On the Keto‐Enol‐Tautomerism of 2‐(1‐Hydroxyethylidene)malononitrile and Its Lewis Adduct Formation with B(C₆F₅)₃

Abstract

2‐(1‐Hydroxyethylidene)malononitrile (4) is commercially sold as the keto isomer and is described in the literature as either a keto (4b) or enol (4a) species. Using NMR techniques and SCXRD, we were able to show that 4 exists exclusively as the enol isomer (4a) in solution as well as in the solid state, which agrees with quantum chemical calculations. By adding one or two equivalents of the Lewis acid B(C6F5)3, the mono‐ or diadducts are formed in exergonic reactions, in which the enol form is further thermodynamically stabilized. The structure and acidity of enol 4a and its borane adducts are discussed based on experimental and theoretical results.