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.
It was proved by our experiments that on microwave irradiation, the mono‐ or bidentate phosphorus ligands generally applied in the palladium(II)‐catalyzed P–C coupling reaction of aryl bromides and dialkyl phosphites or secondary phosphine oxides may be substituted by the excess of the >P(O)H reagent that exists under a tautomeric equilibrium. Taking into account that the reduction of the palladium(II) salt and the ligation of the palladium(0) so formed requires 3 equivalents of the P‐species for the catalyst applied in a quantity of 5–10%, all together, 15–30% of the P‐reagent is necessary beyond its stoichiometric quantity. In the coupling reaction of diphenylphosphine oxide, it was possible to apply diethyl phosphite as the reducing agent and as the P‐ligand. The reactivities of the diethyl phosphite and diphenylphosphine oxide reagents were compared in a competitive reaction. The mechanism and the energetics of this new variation of the Hirao reaction of bromobenzene with Y2P(O)H reagents (Y=EtO and Ph) was explored by quantum chemical calculations. The first detailed study on simple reaction models justified our assumption that, under the conditions of the reaction, the trivalent form of the >P(O)H reagent may serve as the P‐ligand in the palladium(0) catalyst, and shed light on the fine mechanism of the reaction sequence. The existence of the earlier described bis(palladium complex) {[H(OPh2P)2PdOAc]2} was refuted by high level theoretical calculations. This kind of complex may be formed only with chloride anions instead of the acetate anion. The interaction of palladium acetate and Y2P(O)H may result in only the formation of the [(HO)Y2P]2Pd complex that is the active catalyst in the Hirao reaction. The new variation of the Hirao reaction is of a more general value, and represents the greenest protocol, as there is no need for the usual P‐ligands. Instead, the >P(O)H reagent should be used in an excess of up to 30%. Hence, the costs and environmental burdens may be decreased.
The beneficial combination of microwave (MW) and ionic liquid (IL) additives was exploited in the direct esterification of a series of acyclic phosphinic and phosphonic acids giving rise to phenyl-H-phosphinates/methyl-phenylphosphinates/diphenylphosphinates and phenylphosphonic mono- and diesters, respectively. The latter is the first example for the direct esterification of a phosphonic acid.
It is known that phosphinic acids do not undergo direct esterification under conventional conditions. However, the reaction may take place on microwave irradiation. 1‐Hydroxy‐3‐phospholene 1‐oxides, 1‐hydroxy‐phospholane 1‐oxides, and a 1‐hydroxy‐1,2,3,4,5,6‐hexahydrophosphinine 1‐oxide were esterified with n‐pentanol, i‐pentanol, n‐octanol, and i‐octanol(2‐ethylhexanol). The phosphinates prepared in 50%–94% yield are all new compounds, and a number of them are useful intermediates.
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