Experimental and Theoretical Studies on Regiocontrol of Benzyne Reactions Using Silyl and Boryl Directing Groups

The silyl- and boryl-directed regiocontrol of benzyne reactions and their theoretical analyses using density functional theory (DFT) are described herein. The Diels-Alder reactions of both 3-silyl- and 3-borylbenzynes with substituted furans and the [3+2] cycloadditions of 3-silylbenzynes with 1,3-dipoles predominantly produced distal cycloadducts, whereas the [3+2] cycloadditions of the borylbenzynes exclusively produced proximal cycloadducts. Moreover, primary amines selectively underwent nucleophilic addition reactions at the more hindered ortho-position of the silyl group of the benzynes. The origin of these anomalous regiochemistries was theoretically evaluated using the natural bond orbital and reaction pathway analyses including the transition states of these reactions. On the basis of DFT calculations, it is proposed that the reactions of the borylbenzynes were controlled by the electrostatic effect of the boryl group, whereas those of the silylbenzynes were primarily governed by the steric effect of the silyl group.