Regioselective Hydrohydroxyalkylation of Styrene with Primary Alcohols or Aldehydes via Ruthenium-Catalyzed C-C Bond Forming Transfer Hydrogenation

Heteroaryl-Directed Iridium-Catalyzed Enantioselective C-H Alkenylations of Secondary Alcohols

Under iridium-catalyzed conditions, 2-aza-aryl-substituted secondary alcohols undergo C(sp3)-H addition reactions to alkynes to provide alkenylated tertiary alcohols. The processes occur with very high regio- and enantioselectivity. An analogous addition to styrene is shown to provide a prototype C(sp3)-H alkylation process. A mechanism based on directed aza-enolization of the reactant alcohol is proposed.

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.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Air-Stable Coordinatively Unsaturated Ruthenium(II) Complex for Ligand Binding and Catalytic Transfer Hydrogenation of Ketones from Ethanol

Coordinatively unsaturated complexes are interesting from a fundamental level for their formally empty coordination site and, in particular, from a catalytic perspective as they provide opportunities for substrate binding and transformation. Here, we describe the synthesis of a novel underligated ruthenium complex [Ru(cym)(N,N')]+, 3, featuring an amide-functionalized pyridylidene amide (PYA) as the N,N'-bidentate coordinating ligand. In contrast to previously investigated underligated complexes, complex 3 offers potential for dynamic modifications, thanks to the flexible donor properties of the PYA ligand. Specifically, they allow both for stabilizing the formally underligated metal center in complex 3 through nitrogen π-donation and for facilitating through π-acidic bonding properties the coordination of a further ligand L to the ruthenium center to yield the formal 18 e- complexes [Ru(cym)(N,N')(L)]+ (4: L = P(OMe)3; 5: L = PPh3; 6: L = N-methylimidazole; 7: L = pyridine) and neutral complex [RuCl(cym)(N,N')] 8. Analysis by 1H NMR and UV-vis spectroscopies reveals an increasing Ru-L bond strength along the sequence pyridine <1-methylimidazole < PPh3 < P(OMe)3 with binding constants varying over 3 orders of magnitude with log(Keq) values between 2.8 and 5.7. The flexibility of the Ru(PYA) unit and the ensuing accessibility of saturated and unsaturated species with one and the same ligand are attractive from a fundamental point of view and also for catalytic applications, as catalytic transformations rely on the availability of transiently vacant coordination sites. Thus, while complex 3 does not form stable adducts with O-donors such as ketones or alcohols, it transiently binds these species, as evidenced by the considerable catalytic activity in the transfer hydrogenation of ketones. Notably, and as one of only a few catalysts, complex 3 is compatible with EtOH as a hydrogen source. Complex 3 shows excellent performance in the transfer hydrogenation of pyridyl-containing substrates, in agreement with the poor coordination strength of this functional group to the ruthenium center in 3.

Conversion of Primary Alcohols and Butadiene to Branched Ketones via Merged Transfer Hydrogenative Carbonyl Addition-Redox Isomerization Catalyzed by Rhodium

The first examples of rhodium-catalyzed carbonyl addition via hydrogen autotransfer are described, as illustrated in tandem butadiene-mediated carbonyl addition-redox isomerizations that directly convert primary alcohols to isobutyl ketones. Related reductive coupling-redox isomerizations of aldehyde reactants mediated by sodium formate also are reported. A double-labeling crossover experiment reveals that the rhodium alkoxide obtained upon carbonyl addition enacts redox isomerization without dissociation of rhodium at any intervening stage.

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.

Recent Developments in C–C Bond Formation Using Catalytic Reductive Coupling Strategies

Metal-catalyzed reductive coupling processes have emerged as a powerful methodology for the introduction of molecular complexity from simple starting materials. These methods allow for an orthogonal approach to that of redox-neutral strategies for the formation of C–C bonds by enabling cross-coupling of starting materials not applicable to redox-neutral chemistry. This short review summarizes the most recent developments in the area of metal-catalyzed reductive coupling utilizing catalyst turnover by a stoichiometric reductant that becomes incorporated in the final product.

1 Introduction

2 Ni Catalysis

3 Cu Catalysis

4 Ru, Rh, and Ir Catalysis

4.1 Alkenes

4.2 1,3-Dienes

4.3 Allenes

4.4 Alkynes

4.5 Enynes

5 Fe, Co, and Mn Catalysis

6 Conclusion and Outlook

Feedstock Reagents in Metal-Catalyzed Carbonyl Reductive Coupling: Minimizing Preactivation for Efficiency in Target-Oriented Synthesis

Use of abundant feedstock pronucleophiles in catalytic carbonyl reductive coupling enhances efficiency in target-oriented synthesis. For such reactions, equally inexpensive reductants are desired or, ideally, corresponding hydrogen autotransfer processes may be enacted wherein alcohols serve dually as reductant and carbonyl proelectrophile. As described in this Minireview, these concepts allow reactions that traditionally require preformed organometallic reagents to be conducted catalytically in a byproduct-free manner from inexpensive π-unsaturated precursors.

Conversion of Aldehydes to Branched or Linear Ketones via Regiodivergent Rhodium-Catalyzed Vinyl Bromide Reductive Coupling-Redox Isomerization Mediated by Formate

A regiodivergent catalytic method for direct conversion of aldehydes to branched or linear alkyl ketones is described. Rhodium complexes modified by P ^tBu₂Me catalyze formate-mediated aldehyde-vinyl bromide reductive coupling-redox isomerization to form branched ketones. Use of the less strongly coordinating ligand, PPh₃, promotes vinyl- to allylrhodium isomerization en route to linear ketones. This method bypasses the 3-step sequence often used to convert aldehydes to ketones involving the addition of pre-metalated reagents to Weinreb or morpholine amides.

Selective Hydrogen Atom Abstraction through Induced Bond Polarization: Direct α-Arylation of Alcohols through Photoredox, HAT, and Nickel Catalysis

The combination of nickel metallaphotoredox catalysis, hydrogen atom transfer catalysis, and a Lewis acid activation mode, has led to the development of an arylation method for the selective functionalization of alcohol α-hydroxy C-H bonds. This approach employs zinc-mediated alcohol deprotonation to activate α-hydroxy C-H bonds while simultaneously suppressing C-O bond formation by inhibiting the formation of nickel alkoxide species. The use of Zn-based Lewis acids also deactivates other hydridic bonds such as α-amino and α-oxy C-H bonds. This approach facilitates rapid access to benzylic alcohols, an important motif in drug discovery. A 3-step synthesis of the drug Prozac exemplifies the utility of this new method.

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.