Recent Advances in Pd‐Catalyzed Reactions Involving the “On‐Water” Mechanism†

Comprehensive Summary As the chemical industry grapples with the need for more eco‐friendly practices, the use of water as a reaction medium is gaining attraction in organic transformations. This mini‐review delves into Pd‐catalyzed reactions that utilize the "on‐water" mechanism, spanning from 2019 to late 2023. These reactions are neatly categorized into several types: (A) Catalytic C—H activations, (B) Mizoroki‐ Heck‐type reactions, (C) Suzuki‐Miyaura reactions, and (D) Cyclization reactions. By showcasing the potential of water as a sustainable reaction medium for organic transformations, these reactions leave no doubt about the importance of embracing eco‐friendly practices in the chemical industry. Key Scientists In 1980, a seminal work by Breslow et al. showed an acceleration of reaction rate in the Diels‐Alder reaction. Sharpless and co‐workers noted a significant increase in the rate of the [2σ+2σ+2π] cycloaddition of quadricyclane and dimethyl azodicarboxylate (DMAD) when the reaction was conducted in water, as opposed to when it was carried out in organic solvents. The term "on‐water" was then coined to describe this phenomenon. This strategy was further expanded to transition‐metal catalyzed transformations by Ackermann in 2011. Later, Varma and Leazer disclosed a Pd‐catalyzed Mizoroki‐Heck type arylation of alkenes with diaryliodonium salts “on‐water”. The enantioselective version of "on‐water" process was not realized until 2014 by the Zhou group. Later on, the Schaub group described a Pd‐catalyzed Suzuki–Miyaura coupling reaction of electron‐poor aryl chlorides with water, using only 50 ppm of catalyst loading. Very recently, Liu and Lin extended the "on‐water" strategy to Pd‐catalyzed double Mizoroki‐Heck reactions. This mini‐review has focused on Pd‐catalyzed reactions involving the “on‐water” mechanism.