**1. Introduction**

The periodic table comes to the mind when thinking of elements in chemistry, while organic chemistry brings to mind substances such as alcohol, aldehydes, ketones, aromatics, and other compounds based on the functional groups. Aldehydes, ketones, and carbonyl moieties are the most popular and routinely exploited functional groups in synthetic organic chemistry and in the design of organic synthesis since they render the desired synthetic manipulation and spring an easy access to complex molecular architecture. Not only in modern times but also from the times of alchemy, formaldehyde is very well utilized for embalming and preserving dead animal species by biologists; consequently, aldehyde class of compounds ranks to be the parent compounds for many other classes of compounds. Acetyl coenzyme derived from aldehyde functional group or acetaldehyde moiety is responsible for the wide variety of natural products through biosynthesis, while toward the syntheses and manufacture of chemicals on the laboratory and in industries, also the aldehyde functional group is very well synthetically manipulated. They were not only converted into structurally complex compounds through enzymes, catalysts, and thermal process, but also photons convert aldehydes into other molecular architectures by means of eminent photochemical-chemical reactions such as Norrish-type photolysis, cyclobutanol formation through Yang reaction, and [2+2] cycloaddition with alkene (Paternó-Büchi reaction). Apart from conventional catalytic way, traditional synthesis uses name reactions and photochemistry; of late, visible light is being used [1]. Apparently, solar energy is a benign, benevolent, and renewable source of energy. Visible light emerging from the source of sun promotes chemical transformations through single-electron mechanism. Basically using visible light as energy source and in the presence of catalytic amount of metal photosensitizers or organocatalysts, the chemical reactions are carried out, and this process is termed as visible-light photocatalysis and abbreviated as VLPC [2, 3]. This opens a new chapter in the textbook of organic synthesis [4]. Photosensitizers are special molecules which support these lightinduced molecular transformations by electron or energy transfer using its abundant light absorbance and redox property [5]. Aldehydes are subjected to VLPC conditions either protected as acetals or directly during the course of a reaction [6]. Further transformations such as oxidation to COOH are also essential reaction of aldehydes [7]. Aldol condensations and enamines are further variations in their reactions as building blocks in organic synthesis [4]. Thus, the application of aldehydes as building blocks is now elaborated with their VLPC reactions adding to its reaction repertoire. In this chapter we will discuss on the recent developments on VLPC of aldehydes.

presented; the discussion revolves around the recent developments on the chemis-

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

two distinct catalytic cycles, namely, the reductive quenching (RQC) and the oxi-

oxidant into a radical anion species and converts itself into the ground-state cata-

oped due to their superior photophysical and photochemical properties. These photocatalysts are chemically robust and possess long-lived excited states. Their favorable redox properties allow redox-neutral reactions to be carried out as both reductants and oxidants that can be transiently generated during different stages in the catalytic process. This reactivity pattern thus is beneficial allowing exploration of

2+. Based on these viewpoints, cyclometallated Ir complexes have been rapidly devel-

(bpy)3

RuII(bpy)3)

*2+, fac-[Ir(ppy)3], [Ir(ppy)2(dtbbpy)]+*

The polypyridyl complexes of Ru and Ir afford unique chemical reactivities due to their long-lived excited states when excited by visible light [5]. They are chemically robust and possess redox properties that are further fine-tunable by modifying the polypyridyl ancillary ligands. The Ru(bpy)3Cl2 is a widely known and commonly used photoredox molecule. The absorption of visible light leads to excited states that can function both as oxidants and reductants, which allows the generation of radical cations or radical anions under mild conditions. The amphoteric

2+, (\*

RuII(bpy)3)

RuII(bpy)3)

<sup>+</sup> which subsequently reduces an

2+ first oxidizes a reductant

*. Properties of*

2+ to RuIII(bpy)3

2+, enables

3+

try of aldehydes in the domain of VLPC (**Figures 1** and **2**).

redox reactivity of the excited triplet state of RuII(bpy)3

lyst. OQC starts with the oxidation of the complex (\*

*2+, [Ru(bpz)3]*

*Ru redox cycle: A—sacrificial electron acceptor; D—sacrificial electron donor; S—substrate;*

*-bipyridine; MLCT (metal to ligand charge transfer)* � *λ = 452 nm.*

*2+ photocatalyst—[MLCT* � *<sup>λ</sup> = 452 nm].*

dative quenching cycles (OQC). In RQC, (\*

into a radical cation and reduces into RuI

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

followed by its reduction into RuII(bpy)3

**Figure 1.**

*[Ru(bpy)3]*

**Figure 2.**

*bpy—2,2*<sup>0</sup>

**133**

*Photosensitizers: [Ru(bpy)3]*

VLPC is advantageous over the conventional catalysis since it employs the clean, renewable, and readily available visible light from our sun and this state-of-the-art protocol is convenient in its operation. Bench chemists are fascinated by VLPC due to the ease of recycling the heterogeneous catalytic material by simple filtration and because reactions are carried at ambient temperature and the work-up procedure is quite simple. Eventually, this field and phenomenon of synthetic organic chemistry have emerged as an innovative subdiscipline over the last decade; the scientists have made a step forward by carrying out the asymmetric induction [8]; with the advancement in modern analytical tools and the hard work of enthusiastic chemists, VLPC of aldehydes is emerging exponentially.

