Porphyrin complexes, with copper

Kinetic studies have proposed evidence for the formation of heterodinuclear intermediate. In order to study the structure of the intermediate in solution as related to the reaction mechanism, extended X-ray absorption fine structure (EXAFS) measurements have been undertaken for the metal-substitution reaction of mercury(ll) porphyrin complex with copper(II) in an acetate buffer (pH = 5.6). 

Stable structures such as the naturally occurring ferric porphyrin complexes or porphyrins substituted with copper, cobalt, silver or vanadyl probe the active site of heme-containing enzyme [227]. Complexes of copper not associated with heme are also common. They are frequently formed at an amino terminus because the amino group provides a good primary amine donor atom. Two or three amino acid residues beginning at the N-terminus are often flexible until a more rigid portion of the polypeptide, such as the a helix, is encountered. A peptide nitrogen is available to... 

A) Frozen solution EPR spectra of Fe (TPP)(4-MeIm) (top) and Fe (TPP)(4-MeIm)2 (bottom) prepared by addition of 4-methylimidazolate anion (4-MeIm ) to a solution of Fe(TPP)(SbF6). The top spectrum is characteristic of a high-spin Fe" -porphyrin complex, with a resonance atg = 6(g = 2.7, 2.3, and 1.8 are due to formation of a small amount of Fe" (TPP)(4-MeIm)2 ). The bottom spectrum is characteristic of a low-spin ferric-porphyrin bis(imidazole)-type complex.(B) Frozen solution EPR spectrum of Cu"(ImH)4-+ with gjl = 2.06, A = 183 G, and gj = 2.256 (courtesy of Dr. J. A. Roe). This type of spectrum is typical of square-planar Cu complexes, except that the ligand hyperfine splitting of the gi feature is frequently unresolved, especially in copper proteins (for example, see Figure 5.20). (C) Simulated EPR spectrum of a typical organic free radical with no hyperfine interaction. 

Metal Porphyrin Formation.— The rate of formation of cationic porphyrin complexes of copper(ii) is dependent on the basicity of the ligand. Tetra-(4-A, iV, iV"-trimethylanilinium)porphyrin, H2TAP, reacts with a second-order rate constant of 6.2 s, almost thirty times faster than the corresponding... 

The preparation of cyclopropanes by intermolecular cyclopropanation with acceptor-substituted carbene complexes is one of the most important C-C-bond-forming reactions. Several reviews [995,1072-1074,1076,1077,1081] and monographs have appeared. In recent decades chemists have focused on stereoselective intermolecular cyclopropanations, and several useful catalyst have been developed for this purpose. Complexes which catalyze intermolecular cyclopropanations with high enantiose-lectivity include copper complexes [1025,1026,1028,1029,1031,1373,1398-1400], cobalt complexes [1033-1035], ruthenium porphyrin complexes [1041,1042,1230], C2-symmetric ruthenium complexes [948,1044,1045], and different types of rhodium complexes [955,998,999,1002-1004,1010,1062,1353,1401-1405], Particularly efficient catalysts for intermolecular cyclopropanation are C2-symmetric cop-per(I) complexes, as those shown in Figure 4.20. These complexes enable the formation of enantiomerically enriched cyclopropanes with enantiomeric excesses greater than 99%. Illustrative examples of intermolecular cyclopropanations are listed in Table 4.24. 

It is well known that crystal and electronic structures are interdependent and define the reactivity of chemical substances. In Section 1.4.2, it was noted that copper-porphyrin complex gives cation-radicals with significant reactivity at the molecular periphery. This reactivity appears to be that of nucleophilic attack on this cation-radical, which belongs to n-type. The literature sources note, however, some differences in the reactivity of individual positions. A frequently observed feature in these n-cation derivatives is the appearance of an alternating bond distance pattern in the inner ring of porphyrin consistent with a localized structure rather than the delocalized structure usually ascribed to cation-radical. A pseudo Jahn-Teller distortion has been named as a possible cause of this alternation, and it was revealed by X-ray diffraction method (Scheidt 2001). 

Other aspects of solvation have included the use of surfactants (SDS, CTAB, Triton X-100), sometimes in pyridine-containing solution, to solubilize and de-aggregate hemes, i.e., to dissolve them in water (see porphyrin complexes, Section 5.4.3.7.2). An example is provided by the solubilization of an iron-copper diporphyrin to permit a study of its reactions with dioxygen and with carbon monoxide in an aqueous environment. Iron complexes have provided the lipophilic and hydrophilic components in the bifunctional phase transfer catalysts [Fe(diimine)2Cl2]Cl and [EtsBzNJpeCU], respectively. 

Crude oil consists mainly of a mixture of paraffinic, naphthenic, and aromatic hydrocarbons with small amounts of metals-containing heterocyclic compounds. The most abundant metals found in oils are those contained in porphyrin or porphyrin-like complexes (nickel, copper, iron, and vanadium). These... 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

Considerable variation in stereocontrol can also occur, depending on the catalyst employed (equation 125). In general, the various rhodium(II) carboxylates and palladium catalysts show little stereocontrol in intermolecular cyclopropanation162,175. Rhodium(II) acetamides and copper catalysts favour the formation of more stable trans (anti) cyclopropanes162166. The ruthenium bis(oxazolinyl)pyridine catalyst [Ru(pybox-ip)] provides extremely high trans selectivity in the cyclopropanation of styrene with ethyl diazoacetate43. Furthermore, rhodium or osmium porphyrin complexes 140 are selective catalysts... 

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