Alkynes as Electrophilic or Nucleophilic Allylmetal Precursors in Transition-Metal Catalysis

Pd/Brønsted Acid-Catalyzed Atom-Economical [3 + 3] Annulation of 4-Hydroxycoumarins and Skipped Enynes

We report an unprecedented application of skipped enynes as bis-electrophiles under palladium/Brønsted acid catalysis. A [3 + 3] annulation of skipped enynes with 4-hydroxy coumarins as bis-nucleophiles enables a completely atom-economic synthesis of fused O-heterocycles. The reaction is proposed to proceed via the Pd-catalyzed C-H activation of the skipped enyne leading to a diene, which upon subsequent activation by Pd and an O-nucleophilic attack leads to the regioselective formation of a six-membered heterocyclic ring, with a vinyl substituent.

Nickel-Catalyzed Regioselective Hydrothiolation of Allenes Enabled by Visible-Light Photoredox Catalysis

Hydrothiolation presents an attractive way to transform allenes into allylic thioethers. Herein, we described an efficient visible-light photoredox-promoted nickel-catalyzed hydrothiolation of allenes with functionalized aromatic and aliphatic thiols. This synergistic catalytic system exhibits unprecedentedly high reactivities and regiocontrol for the construction of allylic thioethers, representing the unique synthetic utility of the earth-abundant Ni-catalyzed method compared with the related noble-metal-catalyzed allylation reactions.

Leveraging the Stereochemical Complexity of Octahedral Diastereomeric-at-Metal Catalysts to Unlock Regio-, Diastereo-, and Enantioselectivity in Alcohol-Mediated C-C Couplings via Hydrogen Transfer

Experimental and computational studies illuminating the factors that guide metal-centered stereogenicity and, therefrom, selectivity in transfer hydrogenative carbonyl additions of alcohol proelectrophiles catalyzed by chiral-at-metal-and-ligand octahedral d6 metal ions, iridium(III) and ruthenium(II), are described. To augment or invert regio-, diastereo-, and enantioselectivity, predominantly one from among as many as 15 diastereomeric-at-metal complexes is required. For iridium(III) catalysts, cyclometalation assists in defining the metal stereocenter, and for ruthenium(II) catalysts, iodide counterions play a key role. Whereas classical strategies to promote selectivity in metal catalysis aim for high-symmetry transition states, well-defined low-symmetry transition states can unlock selectivities that are otherwise difficult to achieve or inaccessible.

Pd/Brønsted acid catalysed intramolecular N-allylation of indoles and pyrroles with alkynes for the synthesis of N-fused heterocycles

We, herein, report a Pd(0) and Brønsted acid-catalyzed redox-neutral intramolecular N-allylation of indoles and pyrroles with alkynes for the synthesis of biologically important imidazolidinone-fused N-heterocycles. The allylation is completely atom-economical and is applicable to a wide range of substrates. The methodology eliminates the use of a leaving group or an oxidizing agent, often employed for the allylation of nucleophiles. To the best of our knowledge, N-allylation of indoles and pyrroles with alkynes has not been reported to date.

Nickel-catalyzed direct stereoselective α-allylation of ketones with non-conjugated dienes

The development of efficient and sustainable methods for the construction of carbon-carbon bonds with the simultaneous stereoselective generation of vicinal stereogenic centers is a longstanding goal in organic chemistry. Low-valent nickel(0) complexes which promote α-functionalization of carbonyls leveraging its pro-nucleophilic character in conjunction with suitable olefin acceptors are scarce. We report a Ni(0)NHC catalyst which selectively converts ketones and non-conjugated dienes to synthetically highly valuable α-allylated products. The catalyst directly activates the α-hydrogen atom of the carbonyl substrate transferring it to the olefin acceptor. The transformation creates adjacent quaternary and tertiary stereogenic centers in a highly diastereoselective and enantioselective manner. Computational studies indicate the ability of the Ni(0)NHC catalyst to trigger a ligand-to-ligand hydrogen transfer process from the ketone α-hydrogen atom to the olefin substrate, setting the selectivity of the process. The shown selective functionalization of the α-C-H bond of carbonyl groups by the Ni(0)NHC catalyst opens up new opportunities to exploit sustainable 3d-metal catalysis for a stereoselective access to valuable chiral building blocks.

