Ring Expansion to 1-Bromo-1-alumacyclonona-2,4,6,8-tetraene by Insertion of Two Alkyne Molecules into the AlC Bonds

Abstract

Treatment of 1-bromo-2,3,4,5-tetraethylalumole (1) with 3-hexyne afforded the corresponding product 1-bromo-1-alumacyclonona-2,4,6,8-tetraene (2), accompanied by the formation of hexaethylbenzene. In the crystalline state, 2 forms a Br-bridged dimer with a pseudo C2-symmetric and twisted AlC8 nine-membered ring. Deuterium-labeling experiments and DFT calculations on the reaction of 1 with 3-hexyne suggested that 1-bromo-1-alumacyclohepta-2,4,6-triene, which is formed by the insertion of one molecule of 1-hexyne into the AlC bond of alumole 1, is the key intermediate for the generation of 2 as well as hexaethylbenzene.