Reaction mechanism and chemoselectivity of intermolecular cycloaddition reactions between phenyl-substituted cyclopropenone ketal and methyl vinyl ketone

摘要

In this paper, the mechanisms of the intermolecular [3 + 2] and [1 + 2] cycloaddition reactions of 1,1/1,3-dipolar π-delocalized singlet vinylcarbenes, which is obtained from cyclopropenone, with an electron-deficient C=O or C=C dipolarophile, to generate five-membered ring products are first disclosed by the density functional theory (DFT). Four reaction pathways, including two concerted [3 + 2] cycloaddition reaction pathways and two stepwise reaction pathways (an initial [1 + 2] cycloaddition and then a rearrangement from the [1 + 2] cycloadducts to the final [3 + 2] cycloadducts), are investigated at the B3LYP/6-31G(d,p) level of theory. The calculated results reveal that, in contrast to the concerted C=O [3 + 2] cycloaddition reaction pathway, which is 7.1 kcal/mol more energetically preferred compared with its stepwise reaction pathway, the C=C dipolarophile favors undergoing [1 + 2] cycloaddition rather than concerted [3 + 2] cycloaddition (difference of 5.3 kcal/mol). The lowest free energy barrier of the C=O concerted [3 + 2] cycloaddition reaction pathway shows that it predominates all other reaction pathways. This observation is consistent with the finding that the C=O [3 + 2] cycloadduct is the main product under experimental conditions. In addition, natural bond orbital second-order perturbation charge analyses are carried out to explain the preferred chemoselectivity of C=O to the C=C dipolarophile and the origins of cis-stereoselectivity for C=C [1 + 2] cycloaddition. Solvent effects are further considered at the B3LYP/6-31G(d,p) level in the solvents CH_3CN, DMF, THF, CH_2Cl_2, toluene, and benzene using the PCM model. The results indicate that the relative reaction trends and the main products are insensitive to the polarity of the reaction solvent.