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

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 C 2 ‐symmetric and twisted AlC 8 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 AlC bond of alumole 1 , is the key intermediate for the generation of 2 as well as hexaethylbenzene.