ABSTRACT
Monoalkyl phosphonic derivatives obtained by the microwave (MW)- and ionic liquid-promoted direct esterification of alkylphosphonic acids were converted to the corresponding dialkyl alkylphosphonates on reaction with alkyl halides in the presence of triethylamine, under solvent-free MW-assisted conditions. Derivatives with different alkoxy groups were also synthesized. A minor "disproportionation" side reaction was identified during the preparation of dialkyl alkylphosphonates with different alkoxy groups. All together 12 alkylphosphonates were prepared by the efficient method developed.
ABSTRACT
The microwave (MW)-assisted direct esterification of certain P-acids is a green method. Quantum chemical calculations revealed that the activation enthalpy (ΔH#) for the exothermic monoalkylphosphate â dialkylphosphate transformation was on the average 156.6 kJ mol-1, while ΔH# for the dialkylphosphate â trialkylphosphate conversion was somewhat higher, 171.2 kJ mol-1, and the energetics of the elemental steps of this esterification was less favorable. The direct monoesterification may be performed on MW irradiation in the presence of a suitable ionic liquid additive. However, the second step, with the less favorable energetics as a whole, could not be promoted by MWs. Hence, dialkylphosphates had to be converted to triesters by another method that was alkylation. In this way, it was also possible to synthesize triesters with different alkyl groups. Eventually a green, P-chloride free MW-promoted two-step method was elaborated for the synthesis of phosphate triesters.
Subject(s)
Ionic Liquids , Acids , Esterification , Microwaves , ThermodynamicsABSTRACT
It is well-known that the P-acids including phosphonic acids resist undergoing direct esterification. However, it was found that a series of alkylphoshonic acids could be involved in monoesterification with C2-C4 alcohols under microwave (MW) irradiation in the presence of [bmim][BF4] as an additive. The selectivity amounted to 80-98%, while the isolated yields fell in the range of 61-79%. The method developed is a green method for P-acid esterification. DFT calculations at the M062X/6-311+G (d,p) level of theory (performed considering the solvent effect of the corresponding alcohol) explored the three-step mechanism, and justified a higher enthalpy of activation (160.6-194.1 kJ·mol-1) that may be overcome only by MW irradiation. The major role of the [bmim][BF4] additive is to increase the absorption of MW energy. The specific chemical role of the [BF4] anion of the ionic liquid in an alternative mechanism was also raised by the computations.
ABSTRACT
Phosphinic and phosphonic acids are useful intermediates and biologically active compounds which may be prepared from their esters, phosphinates and phosphonates, respectively, by hydrolysis or dealkylation. The hydrolysis may take place both under acidic and basic conditions, but the C-O bond may also be cleaved by trimethylsilyl halides. The hydrolysis of P-esters is a challenging task because, in most cases, the optimized reaction conditions have not yet been explored. Despite the importance of the hydrolysis of P-esters, this field has not yet been fully surveyed. In order to fill this gap, examples of acidic and alkaline hydrolysis, as well as the dealkylation of phosphinates and phosphonates, are summarized in this review.
ABSTRACT
The two-step acidic hydrolysis of α-hydroxybenzylphosphonates and a few related derivatives was monitored in order to determine the kinetics and to map the reactivity of the differently substituted phosphonates in hydrolysis. Electron-withdrawing substituents increased the rate, while electron-releasing ones slowed down the reaction. Both hydrolysis steps were characterized by pseudo-first-order rate constants. The fission of the second P-O-C bond was found to be the rate-determining step.
