Graham H. Barnett

Pyrrole chemistry. XXI. Synthetic approaches to cyanopyrroles

The Vilsmeier–Haack reaction with pyrrole and 1-methylpyrrole has been modified to provide a direct synthesis of the corresponding 2-nitriles. Chlorosulfonyl isocyanate may be used to prepare the same compounds, but less satisfactorily. The latter reagent does enable the synthesis of pyrrole-2,4-dicarbonitrile in one step. This dinitrile may be reduced to pyrrole-2,4-dicarboxaldehyde.

Concerning meso-tetraphenylporphyrin purification

Treatment of crude meso-tetraphenylporphyrin with 2,3-dichloro-5,6-dicyanobenzoquinone affords a high recovery of ‘chlorin-free’meso-tetraphenylporphyrin (1), the excess of quinone and its transformation products being easily removed by adsorption on alumina. This new procedure is also applied to the purification of meso-tetraphenylporphyrin metal complexes.

The proton magnetic resonance spectra of porphyrins—VII: Synthesis and NMR studies of thallium(III) coproporphyrin chelates

Abstract The four coproporphyrin “type-isomers” have been synthesized as their tetramethyl esters ( 1 ) through modifications of existing procedures. When treated with thallium(III) trifluoroacetate, these porphyrins furnish the corresponding aquo porphinatothallium(III) hydroxides ( 2 ) after ligand exchange induced by chromatography on deactivated alumina. The proton NMR spectra of the chloroform solutions of thallium(III) coproporphyrins show a pronounced concentration dependence, all resonances moving to low field upon dilution; the spectra of the type-III and -IV isomers show additional fine structure in solution. Both the meso - and β-Me protons show thallium-proton spin couplings. These results are interpreted in terms of a monomer-dimer equilibrium. In the dimers of the type-I and -II chelates, the rings lie directly one above another, whereas with the type-III and -IV complexes, steric repulsions of the propionate side-chains cause lateral displacement of one molecule in the dimer relative to the other, resulting in the observed fine structure in the spectra. The inter-porphyrin distances and lateral displacements are calculated on this basis and are compared with the corresponding dimers of the parent coproporphyrins, with which there are considerable similarities.

Meso-oxidation of some metalloporphyrins☆

Abstract Treatment of porphyrins with thallium(III) trifluoroacetate in the presence of trifluoroacetic acid results in uncontrolled oxidation at the macrocyclic meso-positions, presumably via radical processes. For example, a mixture of the thallium(III) α γ-dioxoporphodimethene (4a; R = Et), the αβγ-trioxo compound (3), and octaethylxanthoporphyrinogen (6) is obtained when octaethylporphyrin (1; R = Et) is oxidised in the presence of air. More controlled oxidation is achieved when the meso-trifluoroacetoxyporphyrin complexes (8a, b) or metallo-oxophlorins (7a, b) are treated with mild bases in air, the major products being metallo-αγ-dioxoporphodimethenes (4b, c). β-Hydroxy-α-oxophlorins (16) are isolated and characterised for the first time; aspects of the chemistry of this novel oxygenated porphyrin system are reported.

The nuclear magnetic resonance spectra of porphyrins—XI : Self-aggregation of zinc(II) meso-trifluoroacetoxyoctaethylporphyrin☆

Abstract Complex formation in zinc(II) meso -trifluoroacetoxyoctacthylporphyrin ( 1 ) and the corresponding acetate ( 3 ) has been studied by 1 H and 13 C NMR spectroscopy. The large concentration dependence of the 1 H chemical shifts of 1 has been analysed in terms of a monomer-dimer equilibrium to give K ∼ 3.01 mole −1 and monomer-dimer shifts of up to 4.2 ppm (for the γ- meso proton). The dimers are immediately dissociated upon addition of methanol. In complete contrast, 3 shows no concentration dependence nor any change upon addition of methanol. The conformations of the model compounds phenyl acetate ( 4 ) and phenyl trifluoroacetate ( 5 ) were studied by CNDO and solvation calculations and, for 4 , by a LIS experiment. Compound 4 exists as the endo conformer in non-polar media, but the exo conformer is preferentially solvated and is also formed when 4 complexes with Eu(fod) 3 . In compound 5 , the exo-endo energy difference is smaller and is not so affected by solvation; 5 shows no LIS. The large complex shifts found for compound 1 are best explained on the basis of a dimer structure in which the O·CO·CF 3 groups play no part in the association, which is presumably due to a novel metal to porphyrin interaction.

Pyrrole chemistry. XXIV. The Vilsmeier formylation and the cyanation of pyrrole acetals. A synthesis of pyrrole-2,3,5-tricarboxaldehyde

The preparation of the acetals of a number of pyrrole mono- and dicarboxaldehydes is described. It is shown that, provided the reactivity of the unsubstituted ring positions on the pyrrole nucleus is not too low, a carboxyaldehyde or a carbonitrile group may be substituted onto the pyrrole ring using the Vilsmeier reaction or chlorosulfonyl isocyanate respectively. Vilsmeier formylation of the diacetal, 2,4-di(5,5-dimethyl- 1,3-dioxan-2-yl)-pyrrole, followed by hydrolysis gave pyrrole-2,3,5-tricarboxaldehyde.

Reactions of some metalloporphyrin and metallochlorin π-cation radicals with nitrite

Treatment of π-cation radicals of magnesium(II) and cadmium(II) octa-alkylporphyrins with sodium nitrite affords good yields of meso-mononitroporphyrins after demetallation; with zinc(II)trans-octaethylchlorin, meso-substitution occurs adjacent to the partially reduced ring.

Synthesis of meso-pyridinium porphyrin salts

Zinkoctaathylporphyrin (I) liefert bei Behandlung mit Thallium(III)-nitrat und anschliesend mit Pyridinen in meso- Stellung substituierte Pyridiniumsalze (II).

Synthesis and properties of thallium(III) porphyrin chelates

Treatment of porphyrins with thallium(III) acetate or trifluoroacetate affords the corresponding thallium(III) chelates, which suffer ligand exchange during chromatography on alumina to furnish the aquoporphinatothallium-(III) hydroxides [e.g.(2c)]; other examples of ligand exchange are presented. The aquo-hydroxides are obtained directly when porphyrins are metallated with thallium(III) nitrate trihydrate. Demetallation is readily accomplished with acids, by reduction (TlIII→ TlI), or by a combination of both of these methods.Information on metal–ligand interactions is obtained from the 1H n.m.r. spectra; thallium–proton couplings are reported and are shown to occur via a predominantly π-bond mechanism for the meso-protons, but through a σ-bond mechanism for the peripheral protons. The visible absorption spectra indicate no major distortion of the porphyrin ring by the metal atom, and together with n.m.r. evidence, this suggests strongly that the metal atom is situated out of the plane of the porphyrin ring. The mass spectra of thallium(III) porphyrins show several anomalous features, the most notable being extrusion of the metal atom to give the free porphyrin ion, which is often the base peak in the spectra.