Corroles, preparation

Several other examples of metal(III) corroles prepared from free-base corroles have been reported. The first of these was the iron(III) corrole derivative 2.172. This complex was originally prepared in 1973 as the result of treating the free-base corrole 2.6 with phenyllithium in THF, followed by FeCl2 (Scheme 2.1.49). This same complex was later prepared from corrole 2.6 by reacting it with FeCl2 in the presence of pyridine. In both cases, the initially bound iron(II) center undergoes spontaneous oxidation (to iron(III)) during the reaction and/or workup. 

Schemes based on the acid-catalyzed condensation of a bipyrrole with a dipyrrylmethane are in general unsuccessful for the synthesis of corroles. Thus, the bipyrroles 1 fail to give corroles when reacted with the appropriate dipyrrylmethane 2.10,11 However, cobalt(III) corrole 3 can be prepared in moderate yield by a [2 + 2] approach involving bipyrrole T and dipyrrylmethane 2 bearing formyl groups and carboxylic groups, respectively, and heating initial condensation products, presumably norbilenes, with cobalt(II) acetate and triphenylphosphane in methanol.12...

Scheme 6.274 Preparation of corroles from aromatic aldehydes and pyrrole.

One example of a tin porphycene has been reported, but as yet no organometallic derivatives have been reported." A small number of tin corrole complexes are known including one organotin example, Sn(OEC)Ph, prepared from the reaction of Sn(OEC)Cl with PhMgBr. A crystal structure of Sn(OEC)Ph shows it to have both shorter Sn—N and Sn—C bonds than Sn(TPP)Ph2, with the tin atom displaced 0.722 A above the N4 plane of the domed macrocycle (Fig. 6). The complex undergoes reversible one-electron electrochemical oxidation and reduction at the corrole ring, and also two further ring oxidations which have no counterpart in tin porphyrin complexes. " " ... 

One oxochromium(V) complex, CrO(TETMC), containing the trinegative anion of a corrole (279), has been characterized as the solid.1266 It is prepared (Table 102) simply by exposure to air of a solution presumably containing a Cr11 complex. Aerial oxidation of Cr (TPP) produces the oxochromium(IV) complex CrO(TPP) so the corrole ligand apparently facilitates autoxida-tion. The redox behaviour of CrO(TETMC) has been examined by cyclic voltametry.1267... 

Bis(2-pyrrolyl) sulfides, prepared from the reaction of the pyrrole with sulfur dichloride, are useful precursors in the synthesis of corroles and related compounds, as it is possible to cause the extrusion of the sulfur atom with the consequent formation of a bipyrrolic unit within the macrocycle <72JCS(P1)1124). 

The present article reviews the developments of the chemistry of corrole and its metal complexes considering the synthetic procedures that can be followed in order to prepare such compounds, their spectroscopic characterization and redox reactivity and demonstrates the peculiar ligand field effect of this macrocycle. 

Recently a major improvement of this latter procedure had been reported [17]. An efficient route for the preparation of both symmetrical and asymmetrical 1,19-dibromo-dihydrobilins has been developed. The cyclization of these open chain tetrapyrroles has been investigated under a variety of conditions and the best results for the preparation of corroles have been obtained in methanol. 

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. 

The same synthetic procedure has been applied to the preparation of main group metal derivatives of corrole [25]. Thus Sn(OMC)X, Ge(OMC)X and In(OMC) (where X is chloride or acetate) have been prepared starting from SnCl2, GeCl4 and InCl3. 

Two papers have been published to date on meso-substituted derivatives of corrole [30, 31]. Four new cobalt complexes have been prepared and characterized [triphenylphosphine(2,3,7,8,12,13,17,18-octamethyl-1O-phenyl-corrolato)-cobalt (III)] (Co(OMMPC)PPh3), [triphenylphosphine (2,3,7,8,12,13,17,18-octamethyl-5,10,15-triphenylcorrolato)cobalt (III)] (Co(OMTPC)PPh3), [tri-phenylphosphine(2,3,7,8,12,13,17,18-octamethyl-5,10-diphenylcorrolato)cobalt (III)] (Co(5,10-OMDPC)PPh3) and [triphenylphosphine (2,3,7,8,12,13,17,18-octamethyl-5,15-diphenylcorrolato)cobalt(III)] (Co(5,15-OMDPC)PPh3). 

Four different synthetic procedures have been examined for the preparation of the triphenyl derivative, the fourth one suggested by the synthetic conditions developed to obtain the diphenyl derivatives. In the first three procedures it has been impossible to isolate the triphenyl-dihydrobilin. Its formation has been demonstrated, however, by monitoring the electronic spectrum of the reaction mixture and the cyclization to corrole has been carried out in situ. The synthesis that gave the highest yield (20%) and that avoids tedious purification procedures is outlined in Fig. 10. It involved the acidic condensation of benzaldehyde with two equivalents of 3,3, 4,4 -tetramethyl-meso-phenyl-dipyrromethane-5,5 ... 

