Configurational and conformational isomeric antiaromatic [28]tetraoxaporphyrinoids(4.2.4.2) and aromatic [26]tetraoxaporphyrin(4.2.4.2) dications. A new type of molecular dynamics in macrocyclic systems

Abstract

The [28]tetraoxaporphyrinoids(4.2.4.2) I are synthesized by cyclizing Wittig reaction. An alternative path is the McMurry condensation. Four different configurational isomers of I were isolated and characterized by 1H NMR. The inner and outer protons of the (E,E)-dienediyl bridges in (Z,EE,Z,EE)-I exchange by rotation around the adjacent single bonds. In (Z,EE,E,EE)-I, the (E)-ethenediyl bridge is rotationally active, while in (E,ZE,E,EZ)- and (E,EZ,E,EZ)-I, rotation of both (E)-ethenediyl bridges is obsd. When the rotation of the active (E)-double bonds at temps. T <-90° is frozen, all configurational isomers of I appear to be antiarom. and paratropic. The oxidn. of (E,ZE,E,EZ)- and (E,EZ,E,EZ)-I with DDQ yields 2 arom., diatropic [26]tetraoxaporphyrin(4.2.4.2) dications both with (E,EZ,E,EZ)-configuration but different fixed conformations. (Z,EE,Z,EE)-I is oxidized to give the (Z,EE,Z,EE)-dication, while the oxidn. of (Z,EE,E,EE)-I yields a mixt. of isomer dications. The std. formation enthalpies of the obtained and expected [28]tetraoxaporphyrinoids I and [26]tetraoxaporphyrin dications were calcd. with the AM1 method, showing good accordance with exptl. results.