Bacteriochlorins, tautomerism

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].

Figure 14.4 Tautomerization of porphyrin 3 [21, 22] and the hydroporphyrins chiorine 4, bacteriochlorin 5, and iso-bacteriochiorin 6 [23] in the iiquid state.

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