Porphyrins metal-free

Porph-Ph-porph)3 1 A-phenylene bridged porphyrins Metal free and Zn several types of substitutions on the porphyrin. [162]... 

In numerous synthetic studies,9" 6 100 it has been demonstrated that porphyrins react at the chromophore periphery in cycloaddition reactions, rearrangements, conjugative additions and substitution reactions to yield interesting porphyrin derivatives. Thus, metal-free protoporphyrin IX dimethyl ester reacts in Diels-Alder reactions108a b with dienophilcs like ethenetetra-carbonitrile and acetylenedicarboxylates at the diene structural parts to yield, according to the reaction conditions, the corresponding monoadducts 2 and 3 (see also Section 1.2.) and bisadducts 1 (see also Section 1.4.), respectively. 

Oligomerization of the simple monomeric metal-free porphyrin forms the tetramer in only trace amounts. 

Similiar problems of regioselectivity as in reduction reactions are encountered in oxidation reactions of porphyrins and chlorins. The oxidation of chlorins to isobacteriochlorins can be directed by insertion of zinc(II) or nickel(II) into the macrocycle. Again here, the metal-free chlorins give the bacteriochlorins whereas the metal chlorins, e.g. 1, give isobacteriochlorins, e.g. 3.15a,b I 7... 

The cw-isobacteriochlorin 16A formed by the Claisen rearrangement has been utilized for the further transformation into porphyrin dp2311 0 the metal-free ligand of heme dr... 

Oxidation reactions were used in the synthesis of porphyrin d, the metal-free ligand system of naturally occurring heme d,. In a total synthesis of porphyrin d,12d oxo functions were introduced into isobacteriochlorin 3 by selenium dioxide oxidation to yield 4. The selenium dioxide selectively attacks the 3- and 8-positions of the partially reduced pyrrole rings of the chromophore. In another synthesis23a c of porphyrin d, an isobacteriochlorin 5, derived by... 

Several reports of carboranylated porphyrins have appeared for utilization in the BNCT of cancer.150 The examples include both nido- (126) (Fig. 73) and closo-carborane cluster-containing (124 and 125) (Fig. 73) materials. Metal-free nido-carboranylporphyrins were able to deliver a higher amount of boron to cells than the corresponding zinc complexes. 

The q1-coordinated carbene complexes 421 (R = Ph)411 and 422412) are rather stable thermally. As metal-free product of thermal decomposition [421 (R = Ph) 110 °C, 422 PPh3, 105 °C], one finds the formal carbene dimer, tetraphenylethylene, in both cases. Carbene transfer from 422 onto 1,1-diphenylethylene does not occur, however. Among all isolated carbene complexes, 422 may be considered the only connecting link between stoichiometric diazoalkane reactions and catalytic decomposition [except for the somewhat different results with rhodium(III) porphyrins, see above] 422 is obtained from diazodiphenylmethane and [Rh(CO)2Cl]2, which is also known to be an efficient catalyst for cyclopropanation and S-ylide formation with diazoesters 66). 

For the photodiagnostic use of these compounds, a high quantum yield of fluorescence, r, is desirable. The metal complexes of the common first-row transition metals are not suitable, because they show very low 4>f values. On the other hand, porphyrin complexes of d° and d10 elements show appreciable fluorescence, although generally less than that of the metal-free compounds, presumably because of the heavy-atom effect (e.g., TPP ZnTPP, Table 5). The further operation of the heavy-atom effect, which increases the rate of intersystem crossing (/cisc) by... 

The effects in evidence are complex. There is clearly an electrostatic effect due to the central positive charge [Ei/2 MIV > M111 > M11]. Nonetheless, SnIV, GeIV, Fem, and Ptn complexes are more difficult to oxidize than are the metal-free porphyrins while in contrast the Zn11, Cd11, and... 

Weiss et al. have employed the latter method to synthesize the gadolinium analogues [144]. Treatment of Li2Pc with 2equiv. of Gd(acac)3- H20 produces Gd(Pc)(acac), which further reacts with 1 equiv. of H2(TPP) giving GdPc(TPP) in 65% yield and a small amount of Gd2(Pc)(TPP)2. However, by using 4 equiv. of the metal salt and 2.5 of the metal-free porphyrin, the binuclear complex Gd2(Pc)(TPP)2 has been isolated in 78% yield. 

