Hydration of Unsymmetrical Internal Alkynes: Factors Governing the Regioselectivity

Water addition on an unsymmetrical internal alkyne has significance in organic chemistry and has stimulated challenging approaches to access a highly regioselective process. Hydration or formal hydration is presented according to the type of internal alkyne. Methods for electron-rich and -poor diarylalkynes followed by those adapted to arylalkylalkynes are presented. Regioselective hydration reactions of unsymmetrical propargylic and homopropargylic derivatives are then discussed, followed by regioselective aspects of the hydration of internal alkynes directed by carbonyl groups. The following section examines the hydration alkynes linked to CF3, esters, ketones, and heteroatoms as haloalkynes, alkynylphosphonates, ynamides, and others. At the end of this section, we report some recent examples of hydration reactions where the alkyne triple bond is connected to an electron-withdrawing group as an ester or a ketone. Reactions are presented, discussed, and illustrated in each of these items through various representative examples. The mechanistic hypotheses explaining the origin of the regioselectivity in the hydration reactions are discussed. This review of recent advances in the regioselective hydration of internal unsymmetrical alkynes (from 2007 until the present) aims to update the current body of knowledge and aid researchers working in this field.