Tunable Triazole‐Phosphine‐Copper Catalysts for the Synthesis of 2‐Aryl‐1H‐benzo[d]imidazoles from Benzyl Alcohols and Diamines by Acceptorless Dehydrogenation and Borrowing Hydrogen Reactions

Abstract

Triazole‐phosphine‐copper complexes (TAP−Cu) have been synthesized and applied as tunable and efficient catalysts for the selective synthesis of fluoro‐substituted 2‐aryl‐1H‐benzo[d]imidazole and 1‐benzyl‐2‐aryl‐1H‐benzo[d]imidazole derivatives from simple alcohols in only one step. TAP−Cu exhibited excellent and tunable catalytic activity for both dehydrogenation and borrowing hydrogen reactions with more than 80 examples being demonstrated for the first time. It was observed that the ligand played a critical role in catalyst activity. Mechanistic studies and deuterium labeling experiments indicated that the reactions proceeded by an initial and reversible alcohol dehydrogenation resulting in a copper hydride intermediate. This was also supported by the direct observation of a diagnostic copper hydride signal by solid‐state infrared spectroscopy. The TAP−Cu‐H complex showed absorptions at 912 cm−1 that could be assigned to copper−hydride stretches. Furthermore, the direct trapping of an intermediate bisimine was also successfully performed.magnified image

Recent advances in homogeneous borrowing hydrogen catalysis using earth-abundant first row transition metals

“…as base (Scheme 28). 50 A deuterium labelling experiment involving d 7benzyl alcohol implicated a copper hydride species as a catalytic intermediate, which was tentatively supported by solid state infrared spectra (absorptions at 912 cm −1 ). Based on their mechanistic studies, the authors proposed a catalytic cycle involving initial base-assisted coordination of the alcohol to 16.…”

Enhanced Catalytic Properties of Molybdenum Promoted Mesoporous Cobalt Oxide: Structure‐Surface‐Dependent Activity for Selective Synthesis of 2‐Substituted Benzimidazoles

“…This method is preferred in terms of applications as a wide range of 2‐substituted benzimidazole derivatives could be obtained due to the availability of a high variety of aldehydes . However, these reactions require either stoichiometric amounts of hydrogen acceptors or strong bases in order to achieve efficient and selective benzimidazole yields, which has been an issue of high interest …”

Iron-Catalyzed Acceptorless Dehydrogenative Coupling of Alcohols With Aromatic Diamines: Selective Synthesis of 1,2-Disubstituted Benzimidazoles

“…The replacement of noble-metal catalysts by inexpensive and environmentally friendly earth-abundant base metals is an important task for organic chemists. Among the base metals, Cu- (Xu et al, 2017(Xu et al, , 2018, Co- (Daw et al, 2017), Ni-SCHEME 1 | Diverse synthetic strategies for 1,2-disubstituted benzimidazoles. SCHEME 2 | Transition metal catalyzed 1,2-disubstituted benzimidazole synthesis via dehydrogenative coupling of alcohol.…”