Highly Versatile Catalytic Hydrogenation of Carboxylic and Carbonic Acid Derivatives using a Ru-Triphos Complex: Molecular Control over Selectivity and Substrate Scope

The complex [Ru­(Triphos)­(TMM)] (Triphos
= 1,1,1-tris­(diphenylphosphinomethyl)­ethane,
TMM = trimethylene methane) provides an efficient catalytic system
for the hydrogenation of a broad range of challenging functionalities
encompassing carboxylic esters, amides, carboxylic acids, carbonates,
and urea derivatives. The key control factor for this unique substrate
scope results from selective activation to generate either the neutral
species [Ru­(Triphos)­(Solvent)­H2] or the cationic intermediate
[Ru­(Triphos)­(Solvent)­(H)­(H2)]+ in the presence
of an acid additive. Multinuclear NMR spectroscopic studies demonstrated
together with DFT investigations that the neutral species generally
provides lower energy pathways for the multistep reduction cascades
comprising hydrogen transfer to CO groups and C–O bond
cleavage. Carboxylic esters, lactones, anhydrides, secondary amides,
and carboxylic acids were hydrogenated in good to excellent yields
under these conditions. The formation of the catalytically inactive
complexes [Ru­(Triphos)­(CO)­H2] and [Ru­(Triphos)­(μ-H)]2 was identified as major deactivation pathways. The former
complex results from substrate-dependent decarbonylation and constitutes
a major limitation for the substrate scope under the neutral conditions.
The deactivation via the carbonyl complex can be suppressed by addition
of catalytic amounts of acids comprising non-coordinating anions such
as HNTf2 (bis­(trifluoromethane)­sulfonimide). Although the
corresponding cationic cycle shows higher overall barriers of activation,
it provides a powerful hydrogenation pathway at elevated temperatures,
enabling the selective reduction of primary amides, carbonates, and
ureas in high yields. Thus, the complex [Ru­(Triphos)­(TMM)] provides
a unique platform for the rational selection of reaction conditions
for the selective hydrogenation of challenging functional groups and
opens novel synthetic pathways for the utilization of renewable carbon
sources.