A theoretical study of an electronically mismatched Diels-Alder cycloaddition

Both (E)-1-methoxy-4-(prop-1-en-1-yl) benzene and isoprene exhibit electron-rich properties. However, a recent [4 + 2] cycloaddition between them shows quite high selectivity and productivity with low loading of the ruthenium photosensitizer ([Ru(bpz)(3)(2+)]) under visible light. It is an interesting and untraditional D-A reaction because of the electronic mismatch between the diene and dienophile, which are impossible to react with each other theoretically. Presently, under the theoretical level of M062X/cc-pVTZ/LANL2DZ with the solvent effect of CH2Cl2, we design all the possible sixteen pathways in order to understand the most appropriate mechanism of this novel D-A cyclization. The results suggest that the triplet state ruthenium transfers the electrons of dienes to the catalyst by cycloaddition as a stepwise pathway. The analysis of substituent effects indicates that the necessary precondition for the D-A reaction is the electric matching. Finally, we obtain the approximate function between kinetic property and electronic structures. The reactivity can be controlled by tuning electronic structure and the molecular polarization through different substituents.