The reactivity of picket fence porphyrins (meso-tetrakis(o-(alkylcarbonyl)aminophenyl)porphyrins) toward metalation with Cu2+ or Zn2+ shows an extremely wide rate range in aqueous anionic surfactant solutions (sodium dodecyl sulfate (SDS) micelles and Aerosol OT (AOT) reversed micelles, a factor of 35000) compared to homogeneous solution (N,N-dimethylformamide, range = 156). The rates vary systematically as a function of the chain length of the N-alkyl substituent and, more significantly, the atropisomer configuration. A corresponding broad range of apparent porphyrin basicities (core diprotonation constants) spanning 7.0 pK(a) units is observed with the different sets of atropisomers in aqueous SDS, again under conditions where relatively little variation is observed in homogeneous media. The correlation of basicities with metalation rates suggests that anionic interfacial catalysis plays an important role in both processes; the spread in rate and equilibrium constants for different porphyrin atropisomers suggests that topologially controlled orientation of the porphyrin macrocycle with respect to the interface governs reactivity. Specifically, the observed behavior in SDS and AOT is consistent with two separate equilibrating interfacial orientations for the 4,0 atropisomer whose relative proportions vary with side chain length, and a more nearly perpendicular orientation for short chain cis-2,2 and trans-2,2 derivatives. These orientations lead to an inverted reactivity order for the short chain compounds in organized media (i.e. 4,0 < 3,1 < cis-2,2 < trans-2,2) as compared to those in homogeneous solution. Spectra intermediacy of the porphyrin monoacid (H3P+) during acid titration occurs exclusively for the short chain 4,0 derivatives, for which an "inverted" interfacial orientation (hydrophobic core solubilization) is proposed; monoacid "stabilization" is attributed to prerequisite porphyrin migration/reorientation into a more anionic microenvironment prior to diprotonation. Catalytic metalation and elevated interfacial basicities strongly support interfacial solubilization for all of the tetraamidophenyl porphyrins studied such that the porphyrin core is located in close proximity to surfactant head groups while solubilization in sites more remote from the anionic head groups is proposed for hydrophobic porphyrins such as tetraphenylporphyrin (TPP).
