A DFT mechanistic study on [4+3] cycloaddition reactions of oxyallyl cations and five-membered cyclopentadiene derivatives

摘要

The mechanism and stereo-selectivity of [4 + 3] cycloaddition reaction involving oxyallyl cations and five-membered cyclodienes (cyclopentadiene, furan and thiophene) for the construction of seven-membered cycloadducts has been studied with DFT at B3LYP/6-311G** level. The reaction can go through a concerted addition pathway via compact or extended transition states, or through a stepwise pathway. The concerted pathway is slightly favored kinetically (by 4 kcal/mol), via the compact transition state for furan and thiophene and the extended transition state for cylopentadiene, leading to a cis-endo product (W-configuration). Even though the stepwise addition pathway is not completely ruled out, it is not selective as the formation of the trans and cis-endo products through stepwise addition are very close in energy. Generally, the cycloadduct with the U configuration is more stable than the W and sickle configurations, and thus the kinetically-favored W conformer is expected to flip into the themodynamically-favored U conformer. (C) 2020 Elsevier Ltd. All rights reserved.