Photocatalytic Cascade Carbon-Carbon Bond-Forming Radical Reaction in Aqueous Media

Photocatalytic Regeneration of a Nicotinamide Adenine Nucleotide Mimic with Water-Soluble Iridium(III) Complexes

Nicotinamide adenine nucleotide (NADH) is involved in many biologically relevant redox reactions, and the photochemical regeneration of its oxidized form (NAD⁺) under physiological conditions is of interest for combined photo- and biocatalysis. Here, we demonstrate that tri-anionic, water-soluble variants of typically very lipophilic iridium(III) complexes can photo-catalyze the reduction of an NAD⁺ mimic in a comparatively efficient manner. In combination with a well-known rhodium co-catalyst to facilitate regioselective reactions, these iridium(III) photo-reductants outcompete the commonly used [Ru(bpy)₃]²⁺ (bpy = 2,2'-bipyridine) photosensitizer in water by up to 1 order of magnitude in turnover frequency. This improved reactivity is attributable to the strong excited-state electron donor properties and the good chemical robustness of the tri-anionic iridium(III) sensitizers, combined with their favorable Coulombic interaction with the di-cationic rhodium co-catalyst. Our findings seem relevant in the greater context of photobiocatalysis, for which access to strong, efficient, and robust photoreductants with good water solubility can be essential.

Aqueous-Medium Carbon-Carbon Bond-Forming Radical Reactions Catalyzed by Excited Rhodamine B as a Metal-Free Organic Dye under Visible Light Irradiation

The utility of rhodamine B as a water-soluble organic photocatalyst was studied in the cascade radical addition-cyclization-trapping reactions under visible light irradiation. In the presence of (i-Pr)2NEt, the electron transfer from the excited rhodamine B to perfluoroalkyl iodides proceeded smoothly to promote the carbon-carbon bond-forming radical reactions in aqueous media. When i-C3F7I was employed as a radical precursor, the aqueous-medium radical reactions proceeded even in the absence of (i-Pr)2NEt. In these reactions, the direct electron transfer from the excited singlet state of rhodamine B would take place. Furthermore, the cleavage of the C-I bond in less reactive i-PrI could be achieved by the reductive electron transfer from the excited rhodamine B, which was confirmed by the fluorescence quenching of rhodamine B with the addition of i-PrI.

[Multi-component Reactions Based on Formal [2+2] Reaction of Benzyne with Formamide]

Arynes, such as benzyne, are highly strained and kinetically unstable intermediates that have been widely employed in organic synthesis. In particular, since the initial report regarding the in situ generation of arynes under neutral conditions from ortho-(trimethylsilyl)aryl triflates and fluoride ions in 1983, numerous nucleophilic additions to arynes have been utilized in the synthesis of carbocycles, heterocycles, and multi-substituted arenes. Recently, we reported the insertion of arynes into the π-bond in formamides, as well as related cascade reactions. First, we synthesized salicylaldehydes and aminophenols via the insertion of arynes into the C=O bond of sterically less hindered N,N-dimethylformamide (DMF). Notably, computational studies revealed the generation of unstable benzoxetene and ortho-quinone methide as reactive intermediates. We also studied multi-component coupling reactions leading to 2H-chromenes, coumarins, and xanthen-1-ones under mild conditions through the stepwise release of the strain energy in arynes. Furthermore, we studied the synthesis of O-heterocycles such as dihydrobenzofuran, benzofuran, and 4H-chromene as an application of our method. In this review, we will describe these studies in detail.