Effects of Charge, Size, and Shape of Transition States, Bound Intermediates, and Confining Voids in Reactions of Alkenes on Solid Acids

Solid acids with sites varying in acid strength and in confining environments are used here to examine, through kinetic experiments and theory, how size and shape of organic guests and inorganic hosts at ion-pair transition states (TS) influence reactivity. Reactions of C₂−C₄ alkene mixtures serve as the illustrative example; they involve kinetically-relevant C−C coupling between alkenes and bound alkoxides that saturate the surface on microporous (TON) and mesoporous (SiAl) aluminosilicates of similar acid strength and on stronger heteropolytungstic acids (HPW). Rate constants (per H⁺) increase with increasing TS size (number of C-atoms) because of the combined effects of size/substitution on proton affinities of coupling products and on van der Waals (vdW) contacts between carbocations and voids. When the voids and organic moieties are similar in dimension, these contacts require framework distortions, leading to dispersive interactions that are compensated in part by enthalpic penalties associated with the distortions, which are quantified here with DFT-derived atomic displacements and distortion energies. DFT-derived proton affinities of the coupling products and vdW interaction energies between the TS and voids represent the most accurate and complete descriptors of reactivity, as the size and shape of TS structures vary in C−C coupling steps and more generally in acid-catalyzed transformations, and can be used in designing voids for stabilizing specific host-guest pairs.