Junya Higashi

Synthesis of Redox‐Active, Intramolecular Charge‐Transfer Chromophores by the [2+2] Cycloaddition of Ethynylated 2H‐Cyclohepta[b]furan‐2‐ones with Tetracyanoethylene

Ethynylated 2H-cyclohepta[b]furan-2-ones 5-15 have been prepared by Pd-catalyzed alkynylation of 3-iodo-5-isopropyl-2H-cyclohepta[b]furan-2-one (2) with the corresponding ethynylarenes or the reaction of 2-iodothiophene with 3-ethynyl-5-isopropyl-2H-cyclohepta[b]furan-2-one (4) under Sonogashira-Hagihara conditions. Compounds 5-15 reacted with tetracyanoethylene in a formal [2+2] cycloaddition reaction, followed by ring opening of the initially formed [2+2] cycloadducts, cyclobutenes, to afford the corresponding 1,1,4,4-tetracyanobutadienyl (TCBD) chromophores 16-26 in excellent yields. The intramolecular charge-transfer interactions between the 2H-cyclohepta[b]furan-2-one ring and TCBD acceptor moiety were investigated by UV/Vis spectroscopy and theoretical calculations. The redox behavior of the novel TCBD derivatives 16-26 was examined by cyclic voltammetry and differential pulse voltammetry, which revealed multistep electrochemical reduction properties, depending on the number of TCBD units in the molecule. Moreover, a significant color change was observed by UV/Vis spectroscopy under electrochemical reduction conditions.

Synthesis of donor-acceptor chromophores by the [2 + 2] cycloaddition of arylethynyl-2H-cyclohepta[b]furan-2-ones with 7,7,8,8-tetracyanoquinodimethane.

Arylethynyl-2H-cyclohepta[b]furan-2-ones reacted with 7,7,8,8-tetracyanoquinodimethane (TCNQ) in a formal [2 + 2] cycloaddition reaction, followed by ring opening of the initially formed cyclobutene derivatives, to afford the corresponding dicyanoquinodimethane (DCNQ) chromophores in excellent yields. The intramolecular charge-transfer (ICT) interactions between the 2H-cyclohepta[b]furan-2-one ring and DCNQ acceptor moiety were investigated by UV/Vis spectroscopy and theoretical calculations. The redox behavior of the novel DCNQ derivatives was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their multistep electrochemical reduction properties depended on the number of DCNQ units in the molecule. Moreover, a significant color change was observed by visible spectroscopy under electrochemical reduction conditions.

THE [2+2] CYCLOADDITION REACTION OF ETHYNYLATED 2H-CYCLOHEPTA[b]FURAN-2-ONES WITH 2,3-DICHLORO-5,6-DICYANO-1,4-BENZOQUINONE

Ethynylated 2H-cyclohepta[b]furan-2-ones reacted with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in a formal [2+2] cycloaddition reaction to afford the corresponding DDQ-adducts 10–15 in good yields. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is well known as an oxidizing reagent by the formal dehydrogenation reaction in organic synthesis. Recently, Diederich and Trofimov have reported [2+2] cycloaddition reaction of DDQ with various alkyne derivatives to give the corresponding cycloadducts. They have also reported the unusual reactivities and properties of the DQQ-adducts, such as rearrangement of the carbon skeleton, intramolecular charge-transfer absorption and third-order optical nonlinearities. These results show the DQQ-adducts have a potential as a new series of advanced organic materials. 2H-Cyclohepta[b]furan-2-one is known as a heteroazulene, a versatile precursor for azulene derivatives. However, there are few derivatives for the application to the advanced materials with potentially useful electronic properties. For the extension of this chemistry, we have focused our studies on the synthesis of HETEROCYCLES, Vol. 83, No. 10, 2011 2271

Synthesis and Redox Behavior of Cyanovinyl-Substituted 2H-Cyclohepta[b]furan-2-ones

Cyanovinyl-substituted 2H-cyclohepta[b]furan-2-ones 3, 4, 5 and 7 were prepared by the Wittig, Knoevenagel and electrophilic substitution reaction, respectively. The intramolecular charge-transfer (ICT) interactions between the 2H-cyclohepta[b]furan-2-one ring and cyanovinyl acceptor moiety were investigated by UV/Vis spectroscopy and theoretical calculations. The redox behavior of these novel compounds was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Significant color changes in these compounds were observed under electrochemical reduction conditions by visible spectroscopy. INTRODUCTION 2H-Cyclohepta[b]furan-2-one is known as a heteroazulene, a versatile precursor for azulene derivatives. Although unusual reactivity of the compound including its derivatives has already been revealed by many research groups, there are few derivatives for the application to the advanced materials with potentially useful electronic properties. † Dedicated to Prof. Ei-ichi Negishi on the occasion of his 77th birthday. HETEROCYCLES, Vol. 86, No. 1, 2012 305