Corrole analogs

Instead of using classical porphyrins, Verma and coworkers employed analogs of corroles to synthesize meso-meso corrole analog dimers (Fig. 22a) [167]. These dimers were prepared using AgOTf or FeCls as the chemical oxidant and isolated in about 90 % yield. Moreover, Osuka and coworkers also performed the synthesis of a (5-P dimer of A-fused porphyrins using AgOCOCFs as the chemical oxidant, with a yield of 61 % (Fig. 22b) [125]. 

Corrole and sapphyrin analogs are exanqples of how the pocket sizes of imidazolium cyclophanes can be altered to accommodate different sized metals into the pockets. TTie corrole analog 16 would be best suited for first row transition metals because of its decreased size. The pocket cavity of the sapphyrin analog 17 is larger and would be potentially better suited for coordinating both second and third row transition metals. The synthesis of 16 and 17 is outlined below (72). Thermal ellipsoid plots of conq)ounds 16 and 17 are shown in Figure 6 and 7, respectively. 

In the tris-pentafluorophenyl analog (TFPC), in contrast to other Co corroles, aromatic amines can substitute PPh3 to form six-coordinate trivalent bis(amine) complexes.788 Bis-chlorosulfon-ation of TFPC occurs regioselectively to give the 2,17-(pyrrole)-bis-chlorosulfonated derivative fully characterized as its triphenylphosphinecobalt(III) complex.789 The amphiphilic bis-sulfonic acid was also obtained. 

A most interesting example of the corrinoid structure is corrole, a macrocycle where an 18 electron aromatic it system analogous to that of a porphyrin is maintained. Corrole has been shown to be a versatile ligand capable of coordinating transition and main group metals without significant distortion of the macrocycle plane. 

XPS data have been also reported in the past [33] for chromium and molybdenum corrolates and discussed with respect to analogous porphyrinates. 

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... 

Related systems which contain either furan (207 or 212) and thiophene 208 in place of one or more pyrroles, were also synthesized in accordance with the sequence shown in Scheme 36 [3, 24, 26, 27, 154]. Another synthesis of a dioxasapphyrin similar to 207 was achieved when bis(formylfuryl) sulfide (217) was condensed with the tripyrrane 200 (Scheme 37). The product resulting from this condensation is the dioxosapphyrin 212. The mechanism proceeds in analogy to that invoked to rationalize the formation of corrole from we u-thiophlorin [25]. 

Since the initial disclosure of the basic corrin structure, there has been a considerable body of effort devoted toward the synthesis of macrocycles related to this chromophore that may be considered as being intermediates between porphyrin and corrin. These macrocycles, namely the dehydrocorrins (e.g., tetradehydrocor-rin 2.3) and the corroles (e.g., 2.4), represent interesting classes of contracted porphyrins that warrant specific mention here. The interest in these molecules derives in part from the fact that they could represent milestones along the biosynthetic pathway leading to vitamin B12. They are, however, also of interest from a non-biological perspective. Simply stated, this is because corrole-type macrocycles possess unique electronic and chemical characteristics, the study of which can help one to understand better the chemistry of all porphyrin analogs. 

Because of its immense scope, a detailed description of corrins (and vitamin B12) will not be presented here. The reader is instead referred to reviews of B12 chemistry and its biosynthesis that have appeared recently. Further, because they are more directly related to the corrins than are the corroles, the chemistry of the dehydrocorrins will not be discussed here. Also not included in this review are the so-called artificial porphyrins of Floriani, et al. (e.g., 2.5), since it is deemed by these authors in their review that these macrocycles are more dehydrocorrin-like than corrole-like in their nature. Other systems omitted here include the spiro porphyrins of Battersby and coworkers, the tetraphosphole macrocycles of Mathey and coworkers and the tetrapyrrolic systems of Bartczak and Smith and co-workers. Thus, the emphasis will be on those contracted porphyrins that are most removed, in structural and chemical terms, from the macrocyclic unit found in coenzyme B12 and its analogs. 

A more direct, 2 + 2 approach to the synthesis of cobalt(III) corroles has been described. It involves condensing a diformyl bipyrrole such as 2.140 with a diacid dipyrrylmethane such as 2.36. This approach is thus similar to the one used to obtain the bifuran-containing corroles 2.3 2.40 described earlier. In the present instance, the diacid bipyrrole 2,141 may also be reacted with a diformyl dipyrrylmethane such as 2.142. This affords corrole 2.125b (Scheme 2.1.37). In either case, the reaction must be carried out in the presence of Co(II) and PPhs. This requirement for a presumably coordinating metal cation is in stark contrast to what is seen in the case of the bifuran analog there, no metal is needed to template the reaction. ... 

