Substitutional Control of Non-statistical Dynamics in the Thermal Deazetization of Tetracyclic Azo compounds

Abstract

Dynamical control of reactivity for the deazetization of endo,endo-9,10-diazatetracyclo[3.3.2.02,4.06,8]dec-9-ene (3) is studied using on-the-fly quasi-classical trajectory (QCT) calculations at the density functional theory (DFT). Two degenerate homotropilidenes, 4 and 5 are formed simultaneously from a single transition state (TS). The ratio of the cyclohexadienyl substituted product, 4 and the dynamical product ie. bridgehead substituted product, 5 can be neatly controlled by tuning the topology of the potential energy surface (PES). A steep descent post-TS favors the cyclohexadienyl substituted product while a shallow descent increases the dynamical outcome. Chemical demonstration of the same is achieved by symmetrical and asymmetrical substitution of functional groups along the cleaving (C3-C4) bond. Asymmetric mono-functionalization makes the PES broader thereby reducing the slope post-TS. This creates favourable situation for the dynamical products, 5b-5d to become the major ones. On the contrary, symmetric bi-functionalization makes the cyclohexadienyl substituted product, 4m-4o overwhelmingly (>85%) predominating. As a corollary to this phenomena, substitution of C3-C4 bond by the heavier isotopologues of H/C restricts its motion along the IRC path by Newtonian kinetic isotope effect. This facilitates bond-opening along the C10-C11 dynamical pathway. Hence, for isotopic substitution the situation is reversed and the bifunctionalized 3 is more dynamically activated. Simultaneous substitution by heavier isotopologue of C and H causes deviation from the geometric mean of individual isotopic substitution towards the dynamical product, 5. Therefore, the dynamic control in 3 becomes prominent either via functional group asymmetry or through a Newtonian kinetic isotope effect for symmetric bifunctionalization.