The first efforts to prepare corrole were presented by A. W. Johnson in 1960. They involved an attempt to effect ring closure of a metallo-dibromo... 

Johnson, et al. also reported the synthesis of the Pd(II) thiacorrole 2.25. This macrocycle could be prepared from the corresponding tetrapyrrole 2.10 by treatment with sodium sulfide in pyridine (Scheme 2.1.6). In contrast to the imino-corroles... 

Using each of the 2 + 2 synthetic pathways discussed above, Broadhurst, et al. also attempted to prepare bithiophene-containing corroles (cf. Schemes 2.1.8 and 2.1.9). However, in no case could any corrole-like products be obtained. This failure likely reflects the increased steric bulk of the sulfur atoms these presumably hinder effective macrocyclization. 

The synthesis of 10-substituted corroles from dideoxybiladiene-ac dihydrobromides has recently been reported by Paolesse, et al Here, base-catalyzed ring closure of 2.84 was found to afford the 10-carboxymethyl-substituted corroles 2.87. The 10-substituted free-base corroles 2.88 and 2.89 were also prepared in this way (Scheme 2.1.18). Using a similar strategy, but starting from dideoxybiladiene salt 2.90, Paolesse and coworkers synthesized the 5-methoxycarbonyl-substituted corrole 2.91 (Scheme 2.1.19). 

The dideoxybiladiene-ac approach to corroles was also applied to the preparation of metallocorroles. The first report of this sort of reaction appeared in 1964. It described the preparation of the nickel(II) corrole 2.105 from the corresponding metal(II) dideoxybiladiene-ac 2.103 (Scheme 2.1.23). Interestingly, this approach did not work in the case of zinc(II) dideoxybiladiene-ac (2.104). This latter result was rationalized in terms of the known coordination chemistry of Zn(II). In particular, it was postulated that the presumed tetrahedral coordination of the zinc(II) center served to lock the reactive termini of the biladiene into positions that were unsuited for cyclization. 

Trivalent cobalt corroles may also be prepared from dideoxybiladienes-ac as... 

The tedious preparation of the monopyrrolic precursor 2.147 necessary for the tris-(phenyl)cobalt(III) corrole 2.153 led Paolesse, et al. to consider a third approach to preparing tris-(phenyl)cobalt(III) corroles. It centers around an initial acid-catalyzed condensation between the diacid dipyrrylmethane 2.144 with benzaldehyde (2.154) that is followed by a subsequent cyclization step carried out in the presence of Co(OAc)2 and PPh3. In this case, it was found that 2,3-dichloro-5,6-dicyano-I,4-benzoquinone (DDQ) helped mediate the requisite final oxidation step. While this approach did give a slightly lower yield of corrole (18%), it had the advantage of being quite streamlined. 

It is perhaps not surprising that metallocorroles may be prepared from preformed metal-free corroles as well as from linear pyrrolic precursors. In fact, the former metal insertion approach has allowed a considerable number of metallocorroles to be prepared, including complexes containing mono-, di-, tri-, and tetravalent metal cations (as discussed above in Sections 2.1.2.1.1-2.1.2.1.3). The following section will describe examples of the latter approach to metallocorroles, that is, via insertion of a metal center into a pre-formed corrole ring. 

To date, only one example of a monovalent metallocorrole has been reported. It was reported in 1976 by Grigg, et al. and involves a rhodium corrole, which was obtained in 36% yield as the result of reacting free-base diethyl-hexamethyl corrole 2.6 with Rh2(CO)4Cl2. Unlike the trivalent corrole complex 2.134, obtained earlier by the treatment of a dideoxybiladiene-ac with this same metal salt, the complex isolated in this instance analyzed as being the monovalent Rh(CO)2Corrole, 2.157 (Scheme 2.1.42). This complex was later prepared in 72% yield,although it was... 

Cobalt(III) corroles can also be prepared from the free-base macrocycle. The first reported example of this came from Johnson and Kay in 1965. In this report, 8,12-diethyl-2,3,7,13,17,18-hexamethylcorrole 2.6 was treated with Co(OAc)2 in pyridine. The cobalt complex reportedly isolated was formulated as the cobaltic complex represented by structure 2.164 (Scheme 2.1.46). Johnson and Kay further reported in 1965 that heating this complex in methanol afforded a somewhat unstable pyridine-free cobaltous corrole (e.g., 2.165) for which no structure was offered. Treating this supposed cobaltous corrole with pyridine regenerated the cobaltic corrole 2.164. 

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