Role of Heavy Atom Tunneling in Myers–Saito Cyclization of Cyclic Enyne-Cumulene Systems

• Published in: The journal of physical chemistry. B Vol. 120; no. 5; pp. 945 - 950
• Main Authors: Karmakar, Sharmistha, Datta, Ayan
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 11.02.2016
• Subjects: Carbon; Curvature; Dynamical systems; Dynamics; Inclusions; Isotope effect; Mathematical analysis; Tunneling
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.5b12465

Direct dynamics calculation using canonical variational transtition state theory (CVT) inclusive of small curvature tunneling (SCT) reveals heavy atom tunneling in Myers–Saito cyclization of 10- and 9-membered cyclic enyne–cumulene systems like 1,6-didehydro[10]­annulene and derivative of neocarzinostatin, respectively. The pure density functional theory functional, BLYP at a 6-31+G (d,p) basis set reproduce the observed reaction energies and barriers within 1.0 kcal/mol. The calculated rate constants of cyclization inclusive of heavy atom tunneling (k CVT+SCT = 3.26 × 10–4 s–1 at 222 K; t 1/2 = 35 min) are in excellent agreement with experiments (t 1/2 ∼ 21–31 min). Both primary and secondary kinetic isotope effect (KIE) become enhanced significantly upon inclusion of quantum mechanical tunneling. An Arrhenius plot of KIE shows measurable curvature at the experimental temperature of 222 K. The translation vector for the cyclization reactions in the transition-states (TS) show significant motion of primary and secondary carbon atoms explaining the origin of large KIE.