The potential and scope of the microwave tool; A case study on the synthesis of phosphinates 

The microwave (MW) technique became an important tool in organophosphorus chemistry [1]. On the one hand, otherwise reluctant reactions, such as the esterification of P-acids may be enhanced [2], on the other hand, catalyst systems may be simplified on MW irradiation.

Our model reaction was the esterification of phosphinic derivatives. The classical method is the reaction of phosphinic chlorides with alcohols (A) that is, however, not green. Alternative possibilities are the cyclic propylphosphonic acid (T3P^®)-promoted esterifications (B), the alkylating esterifications (C), and the direct esterifications (D) that take place only on MW irradiation [3].

                                               >P(O)Cl             +            ROH               -------------->              >P(O)OR            +                  HCl             (A)

                                               >P(O)OH  +   ROH    +    T3P^®              -------------->              >P(O)OR            +                 T3P-H₂O      (B)

                                               >P(O)OH  +    RX      +   K₂CO₃             -------------->              >P(O)OR   +    KX   +   CO₂    +   H₂O      (C)

                                               >P(O)OH             +           ROH               -------------->              >P(O)OR            +                  H₂O             (D)

                                               R = alkyl or aryl              X = Br or I

The last MW-assited method that is the "greenest" protocol due to the atom-efficiency is discussed in detail together with extensions.

It is also the purpose of this presentation to elucidate the scope and limitations of the MW tool in synthetic organic chemistry in general, and to interpret the special MW effects [4]. The usefulness of MW irradiation for a specific reaction may be predicted on the basis of its energetics. We were the first who modelled the distribution and effect of the local overheatings, and compared the theory with the practice [4]. All these considerations were possible on the basis of the results (eg. enthalpy of activation values) of our quantum chemical calculations, and utilizing the pseudo first order kinetic equation and the Arrhenius equation.

 References

[1] Keglevich, G.; Bálint, E.; Kiss, N. Z. in: Milestones in Microwave Chemistry – SpringerBriefs in Molecular Science, Ed.: Keglevich, G.; Springer, Switzerland, 2016, Ch. 3, pp. 47-76.

[2] Keglevich, G.; Kiss, N. Z.; Grün, A.; Bálint, E.; Kovács, T. Synthesis 2017, 49, 3069-3083.

[3] Keglevich, G.; Rádai, Z.; Kiss N. Z. Green Proc. Synth. 2017, 6, 197-201.

[4] Keglevich, G.; Kiss, N. Z.; Mucsi Z. Pure Appl. Chem. 2016, 88, 931-939.