Ring-Opening of THF via an Intramolecular P/Al-Based Frustrated Lewis Pair: Assistance by C6F5 Groups beyond Electronegativity?

A frustrated Lewis pair composed of an acidic aluminum function (AlR2) and a basic phosphine entity, linked by a xanthene spacer, is capable of cleaving THF. The rate of the ring-opening reaction is higher by a factor of 10 for R = C6F5 than that for R = Mes. Structural and theoretical investigations revealed for the case of C6F5 aromatic interactions in the secondary coordination sphere which─in case they also exist in the transition state─could aid ring-opening through a preorientation of the reacting functions. However, in detailed computational studies these were found to occur without contributing significantly to the lowering of the activation barrier.