A novel amphipathic low-melting complex salt: An efficient homogeneous catalyst for synthesis of pyran-annulated heterocyclic scaffolds and pyrido[2,3-d]pyrimidines

Abstract

Abstract The reaction of triethyl(sulfonyl)ammonium chlorosulfonate with 1,1,3,3-tetramethylguanidine (TMG) gave a low-melting complex salt with amphipathic and acidic characters. The salt has a great tendency towards solvation of organic substrates upon slight heating, so it was employed as a homogeneous acidic and functional ionic liquid catalyst in the expedient synthesis of benzo[b]pyrans, pyrano[4,3-b]pyrans, pyrano[3,2-c]quinolones, and pyrido[2,3-d]pyrimidines. The catalytic activity of the complex salt is much better than the simple salts composing it. Spectroscopic studies and chemical analysis of the salt revealed that its empirical formula is in agreement with neutralization of TMG by two H2SO4 molecules. The HSO4− anions of the salt seem to form extensive H-bonding with the constituting ammonium cations and Cl−, resulting in melting point depression of the complex salt due to the melting points of pure triethylamine and TMG hydrochloride salts. Another significant feature of this solid salt is that it can preserve its composition even after several times of separation from aqueous extracts and can be handled easily without danger.