Improved Synthesis of Covalently Strapped Porphyrins. Aplication to Highly Deformed Porphiryn Synthesis

Article abstract of DOI:10.1021/jo00242a001

The synthesis of porphyrins carrying a hydrocarbon chain linked to diametrically opposite β-positions is described.The chain, in the form of the diacid chloride of an α,ω-dicarboxylic acid (7), was reacted at its termini, with 2 equiv of 2-(ethoxycarbonyl)-3,5-dimethylpyrrole (6), and chain-linked bis<5-(ethoxycarbonyl)pyrrole> 8 so obtained was transformed into the pyrrole-2-carboxaldehyde 13 by using standard methodology.Protection of the formyl groups as the dicyanovinyl derivative and the activation of the 2-methyl substituents with sulfuryl chloride gave the bis<2-(chloromethyl)pyrrole> 16, which on reaction with a 5-unsubstituted 2 -pyrrolecarboxylate (18), in warm gacial acetic acid, afforded the chain-linked dipyrromethane dimer 20 in high yield.Regeneration of the formyl substituents and removal of the ester group (ethyl ester and benzyl ester variations are described) produced the chain-linked 5-formyl-5'-unsubstituted-dipyrromethane dimer 22, which was cyclized intramolecularly, under high dilution, to give the strapped porphyrins 26 in good yield.This strategy of strap first, porphyrin last, enabled the successful synthesis of porphyrins carrying 11-, 10-, or 9-carbon bridges, which enforce a progressively increased distortion of the macrocycle, as evidenced by visible spectroscopy and X-ray crystallography.