P-pyrrole position

Thiadiazole and selenodiazole rings are isoelectronic with benzene and form a fully conjugated, essentially planar structure when fused to porphyrazines at the p-pyrrolic position (168). Compared to pc, the incorporation of sulfur and selenium in addition to nitrogen atoms to the peripheral heterocyclic rings affects the electronic charge density and influences the interactions between adjacent molecular stacks. 

The analogous reactions of Pd(OEP) or Pd(TTP) did not result in formation of Pd(IV) porphyrins, but in excessive chlorination of the meso [295] or peripheral (p-pyrrole) positions [296] of the porphyrin rings. The oxidation of Pt(P) in the absence of hydrogen chloride did not lead to isolatable products. Of course, an oxoplatinum(IV) porphyrin would have been a very interesting compound. 

Porphyrins with Anchoring Groups Connected at P-Pyrrolic Positions... 

Besides to introduce substitutents onto the ra ophenyl ring or the P-pyrrolic positions, modification of the molecular structures by fusing a small aromatic ring to the porphyrin ring has also been an attractive way to extend the absorption spectra and tune the electrochemical properties of porphyrins in the prospect of improving the sensitization behavior. Imahori has investigated the application of a fused five-membered porphyrin (84) as sensitizer in DSSC. Compared with the reference porphyrin, the fused porphyrin shows broaden and red-shifted absorption spectra... 

There are several well known examples where modified porphyrins are used in biology, and in these cases the changes to the macrocycle structure do result in improved properties. The chlorophylls and chlorins are both porphyrinoids that are reduced at P pyrrolic positions (16). The effect of this modification is an increase in excited state lifetimes and quantum yields of... 

The past decade has seen considerable advances in the development of synthetic metallopor-phyrin complexes. " We have now designed numerous practical and efficacious methods of synthesizing porphyrins with halogens at the o-aryl and also the P-pyrrole positions. The methodology provides facile access to a large number of porphyrins in optimum yields and purity (Scheme 29.25). Demetallation with trifluoroacetic acid followed by insertion of iron yielded the corresponding hemins. ... 

More recently, even more robust and more reactive polyhalogenated metalloporphyrins bearing electron-withdrawing substituents on the P-pyrrole positions (Fig.2) have been prepared and used as efficient oxidation catalysts 6-10. The three following examples illustrate the better catalytic activities of these third generation metalloporphyrins. 

Phthalocyanine analogues without fused benzene rings are usually named pophyrazines (PAs). Two types of fluorinated porphyrazines are known - the species with fluorine atoms attached directly in p-pyrrolic positions and compounds with fluorinated P-substituents. 

Many of the p-substituted calix[4]pyrroles known in the literature were likewise synthesized via a condensation strategy. Typically, a functionalized pyrrole bearing substituents on one or both of the p-pyrrolic positions is condensed with a ketone under acid catalyzed conditions. However, a number of mono-substituted calix[4] pyrroles have been prepared by a post-synthetic strategy. Here, a functional group, such as an ester, formyl, or iodine substituent, is introduced onto one of the P-pyrrolic positions within OMCP. In many cases, this is done by subjecting the calix[4]pyrrole to deprotonation with a strong base (e.g., n-butyl lithium), followed by the addition of the appropriate electrophile [9,10]. On the other hand, iodination has typically been achieved by reacting calix[4]pyrrole with iodine-[bis(trifluor-oacetoxy)iodo]benzene [11]. These functionalized systems have been elaborated further to create, inter alia, anion sensors or materials that may be attached to solid or polymeric supports. 

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