Solvent-dependent ambident nucleophilicity of phenoxide ion towards nitroporphyrins: synthesis of 2-hydroxyaryl- and 2-aryloxy-5,10,15,20-tetraphenylporphyrins by displacement of a nitro group

Abstract

The reaction of phenoxide ion with the copper(II), nickel(II) and free-base 2-nitro-5,10,15,20-tetraphenylporphyrins, 1–3 respectively, has been investigated as a means of introducing inwards-directed functionality to the periphery of pre-existing porphyrin systems. It has been found that phenoxide ion shows highly selective solvent-dependent ambident nucleophilicity towards the nitroporphyrin system. Porphyrins 1–3 react with phenoxide ion in refluxing phenol to afford the corresponding 2-(o-hydroxyphenyl)- and 2-(p-hydroxyphenyl)-5,10,15,20-tetraphenylporphyrins in good yields; in each case the ortho isomer is the major product and none of the meta isomer is detected. The scope of the reaction has been extended by the use of the substituted phenols which are blocked from reaction para to the hydroxy (p-cresol and 2,4-dimethylphenol) or blocked from reaction in the ortho-positions (2,6-dimethylphenol). In this way the copper(II) 2-(2-hydroxy-5-methylphenyl)porphyrin 14 (86%), 2-(2-hydroxy-5-methylphenyl)porphyrin 15 (65%), copper(II) 2-(2-hydroxy-3,5-dimethylphenyl)porphyrin 16 (77%), 2-(2-hydroxy-3,5-dimethylphenyl)porphyrin 17 (69%) and 2-(4-hydroxy-3,5-dimethylphenyl)porphyrin 19 (63%) have all been obtained in good yields by reaction of the appropriate 2-nitroporphyrin 1 or 3 with the requisite substituted phenolate in the corresponding phenol. Reaction of the metalloporphyrins 1 and 2 with phenoxide ion in refluxing HCONMe₂ in contrast gives the corresponding 2-phenoxy-metalloporphyrins 10 and 11 in good yield. The results of mechanistic studies using the deuteriated compound, nickel(II) 2-nitro-5,10,15,20-tetraphenyl[3-²H]porphyrin, 37, suggest that both sets of products (from phenoxide as a C-nucleophile and as an O-nucleophile) can arise from ipso-substitution of the nitro group.