Isobacteriochlorins, tautomerism

The tautomerization of porphyrinogens to pyrrocorphins has been reported in detail in connection with the synthesis of chlorins (sec Section 1.2.1.3.), baeteriochlorins (see Section 1.3.1.) and isobacteriochlorins (see Section 1.4.1.3.). Therefore, only the porphyrinogen-pyrrocorphin tautomerization of uroporphyrinogen I octacarbonitrile 8- 96 will be described as it is of importance regarding the biosynthesis and a possible prebiotic formation of corrins.la,b-2 A major problem concerned with using this approach synthetically is the number of possible diastereomeric products (4 in a 1 1 1 1 ratio) obtained in the reaction and also the formation of isobacteriochlorin diastereomers 10. 

Near IR spectra—See Electronic absorption spectra New IR spectra, R sphaeroides reaction centers, 207,208f NH-tautomer structure, porphyrins with nonsymmetrical substitutions, 84 NH-tautomerism, porphyrins with nonsymmetrical substitution, 74-93,89-91 Ni(II)-reconstituted hemoglobin—See Nickel-reconstituted heme proteins Nickel isobacteriochlorin... 

The isobacteriochlorin 8 serves as an example. Two protons distributed between the four nitrogen atoms with a 50% occupancy at each site were found experimentally. It has been suggested that two tautomeric forms (8a, b) contribute, equally to the solid-state structure. Two Ca—N bonds (A = 0.051 A) and two C —Cm bonds (A = 0.077 A) are significantly different as a result of the superposition of the tautomeric forms (82JA2376). 

In porphyrin H2P (71, also known as porphine) the tautomerism is degenerate i.e., tautomers 71a and 71b are identical. Also, in the anion HP-72, the four tautomers are all degenerate the kinetics of proton transfer have been studied. In chlorins, bacteriochlorins, and isobacteriochlorins, all the tautomers are different, but all the evidence indicates that the most stable tautomers are those with the inner protons in a trans disposition, represented by 73, 74, and 75, all having an 18-rr-delocalized system. Gossauer s penta-phyrin, a 22-77-electron pentapyrrole macrocycle with two inner hydrogen atoms, should also be prone to annular tautomerism [83JCS(CC)275]. 

Figure 6.21 Tautomerism of (a) porphyrin, (b) chlorin, (c) isobacteriochlorin and (d) bacteriochlorin. Adapted from Ref [19a].

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