One other approach to metalated M21)-methyl corroles was reported by Johnson and coworkers in the early 1970s. In this instance, the A-methyl thiaph-lorin 2.206 was used as the starting material. It was heated in the presence of Pd(OAc)2 in acetic acid to give the metalated corrole derivative 2.233 directly (see Scheme 2.1.81). The rate of sulfur extrusion in the presence of the palladium(II) ions was found to be dramatically increased in comparison to that of the analogous metal-free system 2.206 or the N-unsubstituted thiaphlorins 2.102 (Scheme 2.1.22). It was proposed that this rate enhancement reflected the extra stabilization that... 

Only very few examples of experiments devoted to the alkylation of heterocorroles have appeared in the literature. These have involved allwlation of the bifuran-containing corrole 2.39 with methyl iodide or ethyl iodide. For instance, the mono-A-methyl corrole 2.241 was prepared in this way from corrole 2.39 in 56% yield (Scheme 2.1.88). In an analogous manner, the mono-A-ethyl corrole 2.242 was prepared in 45% yield starting from 2.39. In both cases, the mono-A-alkyl corroles were isolated as their hydroiodide salts. In fact, these corroles were found to be so basic that they could not be isolated in their free-base forms. ... 

The corrole-corrole dimers of Schemes 2.1.91 and 2.1.92 each exhibit optical spectra that are similar to those of an octaalkylcorrole, with no significant modifications in either the Soret or Q-band regions." Thus, the corrole dimer system 2.250a, for instance, exhibits a Soret band at 400/410 nm (e = 112 000 and 82 000 M cm respectively) and Q-bands at 542 nm and 597 nm (e = 24 000 and 28 000 M cm , respectively). Similarly, the electronic absorption spectra of complexes 2.253 and 2.254 do not differ significantly from spectra of analogous monomeric cobalt(III) corrole complexes. This leads the authors of this monograph to conclude that little electronic interaction exists between the two subunits. 

For the purposes of this review, heterosapphyrins are defined as being sap-phyrin-like macrocycles in which a furan, thiophene, or selenophene subunit serves to replace one or more of the pyrrole rings. Interestingly, like the pentaazasapphyr-ins, this class of macrocycles has its origins in a serendipitous discovery. It occurred during efforts directed toward the synthesis of the heteroatom analogs of corrole. Specifically, as first reported by Johnson and coworkers in 1969, reaction of... 

Abstract The transition metal complexes of the non-innocent, electron-rich corrole macrocycle are discussed. A detailed summary of the investigations to determine the physical oxidation states of formally iron(IV) and cobalt(IV) corroles as well as formally copper(III) corroles is presented. Electronic structures and reactivity of other metallocorroles are also discussed, and comparisons between corrole and porphyrin complexes are made where data are available. The growing assortment of second-row corrole complexes is discussed and compared to first-row analogs, and work describing the synthesis and characterization of third-row corroles is summarized. Emphasis is placed on the role of spectroscopic and computational studies in elucidating oxidation states and electronic configurations. 

The synthesis of Co(tpfc)(PPh3) was reported in 2000 [130]. This complex, and the analogous Co(tpfc)(py)2, have a strong tendency to form C3-C3-linked dimers in aerated solution [131]. The twist angle between the two corrole rings in [Co(tpfc) (py)2]2 is 45° (Fig. 10), with a 1.479(13) A C-C bond between the monomeric units. 

It was observed in a 2005 article that Co(II) porphyrin-Co(III) corrole dimers are more effective dioxygen reduction electrocatalysts than analogous Co(III)-Co(III) corrole dimers or monomeric Co(III) corroles [145], The heterodimers operated effectively at lower overpotentials and promote complete reduction to water (the average number of electrons transferred per 02 molecule approaches 4 in the best porphyrin-corrole catalyst). It was suggested that the inferior catalytic performance of the corrole homodimers could be due to a reduction in the basicity of the activated intermediate when two Co(III) moieties are involved, leading to a less favorable 4-electron reduction. Heterobimetallic catalysts containing formally Co (IV) corroles were also examined as potential dioxygen reduction catalysts [146]. 

Epoxidation of alkenes with iodosylbenzene can be effectively catalyzed by the analogous salen or chiral Schiff base complexes of manganese(in), ruthenium(II), or ruthenium(III). For example, the oxidation of indene with iodosylbenzene in the presence of (/ ,5)-Mn-salen complexes as catalysts affords the respective (15,2/ )-epoxyindane in good yield with 91-96% ee [704]. Additional examples include epoxidation of alkenes with iodosylbenzene catalyzed by various metalloporphyrins [705-709], corrole metal complexes, ruthenium-pyridinedicarboxylate complexes of terpyridine and chiral bis(oxazoUnyl)pyridine [710,711]. 

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