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

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

*Visible-Light Photocatalysis of Aldehyde and Carbonyl Functionalities, an Innovative Domain*

photocatalyst was needed in this reaction.

*DOI: http://dx.doi.org/10.5772/intechopen.92372*

chiral amine is recovered [22] (**Figure 24**).

*Iminium catalysis: β-alkylation of enals and enones.*

**Figure 24.**

**147**

**18. Iminium catalysis: β-alkylation of enals and enones**

reduced to enamine. A tautomerism reaction, that is, enamine-imine

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

photoredox catalysis and organocatalysis, fruitfully accomplished the

Following the footpaths of iminium catalysis, the twin brothers, namely, 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

tautomerization, regenerates photocatalyst TBADT, and finally the work procedure gives rise to the product with excellent enantio- and diastereoselectivities, and the

A major breakthrough in the field of asymmetric radical chemistry was represented by Melchiorre group; they achieved the first enantioselective radical conjugate addition (RCA) to β,β-disubstituted cyclic enones by a combination of photoredox catalysis and iminium-based organocatalysis. The organocatalyst and the primary amine moiety react with a α,β-unsaturated enone forming a chiral iminium ion as the reactive intermediate; the 1,3-benzodioxole present in the reaction medium upon irradiation by means of an UV light-emitting diode and in the presence of tetrabutylammonium decatungstate (TBADT) generates a carboncentered radical. Thus generated carbon-centered radical being short-lived and active, it reacts with the chiral iminium ion producing an α-iminyl radical cation. Then through an intramolecular SET process, the α-iminyl radical cation was

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

**Figure 22.** *Iminium catalysis: [2+2] photocycloaddition reaction of iminium salts.*
