RATIONAL TETRAARYLPORPHYRIN SYNTHESES - TETRAARYLPORPHYRINS FROM THE MACDONALD ROUTE

Four new synthetic routes to meso-tetraarylporphyrins using a MacDonald-type 2 + 2 condensation are described. Self-condensation of a 5-aryldipyrromethane (e.g., 17) with an aryl-substituted one-carbon bridging unit affords a mixture of tetraarylporphyrins due to acid-catalyzed redistribution reactions. The second and third methods presented here show wide applicability for the preparation of 5,10,15,20-tetraaryl-substituted porphyrins (e.g., 31, 37, 48, 49) with 2-fold rotational symmetry and involve self-condensation of 5-aryl-1-aryldipyrromethanecarbinols. Finally, the fourth approach involves a 2 + 2 approach in which one of the two dipyrromethanes bears both of the bridging carbons in the porphyrin products, affording porphyrin 50 which possesses three different aryl rings, with one pair of uniquely opposite identical aryl groups. The last two 2 + 2 methods are further extended to give a tetraarylporphyrin 38 bearing four different aryl groups in a predesignated array, the structure of which is confirmed by a single-crystal X-ray study.