Cyclopentadiene, metal complexes

Fischer EO, Pfab W (1952) Cyclopentadiene-metallic complex, a new type of organo-metallic compound. Z Naturforsch B 7 377-379... 

There is some disagreement concerning assignment of the C—H stretches 16,34). Fritz suggests that there is some interaction of the H hydrogen atoms with the metal. This could cause a lowering of the C—H stretch, in a manner similar to the lowering of the C—H endo stretches of some /x-bonded cyclopentadiene-metal complexes 41, 61). 

In the light of the arguments presented for the configuration of the substituted cyclopentadiene-metal complexes (Chapter 3, C, b) the nucleophilic group is placed in the exo-position. 

Verdet, L. and Stille, J.K., Poly (phenylene oxide) Catalyst Supports Containing (Cyclopentadiene)metal Complexes. Organometalllcs, 1982. 1 p. 380-4. 

Cyclopentadiene itself has been used as a feedstock for carbon fiber manufacture (76). Cyclopentadiene is also a component of supported metallocene—alumoxane polymerization catalysts in the preparation of syndiotactic polyolefins (77), as a nickel or iron complex in the production of methanol and ethanol from synthesis gas (78), and as Group VIII metal complexes for the production of acetaldehyde from methanol and synthesis gas (79). 

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... 

In 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. 

Another recent application to the activation of transition metals was reported (247) by Bonnemann, Bogdavovic, and co-workers, in which an extremely reactive Mg species was used to reduce metal salts in the presence of cyclopentadiene, 1,5-cyclo-octadiene, and other ligands to form their metal complexes. The reactive Mg species, characterized as Mg(THF)3 (anthracene), was produced from Mg powder in THF solutions containing a catalytic amount of anthracene by use of an ultrasonic cleaning bath. A plausible scheme for this reaction has been suggested ... 

As mentioned earlier, ruthenium cymene, rhodium and iridium pentamethyl-cyclopentadiene are good metal complexes, to start with, in a screen. It is diffi-... 

Corey and Ishihara29 report the synthesis of a new bis(oxazoline). This catalyst effects Diels-Alder reaction via a tetracoordinated metal complex. Ligand (.S )-8I is synthesized from (iS )-phenylglycine, as depicted in Scheme 5-25. Treatment of 81 with Mgl2 L gives a dark solution of complex 82, which can be utilized as a Diels-Alder reaction catalyst. Thus, reaction of cyclopentadiene with 71 in the presence of 82 yields product 72a with an enantiomeric ratio of over 20 1 (Scheme 5-26). 

Metal complexes of bis(oxazoline) ligands are excellent catalysts for the enantioselective Diels-Alder reaction of cyclopentadiene and 3-acryloyl-l,3-oxa-zolidin-2-one. This reaction was most commonly utilized for initial investigation of the catalytic system. The selectivity in this reaction can be twofold. Approach of the dienophile (in this case, 3-acryloyl-l,3-oxazolidin-2-one) can be from the endo or exo face and the orientation of the oxazolidinone ring can lead to formation of either enantiomer R or S) on each face. The ideal catalyst would offer control over both of these factors leading to reaction at exclusively one face (endo or exo) and yielding exclusively one enantiomer. Corey and co-workers first experimented with the use of bis(oxazoline)-metal complexes as catalysts in the Diels-Alder reaction between cyclopentadiene 68 and 3-acryloyl-l,3-oxazolidin-2-one 69 the results are summarized in Table 9.7 (Fig. 9.20). For this reaction, 10 mol% of various iron(III)-phe-box 6 complexes were utilized at a reaction temperature of —50 °C for 2-15 h. The yields of cycloadducts were 85%. The best selectivities were observed when iron(III) chloride was used as the metal source and the reaction was stirred at —50 °C for 15 h. Under these conditions the facial selectivity was determined to be 99 1 (endo/exo) with an endo ee of 84%. 

There are also several other examples of bis(oxazoline)-metal complex catalyzed Diels-Alder reactions of cyclopentadiene and other unsaturated esters. 

