Tunneling Control: Competition between 6π-Electrocyclization and [1,5]H-Sigmatropic Shift Reactions in...

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_[1,5]H-shift:k_{6π-electrocyclization} increases from 1:13 at CVT level to 1:2 at CVT+SCT level for room temperature.