Metallocenes, tricarbonyl

Among the compounds that form complexes with silver and other metals are benzene (represented as in 9) and cyclooctatetraene. When the metal involved has a coordination number >1, more than one donor molecule participates. In many cases, this extra electron density comes from CO groups, which in these eomplexes are called carbonyl groups. Thus, benzene-chromium tricarbonyl (10) is a stable compound. Three arrows are shown, since all three aromatic bonding orbitals contribute some electron density to the metal. Metallocenes (p. 53) may be considered a special case of this type of complex, although the bonding in metallocenes is much stronger. 

Irradiation of 1 -ethoxycarbonyl-1 //-azepine with tricarbonyl(cyclobutadiene)iron produces the novel metallocene (166) as a stable orange crystalline solid, the structure of which was confirmed by X-ray crystallography (71JA262). 

The sulfonic acids of these metallocenes can be converted to sulfonyl chlorides, sulfonamides, etc., by appropriate reagents. Reduction of ferrocene-sulfonyl chloride by lithium aluminum hydride produces the rapidly oxidized ferrocenethiol in quantitative yield (43). Both the sulfonic acid of cyclopentadienylmanganese tricarbonyl and the corresponding sulfinic acid (obtained by sodium sulfite reduction of the sulfonic acid) have been converted to sulfones (10). 

The compounds in this section can be divided into two categories those in which the heterocyclic group is a substituent on the metallocene, such as heterocyclic compounds containing cyclopentadienyl-manganese tricarbonyl, and those in which a heteroatom is in a ring of the metallocene, such as in azaferrocene. 

Planar chiral compounds usually (and for the purpose of this review, always) contain unsymmetrically substituted aromatic systems. Chirality arises because the otherwise enantiotopic faces of the aromatic ring are differentiated by the coordination to a metal atom - commonly iron (in the ferrocenes) or chromium (in the arenechromium tricarbonyl complexes). Withdrawal of electrons by the metal centre means that arene-metal complexes and metallocenes are more readily lithiated than their parent aromatic systems, and the stereochemical features associated with the planar chirality allow lithiation to be diastereoselective (if the starting material is chiral) or enantioselective (if only the product is chiral). 

This molecule possesses planar metallocene chirality. Unfortunately, neither this product nor its derivative ferrocenestriol displays any binding activity for activation of the estradiol receptors, whether in the a- or the p-isoform, nor for the androgen or progesterone receptors. This may be due to the absence of a polar OH function on the aromatic A ring, in which case one could envisage attachment of a -(CH2) OH entity to the molecule, as we did in the arene chromium tricarbonyl series [103]. 

Enantiomeric separation of nonpharmaceutical compounds include IV-alkyl-Af-methylaniline W-oxides (ethyl to butyl plus isomers) on a Chiralcel OD column. (A = 210 nm) using 1% to 3% ethanol in hexane [143]. Carrea et al. [144] separated the enantiomers of various substituted chromium and magnesium tricarbonyl metallocenes ( / -benzene and -cyclopentadiene) on a Chiralcel OD column (A = 315 run) with an isocratic mobile phase that varied from 1% to 10% ethanol in hexane depending on the enantiomeric pair involved. Chromium tricarbonyl compounds complexed with a variety of ij -arenes were separated on a Whelk-O column (A = 315 nm) using a 20/80 IPA/hexane as the mobile phase [145]. 

Various strategies for the synthesis of metallocene monomers were described in [198]. Vinylmetallocenes 38 like vinylferrocene or ri -(vinylcyclopentadienyl)-dicarbonylnitro-sylmanganese 39 are prepared several decades ago by synthesizing the vinyl group in the metallo-derivatives [199]. Other 7c-type compounds such as ri -(styrene)tricarbonyl-chromium 40 are obtained by reaction of styrene with triamine-tricarbonylchromium [200]. 

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