Dynamic Effects Dictate the Mechanism and Selectivity of Dehydration-Rearrangement Reactions of Protonated Alcohols [Me_2(R)CCH(OH_2)Me]~+ (R=Me, Et, iPr) in the Gas Phase

摘要

The gas-phase dehydration-rearrangement (DR) reactions of protonated alcohols [Me_2(R)CCH(OH_2)Me]~+ [R= Me (ME), Et (ET), and iPr (I-PR)] were studied by using static approaches (intrinsic reaction coordinate (IRC), Rice-Ramsperger- Kassel-Marcus theory) and dynamics (quasiclassical trajectory) simulations at the B3LYP/6-31G(d) level of theory. The concerted mechanism involves simultaneous water dissociation and alkyl migration, whereas in the stepwise reaction pathway the dehydration step leads to a secondary carbocation intermediate followed by alkyl migration. Internal rotation (IR) can change the relative position of the migrating alkyl group and the leaving group (water), so distinct products may be obtained: [Me(R)CCH(Me)Me···OH_2]~+ and [Me(Me)CCH(R)Me···OH_2]~+. The static approach predicts that these reactions are concerted, with the selectivity towards these different products determined by the proportion of the conformers of the initial protonated alcohols. These selectivities are explained by the DR processes being much faster than IR. These results are in direct contradiction with the dynamics simulations, which indicate a predominantly stepwise mechanism and selectivities that depend on the alkyl groups and dynamics effects. Indeed, despite the lifetimes of the secondary carbocations being short (<0.5 ps), IR can take place and thus provide a rich selectivity. These different selectivities, particularly for ET and I-PR, are amenable to experimental observation and provide evidence for the minor role played by potential-energy surface and the relevance of the dynamics effects (non-IRC pathways, IR) in determining the reaction mechanisms and product distribution (selectivity).