Intermolecular Markovnikov-Selective Hydroacylation of Olefins Catalyzed by a Cationic Ruthenium-Hydride Complex

Ruthenium-Catalyzed 1,6-Hydroalkylation of para-Quinone Methides with Ketones via the in Situ Activation of C(sp3)-H Bonds

A simple and efficient method for the ruthenium-catalyzed 1,6-hydroalkylation of para-quinone methides (p-QMs) with ketones via the in situ activation of C(sp3)-H bonds has been disclosed. Without the need for preactivation of the substrates and oxidant, a broad range of p-QMs and ketones are well tolerated, producing the expected 1,6-hydroalkylation products with moderate to good yields. Step-by-step control experiments and DFT calculation were conducted systematically to gain insights for the plausible reaction mechanism. This finding may have potential application in the selective diarylmethylation of ketones at the α-C position in organic synthesis.

A Quadruple Catalysis Enabling Intermolecular Branch-Selective Hydroacylation of Styrenes

A quadruple N-heterocyclic carbene/cobalt/photoredox/Brønsted base catalysis to realize branch-selective hydroacylation of styrenes with aromatic and aliphatic aldehydes is demonstrated. This protocol allows access to branched ketones from readily available materials in an atom-economical manner. The quadruple catalysis can transfer a formyl hydrogen of aldehydes as a hydrogen radical equivalent onto the terminal carbon of an alkene by controlled electron and proton transfers.

Transition-metal-catalyzed C-H bond alkylation using olefins: recent advances and mechanistic aspects

Transition metal catalysis has contributed immensely to C-C bond formation reactions over the last few decades, and alkylation is no exception. The superiority of such methodologies over traditional alkylation is evident from minimal reaction steps, shorter reaction times, and atom economy while also allowing control over regio- and stereo-selectivity. In particular, hydrocarbonation of alkenes has grabbed increased attention due its fundamental ability to effectively and selectively synthesise a wide range of industrially and pharmaceutically relevant moieties. This review attempts to provide a scientific viewpoint and a systematic analysis of the recent developments in transition-metal-catalyzed alkylation of various C-H bonds using simple and activated olefins. The key features and mechanistic studies involved in these transformations are described briefly.

Exogenous Ligand-Free NiH-Catalyzed Hydroacylation of Aryl Alkenes with Aroyl Fluorides

Acyl fluorides have emerged as efficient acyl group donors, but these attractive reagents have rarely been utilized in transition-metal-catalyzed hydroacylation. Herein we report a nickel hydride-catalyzed hydroacylation of aryl alkenes using aroyl fluorides. The reaction proceeds without recourse to an exogenous ligand under mild conditions. The synthetic utility of the present method is demonstrated by the glovebox-free, gram-scale reaction and the late-stage derivatization of complex molecules containing pharmaceutical frameworks.

Photoredox-Catalyzed Dehydrogenative Csp3-Csp2 Cross-Coupling of Alkylarenes to Aldehydes in Flow

Executing photoredox reactions in flow offers solutions to frequently encountered issues regarding reproducibility, reaction time, and scale-up. Here, we report the transfer of a photoredox-catalyzed benzylic coupling of alkylarenes to aldehydes to a flow chemistry setting leading to improvements in terms of higher concentration, shorter residence times, better yields, ease of catalyst preparation, and enhanced substrate scope. Its applicability has been demonstrated by a multi-gram-scale reaction using high-power light-emitting diodes (LEDs), late-stage functionalization of selected active pharmaceutical ingredients (APIs), and also a photocatalyst recycling method.

Sequential Catalytic Functionalization of Aryltriazenyl Aldehydes for the Synthesis of Complex Benzenes

We demonstrate that aryltriazenes can promote three distinctive types of C–H functionalization reactions, allowing the preparation of complex benzene molecules with diverse substitution patterns. 2-Triazenylbenzaldehydes are shown to be efficient substrates for Rh(I)-catalyzed intermolecular alkyne hydroacylation reactions. The resulting triazene-substituted ketone products can then undergo either a Rh(III)-catalyzed C–H activation, or an electrophilic aromatic substitution reaction, achieving multifunctionalization of the benzene core. Subsequent triazene derivatization provides traceless products.

Cobalt-Catalyzed Markovnikov-Selective Radical Hydroacylation of Unactivated Alkenes with Acylphosphonates

Acylphosphonates having the 5,5-dimethyl-1,3,2-dioxophosphinanyl skeleton are developed as efficient intermolecular radical acylation reagents, which enable the cobalt-catalyzed Markovnikov hydroacylation of unactivated alkenes at room temperature under mild conditions. The protocol exhibits broad substrate scope and wide functional group compatibility, providing branched ketones in satisfactory yields. A mechanism involving the Co-H mediated hydrogen atom transfer and subsequent trapping of alkyl radicals by acylphosphonates is proposed.

