A new step towards solid base catalysis: Azidoproazaphosphatranes immobilized in nanopores of mesoporous silica

Heterogeneous basic catalysts derived from proazaphosphatranes, known as Verkade's superbases, were prepared for the first time by covalent immobilization onto SBA-15 silica. In order to introduce the tether to the surface, three siloxane-containing azido derivatives of the proazaphosphatranes were first synthesized (2a-c) which after post-synthetic grafting onto silica support led to the formation of hybrid materials of different basicity and steric properties, 2a-c@SBA-15. These latter were fully characterized using a wide variety of molecular and solid-state techniques to determine their structural and textural properties. These new solid base catalysts were then evaluated in the Diels-Alder reactions of anthrone and 3-hydroxy-2-pyrone with two electron-deficient dienophiles, N-methylmaleimide and dimethyl fumarate. In general, high activity and selectivity were obtained depending on the catalytic species, the performance decreasing in the order methoxybenzyl>neopentyl> methyl azidophosphatrane. Catalyst recycling was studied for the best catalyst, the methoxybenzyl derivative, 2c@SBA-15, and it was shown that the catalyst could be re-used up to 3 cycles before any loss of activity could be detected.