(20 pts). Transformations, consisting of symmetry (a) Propose a mechanism for the following: provide the following: allowed pericyclic reactions. For each pericyclic reaction, draw curved arrows leading to its structure of the product with stereochemistry, name of the reaction, and what mode it occurs in (e.g., electrocyclic, conrotatory, disrotatory). Predict the stereochemistry in rational conrotatory vs. disrotatory way. Racemic stereo?
The Correct Answer and Explanation is:
To propose a mechanism for a pericyclic reaction, let’s break down the steps using examples of typical pericyclic reactions: electrocyclic, cycloaddition, and sigmatropic rearrangements. These reactions all involve concerted processes, meaning that bonds are made and broken simultaneously in a cyclic transition state.
1. Electrocyclic Reactions
Electrocyclic reactions involve the formation or breaking of a ring by the cyclic movement of electrons. The reaction can be either conrotatory or disrotatory.
Example: Cyclohexene to 1,3-cyclohexadiene
- Reaction Type: Electrocyclic ring-opening.
- Mechanism: The electrons in the π-system of cyclohexene undergo a conrotatory motion when heated (thermal condition) or disrotatory when irradiated with light (photochemical condition).
- Stereochemistry: In the thermal case (conrotatory), both ends of the π-bond break in the same rotational direction, leading to a specific stereochemical outcome. In the photochemical case (disrotatory), the bonds break in opposite rotational directions.
In this case, if we consider the thermal reaction, you would get a cis configuration for the two ends of the new π-bond.
2. Cycloaddition Reactions (Diels-Alder)
Cycloadditions are reactions where two π-systems react to form a cyclic product. The Diels-Alder reaction is one of the most common.
Example: Diene + Dienophile → Cyclohexene Derivative
- Reaction Type: [4+2] Cycloaddition.
- Mechanism: The diene and dienophile undergo a concerted mechanism to form a six-membered ring. This reaction is typically performed thermally and occurs in a suprafacial manner (same face of the molecule) for both the diene and the dienophile.
- Stereochemistry: The reaction can lead to a cis or trans product, depending on the relative positions of substituents on the reactants. If substituents on the dienophile are cis, the product will have them on the same face of the ring.
3. Sigmatropic Rearrangements
Sigmatropic rearrangements involve the migration of a single bond and are classified as [n,m] shifts, where n is the number of atoms moving and m is the number of atoms in the new bond formed.
Example: [1,5]-Hydrogen shift in pentadiene
- Reaction Type: Sigmatropic rearrangement.
- Mechanism: The hydrogen migrates from position 1 to position 5, forming a new C-H bond at position 1 and breaking the C-H bond at position 5.
- Stereochemistry: The reaction can be either suprafacial or antarafacial, depending on the direction of the bond migration. The direction of the electron flow determines the stereochemical outcome, with a potential for a racemic mixture if the shift happens symmetrically.
General Summary:
- Electrocyclic Reactions: Can be conrotatory (same direction of rotation) or disrotatory (opposite direction). Conrotatory motion is typically observed under thermal conditions, and disrotatory is under photochemical conditions.
- Cycloaddition Reactions: The reaction can occur via a suprafacial mechanism for both diene and dienophile. This can yield either a cis or trans product based on the geometry of the reactants.
- Sigmatropic Reactions: Can be either suprafacial or antarafacial with a possibility of a racemic mixture if both pathways are accessible.
These stereochemical outcomes are predicted by understanding the symmetry properties of the molecular orbitals involved, which govern the allowed pericyclic reactions.
