High Reactivity of Strained Seven‐Membered‐Ring trans‐Alkenes

Abstract

trans-Oxasilacycloheptenes are highly reactive strained alkenes. Competition reactions showed that these seven-membered ring trans-alkenes underwent [4+2] cycloaddition reactions faster than a trans-cyclooctene. They also reacted with quinones and dimethyl acetylenedicarboxylate to form adducts with high diastereoselectivity. Kinetic studies showed that ring strain increases nucleophilicity by approximately 10(9).

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

“…As predicted, trans ‐alkenes 13 and 14 were highly reactive in Diels–Alder cycloaddition reactions with furan 5 (Scheme ) . In less than five minutes, trans ‐alkenes 13 and 14 were consumed, forming cycloadducts 20 and 21 in high yields as mixtures of diastereomers.…”

“…Silver‐catalyzed silylene transfer to 1,3‐diene 9 afforded vinylsilacyclopropane 11 in 80 % yield. Addition of benzaldehyde to vinylsilacyclopropane 11 yielded trans ‐oxasilacycloheptene 12 in 99 % yield , . trans ‐Alkene 12 was unstable to atmospheric conditions and it underwent thermal rearrangement .…”

“…trans ‐Cycloalkene 14 differs from alkene 13 only by the presence of a methyl group at the allylic position. Competition experiments, however, revealed that the presence of the allylic methyl group decreased the rate of cycloaddition by over one order of magnitude (Scheme ) . The decrease in rate is likely due to the syn ‐pentane interaction that develops between the allylic methyl group on alkene 14 and the phenyl group on diene 5 in the transition state (Figure ).…”

“…trans ‐Alkenes were predicted to be particularly reactive with electrophilic dienes. Previous studies indicated that the trans double bond of oxasilacycloheptene 12 is nucleophilic , …”

“…In this article, we demonstrate that the reactivity of seven‐membered‐ring trans ‐alkenes in concerted [4+2] cycloadditions is greatly influenced by the substituents on the seven‐membered ring. Preliminary studies showed that unhindered trans ‐oxasilacycloheptenes are more reactive than a trans ‐cyclooctene . Additional studies show that strain energy is not the only factor that influences the reactivity of these strained compounds .…”