Hydroporphyrins

The analysis is different for the intermediate states. TPiBC is predicted to form aromatic cis-intermediates CD and DC which are then lowered in energy as compared to the zwitterionic intermediates AB and BA. Thus, Fig. 6.21c predicts the reactions of iso-bacteriochlorin to be faster than those of porphyrin. On the other hand, the aromatic character of bacteriochlorin is lost in the intermediate states, moreover the trans-tautomers BD and DB of bacteriochlorin exhibit a zwitterionic structure. Thus, one should expect a substantial increase in the barrier height of the exchange between AC and CA in bacteriochlorin as compared to porphyrin. 

These predictions were indeed confirmed experimentally. Schlabach et al. [19b] showed that in the case of TPC the trans-tautomers BD and DB cannot be observed directly by NMR however, rate constants of the HH and the HD reaction could be obtained for the interconversion between AC and CA. According to an analysis similar to that leading to Eq. (6.25) it was shown that in the case of the HH reaction the observed rate constants are given by 

It has been shown [19b] that for the HD reaction two pathways are possible exhibiting reaction profiles similar to those of Fig. 6.13(c) and (d). The rate constants of the HD reaction are given by 

This result can also be directly obtained by inspection of Fig. 6.21(b), setting H = H and H, = D the pathways of the HD reaction are dominated by the steps where D is transferred from ring C to D to A, so that H is transferred from A to B to C. The latter pathway is rate-limiting and exhibits the same rate constant as the corresponding HH reaction. The factor of Vi in Eq. (6.52) arises from the fact that the alternative pathway where D is transferred via ring B is much slower. 

The corresponding Arrhenius curves obtained are included in the lower part of Fig. 6.28(b). Within the margin of error, the kinetic HH/HD isotope effect of about 2 predicted by Eq. (6.52) was confirmed experimentally. Table 6.4 indicates that both and are increased as compared to porphyrin, as expected. The total increase as compared to TPP is about Ilk) rnokk an effect which can be attributed to the loss of the aromaticity of TPP in TPC when the tautomerism occurs. 

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Tire macrocyclic core of porphyrin systems 71 is highly conjugated and a number of effective resonance forms can be written. Tliere are nominally 22 TT-electrons but only 18 of these can be included in any one conjugative path (for a modern discussion on this topic, see references 98AGE177 and 99CEJ267). Chlorins (73, dihydroporphyrins), bacteriochlorins (74, tetra-hydroporphyrins), and isobacteriochlorins (75, tetrahydroporphyrins) also have full 18-7r delocalization available, though the number of possible resonance forms is reduced. 

The tautomerization is induced by cobalt(II) which forms the thermodynamically more stable metalatcd hydroporphyrins from which the cobalt can be removed using trifluoroacctic acid under kinetic control. Experiments with porphyrinogen and hexahydroporphyrin show that the porphyrinogen-hexahydroporphyrin equilibrium can be shifted by complexation of porphyrinogen with metal ions to the more stable metal hexahydroporphyrins and that metal-free hexahydroporphyrins tautomerize back to the more stable metal-free porphyrinogens.29... 

The tetramerization of suitable monopyrroles is one of the simplest and most effective approaches to prepare porphyrins (see Section 1.1.1.1.). This approach, which is best carried out with a-(hydroxymethyl)- or ot-(aminomethyl)pyrroles, can be designated as a biomimetic synthesis because nature also uses the x-(aminomethyl)pyrrole porphobilinogen to produce uroporphyrinogen III. the key intermediate in the biosynthesis of all kinds of naturally occurring porphyrins, hydroporphyrins and corrins. The only restriction of this tetramerization method is the fact that tnonopyrroles with different -substituents form a mixture of four constitutionally isomeric porphyrins named as porphyrins I, II, III, and IV. In the porphyrin biosynthesis starting from porphobilinogen, which has an acetic acid and a propionic acid side chain in the y6-positions, this tetramerization is enzymatically controlled so that only the type III constitutional isomer is formed. 

Porphyrins, Hydroporphyrins, Azaporphyrins, Phthalocyanines, Corroles, Corrins and Related Macrocycles... 

Mashiko, T. Dolphin, D. Porphyrins, hydroporphyrins, azaporphyrins, phthalocyanines, corroles, corrins and related macrocycles. In Comprehensive Coordination Chemistry Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds. Pergamon Oxford, 1987, Vol. 2, Chapter 21.1. 

We examine here possible structural effects that may result from or accompany the generation of the primary photoproducts, and speculate about the consequences of concomitant changes in distances,conformations, relative orientations and charges on the electronic profiles of and interactions between the BChls, BPheos and their radicals. Because the primary events in green plant photosynthesis also involve a series of chlorophyll donors and acceptors ( ), similar trends should therefore prevail for chlorophyll radicals as well. Furthermore, radicals of porphyrins and hydroporphyrins (saturated porphyrins such as chlorins and isobacteriochlorins) have been... 

