Homogeneous Catalysis by Cobalt and Manganese Pincer Complexes

Old Concepts, New Application - Additive-Free Hydrogenation of Nitriles Catalyzed by an Air Stable Alkyl Mn(I) Complex

An efficient additive-free manganese-catalyzed hydrogenation of nitriles to primary amines with molecular hydrogen is described. The pre-catalyst, a well-defined bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dpre)(CO)3(CH3)] (dpre=1,2-bis(di-n-propylphosphino)ethane), undergoes CO migratory insertion into the manganese-alkyl bond to form acyl complexes which upon hydrogenolysis yields the active coordinatively unsaturated Mn(I) hydride catalyst [Mn(dpre)(CO)2(H)]. A range of aromatic and aliphatic nitriles were efficiently and selectively converted into primary amines in good to excellent yields. The hydrogenation of nitriles proceeds at 100 °C with a catalyst loading of 2 mol % and a hydrogen pressure of 50 bar. Mechanistic insights are provided by means of DFT calculations.

The Role of Proton Shuttles in the Reversible Activation of Hydrogen via Metal-Ligand Cooperation

The reversible activation of H₂ via a pathway involving metal-ligand cooperation (MLC) is proposed to be important in many transition metal catalyzed hydrogenation and dehydrogenation reactions. Nevertheless, there is a paucity of experimental information probing the mechanism of this transformation. Here, we present an in-depth kinetic study of the 1,2-addition of H₂ via an MLC pathway to the widely used iron catalyst [(^iPrPNP)FeH(CO)] (1) (^iPrPNP = N(CH₂CH₂PⁱPr₂)₂^-). We report one of the first experimental demonstrations of an enhancement in rate for the activation of H₂ using protic additives, which operate as "proton shuttles". Our results indicate that proton shuttles need to be able to both simultaneously donate and accept a proton, and the best shuttles are molecules that are strong hydrogen bond donors but sufficiently weak acids to avoid deleterious protonation of the transition metal complex. Additionally, comparison of the rate of H₂ activation via an MLC pathway between 1 and two widely used ruthenium catalysts enables more general conclusions about the role of the metal, ancillary ligand, and proton shuttles in H₂ activation. The results of this study provide guidance about the design of catalysts and additives to promote H₂ activation via an MLC pathway.

Polymer-Anchored Bifunctional Pincer Catalysts for Chemoselective Transfer Hydrogenation and Related Reactions

A series of polymer-supported cooperative PC(sp³ )P pincer catalysts was synthesized and characterized. Their catalytic activity in the acceptorless dehydrogenative coupling of alcohols and the transfer hydrogenation of aldehydes with formic acid as a hydrogen source was investigated. This comparative study, examining homogeneous and polymer-tethered species, proved that carefully designing a link between the support and the catalytic moiety, which takes into consideration the mechanism underlying the target transformation, might lead to superior heterogeneous catalysis.

Unmasking the Ligand Effect in Manganese-Catalyzed Hydrogenation: Mechanistic Insight and Catalytic Application

Manganese-catalyzed hydrogenation reactions have attracted broad interest since the first report in 2016. Among the reported catalytic systems, Mn catalysts supported by tridentate PNP- and NNP-pincer ligands have most commonly been used. For example, a number of PNP-Mn pincer catalysts have been reported for the hydrogenation of aldehydes, aldimines, ketones, nitriles, and esters. Furthermore, various NNP-Mn pincer catalysts have been shown to be active in the hydrogenation of less-reactive substrates such as amides, carbonates, carbamates, and urea derivations. These observations indicated that Mn catalysts supported by NNP-pincer ligands exhibit higher reactivity in hydrogenation reactions than their PNP counterparts. Such a ligand effect in Mn-catalyzed hydrogenation reactions has yet to be confirmed. Herein, we investigated the origin and applicability of this ligand effect. A combination of experimental and theoretical investigations showed that NNP-pincer ligands on the Mn complexes were more electron-rich and less sterically hindered than their PNP counterparts, leading to higher reactivity in a series of Mn-catalyzed hydrogenation reactions. Inspired by the ligand effect on Mn-catalyzed hydrogenations, we developed the first Mn-catalyzed hydrogenation of N-heterocycles. Specifically, NNP-Mn pincer catalysts hydrogenated a series of N-heterocycles (32 examples) with up to 99% yields, and the corresponding PNP-Mn pincer catalysts afforded low reactivity under the same conditions. This verified that such a ligand effect is generally applicable in hydrogenation reactions of both carbonyl and noncarbonyl substrates based on Mn catalysis.

