Bacteriochlorins structure

The bacteriochlorin structural type is formally derived from the porphyrin by saturation of two peripheral double bonds in opposite pyrrole rings. The term bacteriochlorin originates from the bacterio-chlorophylls, which are widespread as pigments in bacterial photosynthesis (i, 13, 27a). 

Transformations which alter the bacteriochlorin chromophore are quite rare. An important reaction in the structural elucidation of the bacteriochlorophylls is the dehydrogenation to chlorophyll derivatives. Thus, bacteriopyromethylpheophorbide a (1) can be smoothly dehydrogenated with 3,4,5,6-tetrachloro-l,2-benzoquinone to the corresponding chlorin 3-acetyl-pyromethylpheophorbide a (2) in high yield.1 la,b... 

Starting from the Ni mrao-formyloctaethylporphyrin oxime complex, the meso-cyanooctaethylporphyrin N-oxide complex has been synthesized for the first time. The double addition of the nitrile oxide to 2,5-norbornadiene afford a porphyrin dimer, whose structure has been established by X-ray diffraction analysis (485). The 1,3-dipolar cycloaddition reaction of w< .so-tetraarylporphyrins with 2,6-dichlorobenzonitrile oxide yields isoxazoline-fused chlorins and stereoiso-metric bacteriochlorins. The crystal structure of one of bacteriochlorins has been characterized by X-ray diffraction (486, 487). 

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) ... 

Figure 17.5 Skeleton structures of (a) polypyrrolic photosensitizers (i) porphyrin, (ii) chlorine, (Hi) bacteriochlorin, (iv) phthalocyanine, (v) naphthalocyanine, (vi) texaphyrin (b) examples of metalloderivatives (c) photosensitizers incorporating the metallodrug moieties (x-xii) cisplatin- and/or carboplatin-like structures, (xiii) iron sulphur nilrosy cluster...

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. 

The extremely high sensitivity of bacteriochlorins to various reaction conditions makes their chemistry very diflScult. This might be one reason why methods for the total synthesis of bacteriochlorins had not, until very recently, been developed. Total synthesis of a tolyporphin model 86 which contains geminally dialkylated structural parts was reported by Kishi <97TL6811> using an approach that is very closely related to Eschenmoser s syntheses of hexahydroporphyrins from reduced linear tetrapyrroles by cyclization (see Section 1,5). 

There has been substantial recent interest in the structures of reduced porphyrin derivatives. Reduced derivatives mean the reduction of the porphyrin system by the addition of one or more moles of hydrogen. This chemical change can have important effects on physical properties. However, in this review, the effect of such modified macrocycles on the coordination parameters of the metalloderivatives is the major question. Characterized species include three levels of reduction, di-, tetra- and hexahydroporphyrins. Examples of the dihydroporphyrins are chlorins (G) and porphodimethenes (H). Tet-rahydroporph)nins include bacteriochlorins (I) and isobacteriochlorins (J). Characterized hexahydroporphyrins are the pyrrocorphins (K) and hexahydroporphyrins (L). 

The structural skeleton of I. (tetrahydroporphyrins) is formally derived from porphine by reduction of two peripheral double bonds in neighboring pyrrole rings of the porphyrin skeleton. I. are constitutional isomers of the bacteriochlorins, from which their name is derived. Natural members of this structural type are heme d, siroheme, and sirohydrochlorin. 

Until the mid-1970s the four classic cyclic tetrapyrrolic structures with their porphyrin, chlorin, bacteriochlorin, and corrin skeletons were almost the only representatives in the class of porphyrinoid natural products 1-10). Although other partially reduced porphyrins were conceivable, none of these partially saturated porphyrinoid structures had hitherto been found in nature. 

It is interesting to note that the use of porphyrinic structures in Nature usually deals with processes involving nonfluorescent species such as iron porphyrins in dioxygen storage, transport, or activation. When photoinduced events are performed, porphyrin structures are rather ignored by Nature, and partially reduced species, such as bacteriochlorin (BC) or bacteriopheophytin, are involved instead. Still, most photochemical and photophysical academic studies or applications inspired by natural photosynthetic processes extensively use porphyrins as chromophores. This deliberate choice may have important consequences. 

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