A Metathesis Model for the Dehydrogenative Coupling of Amines with Alcohols and Esters into Carboxamides by Milstein’s [Ru(PNN)(CO)(H)] Catalysts

Manganese-catalyzed hydrogenation, dehydrogenation, and hydroelementation reactions

The emerging field of organometallic catalysis has shifted towards research on Earth-abundant transition metals due to their ready availability, economic advantage, and novel properties. In this case, manganese, the third most abundant transition-metal in the Earth's crust, has emerged as one of the leading competitors. Accordingly, a large number of molecularly-defined Mn-complexes has been synthesized and employed for hydrogenation, dehydrogenation, and hydroelementation reactions. In this regard, catalyst design is based on three pillars, namely, metal-ligand bifunctionality, ligand hemilability, and redox activity. Indeed, the developed catalysts not only differ in the number of chelating atoms they possess but also their working principles, thereby leading to different turnover numbers for product molecules. Hence, the critical assessment of molecularly defined manganese catalysts in terms of chelating atoms, reaction conditions, mechanistic pathway, and product turnover number is significant. Herein, we analyze manganese complexes for their catalytic activity, versatility to allow multiple transformations and their routes to convert substrates to target molecules. This article will also be helpful to get significant insight into ligand design, thereby aiding catalysis design.

Computational Studies on the Mechanisms for Deaminative Amide Hydrogenation by Homogeneous Bifunctional Catalysts

The deaminative hydrogenation of amides is one of the most convenient pathways for the synthesis of amines and alcohols. The ideal source of reducing equivalents for this reaction is molecular hydrogen, though, in practice, this approach requires high pressures and temperatures, with many catalysts achieving only small turnover numbers and frequencies. Nonetheless, during the last ten years, this field has made major advances towards larger turnovers under milder conditions thanks to the development of bifunctional catalysts. These systems promote the heterolytic cleavage of hydrogen into proton and hydride by combining a basic ligand with an acidic metal centre. The present review focuses on the computational study of the reaction mechanism underlying bifunctional catalysis. This review is structured around the fundamental steps of this mechanism, namely the C=O and C–N hydrogenation of the amide, the C–N protonolysis of the hemiaminal, the C=O hydrogenation of the aldehyde, and the competition between hydrogen activation and catalyst deactivation. In line with the complexity of the mechanism, we also provide a perspective on the use of microkinetic models. Both Noyori- and Milstein-type catalysts are discussed and compared.

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

We previously demonstrated that Milstein's seminal diethylamino-substituted PNN-pincer-ruthenium catalyst for ester hydrogenation is activated by dehydroalkylation of the pincer ligand, releasing ethane and eventually forming an NHEt-substituted derivative that we proposed is the active catalyst. In this paper, we present a computational and experimental mechanistic study supporting this hypothesis. Our DFT analysis shows that the minimum-energy pathways for hydrogen activation, ester hydrogenolysis, and aldehyde hydrogenation rely on the key involvement of the nascent N-H group. We have isolated and crystallographically characterized two catalytic intermediates, a ruthenium dihydride and a ruthenium hydridoalkoxide, the latter of which is the catalyst resting state. A detailed kinetic study shows that catalytic ester hydrogenation is first-order in ruthenium and hydrogen, shows saturation behavior in ester, and is inhibited by the product alcohol. A global fit of the kinetic data to a simplified model incorporating the hydridoalkoxide and dihydride intermediates and three kinetically relevant transition states showed excellent agreement with the results from DFT.

Calculations on the non-classical β-hydride elimination observed in trans-(H)(OMe)-Ir(Ph)(PMe3)3: possible production and reaction of methyl formate

