Carbonyl and thiocarbonyl compounds XX. Reaction of hydroxybenzaldehydes with <i>o</i>‐phenylenediamine; newer aspects in benzimidazole synthesis

Abstract

Abstract The nature of the products of the uncatalysed reaction of the o ‐hydroxybenzaldehydes 1a‐d with o ‐phenylenediamine in boiling ethanol is mainly dependent on the molar ratio of the reactants and the nature of the substituents at the phenyl ring of the aldehyde. In contrast, the m ‐ and p ‐hydroxy analogues 1e‐g afford exclusively the corresponding 1‐aralkyl‐2‐arylbenzimidazoles 6 , regardless of these factors. However, at low temperature, the hydroxybenzaldehydes 1a‐g react with the diamine always giving monoanils ( 2 ) regardless of the hydroxy position. The monoanils of the o ‐hydroxy aldehydes afford predominantly 2‐arylbenzimidazoles ( 4 ) when boiled in n ‐butanol. In contrast, those of the m ‐ and p ‐hydroxy analogues give exclusively 1‐aralkyl‐2‐arylbenzimidazoles ( 6 ), whereas, when heated in nitrobenzene, 2‐arylbenzimidazoles ( 4 ) are readily obtained. The dianils 5a‐d , upon boiling in butanol, give a mixture of 2‐arylbenzimidazoles ( 4 ) and 1‐aralkyl‐2‐arylbenzimidazoles ( 6 ), the ratio of which is dependent on the nature of the substituents at the aldehyde. Reaction mechanisms and spectral features of the products are discussed.