A New Step Towards Solid Base Catalysis: Azidoproazaphosphatranes Immobilized in Nanopores of Mesoporous Silica

Abstract

AbstractHeterogeneous 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.