Hydrogen Sulfide: A Reagent for pH-Driven Bioinspired 1,2-Diol Mono-deoxygenation and Carbonyl Reduction in Water

Radical Reactions in Organic Synthesis: Exploring in-, on-, and with-Water Methods

Radical reactions in water or aqueous media are important for organic synthesis, realizing high-yielding processes under non-toxic and environmentally friendly conditions. This overview includes (i) a general introduction to organic chemistry in water and aqueous media, (ii) synthetic approaches in, on, and with water as well as in heterogeneous phases, (iii) reactions of carbon-centered radicals with water (or deuterium oxide) activated through coordination with various Lewis acids, (iv) photocatalysis in water and aqueous media, and (v) synthetic applications bioinspired by naturally occurring processes. A wide range of chemical processes and synthetic strategies under different experimental conditions have been reviewed that lead to important functional group translocation and transformation reactions, leading to the preparation of complex molecules. These results reveal how water as a solvent/medium/reagent in radical chemistry has matured over the last two decades, with further discoveries anticipated in the near future.

Biomimetic Ketone Reduction by Disulfide Radical Anion

The conversion of ribonucleosides to 2'-deoxyribonucleosides is catalyzed by ribonucleoside reductase enzymes in nature. One of the key steps in this complex radical mechanism is the reduction of the 3'-ketodeoxynucleotide by a pair of cysteine residues, providing the electrons via a disulfide radical anion (RSSR^•-) in the active site of the enzyme. In the present study, the bioinspired conversion of ketones to corresponding alcohols was achieved by the intermediacy of disulfide radical anion of cysteine (CysSSCys)^•- in water. High concentration of cysteine and pH 10.6 are necessary for high-yielding reactions. The photoinitiated radical chain reaction includes the one-electron reduction of carbonyl moiety by disulfide radical anion, protonation of the resulting ketyl radical anion by water, and H-atom abstraction from CysSH. The (CysSSCys)^•- transient species generated by ionizing radiation in aqueous solutions allowed the measurement of kinetic data with ketones by pulse radiolysis. By measuring the rate of the decay of (CysSSCys)^•- at λ_max = 420 nm at various concentrations of ketones, we found the rate constants of three cyclic ketones to be in the range of 10⁴-10⁵ M^-1s^-1 at ~22 °C.

Synthesis of a Dihydroxylated Open-Cage [70]Fullerene by a Reductive Ring-Closure Reaction

Upon heating an open-cage diketo C₇₀ derivative in carbon disulfide, the 1,2-dihydroxylated compound was unprecedentedly formed via a reductive ring-closure reaction. The crystallographic analysis of the diketo derivative revealed a benzenoid character on its orifice which suppresses the nucleophilic addition. The 1,2-dihydroxylated derivative could be transformed into a 1,4-substituted one by an acid-catalyzed reaction. The observed unique reactivity of the diketo derivative is inherently different from those of structurally related C₇₀ isomers as well as a C₆₀ analogue.

Photocatalytic redox-neutral hydroxyalkylation of N-heteroaromatics with aldehydes

Hydroxyalkylation of N-heteroaromatics with aldehydes was achieved using a binary hybrid catalyst system comprising an acridinium photoredox catalyst and a thiophosphoric acid organocatalyst. The reaction proceeded through the following sequence: (1) photoredox-catalyzed single-electron oxidation of a thiophosphoric acid catalyst to generate a thiyl radical, (2) cleavage of the formyl C-H bond of the aldehyde substrates by a thiyl radical acting as a hydrogen atom transfer catalyst to generate acyl radicals, (3) Minisci-type addition of the resulting acyl radicals to N-heteroaromatics, and (4) a spin-center shift, photoredox-catalyzed single-electron reduction, and protonation to produce secondary alcohol products. This metal-free hybrid catalysis proceeded under mild conditions for a wide range of substrates, including isoquinolines, quinolines, and pyridines as N-heteroaromatics, as well as both aromatic and aliphatic aldehydes, and tolerated various functional groups. The reaction was applicable to late-stage derivatization of drugs and their leads.