**5.1 Rose Bengal and 9-Fluorenone catalyzed C-N bond formation**

Wei *et al*. in 2018 reported a new and facile visible-light-mediated synthesis of αazido ketones (**48**) via oxyazidation of alkenes (**47**) with TMSN3 in the air at room temperature (**Figure 28**) [32]. Rose Bengal is a metal-free photocatalyst, used for the synthesis of α-azido ketones. This difunctionalized products are easily and efficiently obtained in moderate to excellent yields via the formation of C-N and C=O bonds. The proposed mechanism is shown in (**Figure 28**). At first, visible-light irradiation of Rose Bengal generated the excited RB\*. Subsequently, a single electron transfer (SET) process takes place between TMSN3 and RB\* to produce an azido radical and RB• radical anions. Then, the ground state Rose Bengal and O2 • is formed through the oxidation of RB• by molecular oxygen (air). Furthermore, the attack of azido radical to alkene (**47**) gives the alkyl radical intermediate (A). Next, the interaction between radical intermediate (A) with O2 • and H2O afforded hydroperoxide intermediate (B). The oxidation of (PhSe)2 by hydroperoxide (B) yielded organoselenium intermediate (C) and PhSeOH. Finally, the additions of PhSeOH with reactive intermediate (C) produced the product (**48**) and regenerated (PhSe)2 and elimination of water takes place.

Wang *et al*. in 2019 depicted a visible-light-induce intermolecular azidohydrazination method for the synthesis of β-azido alkyl hydrazines (**50**) from unactivated alkenes (**49**) (**Figure 29**) [33]. This transformation occurs via metalfree and redox neutral conditions and applies to a wide range of alkenes. The βazido alkyl hydrazines are used for the preparations of many valuable synthetic building blocks. As per mechanism (**Figure 29**) initially, photo-excited fluorenone generates a N3 radical via the catalytic oxidation of azide source with the formation of a reductive ketyl radical species (B). Then, the azide radical attacks the alkene to

**Figure 28.** *Synthesis of azido ketones.*

**Figure 30.** *Synthesis of unsymmetrical azo compounds.*

give the alkyl radical intermediate (C). After that, the alkyl radical (C) is trapped by azodicarboxylate giving an *N*-centered radical intermediate (D) which upon the abstraction of a proton give the final product (**50**) (path I). On the other side, the intermediate (D) is reduced by H-N3 to regenerate an azide radical and the cycles continue (path II).

### **5.2 Mes-catalyzed C-N bond formation**

Shen *et al*. in 2021 reported a photo-induced multi-component cascade reaction in the presence of aryldiazonium salts with unactivated alkenes (**51**) and trimethylsilyl azide (TMSN3) under oxidant-free conditions (**Figure 30**) [34]. This protocol provides a new synthetic method for unsymmetrical azo compounds and applies to different aryldiazonium salts and alkenes. According to the proposed mechanism (**Figure 30**) initially, in the presence of visible light, the photocatalyst 'Mes' undergoes an excited state (\*Mes) and take parts in a single electron transfer (SET) process with TMSN3 to generate the azido radical and 'Mes' radical anion. Subsequently, the azido radical attacked the alkene (**51**) to produce an alkyl radical intermediate (A), which is trapped by the aryldiazonium salt to generate a radical cation intermediate (B). Finally, another SET process between the radical cation (B) and the 'Mes' radical anion provide the product (**52**) with simultaneous regeneration of the photocatalyst 'Mes'.

Yang and Lu in 2017 established a suitable method for the formation of hydroxyazidation derivative (**54**) from the reaction of alkenes (**53**) under visible light photo redox catalysis (**Figure 31**) [35]. The important features of the reaction are low catalyst loading, room temperature, broad substrate scope. Readily available starting materials, such as alkenes and air, to construct valuable β-azido alcohols.

Wang *et al.* in 2020, demonstrated a metal-free method for the synthesis of β-trifluoromethyl hydrazines (**56**) by reacting alkene (**55**) with sodium trifluoromethanesulfonate as the CF3 source. (**Figure 32**) [36]. This methodology enabled a radical cascade that incorporates a trifluoromethyl and a hydrazine group across the C=C double bond.

Moon *et al*. in 2020 demonstrated an atom-economical visible-light-mediated synthesis of aminopyridylationproduct (**56**) in the presence of alkenes (**55**) and *N*-aminopyridinium ylides (**Figure 33**) [37]. This environmentally friendly method

**Figure 31.** *Synthesis of Hydroxyazidation derivative.*

**Figure 32.** *Synthesis of β-Trifluoromethyl Hydrazines.*

**Figure 33.** *Synthesis of Aminopyridylation product.*

applies to a wide range of substrates with good functional group tolerance. Both activated, unactivated alkenes and pyridine are smoothly reacted and gave their desired products in moderate to good yields at room temperature.

