A General Approach to Substituted Benzimidazoles and Benzoxazoles via Heterogeneous Palladium‐Catalyzed Hydrogen‐Transfer with Primary Amines

Iodine(III)-Catalyzed Oxidative Cyclization of Aryl Amines to Construct N-Alkylbenzimidazoles

An I(III)-catalyzed oxidative cyclization reaction using selectfluor as the oxidant was developed that converts ortho-substituted anilines to benzimidazoles. The mild reaction requires as little as 0.5 mol % of iodobenzene, and its scope is broad: electron-withdrawing or electron-releasing groups on the aniline portion are tolerated, and cyclic or acyclic N-alkylamines are permitted as ortho-substituents. Preliminary mechanistic investigations suggest that benzimidazole formation occurs via cationic reactive intermediates, and an intramolecular kinetic isotope effect of 1.98 ± 0.05 was measured.

Activation of Ethanol via Conjunction of a Photocatalyst and a HAT Reagent for the Synthesis of Benzimidazoles

The transformation of ethanol to value-added chemicals has tremendous potential. However, generally, harsh reaction conditions are needed for the functionalization of ethanol due to its high dehydrogenation energy. Herein, a metal-free photo-mediated activation of challenging ethanol and higher aliphatic alcohols for the synthesis of differently functionalized benzimidazoles under mild conditions is disclosed. The interplay of a photocatalyst and a HAT reagent facilitated the activation of aliphatic alcohols. A wide array of diamines with different functional groups were well tolerated, and the protocol was also extended to N-substituted diamines for the synthesis of industrially important benzimidazoles. A probable catalytic cycle was proposed based on various mechanistic studies.

An Intramolecular Hydroaminomethylation-Based Approach to Pyrrolizidine Alkaloids under Microwave-Assisted Heating

A general method for the synthesis of pyrrolizidine derivatives using an intramolecular hydroaminomethylation protocol (HAM) under microwave (MW) dielectric heating is reported. Starting from a 3,4-bis(benzyloxy)-2-[(benzyloxy)methyl]-5-vinylpyrrolidine, MW-assisted intramolecular HAM in the presence of gaseous H2 and CO gave the natural alkaloid hyacinthacine A2 protected as benzyl ether. The same approach gave a lentiginosine analogue starting from the corresponding vinyl N-hydroxypyrrolidine. The nature of the reaction products and the yields were strongly influenced by the relative stereochemistry of the starting pyrrolidines, as well as by the catalyst/ligand employed. The use of ethanol as a solvent provides environmentally friendly conditions, while the ligand/catalyst system can be recovered by separating the alkaloid product with an SCX column and recycling the ethanolic solution. HAM worked up to three times with the recycled catalyst solution without any significant impact on yield.

Avoiding hot-spots in Microwave-assisted Pd/C catalysed reactions by using the biomass derived solvent γ-Valerolactone

Herein, we report the use of γ-valerolactone as a new biomass-derived reaction medium for microwave assisted organic synthesis. The interaction of this solvent with microwaves and its heating profile under microwave irradiation has been fully characterized for the first time, demonstrating its stability and the applicability in microwave assisted Pd/C catalysed reactions avoiding the arcing phenomena frequently observed in these conditions. The use of γ-valerolactone demonstrated to be compatible with aliphatic and aromatic amines in the hydrogen transfer Pd/C mediated synthesis of benzimidazoles.

Catalytic transfer hydrogenation of azobenzene by low-valent nickel complexes: a route to 1,2-disubstituted benzimidazoles and 2,4,5-trisubstituted imidazolines

The one-pot synthesis of 1,2-disubstituted benzimidazoles by the transfer hydrogenation of azobenzene, using benzylamine as a hydrogen donor is reported, catalyzed by 2 mol% of [Ni(COD)₂] : dippe.

A bioinspired catalytic aerobic oxidative C-H functionalization of primary aliphatic amines: synthesis of 1,2-disubstituted benzimidazoles

Aerobic oxidative CH functionalization of primary aliphatic amines has been accomplished with a biomimetic cooperative catalytic system to furnish 1,2-disubstituted benzimidazoles that play an important role as drug discovery targets. This one-pot atom-economical multistep process, which proceeds under mild conditions, with ambient air and equimolar amounts of each coupling partner, constitutes a convenient environmentally friendly strategy to functionalize non-activated aliphatic amines that remain challenging substrates for non-enzymatic catalytic aerobic systems.

Domino Hydrogenation-Reductive Amination of Phenols, a Simple Process To Access Substituted Cyclohexylamines

Phenols can be efficiently reduced by sodium formate and Pd/C as the catalyst in water and in the presence of amines to give the corresponding cyclohexylamines. This reaction works at rt for 12 h or at 60 °C under microwave dielectric heating for 20 min. With the exception of aniline, primary, secondary amines, amino alcohols, and even amino acids can be used as nucleophiles. The reductive process is based on a sustainable hydrogen source and a catalyst that can be efficiently recovered and reused. The protocol was developed into a continuous-flow production of cyclohexylamines in gram scale achieving very efficient preliminary results (TON 32.7 and TOF 5.45 h(-1)).

Iron(ii) bromide-catalyzed oxidative coupling of benzylamines with ortho-substituted anilines: synthesis of 1,3-benzazoles

An iron(ii) bromide-catalyzed oxidative coupling of benzylamines with 2-amino/hydroxy/mercapto-anilines has been developed, allowing the synthesis of a diversity of substituted 1,3-benzazoles in good to excellent yields.

Synthesis of α,β-unsaturated aldehydes based on a one-pot phase-switch dehydrogenative cross-coupling of primary alcohols

An efficient one-pot ruthenium-catalyzed hydrogen-transfer strategy for a direct access to α,β-unsaturated aldehydes has been developed. The employment of enolates prepared in situ from alcohols avoided handling unstable aldehydes and provided a very appealing route to different cinnamaldehydes substituted in position 2. A silica-grafted amine was used as phase-switch tag leading to a selective one-pot process in favor of cross-dehydrogenative coupling products.

Metal-free aerobic oxidative C–N bond cleavage of tertiary amines for the synthesis of N-heterocycles with high atom efficiency

An efficient metal-free aerobic C–N bond cleavage of tertiary amines has been developed to construct N-heterocycles using molecular oxygen as the sole oxidant with high atom efficiency. All of the three alkyl groups in tertiary amines can be utilized and transformed into N-heterocycles.