THE KINETICS OF ZINC(II) INCORPORATION INTO 55 FREE BASE PORPHYRINS IN DMF - STRUCTURE REACTIVITY CORRELATIONS

The kinetics of zinc(II) incorporation into fifty-five free base porphyrins of varying charges and structures were investigated in dimethylformamide at 25-degrees-C. The rate law is the same as found by Tanaka and co-workers (Bull. Chem. Soc. Jpn., 57 (1984) 204) for tetraphenylporphyrin. One interpretation is that Zn(II) forms a complex with the free base porphyrin H2-P and in a k1 pathway, the coordinated Zn(II) enters the porphyrin, while in a k2 route, a second zinc is needed to produce the final product. While little correlation is found between the rate constants and overall porphyrin peripheral charge, k2 varies over a 26 000 fold range for uncharged species. The replacement of hydrogens by two bulky substituents in the ortho-phenyl positions of tetraphenyl type porphyrins leads to a 200-500 fold decrease in k2, and such ortho groups might inhibit porphyrin ring deformation. Phenyl rings substituted in the para position by groups such as -NH2,-OCH3 and -OH produce more reactive porphyrins that if such substituents are present in the meso-phenyl positions, indicating electron donation to the porphyrin ring by resonance effects. This implies coplanarity between at least one phenyl ring and the porphyrin plane in a distorted metal-porphyrin activated complex. Mercury(II) markedly catalyzes the Zn(II)/H2-P reactions through the very rapid formation of Hg(II)P, which is 10(4) times more reactive with Zn(II) than H2-P itself. The predeformed octabromo-tetramesitylporphyrin metallates approximately 4 x 10(3) times faster than the tetramesitylporphyrin.