Porphyrins inverted

Inverted porphyrin or 2-aza-21-carbaporphyrin 88 also known as an NCP (Figure 3) was first isolated as a by-product of normal acid-catalyzed condensation of pyrrole and benzaldehyde (1994ACI779, 1994JA767). Latos-Grazynski and coworkers employed a Rothemund reaction between tolualdehyde and pyrrole in the presence of BF3-Et20 in DCM, followed by oxidation with 2,3,5,6-tetrachloro-p-benzoquinone (chloranil) to obtain NCP (1994ACI779). Simultaneously, Furuta and coworkers also reported... 

In Section 2.2.1-2.2.3, a brief discussion on the chemistry, properties and structure of contracted, isomeric, and inverted porphyrins has been presented. Here, a brief sketch of the structural diversity in expanded porphyrins is given. Expanded porphyrins represent those synthetic macrocycles in which pyrrole or heterocyclic rings possess connectivity through mcso-carbon bridges. The conjugation of the porphyrin has been... 

In addition to their biological applications, heteroporphyrins also serve as novel materials. Various porphyrins have been studied, including pyrrole-inverted porphyrins 12 <2000JOC8188>, tetraselenaporphyrins 13... 

Since the initial disclosure of the above synthesis of inverted porphyrins 3.160 and 3.161, two rational synthetic approaches to structures of this type have been devised. The first of these, reported by Lee and coworkers, actually involved the synthesis of the furan-containing system 3.167 (Scheme 3.5.3). In this approach, diol 3.165 was condensed with the tripyrrane derivative 3.166 using boron trifluoride etherate as the acid catalyst. After oxidation with DDQ, the inverted monoxapor-phyrin 3.167 was isolated in 5.5% yield. 

The other more rational synthesis of inverted porphyrins was reported recently by Dolphin and coworkers. This approach has as its key macrocycle-forming step a McDonald-type 2 -t- 2 condensation between the a-p-linked dipyrrylmethane 3.168 and the diformyl dipyrrylmethane 3.169. Following oxidation with air, the heptaalkyl N-inverted porphyrin system 3.170 is isolated in ca. 25% yield (Scheme 3.5.4). 

Perhaps consistent with the conclusion that the external nitrogen atom is the most basic is the fact that methylation of the tetraphenyl-substituted inverted porphyrin 3.160 with methyl iodide was found to occur practically quantitatively at this same peripheral nitrogen atom (affording system 3.171 Scheme 3.5.5). The high selectivity for methylation at this position was attributed both to the steric protection of the three normal nitrogen atoms by the porphyrin pocket, as well as to deformations of the porphyrin ring that would be necessary in order to accommodate an internal methyl group. 

Figure 3.5.2 Single Crystal X-Ray Diffraction Structure of Free-base N-inverted porphyrin 3.160.

The A -methyl inverted porphyrin 3.171 has also been demonstrated as being capable of forming a stable complex with divalent nickel.In this instance, it was found that treatment of 3.171 with nickel(II) acetate tetrahydrate in methanol affords the metal complex 3.172 in 75% yield (Scheme 3.5.6). This complex, like that of the N-unsubstituted inverted porphyrin discussed above, serves as a cogent... 

In 1995, Latos-Grazyhski and coworkers reported an efficient, albeit somewhat low-yielding, synthesis of tetraphenylsapphyrin 5.50. In this work, it was found that the acid-catalyzed, oxidative reaction of benzaldehyde (5.30) with pyrrole (5.29) in a 1 3 molar ratio gave, as expected, tetraphenylporphyrin and an isomeric N-inverted porphyrin as the main macrocyclic products (cf. discussion in Chapter 3). However, careful examination of the minor fractions revealed that one side-product of this reaction was in fact the pentapyrrolic tetraphenylsapphyrin 5.50a (Scheme 5.2.6). Unfortunately, this species could only be isolated in 1.1% yield. Nonetheless, this synthesis represented the first synthetic entry into the area of tetrakis-mc o-substituted sapphyrins. 

The inverted porphyrins (also called N-confused porphyrins) 289 were discovered accidentally as by-products of the well-known acid-catalyzed pyrrole aldehyde cyclocondensation route to porphyrins (Scheme 58) (see Chapter 14 by Latos-Grazynski). Rational multistep synthetic approaches to inverted porphyrins have t peared recently. Dolphin and coworkers. s have reported a method for the preparation of porphyrin 292 involving acid-catalyzed MacDonald-type (2 -I- 2) condensation of an a,j8-dipyrromethane 291 and a,a-dipyrromethane dialdehyde 290. Lee and coworkers have reported a (3 -I- 1) approach to porphyrin analogues with an inverted pyrrole and a thiphene, or a furan ring system 293 (Scheme 59). Pandey et reported the preparation of dibromo-a, c-... 

Scheme 59. Inverted porphyrins other synthetic approaches.

Pd(OAc)2 and an inverted porphyrin (meaning one of the pyrrolic rings being inverted) yield an inner C-tolyl substituted Pd complex 152 in toluene.The X-ray structure was obtained. The possibility of a Pd intermediate has been suggested in the formation of this compound. 

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