In the visible-light photocatalysis, the catalytic species is activated by the action of light, and the photocatalytic material is mostly a semiconducting material which in turn is capable of activating even the small molecules [5]. When the catalytic material is irradiated with light, it undergoes the absorption of photon, and the electron (e) is excited from the valence bond to the conduction band; consequently, a positive electron hole is generated in the semiconducting material (h+ ), and this process is termed as *photoexcitation*. The excited electron then comes to the ground state through the mechanisms such as recombination and dissipates by means of non-radiative mechanism; in a sense, following the photoexcitation process, the catalytic material excited transfers the energy to the molecules in its close proximity through an orthodox redox mechanism in a pure chemistry sense, and the single-electron transfer (SET) occurs. For brevity, the mechanism is provided succinctly; in a nutshell, light source excites the catalytic material and transfers the energy to other molecules close by, and the chemical reaction occurs by means of electron transfer mechanism. This new discipline opened a new science of photophysics and photochemistry of transition metal coordination compounds. In this chapter a discussion of visible-light photocatalysis of aldehyde compounds is

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

presented; the discussion revolves around the recent developments on the chemistry of aldehydes in the domain of VLPC (**Figures 1** and **2**).

The polypyridyl complexes of Ru and Ir afford unique chemical reactivities due to their long-lived excited states when excited by visible light [5]. They are chemically robust and possess redox properties that are further fine-tunable by modifying the polypyridyl ancillary ligands. The Ru(bpy)3Cl2 is a widely known and commonly used photoredox molecule. The absorption of visible light leads to excited states that can function both as oxidants and reductants, which allows the generation of radical cations or radical anions under mild conditions. The amphoteric redox reactivity of the excited triplet state of RuII(bpy)3 2+, (\* RuII(bpy)3) 2+, enables two distinct catalytic cycles, namely, the reductive quenching (RQC) and the oxidative quenching cycles (OQC). In RQC, (\* RuII(bpy)3) 2+ first oxidizes a reductant into a radical cation and reduces into RuI (bpy)3 <sup>+</sup> which subsequently reduces an oxidant into a radical anion species and converts itself into the ground-state catalyst. OQC starts with the oxidation of the complex (\* RuII(bpy)3) 2+ to RuIII(bpy)3 3+ followed by its reduction into RuII(bpy)3 2+.

Based on these viewpoints, cyclometallated Ir complexes have been rapidly developed due to their superior photophysical and photochemical properties. These photocatalysts are chemically robust and possess long-lived excited states. Their favorable redox properties allow redox-neutral reactions to be carried out as both reductants and oxidants that can be transiently generated during different stages in the catalytic process. This reactivity pattern thus is beneficial allowing exploration of

#### **Figure 1.**

responsible for the wide variety of natural products through biosynthesis, while toward the syntheses and manufacture of chemicals on the laboratory and in industries, also the aldehyde functional group is very well synthetically manipulated. They were not only converted into structurally complex compounds through enzymes, catalysts, and thermal process, but also photons convert aldehydes into other molecular architectures by means of eminent photochemical-chemical reactions such as Norrish-type photolysis, cyclobutanol formation through Yang reaction, and [2+2] cycloaddition with alkene (Paternó-Büchi reaction). Apart from conventional catalytic way, traditional synthesis uses name reactions and photochemistry; of late, visible light is being used [1]. Apparently, solar energy is a benign, benevolent, and renewable source of energy. Visible light emerging from the source of sun promotes chemical transformations through single-electron mechanism. Basically using visible light as energy source and in the presence of catalytic amount of metal photosensitizers or organocatalysts, the chemical reactions are carried out, and this process is termed as visible-light photocatalysis and abbreviated as VLPC [2, 3]. This opens a new chapter in the textbook of organic synthesis [4]. Photosensitizers are special molecules which support these lightinduced molecular transformations by electron or energy transfer using its abundant light absorbance and redox property [5]. Aldehydes are subjected to VLPC conditions either protected as acetals or directly during the course of a reaction [6]. Further transformations such as oxidation to COOH are also essential reaction of aldehydes [7]. Aldol condensations and enamines are further variations in their reactions as building blocks in organic synthesis [4]. Thus, the application of aldehydes as building blocks is now elaborated with their VLPC reactions adding to its reaction repertoire. In this chapter we will discuss on the recent developments on

