Pyrrole bromination

Bulky substituents show steric effects which change the usual selectivity, e.g., large 1-alkyl groups increase -substitution in trifluoroacetylation (80JCR(S)42) and in the Vilsmeier formylation (70JCS(C)2572 of iV-alkyl-pyrroles. With larger substituents like triphenylmethyl in position-1 of pyrrole, bromination, formylation, and trifluoroacetylation occur selectively... 

Electron Transfer Processes of -Pyrrole Brominated Porphyrins Structural vs. Electronic Effects... 

Three series of /I-pyrrole brominated porphyrins have been reported in the literature . The first type-1 is represented as (TPPBr c)M where TPPBr c is the dianion of / -pyrrole brominated tetraphenylporphyrin, x = 0-8 and M = 2H, Mn, Fe, Co, Ni, Cu, or Zn - (Figure 9.1). The TPPBr derivatives have been studied in order to ascertain how distortion of the porphyrin ring will quantitatively affect the redox and spectral properties of the porphyrin as a function of the number of halogen substituents at the yS-pyrrole positions which can be varied from 0 to 8. 

Figure 9.1. Structural formula of type-1 yS-pyrrole brominated metalloporphyrins...

Figure 9.2. Structures of type-2 /S-pyrrole brominate water-soluble porphyrins.

The above discussions clearly demonstrates the importance of 6-pyrrole brominated porphyrins in both fundamental and applied fields of porphyrin chemistry. While one aim of these studies has been to gain insights into the role of macrocycle distortion in relation to biological systems containing porphyrins or related macrocycles , another has been to determine which factors influence the robustness of such catalysts in the oxidation of organic substrates ". Both measurements of electrochemical redox potentials and spectral properties of the porphyrins are needed to understand these properties and both are reviewed in this chapter on yS-pyrrole brominated porphyrins. 

Figure 9.3. Structures of highly substituted, type-3 y6-pyrrole brominated porph5uins used in catalytic applications.

Figure 9.4. Partially yff-pyrrole brominated porphyrins obtained by regioselective synthetic procedure.

Synthesis of Water-Soluble / -Pyrrole Brominated Porphyrins... 

The spectral features of the Fe(III), Mn(III), Co(II), and Zn(II) y9-pyrrole brominated porphyrins in various solvents have been investigated as a function of the number of 6-bromo substituents. In general, the Soret and visible bands in each series of compounds are all increasingly red shifted with increase in the number of Br groups on the macrocycle " ". Moreover, as described in subsequent sections of this review, the utilization of spectroelectrochemistry has provided an opportunity to study the spectral behavior of different brominated porphyrins as a function of their different oxidation states. 

Initial electrochemical studies on partially y9-pyrrole brominated porphyrins (x = 0 ) were carried out by the groups of Gross and Callot and others. In... 

The electrochemistry of (TPPBrs)M, where M = Co, Ni, Cu, Zn, Pt, Pd, and Ag generally revealed the expected easier reduction for both metal-centered and ring-centered reactions as a result of the electron withdrawing bromo substituents. Quantitative work carried out on the (TPPBrjt)M and (TPPBr )MCl series " " yielded a wealth of information and allowed for a separation of geometric (steric) from electronic effects caused by the complete or partial yS-pyrrole bromination. As pointed out earlier, investigations performed on the (TPPBrjc)Zn series of compounds revealed two one-electron oxidations and two one-electron reductions, as expected for porphyrins bearing electroinactive metal ions. Since the first oxidation and first reduction both involved the porphyrin ring, it was possible to estimate the HOMO-LUMO gap (potential difference between the first reversible oxidation and first reversible reduction of the macrocycle) and compare the results with data from computational and spectral studies. As discussed earlier, the computational and experimental plots of... 

Electrochemical Oxidation Behavior of )3-Pyrrole Brominated Metalloporphyrins... 

The electrochemical behavior of water-soluble yS-pyrrole brominated porphyrins is more complex than that of their water insoluble analogs. The metal-centered redox reactions of (TMPyP)Mn and (TMPyPBrg)Mn are reversible while the majority of porphyrin ring-centered redox reactions of the free-base, Cu, and Mn derivatives of TMPyPBrs are irreversible The metal-centered oxidation of (TMPyPBr8)Mn is anodically shifted by 420 mV compared to 1/2 for the corresponding reaction of (TMPyP)Mn (Table 9.2). The metal-centered... 

Figure 9.9. Plots of log K vs. Qo 0 for P5ridme binding of different y8-pyrrole brominated zinc and cobalt porphyrins. (Data taken from ref. [77] for construction of the plot).

The -pyrrole brominated porphyrins have proven to be highly versatile N4 ligands to study structural and electronic effects of the compounds as a function of both the number of bromo substituents on the macrocycle and the type or oxidation state of the metal ion in the porphyrin central cavity. As outlined in this chapter, the studies performed so far on these compounds have revealed pronounced spectral shifts as a function of changes in degree of bromo substitution or nature of the solvent. For all the studied brominated MN4 complexes, the electrochemical... 

D Souza, R, Y.Y. Hsieh, and G.R. Deviprasad (1998). Electrochemical and spectro-electrochemical characterization of water-soluble, /i-pyrrole-brominated cobalt porphyrins. J. Porphyrins Phthalocyanines 2, 429-437. 

Ou, Z., J. Shao, F. D Souza, P. Tagliatesta, and K.M. Kadish (2004). 6-pyrrole brominated meso-tetraphenylporph5Tins S5mthesis, spectral and electrochemical properties. J. Porphyrins Phthalocyanines 8, 201-214. 

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