Source:Tetrahedron, Volume 69, Issue 36
Author(s): Pei-Jun Cai , Fu-Qiang Shi , Yi Wang , Xin Li , Zhi-Xiang Yu
Metal-free (5+2) cycloadditions of homopyrrole and homofuran with dienophiles of N-phenylmaleimide, dimethyl acetylenedicarboxylate (DMAD), and maleic anhydride have been investigated by DFT calculations at the (U)B3LYP/6-31+G(d) level. Homopyrrole and homofuran act as 4-electron 1,5-dipoles in these (5+2) reactions. These 1,5-dipoles can exist only in the transition states and undergo competitive concerted or stepwise (5+2) cycloadditions. The (5+2) cycloadditions of homopyrrole with dienophiles are stepwise involving generation of diradical intermediates, and its concerted pathways are disfavored marginally. In contrast, the (5+2) cycloaddition of homofuran with maleic anhydride occurs in a concerted fashion, which is preferred over the stepwise diradical pathway slightly. The distortion energies from homopyrrole and homofuran to their respective 1,5-dipoles in the transition states are not high and this is the main reason why homopyrrole and homofuran can undergo metal-free (5+2) cycloadditions with dienophiles.