ELECTROCYCLIC REACTIONS AND CYCLOADDITIONS
Electrocyclic addition reactions:
Electrocyclic reactions are intramolecular reversible reactions involving a single starting compound in which electrons are simultaneously exchanged between double or triple bonds to form a new sigma bond and a cyclic structure. They are addition reactions because the sum total of the number of atoms in starting materials equals the number of atoms in the product.
Electrocyclic reactions are neither homolytic nor heterolytic reaction.
The reaction is usually illustrated with the full arrow mechanism whereby each full arrow represents a pair of electrons. However, it is sometimes shown as half arrow mechanism whereby half arrows representing single electrons within their π-bonds to be shared to form a new sigma bonds.
π-electron systems can be
- 4n where n=1,2, 3… example 1,3-butadiene
- 4n+2 for n=0,1,2,3……example ethene, benzene, 1,3,5 hexatriene
Electrocyclic reaction will convert one π-system to another.
EXAMPLE :
MOECULAR ORBITAL THEORY and ELECTRON SYSTEM
A p-system is made of p orbitals overlap in different phases. When the same phase lobe overlap bonding occurs but different phases of the p-orbital lobes adjacent on same level forms a NODE and becomes ANTI-BONDING (*) = Zero or no bonding electron cloud occur in that overlap area.
In the excited state the electron in the HOMO absorbs light energy of the right wavelength and the electron is promoted to the LUMO which becomes the new HOMO excited state.
MO THEORY, BONDING and Chemical Reaction conditions:
A Chemical bond occurs when a p-electron cloud overlaps with an empty p-orbital of the same phase.
The front lobes of the orbitals of a p-system should interact with the front lobes of the another p-system to prove if an overlap will give bonding or non-bonding interactions. In other words, frontier orbital overlap shows bonding and non-bonding interactions.
If the back inner end lobes of a p-system are used, they should interact with the back inner end lobes of another p-system to prove that it is bonding or non-bonding interactions. E.g. Diels Alder reaction.
Conrotatory motion: Both p-orbitals of the two =C-R carbons move clockwise or Anti-clockwise directions to form the sigma bond to close the ring.
A Chemical bond occurs when a p-electron cloud overlaps with an empty p-orbital of the same phase.
These rotations are promoted by light or heat.
This directly relates to the way =C-R frontier carbons CAN BE ROTATED so their p-orbitals can overlap in the same phase to form the ring closing sigma bond and establish stereochemistry in the products. The two carbons that closes the ring rotates from horizontal [ p-bond] to vertical [sigma bond] and vice versa for ring opening.
PRACTICE: Use two pencils.
Top and tip represents the two different groups attached to ending =C-R frontier carbons that form the sigma bonds from π-bonds.
The two pencils orient horizontally when starting with alkene =C-R but moves into a perpendicular vertical position to close ring during cyclization.
Conrotatory : both pencils move in same direction to form ring.
Disrotatory: one group =C-R moves opposite in direction to reach vertical position, [that is one moves clockwise and the other moves Anti-clockwise].
Determine the STEREOCHEMISTRY: Assign Dash wedges to groups is pointing downwards but Bold wedges to groups pointing upwards.
Conrotatory cyclization is promoted by light in 4n + 2 π -systems but promoted by heat in 4n π-system.
Disrotatory cyclization is promoted by heat in 4n + 2 π-system but promoted by light in 4n π- system.
MO THEORY, BONDING and Chemical Reaction conditions:
If heat is used for electrocyclic reaction, electrons in the ground state HOMO π-electron system interact with the ground state LUMO to form a new sigma bond and new π-bonds.
EXAMPLE :
TRY Conversion of 1,3,5-hexatriene to 1,3-cyclohexadiene .
