**17. Iminium catalysis: β-benzylation of enals and enones**

The chemistry of ortho-quinodimethanes is very well exploited for the generation of six-membered carbocyclic frameworks by reacting with a diene and through an inter- or intramolecular [4+2] cycloaddition. Under VLPC condition, reports indicate that the photoexcitation of ortho-alkyl-substituted benzaldehydes and benzophenones generates the ortho-quinodimethanes, a diene intermediate, whereas instead of undergoing the [4+2] cycloaddition to yield the carbocyclic product, it gave solely the β-benzylated products through a Michael-type addition reaction. The secondary amine employed in this reaction is solely responsible for the Michael-type product (**Figure 23**).

The reaction mechanism is very interesting for the academic enthusiastic personalities; the chiral secondary amines react with the α,β-unsaturated compound that is used in the reaction medium. When the reaction mixture was irradiated with λ = 365 nm, the photons enolize the ortho-alkyl-substituted benzaldehyde or

*Visible-Light Photocatalysis of Aldehyde and Carbonyl Functionalities, an Innovative Domain DOI: http://dx.doi.org/10.5772/intechopen.92372*

benzophenone to (E)-enol and then the iminium salts in close proximity with the other reactants to deliver the Michael-type addition products rather than [4+2] cycloadducts. A density functional theory (DFT) computational study was carried out by the authors to shed some light on this unusual reactivity, and the results indicated that this transformation proceeds through a water-assisted proton shuttle mechanism. The optical yields are excellent, and it is worth mentioning that no photocatalyst was needed in this reaction.
