**19. Enamine catalysis: β-alkylation of enals and enones**

Following the footpaths of iminium catalysis, the twin brothers, namely, the photoredox catalysis and organocatalysis, fruitfully accomplished the enantioselective α-alkylation of aldehydes; the photoredox catalyst employed was 0.5 mol% of Ru(bpy)3Cl2 (bpy = 2,20-bipyridine), and the photocatalyst utilized was 20 mol% of a chiral imidazolidinone. Under the reaction condition, the

**16. Iminium and enamine catalysis in enantioselective photochemical**

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

Chiral iminium ion photochemistry is an emerging synthetic field; with the creativity of synthetic chemistry, they utilized [2+2] photocycloaddition to arrive at complex molecular architecture. In order to execute the [2+2] photocycloaddition, they have first synthesized an alkene tethered to a chiral iminium perchlorate salt, to procure the chiral product; the iminium salt is produced using a C2-symmetric chiral auxiliary. The bench chemists observed that (i) substituents at positions C2 and C5 of the pyrrolidine were crucial, (ii) the reaction process proceeds through single-electron transfer, (iii) the reaction takes place at the singlet hypersurface, (iv) this notable [2+2] cycloaddition reaction takes place via a concerted pathway resulting from the strong π to π\* absorption, and (v) the iminium salt absorbs light at 280 nm. The reward from this developed protocol was 82% ee and 40% chemical

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

*Iminium catalysis: [2+2] photocycloaddition reaction of iminium salts.*

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

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

**reactions**

yield [21] (**Figure 22**).

**Figure 22.**

**Figure 23.**

**146**

*β-Benzylation of enals and enones.*

Michael-type product (**Figure 23**).

**Figure 25.** *Enamine catalysis.*

substrate aldehydic compound reacts with a chiral enamine (organocatalyst) forming a chiral enamine intermediate; thereby the substrate is tuned to be nucleophilic in nature, and the nucleophilicity arises at the α-position to the aldehydic chromophore. Simultaneously, in the reaction medium, the photocatalyst is electronically excited by the visible light; once excited, it accepts a single electron from the chiral enamine, and the Ru [I] species then reduces the α-bromocarbonyl compound, and in this process the photocatalyst is regenerated. At the work-up the coupled product is released from the enamine with high stereoselectivity and in good yields (**Figure 25**).