Iridium-, Ruthenium-, and Nickel-Catalyzed C-C Couplings of Methanol, Formaldehyde, and Ethanol with π-Unsaturated Pronucleophiles via Hydrogen Transfer

In this Perspective, the use of methanol and ethanol as C1 and C2 feedstocks in metal-catalyzed C-C couplings to π-unsaturated pronucleophiles via hydrogen auto-transfer is surveyed. In these processes, alcohol oxidation to form an aldehyde electrophile is balanced by reduction of an π-unsaturated hydrocarbon to form a transient organometallic nucleophile. Mechanistically related reductive couplings of paraformaldehyde mediated by alcohol reductants or formic acid also are described. These processes encompass the first catalytic enantioselective C-C couplings of methanol and ethanol and, more broadly, illustrate how the native reducing ability of alcohols enable the departure from premetalated reagents in carbonyl addition.

Three-Component Electrochemical Aminoselenation of 1,3-Dienes

Azoles and organoselenium compounds are pharmacologically important scaffolds in medicinal chemistry and natural products. We developed an efficient regioselective electrochemical aminoselenation reaction of 1,3-dienes, azoles, and diselenide derivatives to access selenium-containing allylazoles skeletons. This protocol is more economical and environmentally friendly and features a broad substrate scope; pyrazole, triazole, and tetrazolium were all tolerated under the standard conditions, which could be applied to the expedient synthesis of bioactive molecules and in the pharmaceutical industry.

Synthesis of Diversified Pyrazolo[3,4-b]pyridine Frameworks from 5-Aminopyrazoles and Alkynyl Aldehydes via Switchable C≡C Bond Activation Approaches

A cascade 6-endo-dig cyclization reaction was developed for the switchable synthesis of halogen and non-halogen-functionalized pyrazolo[3,4-b]pyridines from 5-aminopyrazoles and alkynyl aldehydes via C≡C bond activation with silver, iodine, or NBS. In addition to its wide substrate scope, the reaction showed good functional group tolerance as well as excellent regional selectivity. This new protocol manipulated three natural products, and the arylation, alkynylation, alkenylation, and selenization of iodine-functionalized products. These reactions demonstrated the potential applications of this new method.

Mechanistic Versatility at Ir(PSiP) Pincer Catalysts: Triflate Proton Shuttling from 2-Butyne to Diene and [3]Dendralene Motifs

The five-coordinate hydrido complex [IrH(OTf)(PSiP)] (1) catalytically transforms 2-butyne into a mixture of its isomer 1,3-butadiene, and [3]dendralene and linear hexatriene dimerization products: (E)-4-methyl-3-methylene-1,4-hexadiene and (3Z)-3,4-dimethyl-1,3,5-hexatriene, respectively. Under the conditions of the catalytic reaction, benzene, and 363 K, the hexatriene further undergoes thermal electrocyclization into 2,3-dimethyl-1,3-cyclohexadiene. The reactions between 1 and the alkyne substrate allow isolation or nuclear magnetic resonance (NMR) observation of catalyst resting states and possible reaction intermediates, including complexes with the former PSiP pincer ligands disassembled into PSi and PC chelates, and species coordinating allyl or carbene fragments en route to products. The density functional theory (DFT) calculations guided by these experimental observations disclose competing mechanisms for C–H bond elaboration that move H atoms either classically, as hydrides, or as protons transported by the triflate. This latter role of triflate, previously recognized only for more basic anions such as carboxylates, is discussed to result from combining the unfavorable charge separation in the nonpolar solvent and the low electronic demand from the metal to the anion at coordination positions trans to silicon. Triflate deprotonation of methyl groups is key to release highly coordinating diene products from stable allyl intermediates, thus enabling catalytic cycling.

