The Synthesis and Mechanistic Considerations of a Series of Ammonium Monosubstituted H‐Phosphonate Salts. Issue 2 (19th November 2020)

• Record Type: Journal Article
• Authors: Lee, Keng Lung; Feld, Joey; Hume, Paul; Söhnel, Tilo; Leitao, Erin
• Is Part Of: Chemistry. Volume 27:Issue 2(2021)
• Page Start: 815
• Page End: 824
• Publication Date: 2020-11-19
• Subjects: amine -- dealkylation -- H-phosphonate diesters -- ionic salt -- mechanism; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202003090
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: A series of ammonium monosubstituted H ‐phosphonate salts were synthesized by combining H ‐phosphonate diesters with amines in the absence of solvent at 80 °C. Variation of the ester substituent and amine produced a range of ionic liquids with low melting points. The products and by‐products were analyzed by spectroscopic and spectrometric techniques in order to get a better mechanistic picture of the dealkylation and formal dearylation observed. For dialkyl H ‐phosphonate diesters, (RO)2 P(O)H (R=alkyl), the reaction proceeds via direct dealkylation with the reactivity increasing in the order R= i Pr<Et<Me corresponding to DFT calculated activation enthalpies of 22.6, 20.8, and 17.9 kcal mol −1 . For the diphenyl H ‐phosphonate diesters, (PhO)2 P(O)H, the dearylation was found to proceed via phenol‐assisted formation of a 5‐coordinate intermediate, (PhO)3 PH(OH), from which P(OPh)3 and water were eliminated. The presence of an equivalent of water then facilitated the formation of P(OH)2 OPh and the amine, R'NH2, subsequently abstracted a proton from it to yield [(PhO)PH(O)O] ‐ [R'NH3 ] + . Abstract : I on it ! Dialkyl and diphenyl H ‐phosphonates undergo dealkylation and dearylation, respectively, upon heating with amines to afford the corresponding ionic salts. The mechanisms for these processes, which form ammonium monosubstituted H ‐phosphonate salts held together by H‐bonds, were interrogated both experimentally and theoretically, and the results show that they are very different.