Other Heterocyclic Metallocenes

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. 

Bromoacetylcyclopentadienylmanganese tricarbonyl has been converted into 198,199, and 200.161 Action of hydrazine on the appropriate diketone gave 201.155 Friedel-Crafts acylation of cyclopenta-dienylmanganese tricarbonyl gave 202 (X = 0 or S).113 The half-wave potentials of 202 were measured polarographically and compared with ferrocene and benzene analogs.113 

A number of chelates containing oxygen-metal rings have also been prepared in this series.147,155 

Azaferrocene (206) has been prepared by the reaction of sodium pyrrole, cyclopentadienylsodium, and ferrous chloride,157 or better by the reaction of potassium pyrrole with cyclopentadienyliron dicarbonyl iodide.158 It has been found159 that dioxane is a better solvent than benzene for this latter reaction. This reaction has also been applied to 2,4- and 2,5-dimethylpyrrole,158 3-acetyl-2-methyl-and 3-acetyl-2,4-dimethylpyrrole,160 and 2-methylpyrrole.159 The 2-methylazaferrocene from this latter pyrrole has been resolved with (—)6,6 -dinitrodiphenic acid.159 A compound (207) believed to be an intermediate in the formation of azaferrocenes has been isolated.161 

Aza analogs of cyclopentadienylmanganese tricarbonyl have also been prepared.158 160,162 Application of this sequence to pyrazole, imidazole, and 1,2,4-triazole, however, led not to aza analogs but to coordination polymers.168 A mixed manganese and chromium carbonyl complex of 2-benzylpyrrole has been prepared.163a 

Nauk SSSR, Ser. Khim. 2127 (1968) Ghem. Abstr. 70, 29030r 

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The use of the boratabenzene heterocycle as a ligand for transition metal complexes dates back to 1970 with the synthesis of (C H5B-Ph)CpCo+ (1) (Cp = cyclopentadienyl).1 Since boratabenzene and Cp are 6 it electron donors, 1 can be considered isoelectronic to cobaltocenium. Many other transition metal compounds have been prepared that take advantage of the relationship between Cp and boratabenzene.2 In 1996, the synthesis of bis(diisopropylaminoboratabenzene)zirconium dichloride (CsHsB-NPr ZrCh (2) was reported Of particular interest is that 2 can be activated with methylaluminoxane (MAO) to produce ethylene polymerization catalysts with activities similar to those characteristic of group 4 metallocenes.4 Subsequent efforts showed that, under similar reaction conditions, (CsHjB-Ph ZrCh/MAO (3/MAO) gave predominantly 2-alkyl-1-alkenes5 while (CsHsB-OEt ZrCh/MAO (4/MAO) produced exclusively 1-alkenes.6 Therefore, as shown in Scheme 1, it is possible to modulate the specificity of the catalytic species by choice of the exocyclic group on boron. 

Of all of the elements of the Periodic Table, only neighboring carbon and boron share the properties of self-bonding (catenation) and the support of electron-delocalized structures based upon these catenated frameworks. Carbon catenation, of course, leads to the immense field of organic chemistry. Boron catenation provides the nido-, arachno-, and /i p/io-boranes, which may be considered as the borane equivalents of aliphatic hydrocarbons, and the discrete families of c/oso-borane derivatives which bear a for nal resemblance to the aromatic hydrocarbons, heterocycles, and metallocenes. Aside from these analogies, boron and carbon chemistries are also important to each other through their extravagant ability to mix m ways not available to other element-pairs. Thus, the conflux of boron and carbon chemistries effectively provides an element-pair for exploitation in a variety of novel ways. 

Since, for secrecy reasons, information on new processes and the state of their development is not always published, or only after long delays, the classification applied or recent developments may be misleading. For example, the potential of phase-transfer catalyzed processes may already be more important than the present literature indicates. The same statement could apply for areas such as amidocarbonylation, the synthesis of fine chemicals by means of metallocenes, the reductive/oxidative carbonylation of aromatic amines or nitro derivatives, Heck coupling using palladacycles and heterocyclic carbene complexes, catalytic McMurry coupling, or other proposed methods. Recent developments must therefore leave open the stage of development reached, perhaps signaling that at the time of publication no commercialized, licensable process is yet known to the scientific community. 

Interest has also continued in studies of the aromaticity of other unsaturated cyclic phosphorus compounds. A theoretical study of the 1,2-diphosphacyclooctatetraene system (228) has concluded that the ring system is perfectly planar when it has a global charge of —2 or +2, in contrast to the 1,2-diaza analogue which adopts a distorted, non-planar structure in the same oxidation states. However, it is thought unlikely that the phosphorus heterocycle will form planar metallocene complexes with metal ions. Theoretical and experimental studies have also been reported for the lone pair 671-aromatic anions P4 (229) and the arsenic analogue As4 . Both... 

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