Synthesis of conjugation-expanded porphyrins based on the retro Diels-Alder reaction

Abstract

Bicyclo-, benzobicyclo-, and naphthobicyclo- [2.2.2] octadiene-fused porphyrins were prepared from ethano-bridged isoindole derivatives via the common methods such as cyclic tetramerization, MacDonald [2+2], and [3+1] porphyrin syntheses. Heating these porphyrins over 200 °C caused the retro Diels-Alder reaction leading to quantitative extrusion of ethylene to give the corresponding benzo-, naphtho-, and anthra-fused porphyrins, UV absorption of which showed efficient red-shift compared to those of the starting porphyrins. Upon the thermal treatment, the very bulky bicyclic-ring-fused porphyrins readily soluble in common solvents were converted to the flat, insoluble, conjugation-expanded porphyrins, purification of which often encountered difficulty. This methodology provides not only an easy access to highly conjugated porphyrins and related chromophores in very high quality, but also a significant conversion of soluble dyestuffs to insoluble pigments.