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Steric and Stereoelectronic Effects on Metal Ion Incorporation into Picket Fence Porphyrins in Homogeneous Solution: The Relationship of Molecular Conformation to Reactivity
Barber, David C.,Lawrence, David S.,Whitten, David G.
, p. 1204 - 1215 (1992)
Rate constants for the incorporation of Cu(II) and Zn(II) using the perchlorate salts were measured for the four atropisomers of C2 and C16 derivatives of meso-tetrakis(o-alkylamidophenyl)porphyrin, the picket fence porphyrins (PFPs) in N,N-dimethylformamide (DMF).A 156-fold variation in rate constants is noted for Cu(II) incorporation into the 25 PFPs studied in DMF which are 56.5-8800 times less reactive than meso-tetraphenylporphyrin (TPP).For Cu(II) incorporation, the reactivity order is 4,0 > 3,1 > cis 2,2 > trans 2,2 in all cases.The span of metalation rate constants for individual isomers as a function of side chain length follows the opposite order, consistent with increasing steric exclusion of metal ion as the number of substituents (0-2) about the less hindered face of the porphyrin increases.A 2.2 x 105-fold range of facial metalation rate constants calculated for TPP and the PFPs indicates that metalation of the 4,0 isomer occurs predominantly (>99percent) from the unsubstituted face.Increased perpendicularity between the phenyl rings and the porphyrin core with increasing side chain bulk, attributed to (1) steric interactions between ortho substituents and the porphyrin core and (2) transannular side chain interactions, is postulated to result in enhanced core rigidity for the 4,0 isomer and consequent decreased metalation rates.This postulate is supported by 1H NMR and UV-visible spectral data for the set of compounds studied, relative basicities, and molecular models.For the 4,0 isomer, whose metalation is controlled by such stereoelectronic factors, side chain branching more effectively inhibits metalation (range of kCu(2+) = 4.7 M-1s-1) than linear side chain extension (range of kCu(2+) = 2.2 M-1s-1).The results of this study are relevant to the spectroscopy, chemistry, and solution structure of tetraarylporphyrins.
