The Role of Ion−Molecule Pairs in Solvolysis Reactions. Nucleophilic Addition of Water to a Tertiary Allylic Carbocation

Abstract

The acid-catalyzed solvolysis of 2-methoxy-2-phenyl-3-butene (1-OMe) in 9.09 vol % acetonitrile in water provides 2-hydroxy-2-phenyl-3-butene (1-OH) as the predominant product under kinetic control along with the rearranged alcohol 1-hydroxy-3-phenyl-2-butene (2-OH) and a small amount of the rearranged ether 2-OMe. The more stable isomer 2-OH is the predominant product after long reaction time, K(eq) = [2-OH](eq)/[1-OH](eq) = 16. The ether 2-OMe reacts to give 2-OH and a trace of 1-OH. Solvolysis of 1-OMe in (18)O-labeled water/acetonitrile shows complete incorporation of (18)O in the product 1-OH, confirming that the reaction involves cleavage of the carbon-oxygen bond to the allylic carbon. A completely solvent-equilibrated allylic carbocation is not formed since the solvolysis of the corresponding chloride 1-chloro-3-phenyl-2-butene (2-Cl) yields a larger fraction of 1-OH. This may be attributed to a shielding effect from the chloride leaving group. Quantum chemical calculations of the geometry and charge distribution show that the cation should rather be described as a vinyl-substituted benzyl cation than as an allyl cation, which is in accord with its higher reactivity at the tertiary carbon.

Solvation Dynamics and the Nature of Reaction Barriers and Ion-Pair Intermediates in Carbocation Reactions

“…From a wider perspective, the importance of ion pairs in carbocation reactions has long been recognized, and their nature was historically probed by many elegant experiments. − Our results will support an addition to the classical Winstein ion-pair framework and suggest some possible reinterpretations of classically important experimental observations.…”

Regioselective Lewis Acid-Mediated [1,3] Rearrangement of Allylvinyl Ethers

“…In the event both regioisomeric allylvinyl ethers provided the same major product. We hypothesize that if an ionic mechanism is operating, the allyl cation and enolate size determines the observed regioselectivity . Indeed, a crossover experiment using deuterium-labeled vinyl ether illustrated nearly complete scrambling of the label .…”

Competing solvolytic elimination and substitution reactions via very short-lived ion-pair intermediates

“…It has recently been proposed that simple tertiary carbocations react with water from the surrounding solvation shell faster than the ion pair undergoes dissociation. 5, 6 The tertiary carbocation 1 ϩ is more unstable than a simple tertiary carbocation and the direct reaction of the undissociated ion pair 1 ϩ X Ϫ is expected to be even more favoured relative to dissociation. Thus, the rate-limiting process of the addition of a solvent molecule should be the rotation of the nucleophile into a reactive position which should have a rate constant similar to that for rotational relaxation of a water molecule, i.e., k w Ј ∼ 1 × 10 11 s Ϫ1 .…”

“…Also the addition of water to simple tertiary carbocation intermediates has been suggested to occur before dissociation (Scheme 1). 5,6 The role of the chloride ion leaving group in the elimination process is, however, not clear. 2,5 Does it act as a general base that abstracts a β-hydron from the cation, or is a water molecule of the solvation shell the active species?…”