Cyclopentadiene metal derivatives

The monomer, CPD, obtained via cracking of the dimer, DCPD, and the dimer both have extensive uses. Cyclopentadiene is probably the most widely studied conjugated, cycHc diolefin system. Eleven review articles dealing with the chemistry of cyclopentadiene have been pubHshed (1—11). An article dealing specifically with European uses of DCPD has also been pubHshed (12). The discovery ia 1951 of stable metal derivatives has given additional impetus to the study of the chemistry of cyclopentadiene. Eive review articles have been pubHshed on this subject (13—17). 

Cyanoguanidine, 3 43 Cyanuric chloride, formation of, from cyanogen chloride, 2 94 1-Cyclohexyl-l, 1-diethylhydra-zonium chloride, 6 92 Cyclopentadiene, magnesium and other metal derivatives of, 6 11, 15... 

It should not be supposed that all metal derivatives of cyclopentadiehe are sandwichlike molecules. In many cases, the metal is joined to just one of the carbons in the ring through a single (cr) bond, which, however, often has considerable ionic character. Metal-to-earbon bonds almost certainly exist in the cyclopentadiene derivatives of the alkali and alkaline-earth metals, the rare earths, and certain of the post-transitioii elements. 

Cyclopentadiene forms alkali metal derivatives that are essentially ionic ... 

An alternative approach involves displacement of two halide ligands from a metal by an appropriate dilithio or bis-Grignard derivative of a 1,3-diene or a related aryl substrate thus, metallated derivatives of (PhCH=CPh)2, PhCH=CHPh, Ph2 and Ph20 have been used to synthesize metalla-cyclopentadienes, -indenes, -fluorenes and -xanthenes, e.g. 

Another method for the preparation of the complexes was early developed by us. As is well known, Thiele, at the turn of the century, obtained the white saline compound KC5H5 (the first metal derivative of a five-membered ring system) by the action of metallic potassium on freshly prepared cyclopentadiene in benzene solution 191). It was found that a suitable method for the preparation of the alkali metal salts consisted of the action of the metals on cyclopentadiene in liquid ammonia, according to the equation... 

Two general methods have been used for the preparation of cyclopentadienylcobalt dicarbonyl the reaction between dicobalt octacarbonyl and cyclopentadiene and the reaction of bis (cyclopentadienyl) cobalt with carbon monoxide at elevated temperatures and pressures. The method given here is a modification of the second method, and as in the preparation of cyclopentadienylvanadium tetracarbonyl described above, avoids isolation of the intermediate pyrophoric bis(cyclopentadienyl)metal derivative. 

Substituted cyclopentadienes also form metallic derivatives by direct reaction in the liquid phase with elemental lithium, sodium, and potassium. Thus, indene reacts with sodium to form sodium indenide (11,23,24) and lithium, sodium, and potassium react with fluorene (17, 25). Reaction of metals with substituted cyclopentadienes in the vapor phase has not been extended beyond methylcyclopentadiene. 

In 1988 Reetz et al. introduced the concept of metal-free polymerization of acrylates, methacrylates and acrylonitrile [224,225]. Metal-free initiators are salts consisting of a carbanion (A ) having R4N as cationic counterions. They are synthesized by the reaction of neutral CH or NH-acidic compounds such as malonic acid esters, nitriles, sulfones, nitro-alkanes, cyclopentadiene, fluorene derivates, carbazoles and succinimide. Water is removed azeotropically using toluene. 

The preparation of 5-ACETYL-l,2,3,4,5-PENTAMETHYLCYCLO-PENTADIENE is of value in the synthesis of pentamethyleyclo-pentadiene and many pentamethylcyclopentadienyl metal carbonyl derivatives that are more soluble in organic solvents than those derived from cyclopentadiene. Simple preparations of 5,6-DIHYDRO-2-PYRAN-2-0NE and 2-//-PYRAN-2-ONE make these hitherto rather inaccessible intermediates available for cycloaddition and other reactions. The already broad scope of the Michael reaction has been widened further by including an efficient preparation of ETHYL (E)-3-NITROACRYLATE. Workers in the field of heterocyclic chemistry will find a simplified method for the preparation of 2,3,4,5-TETRA-HYDROPYRIDINE of help. 

The cyclopentadienide ion, C H , is a common organic anion that forms very stable complexes with metal cations. The anion is derived by removing a proton from cyclopentadiene, QH, with strong base. The molecule has a five-memhered ring of carbon atoms, with four carbon atoms attached to only one proton and one carbon atom bonded to two. Draw the Lewis... 

Very little is known as yet of the chemistry of cyclopentadienylthallium(I) and the related compounds listed in Table I. The parent compound gives tribromocyclopentane on treatment with bromine and the hexabromo derivative with potassium hypobromite 112). By far the most important use discovered so far for these organothallium(I) compounds is the preparation of metallocenes and cyclopentadiene-transition metal complexes. These preparations are, in general, characterized by manipulative simplicity and high yields, and details of the reactions reported thus far are summarized in Tables II-IV. 

Zeijden [112] used chiral M-functionalized cyclopentadiene ligands to prepare a series of transition metal complexes. The zirconium derivative (82 in Scheme 46), as a moderate Lewis acid, catalyzed the Diels-Alder reaction between methacroleine and cyclopentadiene, with 72% de but no measurable enantiomeric excess. Nakagawa [113] reported l,T-(2,2 -bis-acylamino)binaphthalene (83 in Scheme 46) to be effective in the ytterbium-catalyzed asymmetric Diels-Alder reaction between cyclopentadiene and crotonyl-l,3-oxazolidin-2-one. The adduct was obtained with high yield and enantioselectivity (97% yield, endo/exo = 91/9, > 98% ee for the endo adduct). The addition of diisopropylethylamine was necessary to afford high enantioselectivities, since without this additive, the product was essentially... 

An alternative synthesis of a thermally stable cyclopentadienyl functionalized polymer involved ring bromination of poly(oxy-2,6-diphenyl-l,4-phenylene), followed by lithiation with butyl lithium to produce an aryllithium polymer. Arylation of 2-norbornen-7-one with the metalated polymer yielded the corresponding 2-norbornen-7-ol derivative. Conversion of the 7-ol to 7-chloro followed by treatment with butyl lithium generated the benzyl anion which undergoes a retro Diels-Alder reaction with the evolution of ethylene to produce the desired aryl cyclopentadiene polymer, 6. 

Additions of metallacyclopentadienes to carbon—carbon triple bonds are rare, and only a few examples are known (Eq. 2.45) [37]. The 1,1-addition of zirconacyclopentadienes is quite different from other carbon—carbon bond-forming reactions described in this chapter. This reaction does not require transmetalation of zirconacyclopentadienes to other metals. Thus, in the absence of any added metal halide, zirconacyclopentadienes react with propynoates to give cyclopentadiene derivatives. This reaction requires the use of at least 2 equivalents of the propynoate (Eq. 2.46). 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

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