The photochemical [2+2] cycloaddition of a carbonyl with an olefin to give an oxetane.
Mechanism of the Paterno-Buechi Reaction
The possible transitions (C=O) are shown below:
Once the carbonyl ground state has been photoexcited, either a singlet or triplet state may be formed:
Either type of transition (n,π* and π,π*) and electronic state (singlet, triplet) may participate in the first stage of this reaction, which is rationalized by invoking diradical intermediates:
Breaking of the new σ-bonds requires more energy, and the reverse reaction is not possible using same light frequency.