Mechanistic interpretation of the effects of acid strength on alkane isomerization turnover rates and selectivity

• Skeletal isomerization rates depend on alkene concentrations at acid sites.
• Turnover rates (per proton) increase exponentially with deprotonation energy.
• Ion-pair transition states mediate kinetically-relevant backbone rearrangements.
• Transition states for hexene interconversions depend similarly on deprotonation energies.
• Alkene isomerization product selectivity is independent of Brønsted acid strength.

Acid strength effects on alkane isomerization turnover rates and selectivities are probed using hexene isomers as reactants on bifunctional catalysts containing tungsten Keggin polyoxometalates (POM) with different central atoms and exhibiting well-defined structures amenable to reliable estimates of deprotonation energies (DPE) as rigorous descriptors of acid strength. Titrations of protons with hindered bases during catalysis and mechanistic interpretations of rate data on POM acids in terms of a common sequence of elementary steps give isomerization rate constants that decrease exponentially with increasing DPE. The sensitivity to acid strength is the same for all interconversions among isomeric hexenes because their respective transition states are similar in the amount and localized character of their cationic charges, which determine, in turn, the extent to which the ionic and covalent interactions that determine DPE are recovered upon formation of ion pairs at transition states. The ratios of rate constants for such interconversions, and thus selectivities, are independent of acid strength and their magnitude merely reflects the stability of the gaseous analogs of their respective transition states on all acids.

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