Phenylsubstituted Porphyrins. 2. Synthesis of 5-Arylporphyrins

Abstract

Investigation of porphyrins, that are widely present in the nature, is often carried out with the use of their synthetic analogs as model compounds. meso-Tetraarylporphyrins 1 containing different substituents on the phenyl rings can be easily prepared by condensation reaction of pyrrole with corresponding benzaldehydes in the acidic media.[1-3] Unlike natural species, meso-tetraarylporphyrins don’t have any alkyl or pseudoalkyl substituents in the β-positions of porphyrin macrocycle but, oppositely, have aryl rings in the meso-positions which can contain different active substituents allowing their further structural modification. On the other hand, β-octaalkylporphyrins 2, being also available, contain no active groups what makes difficult their modification. That’s why the porphyrins, combining the advantages of these two classes of porphyrins (i.e. 5-aryloctaalkylporphyrins 3) are of great interest. Presently several synthetic methods for 5-aryloctaalkylporphyrins 3 are known. One of the approaches is condensation of meso-aryl-3,3’,4,4’-tetraalkyldipyrrolylmethanes 4 with 5,5’-diformyl-3,3’,4,4’tetraalkyldipyrrolylmethanes 5 in CH3OH or CH2Cl2 under the action of strong acids (HI, HClO4 or p-toluenesulfonic acid)[4-9] (Scheme 1). In this reaction atmospheric oxygen or derivatives of benzoquinone with electron-withdrawing substituents (por o-chloranil, 2,3-dichloro-5,6-dicyanobensoquinone (DDQ)) act as oxidizers of the intermediate porphodimethene 6. In a similar manner 5-aryloctaalkylporphyrins can be obtained by condensation of meso-aryl substituted dipyrrolylmethanes 4 with hydrobromides of 5,5’-dimethoxymethyl-3,3’,4,4’-tetraalkyldipyrrolylmethenes 7 in refluxing benzene with following oxidation of the intermediate product 8 by benzoquinone derivatives (Scheme 2).[10]