#### **5.3 Eosin-Y-catalyzed C-N bond formation**

Moon *et al*. in 2019 demonstrated an Eosin Y mediated photocatalytic strategy for the synthesis of aminoethyl pyridine derivatives (**60**) in the presence of alkenes (**59**) using a variety of *N*-aminopyridinium salts as both aminating and pyridylating agents (**Figure 34**) [38]. Here concomitant incorporations of amino and pyridyl groups take place into alkenes under mild reaction conditions. In this protocol alkene bearing both electron-withdrawing and electron-donating groups are well tolerated. According to the possible mechanism (**Figure 34**) initially, in the presence of LEDs photocatalyst EY excited to the EY\*, which subsequently form the unstable *N*-pyridine radical (A) via a single electron transfer (SET). Then the homolytic cleavage of the N–N bond takes place and generate the activated *N*centered radical intermediate (B) and release the pyridine. The N-centered radical intermediate (B), attack the alkene and generate the intermediate (C). After that radical additions take place and produce the intermediate (D). Finally, the product (**60**) is obtained via deprotonations and the radical extrusion process.

Alam *et al*. in 2020 developed an elegant visible-light-mediated synthesis of *N*hydroxybenzimidoyl cyanides from aromatic terminal alkenes using Eosin Y as a metal-free photocatalyst (**Figure 35**) [39]. DFT calculation supports a biradical pathway with successive incorporation of two nitrogen atoms, one each from *tert*butyl nitrite (TBN) and ammonium acetate. The difunctionalization product is accomplished by the concomitant installation of an oxime and a nitrile group.

As determined from the DFT calculation and few control experiments a plausible mechanism has been proposed (**Figure 35**). In the influence of visible light Eosin Y (EY) undergoes excitation and generates a PINO radical from NHPI via hydrogen atom transfer (HAT) and returns to the ground state. The PINO radical adds to the

**Figure 34.** *Synthesis of Aminoethyl pyridine derivatives.*

*Construction of C-N Bond* via *Visible-Light-Mediated Difunctionalization of Alkenes DOI: http://dx.doi.org/10.5772/intechopen.98949*

**Figure 35.** *Synthesis of* N*-Hydroxybenzimidoyl cyanides.*

alkene (**61**) to give a benzylic radical intermediate (A) which trap the NO radical originating from TBN to give a nitroso intermediate (B) and is tautomerized to an oxime intermediate (C). Subsequent abstraction of two H atoms from the oxime intermediate (C) by in situ *tert*-butoxyl radical give a 1,4-biradical intermediate (D) which upon intramolecular coupling generates a four-membered cyclic intermediate (E). The strained cyclic intermediate (E) undergoes ring-opening via attack of an OH radical to form a hemiacetal radical intermediate (F). The N O radical intermediate (F) abstracts a proton from *tert*-butanol to generate a neutral hemiacetal intermediate (G). The neutral hemiacetal intermediate loses NHPI providing an oxime aldehyde (H). Condensation between ammonia (generated from ammonium acetate) and the aldehydic intermediate (H) form an iminium intermediate (I). Abstraction of an iminium N H from the intermediate (I) by the <sup>t</sup> BuO radical produce a nitrogen-centered radical (J). Finally, the abstraction of the aldehydic proton from intermediate (J) by *tert*-butoxy radical provided the cyano functionalized product (**62**).

## **6. Conclusion**

In summary, this chapter focus on the recent advancements in visible-lightmediated transition-metal and organic dye catalyzed difunctionalization of alkene leading to the formation of C-N bond. The utilization of visible light by photo catalysis is a burgeoning field in contemporary organic synthesis The ubiquitous nature of the C-N bond predominates the synthetic chemist community. In this regard visible-light-mediated difunctionalization of alkene reactions have emerged as an efficient strategy for the synthesis of functionalized molecules, giving a high atom economy. Organic dye mediated C-N bond formations is even more promising compared to metal-catalyzed C-N bond formation because they overcome the

## *Alkenes - Recent Advances, New Perspectives and Applications*

drawbacks associated with transition metals that limits their use in pharmaceutical industries. With the current momentum of development, a greater impact of photocatalytic C–N bond-formations reactions is foreseeable, for example, in the late-stage modifications of natural products, large-scale syntheses and enantioselective C–N bond formations reactions Visible-light-mediated reactions provide a greener and sustainable approach and mild reaction condition towards the construction of complex molecules. Further developments in this area may open up broad opportunities for straightforward, efficient, and atom economical synthesis of N-compounds from simple alkenes.
