Study of Precatalyst Degradation Leading to the Discovery of a New Ru⁰ Precatalyst for Hydrogenation and Dehydrogenation

Abstract

The complex Ru-MACHO ( 1) is a widely used precatalyst for hydrogenation and dehydrogenation reactions under basic conditions. In an attempt to identify the active catalyst form, 1 was reacted with a strong base. The formation of previously unreported species was observed by NMR and mass spectrometry. This observation indicated that complex 1 quickly degraded under basic conditions when no substrate was present. X-ray crystallography enabled the identification of three complexes as products of this degradation of complex 1. These complexes suggested degradation pathways which included ligand cleavage and reassembly, along with reduction of the ruthenium atom. One of the decomposition products, the Ru 0 complex [Ru(N(CH 2 CH 2 PPh 2 ) 3 )CO] (5), was prepared independently and studied. 5 was found to be active, entirely additivefree, in the acceptorless dehydrogenation of aliphatic alcohols to esters. The hydrogenation of esters catalyzed by 5 was also demonstrated under base-free conditions with methanol as an additive. Protic substrates were shown to add reversibly to complex 5, generating Ru II −hydrido species, thus presenting a rare example of reversible oxidative addition from Ru 0 to Ru II and reductive elimination from Ru II to Ru 0 .

Hydrogenation/dehydrogenation of N-heterocycles catalyzed by ruthenium complexes based on multimodal proton-responsive CNN(H) pincer ligands

“…Interestingly, solutions of the Ru(0) derivative 8 were active in the hydrogenation of N-heterocycles. 14,15 Thus, THF-d 8 solutions containing this complex were able to hydrogenate (3 bar H 2 ) 2-methylquinoxaline (10 equiv.) at 60°C, being 8 the only detectable metal species during the catalytic reaction and suggesting that this is the catalyst resting state (ESI †).…”

Optimization and sustainability assessment of a continuous flow Ru-catalyzed ester hydrogenation for an important precursor of a β2-adrenergic receptor agonist

“…Interestingly, the catalyst has been previously reported to slowly decompose in the presence of base. 31 Furthermore, as stated previously, the presence of a base results in the for- mation of side product 3. Thus, to avoid any undesired reactions occurring within the feed solutions, the catalyst and substrate in MeOH were introduced as one feed and the NaOMe in MeOH as the second feed.…”

The Guerbet Reaction Network – a Ball‐in‐a‐Maze‐Game or: Why Ru‐MACHO‐BH is Poor in Coupling two Ethanol to n‐Butanol

“…A recent study shows that Ru-MACHO degrades to a mixture of several catalytically active complexes in alkaline media. [28] In our work, we could only reproduce that Ru-MACHO is not a suitable catalyst for the Guerbet reaction from ethanol to butanol. The additional benefit of the microkinetic simulation consists of understanding why the catalyst fails in the reaction.…”