Electrocyclic reactions, a type of pericyclic reaction is a reversible reaction that involves ring closure of conjugated polyene to cycloalkane or ring opening of cycloalkane to conjugated polyene. It is a concerted reaction and proceeds through a cyclic transition state. The reaction requires heat or light and is completely stereospecific.
Electrocyclic reaction definition
Electrocyclic reaction is defined as “the reaction that involves ring closing or ring opening of a conjugated polyene and a cycloalkene occurs, respectively. The ring closure of conjugated polyene to cycloalkene takes place by the conversion of one π bond to one σ bond, while the ring opening of cycloalkene to conjugated polyene involves the conversion of one σ bond to one π bond.
Stereochemistry of Electrocyclic reactions
Electrocyclic reactions are completely stereospecific. In the case of (2E,4Z,6E)-2,4,6-octatriene, ring closure results in a single product having cis methyl groups on the ring. One conjugated diene containing one Z-alkene and one E-alkene is formed when the ring of cis-3,4-dimethyl cyclobutene undergoes ring opening.
However, whether an electrocyclic reaction takes place in a thermal or photochemical environment affects the stereochemistry of the product. Heat-induced cyclization of (2E 4E)-2,4-hexadiene results in cyclobutene with trans methyl groups, whereas light-induced cyclization results in cyclobutene with cis methyl groups.
The exact stereochemistry of electrocyclic reactions depends on:
- Number of double bonds in the polyene.
- Photochemical or thermal conditions of the reaction.
|Number of π electrons||Reactions||Mode of rotation|
An electrocyclic reaction takes place when orbitals of a similar phase overlap to form a bond, and such a reaction is said to be symmetry allowed. However, an electrocyclic reaction cannot take place when overlap occurs between orbitals of opposite phases, and the reaction is said to be symmetry forbidden.
To create a new sigma bond, similar phases must interact by rotating the p orbital on the terminal carbons. Rotations can occur in two different ways; Conroatatory and Disrotatory.
The two orbitals must rotate in the opposite direction when like phases of the p orbitals are on the same side of the molecule. One direction is counterclockwise, and the other is clockwise. Disrotatory rotation is the opposite of rotatory rotation i.e. rotating in opposite direction. When lobes from opposing phases are brought together through disrotatory motion, the interaction is antibonding and repulsive.
The two orbitals, however, must rotate in the same direction and are referred to as being conrotatory when like phases of the p orbitals are on opposite sides of the molecule. The overlapping of lobes from the same phase brought together by conrotatory motion results in the formation of a bond.
Electrocyclic reactions (Photochemical/Thermal)
Thermal ring closure of electrocyclic reaction
Let us consider thermal electrocyclization reactions of butadiene. The HOMO of conjugated diene is ψ2. The electron in this orbital is responsible for the bond formation which involves overlapping of the lobes on C-1 and C-4 of the diene. Rotation about the bonds C(1)-C(2) and C(3)- (4)must occur in order to bring these lobes into an overlapping position. There are two ways that this rotation can occur: either conrotatory motion, in which the bonds rotate in the same direction or disrotatory motion, in which the bonds rotate in the opposite directions.
According to Woodward Hoffmann’s rule, there are 4n electrons in butadiene, and hence the conroratory rotation takes place in thermal conditions resulting in bonding between the lobes.
Photochemical ring closure of electrocyclic reaction
In photochemical ring closure of butadiene, the disrotatory motion leads to bonding while conrotatory motion leads to antibonding, which is the reverse of thermal ring closure. It is because of the excitation of an electron from ψ2 to ψ3 on the absorption of light. As a result, ψ3 becomes the HOMO (Highest occupied molecular orbital) whose symmetry of the terminal carbon is opposite to that of ψ2. In order to bring lobes of the similar phase together, disrotatory motion takes place.
According to Woodward Hoffmann’s rule, there are 4n electrons in butadiene, and hence the disrotatory rotation takes place in photochemical conditions resulting in bonding between the lobes.
Thermal ring opening of electrocyclic reaction
The symmetry of the ground state HOMO governs the stereochemistry in the ring opening of electrocyclic reactions. The mode of rotation must be disrotatory on the ring opening reaction in order to attain m-symmetry in the HOMO of the product, while the conrotatory mode of rotation to attain C2-symmetry in the product.
Let us consider thermal ring opening of electrocyclic reaction.
The total electrons involved in the reaction are 8e- (6π and 2σ). According to Woodward Hoffmann selection rules, it corresponds to the 4n system, and hence undergoes conrotatory ring opening at thermal conditions.
Photochemical ring opening of electrocyclic reaction
There are 8e– (6π and 2σ) involved in this ring-opening reaction, which according to Woodward Hoffmann selection rules corresponds to the 4n system, and hence undergoes disrotatory motion at photochemical conditions.
Electrocyclic reactions video
- R. B. Woodward and R. Hoffmann, Conservation of Orbital symmetry, Verlag Chemie GmbH, Academic Press, 1971.
- Charles DePuy and Orville L. Chapman, Molecular Reaction and Photochemistry, Prentice-Hall, 1972.
- Peter Skyes, A Guide book to mechanism in organic chemistry, sixth edition.