An improved and greener protocol for the synthesis of benzimidazole derivatives, starting from o-phenylenediamine, with different aldehydes is reported. Double-condensation products were selectively obtained when Er(OTf)₃ was used as the catalyst in the presence of electron-rich aldehydes. Conversely, the formation of mono-condensation products was the preferred path in absence of this catalyst. One of the major advantages of these reactions was the formation of a single product, avoiding extensive isolation and purification of products, which is frequently associated with these reactions.Theoretical calculations helped to understand the different reactivity established for these reactions. Thus, we found that the charge density on the oxygen of the carbonyl group has a significant impact on the reaction pathway. For instance, electron-rich aldehydes better coordinate to the catalyst, which favours the addition of the amine group to the carbonyl group, therefore facilitating the formation of double-condensation products.Reactions with aliphatic or aromatic aldehydes were possible, without using organic solvents and in a one-pot procedure with short reaction time (2–5 min), affording single products in excellent yields (75–99%). This convenient and eco-friendly methodology offers numerous benefits with respect to other protocols reported for similar compounds.

A series of benzyl-substituted thiobenzoazoles were synthesized by an environmentally friendly approach, to search for new antifungal agrochemicals. Compounds were prepared starting from 2-mercaptobenzoazoles, using KOH, benzyl halides, and water, resulting in a simple and ecological method. New antifungals were tested against a group of phytopathogenic fungi. Two compounds showed an interesting activity against Botrytis cinerea, Fusarium oxysporum, and Aspergillus spp.: 2-((4-(trifluoromethyl)benzyl)thio)benzo[d]thiazole, 3ac, and 2-((4-methylbenzyl)thio)benzo[d]thiazole, 3al. Thus, 3ac and 3al can be considered as broad spectrum antifungal agents. Furthermore, two new compounds, 2-((4-iodobenzyl)thio)benzo[d]thiazole, 3aj, and 2-(benzylthio)benzo[d]oxazole, 3ba, showed better inhibitory effect against Botrytis cinerea and Fusarium oxysporum when compared to the commercial fungicide Captan. Thus, 3aj and 3ba can be considered reduced-spectrum antifungals.