Paterno-Buchi reaction photochemical 2+2 cycloaddition NTA CSIR NET GATE SET IIT JAM Adichemistry

@AdiChemistry Aditya Vardhan #paterno_buchi #adichemistry_online_classes
Paternò–Büchi reaction is a a 2+2 photocycloaddition reaction, specifically , which forms four-membered oxetane rings from an excited carbonyl and reacting with an alkene.
This reaction involves 2+2 cycloaddition of a carbonyl compound with an alkene in presence of light to furnish four membered oxetane rings.
An oxetane is four membered ring with one oxygen atom.

A mixture of carbonyl compound like aldehyde or ketone and an olefinic compound is irradiated with light of suitable wavelength, oxetane rings are formed as a result of 2+2 cycloaddition.

Recall that 2+2 cycloadditions are allowed under photochemical conditions and not under thermal conditions.
Now let us go through the mechanism part.

In the carbonyl compound there are two electrons in the nonbonding molecular orbital of the carbonyl group.

When irradiated with light, one of the non-bonding electrons is excited to pi star orbital to give a singlet excited state. Recall that in the singlet state, all the electrons are paired up and the total spin angular momentum is zero.

This singlet state may undergo intersystem crossing to a triplet excited state. Here, you can see there are two electrons with parallel spin and the total spin angular momentum is one.