Rhodaelectro-catalyzed access to chromones via formyl C-H activation towards peptide electro-labeling

Chromones represent a privileged scaffold in medicinal chemistry and are an omnipresent structural motif in natural products. Chemically encoded non-natural peptidomimetics feature improved stability towards enzymatic degradation, cell permeability and binding affinity, translating into a considerable impact on pharmaceutical industry. Herein, a strategy for the sustainable assembly of chromones via electro-formyl C–H activation is presented. The rational design of the rhodaelectro-catalysis is guided by detailed mechanistic insights and provides versatile access to tyrosine-based fluorogenic peptidomimetics.

The chromone scaffold is present in drugs and bioactive natural products, but conventional approaches to access chromones require stoichiometric amounts of oxidants. Here, the authors report rhodaelectro-catalyzed assembly of chromones by electrochemical formyl C–H activations, providing the basis for late-stage peptide diversification.

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.

Construction of Highly Functionalized Xanthones via Rh-Catalyzed Cascade C-H Activation/O-Annulation

A facile and efficient strategy for obtaining functionalized and multihydroxylated xanthones via Rh catalysis under redox-neutral conditions is developed. Diverse salicylaldehydes bearing heterocycles, aromatics, and fused aromatics can be rapidly coupled with 1,4-benzoquinones or 1,4-hydroquinones to afford valuable xanthones via cascade C-H/O-H functionalization and annulation. This protocol provides a rapid synthetic approach to obtain biologically active materials through late-stage functionalization and prepares natural products such as subelliptenone, pruniflorone N, and ravenelin.

Teaching Aldehydes New Tricks Using Rhodium- and Cobalt-Hydride Catalysis

By using transition metal catalysts, chemists have altered the "logic of chemical synthesis" by enabling the functionalization of carbon-hydrogen bonds, which have traditionally been considered inert. Within this framework, our laboratory has been fascinated by the potential for aldehyde C-H bond activation. Our approach focused on generating acyl-metal-hydrides by oxidative addition of the formyl C-H bond, which is an elementary step first validated by Tsuji in 1965. In this Account, we review our efforts to overcome limitations in hydroacylation. Initial studies resulted in new variants of hydroacylation and ultimately spurred the development of related transformations (e.g., carboacylation, cycloisomerization, and transfer hydroformylation).Sakai and co-workers demonstrated the first hydroacylation of olefins when they reported that 4-pentenals cyclized to cyclopentanones, using stoichiometric amounts of Wilkinson's catalyst. This discovery sparked significant interest in hydroacylation, especially for the enantioselective and catalytic construction of cyclopentanones. Our research focused on expanding the asymmetric variants to access medium-sized rings (e.g., seven- and eight-membered rings). In addition, we achieved selective intermolecular couplings by incorporating directing groups onto the olefin partner. Along the way, we identified Rh and Co catalysts that transform dienyl aldehydes into a variety of unique carbocycles, such as cyclopentanones, bicyclic ketones, cyclohexenyl aldehydes, and cyclobutanones. Building on the insights gained from olefin hydroacylation, we demonstrated the first highly enantioselective hydroacylation of carbonyls. For example, we demonstrated that ketoaldehydes can cyclize to form lactones with high regio- and enantioselectivity. Following these reports, we reported the first intermolecular example that occurs with high stereocontrol. Ketoamides undergo intermolecular carbonyl hydroacylation to furnish α-acyloxyamides that contain a depsipeptide linkage.Finally, we describe how the key acyl-metal-hydride species can be diverted to achieve a C-C bond-cleaving process. Transfer hydroformylation enables the preparation of olefins from aldehydes by a dehomologation mechanism. Release of ring strain in the olefin acceptor offers a driving force for the isodesmic transfer of CO and H₂. Mechanistic studies suggest that the counterion serves as a proton-shuttle to enable transfer hydroformylation. Collectively, our studies showcase how transition metal catalysis can transform a common functional group, in this case aldehydes, into structurally distinct motifs. Fine-tuning the coordination sphere of an acyl-metal-hydride species can promote C-C and C-O bond-forming reactions, as well as C-C bond-cleaving processes.

Aerobic Copper-Catalyzed Salicylaldehydic Cformyl -H Arylations with Arylboronic Acids

We report a challenging copper-catalyzed C_formyl -H arylation of salicylaldehydes with arylboronic acids that involves unique salicylaldehydic copper species that differ from reported salicylaldehydic rhodacycles and palladacycles. This protocol has high chemoselectivity for the C_formyl -H bond compared to the phenolic O-H bond involving copper catalysis under high reaction temperatures. This approach is compatible with a wide range of salicylaldehyde and arylboronic acid substrates, including estrone and carbazole derivatives, which leads to the corresponding arylation products. Mechanistic studies show that the 2-hydroxy group of the salicylaldehyde substrate triggers the formation of salicylaldehydic copper complexes through a Cu^I /Cu^II /Cu^III catalytic cycle.

