Catalytic Dehydrogenative Amide and Ester Formation with Rhenium-Triphos Complexes

Dynamic Processes of Rhenium Polyhydride Complexes

Studies have demonstrated that high-coordination-number rhenium polyhydride complexes are precursors to catalysts that transform a variety of organic molecules. While rhenium polyhydride complexes lead to active catalysts, little has been reported on the mechanisms for the transformations. High-coordination-number rhenium polyhydride complexes exhibit several dynamic processes that make characterizations of the chemical properties for individual atoms difficult, at best, for room-temperature solutions. This review describes what is known of the dynamic processes that occur at high-coordination-number rhenium polyhydride complexes and how that knowledge may lead to the design of catalytic precursors in which the chemical properties of individual atoms can be more readily identified in room-temperature solutions.

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.

Homogeneous catalysis with polyhydride complexes

This review analyzes the role of transition metal polyhydrides as homogeneous catalysts for organic reactions. Discussed reactions involve nearly every main organic functional group.

Describing Polytopal Rearrangement Processes of Octacoordinate Structures. I. Renewed Insights into Fluxionality of the Rhenium Polyhydride Complex ReH5(PPh3)2(Pyridine)

Hydride ligands of transition metal polyhydride complexes with a high coordination number are prone to fluxionality leading to interesting structural dynamics. However, the underlying polytopal rearrangement pathways have been rarely studied. Based on quantum chemical calculations carried out in this work with density functional theory and coupled-cluster theory, two new fluxional mechanisms have been identified for the rhenium polyhydride complex ReH₅(PPh₃)₂(pyridine) to jointly account for two consecutive coalescence events in the variable-temperature NMR spectra upon heating: lateral and basal three-arm turnstile rotation. The frequently cited pseudorotation in ReH₅(PPh₃)₂(pyridine) (Lee et al. Inorg. Chem. 1996, 35, 695) turns out to be a three-step process including two lateral three-arm turnstile steps and one basal turnstile step in between. The new fluxional mechanisms discovered in this work may also exist in other transition metal polyhydrides.

Synthesis of Quinolines Through Acceptorless Dehydrogenative Coupling Catalyzed by Rhenium PN(H)P Complexes

A practical and sustainable synthesis of substituted quinolines was achieved through the annulation of 2-aminobenzyl alcohol with various secondary alcohols, ketones, aldehydes, or nitriles, under hydrogen-borrowing conditions. Under the catalysis of well-defined rhenium complexes bearing tridentate diphosphinoamino ligands, the reaction proceeded efficiently (31 examples were isolated with yields up to 96 %) affording a variety of quinoline derivatives.

Chemoselective Hydrogenation of Aldehydes under Mild, Base-Free Conditions: Manganese Outperforms Rhenium

Several hydride Mn(I) and Re(I) PNP pincer complexes were applied as catalysts for the homogeneous chemoselective hydrogenation of aldehydes. Among these, [Mn(PNP-iPr)(CO)₂(H)] was found to be one of the most efficient base metal catalysts for this process and represents a rare example which permits the selective hydrogenation of aldehydes in the presence of ketones and other reducible functionalities, such as C=C double bonds, esters, or nitriles. The reaction proceeds at room temperature under base-free conditions with catalyst loadings between 0.1 and 0.05 mol% and a hydrogen pressure of 50 bar (reaching TONs of up to 2000). A mechanism which involves an outer-sphere hydride transfer and reversible PNP ligand deprotonation/protonation is proposed. Analogous isoelectronic and isostructural Re(I) complexes were only poorly active.

Concerted Functions of Surface Acid-Base Pairs and Supported Copper Catalysts for Dehydrogenative Synthesis of Esters from Primary Alcohols

Dehydrogenative synthesis of esters from primary alcohols proceeded efficiently over a ZrO₂-supported copper catalyst. A variety of esters were obtained from primary alcohols as well as diols in good to high yields. The key to the dehydrogenative synthesis of esters is the concerted effect of the acid-base pairs on ZrO₂ and metallic copper.

