Dihydroboration of cyclic allenes

Abstract

The study on the dihydroboration of a large ring cyclic allene, 1,2-cyclotridecadiene and the smallest stable allene, 1,2-cyclononadiene is described. For example, a mixture of products containing isomeric cyclotridecene, bicyclo(10.1.0)-tridecane, cyclotridecanone, cyclotridecanol, isomeric cyclic 1,2-cyclotridecandiol and isomeric 1,3-cyclotridecandiol is obtained from dihydroboration-oxidation of 1,2-cyclotridecadiene. However, dihydroboration-hydrolysis-oxidation of 1,2-cyclotridecadiene affords mainly cyclotridecanol, whereas dihydroboration-oxidation with chromium trioxide-pyridine yields mainly a mixture of Z- and E-cyclotridecene. Reasonable mechanistic pathways have been suggested for the formation of products. The proposed unusual elimination reaction of 1,2-diorganoboranes with chromium trioxide has been substantiated using an authentic 1,2-diorganoborane from diphenyl acetylene. The reaction appears to be stereospecific withthreo-diorganoborane furnishing predominantly E-alkene. The results with 1,2-cyclononadiene has also been rationalised on a similar basis.