Photoinduced decarboxylative radical cascade alkylation/cyclization of benzimidazole derivatives with aliphatic carboxylic acid via ligand-to-iron charge transfer
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Abstract:
The photoinduced decarboxylative radical cascade alkylation/cyclization method with various benzimidazole moieties bearing unactivated alkenes efficiently synthesizes polycyclic benzimidazoles via LMCT processes.Keywords:
Benzimidazole
Decarboxylation
Radical cyclization
Abstract For the reaction mechanism of the ketonic decarboxylation of two carboxylic acids, a β‐keto acid is favored as key intermediate in many experimental and theoretical studies. Hydrogen atoms in the α‐position are an indispensable requirement for the substrates to react by following this mechanism. However, isolated observations with tertiary carboxylic acids are not consistent with it and these are revisited and discussed herein. The experimental results obtained with pivalic acid indicate that the ketonic decarboxylation does not occur with this substrate. Instead, it is consumed in alternative reactions such as disintegration into isobutene, carbon monoxide, and water (retro‐Koch reaction). In addition, the carboxylic acid is isomerized or loses carbon atoms, which converts the tertiary carboxylic acid into carboxylic acids bearing α‐proton atoms. Hence, the latter are suitable to react through the β‐keto acid pathway. A second substrate, 2,2,5,5‐tetramethyladipic acid, reacted by following the same retro‐Koch pathway. The primary product was the monocarboxylic acid 2,2,5‐trimethyl‐4‐hexenoic acid (and its double bond isomer), which might be further transformed into a cyclic enone or a lactone. The ketonic decarboxylation product, 2,2,5,5‐tetramethylcyclopentanone was observed in traces (<0.2 % yield). Therefore, it can be concluded that the observed experimental results further support the proposed mechanism for the ketonic decarboxylation via the β‐keto acid intermediate.
Decarboxylation
Pivalic acid
Cyclohexanecarboxylic acid
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1-(p-Methoxybenzyl)-2-(p-thoxyphenyl)-benzimidazole and 1-(p-hydroxybenzyl)-2-(p-hydroxyphenyl)-benzimidazole were snythesized by the condensation of o-phenylenediamine with p-methoxybenzaldehyde and p-hydroxyheozaldehyde under microwave irradiation,and characterized by melting point determination,elements analysis,IR and 1HNMR spectra.Antibacterial activities of the two benzimidazole derivatives were studied.
Benzimidazole
Condensation reaction
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The kinetic effect of substituents at C(3) of βγ-unsaturated acids is consistent with the development of a partial positive charge at that position during decarboxylation. The OMe group increases the rate of decarboxylation as much as 105–106 fold.
Decarboxylation
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Benzimidazole
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Abstract Benzimidazole reactant: 54 g. (0.5 mole) of o ‐phenylenediamine product: benzimidazole product: 2‐methyl‐benzimidazole
Benzimidazole
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A new approach to platencin, a potent antibiotic isolated from Streptomyces platensis, has been established. The highly congested tricyclic core of the natural product was successfully constructed by decarboxylative radical cyclization of an alkynyl silyl ester with Pb(OAc)4 in the presence of pyridine in refluxing 1,4-dioxane. The key decarboxylation, which likely takes place via lead(IV) esterification followed by carbon-centered radical generation and subsequent capture of the radical with a triple bond, allows the rapid construction of the twisted polycyclic system.
Decarboxylation
Radical cyclization
Natural product
Triple bond
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Decarboxylation
Carboxylate
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Novel benzimidazole and fused benzimidazole derivatives such as triazinobenzimidazoles 3a-f, oxadia-zolylthiomethyl-1H-benzimidazoles 5a-e, triazolylthioimethyl-1H-benzimidazoles 7a,b. thiazolidinyl-methyl-1H-benzimidazoles 9a-c and pyrimidopyrrolobenzimidazoles 12a,b have been synthesized via several reactions of the key intermediate 2-chloromethyl-1H-benzimidazole with various reagents. Moreover, triazinobenzimidazoles 15a-c have been prepared starting with 2-cyanomethyl-1H-benzimidazole. Finally, another series of oxadiazoles 21a,b and triazoles 23a,b linked to benzimidazole moiety at one position have been synthesized starting with 2-mercapto-1H-benzimidazole. The structures of the newly synthesized compounds have been confirmed on the basis of elemental analysis and spectral studies. Some of the newly synthesized compounds exhibit significant antimicrobial activity, the most active compound is 21a.
Benzimidazole
Moiety
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Grenda's method to convert arylamidine hydrochloride into benzimidazole with the aid of sodium hypochloride and a base has been applied to the previously untried N-arylamidines to synthesize the following : 5-chloro-2-phenylbenzimidazole (Y=96%), 5-methyl-2-phenylbenzimidazole (Y=96%), 2-benzylbenzimidazole (Y=50%), 2-benzyl-5-chlorobenzimidazole (Y=70%), 2-(2'-naphthyl)-benzimidazole (Y=75%), 5-chloro-2-(2'-naphthyl)-benzimidazole (Y=90%), 2-(p-tolyl)-benzimidazole (Y=95%), 5-chloro-2-(p-tolyl)-benzimidazole (Y=80%), 5-methyl-2-(p-tolyl)-benzimidazole (Y=80%).
Benzimidazole
Hydrochloride
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