Hydrogen autotransfer and related dehydrogenative coupling reactions using a rhenium(i) pincer catalyst

A novel rhenium complex bearing a non-innocent PNP pincer ligand was prepared.

Dehydrogenative coupling promoted by copper catalysts: a way to optimise and upgrade bio-alcohols

A one-pot one-step transformation of butanol into butyl butanoate takes place with excellent yield on a Cu/ZrO₂ catalyst.

Copper-Catalyzed Aerobic Oxidative Amidation of Benzyl Alcohols

A Cu-catalyzed synthesis of amides from alcohols and secondary amines using the oxygen in air as the terminal oxidant has been developed. The methodology is operationally simple requiring no high pressure equipment or handling of pure oxygen. The commercially available, nonprecious metal catalyst, Cu(phen)Cl₂, in conjunction with di-tert-butyl hydrazine dicarboxylate and an inorganic base provides a variety of benzamides in moderate to excellent yields. The pK_a of amine conjugate acid and electronics of alcohol were shown to impact the selection of base for optimal reactivity. A mechanism consistent with the observed reactivity trends, KIE, and Hammett study is proposed.

CO-enabled rhenium hydride catalyst for directed C(sp2)–H bond alkylation with olefins

We report the first example of CO-enabled rhenium hydride complex-catalyzed inter- and intra-molecular hydroarylation of activated alkenes, with a broad reaction scope.

System with potential dual modes of metal-ligand cooperation: highly catalytically active pyridine-based PNNH-Ru pincer complexes

Metal-ligand cooperation (MLC) plays an important role in catalysis. Systems reported so far are generally based on a single mode of MLC. We report here a system with potential for MLC by both amine-amide and aromatization-dearomatization ligand transformations, based on a new class of phosphino-pyridyl ruthenium pincer complexes, bearing sec-amine coordination. These pincer complexes are effective catalysts under unprecedented mild conditions for acceptorless dehydrogenative coupling of alcohols to esters at 35 °C and hydrogenation of esters at room temperature and 5 atm H2. The likely actual catalyst, a novel, crystallographically characterized monoanionic de-aromatized enamido-Ru(II) complex, was obtained by deprotonation of both the N-H and the methylene proton of the N-arm of the pincer ligand.

Activation of nitriles by metal ligand cooperation. Reversible formation of ketimido- and enamido-rhenium PNP pincer complexes and relevance to catalytic design

The dearomatized complex cis-[Re(PNP(tBu)*)(CO)2] (4) undergoes cooperative activation of C≡N triple bonds of nitriles via [1,3]-addition. Reversible C-C and Re-N bond formation in 4 was investigated in a combined experimental and computational study. The reversible formation of the ketimido complexes (5-7) was observed. When nitriles bearing an alpha methylene group are used, reversible formation of the enamido complexes (8 and 9) takes place. The reversibility of the activation of the nitriles in the resulting ketimido compounds was demonstrated by the displacement of p-CF3-benzonitrile from cis-[Re(PNP(tBu)-N═CPh(pCF3))(CO)2] (6) upon addition of an excess of benzonitrile and by the temperature-dependent [1,3]-addition of pivalonitrile to complex 4. The reversible binding of the nitrile in the enamido compound cis-[Re(PNP(tBu)-HNC═CHPh)(CO)2] (9) was demonstrated via the displacement of benzyl cyanide from 9 by CO. Computational studies suggest a stepwise activation of the nitriles by 4, with remarkably low activation barriers, involving precoordination of the nitrile group to the Re(I) center. The enamido complex 9 reacts via β-carbon methylation to give the primary imino complex cis-[Re(PNP(tBu)-HN═CC(Me)Ph)(CO)2]OTf 11. Upon deprotonation of 11 and subsequent addition of benzyl cyanide, complex 9 is regenerated and the monomethylation product 2-phenylpropanenitrile is released. Complexes 4 and 9 were found to catalyze the Michael addition of benzyl cyanide derivatives to α,β-unsaturated esters and carbonyls.