Transition-Metal-Catalyzed Carbonylative Multifunctionalization of Alkynes

Currently, the construction of new carbon-carbon bonds and value-added structures in an atom- and step economical manner has become a continuous pursuit in the synthetic chemistry community. Since the first transition-metal-catalyzed hydroformylation of ethylene was reported by Otto Roelen in the 1930s, impressive progress has been achieved in the carbonylative functionalization of unsaturated C-C bonds. In contrast to alkenes, the carbonylative functionalization of alkynes offers tremendous potential for the construction of multisubstituted carbonyl-containing derivatives because of their two independently addressable π-systems. This review provides a timely and necessary investigation of transition-metal-catalyzed carbonylative mutifunctionalization of alkynes with the exclusion of carbonylative hydrofunctionalizations. Different transition metals including palladium, rhodium, iridium, ruthenium, iron, copper, etc. were applied to the development of novel carbonylative transformation. Various C-C, C-N, C-O, C-S, C-B, C-Si, and carbon-halogen bonds were formed efficiently and give the corresponding tri- or tetrasubstituted α,β-unsaturated ketones, diesters, and heterocycles.

Nitrogen-Doped Carbon-Incarcerated Zinc Electrodes as Heterogeneous Catalysts for Electrochemical Allylation of Carbonyl Compounds

Electrochemical allylation reactions of carbonyl compounds using cathodes prepared from nitrogen-doped carbon (NDC)-incarcerated zinc catalysts have been developed. A range of aldehydes and ketones afforded the desired allylic alcohols in high yields with <10 mol % zinc leaching, and the heterogeneous nature of the active species was suggested. Compared with bulk zinc electrodes, NDC-stabilized zinc nanoparticle species were compatible with a broader range of heteroaromatic substrates and enabled the use of an undivided cell.

V. K, Reddy VS. Regioselective Allenylation and Propargylation of Various para-Quinone Methides Using Alkynyl Azaarenes as Pronucleophile

We have developed a Brønsted base-mediated regioselective allenylation and propargylation of various para-quinone methides using unfunctionalized 2-alkynyl azaarenes as the pronucleophile. The appropriate choice of a base provides an opportunity...

SOMOphilic Alkynylation of Unreactive Alkenes Enabled by Iron-Catalyzed Hydrogen Atom Transfer

We report an efficient and practical iron-catalyzed hydrogen atom transfer protocol for assembling acetylenic motifs into functional alkenes. Diversities of internal alkynes could be obtained from readily available alkenes and acetylenic sulfones with excellent Markovnikov selectivity. An iron hydride hydrogen atom transfer catalytic cycle was described to clarify the mechanism of this reaction.

Stereodivergent Palladium- and Rhodium-Catalyzed Intramolecular Addition of Tosylureas to Allenes: Diastereoselective Synthesis of Tetrahydropyrimidinones

The intramolecular addition of tosylureas to allenes is highly syn-/anti-diastereoselective when employing a palladium or rhodium-based catalytic system and affords 1,3-cyclic ureas. Under palladium catalysis a range of thermodynamic anti-tetrahydropyrimidinones are accessible, while rhodium catalysis allows synthesis of the kinetic syn-tetrahydropyrimidinones. For a representative scope of substrates both cyclic ureas were obtained in excellent yields and diastereoselectivities. The obtained tetrahydropyrimidinones were shown to be easily deprotected and modified to demonstrate the synthetic value.

Understanding Halide Counterion Effects in Enantioselective Ruthenium-Catalyzed Carbonyl (α-Aryl)allylation: Alkynes as Latent Allenes and Trifluoroethanol-Enhanced Turnover in The Conversion of Ethanol to Higher Alcohols via Hydrogen Auto-transfer

Crystallographic characterization of RuX(CO)(η3-C3H5)(JOSIPHOS), where X = Cl, Br, or I, reveals a halide-dependent diastereomeric preference that defines metal-centered stereogenicity and, therefrom, the enantioselectivity of C-C coupling in ruthenium-catalyzed anti-diastereo- and enantioselective C-C couplings of primary alcohols with 1-aryl-1-propynes to form products of carbonyl anti-(α-aryl)allylation. Computational studies reveal that a non-classical hydrogen bond between iodide and the aldehyde formyl CH bond stabilizes the favored transition state for carbonyl addition. An improved catalytic system enabling previously unattainable transformations was developed that employs an iodide-containing precatalyst, RuI(CO)3(η3-C3H5), in combination with trifluoroethanol, as illustrated by the first enantioselective ruthenium-catalyzed C-C couplings of ethanol to form higher alcohols.

