Electron deficient porphyrin macrocycl

Naruta et al. [225, 226] designed the twin-coronet porphyrin ligands (62) and (63) with binaphthyl derivatives as chiral substituents (Figure 13). Each face of the macrocycle is occupied by two binaphthyl units and the ligand has C2 symmetry. Iron complexes of these compounds can be very effective catalysts in the epoxidation of electron-deficient alkenes. Thus, nitro-substituted styrenes are readily epoxidized in 76-96% ee [226]. The degree of enantioselectivity can be explained on the basis of electronic interactions between the substrate aromatic ring and the chiral substituents rather than on the basis of steric interactions. 

Similar to quinones and NAD+ the flavins are one- and two-electron redox systems (Fig. 7.2.10a). Upon half-reduction a quinhydrone-type dimer appears, producing a charge transfer band at 820 nm (Fig. 7.2.10b). The noncovalent interactions of flavins arise from electrostatic attraction between electron-rich donor atoms, in particular the basic oxygen atoms of amide groups, and electron-deficient aromatic systems, the inner conjugation system of oxidized flavins (Breinlinger et al., 1998). This is reminescent of the interactions between porphyrin macrocycles (see Fig. 6.2.15). 

Electron-deficient species, such as carbenes and nitrenes, attack the porphyrin macrocycle, but in completely different ways. Thus, carbenes tend to attack at the 3-pyrrole positions [Figure 3.18(a)] to give mainly cyclo-... 

Fig. 3. Catalytic cycle of cytochrome P-450 and the postulated partial structures of the intermediates. The dianionic porphyrin macrocycle is abbreviated as a parallelogram with nitrogens at the comers, in this and subsequent figures. Oxy-P-450 (4) is shown as a complex of ferric porphyrin and superoxide anion, but could also be describe as an adduct of neutral dioxygen and ferrous porphyrin. States 6 and 7 are hypothetical intermediates whose structures have not been established. Structures /, 2, and 7 are neutral (the dot and the positive charge on 7 indicate the radical state and electron deficiency of the n electron system of the porphyrin ring), while the overall charge on structures 3, 4, and 5 is minus one and on structure 6 is minus two. Adapted from Ref. [22]...

The electroreduction of Ni(II) porphyrins and related macrocycles was investigated by the groups of Stolzen-berg [330-332], Fajer [333], Kadish [116, 323, 328, 334, 335], Saveant [324] and more recently, by Fajer, Mansuy and coworkers [336], who showed that the very electron-deficient ((NO2 lyT (0X0-CI2) PP) Ni was reduced in three reversible one-electron reductions. Earlier work had shown that Ni(II) chlorins or isobacteri-ochlorins could be reduced to their Ni(I) state [330, 331, 333], as opposed to porphyrins, where a Ni(II) porphyrin jr-anion radicals was generally formed upon reduction [7]. However, the singly reduced products of Ni(II) porphyrins have been described as Ni(I) porphyrins, Ni(II) porphyrin r-anion radicals or Ni(II) porphyrin jr-anion radicals with some Ni(I) character [116]. The exact site of electron transfer and formulation of the reduction product will depend on several factors, two of which include the solution conditions and the nature of the porphyrin macrocycle [7]. 

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