Abstract
Although dyes have received much attention as the visible light-activated photocatalysts, the use of metal-free organic dyes in synthetic organic chemistry is still limited. This chapter summarizes the recent progress in the visible light photocatalysis promoted by metal-free organic dyes. Eosin Y is the typical organic dyes to induce the photoredox catalysis. Recently, other organic dyes such as Rose Bengal, fluorescein, and methylene blue have been studied as photocatalysts to promote the single-electron transfer processes.
Keywords
- photocatalyst
- organocatalyst
- dye
- catalysis
- visible light
- radical
1. Introduction
The use of abundant sunlight as a clean source of energy is an important aim of green chemistry. In recent years, dyes have attracted a great deal of attention as the visible light-activated photocatalysts in synthetic organic chemistry. However, these studies have mainly concentrated on the redox transformations using transition metal dyes such as ruthenium or iridium photocatalysts [1, 2, 3, 4, 5, 6, 7, 8, 9]. In contrast, the use of metal-free dyes still remains rather underdeveloped, although organic dyes are more environmentally friendly and cheaper. Eosin Y is the typical organic dyes to induce the photoredox catalysis [10]. Recently, Rose Bengal, fluorescein, methylene blue, and other organic dyes have been studied as photocatalysts to promote the single-electron transfer processes [11, 12, 13]. Additionally, 3-cyano-1-methylquinolinium, 9-mesityl-10-methylacridinium ion, and acridinium salts were developed as organic photocatalysts [12, 13].
The photoredox cycle is initiated by the visible light irradiation of dye in the ground state to produce the high-energy excited state of dye (Dye*) (Figure 1). Two distinctive pathways from dye in the excited state (Dye*) are described for the mechanism of visible light photoredox catalysis. The reductive property of Dye* can be used in the presence of a sacrificial electron acceptor. In other words, Dye* serves as an electron donor leading the radical cation species of Dye. In contrast, Dye* also acts as an electron acceptor in the presence of a sacrificial electron donor.
2. Eosin Y and eosin B
Eosin Y (EY) is the typical organic dye to induce the synthetically useful photoredox transformations [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]. EY that absorbed visible light populates in the lowest excited singlet state. The subsequent spin-forbidden singlet-triplet intersystem crossing affords EY in the excited triplet state. A variety of photoredox transformations are induced by the single-electron transfer from EY in the excited triplet state. In 2014, König provides the review article concerning the utility of EY as a photocatalyst in synthetic organic chemistry [10]. Therefore, this section highlights the recent remarkable progress in the EY-catalyzed photoredox transformations.
The EY-catalyzed generation of aryl radicals from aryl diazonium salts was studied by König’s group [14]. The direct C−H bond arylation of heteroarenes with aryl diazonium salts was achieved by employing only 1 mol% of EY (Figure 2). The arylation of furan with diazonium salt
The vinyl sulfones were synthesized by the EY-catalyzed reaction of alkenes with sodium aryl sulfinates [21]. The reaction of 1,2-dihydronaphthalene
The oxidative cyclization reaction between 3-phenylpropiolate
The EY-catalyzed cyclization of 2-isocyanobiphenyls with arylsulfonyl chlorides took place under the oxidant-free visible light irradiation conditions [23]. In the presence of K2HPO4 as a base, the EY-catalyzed reaction of benzenesulfonyl chloride
EY could be used as the photocatalyst for the 5-
The EY-induced photocatalysis was applied to the radical cascade cyclization of polyenes [25]. The photocatalytic cascade cyclization of polyene
Eosin B is also the active catalyst under the visible light irradiation [31]. The C−H functionalization of thiazole derivatives with diarylphosphine oxides was achieved by the eosin B-catalyzed photoredox process. When eosin B was employed as a photocatalyst, the phosphorylation of benzothiazole
3. Rose Bengal
Rose Bengal (RB) was widely used as a visible light-activated photocatalyst [32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44]. Tan’s group studied the photoredox catalysis using RB [32, 33, 34, 35, 36]. RB was a good catalyst for the dehydrogenative coupling reaction between tetrahydroisoquinolines and nitroalkanes (Figure 9) [33]. In the presence of RB (5 mol%), the reaction of
Next, the combination of graphene oxide and RB was studied in the reaction between tetrahydroisoquinolines with TMSCN or TMSCF3 [34]. In the presence of RB (5 mol%) and graphene oxide (50 wt%), the reaction of
New method for the synthesis of Meyers’s bicyclic lactams was developed by using RB photocatalysis [37, 38, 39]. This cascade transformation is the one-pot reaction which begins from furan substrates (Figure 10) [37]. Despite the extraordinary complexity of reaction cascade, the reaction between 2-methylfuran
The aerobic visible light-promoted indole C3 formylation reaction was achieved by using RB as a photocatalyst and
The new method for the synthesis of quinazolines was developed by using RB as a photocatalyst (Figure 12) [42]. In the presence of RB (1 mol%), CBr4 as an oxidant, and K2CO3 as a base, the oxidative carbon-carbon bond-forming cyclization of
4. Fluorescein and rhodamine B
The utility of fluorescein was demonstrated in the alkoxycarboxylation of aryldiazonium salts using CO gas [45]. In the presence of 0.5 mol% of fluorescein as a photocatalyst, treatment of diazonium tetrafluoroborate
The utility of rhodamine B as a water-soluble photocatalyst was demonstrated in the aqueous-medium carbon-carbon bond-forming radical reactions [47]. In the presence of (
5. Methylene blue and acridine red
Methylene blue (MB) is a member of the thiazine dye family. Scaiano’s group used MB as a photocatalyst under the visible light irradiation [48]. The radical trifluoromethylation of electron-rich heterocycles was studied by the use of Togni’s reagent
The one-pot transformation of furans into 5-hydroxy-1
New phenothiazine-based organic dye was also developed as a visible light-activated photocatalyst [51].
Acridine red was used as a photocatalyst for the visible light-induced direct thiolation of ethers (Figure 17) [52]. The thiolation of tetrahydrofuran (THF) using diphenyl disulfide
6. Riboflavin tetraacetate
Riboflavin tetraacetate (RFT) is an effective photocatalyst for the visible light-driven organic reactions. The aerobic oxidation of alkyl benzenes to ketones and carboxylic acids was investigated through a dual catalysis using RFT and the tris(2-pyridylmethyl)amine-iron complex [Fe(TPA)(MeCN)2](ClO4)2 (TPA=tris(2-pyridylmethyl)amine) [53]. When a mixture of RFT (10 mol%) and [Fe(TPA)(MeCN)2](ClO4)2 (2 mol%) was employed, the oxidation of 4-ethylanisole
RFT also catalyzed the aerobic oxidation of sulfides to sulfoxides without overoxidation to sulfones [54]. In the presence of RFT (2 mol%), sulfide
7. Concluding remarks
Organic dyes that absorbed visible light induce the synthetically valuable photochemical transformations. The metal-free photocatalysis using organic dyes rapidly progresses in the last few years. In addition to the organic dyes shown in this chapter, Fukuzumi’ group has developed 3-cyano-1-methylquinolinium and 9-mesityl-10-methylacridinium ions as photocatalysts [13]. More recently, Nicewicz’ group has studied the photocatalysis using acridinium salts [12]. These visible light-induced catalysis disclosed a broader aspect of the utility of organic photocatalysts for synthetic organic chemistry. This chapter will inspire creative new contributions to organic chemists.
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