sigmatropic rearrangement
/ SIG-muh-TROH-pik /
In a sigmatropic rearrangement, a single bond gets up and walks. More precisely, a sigma bond breaks at one place and a new sigma bond forms somewhere else along a conjugated pi system, while the double bonds slide over to fill in behind it — all in one concerted, pericyclic step. The molecule's connectivity reshuffles, but no atoms ever leave; it is an internal migration, like a person stepping from one rung of a ladder to another while the ladder shifts under them.
The bookkeeping is captured by a pair of numbers in brackets, as in a [3,3]-shift: you count along the chain from the breaking bond to find where the new bond forms on each side. The two most famous examples are both [3,3]-sigmatropic rearrangements. In the Cope rearrangement, a 1,5-diene reorganizes into a different 1,5-diene, the sigma bond migrating across the carbon framework. In the Claisen rearrangement, the same [3,3] machinery acts on an allyl vinyl ether (a C-O system), converting it into a carbonyl compound — the migration drives the molecule downhill to a more stable product. Both proceed through a tidy six-membered, chair-like cyclic transition state.
Sigmatropic shifts matter because they are powerful, predictable, byproduct-free ways to rearrange a carbon skeleton, and because they round out the great triad of pericyclic reaction types alongside cycloadditions and electrocyclic reactions. The Claisen rearrangement in particular is a synthetic workhorse for building new carbon-carbon bonds with reliable stereochemistry. And, like all pericyclic reactions, whether a given sigmatropic shift is allowed and what stereochemistry it shows is governed by orbital symmetry through the Woodward-Hoffmann rules.
The Claisen rearrangement heats an allyl vinyl ether (CH2=CH-O-CH2-CH=CH2) and, through a six-membered cyclic transition state, shifts the sigma bond to give a gamma,delta-unsaturated carbonyl compound — a new C-C bond forged with no reagent but heat.
A sigma bond migrates along the pi system in one concerted [3,3] step — atoms reshuffle, none leave.
Despite the word 'rearrangement,' a sigmatropic shift is concerted and intermediate-free — it is not a stepwise process through a carbocation, so do not picture an ion or radical breaking loose and re-landing; the bonds reshuffle all at once.