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Reaction Mechanisms

Vinyl cation busts some new moves

By tweaking its counter ion, classic carbocation inserts into C-H bonds

by Bethany Halford
July 26, 2018 | A version of this story appeared in Volume 96, Issue 31


Cyclohexyl triflate reacts with triethylsilane and a weakly coordinating anion to form dicyclohexane.
Triethyl silane and a weakly coordinating anion catalyze formation of a vinyl cation from the reaction of cyclohexenyl triflate. This vinyl cation can insert into C–H bonds, such as those in cyclohexane.

Chemists have coaxed vinyl cations to insert into C–H bonds of alkanes and arenes. In doing so, they’ve shown that this classic carbocation is capable of much more than previously thought. Although vinyl cations have been studied for more than 50 years, until now their use as reagents was limited to intramolecular reactions and reactions with heteroatom solvents.

Because they are so reactive, vinyl cations tend to be trapped by their counter ions, via deprotonation or nucleophilic attack, explains Hosea M. Nelson, a University of California, Los Angeles chemist who led the study along with UCLA colleague Kendall N. Houk. By generating vinyl cations using silylium catalysis and a weakly coordinating anion, which won’t take part in either deprotonation or nucleophilic attack, Nelson says, the vinyl cation had the freedom to engage in the C–H insertion reactions (example shown).

The study “lays the groundwork for the discovery of new reactions in the area of hydrocarbon carbon-hydrogen bond functionalization,” comments Peter P. Gaspar, who specializes in reactive intermediates at Washington University in St. Louis.

Computational studies reveal that the reaction proceeds through a single, nonclassical cation transition state that generates multiple products, depending upon molecular motions (Science 2018, DOI: 10.1126/science.aat5440). That’s unusual, Houk points out, because multiple products typically arise from multiple transition states. “There are a lot of reactions that go through carbocation intermediates,” he says. “Looking at these reactions with molecular dynamics is a fruitful way of understanding how these things happen.”

Structures generated from calculations showing how a single transition state can lead to multiple products.
Calculations indicate that the vinyl cation generates a single, nonclassical cation in the transition state, which forms multiple products depending upon a 1,2-hydride shift.

Nelson notes that there’s work to be done before vinyl cations become commonplace tools for synthetic chemists, but he thinks the mechanistic insight they’ve gained is important. He notes that carbocations are involved in everything from biosynthesis to astrochemistry. “The means by which we make carbon-carbon bonds is fundamentally important to all those processes,” Nelson says. “This is just another description of how a carbocation can make a carbon-carbon bond.”


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