Template catalysis by manganese pincer complexes: oxa- and aza-Michael additions to unsaturated nitriles

Activation of C[triple bond, length as m-dash]N bonds by metal-ligand cooperation provides a new route for the functionalization of nitriles. Herein, we report the electrophilic activation of unsaturated nitriles by dearomatized manganese pincer complexes for the oxa- and aza-Michael addition reactions under very mild and neutral conditions. Derivatives of acrylonitrile and allyl cyanide furnished the corresponding β-addition products by reacting with alcohols and amines. Mechanistically, the catalysis is mostly ligand based. Reaction of the dearomatized PNN-Mn complex with 2-pentenenitrile or 3-pentenenitrile furnished an enamido-Mn complex. The equilibrium between an enamido complex and a ketimido complex, and reversible C-C bond formation with the ligand are proposed to play central roles in the catalysis.

Nonbifunctional Outer-Sphere Strategy Achieved Highly Active α-Alkylation of Ketones with Alcohols by N-Heterocyclic Carbene Manganese (NHC-Mn)

The unusual nonbifunctional outer-sphere strategy was successfully utilized in developing an easily accessible N-heterocyclic carbene manganese (NHC-Mn) system for highly active α-alkylation of ketones with alcohols. This system was efficient for a wide range of ketones and alcohols under mild reaction conditions, and also for the green synthesis of quinoline derivatives. The direct outer-sphere mechanism and the high activity of the present system demonstrate the potential of nonbifunctional outer-sphere strategy in catalyst design for acceptorless dehydrogenative transformations.

Emergence and Applications of Base Metals (Fe, Co, and Ni) in Hydroboration and Hydrosilylation

Base metal catalysis offers an alternative to reactions, which were once dominated by precious metals in hydrofunctionalization reactions. This review article details the development of some base metals (Fe, Co, and Ni) in the hydroboration and hydrosilylation reactions concomitant with a brief overview of recent advances in the field. Applications of both commercially available metal salts and well-defined metal complexes in catalysis and opportunities to further advance the field is discussed as well.

Iron-PNP-Pincer-Catalyzed Transfer Dehydrogenation of Secondary Alcohols

The well-defined iron PNP pincer complex catalyst [Fe(H)(BH₄ )(CO)(HN{CH₂ CH₂ P(iPr)₂ }₂ ] was used for the catalytic dehydrogenation of secondary alcohols to give the corresponding ketones. Using acetone as inexpensive hydrogen acceptor enables the oxidation with good to excellent yields. DFT computations indicate an outer-sphere mechanism and support the importance of an acceptor to achieve this transformation under milder conditions.

Manganese(I)-Catalyzed Cross-Coupling of Ketones and Secondary Alcohols with Primary Alcohols