Compounds with a narrow HOMO-LUMO gap (Figure 5.5d) are kinetically reactive and subject to dimerization (e.g., cyclopentadiene) or reaction with Lewis acids or bases. Polyenes are the dominant organic examples of this group. The difficulty in isolation of cyclobutadiene lies not with any intrinsic instability of the molecule but with the self-reactivity which arises from an extremely narrow HOMO-LUMO gap. A second class of compounds also falls in this category, coordinatively unsaturated transition metal complexes. In transition metals, the atomic n d orbital set may be partially occupied and/or nearly degenerate with the partially occupied n + 1 spn set. Such a configuration permits exceptional reactivity, even toward C—H and C—C bonds. These systems are treated separately in Chapter 13. 

The strategies used to synthesize metal complexes with the cyclopentadienyl ligands portrayed in Scheme 13 are briefly described below. The most widely used procedure begins with the corresponding cyclopentadiene derivative, which can be transformed into a cyclopentadienyl metal complex by a variety of metallation methods as described in... 

There is a certain analogy between the aromatic anions of cyclopentadienide (C5H5 ) and boratabenzene (C5H6B ). l-Methylbora-2,5-cyclohexadienehas a more acidic proton connected to the, sp3-hybridized ring carbon atom than cyclopentadi-ene, due to the same tendency of aromatic anion formation [252, 253]. The related 1-phenyl-1,4-dihydroborabenzene affords the lithium salt of 1-phenylborataben-zene on treatment with tert-butyllithium. Like metallic complexes such as ferrocene formed by cyclopentadiene, boratabenzene also forms such sandwich -complexes with iron and cobalt. The iron complex can be acetylated under Friedel-Crafts conditions. 

One of the most important carbon 7i-donors are the cyclopentadienes and their heteroanalogues, for instance 15 and 16. In general, cyclopentadiene itself forms three general types of mononuclear Cp transition metal complexes Cp2M 3 (symmetric molecules with mutually parallel Cp rings examples M = Fe, Cr, Ni), Cp2ML 124 [bent metallocenes, L = H, R, CO, etc. (n=l-3)], and half-sandwich compounds CpML 125 (n — 1 4) [21b], In the bent sandwich complexes... 

Treatment of hydrogermanium cyclopentadiene transition metal complexes with LDA can lead initially to a competition between the deprotonation of the hydrogen linked to germanium or to the cyclopentadienyl ring, but a migration of the germyl group to cyclopentadiene was actually observed (equation 192)9. 

To date there are no examples of the very highly strained cyclopropadiene (288), 1,2-cyclobutadiene (289), or 1,2-cyclopentadiene (290), either free or complexed to transition metals. Complexes of a valence isomeric form of cyclopropadiene (291) have been prepared,110 but as with the free hydrocarbon,111112 there is no evidence for any converting to the allene form [Eq. (46)]. 

Unlike the compounds obtained with the heavier halogens, polyfluori-nated metallocene derivatives cannot be prepared from the permercurated metallocenes. Thermally stable pentafluorinated ruthenocenes have been made from the (oxocyclohexadienyl)(cyclopentadienyl)metal complexes by flash vacuum pyrolysis (Scheme 6).53 54 Decafluorometallocenes, [M(C5F5)2], are still unknown potential routes to these compounds starting from fluorinated cyclopentadiene (C5F5H)55 or from the decachlorometal-locenes48 have been unsuccessful. However, there seems to be no fundamental steric or electronic barrier to their eventual preparation. 

This rather elaborate collection of synthetic routes leading to supra-Cp systems is also intended to provide the reader with some potential solutions for problems he or she might have in designing bulky Cp s. In addition to the substituted cyclopentadiene hydrocarbons whose syntheses are outlined above, the respective bromo-Cp s are also ligand precursors of considerable utility for Cp-metal complex-forming reactions (see Section III and... 

In a first approximation supra-Cp metal complexes can be prepared the same way as normal or other Cp-metal and organo-metal bonds in general. The methods used most often (see Appendix) are the metathesis reaction [Eq. (1)] followed in number by oxidative additions (Eq. (2)] and metallation/deprotonation reactions [Eq. (3)]. The latter is especially important for the cyclpentadienyl alkali metal compounds. A useful variation of reaction (3) is the formation of CpTl in an acid/base reaction from cyclopentadiene and thallium ethoxide [Eq. (3b)]. This represents a convenient route to cyclopentadienylthallium compounds, which are also valued (in place of Cp alkalis) as mild Cp-transfer reagents for the synthesis of difficultly isolable cyclopentadienyl derivatives (77). 

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