Bell-clapper rearrangement reaction via boron atom tunneling

Quantum mechanical tunneling (QMT) is a well-known effect that has the potential to significantly enhance the rates of chemical reactions even at extremely low temperatures, where classical, “over the barrier” thermal reactions are virtually non-existence. Since this effect is dependent on the mass of the shifting atoms, QMT has been studied widely for lightest element i.e., hydrogen atom transfer. However, a slowly growing body of evidence has shown that “heavy” atom tunneling (mainly second row elements of the periodic table) is indeed possible as long as the barrier height is low, more important, the barrier width is markedly narrow. In this computational research intra-molecular SN² degenerate reactions of the “Bell Clapper mechanism” style have been studied, where a boron atom jumps from one side to another over an anthracene scaffold forming a covalent bond with the apical sp³ atom or a charge transfer with the boron. This reaction possesses a double-well potential energy surface where the atomic trajectory to pass from one minimum to another is extremely short, and the barrier height is too high to surmount via a classical thermal pathway at low temperatures, thus making these reactions ideal for QMT.