Rapid Microwave-Assisted Synthesis of γ-Phosphonic Acid Quaternary Ammonium Antimicrobials for Biomedical Applications

Novel water soluble γ-phosphonates and phosphonic acid quaternary ammonium compounds (QAC) with different alkyl chain lengths, a dansyl (DNS) fluorescent tag ([RN+Me2(CH2)3P(O)(OR1)2]Br-; R = CH3, C18H37, -DNS-NH- (CH2)3; R1 = Et, iPr, or H) or pyridinium ([C5H5N+(CH2)3P(O)(OR1)]Br-; R1 = iPr, or H) were synthesized in two or three steps (Arbuzov, Menshutkin and bis-dealkylation reactions) under MW conditions within minutes in good yields. These protocols were then compared to thermal heating (TH). Both H2O and MeCN were found to be effective solvents for the Menshutkin and the bis-dealkylation of phosphonate QAC. These reactions were carried out for the first time under MW heating sequentially or in a one-vessel procedure with mineral acids. New compounds were characterized by NMR spectroscopy, HRMS spectrometry and in two instances (17, 20) by X-ray crystallography. We provide X-ray crystallographic evidence for the isolation of the QAC phosphonic acids as the free acids containing the Br- counter ion instead of the previously suggested phosphobetaine internal salts. Self-assembled monolayers of the phosphonic acid QAC prepared in this study are expected to be suitable antimicrobial coatings for metal surfaces used for biomedical applications.