Meso-Sapphyrins

Electrophilic proton exchange occurs readily for sapphyrin (39) and meso deuteration is completed in trifluoroacetic acid and at room temperature overnight.274 The substitution proceeds at a moderate rate for two of the meso protons but very slowly for the other two in the dioxa analogue... 

Removal of one meso-carbon from the sapphyrin skeleton leads to relatively unstable non-sapphyrin 22ji smaragdyrin 144 (Scheme 59) (1983JA6429). These (1972JCSP(1)2111) bear a structural relationship analogous to that between a porphyrin and a corrole. The earlier attempts to... 

With the original reports of the successM synthe of the sapphyrins [26,66,152] and uranyl superphthalocyanine [112, 118, 119], interest in other expanded porphyrin systems, was kindled. The next logical step (after sapphyrin), in the expanding series of all-pyrrole systems, was the pentaphyrin macrocycle 231 which contains five pyrroles and five meso-like methine bridgra. In 1983 Gossauer et al. reported the synthesis of the first prototypical member 231 of this macrocyclic family [158, 182, 183, 185-187]. This first synthesis was achieved by a 2 + 3 MacDonald-type condensation between an oc-firee dipyrromethane 233 and a tripyrrane dialdehyde 236. More recently, the synthesis of pentaphyrin 231 has l n achieve by using a dipyrromethane 5,5 -dicarboxylic acid 235 in place of an a-firee dipyrromethane [21]. Here, as is the case in many of these kind of reactions [21,26,27,66,155], decarboxylation occurs under the reaction conditions to produce the corresponding a-free species 233 in situ. (Scheme 40) [21]. 

Prior to developing the above synthesis, Sessler and coworkers discovered a different route to meso-axy sapphyrins. This synthetic strategy, like so many in the area of sapphyrin-related research, is based upon a reaction that was not initially intended to produce sapphyrins. Specifically, it involves the reaction between bis(pyrrolyl)bipyrrole 5,38 and diformyl dipyrrylmethane 5.39, and was found to produce, quite unexpectedly, sapphyrin 5,41 in ca. 5% yield, with none of the intended hexapyrrolic macrocycle 5,40 being detected (Scheme 5.2.3). It was thus considered that such a 4 + 2 = 5 approach might prove useful for the synthesis of we o-phenyl sapphyrins. This indeed proved to be the case. In fact, this strategy has... 

The me o-phenyl-substituted sapphyrins 5.33-5.35, 5.37, 5.45-5.47, and 5.49-5.50 (as HCl salts) display UV-vis absorption bands that resemble qualitatively those of the better studied all P-alkyl-substituted sapphyrins. For instance, 10,15-diphenylsapphyrin 5.33, when dissolved in CH2CI2, exhibits a Soret band that is bathochromically shifted by ca. 5-10 nm relative to that of the meso-unsubstituted alkylsapphyrins to 462 nm. A further bathochromic shift of ca. 30 nm (out to A-max = 484 nm) is observed for the Soret band of the bis-HCl salt of wc50-tetraphenylsapphyrin 5.50. Each of the phenyl-substituted sapphyrin salts also exhibit two to three (as opposed to four for alkyl-sapphyrin salts) weak Q-type bands in their absorption spectra. These bands fall in the 625-780 nm region and are thus red-shifted relative to the corresponding bands of alkyl-substituted sapphyrins. 

Several years after the initial report of Woodward, et al., Sessler and coworkers found that sapphyrin 5.21 would in fact form a stable complex with uranyl cation. It was determined, however, that 1102 " complexation was accompanied by reduction of the sapphyrin macrocycle. Specifically, it was found that metal insertion occurred concurrent with addition of methanol to one of the meso-Xike carbon centers. The net result was an overall neutral complex of a modified, nonaromatic sapphyrin-like system (Scheme 5.5.2). 

A separate single crystal X-ray structural analysis revealed that the partially reduced macrocycle 6.27b also adopts a near-planar conformation in the solid state (Figure 6.3.4). It also confirmed the anticipated trans-stereochemistry about the meso-C = C bond in that one of the protons was found to be directed into the core of the macrocycle, while the other points outward. Taken together these solid-state structural findings are thus consistent with the proposal that 6.27 is a true isomer of sapphyrin. 

Uranyl complex of the meso-methoxy isosapphyrin Scheme 34 Oxidative Sn (AO) methoxylation of sapphyrin... 

Heating a mixture of free sapphyrin and methanol in pyridine/triethylamine solution for 72 h does not lead to substitution [130]. However, after the addition of the uranyl complex (NMe3)2U02Cl2 in the presence of air oxygen, the reaction completes in 2 h to give a thermodynamically more stable tautomeric complex of uranyl and meso-methoxy substituted so-sapphyrin. A plausible mechanism of this reaction is given below (Scheme 34) [131]. 

Two new meso-aryl core-modified fused sapphyrins (X = S, Se) were synthesized by a simple acid-catalyzed condensation of the electron-rich and rigid precursor, dithienothiophene diol, with a core-modified tripyr-rane (140L3472). 

Many years after the seminal studies of Marks and Woodward, the present authors became intrigued by the above enigma. The reaction between 002 " and sapphyrin 3b was thus reexamined. It was discovered, in contrast to what was reported by Woodward and coworkers, that sapphyrin does indeed react with uranyl salts under appropriate conditions (e.g., UO2CI2, methanol, pyridine, triethylamine). Interestingly, however, the product obtained is not the sapphyrin uranyl complex but rather a nonaromatic derivative (c.f. 5) in which a methoxy substituent has been attached onto what was initially a meso- it bridging methine position. 

Porphyrin has fourfold symmetry, with the pyrrole rings connected by alternate meso bridges. About eight isomers can be sketched down by changing the sequence of the subunits, of which porphycene (37), hemiporphycene, corrphycene, and isoporphycene have been synthesized to date. Expanded porphyrins are, by definition, those possessing more than 16 atoms in the smallest circuit of the macrocycle. Sapphyrin (39) is the common expanded porphyrin derived by the insertion of an additional pyrrole ring into the macrocycle [40]. 

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