Synthesis of a spheroidal bis-porphyrin: a ligand designed to accept two catalytic metal ions in an isolated environment

Abstract

A spheroidal bis-porphyrin (dual capped quadruply cofacial dimeric tetraphenylporphyrin, 1), designed to be employed as a ligand for a class of catalysts that mimic the combined enzyme activities of superoxide dismutase and catalase, has been synthesized in 9 steps. C-Alkylation of 2-(2′-lithio-5′- tert-butyldimethylsiloxy-methyl)phenyl-1,3-dioxolane ( D) with tetra-bromide 6, prepared from bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride by 5 steps, afforded the key intermediate, tetra-alkylated compound 3a, in 68% yield. An alternative route to 3, O-alkylations of tetrol 4 with aliphatic halides 5, was also tested. Conversion of the four protected hydroxy groups to bromo substituents is crucial in the preparation of porphyrin 2. Therefore, tetra-benzyl bromide 24b was prepared and then treated with pyrrole in the presence of boron trifluoride etherate, followed by DDQ oxidation to obtain the capped porphyrin 2. The coupling of two molecules of 2 to form a spheroidal porphyrin 1 was carried out in dilute DMF solution, using 0.8 equivalent of 2-trimethylsilylethane sulfonamide (H 2NSO 2CH 2CH 2Si(CH 3) 3, SES-NH 2) and cesium carbonate. The structure of 1 was confirmed by 1H NMR, UV/VIS, FABMS and laser desorption high resolution MS. The two isomers of 1, eclipsed and gauche, formed due to two possible modes of approach of the units of 2, were observed in approximately 1 : 1 ratio by both 1H NMR and HPLC techniques.