Allylic Hydrogen Acidity of 1-Butene Derivatives Coordinated to Transition Metals─A Mechanistic Insight Including Carbonyl-Olefin Metathesis

The coordination of organic molecules to transition metals significantly alters the electron density distribution, influencing the acidity of specific hydrogen atoms. This study scrutinizes the acidity of allylic hydrogens in transition metal-coordinated alkenes, delving into the factors that govern allylic proton abstraction. Employing density functional theory, we investigate the effects of various parameters, including the electronic nature of substituents on the vinylic carbons of the alkene, the oxidation state of the metal, and the identity of the transition metal center on the allylic hydrogens’ acidity. Our findings reveal that the impact on the acidity of allylic hydrogens in alkenes coordinated to gold(III), a third-row transition metal, is considerably substantial both kinetically and thermodynamically. Conversely, the impact is minimal for cobalt(III) from the first row and moderate for rhodium(III) from the second row of transition metals. Furthermore, our results indicate that electron-withdrawing substituents on vinylic carbons generally enhance the acidity of allylic hydrogens. The influence of oxidation state is also profound, as gold(I) exhibits markedly weaker effects compared to gold(III). To illustrate the practical application of these insights, we present a case study involving the use of AuCl₃ to catalyze an organic transformation [Chem. Eur. J. 2020, 26, 1941-1946], elucidating the mechanism initiated by the deprotonation of the allylic hydrogen in the coordinated alkene.