Abstract
A reaction of an acid chloride with a diamine yielded a diamide. m-Toluic acid was chlorinated to m-toluoyl chloride and subsequently reacted with 4-methyl-o-phenylenediamine in pyridine to obtain 3-methyl-N-[2-(3-methylbenzamido)phenylbenzamide (I). 2-(3-Methylphenyl)-1H-benzimidazole (II) has been obtained upon reacting o-phenylenediamine with m-toluic acid in polyphosphoric acid and toluene. The compounds have been characterized by IR, NMR, microanalyses and GC-MS. The crystal structures of the compounds have been discussed. DFT calculations of the frontier orbitals of the precursor compounds have been carried out to ascertain the groups that contribute to the HOMO and LUMO, and to study their contribution to the reactivity in the formation of the diamides and benzimidazoles. The synthesis of the amide from a diamine was seen to be favoured in the presence of a good leaving group attached to the carbonyl as in the case of acid chloride. However, the synthesis of benzimidazoles was found to be favoured in the presence of an excess of a protonating agent and high temperature.
Highlights
There are several reported means to access benzimidazoles; v.i.z. condensation of o-phenylenediamine with carboxylic acids in the presence of ring closing agents such as polyphosphoric acid and hydrochloric acid,[1] zinc acetate catalysis at room temperature using o-phenylenediamine and aldehydes,[2] and copper(I) catalyzed intramolecular arylation of formamidines to form 2-unsubstituted N-substituted benzimidazoles.[3]
Extension to other catalysts have been reported, with catalytic amounts of indium trioxide being used in the conversion of carboxylic acid esters to primary amides,[9] and Han et al,[10] have reported a process for preparation of amides from esters and amines using a catalytic system comprised of Group IV metal alkoxides in conjunction with additives including 1-hydroxy-7-azabenzotriazole (HOAt)
Analytical grade reagents and solvents for synthesis and analysis which included m-toluic acid and o-phenylenediamine were obtained from Sigma Aldrich (USA) whilst pyridine, thionyl chloride, hexane, methanol, ethyl acetate and dimethyl sulfoxide were obtained from Merck Chemicals (SA)
Summary
There are several reported means to access benzimidazoles; v.i.z. condensation of o-phenylenediamine with carboxylic acids in the presence of ring closing agents such as polyphosphoric acid and hydrochloric acid,[1] zinc acetate catalysis at room temperature using o-phenylenediamine and aldehydes,[2] and copper(I) catalyzed intramolecular arylation of formamidines to form 2-unsubstituted N-substituted benzimidazoles.[3]. Extension to other catalysts have been reported, with catalytic amounts of indium trioxide being used in the conversion of carboxylic acid esters to primary amides,[9] and Han et al,[10] have reported a process for preparation of amides from esters and amines using a catalytic system comprised of Group IV metal alkoxides in conjunction with additives including 1-hydroxy-7-azabenzotriazole (HOAt). In this case, the ester-amide exchange proceeds using a variety of structurally diverse esters and amines without azeotropic reflux to remove the alcohol by-product. The lack of reactivity of 2-(3-methylphenyl)-1H-benzimidazole towards oxidation reactions has been explained and contrasted with compounds that undergo oxidation of their methyl groups
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