The examples cited above represent part of an increasing body of structural information on chlorophylls, chlorins, bacteriochlorins and isobacteriochlorins (10-14 and references therein) that points to the remarkable flexibility of these molecules This ability of the macrocycle to adjust is not limited to hydroporphyrins but is also observed in porphyrins 5,10,15,20-tetra-n-propylporphinato lead (II) assumes a "roof" shape by folding along an axis defined by two opposite methine carbons with the two planes of the "roof" inclined at 22 to one another (15) In contrast, triclinic 5,10,15, 20-tetraphenylporphinato cobalt (II) is distinctly saddle shaped with the 3 carbons of adjacent pyrrole rings lying 40 66 and -0 66A above and below the plane of the four nitrogens (16) ... 

The NIR absorptions between 1100 and 1500 nm therefore are a common property of two closely associated porphyrin or hydroporphyrin rings one of which is oxidized to a radical. Especially noteworthy is the parallelism between the redox pairs defined in Equations 1 and 2 which is presently further investigated in our laboratories (1 ). 

A second reaction involves the chlorin-to-porphyrin conversion. Any chlorin which has hydrogen atoms at the sp -hybridized centers of the reduced ring can be oxidized to the respective porphyrin. Oxidation may be achieved by various oxidants including oxygen . Likewise, rednctions to hydroporphyrins and other reactions of the macrocycle are possible. However, most of these are of interest only for the specialist. Under... 

Porphyrin can add up to six hydrogen atoms on the pyrrolic j8-carbons without loss of aromaticity. Except for 2,3,12,13-tetrahydro derivatives (Bchl) (27), the porphyrin normal is no more than a Cs axis and the four nitrogens are nonequivalent in the other hydroporphyrins (Chi (25), iBchl (26) and hexahydroporphyrin). The X-ray structures of some metallochlorins [M(Chl)] show the asymmetry of the macrocycles (Figure 9 and Table 8),21,162,163... 

The mass spectral features of hydroporphyrins are dominated by the benzylic cleavage of / substituents on the saturated carbons.2,14... 

More conveniently, hydroporphyrins are obtained by reduction of porphyrins.2,14,165 For example, photochemical reduction of M(Por) (M = H2, Zn, SnCl2, GeCl2) with reductants such as R3N,... 

The j8 hydrogens of the pyrroline rings are substituted with deuterium under basic conditions (BulOK-BulOD/THF, r.t) without isomerization to other hydroporphyrins. The acidic proton-... 

The metallation and demetallation procedures for porphyrins are applicable for hydroporphyrins, though M111 salts may cause some dehydrogenation. Perchlorate or acetate salts are usually used as the metal carrier for both metallations and transmetallations (Scheme 48).17,167,168... 

Mg11 insertion into labile hydroporphyrins is effected by the use of IMg(2,6-di-f-butyl-4-methyl-phenoxide)-Li(2,2,6,6-tetramethylpiperidide).169... 

There are several other nonaromatic hydroporphyrins known.2 14 Phlorins (5,22-dihydropor-phyrins) are stable only in acidic media (El/2 —0.04 V vs. SCE, pH2/HCl) but easily oxidized to porphyrins under neutral to basic conditions (El/2 = — 0.5 to — 0.7 V vs. SCE) unless the methylene bridge is blocked with substituents. Their spectra are characterized by two bands at 430nm (e 105) and 650 nm (broad). The former is accompanied by a band at 370nm (s 104), and both red-shift by 20 nm on metallation. The latter shifts to 750 nm on protonation and to more than 800 nm on metallation. Phlorins are at the same oxidation level as chlorins, and are the intermediates of photochemical conversions of porphyrins to chlorins. 

Hydrogen ligands, 689-711 Hydrogen selenide metal complexes, 663 Hydrogen sulfide metal complexes, 516 Hydrogen telluride metal complexes, 670 Hydroporphyrins, 814-856 basicity, 853 dehydrogenation, 853 demetallation, 854 deuteration, 853 mass spectra, 852 metallation, 854 NMR, 852 non-aromatic, 855 photochemistry, 854 redox chemistry, 855 synthesis, 852... 

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

The structure of iron(II) octaethylchlorin shows the iron and four nitrogen atoms to be rigorously planar, but the rest of the chlorin macrocycle to be significantly S4 ruffled.732 Hydroporphyrins intrinsically have larger cores than porphyrins, but the distortion from planarity leads to a reduction in core size and shorter metal-nitrogen distances. The enhanced ability of hydroporphyrins to undergo distortion so as to adjust their core size in response to the size of the metal may be responsible for the differences between iron porphyrins and hydroporphyrins.733... 

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