Chiral Brønsted Acid Catalysts. Activation of Methyl 3,3,3-Trifluoropyruvate by Hydroxymethylpyridine-Containing Half-Sandwich Complexes

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-C5Me5)­Ir, (η6-p-MeC6H4iPr)­Ru; NOH = hydroxypyridine ligand; (R)-P1 = (R)-monophos)
is deprotonated by Na2CO3, 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}]­[SbF6] (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.