Electronic Structures and Oxidation Potentials of Some Azulene Derivatives

The electronic structures of azulene (1) and its alkylated derivatives were investigated on the basis of MO calculations by MNDO and PM3 method. There results were experimentally evaluated by measuring the oxidation potentials (Eox) of 1 and its alkylated derivatives, from which the additive property on Eox was suggested as shown by Eox(Az.subst) = Eox(AZ) − 0.10n1,3 + 0.06n2 − 0.02n4,8 − 0.11n5,7 − 0.01n6. Thus, the alkyl groups at the 1,3- and 5,7-positions lower the Eox values by ca. 0.1 V, and stronger donating properties were observed for 3-methylguaiazulene (Eox = +0.53 V) and 3,3′-biguaiazulene (Eox = +0.40 V). The redox properties of guaiazulenyl sulfides which were prepared from guaiazulene via a phenylsulfonylthio derivative were also studied. Unlike the alkylated azulenes they underwent reversible one-electron oxidations and exhibited stronger donating properties (Eox = +0.40 – +0.44).