Allenes and Dienes as Chiral Allylmetal Pronucleophiles in Catalytic Enantioselective C=X Addition: Historical Perspective and State-of-The-Art Survey

The use of allenes and 1,3-dienes as chiral allylmetal pronucleophiles in intermolecular catalytic enantioselective reductive additions to aldehydes, ketones, imines, carbon dioxide and other C=X electrophiles is exhaustively catalogued together with redox-neutral hydrogen auto-transfer processes. Coverage is limited to processes that result in both C-H and C-C bond formation. The use of alkynes as latent allylmetal pronucleophiles and multicomponent C=X allylations involving allenes and dienes is not covered. As illustrated in this review, the ability of allenes and 1,3-dienes to serve as tractable non-metallic pronucleophiles has evoked many useful transformations that have no counterpart in traditional allylmetal chemistry.

Redox-Switchable Cycloisomerization of Alkynoic Acids with Napthalenediimide-Derived N-Heterocyclic Carbene Complexes

Two naphthalene-diimide (NDI) bis-imidazolium salts have been used as N-heterocyclic carbene (NHC) precursors for the preparation of NDI-functionalized complexes of rhodium and iridium of general formula [MCl(NDI-NHC)(COD)] (M=Rh, Ir; NDI-NHC=NDI-functionalized NHC ligand). Comparison of the IR spectra of the complexes [IrCl(NDI-NHC)(CO)2 ] and their related one- and two-electron reduced forms, reveal that each one-electron reduction produces a decrease of the average ν(CO) of 9-10 cm-1 , indicating a significant enhancement of the electron-richness of the metal. The [MCl(NDI-NHC)(COD)] complexes were tested in the catalytic cycloisomerization of alkynoic acids. The one-electron reduced forms showed greatly enhanced activities. For the cyclization of 5-hexynoic acid, the two-electron reduction of the ligand produced further enhancement of the catalytic activity, therefore showing that the catalyst can switch between three redox species with three distinct catalytic activities.

Reducing Challenges in Organic Synthesis with Stereoselective Hydrogenation and Tandem Catalysis

Tandem catalysis enables the rapid construction of complex architectures from simple building blocks. This Perspective shares our interest in combining stereoselective hydrogenation with transformations such as isomerization, oxidation, and epimerization to solve diverse challenges. We highlight the use of tandem hydrogenation for preparing complex natural products from simple prochiral building blocks and present tandem catalysis involving transfer hydrogenation and dynamic kinetic resolution. Finally, we underline recent breakthroughs and opportunities for asymmetric hydrogenation.

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.

Comparative DFT Study on Dehydrogenative C(sp)-H Elementation (E = Si, Ge, and Sn) of Terminal Alkynes Catalyzed by a Cationic Ruthenium(II) Thiolate Complex

Described herein is a comparative theoretical study of dehydrogenative C(sp)-H functionalizations of a terminal alkyne with group-14-based hydrides (HEEt₃; E = Si, Ge, Sn) catalyzed by an Ohki-Tatsumi complex-a cationic Ru(II) complex with a tethered thiolate ligand ([Ru-S] = [(DmpS)Ru(PiPr₃)][BAr₄^F]; Dmp = 2,6-(dimesityl)₂C₆H₃; Ar^F = 3,5-(CF₃)₂C₆H₃). The calculations indicate that the energy barriers for heterolytic cleavage of the H-EEt₃ bonds at the Ru-S sites of the Ohki-Tatsumi complex highly vary depending on the group 14 elements from 3.8 kcal/mol (E = Sn) to 10.5 kcal/mol (E = Ge) and 18.5 kcal/mol (E = Si), where Ru and S elements cooperatively serve as the Lewis acid and base, respectively. Likewise, the transfer of the group 14 cation (Et₃E⁺) to the C-C triple bond to generate the β-element-stabilized vinyl cations-the rate-determining step (RDS) of the overall reaction-is predicted to be susceptible to the element's identity [E_a = 36.8 for Sn < 42.9 and Ge < 50.7 for Si (kcal/mol)]. The key transition states involved in the RDS are compared in terms of energy and structure within each system of the group 14 hydrides. The distortion/interaction-activation strain (DIAS) model analysis of the transition states responsible for dehydrogenative stannylation and hydrostannation of a terminal alkyne sheds light on the origin of the experimentally observed kinetic preference toward dehydrogenative C-H stannylation over hydrostannation.

