Insights into a surprising reaction: The microwave-assisted direct esterification of phosphinic acids

Abstract

It is well-known that phosphinic acids do not undergo direct esterifications with alcohols under thermal conditions. However, the esterifications take place under microwave (MW) irradiation due to the beneficial effect of MW. As a comparison, maximum 12-15% conversions were observed on traditional heating. It was proved experimentally that the MW-assisted esterifications are not reversible under the conditions applied that may be the consequence of the hydrophobic medium established by the long chain alcohol/phosphinic ester. Neither the thermodynamic, nor the kinetic data obtained by high level quantum chemical calculations justify the direct esterification of phosphinic acids under thermal conditions. The thermodynamic data show that there is no driving force for the reactions under discussion. As a consequence of the relatively high values of activation enthalpy (102-161 kJ mol(-1)), these esterifications are controlled kinetically. Comparing the energetics of the esterification of phosphinic acids and the preparative results obtained under MW conditions, one can see the potential of the MW technique in the synthesis of phosphinates. During our study, a series of new cyclic phosphinates with lipophilic alkyl groups was synthesized.

Environmentally Friendly Chemistry with Organophosphorus Syntheses in Focus

“…The method elaborated by us is of a more general value. It was also found that the esterification of phosphinic acids is thermoneutral, and controlled kinetically [47].…”

“…A series of phosphinic acids underwent efficient esterification using alcohols with longer chain at around 200°C on MW irradiation (Scheme 7) [43,[46][47][48]. The esterification of cyclic phosphinic acids, such as 1-hydroxy-3-phospholene oxides (8a and 8b), 1-hydroxyphospholane oxides (8c and 8d) and 1-hydroxy-1,2,3,4,5,6-hexahydrophosphinine oxide (8e) was carried out in a closed vessel to afford the phosphinates (9a-e) in acceptable to excellent yields [43,[46][47][48]. The method elaborated by us is of a more general value.…”

“…Our idea is that 1 equivalent of the phosphorus trichloride is needed to convert the acid to the corresponding acid chloride (47). In MSA as the solvent, a mixed anhydride RC(O)O(O) 2 SMe (48) may also be formed from the acid chloride and a molecule of solvent, or in another way.…”

The Use of MW in Organophosphorus Chemistry

“…It was also found that the esterification of phosphinic acids is thermoneutral and controlled kinetically. The reaction enthalpies were found to fall in the range of 0.1-4.0 kJ mol −1 , while the activation enthalpies were in the range of 102.0-161.0 kJ mol −1 [25,27]. Interestingly, the analogous thioesterifications were reluctant and incomplete under MW conditions [28].…”

“…However, when ethanolamine was applied in only a 1-2 equivalents quantity, not alcoholysis, but mono-and diethylation of the ethanolamine took place. Using ethanolamine in a 4-10-fold quantity at 140°C for 20 min, different ratios of the mixed phosphinate (24) and the fully transesterified product (25) Scheme 3.14 MW-assisted alcoholysis of diethyl phosphite with ethylene glycol From the point of view of bis(aminoethyl)phosphite (25), the best experiment was, when ethanolamine was measured in a 10 equivalents quantity and the reaction was performed at 140°C for 20 min.…”