Ruthenium-catalyzed C-C coupling of amino alcohols with dienes via transfer hydrogenation: redox-triggered imine addition and related hydroaminoalkylations

Electroinduced Reductive and Dearomative Alkene-Aldehyde Coupling

The direct coupling of alkene feedstocks with aldehydes represents an expedient approach to the generation of new and structurally diverse C(sp3)-hybridized alcohols that are primed for elaboration into privileged architectures. Despite their abundance, current disconnection strategies enabling the direct coupling of carbon-carbon π-bonds and aldehydes remain challenging because contemporary methods are often limited by substrate or functional group tolerance and compatibility in complex molecular environments. Here, we report a coupling between simple alkenes, heteroarenes and unactivated aliphatic aldehydes via an electrochemically induced reductive activation of C-C π-bonds. The cornerstone of this approach is the discovery of rapid alternating polarity (rAP) electrolysis to access and direct highly reactive radical anion intermediates derived from conjugated alkenes and heterocyclic compounds. Our developed catalyst-free protocol enables direct access to new and structurally diverse C(sp3)-hybridized alcohol products. This is achieved by the controlled reduction of conjugated alkenes and the C2-C3 π-bond in heteroarenes via an unprecedented reductive dearomative functionalization for heterocyclic compounds. Experimental mechanistic studies demonstrate a kinetically biased single-electron reduction of C-C π-bonds over aldehydes. Application of rAP enables chemoselective generation of olefinic radical anion intermediates and avoids undesired saturative overreduction. Overall, this technology provides a versatile approach to the reductive coupling of olefin and heterocycle feedstocks with aliphatic aldehydes, offering straightforward access to diverse C(sp3)-rich oxygenated scaffolds.

Ligand-Controlled Regiodivergent Nickel-Catalyzed Hydroaminoalkylation of Unactivated Alkenes

Ligand modulation of transition-metal catalysts to achieve optimal reactivity and selectivity in alkene hydrofunctionalization is a fundamental challenge in synthetic organic chemistry. Hydroaminoalkylation, an atom-economical approach for alkylating amines using alkenes, is particularly significant for amine synthesis in the pharmaceutical, agrochemical, and fine chemical industries. However, the existing methods usually require specific substrate combinations to achieve precise regio- and stereoselectivity, which limits their practical utility. Protocols allowing for regiodivergent hydroaminoalkylation from the same starting materials, controlling both regiochemical and stereochemical outcomes, are currently absent. Herein, we report a ligand-controlled, regiodivergent nickel-catalyzed hydroaminoalkylation of unactivated alkenes with N-sulfonyl amines. The reaction initiates with amine dehydrogenation and involves aza-nickelacycle intermediates. Tritert-butylphosphine promotes branched regioselectivity and syn diastereoselectivity, whereas ethyldiphenylphosphine enables linear selectivity, yielding regioisomers with inverse orientation. Systematic evaluation of diverse monodentate phosphine ligands reveals distinct regioselectivity cliffs, and % Vbur (min), a ligand steric descriptor, was established as a predictive parameter correlating ligand structure to regioselectivity. Computational investigations supported experimental findings, offering mechanistic insights into the origins of regioselectivity. Our method provides an efficient and predictable route for amine synthesis, demonstrating broad substrate scope, excellent tolerance toward various functional groups, and practical advantages. These include the use of readily available starting materials and cost-effective nickel(II) salts as precatalysts.

Historical perspective on ruthenium-catalyzed hydrogen transfer and survey of enantioselective hydrogen auto-transfer processes for the conversion of lower alcohols to higher alcohols

Ruthenium-catalyzed hydrogen auto-transfer reactions for the direct enantioselective conversion of lower alcohols to higher alcohols are surveyed. These processes enable completely atom-efficient carbonyl addition from alcohol proelectrophiles in the absence of premetalated reagents or metallic reductants. Applications in target-oriented synthesis are highlighted, and a brief historical perspective on ruthenium-catalyzed hydrogen transfer processes is given.

CuH-Catalyzed Enantioselective Reductive Coupling of 1,3-Dienes and Trifluoromethyl Ketoimines or α-Iminoacetates

The intermolecular addition of allylic copper species generated from diene and copper hydride remains elusive. Herein copper hydride catalyzed asymmetric cross reductive coupling of conjugated dienes and ketoimines including trifluoromethyl ketoimines and α-iminoacetates was first achieved using chiral Ph-BPE as the ligand, providing rapid access to structurally and optically enriched homoallylic amines containing two vicinal stereogenic centers with up to 95% yield, 99% ee, and 11:1 diastereoselectivities.

