New Method for the Synthesis of Polyether-Bridged Azulenes:  Reactions of Conjugated Ketocarbenes Generated from the Corresponding Azulenoquinone Diazides

The diazide of 2,4-azulenoquinone carrying 1,3-cyano and carbomethoxy substituents was synthesized and photolyzed in cyclic ethers (THF, dioxane, and tetrahydropyran) to give good yields of 2,4-polyether-bridged azulenes. The shapes of some azulene polyethers were examined by X-ray structural determination to show the cavity size ranging from 6.4 to 4.1 Å (long−short diameters) to 7.36−6.6 Å encircled by an azulene and polyether arc. In analogy to 2,5-cyclohexadien-4-onylidenes, photogenerated conjugated azulenone carbenes were assumed to have energetically closely spaced triplet and singlet ground states that can interconvert readily and react from one of these states under ambient conditions. The spanning of a polyether bridge is initiated by the electrophilic attack of a singlet state on the oxygen of solvent ethers, followed by one or two propagation steps and a ring closure by phenolate attack. The 1,3-dicarbomethoxy group could exert steric effects to hinder the approach of the ether oxygen and control the polyether spanning processes and ring size; this caused a triplet state reaction to supersede the overall reaction. While radical and insertion reactions are common for ketocarbene reactions, the present description adduces a well manifested electrophilic reaction of the allied transients.