Sigmatropic and Electrocyclic Reactions

Sigmatropic rearrangements migrate a sigma bond across a pi system, and electrocyclic reactions open or close rings, both as concerted pericyclic processes governed by orbital symmetry.

Definition

Sigmatropic reactions relocate a sigma bond to a new position along a conjugated framework, while electrocyclic reactions interconvert an open-chain conjugated polyene and a cyclic isomer by forming or breaking a single sigma bond; both are concerted and stereospecific.

Scope

This topic covers electrocyclic ring opening and closing and their conrotatory/disrotatory stereochemistry, [3,3]-sigmatropic rearrangements such as the Cope and Claisen, [1,5]- and [1,7]-hydrogen shifts, and the Woodward–Hoffmann rules that determine the allowed pathways.

Core questions

• What determines whether an electrocyclic reaction proceeds conrotatory or disrotatory?
• How do the order and stereochemistry of a sigmatropic shift follow from orbital symmetry?
• Why do thermal and photochemical conditions give opposite stereochemical outcomes?

Key theories

Electrocyclic stereochemistry rules

The conrotatory or disrotatory mode of ring closure/opening is dictated by the number of pi electrons and whether the reaction is thermal or photochemical, following the Woodward–Hoffmann analysis.

Sigmatropic rearrangements

The Cope and Claisen [3,3]-shifts and [1,n]-hydrogen shifts proceed through ordered cyclic transition states whose allowed suprafacial/antarafacial geometry is set by orbital symmetry.

Mechanisms

Electrocyclic reactions form or break a terminal sigma bond through rotation of the terminal p orbitals, conrotatory or disrotatory depending on the symmetry of the controlling orbital. Sigmatropic shifts migrate a sigma-bonded group to a new terminus of an allylic or polyene system through a cyclic, often chair-like transition state, with the Claisen rearrangement converting allyl vinyl ethers to gamma,delta-unsaturated carbonyls.

Clinical relevance

These rearrangements are exploited in synthesis to construct stereodefined frameworks under mild, byproduct-free conditions; the Claisen rearrangement in particular is used to set quaternary centers and is mirrored by the enzyme chorismate mutase in biosynthesis.

History

Long-known thermal rearrangements such as the Claisen (1912) and Cope reactions found a unifying explanation in the Woodward–Hoffmann orbital-symmetry rules of the 1960s, which predicted their stereochemistry from first principles.

Key figures

• Robert Burns Woodward
• Roald Hoffmann
• Arthur C. Cope
• Rainer Ludwig Claisen

Seminal works

• woodward1969
• careysundberg2007a

Frequently asked questions

What does conrotatory versus disrotatory mean?

They describe the two ways the terminal carbons of a polyene rotate during electrocyclic ring closure: conrotatory means both rotate in the same sense, disrotatory in opposite senses, and orbital symmetry dictates which is allowed for a given electron count and condition.

What is a [3,3]-sigmatropic rearrangement?

It is a concerted reaction, such as the Cope or Claisen, in which a sigma bond migrates so that the new bond forms three atoms away on each side of the original, reorganizing the molecule through a six-membered cyclic transition state.