Although direct reaction of lanthanide mono-porphyrins with free phthalo-cyanine or its lithium derivatives is generally more efficient than the template synthesis, and gives rise to mixed-ligand complexes, the template strategy can also be applied for synthesis of phthalocyanine-porphyrin complexes, as in the case of unsymmetric bisphthalocyanine complexes (Scheme 8.2, B(b)) [106, 136, 145, 146]. Thus, metallation of free porphyrins with lanthanide salts in TCB or n-octanol leads to single-decker complexes, which then react with phthalonitriles under the action of DBU in alcoholic media to give the desired compounds. 

Most clinically used PDT sensitizers are metal-free porphyrins and porphyrin derivatives such as 56. Hematoporphyrin and its deriva-... 

Many metal-free porphyrins or porphyrins lacking a metal and therefore unable to ligate with His residues are nonetheless also bound by hemopexin, and changes in absorbance and fluorescence of both hemopexin and the ligand result. However, the affinity of binding... 

The absorption cross sections of the Sn Si transition are determined from the kinetic analyses based on the two-photon absorption measurement. In contrast, tetraphenylporphin scarcely show the S2 state fluorescence. In order to elucidate the effect of the imino hydrogens on the radiationless transitions, deuterium isotope effect on the relaxation processes from the lowest excited singlet state of metal free porphyrins has been investigated. 

Porphyrins in nature are usually synthesized in a metal-free form, the metal ions being inserted subsequently. This latter process is catalyzed by enzymes (90). 

The porphyrin macrocycle is an ampholyte with two pyrrolenine (=N—) nitrogen atoms capable of accepting protons, and two NH groups capable of deprotonation. The most useful scheme for assigning pK values is due to Phillips (60MI30700) in this the metal-free porphyrin is abbreviated PH2, the dianion P2 and the dication PH42+ ... 

The dianions (P2 ) of the weakly acidic, metal-free porphyrins are generated for spectroscopic observation using metal alkoxides. The pK and pK2 values for etioporphyrin-I (Table 1) have been estimated to be about +16 pK3 for octaalkylporphyrins such as etioporphyrin-I, measured for solubility reasons in 2.5% sodium dodecyl sulfate, is about 5.5, but spectroscopic observation of the monocation is difficult owing to the resonance stabilization available in the symmetrical dication (PH42+). Generally, pK3 and pK4 differ... 

Most structural information is gained from the less intense (e 2000-20 000) satellite peaks which appear, for metal-free, neutral porphyrins, between 500 and 700 nm. Visual inspection of the intensity ratios of these minor peaks enables four types of peripheral or methine substituent patterns to be identified, and these are shown in Figure 2. The etio ... 

In acidic solution, where the dication (PH42+) of metal-free porphyrins is produced, the Soret band usually sharpens noticeably and is displaced to longer wavelength. Owing to the symmetrical nature of the -rr-cloud in the dication, its satellite bands simplify to give two major peaks, with minor absorptions giving rise to shoulders (Figure 3a). Symmetry... 

Early workers appeared to show that electrophilic substitution reactions could not be carried out on porphyrins, and began to question the aromaticity of porphyrins since this classical pre-requisite of aromatic character could not be accomplished. However, they had concentrated on reactions of metal-free systems, and since many electrophilic substitution reactions utilize acidic conditions (nitration, sulfonation), they were actually dealing with the non-nucleophilic porphyrin dication. But, as early as 1929, H. Fischer had realised that diacetylation of deuteroporphyrin-IX (Table 1) had to be carried out on a metal complex, such as the iron (III) derivative chelation with a metal ion which cannot be removed under the acid conditions of the subsequent reaction, effectively eliminates dication formation. A judicious choice of metal complex therefore needs to be made for any particular reaction. For example, though magnesium(II) produces an extremely reactive substrate for electrophilic substitution reactions, it is removed by contact with the mildest of acids and is, consequently, of little use for this purpose. 

Porphyrins can also be sulfonated a t the peripheral positions, and copper(II) porphyrins react with thiocyanogen to give the methine-thiocyanatoporphyrin which can be hydrolyzed to the corresponding mercapto derivative. Reactions between metalloporphyrins and car- benes are known, but they tend to give mixtures of products owing to addition at methine positions as well as across peripheral double bonds. Nitrenes also react with metal-free porphyrins to give insertion products and methine-substituted derivatives. 

Reduction potentials of a large number of peripherally substituted metal-free porphyrins have been measured, and substituent partial potentials have been derived the reduction potentials can also be correlated with porphyrin basicity (p/Cs) and reactivity (80MI30701). 

Fig. 1. Suggested structure of a metal-free porphyrin molecule...

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