New efficient bifunctional catalysts for condensation: (2-(pyridin-2-yl)ethyl)phosphonic acid and (2-(pyridin-4-yl)ethyl)phosphonic acid grafted on alumina (Al2-pyrPA and Al4-pyrPA)

(2-(Pyridin-2-yl)ethyl)phosphonic acid and (2-(pyridin-4-yl)ethyl)phosphonic acid were first synthesized from 2-vinylpyridine and 4-vinylpyridine respectively and characterized by spectroscopic methods.

While phosphonic acids are well known to graft easily onto metal oxides,¹ in this study, these specific phosphonic acids were grafted onto alumina under microwave irradiation. The resulting new materials were thoroughly characterized using elemental microanalysis, FTIR spectroscopy, BET analysis, TGA, and DTA.

Both Al2-pyrPA and Al4-pyrPA were then evaluated as heterogeneous catalysts in Knoevenagel condensations involving a wide variety of active methylene or methyl compounds under microwave irradiation.² All condensation products were characterized by spectroscopic methods. Many of them constitute useful synthons or potential biologically active compounds.

It is particularly worth noting that Al2-pyrPA has proved to be one of the most efficient heterogeneous catalysts reported to date for the Knoevenagel condensation of weakly acidic carbon containing compounds. In the case of Al2-pyrPA, a cooperative mechanism has been proposed showing how the basic pyridine nitrogen activates the nucleophile while the acidic alumina surface or residual phosphonic protons activate the carbonyl electrophile.

References:

1- P. H. Mutin, G. Guerrero, A. Vioux, Hybrid materials from organophosphorus coupling molecules, J. Mater. Chem. 2005, 15 3761-3768.

2- A. Ben Alloum, B. Labiad, D. Villemin, Application of microwave heating techniques for dry organic reactions. J. Chem. Soc. Chem. Commun., 1989, 386-387; doi: 10.1039/C39890000386