Pd-Catalyzed tandem C–C/C–O/C–H single bond cleavage of 3-allyloxybenzocyclobutenols

A Pd-catalyzed skeletal rearrangement of 3-allyloxybenzocyclobutenols was achieved, which involved tandem C–C/C–O bond cleavage and C–H allylic substitution.

Rh-Catalyzed domino synthesis of 4-hydroxy-3-methylcoumarins via branch-selective hydroacylation

A Rh-catalyzed domino synthesis of 4-hydroxy-3-methylcoumarins via branch-selective hydroacylation of acrylates and acrylamides using salicylaldehydes is described.

Branch-Selective Addition of Unactivated Olefins into Imines and Aldehydes

Radical hydrofunctionalization occurs with ease using metal-hydride hydrogen atom transfer (MHAT) catalysis to couple alkenes and competent radicalophilic electrophiles. Traditional two-electron electrophiles have remained unreactive. Herein we report the reductive coupling of electronically unbiased olefins with imines and aldehydes. Iron catalysis allows addition of alkyl-substituted olefins into imines through the intermediacy of free radicals, whereas a combination of catalytic Co(Sal ^{t-Bu, t-Bu}) and chromium salts enables a branch-selective coupling of olefins and aldehydes through the formation of a putative alkyl chromium intermediate.

Direct Synthesis of Highly Substituted Pyrroles and Dihydropyrroles Using Linear Selective Hydroacylation Reactions

Rhodium(I) catalysts incorporating small bite-angle diphosphine ligands, such as (Cy2 P)2 NMe or bis(diphenylphosphino)methane (dppm), are effective at catalysing the union of aldehydes and propargylic amines to deliver the linear hydroacylation adducts in good yields and with high selectivities. In situ treatment of the hydroacylation adducts with p-TSA triggers a dehydrative cyclisation to provide the corresponding pyrroles. The use of allylic amines, in place of the propargylic substrates, delivers functionalised dihydropyrroles. The hydroacylation reactions can also be combined in a cascade process with a Rh(I) -catalysed Suzuki-type coupling employing aryl boronic acids, providing a three-component assembly of highly substituted pyrroles.

A pentacoordinated norbornenyl-acyl-rhodium(iii) complex as a likely intermediate in the catalytic hydroacylation of norbornadiene

The isolated and characterized compounds hydrido-acyl-Rh(iii) and norbornenyl-acyl-Rh(iii) are active species in the hydroacylation of norbornadiene with quinoline-8-carbaldehyde.

Rhodium(I)-Catalyzed Intermolecular Hydroacylation of α-Keto Amides and Isatins with Non-Chelating Aldehydes

The application of the bidentate, electron-rich bisphosphine ligand, 1,3-bis(dicyclohexyl)phosphine-propane (dcpp), in rhodium(I)-catalyzed intermolecular ketone hydroacylation is herein described. Isatins and α-keto amides are shown to undergo hydroacylation with a variety of non-chelating linear and branched aliphatic aldehydes. Also reported is the synthesis of new bidentate chiral phosphine ligands, and their application in hydroacylation is discussed.

Mechanistic insights into hydroacylation with non-chelating aldehydes†Electronic supplementary information (ESI) available: Materials and methods, reaction procedures, characterization data. CCDC 1012849. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4sc02026jClick here for additional data file

The combination of a small-bite-angle diphosphine bis(dicyclohexylphosphino)methane (dcpm) and [Rh(cod)OMe]2 catalyses the hydroacylation of 2-vinylphenols with a wide range of non-chelating aldehydes. Here we present a detailed experimental study that elucidates the factors contributing to the broad aldehyde scope and high reactivity. A variety of catalytically relevant intermediates were isolated and a [Rh(dcpm)(vinylphenolate)] complex was identified as the major catalytically relevant species. A variety of off-cycle intermediates were also identified that can re-enter the catalytic cycle by substrate- or 1,5-cyclooctadiene-mediated pathways. Saturation kinetics with respect to the 2-vinylphenol were observed, and this may contribute to the high selectivity for hydroacylation over aldehyde decarbonylation. A series of deuterium labelling experiments and Hammett studies support the oxidative addition of Rh to the aldehyde C-H bond as an irreversible and turnover-limiting step. The small bite angle of dcpm is crucial for lowering the barrier of this step and providing excellent reactivity with a variety of aldehydes.

