Mechanism of Iron-Catalyzed C—H Alkenylation of Pivalophenone Derivatives With Unsymmetric Internal Alkynes

The mechanism of an iron-catalyzed C—H alkenylation of pivalophenone derivatives with unsymmetric internal alkynes is investigated in details by means of density functional theory calculations. It is shown that the reaction begins with two consecutive ligand exchanges, followed by a fast and reversible oxidative addition C—H activation step. Next, an alkyne insertion into the Fe—H bond, two isomerization steps, and a reductive elimination afford the final product. The reductive elimination is the turnover-limiting step of the process, and also determines the regiochemical outcome of the reaction. The origin of the regioselectivity is proposed to be the steric repulsion between the bulky trimethylsilyl group and the aromatic fragment in the reductive elimination step leading to the not observed regioisomer.