Synthesis of imines from the coupling reaction of alcohols and amines catalyzed by phosphine-free cobalt(ii) complexes

Abstract

Phosphine-free, air stable cobalt(II) based complexes (1a and 1b) consisting of ligands L¹H₂ and L²H₂ (L¹H₂ = N,N′-((1,2-phenylenebis(azaneylylidene))bis(methaneylylidene))diphenol and L²H₂ = N,N′-bis(4-diethylaminosalicylidene)-4,5-dichloro-1,2-phenylenediamine) were synthesized and utilized as catalysts in the coupling reaction of alcohols with amines into imines following an acceptorless dehydrogenative pathway. The reactions were carried out in the presence of t-BuOK base with low catalyst loading (1 mol%) in an open atmosphere. The corresponding imines were isolated in moderate to excellent yields. The methodology was screened with different substituted alcohols and amines. The proposed mechanistic pathway of this reaction was ascertained through intermediate mass and ¹H NMR analyses. Most of the previously reported 3d transition metal catalysts used in imine synthesis reactions have a phosphine ligand environment, and the reactions were performed under inert conditions. Herein we have developed a sustainable route for the synthesis of imines from the coupling reaction of alcohols with amines under aerial reaction conditions using phosphine-free air stable cobalt catalysts.