**3. Cooperative NHC catalysis with photocatalysts**

The combined use of NHC and photocatalyst has gained increasing attention as novel redox catalysis. The compatibility of NHC with ruthenium photocatalyst was demonstrated (**Figure 7**) [37]. Acylation of *N*-phenyltetrahydroisoquinoline **34** with butanal **33** was promoted by using chiral NHC, generated from precursor **(5a***S***,10b***R***)-A9** and photocatalyst Ru(bpy)3Cl2 in the presence of *m*-dinitrobenzene *Recent Advances in Cooperative N-Heterocyclic Carbene Catalysis DOI: http://dx.doi.org/10.5772/intechopen.101328*

**Figure 7.**

*Compatibility of NHC with ruthenium photoredox catalyst.*

as an oxidant. This transformation proceeds *via* the generation of Breslow intermediate, which undergoes the addition to iminium intermediate generated by the photocatalytic oxidation of **34**. Finally, the release of free NHC catalyst results in the formation of acylated product **35**. The decarboxylative carbonylation reaction also proceeded *via* a similar mechanism [38].

The cooperative catalysis was applied to the oxidative transformation of aldehydes (**Figure 8**) [39–42]. The oxidative esterification of cinnamaldehyde **2** was achieved by the dual organocatalysis based on the cooperation between NHC and rhodamine 6G as an organophotocatalyst [39]. In this reaction, Breslow intermediate is photocatalytically oxidized to acyl azolium *via* the radical intermediate. The subsequent reaction of acyl azolium with MeOH gives ester **36**. Furthermore, the alkylation and esterification reaction of γ-oxidized enal **37** was developed [40]. When racemic precursor **A1** and photocatalyst Ru(bpy)3(PF6)2 were employed, the reaction of γ-oxidized enal **37** with iodoacetonitrile **38** and MeOH gave γ-alkylated ester **39** in 86% yield. In this Ru-photocatalysis, iodoacetonitrile **38** acts as not only a radical source but also an oxidant. The oxidative Smiles rearrangement was also reported [42]. Under the cooperative catalysis conditions using NHC and 9-mesityl-10-methyl-acridin-10-ium as an organophotocatalyst, the oxidative Smiles rearrangement of *O*-aryl salicylaldehyde **40** proceeded effectively to give the aryl salicylate **41** in 79% yield. Initially, the photocatalytic oxidation of Breslow intermediate, generated from salicylaldehyde **40** to acyl azolium leads to the generation of the acid intermediate *via* the subsequent reaction of acyl azolium with H2O. The subsequent oxidation of acid intermediate by photocatalysis promotes Smiles rearrangement to give another radical *via* the spirocyclic intermediate. Finally, the photocatalytic reduction of this radical gives the aryl salicylate **41**.

The cooperative catalysis for preparing ketones from carboxylic acid derivatives was studied (**Figure 9**) [43, 44]. The synthesis of ketone **44** was achieved by the combined NHC and Ir-photoredox catalysis of acyl imidazole **42** with benzyl Hantzsch ester **43** as a benzyl radical source [43]. In the presence of precursor **A12** (15 mol%), photocatalyst [Ir(dFCF3ppy)2(dtbpy)]PF6 (1 mol%), and Cs2CO3, the reaction between imidazole **42** and Hantzsch ester **43** led to the formation of ketone **44** in 79% yield under the LED irradiation. In this catalysis, the iridiumphotocatalyzed one-electron reduction of acyl azolium, generated from NHC and acyl imidazole **42**, affords a radical intermediate. This radical undergoes the subsequent radical-radical coupling with a benzyl radical generated by the iridiumphotocatalyzed one-electron oxidation of benzyl Hantzsch ester **43**. The cooperative triple catalysis using NHC catalyst, Ru-photocatalyst, and sulfinate catalyst was

#### **Figure 8.**

*Cooperative catalysis via oxidation of Breslow intermediates.*

developed [44]. When precursor **A13** (15 mol%), Ru(bpy)3(PF6)2 (1.5 mol%), and 4-Cl-PhSO2Na (25 mol%) were employed, the acylation of 4-methylstyrene **46** with benzoyl fluoride **45** was promoted under the CFL irradiation to give ketone **47** in 78% yield. This triple catalysis involves photocatalysis, NHC catalysis, and sulfinate catalysis. The NHC catalysis gives a ketyl radical *via* the photocatalytic reduction of acyl azolium, generated from benzoyl fluoride **45** and NHC. In the sulfinate catalysis, the photocatalytic oxidation of 4-Cl-PhSO2Na affords sulfonyl radical, which adds to 4-methylstyrene **46** leading to the adduct radical. Finally, radical/ radical cross-coupling between these two radicals leads to the acylated product **47**.

*Recent Advances in Cooperative N-Heterocyclic Carbene Catalysis DOI: http://dx.doi.org/10.5772/intechopen.101328*

#### **Figure 9.**

*Cooperative catalysis for preparing ketones.*

In addition to the cooperative NHC catalysis with photocatalysts, the combined use of NHC catalysis and photoredox reaction in the absence of a photocatalyst has gained increasing attention as novel catalysis [45–47].

## **4. Conclusions**

The N-heterocyclic carbenes (NHCs) are powerful and versatile organocatalysts that induce synthetically valuable chemical transformations. In addition to the cooperative catalysis using NHC/Lewis acid, NHC/Brønsted acid, and NHC/ hydrogen-bonding organocatalyst, the cooperative NHC catalysis combined with transition-metal catalysts are emerging continuously. In the last few years, the combined use of NHC and photocatalyst has gained increasing attention as dual redox catalysis. The recent dramatic progress in NHC-induced cooperative catalysis disclosed a broader aspect of the utility of NHC-organocatalysis for synthetic organic chemistry. This chapter will inspire creative new contributions to organic chemists.

#### **Acknowledgements**

Our work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) Grant Number 16K08188.

*Carbene*
