Pyrrole metallic derivatives

Inhaltsiibersicht Struktur des Pyrrol-Molekiils. — Ana-lytische Methoden. — Reaktivitat der Pyrrole. — Pyrrol-Metall-Derivate. — Pyrrole als Naturprodukte. — Pyrrol-Ringsynthesen. — Synthetische Methoden. 

In another series of experiments, the nitroporphyrin 6 (M = H2) or its metal derivatives 6 (M = Cu, Ni) have been used to attach heterocyclic rings to the nitro-substituted pyrrole ring of the macrotetracycle.112,113... 

Enantiomerically pure open-chain a-pyrrylalditols such as C-nucleoside analogs 1215 have been synthesized by direct coupling of sugar derivatives 1214 with pyrrole metal reagents based on magnesium-titanium" or mag-... 

The a- (171-) complexes are known for pyrroles and phospholes. For pyrroles, both N-metal derivatives (10) and organometallic compounds (82) and (83) are described. A similar situation is observed in pyrrolylimido complexes Pwu-[MX(NNC4H4)(dppe)2]+ and cis,mer-[WX2(NNC4H4) (PMe2Ph)3], whose reaction ability has been studied comprehensively (95JA12181). A mixed 771 175 coordination is realized in the binuclear Cr(CO)5 complex of azaferrocene (90MI4). 

Unsaturated heterocyclic ligands like pyrrols or porphyrins are formally derived from secondary organic amines and their metal derivatives are therefore amide complexes. The Ln-N bond lengths formed with simple monocyclic ligands... 

Potassium salts of pyrrole, 2-acetylpyrrole, 1,2,3,4-tetrahydrocarbazole, 9-carbazole, indole, imidazole, and 1,2,4-triazole react with [Tr-CjHjFe ( 0)2 ] to form ct-N derivatives, e.g., [7r-C5H5Fe(CO)2(CT-N-pyrrolyl)] (III) (400, 421), shown to be intermediates in the formation of the n-complexes e.g., (IV) (400). Anions [M(CO)5L] (M = Cr, Mo, W) were similarly prepared from the hexacarbonyls and alkali metal derivatives of succinimide, phthalimidine, and saccharin (49). 

Baeyer first reported a 4 4 cyclization product for the reaction of pyrrole and dry acetone catalyzed by the addition of one drop of hydrochloric acid (Baeyer, 1886). The macrocycle was later isolated in an 88% yield in a less violent reaction (Rothemund and Gage, 1955). Metal derivatives of the phthalocyanines were obtained when o-diaminobenzene, o-cyanobenzamide, or related compounds were pyrolized with metals or their salts (Braun and Tcherniac, 1907). This was possibly the first observed template synthesis of a macrocyclic ligand. 

Andrieux, C. P., Audebert, R, and Saveant, J. M., Studies of modified electrodes starting from pyrrole monomers derivated by anthraquinone, phenothiazine or anthracene moieties on the nitrogen or in the 3-position, synthesis, cyclic voltammetry and a study of charge-transfer reactions, Synth. Metals, 35, 155-158 (1990). 

The macro-ring system of four partially reduced pyrrole nuclei joined through three bridge-carbon atoms is designated corrin, metal derivatives of this compound are designated cobalto-corrin, cohalti-corrin, etc. 

The alkali metallic derivatives are white, crystalline compounds which are decomposed by water to give back pyrrole and which have some value in synthesis. From colorimetric and spectroscopic studies, McEwen o derived the value Ka — 16 5 for pyrrole (as an acid). It may be noted that... 

The competition between N- and C-substitution in the alkali metal derivatives of pyrrole is, of course, only one example of the general problem of substitution into ambident ions 225-26 ... 

A number of instances have already been mentioned of reactions in which both N- and G-substitution occur. These are reactions of pyrroles with acid anhydrides (p. 64), alkylation and alkenylation of metallic derivatives of pyrroles (p. 66) and ethoxycarbonylation of metallic derivatives (p. 66 see also below). Most of the reactions of pyrrole Grignard reagents result in G-substitution, but occasionally N-substitution is also observed (p. 106). The Mannich reaction does not cause N-substitution (p. 70). Deuteration proceeds most easily at the nitrogen atom (p. 75), and reactions with metals (p. 61) cause displacement of hydrogen from nitrogen. 

