Veejendra K. Yadav

Conservation of orbital symmetry rule: Reactions occur readily when there is a congruence between the orbital symmetry characteristics of the reactants and the products, and only with difficulty when that congruence does not obtain. In other words, the orbital symmetry is conserved in concerted a reaction.

If x1 and x2 are two atomic orbitals,

(a) x1 + x2 shall represent the bonding combination characterized by +ve overlap and by the concentration of maximum electron density in the region between the two nuclei, and

(b) x1 – x2 shall represent the antibonding combination characterized by –ve overlap and by a nodal plane in the region between the two nuclei where the electron density is nil.

Because an orbital is a mathematical representation of a wave function and because multiplying an entire wave function by –1 does not change its energy characteristics, overlap of a minus lobe with another minus lobe is precisely the same as the overlap of a plus lobe with another plus lobe.

(x1 + x2) (-x1 - x2)

1 + 2

1

2

1

2

1

2

1

2

1 + 2

SS

1 – 2

SA

1*

+ 2*

AS

1*

– 2*

AA

x1 + x2

x1 - x2

x1 and x2 are p-orbitals and interact in manner

x1 + x2

x1 - x2

x1 and x2 are p-orbitals and interact in manner

1

2

1

2

1

2

1

2

1 2 1* 2

*

Disrotation:

Conrotation:

4

3

2

1

A

A

S

S

A

A

S

S

*

*

4

3

2

1

S

S

A

A

A

S

A

S

*

*

Orbitals symmetry correlation diagram

with respect to a mirror plane

S = Symmetric, A = Antisymmetric

Orbitals symmetry correlation diagram

with respect to a C2 rotation axis

S = Symmetric, A = Antisymmetric

E

E

Ph

Ph

Ph

Ph

H

E

E

Ph

Ph

Ph

Ph

MA

MA

E

E

H

H

O

O

O

Ph

Ph

O

O

O

Ph

Ph

A conrotatory ring closure under thermalconditions shall furnish this cyclobutene derivative having the two ester functions cis on the ringframe work as shown. A photochemicaldisrotatory ring closure shall have these two esterfunctions trans.

Both of these reactions involve conrotatorythermal ring opening of thebenzocyclobutanes, which differ in therelative stereochemistry of the two phenyls. The resultant quinodimetanes are thentrapped by MA in Diels-Alder fashion tofurnish the adducts wherein the relativestereochemistry of the two phenyls in eachcase was easily determined as shown. Theobserved stereochemistries confim to thepredictions one shall make based onconrotatory ring openings.

MA = maleic anhydride

heat

heat

heat

O

H

H

O

XO

O

H

H

O

O

H

H

Although both the reactions are symmetry allowed, thethermal pathway is disfavored simply for thegeometrically difficult trans nature of the ring junction.Similarly, if the ring size of the reactant were seven, the thermal conrotatory cyclization would not take place.

h

N NH

PhO

N NH

O

N NH

PhO

H

HSo, if the requirement is that of trans-3,4-dimethyl-1-cyclobutene, one shall choose to perform the reaction thermally.The reaction ought to be photochemical for the generation of the cis-isomer.

The photochemical forward reaction is fine and we understand it in thelight of what has been discused above. But, if one is to believe in theprinciple of microscopic reversibility, the reverse thermal reaction doesnot seem to fit in place. However, inversion at the ring junction nitrogen to produce a trans fused ring following conrotatory thermal ringopening explains the observation. The inversion at N shall not be sodifficult for its pyramidal nature.

h

h