[4+2] Cycloadditions of Seven-Membered-Ring trans-Alkenes: Decreasing Reactivity with Increasing Substitution of the Seven-Membered Ring

The reactivity of trans-oxasilacycloheptenes in [4+2] cycloadditions depends on the substitution pattern on the seven-membered ring. Unhindered trans-alkenes undergo [4+2] cycloadditions with 1,3-diphenylisobenzofuran faster than the most reactive trans-cyclooctene. Increasing the substitution of the seven-membered ring or increasing the electron density of the trans-alkene decreases reactivity with 1,3-dienes in concerted cycloaddition reactions. Although highly substituted trans-alkenes are unreactive in concerted cycloaddition reactions, these alkenes react rapidly in stepwise reactions with diethyl azodicarboxylate (DEAD), an electrophilic diene.