Corrole protonation

Protonation and deprotonation reactions of corroles have already been mentioned (see Introduction). Attempts to achieve electrophilic substitution reactions, at the corrole, e.g. Friedel-Crafts acylation, have been unsuccessful.1 Heating corroles with acetic anhydride yields the corresponding 21-acetyl derivatives l.8a,b... 

Corrole contains an 1 S-7t-electron system and shows aromaticity similar to the porphyrin chromophore, with strong ring current effects on the peripheral and inner-proton NMR, relatively stable parent mass ion and intermediary C—C bond length between single and double bonds. 

The corrole ring is not strictly planar in the metal-free form.238 Because of the steric repulsion between inner protons, pyrrole ring A is tilted out of the mean plane. According to 7t-electron distribution calculations the three inner protons are attached to N-21, N-22 and N-24.aa>... 

Corroles are stronger acids and weaker bases than porphyrins.239 They are deprotonated in dilute alkali to form aromatic anions. In acidic media, the first protonation occurs on the pyrrole nitrogen, but the second proton adds to the meso carbon causing loss of the aromaticity (Scheme 72). 

Proton exchange at the meso position of corrole is facile, and the deuteration is complete in 15 min in trifluoroacetic acid at room temperature. Under similar condition, porphyrin does not exchange the meso protons. 

Dioxacorroles are 18-7r-electron aromatic systems like corroles. They exhibit basicity intermediate between that of porphyrin and corrole, and require 1 h at 100 °C in TFA for complete deuteration of the meso positions. The furan protons are also substituted by deuterium under the same conditions after lOOh. Friedel-Crafts acylation occurs at C-5 while alkyl halides attack on the pyrrolic nitrogens to give a mixture of mono- and di-alkyl derivatives. 

Zn2+ correlate can be obtained, as pyridinium salt, by reaction of corrole with zinc acetate in pyridine [25] in a procedure similar to that reported for the preparation of nickel and palladium complexes of corrole [11]. The zinc derivative is not paramagnetic and its formulation has been made on the basis of its proton NMR spectrum. Attempts to isolate the neutral zinc complex have been unsuccessful. 

All resonances in the proton NMR spectra of diamagnetic metallocorrolates show a strong upheld shift due to anisotropic effects caused by the macrocycle ring current [44]. This is another demonstration of the aromatic character of the corrole ring [21]. Spectral properties of several derivatives of octamethylcorrole are reported in Table 14. 

Data relative to the 400 MHz H-NMR spectra of cobalt corrolates are reported in Table 16. The strong shift due to the macrocycle ring current demonstrates that the presence of the meso-phenyl substituents does not cause the loss of aromaticity. The meso-protons of the monophenyl derivative resonate at the same chemical shift value observed for the meso-unsubstituted complex. 

A general feature of weso-phenyl substituted Co3 + corrolates is then that the planarity of the macrocyclic ligand is maintained in solution the pattern shown by the resonances due to the peripheral methyl groups in fact are indicative of the existence of a C2 symmetry axis the direct pyrrole-pyrrole bond typical of a planar corrole skeleton, confirmed also by the signals due to the meso-protons, if present. 

Corroles are tetrapyrrole macrocycles that are closely related to porphyrins, with one carbon atom less in the onter periphery and one NH proton more in their inner core. They may also be considered as the aromatic version (identical skeleton) of the only partially conjugated corrin, the cobalt-coordinating ligand in Vitamin B. Two potential application of corroles are in tumor detection and their use in photovoltaic devices. Selective snbstitntion of corroles via nitration, hydroformylation, and chlorosulfonation for the gallinm were studied in detail and the respective mechanistic pathways and spectroscopic data were reported, (an example is shown in Fignre 27). Overall, over 139 varions corroles were synthesized and the effect of various metal complexation pertaining to their selective reactivity examined. ... 

A single crystal X-ray structure of a free-base corrole was obtained in 1971, and is shown below in Figure 2.1.3. Based upon this structure and upon molecular calculations, free-base corrole was determined to bear a proton hole at N(22) (as drawn for 2.4), with the other three nitrogen atoms protonated. Where appropriate throughout this discussion, the corrole framework will be shown in this manner. This does not mean, however, that a specific location is being designated for what are, presumably, highly delocalized double bonds. 

Corrole also forms stable protonated species. For example, corrole can be monoprotonated by treatment with weak acid to afford adducts such as 2.8, which, like corrole, display strong visible absorption bands vide supra) (Scheme... 

