**5. C**d**C bond formation enhanced by VPLC and alkylation of aldehydes through alkenes as alkylating agents**

The α-alkylation of carbonyl compounds is a routine affair for synthetic chemists both for making substituents and also to synthesize pharmaceutically active ingredients (**Figures 7** and **8**). In the case of α-alkylation of aldehydes, the acidic methylenic (CH2) hydrogen atoms are acidic in nature, and they can be removed; as a result an enol form is produced which directs the alkylating agents attached to the α-alkylation. A skillful execution of three catalytic materials together in a synergistic fashion enables an enantiomeric α-alkylation of aldehydes; mechanistically, the triple catalytic process is sequenced to deliver a hydrogen atom transfer, electron

**Figure 7.** *Intermolecular alkylation with alkenes.*

**Figure 8.** *Intramolecular alkylation.*

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

borrowing tendency, and chirality induction through chiral imidazolidinones or prolinols with a thiophenol where the iridium catalyst transfers the activation of molecules by means of light energy (λ). The α-alkylation is carried out both by inter- and intramolecularly where the alkenes are alkylated at the α-position to the aldehyde functional group to furnish cyclic and acyclic products. The process is atom economical with a stereoselective process, allowing the production of valueadded molecules from feedstock chemicals in a single step while consuming only one photon [10].

The mechanistic pathway is based on the excitation of Ir complex, and simultaneously the chiral reagent adds to the aldehyde compound through elimination of water and forms an enamine. The excited iridium complex oxidizes the enamine present in the reaction medium through a single-electron transfer mechanism; thus formed enaminyl radical adds to the alkene substrate producing a carbon radical which is finally trapped by the hydrogen atom transfer catalyst. During the workup procedure, the iminium ion is hydrolyzed to get the enantiomerically enriched product, and the organocatalyst is regenerated. Finally the reduction of the thiyl radical by the Ir(I) species regenerates the thiol catalyst as well as the Ir(III) catalyst to complete the redox cycle.

With the success in the α-alkylation protocol, its intramolecular version also achieved where an intramolecular cyclization with tethered alkenes was first attempted to determine the feasibility of enantioselective ring formation reaction. Interestingly, carbocycles and heterocycles were synthesized with high yield and enantiocontrol. Tosamide- or carbamate-protected N-tethered aldehydic alkenes gave rise to the corresponding piperidines, ether-linked systems provided transsubstituted tetrahydropyrans, and carbocycles were also attained. Pyrrolidines were also formed as well as seven-membered rings such as azepanes or cycloheptanes. High stereocontrol was obtained with trisubstituted alkenes, and where multiple alkenes were available, only proximal alkenes reacted to provide the corresponding cyclic molecule.

Following this successful reaction, intermolecular reactions with styrene was attempted. A variety of substituted aldehydes provided the alkylated products in high yields and selectivity. Terminal alkenes were suitable substrates though 1,1-disubstituted alkenes reacted with moderate efficiency.
