Ferrocene homoannular

Wally et al.115 report a homoannularly bridged hydroxyamino ferrocene (+)-123 as an efficient catalyst for enantioselective ethylation of aromatic or aliphatic aldehydes. 

The major product is the heteroannular disubstituted derivative, l,l -diacetyl-ferrocene (I), while a very small amount of a homoannular isomer, 1,2-diacetylfer-rocene (II), is also obtained. The first acetyl group therefore appears to deactivate the substituted ring toward further electrophilic substitution, and the second acetyl group preferentially enters the opposite ring. 

Diarene metals generally decompose under conditions of normal electrophilic substitution, but dibenzene chromium can be metalated with amyl sodium in alkanes, and subsequent reaction with carbon dioxide and dimethyl sulfate results in a complex mixture of mono-, di-, and poly-substituted dibenzene chromium methyl esters. It is interesting that these polysubstituted compounds are homoannular in contrast to the corresponding ferrocene compounds. In view of the scanty evidence and ease of oxidation, it is impossible to conclude whether the diarene metals are less aromatic than the dicyclopentadienyl metals as predicted by simple theory (Sec. III.A). 

In other examples, compounds in which a metal atom is already coordinated in a molecule can be used as a comonomer in an addition polymerization. Two examples involve the ferrocenes discussed in Chapter 6. The vinyl ferrocene molecule is shown in 7.14, and a similar vinyl manganese complex in 7.15.30 An alternative approach involves condensation polymerization. For example, if the R group in the ferrocene unit shown in 7.16 contains a hydroxyl group, it can be copolymerized with a diacid chloride. If it is an acid chloride, it can be copolymerized with a diamine. (This type of polymer is called a heteroannular chain if only one of the rings in the repeat unit is in the backbone, the polymer is called homoannular.)7 Similarly, the titanium complex shown in 7.17 is copolymerized with diacids or diols.30 Numerous other examples involving ferrocenes are discussed in Chapter 6. 

Just as homoannular ferroeenedicarboxylic acid leads to a cyclic anhydride (23), so does the heteroannular ferroeenedicarboxylic acid. Thus treatment of the dichloride of ferrocene-l,l -dicarboxylic acid with water in chloroform-pyridine gave the anhydride (34).21 Treatment of 34 with ammonium hydroxide led to ring opening and formation of the diamide and the monoamide-monoacid.21... 

Since all aromatic organometallic 7T-complex systems possess several possible sites for which substituent effects could be analyzed, it seems best that the various sites be studied independently of one another. One particular site, the substituted ring, has been shown to be amenable to such analysis. The electronic effects on the substituted ring in ferrocenes, i.e., homoannular electronic effects, have recently been the subject of an extensive review (75). For this reason discussion of ferrocene systems in this section will be abbreviated and attention will be focused upon recent advances in related metallocene and other aromatic organometallic systems. 

Two distinct approaches to the elucidation of homoannular electronic effects within the transition metal-complexed aromatic ring have been utilized (1) relative site reactivities and (2) NMR spectroscopy. Because of the relative availability of many ferrocene derivatives considerable attention has been devoted to this system. In several instances results for the ferrocene system have provided a basis for the analysis of electronic effects in related organometallic Tr-complexes. 

The study of relative site reactivities for aromatic electrophilic substitution reactions has provided useful information for assessing homoannular electronic effects within the ferrocene system. Ferrocenes of the type (I)... 

Most of the data concerning ferrocene electronic effects correlate rather well, f errocene emerges as a system which homoannularly allows electron withdrawal to take place principally from the 2-position and secondarily... 

A word of caution should be added with respect to Chemical Abstracts, as far as chirality assignments of homoannular substituted ferrocene derivatives are concerned. Until the 8th collective index, only (-f) and (—) are found as chirality indicators. For quite a long time, no descriptors were given at all, only the remark stereoisomer , followed by the registry number, which does not allow identification of a compound easily. This fact is in sharp contrast to the claims of Chemical Abstracts Service authors that they would consequently use Schlogl s central descriptors [20, 21]. Since volume 114, the (R, S ) nomenclature for ferrocene derivatives begins to appear, but its application is not very consequent, at least at the time where the book was written, and it is advisible to examine the orginal article rather than trust Chemical Abstract s descriptors. 

This latter aspect of the proposed mechanism provided the grounds upon which its validity could be tested by examining the relative rates at which an epimeric pair of ferrocene carboxylic acids, constrained by their stereochemistry to either exocyclic or endocyclic electrophilic attack, would undergo homoannular cyclization... 

The preparation and structure of homoannular diacetyl ferrocenes 164), and intramolecular cyclizations, both homoannular 168) and heteroannular I48, 168), have been exhaustively investigated. As part of this work, ferrocene carboxylic acids have been prepared 170). A rearrangement with ortho-substitution of a quaternary ammonium ion derived from ferrocene is also noteworthy 99). Work has been done on the isolation of optical isomers of substituted ferrocene derivatives 6, 166) and on cis-trans isomerism 169). 

A443 nm) to [4]superferrocenophane (77) (A403 nm). Alkyl substituted fcp as well as fcp containing homoannular bridges, do not show this dependency consequently slight distortions of the ferrocene nucleus caused by heteroannular bridging, must be responsible for the hypsochromic shift [71a],... 

The formation of polymetaUocenes through homoannular or heteroannular substitution is not the only method for obtaining the polymers they can also be formed by polymerization via the metal. The metallocene can be attached to the polymer chain as in poly(vinyl ferrocene) (8), or it can be an integral part of the polymer backbone (9). ... 

Research in metallocene-containing polyamides, polyureas, polyuretiianes, and polyesters continued to be conducted in the 1980s, and since that time, research into these classes of materials has continued to grow7 Homoannular and heteroaimular liquid-crystalline polymetallocenes have been prepared by Zentel and coworkers through condensation reactions. Ferrocene-based liquid-crystalline polymers have also been reported by Senthil and Kannan. The synthesis of ferrocene polymers containing backbone organophosphorus moieties 32 is shown in scheme 9. 

Dialkylation preferentially occurs homoannularly, as expected, and indeed the occurrence of large yields of polyalkylated products may be understood since alkyl substituents increase the reactivity of the substituted ring. Direct alkylation of ferrocene may also be achieved by the condensation of ferrocene with formaldehyde in liquid hydrogen fluoride [314, 315]. 

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