Pyrroles metal complexes

Preparation of pyrrole metal complexes and their use in organic synthesis 92MI29. 

Merlic demonstrated the direct, non-photochemical insertion of carbon monoxide from acylamino chromium carbene complexes 14 to afford a presumed chromium-complexed ketene 15 <00JA7398>. This presumed metal-complexed ketene leads to a munchnone 16 or munchnone complex which undergo dipolar cycloaddition with alkynes to yield the pyrroles 17 upon loss of carbon dioxide. 

Proton ENDOR. In both metal complexes four pairs of magnetically equivalent pyrrole protons along with some weakly coupled phenyl protons have been observed. 

All polycyclic pigments, with the exception of triphenylmethyl derivatives, comprise anellated aromatic and/or heteroaromatic moieties. In commercial pigments, these may range from systems such as diketopyrrolo-pyrrol derivatives, which feature two five-membered heteroaromatic fused rings (DPP pigments) to such eight-membered ring systems as flavanthrone or pyranthrone. The phthalo-cyanine skeleton with its polycylic metal complex is somewhat unique in this respect. 

The first class of amine-based nucleophilic catalysts to give acceptable levels of selectivity in the KR of aryl alkyl. yec-alcohols was a series of planar chiral pyrrole derivatives 13 and 14, initially disclosed by Fu in 1996 [25, 26]. Fu and co-workers had set out to develop a class of robust and tuneable catalysts that could be used for the acylative KR of various classes of. yec-alcohols. Planar-chiral azaferrocenes 13 and 14 seemed to meet their criteria. These catalysts feature of a reasonably nucleophilic nitrogen and constitute 18-electron metal complexes which are highly stable [54-58]. Moreover, by modifying the substitution pattern on the heteroaromatic ring, the steric demand and hence potentially the selectivity of these catalysts could be modulated. 

Metal complexes of pyrrole have also been investigated as substrates for lithiation reactions, with both iron and rhenium Tj -pyrrole derivatives having been found to undergo a-lithiation [90H(31 )383]. Azaferrocene was the first derivative of this type to be studied [83JOM(25l)C41], but it was found that lithiation was not selective and occurred equally in both rings. However, notwithstanding this, it has recently been reported that isomer-ically clean products can be obtained in certain circumstances from reaction with certain carbonyl compounds (Scheme 12) (89MI2). 

Concerning the question of phosphole aromaticity there are some indications of 6-rr-delocalization from NMR spectroscopy and other physical methods. The formation of metal complexes with derivatives of Mn, Fe and other metals, however, gives the best chemical proof of such 6ir- delocalization but there are still some doubts arising from the considerable differences in chemical behaviour compared with pyrroles, furans and thiophenes. This once more points to the very different properties of phosphorus, compared to nitrogen in particular. Therefore the problem is still open and much work, both practical and theoretical, remains. 

Template and capping reactions feature in recent work. A number of sterically hindered H20 and N4 ligands have been prepared in which a quadridentate Schiff base is capped by condensation of salicylaldehyde or pyrrole-2-carbaldehyde with a series of bis(8-aminonaphthyl)alkyl ethers. These ligands combine the versatility of the Schiff base with the protective features well known for certain model porphyrin systems, and appear to be of some interest. The zinc(II) complex and other transition metal complexes have been prepared.340... 

The synthesis and metal complexes of the pyrrole dithiocarbamate ligand were reported by Kellner et al. (386). At a later date two independent studies dealt with the coordination chemistry of this ligand (26, 211). An outstanding feature of this molecule is the pyrrole ring, which, by preserving aromaticity makes the contribution of resonance form B (Fig. 4) insignificant (26) and creates a dominant -accepting character at the sulfur atoms (26). 

Several porphyrin analogues with the pyrrolic nitrogens substituted by heteroatoms have been synthesized, but only the oxa analogues (23 X = O, Y = NH) are reported to form stable metal complexes (Figure 8). 60 Formation of a Zn complex of the thia analogue (23 X = S, Y = NH) requires the presence of a large excess of Zn" ion. An iron complex of dithiaporphyrin (23 X = Y = S) is also known. 

A crystal structure of a cationic corrin salt has been analyzed.251 The macrocycle is significantly ruffled and the pyrrole ring A is tipped out of the mean plane. The two inner protons are placed on N-21 and N-23. The deviation from the mean plane is small in the metal complexes.252... 

Reynolds et al. (1985, 1987) examined petroleum porphyrin and nonporphyrin metals independently with EPR after methanol extraction of porphyrins from the petroleum. Table IV shows the EPR-derived coordination spheres of vanadyl nonporphyrin fractions of various crudes. The Ni and V coordinated to four nitrogens (N4 type) could be petropor-phyrin precursors and derivatives as previously discussed or tetradentate-mixed ligands in which N does not necessarily come from a pyrrole group. Considerable variability in the vanadyl coordination among different crude oils is evident. Figure 4 illustrates some proposed metal complexes that may exist in petroleum (Yen, 1975). 

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