Abstract
The use of tri- and tetraphosphonate cavitand hosts in conjunction with tetrabutylammonium halide catalysts led to improved activity in the coupling of 1,2-epoxyhexane and CO2. Catalytic efficiency of the supramolecular assembly strongly relies on both the structural features and binding properties of the cavitand host. Tetraphosphonate cavitand host (4iPO 1) was shown to impact more markedly n-Bu4NCl reactivity than triphosphonate counterpart (3iPO 3) due to its greater binding strength toward the ammonium cation as evidenced by titration experiments. The participation of acidic functional groups within the cavitand structure to activity enhancement was also clearly demonstrated. The best result was obtained with n-Bu4NI/3iPO 2 supramolecular assembly affording hexene carbonate in quantitative yield after 18h of reaction. The improved performance observed with the cavitand 3iPO 2 relies on a double activation of both the nucleophile (halide) through efficient encapsulation of the ammonium cation and the substrate (epoxide) through hydrogen bonding.
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