Direct, Catalytic α-Alkylation of N-Heterocycles by Hydroaminoalkylation: Substrate Effects for Regiodivergent Product Formation

Saturated N-heterocycles are prevalent in pharmaceutical and agrochemical industries, yet remain challenging to catalytically alkylate. Most strategies for C-H activation of these challenging substrates use protected amines or high loadings of precious metal catalysts. We report an early transition-metal system for the broad, robust, and direct alkylation of unprotected amine heterocycles with simple alkenes. Short reaction times are achieved using an in situ generated tantalum catalyst that avoids the use of bases, excess substrate, or additives. In most cases, this catalyst system is selective for the branched reaction product, including examples of products that are generated with excellent diastereoselectivity. Alkene electronic properties can be exploited for substrate-modified regioselectivity to access the alternative linear amine alkylation product with a group 5 catalyst. This method allows for the facile isolation of unprotected N-heterocyclic products, as useful substrates for further reactivity.

From Hydrogenation to Transfer Hydrogenation to Hydrogen Auto-Transfer in Enantioselective Metal-Catalyzed Carbonyl Reductive Coupling: Past, Present, and Future

Atom-efficient processes that occur via addition, redistribution or removal of hydrogen underlie many large volume industrial processes and pervade all segments of chemical industry. Although carbonyl addition is one of the oldest and most broadly utilized methods for C-C bond formation, the delivery of non-stabilized carbanions to carbonyl compounds has relied on premetalated reagents or metallic/organometallic reductants, which pose issues of safety and challenges vis-à-vis large volume implementation. Catalytic carbonyl reductive couplings promoted via hydrogenation, transfer hydrogenation and hydrogen auto-transfer allow abundant unsaturated hydrocarbons to serve as substitutes to organometallic reagents, enabling C-C bond formation in the absence of stoichiometric metals. This perspective (a) highlights past milestones in catalytic hydrogenation, hydrogen transfer and hydrogen auto-transfer, (b) summarizes current methods for catalytic enantioselective carbonyl reductive couplings, and (c) describes future opportunities based on the patterns of reactivity that animate transformations of this type.

Access to Chiral Diamine Derivatives through Stereoselective Cu-Catalyzed Reductive Coupling of Imines and Allenamides

Chiral 1,2-diamino compounds are important building blocks in organic chemistry for biological applications and as asymmetric inducers in stereoselective synthesis that are challenging to prepare in a straightforward and stereoselective manner. Herein, we disclose a cost-effective and readily available Cu-catalyzed system for the reductive coupling of a chiral allenamide with N-alkyl substituted aldimines to access chiral 1,2-diamino synthons as single stereoisomers in high yields. The method shows broad reaction scope and high diastereoselectivity and can be easily scaled using standard Schlenk techniques. Mechanistic investigations by density functional theory calculations identified the mechanism and origin of stereoselectivity. In particular, the addition to the imine was shown to be reversible, which has implications toward development of catalyst-controlled stereoselective variants of the identified reductive coupling of imines and allenamides.

Intermolecular Hydroaminoalkylation of Propadiene

Intermolecular hydroaminoalkylation reactions of propadiene with selected secondary amines take place in the presence of a 2,6-bis(phenylamino)pyridinato titanium catalyst. The corresponding products, synthetically useful allylamines, are formed in convincing yields and with high selectivities. In addition, propadiene easily inserts into the titanium-carbon bond of a titanaaziridine.

Microwave-Assisted Synthesis of Zirconium Phosphate Nanoplatelet-Supported Ru-Anadem Nanostructures and Their Catalytic Study for the Hydrogenation of Acetophenone

