Hydrogenation of Alkyl Carboxylic Acids with Tetrahydropyrimidine-Derived Iridium Complexes under Mild Conditions

The hydrogenation of carboxylic acids to alcohols is
of great significance
in synthetic chemistry. Reported herein are competent iridium catalysts
[Cp*Ir­(N,N′)­(H2O)]­[OTf]2 (N,N′
= three different 2,2′-bi-1,4,5,6-tetrahydropyrimidines) for
application in the catalytic hydrogenation of a series of alkyl carboxylic
acids under mild conditions (25–100 °C) without any additives.
Among these structurally diverse catalysts evaluated for reactivity,
complex 4 bearing an electron-deficient ligand (N,N′
= 2,2′-bi-5,5′-hydroxyl-1,4,5,6-tetrahydropyrimidine)
is the most robust, which can hydrogenate acetic acid smoothly at
25 °C to give the alcohol and ester products with a TON of 1106.
Detailed mechanistic studies suggest that the generation of an [Ir–H]
intermediate is involved in the rate-limiting step. With this catalyst
system, a broad range of alkyl carboxylic acids were hydrogenated
with moderate to high TONs.