M(κ2N,O-μ-O- NO}2][SbF6]2 (NOH = NOH-1, (ηn- ring)M = (η5-C5Me5)Rh, 8a; (η6-p-MeC6H4iPr)Ru, 8c) and [(η5-C5Me5)Ir(NO){(R)-P1}][SbF 6] (NOH = (R)-NOH-2; (R)-11b) by X-ray diffractometric methods. In complexes [(ηn-ring)M(NOH)(P)][SbF6]2 (P* = chiral phosphoramidite ligand) the proton of the coordinated hydroxypyridine ligand is able to activate the carbonyl group of methyl 3,3,3-trifluoropyruvate toward the Friedel-Crafts addition of indoles. In most cases, quantitative conversion is achieved in a few minutes, at -70 °C, with an ee of up to 82%. NMR data support the activation of the pyruvate by interaction between its carbonyl oxygen and the OH group of the coordinated hydroxymethylpyridine. Therefore, the metallic complexes act as Lewis acid assisted Brønsted acid catalysts. © 2014 American Chemical Society.