**20. Enamine catalysis: α-benzylation of aldehydes and ketones, α-hydroxylation, β-arylation**

α-Benzylation and α-alkylation are thematically one and the same; however, much recent advances in α-benzylation of carbonyl compounds were reported with stereoselectivity (**Figure 26**).

A variety of electron-deficient aryl and heteroaryl methylene bromides were examined as the benzylating agents, and they were coupled with a range of aldehydes bearing different functional groups efficiently with excellent enantioselectivity. The benzylation reaction proceeds via an oxidative quenching cycle, in contrast to the reductive quenching cycle operation in the α-alkylation reaction. The hybrid organocatalytic cycle and photoredox catalytic cycle are similar to the reaction of aldehydes with alkyl halides described in **Figure 11**. The Ir photocatalyst fac-Ir(ppy)3 and imidazolidinone organocatalyst generate the benzyl radical from electron-deficient benzyl halides. This benzyl radical then couples with the chiral enamine providing α-amino radical which is oxidized by the intermediate Ir(IV) species. Hydrolysis of the iminium ion releases the α-benzyl aldehyde. A range of electron-deficient heteroaromatics such as pyridines, pyrazines, pyrimidines, quinolines, and benzimidazoles undergo facile reaction (**Figure 27**).

**21. Enamine catalysis: α-hydroxylation of aldehydes and ketones**

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

*A catalytic cycle—enantioselective benzylation of aldehydes.*

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

**Figure 27.**

**Figure 28.**

**149**

*Enamine hydroxylation.*

The α-hydroxylation of carbonyl compounds is a very important class of reaction in the design of drug molecules; mostly the hydroxylation is appended in a stereoselective fashion. Conveniently it is achieved by enolizing the carbonyl, and the oxidation is done by oxidizing agents such as epoxides, OsO4, and so on. Rarely the singlet oxygen is used for this functional group introduction. In this VLPC condition, the hardships related to the α-hydroxylation reactions are tackled with ease; an amine-catalyzed enantioselective α-hydroxylation of aldehydes under photochemical condition was achieved where (L)-α-Me proline-based organocatalyst was exploited and singlet oxygen is employed instead of explosive oxidizing agents. Mechanistically the amino acid-based organocatalyst activated the aldehyde, and

**Figure 26.** *Enamine benzylation.*

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

**Figure 27.** *A catalytic cycle—enantioselective benzylation of aldehydes.*