A complete switch of the directional selectivity in the annulation of 2-hydroxybenzaldehydes with alkynes

Controlling reaction selectivity is an eternal pursuit for chemists working in chemical synthesis. As part of this endeavor, our group has been exploring the possibility of constructing different natural product skeletons from the same simple starting materials by using different catalytic systems. In our previous work, an isoflavanone skeleton was obtained from the annulation of a salicylaldehyde and an alkyne when a gold catalyst was employed. In this paper, it is shown that a coumarin skeleton can be efficiently obtained through an annulation reaction with the same starting materials, that is, terminal alkynes and salicylaldehydes, by simply switching to a rhodium catalyst. A plausible reaction mechanism is proposed for this new annulation based on isotopic substitution experiments.

Regioselective hydroacylation of 1,3-dienes by cobalt catalysis

We describe a cobalt-catalyzed hydroacylation of 1,3-dienes with non-chelating aldehydes. Aromatic aldehydes provide 1,4-addition products as the major isomer, while aliphatic aldehydes favor 1,2-hydroacylation products. The kinetic profile supports an oxidative cyclization mechanism involving a cobaltacycle intermediate that undergoes transformation with high regio- and stereoselectivity.

Substrate-directed hydroacylation: rhodium-catalyzed coupling of vinylphenols and nonchelating aldehydes

We report a protocol for the hydroacylation of vinylphenols with aryl, alkenyl, and alkyl aldehydes to form branched products with high selectivity. This cross-coupling yields α-aryl ketones that can be cyclized to benzofurans, and it enables access to eupomatenoid natural products in four steps or less from eugenol. Excellent reactivity and high levels of regioselectivity for the formation of the branched products were observed. We propose that aldehyde decarbonylation is avoided by the use of an anionic directing group on the alkene and a diphosphine ligand with a small bite angle.

Rhodium catalyzed oxidative coupling of salicylaldehydes with diazabicyclic olefins: a one pot strategy involving aldehyde C-H cleavage and π-allyl chemistry towards the synthesis of fused ring chromanones

An efficient one pot strategy for the synthesis of cyclopentene fused chromanone derivatives through the direct oxidative coupling of salicylaldehydes with bicyclic olefins in the presence of a rhodium-copper catalyst system is described. This is the first report on the ring opening-ring closing of bicyclic hydrazines via metal catalyzed oxidative coupling reaction.

Rhodium(iii)-catalyzed coupling of N-sulfonyl 2-aminobenzaldehydes with oxygenated allylic olefins through C–H activation

N-Sulfonyl-2-aminobenzaldehyde undergoes C–H activation and coupling with oxygenated allylic olefins under redox-neutral conditions with high efficiency.

Recent catalytic approaches to chemical synthesis from carbon feedstocks

Traditional organic synthesis is driven by the need for functional molecules. The development of green chemical methods, however, is an increasingly important challenge in the context of global sustainability. To this end, the direct use of abundant carbon feedstocks in synthesis (such as CO, CO2, methanol, arenes, alkanes, α-olefins, and biological raw materials) aims to minimize waste production and increase efficiency.

A method for the synthesis of pyridines from aldehydes, alkynes and NH4OAc involving Rh-catalyzed hydroacylation and N-annulation

A new method for regiocontrolled pyridine synthesis has been developed involving sequential Rh(I)-catalyzed chelation-assisted hydroacylation of alkynes with aldehydes followed by Rh(III)-promoted N-annulation of the resulting α,β-enones with another alkyne and NH(3).

Catalysis through temporary intramolecularity: mechanistic investigations on aldehyde-catalyzed Cope-type hydroamination lead to the discovery of a more efficient tethering catalyst

Mechanistic investigations on the aldehyde-catalyzed intermolecular hydroamination of allylic amines using N-alkylhydroxylamines are presented. Under the reaction conditions, the presence of a specific aldehyde catalyst allows formation of a mixed aminal intermediate, which permits intramolecular Cope-type hydroamination. The reaction was determined to be first-order in both the aldehyde catalyst (α-benzyloxyacetaldehyde) and the allylic amine. However, the reaction displays an inverse order behavior in benzylhydroxylamine, which reveals a significant off-cycle pathway and highlights the importance of an aldehyde catalyst that promotes a reversible aminal formation. Kinetic isotope effect experiments suggest that hydroamination is the rate-limiting step of this catalytic cycle. Overall, these results enabled the elaboration of a more accurate catalytic cycle and led to the development of a more efficient catalytic system for alkene hydroamination. The use of 5-10 mol % of paraformaldehyde proved more effective than the use of 20 mol % of α-benzyloxyacetaldehyde, leading to high yields of intermolecular hydroamination products within 24 h at 30 °C.