Mechanism of Isobutanal–Isobutene Prins Condensation Reactions on Solid Brønsted Acids

Abstract

The selectivity to 2,5-dimethyl-hexadiene isomers (2,5-DMH) via acid-catalyzed isobutanal–isobutene Prins condensation is limited by isobutene oligomerization reactions (to 2,4,4-trimethyl-pentene isomers) and by skeletal isomerization and cyclization of the primary 2,5-DMH products of Prins condensation. Experiment and theory are used here to assess and interpret acid strength effects on the reactivity and selectivity for isobutanal–isobutene Prins condensation routes to 2,5-DMH, useful as precursors to <i>p</i>-xylene. Non-coordinating 2,6-di-<i>tert</i>-butylpyridine titrants fully suppress reactivity on Keggin heteropolyacids, niobic acid, and mesoporous and microporous aluminosilicates, indicating that Prins condensation, parallel isobutene oligomerization, and secondary skeletal isomerization and cyclization of primary 2,5-DMH products occur exclusively on Brønsted acid sites. The number of titrants required to suppress rates allows site counts for active protons, a requirement for comparing reactivity among solid acids as turnover rates, as well as for the rigorous benchmarking of mechanistic proposals by theory and experiment. Kinetic and theoretical treatments show that both reactions involve kinetically relevant C–C bond formation elementary steps mediated by cationic C–C coupling transition states. Transition state charges increase with increasing acid strength for Prins condensation, becoming full carbenium-ions only on the stronger acids. Oligomerization transition state structures, in contrast, remain full ion-pairs, irrespective of acid strength. Turnover rates for both reactions increase with acid strength, but oligomerization transition states preferentially benefit from the greater stability of the conjugate anions in the stronger acids, leading to higher 2,5-DMH selectivities on weaker acids (niobic acid, aluminosilicates). These trends and findings are consistent with theoretical estimates of activation free energies for Prins condensation and oligomerization elementary steps on aluminosilicate slab and Keggin heteropolyacid cluster models. High 2,5-DMH selectivities require weak acids, which do not form a full ion-pair at transition states and thus benefit from significant stabilization by residual covalency. These trends demonstrate the previously unrecognized consequences of incomplete proton transfer at oxygen-containing transition states in dampening the effects of acid strength, which contrast the full ion-pair transition states and stronger acid strength effects in hydrocarbon rearrangements on solids acids of catalytic relevance. These mechanistic conclusions and the specific example used to illustrate them led us to conclude that reaction routes involving O-containing molecules become prevalent over hydrocarbon rearrangements on weak acids when parallel routes are accessible in mixtures of oxygenate and hydrocarbon reactants