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

VLPC is advantageous over the conventional catalysis since it employs the clean, renewable, and readily available visible light from our sun and this state-of-the-art protocol is convenient in its operation. Bench chemists are fascinated by VLPC due to the ease of recycling the heterogeneous catalytic material by simple filtration and because reactions are carried at ambient temperature and the work-up procedure is quite simple. Eventually, this field and phenomenon of synthetic organic chemistry have emerged as an innovative subdiscipline over the last decade; the scientists have made a step forward by carrying out the asymmetric induction [8]; with the advancement in modern analytical tools and the hard work of enthusiastic chemists,

In the visible-light photocatalysis, the catalytic species is activated by the action of light, and the photocatalytic material is mostly a semiconducting material which in turn is capable of activating even the small molecules [5]. When the catalytic material is irradiated with light, it undergoes the absorption of photon, and the electron (e) is excited from the valence bond to the conduction band; consequently, a positive electron hole is generated in the semiconducting material (h+

and this process is termed as *photoexcitation*. The excited electron then comes to the ground state through the mechanisms such as recombination and dissipates by means of non-radiative mechanism; in a sense, following the photoexcitation process, the catalytic material excited transfers the energy to the molecules in its close proximity through an orthodox redox mechanism in a pure chemistry sense, and the single-electron transfer (SET) occurs. For brevity, the mechanism is provided succinctly; in a nutshell, light source excites the catalytic material and transfers the energy to other molecules close by, and the chemical reaction occurs by means of electron transfer mechanism. This new discipline opened a new science of

photophysics and photochemistry of transition metal coordination compounds. In this chapter a discussion of visible-light photocatalysis of aldehyde compounds is

),

VLPC of aldehydes.

**132**

VLPC of aldehydes is emerging exponentially.

*Photosensitizers: [Ru(bpy)3] 2+, [Ru(bpz)3] 2+, fac-[Ir(ppy)3], [Ir(ppy)2(dtbbpy)]+ . Properties of [Ru(bpy)3] 2+ photocatalyst—[MLCT* � *<sup>λ</sup> = 452 nm].*

#### **Figure 2.**

*Ru redox cycle: A—sacrificial electron acceptor; D—sacrificial electron donor; S—substrate; bpy—2,2*<sup>0</sup> *-bipyridine; MLCT (metal to ligand charge transfer)* � *λ = 452 nm.*

alternate reaction pathways under benign reaction conditions. They have thus been used as photoredox catalysts and serve as photosensitizers in organic synthesis [5].

**4. Direct C**d**H arylation and alkylation of aldehydes**

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

excellent [9] (**Figure 5**).

*Direct C*d*H arylation and alkylation of aldehydes [9].*

**Figure 5.**

**Figure 6.**

**135**

*Synthesis of haloperidol.*

Direct and catalytic CdH activation or functionalization comprising of arylation, alkenylation, alkylation, allylation, and annulation reaction is an important field in the synthetic organic chemistry in the manufacture and the process development of pharmacologically and biologically active ingredients. Knowing the importance of CdH activation, direct arylation of aldehydes has been achieved in a synergistic manner, where nickel catalyst was employed in combination with VLPC system. In this outstanding redox system, a hydrogen atom transfer (HAT) was achieved on the reactions in between commercially available aldehydes and aryl and alkyl bromides under milder conditions; it is interesting to note that the yields are

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

The mechanism is based on the photoexcitation of the Ir photocatalyst which gives rise to the highly oxidizing species Ir\* Ir[dF(CF3)ppy]2(dtbbpy) which oxidizes quinuclidine to form a cation radical. This radical cation then engages in a HAT event with any aldehyde to generate the acyl radical. Simultaneously oxidative addition of aryl bromide to LnNi (0) generates the aryl-Ni (II) species which is intercepted by the acyl radical to form the acyl-Ni complex. Both the Ni and the Ir photoredox catalysts then turn over in a critical reductive elimination step to the desired ketone product while regenerating the Ir and Ni catalysts. It is interesting to note that using this protocol, a pharmacologically active ingredient, namely, halo-

A two-step synthesis of haloperidol was achieved by this photoredox methodol-

ogy. 1,4-Chlorobutanal was merged with 1-bromo-4-fluorobenzene using the

peridol, a typical antipsychotic drug, was synthesized [9] (**Figure 6**).