Conjugated ynones in catalytic enantioselective reactions

Conjugated ynones are easily accessible feedstock and the existence of an alkyne bond endows ynones with different attractive reactivities, thus making them unique substrates for catalytic asymmetric reactions. Their compatibility under organocatalytic, metal-catalyzed as well as cooperative catalytic conditions has resulted in numerous enantioselective transformations. Importantly, conjugated ynones can act as nucleophiles or electrophiles, and serve as easily accessed synthons for different cyclization pathways. This review summarizes the recent literature examples of the catalytic reactions of conjugated ynones and related compounds such as alkyne conjugated α-ketoesters, and classifies these reaction types alongside mechanistic insights whenever possible. We aim to trigger more intensive research in the future to render the asymmetric transformation of ynones as a common and reliable tool for asymmetric synthesis.

Orthogonal Regulation of Nucleophilic and Electrophilic Sites in Pd-Catalyzed Regiodivergent Couplings between Indazoles and Isoprene

Depending on the reactant property and reaction mechanism, one major regioisomer can be favored in a reaction that involves multiple active sites. Herein, an orthogonal regulation of nucleophilic and electrophilic sites in the regiodivergent hydroamination of isoprene with indazoles is demonstrated. Under Pd-hydride catalysis, the 1,2- or 4,3-insertion pathway with respect to the electrophilic sites on isoprene could be controlled by the choice of ligands. In terms of the nucleophilic sites on indazoles, the reaction occurs at either the N¹ - or N² -position of indazoles is governed by the acid co-catalysts. Preliminary experimental studies have been performed to rationalize the mechanism and regioselectivity. This study not only contributes a practical tool for selective functionalization of isoprene, but also provides a guide to manipulate the regioselectivity for the N-functionalization of indazoles.

Enantioselective Addition of α-Nitroesters to Alkynes

By using Rh-H catalysis, we couple α-nitroesters and alkynes to prepare α-amino-acid precursors. This atom-economical strategy generates two contiguous stereocenters, with high enantio- and diastereocontrol. In this transformation, the alkyne undergoes isomerization to generate a Rh^III -π-allyl electrophile, which is trapped by an α-nitroester nucleophile. A subsequent reduction with In powder transforms the allylic α-nitroesters to the corresponding α,α-disubstituted α-amino esters.

Diastereo- and Enantioselective Ruthenium-Catalyzed C-C Coupling of 1-Arylpropynes and Alcohols: Alkynes as Chiral Allylmetal Precursors in Carbonyl anti-(α-Aryl)allylation

Highly tractable 1-aryl-1-propynes, which are readily accessible via Sonogashira coupling, serve as chiral allylmetal pronucleophiles in ruthenium-JOSIPHOS-catalyzed anti-diastereo- and enantioselective aldehyde (α-aryl)allylations with primary aliphatic or benzylic alcohol proelectrophiles. This method enables convergent construction of homoallylic sec-phenethyl alcohols bearing tertiary benzylic stereocenters. Both steric and electronic features of aryl sulfonic acid additives were shown to contribute to the efficiency with which a more selective and productive iodide-bound ruthenium catalyst is formed. As corroborated by isotopic labeling studies, a dual catalytic process is operative in which alkyne-to-allene isomerization is followed by allene-carbonyl reductive coupling via hydrogen auto-transfer. Crossover of ruthenium hydrides emanating from these two discrete catalytic events is observed. The utility of this method is illustrated by conversion of selected reaction products to the corresponding phenethylamines and the first total syntheses of the neolignan natural products (-)-crataegusanoids A-D.

Asymmetric α-Pentadienylation of Aldehydes with Cyclopropylacetylenes

By employing readily available cyclopropylacetylene and its derivatives as the pentadienylation reagent, an asymmetric regioselective asymmetric α-pentadienylation reaction of aldehydes is developed by cooperative catalysis of a chiral Pd(0) catalyst and a chiral Brønsted acid in the presence of a subschoichmetric amount of an achiral amine. α-Pentadienylated aldehydes are afforded with high yields and enantioselectivities as well as excellent E/Z ratios.

Rhodium(i)/bisoxazolinephosphine-catalyzed regio- and enantioselective amination of allylic carbonates: a computational study

DFT calculations were performed to investigate the rhodium(i)/bisoxazolinephosphine-catalyzed regio- and enantioselective amination of allylic carbonates.