Chemical reactions can proceed either by passage of the reactants over the reaction barrier or by tunneling through the barrier. During the past 40 years, it has become clear that tunneling, not only by hydrogen but also by carbon, can occur, provided that the reaction barrier is narrow and that tunneling occurs at energies that are not too far below the top of the barrier. This review discusses those reactions in which tunneling by carbon has been predicted to occur by calculations and/or been found to occur by experiments. WIREs Comput Mol Sci 2016, 6:20–46. doi: 10.1002/wcms.1235



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Reactions that involve tunneling by carbon and the role that calculations have played in their study

We use ab initio CASSCF and CASPT2 computations to construct the composite multistate relaxation path relevant to cycloocta-1,3,5,7-tetraene singlet photochemistry. The results show that an efficient population of the dark excited state (S1) takes place after ultrafast decay from the spectroscopic excited state (S2). A planar D8h-symmetric minimum represents the collecting point on S1. Nonadiabatic transitions to S0 appear to be controlled by two different tetraradical-type conical intersections, which are directly accessible from the S1 minimum following specific excited-state reaction paths. The higher-energy conical intersection belongs to the same type of intersections previously documented in linear and cyclic conjugated hydrocarbons and features a triangular −(CH)3− kink. This point mediates both cis → trans photoisomerization and cyclopropanation reactions. The lowest energy conical intersection has a boat-shaped structure. This intersection accounts for production of semibullvalene or for double-b...

Cyclooctatetraene computational photo- and thermal chemistry: a reactivity model for conjugated hydrocarbons.

Calculations predict that carbon tunneling allows the degenerate cope rearrangement of semibullvalene to occur rapidly at cryogenic temperatures.

[structure: see text] B3LYP/6-31G* calculations have been used to investigate the origins of the relative barrier heights for the degenerate Cope rearrangements of semibullvalene (1), barbaralane (2), bullvalene (3), and dihydrobullvalene (4). We conclude from our calculations that, of the four transition structures (TSs), that for rearrangement of 1 has the smallest amount of interallylic bonding. Nevertheless, relief of strain in the reactant confers on 1 the lowest barrier to Cope rearrangement. Conjugation between the cyclopropane ring and the pi bond of the etheno bridge in 3 makes the barrier for its Cope rearrangement higher than that for 4 and also contributes to making the barrier for 3 higher than that for 2. However, the relatively low barrier to the Cope rearrangement of 2 is largely due to the TS for this reaction having the largest amount of interallylic bonding of all four TSs.

How important is bishomoaromatic stabilization in determining the relative barrier heights for the degenerate Cope rearrangements of semibullvalene, barbaralane, bullvalene, and dihydrobullvalene?

The Effects of Substituents on the Degenerate Cope Rearrangement of Semibullvalenes and Barbaralanes

Die Kinetik der entarteten Cope-Umlagerungen der Titelverbindungen wurden mittels dynamischer 13C-NMR-Spectroskopie gemessen. Die aus den NMR-Linienformanderungen abgeleiteten Reaktionsgeschwindigkeitskonstanten k lieferten auf der Basis der Eyring-Gleichung folgende Aktivierungsparameter: Semibullvalen (1) ΔH≠ = 5.2 kcal mol−1, ΔS≠ = –3.2 cal K−1 mol−1, ΔG≠298 = 6.2 kcal mol−1; 1,5-Dimethylsemibullvalen (2) ΔH≠ = 4.5 kcal mol−1, ΔS≠ = –1.6 cal K−1 mol−1, ΔG≠298 = 5.0 kcal mol−1; 2,6-Dibrom-1,5-dimethylsemibullvalen (3) ΔH≠ = 7.5 kcal mol−1, ΔS≠ ≈ 0 cal K−1 mol−1, ΔG≠298 = 7.4 kcal mol−1. Die Substituenteneffekte werden mit denen anderer substituierter Semibullvalene verglichen und im Hinblick auf die Natur des Ubergangszustandes der Cope-Umlagerung in uberbruckten Homotropilidenen diskutiert. Die fur 2 bestimmte Barriere ist die niedrigste, die bisher mit der dynamischen NMR-Spektroskopie gemessen wurden.



Applications of 13C-NMR Spectroscopy, XXVII. – Activation Parameters off the Cope Rearrangements of semibullvalene, 1,5-Dimethylsemibullvalene, and 2,6-Dibromo-1,5dimethylsemibullvalene



The kinetics of the degenerate Cope rearrangements of the title compounds were measured by dynamic 13C-NMR spectroscopy. The rate constants derived from the changes in NMR lineshape yielded, on the basis of the Eyring equation, the following activation parameters: semibullvalene (1) ΔH≠ = 5.2 kcal mol−1, ΔS≠ = –3.2 cal K−1 mol−1, ΔG≠298 = 6.2 kcal mol−1; 1,5-dimethylsemibullvalene (2) ΔH≠ = 4.5 kcal mol−1, ΔS≠ = –1.6 cal K−1 mol−1, ΔG≠298 = 5.0 kcal mol−1; 2,6-dibromo-1,5-dimethylsemibullvalene (3) ΔH≠ = 7.5 kcal mol−1, ΔS≠ ≈ 0 cal K−1 mol−1, ΔG≠298 = 7.4 kcal mol−1. The substituent effects are compared with those of other substituted semibullvalenes and are discussed with respect to the nature of the transition state for the Cope rearrangement of bridged homotropilidenes. The barrier for 2 is the lowest measured so far by dynamic NMR spectroscopy.

Anwendungen der 13C‐NMR‐Spektroskopie, XXVII. Die Aktivierungsparameter der Cope‐Umlagerung von Semibullvalen, 1,5‐Dimethylsemibullvalen und 2,6‐Dibrom‐1,5‐dimethylsemibullvalen

Libby S. Miller

Papers

Anwendungen der 13C‐NMR‐Spektroskopie, XXVII. Die Aktivierungsparameter der Cope‐Umlagerung von Semibullvalen, 1,5‐Dimethylsemibullvalen und 2,6‐Dibrom‐1,5‐dimethylsemibullvalen