Reaction of metallo-2-nitro-5,10,15,20-tetraphenylporphyrins with oxyanions. Temperature-dependent competition between nucleophilic addition and single-electron transfer processes

Abstract

The outcome of the reaction of metallo-2-nitro-5,10,15,20-tetraphenylporphyrins with oxyanions is dependent both on the nature of the coordinated metal ion and on the reaction temperature. Metallo-2-nitro-5,10,15,20-tetraphenylporphyrins with relatively electronegative chelated metal ions (Cu^II, Ni^II and Fe^IIICl)react with methoxide ion in N,N-dimethylformamide to afford mainly the corresponding metallated 2-methoxy-3-nitroporphyrins and 2,2-dimethoxy-3-nitro-2,3-dihydroporphyrins. The zinc^II chelate 7 is unreactive towards methoxide under identical conditions. When the room temperature reaction of (2-nitro-5,10,15,20-tetraphenylporphyrinato)nickel(II) 5 with methoxide is quenched with water after 1 min, the trans-(2-methoxy-3-nitro-2,3-dihydroporphyrinato)nickel(II) 14 is obtained. Inclusion of nitrobenzene in the reaction of the copper(II) chelate 4 with methoxide is beneficial and results in the formation of the 2,2-dimethoxy-3-nitro-2,3-dihydroporphyrin 8 in 90% yield. Reaction of 4 with benzyl oxide ion gives analogous products but in lower yield, while the major product from reaction of 4 with hydroxide ion is the 2-hydroxy-3-nitroporphyrin 16. At elevated temperatures the nature of the reaction of the copper(II) chelate 4 with methoxide changed to afford the corresponding copper(II) 2-alkoxyporphyrin 10 and the denitrated parent porphyrin 20; the later compounds arise from radical reactions. A mechanistic rationale for the processes involved in these temperature-dependent reactions is presented. The ease with which these reactions occur is a consequence of the fact that all the reaction intermediates are able to retain macrocyclic aromaticity.