The catalytic hydrogenation of organic compounds containing carbonyl groups has been extensively studied and widely used in industrial processes. Herein, we report the preparation of a novel nanomaterial, α-zirconium phosphate (α-ZrP) nanoplatelet-supported ruthenium nano-anadem catalyst, which possesses high selectivity in the catalytic hydrogenation of aromatic ketones. The α-ZrP nanoplatelets were prepared using a modified reflux method. Through an ion-exchange and reduction reaction pathway, ruthenium nanoparticles were loaded on ZrP to produce Ru-ZrP with a nano-anadem structure. The successful synthesis of Ru-ZrP composites is supported by a series of characterization techniques (PXRD, SEM, TEM, EDS, XPS, FT-IR, etc.). Compared with pure ZrP nanoplatelets, the catalytic hydrogenation of acetophenone has been dramatically improved when using Ru-ZrP. Full conversion was achieved at room temperature, and the yield of 1-cyclohexylehtanol was up to 95%. The effects of reaction time, reaction temperature, and hydrogen pressure were investigated. The investigation illustrates that there are two proposed reaction pathways in the hydrogenation of acetophenone, which are further supported by computational analyses. Recycling experiments indicate that the Ru-ZrP material could be reused four times without a noticeable activity decrease.

A general carbonyl alkylative amination for tertiary amine synthesis

The ubiquity of tertiary alkylamines in pharmaceutical and agrochemical agents, natural products and smallmolecule biological probes^1,2 continues to stimulate enormous efforts towards their streamlined synthesis^3–9. Arguably, the most robust method for tertiary alkylamine synthesis is carbonyl reductive amination³: comprising two elementary steps, condensation of a secondary alkylamine with an aliphatic aldehyde forms an all alkyl-iminium ion, which is reduced by a hydride reagent. Chemists have sought to develop direct strategies for a ‘higher order’ variant of this reaction via the union of an alkyl fragment with an in-situ generated all alkyl-iminium ion^10–14. However, despite more than 70 years of research, the successful realization of a ‘carbonyl alkylative amination’ has remained elusive. Herein, we report that a practical and general solution can be accomplished by the addition of alkyl-radicals to all alkyl-iminium ions. The process is facilitated by visible-light and a silane reducing agent, which, together with the other reaction components, trigger a distinct radical initiation step to establish a chain process. An attractive feature of this operationally straightforward, metal-free and modular transformation is the unbiased nature of tertiary amines that arise from the traceless union of aldehydes and secondary amines with alkyl-halides. As such, the structural and functional diversity within these classes of abundant feedstocks provides a versatile and flexible strategy for the streamlined synthesis of complex tertiary amines.

Pyridine(diimine) Iron Diene Complexes Relevant to Catalytic [2+2]-Cycloaddition Reactions

The synthesis, characterization, and catalytic activity of pyridine(diimine) iron piperylene and isoprene complexes are described. These diene complexes are competent precatalysts for (i) the selective cross-[2+2]-cycloaddition of butadiene or (E)-piperylene with ethylene and α-olefins and (ii) the 1,4-hydrovinylation of isoprene with ethylene. In the former case, kinetic analysis implicates the diamagnetic η4-piperylene complex as the resting state prior to rate-determining oxidative cyclization. Variable temperature 1H NMR and EXSY experiments established that diene exchange from the diamagnetic, 18e- complexes occurs rapidly in solution at ambient temperature through a dissociative mechanism. The solid-state structure of (Me(Et)PDI)Fe(η4-piperylene) (Me(Et)PDI = 2,6-(2,6-Me2-C6H3N═CEt)2C5H3N), was determined by single-crystal X-ray diffraction and confirmed the s-trans coordination of the monosubstituted 1,3-diene. Possible relationships between ligand-controlled diene coordination geometry, metallacycle denticity, and chemoselectivity of iron-mediated cycloaddition reactions are discussed.

A General Acid-Mediated Hydroaminomethylation of Unactivated Alkenes and Alkynes

In comparison to the extensively studied metal-catalyzed hydroamination reaction, hydroaminomethylation has received significantly less attention despite its considerable potential to streamline amine synthesis. State-of-the-art protocols for hydroaminomethylation of alkenes rely largely on transition-metal catalysis, enabling this transformation only under highly designed and controlled conditions. Here we report a broadly applicable, acid-mediated approach to the hydroaminomethylation of unactivated alkenes and alkynes. This methodology employs cheap, readily available, and bench-stable reactants and affords the desired amines with excellent functional group tolerance and impeccable regioselectivity. The broad scope of this transformation, as well as mechanistic investigations and in situ domino functionalization reactions are reported.

