Advances in Photocatalytic Organic Synthetic Transformations in Water and Aqueous Media

Abstract

Advances in visible light-photocatalyzed preparative synthetic organic transformations in water or aqueous media are studied. Both the convenience of visible light photocatalytic strategies to achieve synthetic organic transformations and the employment of water or aqueous mixtures make this approach a very resourceful methodology. Mechanistic aspects for such photocatalyzed processes are discussed. The effects of water in the stereoselectivity of some reactions and considerations regarding the improved enantioselectivity in chiral catalytic photoredox reactions are manifested with the use of water. The use of water as a result of a protocol improvement or environmentally friendlier methodology as compared to the same reaction performed in organic solvents are also considered. All these combined effects of water as a reaction medium or its very presence in photocatalytic reactions have an ultimate goal which is to diminish the environmental impact of synthetic and photocatalytic processes. This perspective highlights the progress and state of the art organic transformations realized by photocatalytic techniques in water or aqueous systems as well as water as a reactant. In this regard, aromatic substitution reactions, radical additions to carbon–carbon double bonds, photooxidation reactions, decarboxylation processes, and miscellaneous reactions are presented.

Direct C−H Trifluoromethylation of (Hetero)Arenes in Water Enabled by Organic Photoredox‐Active Amphiphilic Nanoparticles

“…After sealing the capped vial with parafilm and degassing the aqueous phase with argon for 5 min, the reaction mixture was illuminated with a focused 385 nm LED for the indicated time while being stirred and cooled with compressed air. Afterwards, trifluorotoluene (30 μmol, 1.0 equiv) as the internal 19 F NMR standard was added and the mixture was extracted three times with 0.5 mL of deuterated chloroform. Product formation was determined via quantitative 19 F NMR spectroscopy and verified by GC-MS if applicable.…”

“…To this end, the chemical structure of PTH was slightly altered by tying an ethylamine linker to the 4position of the phenyl group. The degree of functionalization after each addition step was conveniently determined by comparing the signals of released pentafluorophenol and remaining polymer in 19 F NMR spectroscopy. After dialysis, the set of amphiphilic polymers P1-P4 was fully characterized.…”

“…After adding the hydrophobic substrates, the homogenous SCPN solution turned into a heterogenous mixture (see Figure S12 in the Supporting Information). The reaction mixture was illuminated until (near) full conversion of the CF 3 reagent was reached and product formation was determined by quantitative 19 F NMR spectroscopy with trifluorotoluene as internal standard. Employing polymer P1 (catalyst loading of 7 mol % under the applied conditions) showed the generation of singly trifluoromethylated species 2 b as the main product (22 % with respect to 1 a) and bistrifluoromethylated compound 2 c as the minor product (10 %) with a molar ratio of 4 : 1 (see bar 1 in Figure 3b).…”

“…Direct CÀ H trifluoromethylation of various (hetero)aromatic substrates under optimized reaction conditions in water. Product formation was determined by quantitative 19 F NMR spectroscopy using trifluorotoluene as internal standard. Product formation includes bistrifluoromethylated products or regioisomers if applicable.…”

“…[17] Surprisingly, despite these considerable achievements, the performance of photoredox catalysis in aqueous solutions is still in its infancy. [18,19] Yet, using water as an alternate reaction medium has multiple advantages. [20][21][22] Water is abundant, inexpensive, non-toxic, biocompatible, and by far the most environmentally benign solvent.…”