Tunneling Control: Competition between 6π-Electrocyclization and [1,5]H-Sigmatropic Shift Reactions in Tetrahydro-1H-cyclobuta[e]indene Derivatives

Abstract

Direct dynamics calculation using canonical variational transtition state theory (CVT) inclusive of small curvature tunneling (SCT) reveals the influential role of quantum mechanical tunneling (QMT) for 2,2a,5,7b-tetrahydro-1H-cyclobuta[e]indene derivatives (2a-2j) in governing their product selectivity. 2a-2j follow two distinct reaction channels, namely, 6π-electrocyclization (2 → 3) and [1,5]H-sigmatropic shift (2 → 4), among which the activation barrier is higher for [1,5]H-shift (2 → 4), thereby favoring the kinetically controlled product (3a-3j) as anticipated. However, SCT calculations show that a narrower barrier and smaller mass of participating atoms make QMT more pronounced for [1,5]H-shift reaction despite its higher activation energy, which results in a competition between kinetic controlled (2 → 3) and tunneling controlled (2 → 4) products. At low temperature (T ≤ 170 K), when QMT is the dominant pathway, the tunneling controlled product (4a-4j) is formed exclusively. As the reaction temperature increases, the role of QMT becomes less prominent and eventually gets kinetically controlled at room temperature. Nevertheless, QMT strongly tunes the product ratio at ambient temperatures by favoring the [1,5]H-shift reaction over 6π-electrocyclization. For 2a, k:k increases from 1:13 at CVT level to 1:2 at CVT+SCT level for room temperature.

Tunneling Control of Chemical Reactions: The Third Reactivity Paradigm

“…Quite the contrary: when H-atom, proton, or hydride transfers (or that of one of their isotopologues) are part of the rate-limiting step of a reaction, QMT must nearly always be invoked to compute accurate rate constants . There is a multitude of experimental examples in enzymatic catalysis (not without criticism), sigmatropic rearrangements, radical abstractions, eliminations, organometallic reactions, and, in particular, the reactions of carbenes as well as, just recently, nitrenes . With regard to H-tunneling, we are now at a point where the “expectation has changed from being surprised to see the dominance of tunneling in hydrogen transfer to where we would be very surprised if we did not see it.”…”

A vector‐based representation of the chemical bond for predicting competitive and noncompetitive torquoselectivity of thermal ring‐opening reactions

“…In particular, we seek to find a new criterion to determine whether a reaction is competitive or noncompetitive based on the degeneracy or nondegeneracy of scalar chemical topological properties rather than the somewhat arbitrary Δ TS < 1.0 kcal/mol, see Table that has been previously used . According to Karmakar and Datta a difference in activation barrier Δ TS between two competitive reaction channels cannot entirely dictate the product ratios . They stated that their prediction could be verified experimentally by measuring the relative product ratio of two competitive reaction channels with varying temperatures (low to high).…”

Isotope-Controlled Selectivity by Quantum Tunneling: Hydrogen Migration versus Ring Expansion in Cyclopropylmethylcarbenes

“…6,8,9,11 This type of change in selectivity on going from high to low temperatures due to tunneling has been termed tunneling control, 12 to distinguish it from the traditional kinetic and thermodynamic control schemes. 3,8,9,11,13 Since the two competing reactions depicted in Scheme 1 can conceivably both occur via tunneling at very low temperatures, where there is insufficient energy to overcome the thermal barriers, we ask: Can the preferred product of tunneling be switched f rom HS to RE by isotopic substitution? HS should be much more sensitive to deuterium substitution in the methyl group than RE.…”