Formal transfers of hydride from carbon–hydrogen bonds. Attempted generation of H₂ by intramolecular protonolyses of the activated carbon–hydrogen bonds of dihydrobenzimidazoles

Abstract

Protonolyses of carbon-hydrogen bonds can occur under suitable conditions to produce carbocations and H,. In an effort to accelerate these fundamental reactions, we have attempted to make them intramolecular by devising compounds in which carbon-hydrogen bonds designed to be particularly good formal donors of hydride are held in close proximity to acidic sites. Dihydrobenzimidazoles 4 and 11 are compounds of this type, since the carbon-hydrogen bonds at C2 are activated as formal donors of hydride by adjacent lone pairs in a dihydroaromatic ring, and acidic anilinium and carboxylic acid groups are held nearby. Unfortunately, this proximity does not lead to the formation of Hz by protonolysis; instead, other reactions intervene when compounds 4 and 11 are subjected to pyrolysis.Key words: intramolecular protonolysis of carbon-hydrogen bonds, formal donors of hydride, generation of H,, dihydrobenzimidazoles.RksumC : La protonolyse des liaisons carbone-hydrogkne peuvent se produire dans des conditions approprikes pour conduire i des carbocations et du H,. Dans le but d'accC1krer les vitesses de ces rkactions fondamentales, on a essay6 de faire des rCactions intramolkculaires en construisant des composts dans lesquels des liaisons carbone-hydrogkne comportant des donneurs formels d'hydrures particulikrement bons seront maintenues faible distance des sites acides. Les dihydrobenzimidazoles 4 et 11 sont des composCs de ce type puisque les liaisons carbone-hydrogkne en C(2) sont activCes comme donneurs formels d'hydrure par les paires non partagkes adjacentes du noyau dihydroaromatique alors que les groupes acides anilinium et acide carboxylique sont maintenus B proximitk. Malgrk cette proximitk, la protonolyse ne conduit malheureusement pas h la formation de H,; quand les composks 4 et 11 sont soumis a une pyrolyse, il y a plutBt intervention d'autres reactions.Mots c l b : protonolyse intramolCculaire de liaisons carbone-hydrogkne, donneurs formels d'hydrure, gknkration de H,, dihydrobenzimidazoles.[Traduit par la rCdaction]

A novel paradigm for metal-induced ring flipping in the copper(II) complex of 1,2-bis( N -methylbenzimidazol-2 ′ -yl)benzene triflate

“…The ligand bmbz was obtained as a byproduct and in low yield in an earlier attempt to synthesize the compound [15]. We have prepared bmbz with increased yield and in one step by the adaptation of the published procedures for N -methylbenzimidazoles and 2-arylbenzimidazoles [16,17] Then, the copper complex was prepared by slowly adding a solution of copper(II) triflate (1.2 mmol) in methanol to a stirred solution of the ligand (0.41 g, 1.2 mmol) in methanol.…”

Hydrogen evolution from organic “hydrides”

“…Following the literature report we prepared the N,N9-dimethyl dihydrobenzimidazole compound 1 15 and confirmed it to be quite stable both in solution and in the solid-state at temperatures up to 80 uC, in the absence of air and catalyst. However, when treated with 3 mol% Pearlman's catalyst (Pd(OH) 2 on carbon) in acetonitrile at 80 uC, 1 lost H 2 16 with concomitant generation of…”

“…Stoichiometric, non-oxidatively-driven hydrogen evolution from dihydrobenzimidazole compounds has not been reported, although one research group made several deliberate and determined attempts to force hydrogen evolution from dihydrobenzimidazoles by reaction with intramolecular proton sources. 15 Because hydrogen was not found among the reaction products, the authors reluctantly came to the conclusion that dihydrobenzimidazoles are thermodynamically incompetent to react with acid to evolve hydrogen, despite being specially constructed so as to facilitate intramolecular hydrogen elimination. 15 Upon considering this system we suspected that the failure to eliminate hydrogen from dihydrobenzimidazole-acid combinations arises from an intrinsic kinetic barrier, not thermodynamic incompetence, and by studying the effects of catalysts we have demonstrated the exothermic and exergonic evolution of hydrogen at ambient temperature.…”

Zinc Complexes with Nitrogen Donor Ligands as Anticancer Agents

“…Bidentate benzimidazole ligands L 30a ,b [ 168 , 169 , 170 ] allowed the formation of structurally characterized 30a and 30b ( Figure 17 ), in which each Zn 2+ core is tetrahedrally coordinated to two N and two Cl atoms. The cytotoxicity against MB-MDA-231 cells of Zn complexes, free ligands and cisplatin was investigated by CCK-8 assay ( Table 9 ).…”