Pyrrolenine unit

Rearrangement of metallophlorins to air-stable chlorins has been observed in various instances [e.g. (Buckler (20), Closs (35), Fuhrhop (75)]. The further reduction to chlorinphlorins of natural chlorophyll derivatives occurs exclusively on the /3 bridge, which again proves the special vulnerability to nucleophilic attack of methine bridges between pyrrole and pyrrolenine units [Inhoffen (101, 102, 103)]. 

Figure 6.2.2 Schematic NMR spectra of hexamethyldipyrromethene. IH, 13C, and 15N spectra do not differentiate between pyrrole and pyrrolenine units. Proton exchange is too rapid.

Here we see that the macrocycle consists of two pyrrole units and two pyrrolenine units. After a ir-electron count, the great American chemist, R.B. Woodward, pointed out that the two pyrrole units could be considered to have their own aromatic sextet of electrons. Each pyrrolenine unit, on the other hand, is one electron short of the aromatic sextet, having only five tt-electrons. In order to make up this deficiency, they can be thought of as borrowing electron density from the neighbouring meso-carbons. This means that, compared with the pyrrole carbons, the meso-carbons will have a tendency to be electron deficient and, therefore, would be less enthusiastic about electrophilic substitution. 

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