Experimental and Computational Study of the ( Z)-Selective Formation of Trisubstituted Olefins and Benzo-Fused Oxacycles from the Ruthenium-Catalyzed Dehydrative C-H Coupling of Phenols with Ketones

The cationic Ru-H complex was found to be an effective catalyst for the dehydrative C-H coupling of phenols with ketones to form the trisubstituted olefin products. The coupling of phenol with linear ketones led to highly stereoselective formation of the ( Z)-olefin products. The dehydrative coupling of phenol with enones and diones efficiently formed the benzopyrene and related oxacyclic derivatives. The reaction of 3,5-dimethoxyphenol with cyclohexanone-2,2,6,6- d₄ showed a significant H/D exchange to both vinyl and α-CH₂ positions on the olefin product (72-75% D). A significant carbon isotope effect was observed on the ortho-arene carbon of the olefin product. The free energies of intermediate species for the entire catalytic cycle were successfully computed by using the DFT method. The DFT study revealed that the E/ Z stereoselectivity is a result of the energy difference in the insertion step of ortho-metalated phenol to an enol form of the ketone substrate (ΔΔ E = 9.6 kcal/mol). The coupling method provides a direct catalytic C-H olefination method for ketones to form trisubstituted olefins without employing any reactive reagents or forming any wasteful byproducts.

Rh(DPEPhos)-Catalyzed Alkyne Hydroacylation Using β-Carbonyl-Substituted Aldehydes: Mechanistic Insight Leads to Low Catalyst Loadings that Enables Selective Catalysis on Gram-Scale

The detailed mechanism of the hydroacylation of β-amido-aldehyde, 2,2-dimethyl-3-morpholino-3-oxopropanal, with 1-octyne using [Rh( cis-κ²-_P,P-DPEPhos)(acetone)₂][BAr^F₄]-based catalysts, is described [Ar^F = (CF₃)₂C₆H₃]. A rich mechanistic landscape of competing and interconnected hydroacylation and cyclotrimerization processes is revealed. An acyl-hydride complex, arising from oxidative addition of aldehyde, is the persistent resting state during hydroacylation, and quaternary substitution at the β-amido-aldehyde strongly disfavors decarbonylation. Initial rate, KIE, and labeling studies suggest that the migratory insertion is turnover-limiting as well as selectivity determining for linear/branched products. When the concentration of free aldehyde approaches zero at the later stages of catalysis alkyne cyclotrimerization becomes competitive, to form trisubstituted hexylarenes. At this point, the remaining acyl-hydride turns over in hydroacylation and the free alkyne is now effectively in excess, and the resting state moves to a metallacyclopentadiene and eventually to a dormant α-pyran-bound catalyst complex. Cyclotrimerization thus only becomes competitive when there is no aldehyde present in solution, and as aldehyde binds so strongly to form acyl-hydride when this happens will directly correlate to catalyst loading: with low loadings allowing for free aldehyde to be present for longer, and thus higher selectivites to be obtained. Reducing the catalyst loading from 20 mol % to 0.5 mol % thus leads to a selectivity increase from 96% to ∼100%. An optimized hydroacylation reaction is described that delivers gram scale of product, at essentially quantitative levels, using no excess of either reagent, at very low catalyst loadings, using minimal solvent, with virtually no workup.

Cobalt-catalyzed intermolecular hydroacylation of aldehydes: initiation of hydride transfer enables turnover

Intermolecular hydroacylation of aldehydes catalyzed by a low-valent cobalt system was developed. Mechanistic studies disclosed that the rate of the insertion step is significantly promoted in the presence of iPr₂NEt, which functions as an initiator of hydride transfer.

Enantioselective CuH-Catalyzed Hydroacylation Employing Unsaturated Carboxylic Acids as Aldehyde Surrogates

The direct asymmetric copper hydride (CuH)-catalyzed coupling of α,β-unsaturated carboxylic acids to aryl alkenes to access chiral α-aryl dialkyl ketones is reported. A variety of substrate substitution patterns, sensitive functional groups, and heterocycles are tolerated in this reaction, which significantly expands the range of accessible products compared with existing hydroacylation methodology. Although mechanistic studies are ongoing, we propose that CuH-catalyzed silylation of unsaturated acids occurs to access a uniquely effective acyl electrophilic coupling partner.

Two-Component Assembly of Thiochroman-4-ones and Tetrahydrothiopyran-4-ones Using a Rhodium-Catalyzed Alkyne Hydroacylation/Thio-Conjugate-Addition Sequence

β'-Thio-substituted-enones, assembled from the combination of β-tert-butylthio-substituted aldehydes and alkynes, using rhodium catalysis, are shown to smoothly undergo in situ intramolecular S-conjugate addition to deliver a range of S-heterocycles in a one-pot process. Aryl, alkenyl, and alkyl aldehydes can all be employed, to provide thiochroman-4-ones, hexahydro-4H-thiochromen-4-ones, and tetrahydrothiopyran-4-ones, respectively. A variety of in situ oxidations are also performed, allowing access to S,S-dioxide derivatives, as well as unsaturated variants.

Traceless Rhodium-Catalyzed Hydroacylation Using Alkyl Aldehydes: The Enantioselective Synthesis of β-Aryl Ketones

A one-pot three-step sequence involving Rh-catalyzed alkene hydroacylation, sulfide elimination and Rh-catalyzed aryl boronic acid conjugate addition gave products of traceless chelation-controlled hydroacylation employing alkyl aldehydes. The stereodefined β-aryl ketones were obtained in good yields with excellent control of enantioselectivity. Good variation of all three reaction components is possible.