Joy and Challenges of Alkynylation of Arenes and Heteroarenes through Double C−H Functionalizations

Regiodivergent Metal-Catalyzed Oxidative Alkynylation of 2-Arylthiazoles with Terminal Alkynes under Air Conditions

Regiodivergent transition-metal-catalyzed oxidative C5- and ortho-alkynylation of 2-arylthiazoles have been demonstrated. Namely, Pd(II)-catalysis selectively generated C5-alkynylated products from the reaction of 2-arylthiazoles and terminal alkynes. In contrast, Ru(II)-catalysis exclusively provided ortho-alkynylated products from the same substrates. This protocol features a wide substrate scope, good functional group tolerance, high atom-economy, and exclusive regioselectivity. The alkynylated products can be readily converted into highly valuable synthons, which hold potential for applications in the fields of medicinal chemistry and materials science.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

The C-H Bond Activation Triggered by Subsurface Mo Dopant on MgO Catalyst in Oxidative Coupling of Methane

In this work, density functional theory calculations are performed to explore the unique role of Mo dopant on MgO in oxidative coupling of methane. It is revealed that subsurface Mo dopant significantly enhanced the adsorption and activation of oxygen molecules. The combination of adsorbed oxygen and surface Mg exhibited a balanced activity for C-H bond activation and release of methyl radical which paves the way to activate methane with a promising yield.

Pd/Cu-catalyzed access to novel 3-(benzofuran-2-ylmethyl) substituted (pyrazolo/benzo)triazinone derivatives: their in silico/in vitro evaluation as inhibitors of chorismate mutase (CM)

In view of the reported chorismate mutase (CM or MtbCM) inhibitory activities of 3-indolylmethyl substituted (pyrazolo/benzo)triazinone derivatives the structurally similar 3-(benzofuran-2-ylmethyl) substituted (pyrazolo/benzo)triazinones were designed and evaluated in silico against CM. The docking of target molecules was performed at the interface site of MtbCM (PDB: 2FP2). All the best ranked molecules participated in a strong H-bonding with the ILE67 of the B chain at the backbone position in addition to several hydrophobic/van der Waals interactions with the hydrophobic residues. Based on encouraging docking results, the one-pot synthesis of newly designed benzofuran derivatives was carried out using tandem Pd/Cu-catalyzed Sonogashira cross-coupling followed by intramolecular cyclization of 2-iodophenols with appropriate terminal alkynes. A range of novel 3-(benzofuran-2-ylmethyl) substituted (pyrazolo/benzo)triazinone derivatives were prepared in high (>80%) yields. Three molecules i.e.3h, 3i and 3m that participated in good interaction with CM in silico showed encouraging (64–65%) inhibition at 30 μM in vitro. An SAR within this class of molecules suggested that the benzotriazinone series in general was better than the pyrazolotriazinone series. Based on molecular docking in silico, CM inhibition in vitro and computational ADME prediction the benzofuran derivatives 3i and 3m seemed to be of further medicinal interest in the context of discovery and development of new anti-tubercular agents.

We report the Pd/Cu-catalyzed synthesis, in silico molecular docking, in vitro CM inhibition and computational ADME prediction of novel 3-(benzofuran-2-ylmethyl) substituted (pyrazolo/benzo)triazinone derivatives.

Straightforward synthesis of [Cu(NHC)(alkynyl)] and [Cu(NHC)(thiolato)] complexes (NHC = N-heterocyclic carbene)

Synthetic access to monomeric copper-alkynyl and copper-thiolato complexes of the type [(NHC)Cu(R)] (R = alkynyl or thiolato) using a weak base approach is reported. All reported reactions proceed under mild conditions in air and in environmentally acceptable solvents. The novel complexes are fully characterized and single crystal X-ray analyses unambiguously establish the atom connectivity in these mononuclear complexes. The importance of the supporting NHC ligand's steric properties in stabilizing mononuclear complexes is discussed.

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.

Ligand-free Pd/Ag-mediated dehydrogenative alkynylation of imidazole derivatives

A variety of 2-alkynyl(benzo)imidazoles have been synthesized by dehydrogenative alkynylation of (benzo)imidazoles with terminal alkyne in NMP under air in the presence of Ag₂CO₃ as the oxidant and Pd(OAc)₂ as the catalyst precursor. The data obtained in this study support a reaction mechanism involving a non-concerted metalation deprotonation (n-CMD) pathway.

The regioselective synthesis of 2-alkynyl(benz)imidazoles was successfully achieved by Pd(ii)/Ag(i)-mediated dehydrogenative alkynylation of the corresponding (benz)imidazoles with terminal alkynes in an open vessel.

Facilitating Ir-Catalyzed C-H Alkynylation with Electrochemistry: Anodic Oxidation-Induced Reductive Elimination

An electrochemical approach in promoting directed C-H alkynylation with terminal alkyne via iridium catalysis is reported. This work employed anodic oxidation of Ir(III) intermediate (characterized by X-ray crystallography) to promote reductive elimination, giving the desired coupling products in good yields (up to 95%) without the addition of any other external oxidants. This transformation is suitable for various directing groups with H2 as the only by-product, which warrants a high atom economy and practical oxidative C-C bond formation under mild conditions.

Cross-Dehydrogenative Alkynylation: A Powerful Tool for the Synthesis of Internal Alkynes

Alkynes are among the most fundamentally important organic compounds and are widely used in synthetic chemistry, biochemistry, and materials science. Thus, the development of an efficient and sustainable method for the preparation of alkynes has been a central concern in organic synthesis. Cross-dehydrogenative coupling utilizing E-H and Z-H bonds in two different molecules can avoid the need for prefunctionalization of starting materials and has become one of the most straightforward methods for the construction of E-Z chemical bonds. This Review summarizes recent progress in the preparation of internal alkynes by cross-dehydrogenative coupling with terminal alkynes.