Synthesis of α-aminophosphonates and related derivatives under microwave conditions

α-Aminophosphonates and related derivatives represent one of the most important class of organophosphorus compounds. The most widely applied synthetic routes towards α-aminophosphonates involve the Kabachnik-Fields condensation and the Pudovik reaction. In this paper the microwave (MW)-assisted catalystand mostly solvent-free syntheses of various α-aminophosphonates and α-aminophosphine oxides are discussed. Bis(aminophosphonate) derivatives were also prepared by the double Kabachnik-Fields reaction. Furthermore, the synthesis of amino-methylenebisphosphonates and amino-methylenebisphosphine oxides is also described.

In this paper, the MW-assisted synthesis of α-aminophosphonates and α-aminophosphine oxides by the catalyst-and solvent-free Kabachnik-Fields condensation is summarized.The preparation of bis(aminophosphonates) and bis(aminophosphine oxides) by the double Kabachnik-Fields reaction, as well as the utilization of the latter compounds are also discussed.Another "green" accomplishment of the synthesis of α-aryl-α-aminophosphonate derivatives, the Pudovik reaction is also summarized.Finally, the synthesis of amino-methylenebisphosphonates and amino-methylenebisphosphine oxides is described.
The Kabachnik-Fields reaction of benzylamine, benzaldehyde and triethyl phosphite or diethyl phosphite was also studied in water.It was found that in case of triethyl phosphite, p-toluenesulfonic acid (PTSA) had to be used as the catalyst, however, using diethyl phosphite, the reaction could be performed without any catalyst, in a solvent-free manner [24].
As a new development, the flow Kabachnik-Fields reaction is also being studied in a self-designed continuous flow MW system [25].Scheme 1. Synthesis of α-aminophosphonates and α-aminophosphine oxides.

Conclusion
In summary, several novel mono-and bis(aminophosphonates), as well as related aminophosphine oxides were synthesized by the single and double MW-assisted Kabachnik-Fields reactions.It was proved that under the conditions investigated, there is no need for any catalyst and, in most cases, for any solvent.The bis(aminophosphines) obtained by the deoxygenation of bis(aminophosphinoyl)amines were utilized as bidentate P-ligands in platinum complexes.Furthermore, α-aryl-α-aminophosphonate derivatives were prepared by the (aza-)Pudovik reaction under MW irradiation.
Finally, the synthesis of amino-methylenebisphosphonates and amino-methylenebis(phosphine oxides) was accomplished by the three-component condensation of amines, orthoformiates and >P(O)H reagents.