Chair Transition States in [3,3] Shifts

Introduction

A [3,3] rearrangement can create a new σ bond and move double bonds with high stereochemical order. Its six-atom transition-state array can fold into a chair-like or boat-like shape. Drawing that shape and keeping substituents attached to the same atoms gives a practical way to predict alkene geometry and new stereocentres in Cope and Claisen products.

Core explanation

Both Cope and Claisen rearrangements involve a six-electron [3,3] orbital array. A six-membered transition-state model can often adopt a chair-like geometry with relatively favorable staggered relationships. A boat-like alternative may place substituents in more crowded or eclipsed positions. However, “chair always wins” is not a law: a rigid ring, tether, heteroatom coordination or specific attractive interaction can favor another shape. The model should compare actual candidate transition states rather than just repeat a mnemonic.

For a chair drawing, mark the six atoms in order around the cyclic array and highlight the old σ bond that is breaking and the new σ bond that is forming. Place each substituent on its proper atom. Bulky groups frequently prefer pseudoequatorial placement to avoid close contacts, much as equatorial substituents are commonly favored in a cyclohexane chair. This preference can make one chair orientation lower in energy than a second chair that places the same group pseudoaxial. The word pseudo matters because the transition state is not a stable cyclohexane molecule with ordinary chair bonds.

After choosing a plausible transition-state arrangement, unfold it into the product . Do not redraw substituents from memory. Trace the original atom labels through the bond changes, then assign E/Z geometry to new double bonds and wedge/dash relationships to any stereocentres. In a Claisen reaction, first draw the rearranged enol and then the carbonyl tautomer; tautomerisation can remove or alter stereochemical features at the carbonyl-adjacent position, so the final product must be analysed after that step. In an oxy-Cope sequence, the same caution applies.

The starting alkene geometry constrains which substituent placements are available in a chair-like transition state. Changing an E alkene to Z can change which new stereoisomer is reached even though the same [3,3] bond map occurs. A racemic pair of chair conformations can form mirror-image products from an achiral substrate if no chiral influence selects one. For a chiral starting molecule, competing chair arrangements may give diastereomers. The OpenStax account of Cope and Claisen examples provides the foundational cyclic transition-state framework.

Steric intuition is a starting point, not final proof. Relative transition-state free energy includes bond distortion, orbital overlap, electrostatics, solvent and temperature. A large group might avoid a pseudoaxial position, yet a strong intramolecular hydrogen bond can favor a less obvious arrangement. When a problem supplies no structural details, the defensible conclusion is a qualitative chair preference rather than a precise numerical product ratio.

Step-by-step reasoning

Number the six reacting atoms and identify old and new σ bonds. Draw both feasible chair-like transition-state orientations, keeping fixed E/Z starting geometries. Put each substituent on its original atom and label it pseudoaxial or pseudoequatorial in each drawing. Compare close contacts and any stated coordinating or hydrogen-bonding effects. Carry the preferred arrangement into the immediate [3,3] product, then perform tautomerisation separately if relevant and assign product stereochemistry.

Visual explanation

Draw a six-membered chair outline with the breaking bond colored blue and the forming bond dashed red. Add a large substituent as an outward-facing pseudoequatorial group in one diagram and an inward or vertical pseudoaxial group in the alternative. Draw the product below each chair with atom numbers preserved so the origin of the different E/Z or wedge/dash relationships can be checked.

Real-world analogy

Six linked panels can fold into two box-like arrangements while preserving their order. A bulky handle on one panel points outward in one fold and into a crowded interior in the other. The less crowded fold may be preferred, and opening that fold back out determines where the handle ends up. The transition-state chair and product stereochemistry have the same relation.

Real-world example

Claisen rearrangements of substituted allyl vinyl ethers are used to set relative stereochemistry during complex-molecule synthesis. Choosing the starting E/Z geometry and substituent pattern can bias the chair-like transition state, enabling a new carbon–carbon bond and a useful unsaturated carbonyl product in a stereocontrolled sequence.

Why?

A six-electron [3,3] transition state must bring distant atoms into bonding distance while maintaining cyclic overlap. A folded chair-like geometry often achieves this with less steric crowding than alternatives. Because the same folded geometry constrains substituent positions, relative product stereochemistry records the path taken through the transition-state region.

Common misconception

“The bulky group goes equatorial” is not enough to draw the product. The atom map, initial E/Z geometry and subsequent tautomerisation must all be followed. Also, pseudoaxial and pseudoequatorial describe a transition-state model; they are not stable conformations that can be isolated and directly observed in most reactions.

Worked example

Question: Two proposed chair-like Claisen transition states differ only in whether a large alkyl substituent is pseudoequatorial or pseudoaxial, and no special attractive interactions are specified. Which is the first qualitative prediction? Reasoning: The pseudoequatorial arrangement usually avoids close nonbonded contacts. The same [3,3] bonds form in either pathway, but their barriers can differ. Answer: Predict the pseudoequatorial chair as the lower-barrier candidate, then map its substituent positions through the enol and final carbonyl before assigning a major stereoisomer.

Quick check

1. Why should tautomerisation be drawn after a Claisen chair-based stereochemical prediction? Answer: The [3,3] step first makes an enol, and the later carbonyl-forming step may change stereochemical features near that site.

Exam focus

Draw at least two competing chair arrangements when selectivity is asked. Keep atom labels fixed, mark bulky-group placement, and assign the product stereochemistry from the chosen drawing rather than by a memorised shortcut.

Advanced insight

Transition-state conformer populations obey kinetics, not merely reactant ground-state chair populations. A more populated starting conformer might react through a higher barrier than a less populated one, so product ratios can reflect conformer interconversion and transition-state energies together. When this Curtin–Hammett-like situation applies, comparing only the most visible reactant conformation can give the wrong major product.

Summary

Chair-like six-membered transition states help predict stereochemistry in [3,3] Cope and Claisen shifts. Substituent placement, especially pseudoaxial versus pseudoequatorial, can bias pathways. A reliable prediction preserves atom mapping, starting E/Z geometry and any later tautomerisation; chair preference alone is an incomplete product drawing.

Practice questions

1. What bond changes define a [3,3] shift regardless of chair orientation? Answer: One old σ bond breaks, one new σ bond forms and two π bonds reorganise through a six-atom array.

2. Why might a bulky substituent prefer a pseudoequatorial position? Answer: It can reduce close nonbonded contacts in the chair-like transition-state geometry.

3. Can a boat-like transition state ever compete? Answer: Yes. Tethers, rings or specific interactions can change relative barriers, so chair is a useful common model rather than an absolute rule.

4. What must be preserved when converting a transition-state sketch to a product? Answer: Atom labels, starting substituent geometry and the exact old and new bond map.