Chemists Confirm The Identity Of Pivotal Intermediate In Carbocation Rearrangements | Chemical & Engineering News
Volume 94 Issue 7 | p. 23 | Concentrates
Issue Date: February 15, 2016

Chemists Confirm The Identity Of Pivotal Intermediate In Carbocation Rearrangements

Reaction mechanisms: Computational study solves a long-standing chemical conundrum on alkane branching
Department: Science & Technology
News Channels: Organic SCENE
Keywords: carbocation, rearrangement, reaction mechanism

Chemists have known for 70 years that carbocation rearrangements involve slow steps and fast steps. The slow steps are crucial in that they control the degree of branching in alkanes, and understanding how that works is vital for predicting product distributions in processes such as biomass and petroleum refining. But why the key steps are slow has been a confounding mystery. Daniel J. S. Sandbeck, Daniel J. Markewich, and Allan L. L. East of the University of Regina now appear to have found the answer via a set of computer simulations (J. Org. Chem. 2016, DOI: 10.1021/acs.joc.5b02553). The team first revisited decades-old studies in which chemists proposed that the rearrangements proceed through a protonated cyclopropane intermediate. Then using hexyl ion as a model, the researchers ran simulations and uncovered 70 transition states connecting primary, secondary, and tertiary ion versions. In the past, most chemists thought the cyclopropane intermediate must be protonated on an edge of the ring or at one of the corners. The Regina researchers found that neither assumption was correct but that the intermediate takes on a mesomeric structure that is a hybrid of the two. The finding suggests that branching is slow because the reaction pathway must pass through an unstable primary carbocation that involves the mesomeric structure.

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Robert Buntrock (February 21, 2016 9:06 PM)
Interesting. Has this method been applied to (in)famous non-classical carbonium ions (NCCIs) in the norbornane series?
Allan East (February 26, 2016 11:07 AM)
As a matter of fact we started in mid-January looking at norbornyl and related structures! I do not know if anyone has done such a study yet.

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