The key role of the latent N–H group in Milstein's catalyst for ester hydrogenation

Abstract

We report a detailed mechanistic study of ester hydrogenation catalyzed by the activated form of Milstein’s catalyst. Catalyst activation leads to the replacement of a dialkylamino side group with an NHEt group, which has a key role in catalysis.

Near-Ambient-Temperature Dehydrogenative Synthesis of the Amide Bond: Mechanistic Insight and Applications

“…Several ruthenium and manganese pincer complexes were screened for the amide synthesis under Et 2 O reflux (boiling point 34.6 °C) in the presence of catalytic amounts of t -BuOK. Interestingly, among these complexes, Ru–PNNH complexes 1 and 2 , featuring a terminal N–H moiety, displayed catalytic activities toward amide formation even at this low temperature (entries 1–2). Ru–PNNH complexes 1–3 are capable of two distinct modes of metal–ligand cooperation (MLC), amido–amine and aromatization–dearomatization, unlike complexes 4–6 , where only aromatization–dearomatization is possible.…”

“…Interestingly, among these complexes, Ru–PNNH complexes 1 and 2 , featuring a terminal N–H moiety, displayed catalytic activities toward amide formation even at this low temperature (entries 1–2). Ru–PNNH complexes 1–3 are capable of two distinct modes of metal–ligand cooperation (MLC), amido–amine and aromatization–dearomatization, unlike complexes 4–6 , where only aromatization–dearomatization is possible. Ru–P Ph NNH complex 3 , with electron-withdrawing Ph substituents on the P donor atom, did not show catalytic activity at this temperature (entry 3).…”

“…Mechanistic studies were carried out to further understand the mechanism of the low-temperature catalytic activities of the PNNH complexes (Scheme ). Complex 1 , upon addition of 2 eq of t -BuOK, forms the anionic complex 1a which is intensely violet in diethyl ether solution (Scheme a) . Addition of 1-hexanol (4 equiv) to this complex results in the formation of the aromatic alkoxy complex 1b (see Figure S20 for reaction progress).…”

Cooperative approaches in catalytic hydrogenation and dehydrogenation

“…27 Recent work on PNN systems bearing an NEt 2 side-arm donor has suggested that dehydroalkylation of the CH 2 NEt 2 arm can occur during catalytic ester hydrogenation to form a secondary amine donor NHEt. 43 The secondary amine PNN ligand is thought to favour a traditional Lewis base assisted MLC mechanism via protonation-deprotonation of the NHEt moiety, rather than an aromatisation-dearomatisation mechanism. 43 The fact that mechanisms through which prominent catalysts operate are the subject of ongoing debate decades after their discoveries highlights the difficulty in establishing the role of cooperative processes in catalysis.…”

Bulky PNP Ligands Blocking Metal‐Ligand Cooperation Allow for Isolation of Ru(0), and Lead to Catalytically Active Ru Complexes in Acceptorless Alcohol Dehydrogenation

“…Heating the complex at 40 °C for two weeks does however lead to the establishment of the same cis/trans equilibrium, without any deuteration of the complex hydrides, or of the dissolved H 2 signal, allowing us to establish a ratio of ~10 : 1.2 for the trans:cis 9 b : 9 a H 2 complex equilibrium (Supporting Information, pp. [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43].…”