Pyrrole compounds metalation

A further approach to the synthesis of pyrroles other than that shown in Scheme 47 involves formation of l-metalla-5-aza-l,3,5-trienes 123 (M = W) by insertion of an alkyne into the M=C bond of an (imino)carbene complex 121, leading to chain extension by two carbon atoms ( alkyne route to pyrroles). Compounds 123 cyclize spontaneously to 2//-pyrrole complexes 124, from which ///-pyrroles 122 are obtained by disengagement of the metal unit (Scheme 48).l2d 2H-Pyrrole complexes of type 124 have been characterized by X-ray analysis, and 2/Z-pyrroles could be eliminated from such complexes if substituents other than hydrogen are attached to C2 of the compound.136125 137 l2al2b... 

Furthermore, porphyrins react like extremely electron-rich aromatic compounds, comparable to phenol, aniline, or pyrrole. Central metal ions, in particular zinc(II) and copper(II), further activate them in electrophilic substitution reactions, l pical examples of the di- and tetrachlorination, mono- and dinitration, and most important Vilsmeier mono me o-formylation are given (Scheme 6.4.3) (Bonnett and Stephenson, 1965). Vilsmeier formylation of tetraphenylpor-phyrins gives mono (3-formyl products, which have been used to attach side chains to an imidazole end. Five coordinated metal complexes thus become accessible (not shown). 

Azaferrocene reacts with aromatic hydrocarbons in the presence of aluminium chloride, giving rise to the cationic complexes of the type (Ti -arene)(Ti -cyclopenta-dienyl)iron(l+) isolated as BF4 salts [87JOM(333)71]. The complex 28 is obtained by reaction of the sulfane compound [Cp(SMc2)3Fe]BF4 with pentamethyl-pyrrole [88AG(E)579 88AG(E)1468 90ICA(170)155]. The metallic site in this center reveals expressed Lewis acidity (89CB1891). 

The fourth chapter of this volume comprises the second part of an ongoing series by Professor A. P. Sadimenko (Fort Hare University, South Africa) dealing with organometallic compounds of pyrrole, indole, carbazole, phospholes, siloles, and boroles. This follows the review in Volume 78 of Advances covering organometallic compounds of thiophene and furan. The enormous recent advances in this area are summarized and classified according to the nature of the heterocycle and of the metals. 

Vitamin B12 (Fig. 1) is defined as a group of cobalt-containing conoids known as cobalamins. The common features of the vitamers are a corrin ting (four reduced pyrrole rings) with cobalt as the central atom, a nucleotide-like compound and a variable ligand. Vitamin B12 is exceptional in as far as it is the only vitamin containing a metal-ion. The vitamers present in biological systems are hydroxo-, aquo-, methyl-, and 5 -deoxyadenosylcobalamin. 

A porphyrin compound with a 2,9-dimethyl- 1,10-phenanthroline functionality fused at the beta-pyrrole positions is a phthalocyanine analog, and formed a complex with zinc in the cavity and a further zinc binding to the phenanthroline group. The absorption and emission spectra of the compound with and without the external zinc demonstrated the strong effects of the second metal binding on the porphyrin 7r-system.840... 

In this method, Furstner converts N-BOC protected pyrrole to the 2,5-dibromo compound (122) with NBS and this is followed by metalation and carbomethoxylation with t-butyl lithium in THF and subsequent trapping of the metalated species with methyl chloroformate to yield a pyrrole diester (123). Bromination of this diester at positions 3 and 4 with bromine in water followed by Suzuki cross-coupling with 3,4,5-trimethoxyphenyl boronic acid yields the symmetrical tetrasubstituted pyrrole (125). Base-mediated N-alkylation of this pyrrole with 4-methoxyphenethyl bromide produces the key Boger diester (126) and thereby constitutes a relay synthesis of permethyl storniamide A (120). 

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