The octahedral trans hydrido-alkoxide complex trans-(H)(OMe)-Ir(Ph)(PMe3)3 (2-OCH3) was prepared by Milstein and coworkers by addition of methanol to Ir(Ph)(PMe3)3 (1). 2-OCH3 was discovered to undergo a methanol catalyzed outer-sphere carbonyl de-insertion in which a vacant coordination site is not required. The reaction yields the octahedral trans dihydride complex trans-(H)2-Ir(Ph)(PMe3)3 (2-H) as a kinetic product along with formaldehyde derivatives reported as [CH2=O]x. We investigate the mechanism and products of this reaction using density functional theory. The de-insertion transition state has an ion-pair character leading to a high barrier in benzene continuum: ΔG ‡ = 27.9 kcal/mol. Adding one methanol molecule by H-bonding to the alkoxide of 2-OCH3 lowers the barrier to 22.7 kcal/mol. When the calculations are conducted in a methanol continuum, the barrier drops to 8.8 kcal/mol. However, the thermodynamics of de-insertion are endergonic by near 5 kcal/mol in both benzene and methanol. The calculations identify a low energy outer-sphere H/OMe metathesis pathway that transforms the formaldehyde and another 2-OCH3 molecule directly into a second 2-H complex and methyl formate. Likewise, a second H/OCH3 metathesis reaction interconverting methyl formate and 2-OCH3 into 2-H and dimethyl carbonate is computed to be exergonic and kinetically facile. These results imply that the production of methyl formate and dimethyl carbonate from 2-OCH3 is plausible in this system. The net transformation from the square planar 1 and methanol to 2-H and either methyl formate or dimethyl carbonate would represent a unique stoichiometric dehydrogenative coupling reaction taking place at room temperature by an outer-sphere mechanism.

Synthesis, structure and catalytic activity of manganese(ii) complexes derived from bis(imidazole)methane-based ligands

New mononuclear manganese(ii) complexes [Mn(κ2-L1)(OAc)2] ([Mn]-1), [Mn(κ2-L2)(OAc)2] ([Mn]-2) and [Mn(κ2-L3)(OAc)2] ([Mn]-3) with imidazole based ligands {4,4'-(phenylmethylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L1), {(4,4'-((2-methoxy phenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L2) and {4,4'-((2-chlorophenyl) methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L3) are synthesized and fully characterized by a variety of techniques. Furthermore, the molecular structures of complexes [Mn]-1 and [Mn]-2 are established by single crystal X-ray structure analysis. The synthesized manganese(ii) complexes exhibited efficient catalytic oxidative coupling of primary amines in air under solvent-free conditions to the corresponding imines in moderate to good yields.

Dehydroalkylative Activation of CNN- and PNN-Pincer Ruthenium Catalysts for Ester Hydrogenation

Ruthenium-pincer complexes bearing CNN- and PNN-pincer ligands with diethyl- or diisopropylamino side groups, which have previously been reported to be active precatalysts for ester hydrogenation, undergo dehydroalkylation on heating in the presence of tricyclohexylphosphine to release ethane or propane, giving five-coordinate ruthenium(0) complexes containing a nascent imine functional group. Ethane or propane is also released under the conditions of catalytic ester hydrogenation, and time-course studies show that this release is concomitant with the onset of catalysis. A new PNN-pincer ruthenium(0)-imine complex is a highly active catalyst for ester hydrogenation at room temperature, giving up to 15 500 turnovers with no added base. This complex was shown to react reversibly at room temperature with two equivalents of hydrogen to give a ruthenium(II)-dihydride complex, where the imine functionality has been hydrogenated to give a protic amine side group. These observations have potentially broad implications for the identities of catalytic intermediates in ester hydrogenation and related transformations.

Mechanistic insights into the iridium catalysed hydrogenation of ethyl acetate to ethanol: a DFT study

Density functional theory study of the hydrogenation of ethyl acetate catalysed by iridium complexes [Cp*Ir(bpy)OH2]2+ reveals a direct C-O bond cleavage mechanism with two cascade catalytic cycles for the hydrogenation of ethyl acetate to aldehyde and the hydrogenation of aldehyde to ethanol. Calculation results indicate that the rate-determining state in the whole catalytic reaction is the direct C-O bond cleavage for the formation of aldehyde and ethanol with a total free energy barrier of 25.5 kcal mol-1, which is 0.6 kcal mol-1 more favorable than the mechanism proposed by Goldberg and co-workers in their experimental study.

Direct access toN-alkylated amines and iminesviaacceptorless dehydrogenative coupling catalyzed by a cobalt(ii)-NNN pincer complex

A Co(ii)-NNN pincer complex catalyzed directN-alkylation of anilines with alcoholsviahydrogen auto-transfer and selective acceptorless dehydrogenative coupling of benzylamines with alcohols affording imines with the liberation of molecular hydrogen and water is reported.

Synthesis, characterization and Pd(ii)-coordination chemistry of the ligand tris(quinolin-8-yl)phosphite. Application in the catalytic aerobic oxidation of amines

The ligand P(Oquin)₃ is reported and was coordinated to Pd(ii). This complex is a catalyst precursor for the homocoupling of amines.