Heterogeneous Photocatalysis as a Potent Tool for Organic Synthesis: Cross-Dehydrogenative C–C Coupling of N-Heterocycles with Ethers Employing TiO2/N-Hydroxyphthalimide System under Visible Light

Abstract

Despite the obvious advantages of heterogeneous photocatalysts (availability, stability, recyclability, the ease of separation from products and safety) their application in organic synthesis faces serious challenges: generally low efficiency and selectivity compared to homogeneous photocatalytic systems. The development of strategies for improving the catalytic properties of semiconductor materials is the key to their introduction into organic synthesis. In the present work, a hybrid photocatalytic system involving both heterogeneous catalyst (TiO2) and homogeneous organocatalyst (N-hydroxyphthalimide, NHPI) was proposed for the cross-dehydrogenative C–C coupling of electron-deficient N-heterocycles with ethers employing t-BuOOH as the terminal oxidant. It should be noted that each of the catalysts is completely ineffective when used separately under visible light in this transformation. The occurrence of visible light absorption upon the interaction of NHPI with the TiO2 surface and the generation of reactive phthalimide-N-oxyl (PINO) radicals upon irradiation with visible light are considered to be the main factors determining the high catalytic efficiency. The proposed method is suitable for the coupling of π-deficient pyridine, quinoline, pyrazine, and quinoxaline heteroarenes with various non-activated ethers.

t-BuOOH/TiO2 Photocatalytic System as a Convenient Peroxyl Radical Source at Room Temperature under Visible Light and Its Application for the CH-Peroxidation of Barbituric Acids

“…In the first step, we studied the decomposition of organic peroxides of various classes (hydroperoxides, dialkyl peroxides, diacyl peroxides and peroxyacids) under visible light in a suspension of commercially available nanosized titanium dioxide (Table 1). Based on our previous research [37,38], we used TiO 2 Hombikat UV100 with a high specific surface area (300 m 2 •g −1 ) as a heterogeneous photocatalyst and a 443 nm blue LED with 10 W input power as a light source. In all cases, corresponding alcohols 2 (or benzoic acid, in the case of mCPBA) were detected as the main decomposition products.…”

“…Numerous attempts were directed to the photosensitization of titanium dioxide in the visible light region by another semiconductors [19][20][21][22], organic photocatalysts [23] or via element doping [24][25][26][27][28][29]. Another opportunity is to use organic compounds, especially bearing hydroxyl or carboxyl groups, such as phenols [30,31], salicylic acid [32,33], carboxylic acids [34][35][36] or N-hydroxyphthalimide [37,38], which can modify the TiO 2 surface to absorb visible light. In our work, we discovered that peroxides can also lead to visible light sensitization: the mixing of TiO 2 suspension in MeCN with tert-butylhydroperoxide resulted in a change in color from white to pale yellow (the appearance of the mixture can be found in the Supplementary Materials).…”

“…In our previous work, it was found that some hydroperoxides, in particular ethylbenzene hydroperoxide [37] and tert-butyl hydroperoxide (Scheme 1a) [38], can be decomposed on the TiO 2 surface when irradiated with visible light (443 nm). Previously, studies of peroxide-TiO 2 photochemistry were mainly focused on H 2 O 2 and UV light usage [39][40][41][42][43][44][45].…”

Metal- and photosensitizer-free cross-dehydrogenative coupling through photoinduced energy transfer

“…21 More recently, there have been reports of employing photocatalysts in conjunction with peroxides to facilitate this reaction (Scheme 1a). 22 Nevertheless, the high cost associated with photocatalysts, typically composed of precious metal–polypyridyl complexes or intricately synthesized organic dyes, poses a significant obstacle to their broader adoption in extensive industrial processes. In recent years, there have been advancements in the direct cleavage of peroxides, including H 2 O 2 and di- tert -butyl peroxide (DTBP) (Scheme 1b), under visible light irradiation.…”