The direct functionalization of heterocyclic compounds has emerged as one of the most important topics in the field of metal-catalyzed C-H bond activation due to the fact that products are an important synthetic motif in organic synthesis, the pharmaceutical industry, and materials science. This critical review covers the recent progresses on the regioselective dehydrogenative direct coupling reaction of heteroarenes, including arylation, olefination, alkynylation, and amination/amidation mainly utilizing transition metal catalysts (113 references).

The catalytic transfer hydrogenation (CTH) reaction is considered as a potential route for upgrading bio-based carbonyls to their corresponding alcohols. Herein, a series of Zr-based metal−organic frameworks (Zr-MOFs) containing various types of metal node to ligand coordinations were synthesized and tested for CTH of furfural (FUR) to furfuryl alcohol (FOL). It was found that metal node coordination plays a more important role than porosity in Zr-MOFs. MOF-808 (synthesized using a scaled-up approach to achieve a higher batch yield), with the lowest metal node to ligand coordination (coordination number 6), was found to be the most active catalyst among the various tested Zr-MOFs. Furthermore, M-MOF-808, modified by simple methanol activation (M), outperformed the pristine MOF-808 in CTH of FUR to FOL even at 30 °C in the presence of 2-propanol (IPA) as the hydrogen source. The simple modification of the metal node in the Zr-MOF changed the acid− base properties of the MOF-808 surface through the development of coordinatively unsaturated sites (CUS), hydroxyl and methoxy groups in the framework of the Zr-MOF, which probably help to facilitate the adsorption of FUR and IPA onto the metal node surfaces of the catalyst. To evaluate the versatility of methanol activation in CTH, further substrates, including other types of biomass and representative carbonyl compounds over M-MOF-808, were tested. To demonstrate heterogeneous catalysis, the catalyst was recycled for five consecutive cycles, with little loss after the first cycle, and was fully characterized to observe any changes in its structure. Mechanistic insights were provided by isotopically labeled 2-propanol-d 8 experiments, indicating FUR reduction through transfer hydrogenation. Finally, the reaction mechanism for CTH of FUR to FOL was proposed in detail using density functional theory (DFT) calculations over metal node modified model systems of a 6-connected Zr-MOF.

Direct C-H amination of arenes offers a straightforward route to aniline compounds without necessitating aryl (pseudo)halides as the starting materials. The recent development in this area, in particular in the metal-mediated transformations, is significant with regard to substrate scope and reaction conditions. Described herein are the mechanistic details on the Rh-catalyzed direct C-H amination reaction using organic azides as the amino source. The most important two stages were investigated especially in detail: (i) the formation of metal nitrenoid species and its subsequent insertion into a rhodacycle intermediate, and (ii) the regeneration of catalyst with concomitant release of products. It was revealed that a stepwise pathway involving a key Rh(V)-nitrenoid species that subsequently undergoes amido insertion is favored over a concerted C-N bond formation pathway. DFT calculations and kinetic studies suggest that the rate-limiting step in the current C-H amination reaction is more closely related to the formation of Rh-nitrenoid intermediate rather than the presupposed C-H activation process. The present study provides mechanistic details of the direct C-H amination reaction, which bears both aspects of the inner- and outer-sphere paths within a catalytic cycle.

Reported herein is the development of the Ir(III)-catalyzed direct C-H amidation of arenes and alkenes using acyl azides as the nitrogen source. This procedure utilizes an in situ generated cationic half-sandwich iridium complex as a catalyst. The reaction takes place under very mild conditions, and a broad range of sp(2) C-H bonds of chelate group-containing arenes and olefins are smoothly amidated with acyl azides without the intervention of the Curtius rearrangement. Significantly, a wide range of reactants of aryl-, aliphatic-, and olefinic acyl azides were all efficiently amidated with high functional group tolerance. Using the developed approach, Z-enamides were readily accessed with a complete control of regio- and stereoselectivity. The developed direct amidation proceeds in the absence of external oxidants and releases molecular nitrogen as a single byproduct, thus offering an environmentally benign process with wide potential applications in organic synthesis and medicinal chemistry.