Selective Cycloaddition of Tetracyanoethene (TCNE) and 7,7,8,8‐Tetracyano‐p‐quinodimethane (TCNQ) to Afford meso‐Substituted Phenylethynyl Porphyrins

π-Extended TCBD-porphyrins that contained a 1,1,4,4-tetracyanobuta-1,3-diene unit were prepared by a highly efficient [2+2] cycloaddition of tetracyanoethene (TCNE) or 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) with meso-substituted trans-A(2)B(2)-porphyrins that contained two phenylethynyl groups, followed by a retro-electrocyclization reaction. Depending on the electronic properties of the arylethynyl groups, the cycloaddition reaction took place exclusively on either one or two ethynyl moieties with high yield. The addition of TCNQ proceeded with complete regioselectivity. The resulting π-expanded TCBD-porphyrins had a hypsochromically shifted Soret band and showed unique, broad absorption in the visible region.