Pd-Catalyzed Intermolecular Dehydrogenative Heck Reactions of Five-Membered Heteroarenes

Transition metal-catalyzed double Cvinyl-H bond activation: synthesis of conjugated dienes

Conjugated dienes have occupied a pivotal position in the field of synthetic organic chemistry and medicinal chemistry. They act as important synthons for the synthesis of various biologically important molecules and therefore, gain tremendous attention worldwide. A wide range of synthetic routes to access these versatile molecules have been developed in the past decades. Transition metal-catalyzed cross-dehydrogenative coupling (CDC) has emerged as one of the utmost front-line research areas in current synthetic organic chemistry due to its high atom economy, efficiency, and viability. In this review, an up-to-date summary including scope, limitations, mechanistic studies, stereoselectivities, and synthetic applications of transition metal-catalyzed double C_vinyl-H bond activation for the synthesis of conjugated dienes has been reported since 2013. The literature reports mentioned in this review have been classified into three different categories, i.e. (a) C_vinyl-C_vinyl bond formation via oxidative homo-coupling of terminal alkenes; (b) C_vinyl-C_vinyl bond formation via non-directed oxidative cross-coupling of linear/cyclic alkenes and terminal/internal alkenes, and (c) C_vinyl-C_vinyl bond formation via oxidative cross-coupling of directing group bearing alkenes and terminal/internal alkenes. Overall, this review aims to provide a concise overview of the current status of the considerable development in this field and is expected to stimulate further innovation and research in the future.

Pd-catalysed, Ag-assisted C2–H alkenylation of benzophospholes

A palladium-catalysed, silver-assisted C2–H alkenylation of benzophospholes with terminal alkenes has been developed to form the corresponding benzophosphole–vinylene conjugations.

Design and synthesis of Fe3O4@SiO2@KIT-6@DTZ-Pd0 as a new and efficient mesoporous magnetic catalyst in carbon-carbon cross-coupling reactions

In this paper, a new type of mesoporous material based on KIT-6 has been introduced. In this aim, magnetic Fe3O4 nanoparticles and mesoporous silica KIT-6 have been combined to obtain mesoporous MNPs. The prepared magnetic mesoporous catalyst has been applied in different carbon-carbon cross-coupling reactions including Mizoroki-Heck, Suzuki-Miyaura, and Stille reactions. This magnetic mesoporous compound is characterized by various techniques including FT-IR, BET, VSM, SEM, XRD, and TGA.

Sterically controlled C-H alkenylation of pyrroles and thiophenes

Pd-catalyzed C-H alkenylations targeting the least hindered position of N-alkyl pyrroles and 3-substituted thiophenes, as opposed to electronically controlled approaches, are developed. The steric demand and stable bidentate binding mode of the pyrazolonaphthyridine ligand are key to the success of these sterically controlled alkenylations using oxygen as an oxidant.

Combustion-Synthesized Porous CuO-CeO2-SiO2 Composites as Solid Catalysts for the Alkenylation of C(sp3)-H Bonds Adjacent to a Heteroatom via Cross-Dehydrogenative Coupling

A series of mixed oxides of CuO, CeO2, and SiO2 were prepared by gel combustion and employed for the first time as efficient solid catalysts in a solvent-less liquid-phase cross-dehydrogenative coupling. The facile one-pot catalyst synthesis resulted in highly porous materials presenting large specific surface areas and strong metal–support interactions. The interaction with highly dispersed CeO2 enhanced the redox properties of the CuO species. The CuO-CeO2-SiO2 composites exhibited excellent catalytic performance for the selective coupling between 1,1-diphenylethylene and tetrahydrofuran with a yield up to 85% of 2-(2,2-diphenylvinyl)-tetrahydrofuran in the presence of di-tert-butyl peroxide (DTPB) and KI. Albeit both CuO and CeO2 species are proved to be responsible for the catalytic conversion, a great synergistic improvement in the catalytic activity was obtained by extended contact between the oxide phases by high porosity in comparison with the reactions using individual Cu or Ce catalysts. The activity of the composite catalyst was shown to be highly stable after five successive reaction cycles. Furthermore, the study scope was extended to the synthesis of different derivatives via composite-catalyzed coupling of C(sp2)-H with C(sp3-H) adjacent to a heteroatom. The good yields recorded proved the general validity of this composite for the cross-dehydrogenative coupling reaction rarely performed on solid catalysts.

C-H activation reactions of nitroarenes: current status and outlook

Ring substitution reactions of nitroarenes remain an under-developed area of organic synthesis, confined to the narrow domains of S_NAr and S_NArH reactions. While searching for alternative methodologies, we took stock of the C-H activation reactions of nitroarenes which unearthed a variety of examples of nitro directed regioselective C-H functionalization reactions such as ortho-arylation, -benzylation/alkylation, and -allylation, oxidative Heck and C-H arylation reactions on (hetero)aromatic rings. A collective account of these reactions is presented in this review to showcase the existing landscape of C-H activation reactions of nitroarenes, to create interest in this field for further development and propagate this strategy as a superior alternative for ring substitution reactions of nitroarenes. The prospect of merging the C-H activation of nitroarenes with C-NO₂ activation, thereby harnessing NO₂ as a transformable multitasking directing group, is also illustrated.

Palladium-Catalyzed Dehydrogenative C-2 Alkenylation of 5-Arylimidazoles and Related Azoles with Styrenes

The construction of carbon–carbon bonds by direct involvement of two unactivated carbon–hydrogen bonds, without any directing group, ensures a high atom economy of the entire process. Here, we describe a simple protocol for the Pd(II)/Cu(II)-promoted intermolecular cross-dehydrogenative coupling (CDC) of 5-arylimidazoles, benzimidazoles, benzoxazole and 4,5-diphenylimidazole at their C-2 position with functionalized styrenes. This specific CDC, known as the Fujiwara–Moritani reaction or oxidative Heck coupling, also allowed the C-4 alkenylation of the imidazole nucleus when both 2 and 5 positions were occupied.

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.

Transition-metal-catalyzed C–H functionalization of pyrazoles

This review describes recent advances in transition-metal-catalyzed C–H functionalization reactions of pyrazoles to form new C–C and C–heteroatom bonds on the pyrazole ring.