Does the Neophyl-like Rearrangement Play a Decisive Role in Intramolecular Cyclization of Iminyl Radicals? A Combined Quantum Chemistry and Numerical Simulation Investigation of the Cyclization Mechanism and Product Distributions of Bicyclic 2-Allyl-2-methyl-2,3-dihydro-1 H-inden-1-iminyl Radical and Several Iminyl Model Compounds.

Abstract

In this study, we performed a theoretical investigation of the intramolecular cyclization of bicyclic 2-allyl-2-methyl-2,3-dihydro-1 H-inden-1-iminyl radical 1 along with several iminyl model compounds. The results were used to comparatively evaluate the reaction mechanism suggested previously, in which the neophyl-like rearrangement was deemed to play a decisive role. The present computation and numerical simulation identify the experimentally observed endo product in the high-temperature cyclization of 1. The product results from a kinetically controlled endo cyclization-reduction pathway involving an initial reversible 5- exo ring-closure/ring-opening process, not via 5- exo cyclization/neophyl-like rearrangement/ endo-radical reduction pathway as proposed previously. Considering many available theoretical and experimental results, the neophyl-like rearrangement seems to play only a minor role in the intramolecular cyclization of N- and C-centered radicals. The structural effect of cyclized radical intermediates of bicyclic 1 leads to a lower thermodynamic reaction energy of exo cyclization than of endo cyclization, which together with the temperature effect should be responsible for the formation of the dominant endo product in the high-temperature region. Additionally, this investigation provided further insight into the cyclization of 1 and compounds structurally similar to 1; that is, control of endo- or exo-regioselective products is readily available by regulating the reaction temperature.