Four-atom-linked capped porphyrins: Synthesis and characterization

Three new capped porphyrins, H-2(OC(2)OPor) (7), H-2(OC(CO)NPor) (13), and H-2(OC(2)NPor) (14), to be used as heme model compounds, have been synthesized and characterized. These compounds have the shortest linkages between cap and porphyrin plane of any four-arm capped porphyrins synthesized to date. The general synthetic procedure for H-2(OC(2)OPor) involves the reaction of a tetraaldehyde ''cap'' with pyrrole to form the capped porphyrin. In the synthesis of the tetraamide-capped porphyrin, H-2(OC(CO)NPor), a tetraacid chloride cap is reacted with meso-alpha,alpha,alpha,alpha-tetra-(o-aminophenyl)porphyrin. The amide groups may be reduced to form H-2(OC(2)NPor). With the use of FeBr2, Fe2+ may be inserted in high yield into Ha(2)(OC(2)OPor) and H-2(OC(CO)NPor). The crystal structures of H-2(OC(2)NPor), Fe(OC(2)OPor)(OMe), and [Fe(OC(2)OPor)](2)(mu-O) provide details on the types and amounts of cap expansion necessary to accommodate small ligands of biological interest, such as O-2 and CO.