The cobalt(II) corrole anion prepared as above was characterized primarily by electron spin resonance (esr) and absorption spectroscopy. When prepared via sodium film reduction, the cobalt(II) corrole oxidizes rapidly to the corresponding Co(III) corrole on exposure to air. When prepared by the other methods, it is moderately stable in air in the presence of a reducing agent. Attempts to prepare the neutral form of the initial Co(II) corrole anion, by protonation with perchloric acid, resulted in formal oxidation to the Co(III) derivative. Interestingly, further protonation of the Co(III) corrole with perchloric acid led to what appeared to be a protonated Co(III) corrole. Certainly, the absorption spectrum of this species is similar to that of the corresponding neutral nickel(II) corrole complex. However, the exact nature of this protonated material has not been fully elucidated. 

In terms of spectroscopic characteristics, it was determined that the UV-vis absorption spectrum of the 7V(21)-methyl corrole 2.194 is very similar to that of the normal, N-unsubstituted corrole. On the other hand, the UV-vis spectrum of the V(22)-methyl corrole 2.195 was found to be markedly different from that of either the N(21)-methyl-substituted species 2.194 or the N-unsubstituted corrole 2.6. Furthermore, in the relevant H NMR spectra, the signals of the internal and external protons of the AT(22)-methyl corrole 2.195 were found to be spread out compared to those of the N(21)-methyl or N-unsubstituted species. This latter finding was... 

In the case of the pyridinium salt of the palladium(II) corrole derivative 2.161, methylation gave a mixture of two products, namely A (21)-methyl corrole 2.221 and 3,3-dimethyl corrole 2.222 (Scheme 2.1.75). As above, the regioselectivity of the C-methylation was elucidated by examining the methylation products of the 3,17-diethylcorrole derivative 2.160. Further, in this specific instance, it was determined that under the methylating conditions, compound 2.223 could not be obtained from 2.224, nor could this latter be made from 2.223. This was taken as an indication that these methylation reactions were under kinetic control. Interestingly, however, both of the methylated products were found to undergo reversible protonation in the presence of TFA, in both cases, the site of protonation was determined (by NMR spectroscopic analysis) to be at the C(5) mejo-position. 

When 3,3-dimethyl Pd(II) corrole 2.222 was heated to reflux in allyl bromide and then treated with sodium perchlorate, the main product obtained was the C(17)-allyl derivative 2.237 (isolated in 73% yield as the perchlorate salt) (Scheme 2.1.85). The site of addition of the allyl fragment is also the site at which protonation of the... 

Initial reports concerning the protonation behavior of corrole involved claims of having observed both a mono- and a diprotonated species. In the case of the monoprotonated macrocycle, the UV-vis absorption bands remained relatively unchanged compared to those of the free-base species. In the case of diprotonated corroles, on the other hand, the second protonation event was presumed to occur at one of the meso carbons, resulting in a disruption of the n-electron conjugation. This latter conclusion was based upon the dramatic changes observed to occur in the UV-vis absorption spectrum. 

In order to gain a more complete understanding of the protonation chemistry of corroles, a study of the behavior of A -methyl corroles in acidic media was carried out. In this study it was found, for instance, that A (21)-methyl corrole (2.194) exists... 

Consistent with its 22 n-electron formulation, the free-base form of isosmar-agdyrin (6.11b) appears to be aromatic, as judged from a preliminary H NMR spectral analysis. On the other hand, the protonated form (6.11a) appears to be non-aromatic, as evidenced by the lack of any diamagnetic ring-current effects in its H NMR spectrum. Based upon integration of the NMR signals, the lack of aromaticity for 6.11a appears to be the result of protonation at one of the meso-positions of the macrocycle. This protonation effect thus resembles that observed for the 18 7c-electron corroles discussed in Chapter 2. 

The numbering schemes of porphyrins and corroles are essentially the same (Fig. 4). Throughout this chapter, the protonated macrocycles are denoted by the addition of Hn to the appropriate abbreviation, where n = the number of protons in the N4 core. The abbreviations cor and por are used to denote the bare correlate and porphyrinate macrocycles, respectively. The 5, 10, 15, and 20 positions on the ring are referred to as meso positions, while the 2, 3, 7, 8, 12, 13, 17, and 18 ring positions are referred to by the shorthand p (they are p-pyrrole carbon atoms). 

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