The formulation of the pyrrole Grignard reagents as ionic, but not highly dissociated, structures related to (104) enables the predominant C-substi-tution reactions to be formulated in the same way as the similar processes leading to C-substitution in the alkenylation of other metallic derivatives of pyrrole (p. 67). The reagents should be regarded as sources of more or less dissociated ambident pyrrole anions (p. 68). 

Structure of the Haems.— The iron-porphyrans, or haems, like the other porphyrans are mono-metallic derivatives of a porphyrin. In haem the iron is ferrous (divalent) and united to one of the pyrrole nitrogens, conventionally regarded as belonging to ring I. In haematin (methaem) and in haematin chloride, the iron is ferric (trivalent) and united to two nitrogen atoms in the parent porphyran, protoporphyrin. 

Reduction of isoindoles with dissolving metals or catalytically occurs in the pyrrole ring. Reduction of indolizine with hydrogen and a platinum catalyst gives an octahydro derivative. With a palladium catalyst in neutral solution, reduction occurs in the pyridine ring but in the presence of acid, reduction occurs in the five-membered ring (Scheme 38). Reductive metallation of 1,3-diphenylisobenzofuran results in stereoselective formation of the cw-1,3-dihydro derivative (Scheme 39) (80JOC3982). 

Nontransition metal cyclopentadienyl derivatives of pyrrole are quite scarce (99CRV969). The derivatives M(2,5-C4H2 Bu2N)2 (M = Sn, Pb) were studied [92AG(E)778 92JCS(CC)760]. 

An approach to isobacteriochlorins1 ln-e makes use of Pd(II) or metal-free bilatrienes 1 as starting materials. Cyclization of the corresponding bilatriene derivatives is induced by base in the presence of palladium(II) or zinc(II) which exercise a template effect. Zinc can be readily removed from the cyclized macrotetracycles by acid whereas palladium forms very stable complexes which cannot be demetalated. Prior to the cyclization reaction, an enamine is formed by elimination of hydrogen cyanide from the 1-position. The nucleophilic enamine then attacks the electrophilic 19-position with loss of the leaving group present at the terminal pyrrole ring. 

Oxidation reactions were used in the synthesis of porphyrin d, the metal-free ligand system of naturally occurring heme d,. In a total synthesis of porphyrin d,12d oxo functions were introduced into isobacteriochlorin 3 by selenium dioxide oxidation to yield 4. The selenium dioxide selectively attacks the 3- and 8-positions of the partially reduced pyrrole rings of the chromophore. In another synthesis23a c of porphyrin d, an isobacteriochlorin 5, derived by... 

The reaction of JV,iV-dimethylhydrazones (1-amino-1-azadienes) and alkenylcarbene complexes mainly produces [3C+2S] cyclopentene derivatives (see Sect. 2.6.4.5). However, a minor product in this reaction is a pyrrole derivative which can be considered as derived from a [4S+1C] cycloaddition process [75]. In this case, the reaction is initiated by the nucleophilic 1,2-addition of the nitrogen lone pair to the metal-carbon double bond followed by cyclisation and... 

The last method for the preparation of 2-quinolones described in this chapter relies on a intramolecular Heck cyclization starting from heteroaryl-amides (Table 2) [57]. These are synthesized either from commercially available pyrrole- and thiophene-2-carboxylic acids (a, Table 2) or thiophene-and furan-3-carboxylic acids (b, Table 2) in three steps. The Heck cyclization is conventionally performed with W,Ar-dimethylacetamide (DMA) as solvent, KOAc as base and Pd(PPh3)4 as catalyst for 24 h at 120 °C resulting in the coupled products in 56-89% yields. As discussed in Sect. 3.4, transition metal-catalyzed reactions often benefit from microwave irradiation [58-61], and so is the case also for this intramolecular reaction. In fact, derivatives with an aryl iodide were successfully coupled by conventional methods, whereas the heteroarylbromides 18 and 19, shown in Table 2, could only be coupled in satisfying yields by using MAOS (Table 2). 

In a related reaction, heating ketones in the presence of TlClsOTf leads to 1,3,5-trisubstituted arenes. " Nitriles react with 2 mol of acetylene, in the presence of a cobalt catalyst, to give 2-substituted pyridines. " Triketones fix nitrogen gas in the presence of TiCU and lithium metal to form bicyclic pyrrole derivatives. " ... 

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