Copper-catalyzed enantioselective 1,2-borylation of 1,3-dienes

A highly enantioselective Cu-catalyzed borylation of 2-substituted 1,3-dienes is reported. The use of a chiral phosphanamine ligand is essential in achieving high chemo-, regio-, diastereo- and enantioselectivity. It provides access to a variety of homoallylic boronates in consistently high yield and enantiomeric excess with 2-aryl and 2-heteroaryl 1,3-dienes as well as sterically demanding 2-alkyl 1,3-dienes. Preliminary investigations based on a non-linear effect study point to a mechanism involving more than one metal center.

Enantioselective Synthesis of anti-1,2-Diamines by Cu-Catalyzed Reductive Couplings of Azadienes with Aldimines and Ketimines

Here we report highly efficient and chemoselective azadiene-imine reductive couplings catalyzed by (Ph-BPE)Cu-H that afford anti-1,2-diamines. In all cases, reactions take place with either aldimine or ketimine electrophiles to deliver a single diastereomer of product in >95:5 er. The products' diamines are easily differentiable, facilitating downstream synthesis.

Intermolecular Metal-Catalyzed Reductive Coupling of Dienes, Allenes, and Enynes with Carbonyl Compounds and Imines

Metal-catalyzed reductive coupling has emerged as an alternative to the use of stoichiometric organometallic reagents in an increasingly diverse range of carbonyl and imine additions. In this review, the use of diene, allene, and enyne pronucleophiles in intermolecular carbonyl and imine reductive couplings are surveyed, along with related hydrogen autotransfer processes.

Early transition metal-catalyzed C–H alkylation: hydroaminoalkylation for Csp3–Csp3 bond formation in the synthesis of selectively substituted amines

Protecting group, directing group, and external oxidant free synthesis of structurally diverse amines.

Reductive Coupling of Acrylates with Ketones and Ketimines by a Nickel-Catalyzed Transfer-Hydrogenative Strategy

Nickel-catalyzed coupling of benzyl acrylates with activated ketones and imines provides γ-butyrolactones and lactams, respectively. The benzyl alcohol byproduct released during the lactonization/lactamization event is relayed to the next cycle where it serves as the reductant for C-C bond formation. This strategy represents a conceptually unique approach to transfer-hydrogenative C-C bond formation, thus providing examples of reductive heterocyclizations where hydrogen embedded within an alcohol leaving group facilitates turnover.

Reductive C-C Coupling via Hydrogenation and Transfer Hydrogenation: Departure from Stoichiometric Metals in Carbonyl Addition

Metal catalyzed reductive couplings of π-unsaturated reagents with carbonyl compounds via hydrogenation or transfer hydrogenation has emerged as an alternative to the use of stoichiometric organometallic reagents in carbonyl addition.

Arylative Intramolecular Allylation of Ketones with 1,3-Enynes Enabled by Catalytic Alkenyl-to-Allyl 1,4-Rhodium(I) Migration

Alkenyl‐to‐allyl 1,4‐rhodium(I) migration enables the generation of nucleophilic allylrhodium(I) species by remote C−H activation. This new mode of reactivity was employed in the diastereoselective reaction of arylboron reagents with substrates containing a 1,3‐enyne tethered to a ketone, to give products containing three contiguous stereocenters. The products can be obtained in high enantioselectivities using a chiral sulfur‐alkene ligand.

Rhodium-Catalyzed Hydrofunctionalization: Enantioselective Coupling of Indolines and 1,3-Dienes

We communicate a strategy for the hydrofunctionalization of 1,3-dienes via Rh-hydride catalysis. Conjugated dienes are coupled to nucleophiles to demonstrate the feasibility of novel C-C, C-O, C-S, and C-N bond forming processes. In the presence of a chiral JoSPOphos ligand, hydroamination generates chiral allylic amines with high regio- and enantioselectivity. Tuning both the pKa and steric properties of an acid-additive is critical for enantiocontrol.

Regiodivergent and Diastereoselective CuH-Catalyzed Allylation of Imines with Terminal Allenes

A copper-catalyzed, chemoselective hydrometalation process enables the use of simple allenes as allylmetal nucleophile surrogates in imine allylation reactions. By modulating the nitrogen-protecting group, either highly branched- or linear-selective addition can be achieved from the same allene. Both reactions exhibit excellent diastereoselectivity and broad functional-group tolerance. Preliminary results indicate that good enantioselectivity can also be achieved in the linear-selective reaction. Finally, a mechanistic model for the regiodivergence is proposed on the basis of density functional theory calculations.