Catalytic cross-coupling of ketones and secondary alcohols with primary alcohols is reported. An abundant manganese-based pincer catalyst catalyzes the reactions. Low loading of catalyst (2 mol %) and catalytic use of a mild base (5-10 mol %) are sufficient for efficient cross-coupling. Various aryl and heteroaryl ketones are catalytically cross-coupled with primary alcohols to provide the selective α-alkylated products. Challenging α-ethylation of ketones is also attained using ethanol as an alkylating reagent. Further, direct use of secondary alcohols in the reaction results in in situ oxidation to provide the ketone intermediates, which undergo selective α-alkylation. The reaction proceeds via the borrowing hydrogen pathway. The catalyst oxidizes the primary alcohols to aldehydes, which undergo subsequent aldol condensation with ketones, promoted by catalytic amount of Cs₂CO₃, to provide the α,β-unsaturated ketone intermediates. The hydrogen liberated from oxidation of alcohols is used for hydrogenation of α,β-unsaturated ketone intermediates. Notably either water or water and dihydrogen are the only byproducts in these environmentally benign catalytic processes. Mechanistic studies allowed inferring all of the intermediates involved. Dearomatization-aromatization metal-ligand cooperation in the catalyst facilitates the facile O-H bond activation of both primary and secondary alcohols, and the resultant manganese alkoxide complexes produce corresponding carbonyl compounds, perhaps via β-hydride elimination. The manganese(I) hydride intermediate plays dual role as it hydrogenates α,β-unsaturated ketones and liberates molecular hydrogen to regenerate the catalytically active dearomatized intermediate. Metal-ligand cooperation allows all of the manganese intermediates to exist in same oxidation state (+1) and plays an important role in these catalytic cross-coupling reactions.

Additive-Free Isomerization of Allylic Alcohols to Ketones with a Cobalt PNP Pincer Catalyst

Catalytic isomerization of allylic alcohols in ethanol as a green solvent was achieved by using air and moisture stable cobalt (II) complexes in the absence of any additives. Under mild conditions, the cobalt PNP pincer complex substituted with phenyl groups on the phosphorus atoms appeared to be the most active. High rates were obtained at 120 °C, even though the addition of one equivalent of base increases the speed of the reaction drastically. Although some evidence was obtained supporting a dehydrogenation-hydrogenation mechanism, it was proven that this is not the major mechanism. Instead, the cobalt hydride complex formed by dehydrogenation of ethanol is capable of double-bond isomerization through alkene insertion-elimination.

A Microwave-Assisted SmI2-Catalyzed Direct N-Alkylation of Anilines with Alcohols

A new protocol for the alkylation of aromatic amines has been described using alcohols in the presence of SmI₂ as a catalyst with the generation of water as the sole byproduct. The reaction proceeds under MW conditions and selectively generates monoalkylated amines. This protocol features a broad substrate scope and good functional-group tolerance with moderate to high yields.

Enabling Two-Electron Pathways with Iron and Cobalt: From Ligand Design to Catalytic Applications

Homogeneous catalysis with Earth-abundant, first-row transition metals, including iron and cobalt, has gained considerable recent attention as a potentially cost-effective and sustainable alternative to more commonly and historically used precious metals. Because fundamental organometallic transformations, such as oxidative addition and reductive elimination, are two-electron processes and essential steps in many important catalytic cycles, controlling redox chemistry-in particular overcoming one-electron chemistry-has been as a central challenge with Earth-abundant metals. This Perspective focuses on approaches to impart sufficiently strong ligand fields to generate electron-rich metal complexes able to promote oxidative addition reactions where the redox changes are exclusively metal-based. Emphasis is placed on how ligand design and exploration of fundamental organometallic chemistry coupled with mechanistic understanding have been used to discover iron catalysts for the hydrogen isotope exchange in pharmaceuticals and cobalt catalysts for C(sp²)-H borylation reactions. A pervasive theme is that first-row metal complexes often promote unique chemistry from their precious-metal counterparts, demonstrating that these elements offer a host of new opportunities for reaction discovery and for more sustainable catalysis.

Base Metal-Catalyzed Direct Olefinations of Alcohols with Sulfones

Herein, a base-metal nickel-catalyzed direct olefination of alcohols with sulfones is reported. The reaction operates under low catalyst loading and does not require an external redox reagent. A wide range of trans-stilbenes and styrenes were synthesized in good yields and selectivities. Biologically active stilbene DMU-212 could also be synthesized in a single step under these conditions. Mechanistic studies involving kinetic isotope effect, deuterium labeling experiments, and catalytic and stoichiometric reactions with possible catalytic intermediates were performed to elucidate a plausible mechanism.