92396-64-0Relevant academic research and scientific papers
Kinetics of dealkylation reactions of N-alkylporphyrin complexes. 4. Effect of N-alkyl and N-aryl substituents on the dealkylation process
Lavallee, David K.,Kuila, Debasish
, p. 3987 - 3992 (2008/10/08)
The dealkylation reactions of N-alkylporphyrins are of interest with respect to the formation of N-alkylporphyrins from the cytochrome P-450 prosthetic group and to the use of N-alkylporphyrins as intermediates for the rapid synthesis of non-N-alkylated porphyrin complexes. Previous work has demonstrated that the rate of dealkylation is strongly influenced by the nature of the metal ion, the nucleophile, and the solvent medium but that typical porphyrin ring substituents have little effect. In this report, we have examined the effect of the alkyl or aryl group on the dealkylation rate using Cu(II) complexes of monosubstituted 5,10,15,20-tetraphenylporphyrin in acetonitrile with di-n-butylamine as the nucleophile. The dealkylation reactions of Cu(II) complexes show biphasic kinetics, with the predominant path at low amine concentration being attributed to the reaction of the complex with solvent as axial ligand. At 25°C (with the N-alkyl or N-ary substituent in parentheses), the rate constants for this path are 2.94 × 10-3 M-1 s-1 (CH3), 2.27 × 10-4 M-1 s-1 (CH2CH3), 0.271 M-1 s-1 (p-nitrobenzyl), and -6 M-1 s-1 (phenyl). The activation parameters are as follows: ΔH≠ = 16.9 ± 1.0 kcal/mol, ΔS≠ = -13 ± 3 eu (CH3); ΔH≠ = 18.2 ± 1.0 kcal/mol, ΔS≠ = -14 ± 3 eu (CH2CH3); ΔH≠ = 14.8 ± 1.0 kcal/mol, ΔS≠ = -12 ± 3 eu (p-nitrobenzyl). The rate constants for the path that is second order in amine are 3.0 × 10-4 M-1 s-1 (CH3), 3.7 × 10-5 M-1 s-1 (CH2CH3), and 7.9 × 10-2 M-1 s-1 (p-nitrobenzyl). Thus, the alkyl group has a major effect on the rate, and the trend in rates suggests that good carbocation stabilization facilitates the reaction and steric hindrance has a significant but not as pronounced an effect. The rates are relatively slow for simple alkyl groups, suggesting that dealkylation of an N-alkylated protoheme intermediate in the decomposition of cytochrome P-450 would be an inefficient process.
