Sustainable hydration of alkynes promoted by first row transition metal complexes: background, highlights and perspectives

The review describes the state of the art in hydration reactions of alkynes by direct or indirect catalysis of first row transition metal complexes, covering mostly the literature of the last decade in which many examples based on these metal ions have been reported. Previously limited to few cases, catalytic metal ions of the fourth period allow for the regioselective conversion of terminal and internal alkynes with different electronic properties or even sterically hindered into the corresponding ketones. The substrate scope includes aromatic, heteroaromatic and aliphatic alkynes under reaction conditions compatible with various functional groups, in moderately acidic conditions, as for iron catalysts, or even in absence of Brønsted acids, as for cobalt and copper salts. The mechanism of activation of the triple bond varies largely depending on the metal ion and includes attack of proton under combined acid catalysis (Lewis acid-Brønsted acid) or intermediacy of π-alkyne or σ-acetylide late transition metal-complexes. The use of 3d metal complexes as alternatives to conventional noble metal catalysts for water addition reactions of alkynes entails significant implications on cost and environmental issues. Selected examples of applications in cascade and multi step reactions involving acid sensitive organic compounds are also described.