Ping-pong tunneling reactions, part 2: boron and carbon bell-clapper rearrangement

Abstract

Anthracene can be used as a scaffold for intramolecular SN2 degenerate reactions of the “bell clapper” type, where a central boron atom or its isoelectronic carbocation bonds alternatively towards one or the other lateral Lewis bases at the first and eight anthracene positions. This ping-pong bond-switching reaction possesses a symmetrical double-well potential with low activation barrier and relatively narrow barrier width. Herein we show by computational means the active role played by heavy atom quantum tunneling in this degenerate rearrangement reaction at cryogenic temperatures. At these conditions the thermal “over the barrier” reaction is forbidden, whereas the tunneling effect enhances the rate of reaction up to an experimentally measurable half-life. Kinetic isotope effects and cryogenic NMR spectroscopy can, in principle, experimentally demonstrate the tunneling mechanism.

Heavy‐Atom Tunneling in Organic Reactions

“…A similar analysis of “bell‐clapper” rearrangements of hypothetical anthracenes 27 and 28 (related to known systems) revealed that reducing the Lewis basicity of the lateral Y groups (for example, NMe 2 to NCl 2 ) reduces the effective barrier width (and height), allowing for surprisingly high tunneling rate constants near absolute zero . The carbocation system 27 (Y=Cl) was predicted to have k SCT =6×10 −5 s −1 at 6 K ( τ 1/2 =3 h), more than 115 orders of magnitude larger than the semiclassical rate constant.…”

Boron Tunneling in the “Weak” Bond‐Stretch Isomerization of N−B Lewis Adducts

“…In principle there should be a correlation between the magnitude of these vectors and the KIE, and indeed we see a correlation between these magnitudes in Figure 4. The highest KIE and the most significant vector reside in the boron, and therefore in the N−B bond‐stretch we can safely say that this is the “tunneling‐determining atom”, in a rare case of boron‐atom tunneling [79] …”

“…The highest KIE and the most significant vector reside in the boron, and therefore in the NÀ B bond-stretch we can safely say that this is the "tunneling-determining atom", in a rare case of boron-atom tunneling. [79] Note that to keep the symmetry we studied the KIEs substituting all the equivalent atoms together (i. e. the three hydrogens by three deuteriums, or the three 35 Cl by three 37 Cl); taken like this, the individual KIEs for these atoms was approximated by the cube root of the triply substituted KIE. Figure 4 depicts the KIE versus the inverse of the temperature, showing a flat curve below 10 K for all atoms (i. e. ground state tunneling), reaching 1.43 for H/D, 1.38 for 10 B/ 11 B, and smaller values for the other atoms.…”

Heavy-Atom Tunneling in the Covalent/Dative Bond Complexation of Cyclo[18]carbon–Piperidine

“…Indeed, several experimental and/or computational studies have demonstrated that the characteristic features of reactions driven by heavy-atom QMT are their low and narrow barriers. 25 , 28 , 29 The few documented cases include pericyclic 32 and degenerate rearrangement reactions involving carbon, 33 − 35 fluoride, 36 , 37 and boron 38 tunneling. Carbon and nitrogen tunneling in highly exergonic reactions in reactive intermediate species, such as carbene or nitrene, has also been reported.…”