Sigmatropic Rearrangements in Pericyclic Reactions | Organic Chemistry PDF Download

SIGMATROPIC REARRANGEMENTS

• A σ-bonded substituent atom or group migrates across a π-electron system from one position to another.
• A σ-bond is broken in the reactant, the π-bonds move, and a new sigma-bond is formed in the product.

Sigmatropic Notation

• Numbers in brackets refer to the two groups connected by the sigma-bond and designate the positions in those groups to which migration occurs
• In a [1,5] sigmatropic rearrangement of a diene migration occurs to position 1 of the H group (the only possibility) and to position 5 of the pentadienyl group
• n a [3,3] Claisen rearrangement migration occurs to position 3 of the allyl group and also to position 3 of the vinylic ether

Sigmatropic Stereospecificity: Suprafacial and Antarafacial

• Migration of a group across the same face of the  system is a suprafacial rearrangement
• Migration of a group from one face of the  system to the other face is called an antarafacial rearrangement

[1,3] Sigmatropic Rearrangements (H migration)

Construct TS by considering an allyl anion and the proton (or a radical pair)

Suprafacial migration is forbidden and the bridging distance too great for the antarafacial migration. Hence, [1,3] hydrogen migrations are not observed under thermal conditions. Under photochemical conditions, rearrangement is allowed suprafacially.

[1,3]-Sigmatropic Rearrangements (C migration)

Using a similar analysis, one can prove that [1,5] hydrogen and alkyl shifts should be allowed when suprafacial on the pi component and proceeding with retention.

[1,5] Sigmatropic Rearrangements

• A [1,5] sigmatropic rearrangement involves three electron pairs (two π bonds and one s bond)
• Orbital-symmetry rules predict a suprafacial reaction
• 5-methylcyclopentadiene rapidly rearranges at room temperature

• Heating 5,5,5-trideuterio-(1,3Z)-pentadiene causes scrambling of deuterium between positions 1 and 5

Orbital Picture of a Suprafacial [1,5] H Shift

[1,7] Sigmatropic Rearrangements

A [1,7]H shift occurs in the final stages of the human body’s synthesis of vitamin D from cholesterol. This step happens spontaneously, without the need for light, so the shift must be antarafacial. The reason the body does need light to make vitamin D is the previous step, which only occurs when light shines on the skin.

[2,3] Sigmatropic Rearrangements

Transition state model for rationalizing stereoselectivity

[3,3] Rearrangements

A thermally allowed reaction is possible in either of two geometries, the "chair" or the "boat" geometry.