Copper-Catalyzed Enantioselective Addition of Styrene-Derived Nucleophiles to Imines Enabled by Ligand-Controlled Chemoselective Hydrocupration

The copper-catalyzed intermolecular enantioselective addition of styrenes to imines has been achieved under mild conditions at ambient temperature. This process features the use of styrenes as latent carbanion equivalents via the intermediacy of catalytically generated benzylcopper derivatives, providing an effective means for accessing highly enantiomerically enriched amines bearing contiguous stereocenters. Mechanistic studies shed light on the origin of the preferential styrene hydrocupration in the presence of an imine with the Ph-BPE-derived copper catalyst.

Ruthenium-Catalyzed Transfer Hydrogenation for C-C Bond Formation: Hydrohydroxyalkylation and Hydroaminoalkylation via Reactant Redox Pairs

Merging the chemistry of transfer hydrogenation and carbonyl or imine addition, a broad new family of redox-neutral or reductive hydrohydroxyalkylations and hydroaminomethylations have been developed. In these processes, hydrogen redistribution between alcohols and π-unsaturated reactants is accompanied by C-C bond formation, enabling direct conversion of lower alcohols to higher alcohols. Similarly, hydrogen redistribution between amines to π-unsaturated reactants results in direct conversion of lower amines to higher amines. Alternatively, equivalent products of hydrohydroxyalkylation and hydroaminomethylation may be generated through the reaction of carbonyl compounds or imines with π-unsaturated reactants under the conditions of 2-propanol-mediated reductive coupling. Finally, using vicinally dioxygenated reactants, that is, diol, ketols, or diones, successive transfer hydrogenative coupling occurs to generate 2 C-C bonds, resulting in products of formal [4+2] cycloaddition.

Catalysis by Design: Well-Defined Single-Site Heterogeneous Catalysts

Heterogeneous catalysis, a field important industrially and scientifically, is increasingly seeking and refining strategies to render itself more predictable. The main issue is due to the nature and the population of catalytically active sites. Their number is generally low to very low, their "acid strengths" or " redox properties" are not homogeneous, and the material may display related yet inactive sites on the same material. In many heterogeneous catalysts, the discovery of a structure-activity reationship is at best challenging. One possible solution is to generate single-site catalysts in which most, if not all, of the sites are structurally identical. Within this context and using the right tools, the catalyst structure can be designed and well-defined, to reach a molecular understanding. It is then feasible to understand the structure-activity relationship and to develop predictable heterogeneous catalysis. Single-site well-defined heterogeneous catalysts can be prepared using concepts and tools of surface organometallic chemistry (SOMC). This approach operates by reacting organometallic compounds with surfaces of highly divided oxides (or of metal nanoparticles). This strategy has a solid track record to reveal structure-activity relationship to the extent that it is becoming now quite predictable. Almost all elements of the periodical table have been grafted on surfaces of oxides (from simple oxides such as silica or alumina to more sophisticated materials regarding composition or porosity). Considering catalytic hydrocarbon transformations, heterogeneous catalysis outcome may now be predicted based on existing mechanistic proposals and the rules of molecular chemistry (organometallic, organic) associated with some concepts of surface sciences. A thorough characterization of the grafted metal centers must be carried out using tools spanning from molecular organometallic or surface chemistry. By selection of the metal, its ligand set, and the support taken as a X, L ligands in the Green formalism, the catalyst can be designed and generated by grafting the organometallic precursor containing the functional group(s) suitable to target a given transformation (surface organometallic fragments (SOMF)). The choice of these SOMF is based on the elementary steps known in molecular chemistry applied to the desired reaction. The coordination sphere necessary for any catalytic reaction involving paraffins, olefins, and alkynes also can thus be predicted. Only their most complete understanding can allow development of catalytic reactions with the highest possible selectivity, activity, and lifetime. This Account will examine the results of SOMC for hydrocarbon transformations on oxide surfaces bearing metals of group 4-6. The silica-supported catalysts are exhibiting remarkable performances for Ziegler-Natta polymerization and depolymerization, low temperature hydrogenolysis of alkanes and waxes, metathesis of alkanes and cycloalkanes, olefins metathesis, and related reactions. In the case of reactions involving molecules that do not contain carbon (water-gas shift, NH3 synthesis, etc.) this single site approach is also valid but will be considered in a later review.