The pivotal role of the counterion in gold catalyzed hydration and alkoxylation of alkynes

Hydration and alkoxylation of alkynes are key processes for the industrial production of carbonyl derivatives. In this review, the pivotal role of ion pairing in the mechanism of hydration and alkoxylation of alkynes promoted by gold(I) catalysts L-Au-X is deeply analyzed and discussed, that has been elucidated by means of experimental findings supported by some theoretical calculations. In particular, the crucial role of the counterion X- is fully described in chapter 2. The catalytic performances in the alkoxylation and hydration of alkynes promoted by gold(I) are influenced by the coordination ability and basicity (proton affinity) of the counterion (paragraph 2.3) and the anion/cation relative orientation (paragraph 2.3). Also the appropriate matching of X- and the neutral ligand L must be taken into account to improve the catalytic performance of gold catalysts (paragraph 2.4). A survey of other non-covalent interactions, which however play a kinetic important role (hydrogen bonds, those triggered by suitable functionalities present on the ligand L, formation of supramolecular catalytic systems or micelles, and others), is presented in chapter 3. The progress in the development of sustainable methodologies for the gold(I)-promoted hydration of alkynes is discussed in chapter 4. In particular, paragraph 4.1 focuses on the unique role played by the anion in the L-Au-X catalyzed hydration of alkynes conducted in solvent-, silver-, and acid-free conditions. To conclude this critical review, in paragraph 4.2 it is highlighted how the amount of ion pairing, combined with the presence of suitable functionalities in neoteric solvent, may allow the development of green protocols for gold(I) catalyzed hydration of alkynes. (C) 2019 Elsevier B.V. All rights reserved.