Preparation and Stille cross-coupling reaction of the first organotin reagents of azulenes. Easy access to poly(azulen-6-yl)benzene derivatives

Abstract

The first versatile organometallic reagents derived from azulenes, i.e., 6-(tri-n-butylstannyl)azulene (1a) and its 1,3-diethoxycarbonyl derivative (1b), have been prepared by Pd(0)-catalyzed direct stannylation of 6-bromoazulenes with bis(tri-n-butyltin). We demonstrate the utility of the reagents in the Stille cross-coupling reaction with aryl, acyl and azulenyl halides to afford 6-aryl-, 6-acyl- and bi-azulenes in good yield. Furthermore, the methodology was applied to the synthesis of poly(azulen-6-yl)benzene derivatives. The reaction of 1b with 1,4-di-, 1,3,5-tri-, 1,2,4,5-tetra- and hexabromobenzenes afforded 1,4-di-, 1,3,5-tri-, 1,2,4,5-tetra-, 1,2,4-tri- and 1,2,3,5-tetra(azulen-6-yl)benzene derivatives (18, 20, 22, 24 and 25). The redox behavior of 18 and 22 was examined by cyclic voltammetry (CV) and compared with those of 20 and 24 reported previously. In contrast to the three-step reduction of 20, the compound 18 exhibited a reversible one-step two-electron reduction wave at −1.30 V upon CV, which revealed the formation of a closed-shell dianion. The four azulen-6-yl substituents on benzene in a 1,2,4,5 relationship increased electron-accepting properties because of the formation of a closed-shell dianion stabilized by four azulen-6-yl groups. As expected, the compound 22 exhibited a color change during the electrochemical reduction. However, the reverse oxidation did not regenerate the spectrum of 22 due to the low stability of the presumed dianionic species under the conditions of the UV–vis measurement.