Polyborylation of azulenes

In the presence of an active Ir-based catalyst, azulene underwent stepwise borylation with bis(pinacolato)diboron to produce polyborylated products. The reactivity of the ring atoms toward borylation was found to decrease in the following order: 2-position>1,3-positions>6-position>5,7-positions. Extension of the borylation to some azulenes substituted at the five-membered ring was also examined. Furthermore, the reaction of 2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl)azulene (3a) with an equivalent of hydrogen peroxide revealed that the oxidation proceeds preferentially at the 6-position. This indicates that the reactivity of the boryl group is governed by the π-polarization of the azulenyl skeleton.