N-Heterocycles, such as pyrroles, pyrimidines, quinazolines, and quinoxalines, are important building blocks for organic chemistry and the fine-chemical industry. For their synthesis, catalytic borrowing hydrogen and acceptorless dehydrogenative coupling reactions of alcohols as sustainable reagents have received significant attention in recent years. To overcome the problems of product separation and catalyst reusability, several metal-based heterogeneous catalysts have been reported to achieve these transformations with good yields and selectivity. In this Minireview, we summarize recent developments using both noble and non-noble metal-based heterogeneous catalysts to synthesize N-heterocycles from alcohols and N-nucleophiles via acceptorless dehydrogenation or borrowing hydrogen methodologies. Furthermore, this Minireview introduces strategies for the preparation and functionalization of the corresponding heterogeneous catalysts, discusses the reaction mechanisms and the roles of metal electronic states, and the influence of support Lewis acid-base properties on these reactions.

Synthesis of N-Heterocycles via Oxidant-Free Dehydrocyclization of Alcohols Using Heterogeneous Catalysts.

Recent Progress in the Synthesis of Heterocycles through Base Metal-Catalyzed Acceptorless Dehydrogenative and Borrowing Hydrogen Approach

We present a convenient and efficient protocol to synthesize quinolines and quinazolines in one pot under mild conditions. A variety of substituted quinolines were synthesized in good to excellent yields (up to 97% yield) from the dehydrogenative cyclizations of 2-aminoaryl alcohols and ketones catalyzed by readily available Co(OAc)2·4H2O. This cobalt catalytic system also showed high activity in the reactions of 2-aminobenzyl alcohols with nitriles, affording various quinazoline derivatives (up to 95% yield). The present protocol offers an environmentally benign approach for the synthesis of N-heterocycles by employing an earth-abundant cobalt salt under ligand-free conditions.

Dehydrogenative Synthesis of Quinolines and Quinazolines via Ligand-Free Cobalt-Catalyzed Cyclization of 2-Aminoaryl Alcohols with Ketones or Nitriles.

Finite size effect is closely correlated with heterogeneous catalytic activity and selectivity as reducing the metal to a few nanometers, but may suffer from deficient active sites for cascade reactions. Here, a type of dual-size heterogeneous N-doped cobalt catalysts (CoSAs-NPs/NC(n)) with the coexistence of Co single-atoms (Co-SAs) and Co nanoparticles (Co-NPs) were prepared for efficient synthesis of benzimidazoles (up to 91% yield) from 2-nitroanilines and bio-based alcohols via cascade hydrogen transfer-dehydrogenative cyclization. Experimental results and density functional theory calculations suggested that the bio-alcohol dehydrogenation mainly occurs at Co-SAs sites, while Co-NPs sites remarkably facilitate the subsequent reduction of nitro species, indicating the synergistic effect between Co-SAs and Co-NPs in the coupled hydrogen transfer process. The CoSAs-NPs/NC(0.25) catalyst can be recycled and applied to a wide range of substrates (28 examples). This catalytic approach based on the metal size effect opens a new route for the direct upgrading of biomass feedstocks involving multiple reactions. 

Synergistic effect between Co single atoms and nanoparticles enables selective synthesis of bio-based benzimidazoles

Visible-Light- and PPh3-Mediated Direct C-N Coupling of Nitroarenes and Boronic Acids at Ambient Temperature.

A straightforward and selective reduction of nitroarenes with various alcohols was efficiently developed using an iron catalyst via a hydrogen transfer methodology. This protocol led specifically t...

Iron-Catalyzed Hydrogen Transfer Reduction of Nitroarenes with Alcohols: Synthesis of Imines and Aza Heterocycles.

Starting from nitroarenes, under hydrosilylation conditions, using a well-defined N-heterocyclic carbene iron(0) catalyst, (IMes)Fe(CO)(4), the corresponding aniline derivatives were produced in 61-92% isolated yields. More impressively, a selective synthesis of cyclic amines such as pyrrolidines, piperidines and azepanes were conducted from levulinic acid, 1,5- and 1,6-keto acids, respectively. The sequential procedure proceeded under both visible light irradiation and thermal conditions with 20 examples in isolated yields up to 69%.

A Concise Route to Cyclic Amines from Nitroarenes and Ketoacids under Iron-Catalyzed Hydrosilylation Conditions

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Iron-Catalyzed Hydrogen Transfer Reduction of Nitroarenes with Alcohols: Synthesis of Imines and Aza Heterocycles.

A Concise Route to Cyclic Amines from Nitroarenes and Ketoacids under Iron-Catalyzed Hydrosilylation Conditions