Using Bulky Cyclopentadienes

The induction of chirality in Cp- metal derivatives may also be studied. There are different ways that even achiral substituents on a cyclopentadienyl ring can give chiral metal complexes. The induction of chirality can proceed through their substitution pattern and/or a hindered ring or substituent rotation. The isotactic polymerization of propylene by means of metallocene catalysts is one example where such a metallocenic chirality has already been employed in an important stereoselective synthesis. 

In the following each of the above concepts is elaborated in more detail and illustrated with specific examples. 

A brief comment is in place here, to avoid any confusion between the thermodynamic terms stable/unstable and the kinetic descriptors labile/ inert. Inert complexes simply have no suitable low-energy pathway for the reaction available, or, in other words, the free enthalpy of activation (AGt) is very high even if there are more (thermodynamically) stable products. A stable complex has a large positive free enthalpy of reaction (AG°) for its decomposition (702). This is illustrated in Fig. 1. Quite often, however, the terms inert/labile are replaced by kinetically stable/unstable. For an interesting essay on the different meaning of the simple term stable in the chemistry and physics community, see Ref. 103. 

Experimentally an enhanced inertness is clearly seen in a comparison between the normal and decabenzylmetallocenes of Ge and Sn, as an example. Although the normal or even pentamethyl-Cp metallocenes are air and water sensitive (110-112), the decabenzylmetallocene analogs can be stored in air for days or weeks without apparent decomposition (105,106). 

A shielding effect which is the basis for the stabilization by bulky groups raises the barrier for a associative mechanism, thereby increasing 

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Alkali metal alkyls, particularly n-butyl lithium, are the most frequently used reagents to form metallated intermediates.246 247 In certain cases (di- and triphenyl-methane, acetylene and 1-alkynes, cyclopentadiene) alkali metals can be directly applied. Grignard reagents are used to form magnesium acetylides and cyclopenta-dienyl complexes.248 Organolithium compounds with a bulky alkoxide, most notably M-BuLi-ferf-BuOK in THF/hexane mixture, known as the Lochmann-Schlosser reagent or LICKOR superbase, are more active and versatile reagents.249-252... 

Diels-Alder catalyst. The Diels-Alder reaction of cyclopentadiene with acrolein (equation I) catalyzed by BF3 etherate gives a mixture of endo- and exoadducts in the ratio 9 1. The endo-selectivity is improved by use of trimethylalu-minum. Introduction of a more bulky group on aluminum by use of DAD results... 

Planar chiral compounds should also be accessible from the chiral pool. An example (with limited stereoselectivity) of such an approach is the formation of a ferrocene derivative from a -pinene-derived cyclopentadiene (see Sect. 4.3.1.3 [81]). A Cj-symmetric binuclear compound (although not strictly from the chiral pool, but obtained by resolution) has also been mentioned [86]. Another possibility should be to use the central chiral tertiary amines derived from menthone or pinene (see Sect. 4.3.1.3 [75, 76]) as starting materials for the lithiation reaction. In these compounds, the methyl group at the chiral carbon of iV,iV-dimethyl-l-ferrocenyl-ethylamine is replaced by bulky terpene moieties, e.g., the menthane system (Fig. 4-2 le). It was expected that the increase in steric bulk would also increase the enantioselectivity over the 96 4 ratio, as indicated by the results with the isopropyl substituent [118]. However, the opposite was observed almost all selectivity was lost, and lithiation also occurred in the position 3 and in the other ring [134]. Obviously, there exists a limit in bulkiness, where blocking of the 2-position prevents the chelate stabilization of the lithium by the lone pair of the nitrogen. 

Most of the time, the addition is predominantly endo that is, the more bulky side of the alkene is under the ring, and this is probably true for open-chain dienes also. However, exceptions are known, and in many cases mixtures of exo and endo addition products are found. ° ° An imidazolidone catalyst was used to give a 1 1.3 mixture favoring the exo isomer in a reaction of conjugated aldehydes and cyclopentadiene. It has been argued that facial selectivity is not due to torsional angle decompression. Secondary orbital interactions. ° have been invoked, but this approach has been called into question. ° " There has been a direct evaluation of such interactions, however. The endo/exo ratio can be influenced by the nature of the solvent. ... 

Finally, acyclic primary alcohols have proved to be less efficient than cyclic secondary alcohols28, even with bulky anchoring substituents. Ethyl lactate, when employed as a chiral auxiliary for the acrylate 15, results in an inversion of the sense of induction according to the nature of the Lewis acid used to promote its addition to cyclopentadiene (9)29. 

Thus, in the presence of a chiral Yb triflate prepared from Yb(OTf )3, ( )-(+)-binaphthol, and triethylamine at 0 °C for 0.5 h in dichloromethane, 29 reacted with cyclopentadiene at room temperature to afford the Diels-Alder adduct in an 87% yield (endolexo=76l24) and the enantiomeric excess of the endo adduct was shown to be 33%. After screening several reaction conditions, it was found that the amine employed at the stage of the preparation of the chiral ytterbium triflate strongly influenced the diastereo- and enantioselectivities. In general, bulky amines gave better results and 70%, 75%, and 71% ees were observed when diisopropylethylamine, ris-2,6-dimethylpiperidine, and cis-1,2,6-trimeth-ylpiperidine were used, respectively. In addition, a better result was obtained when the amine was combined with molecular sieves 4 A (cis-l,2,6-trimethyl-piperidine, 91% yield, endo/exo=86H4, endo=90% ee), and the enantiomeric excess was further improved to 95% when the reaction was carried out at 0 °C [72]. 

Various azodicarboxylate esters have been used extensively as dienophiles.129 Several representative examples of this type of cycloaddition are listed in Table 6-III.33-40 As mentioned above, these reactions appear to be concerted processes that retain the 1,3-diene configuration in the adducts.303 The more bulky esters tend to react more slowly with dienes. Thus, dimethylazodicarboxylate reacts 5-6 times more rapidly with cyclopentadiene than does the ethyl ester,423 whereas the fert-butyl compound reacts sluggishly.42b... 

Cyclopentadiene, whose deprotonation in the presence of a base gives the relatively stable cyclopentadienyl anion, has been found to be a suitable substrate for palladium-catalyzed arylation [74, 75]. By using excess aryl bromides, it is completely arylated to produce pentaarylcyclopentadienes (Eq. 29). Metallocenes, typically zirconocene dichloride, are perarylated similarly [74, 76]. Pentaarylcyclopentadienes have been applied as the components of electroluminescent devices [77] and bulky metallocene ligands [78].Di-tcrt-butylphos-phinoferrocene is also pentaarylated efficiently (Eq. 30) [79]. In this case, another mechanism other than Scheme 1 (B) seems to be involved possibly coordination-assisted palladation participates (see Sect. 3.2). 

The reaction between cyclopentadiene and methyl acrylate (14) catalyzed by BH3 has been studied and the results obtained used to develop a force field applicable to the prediction of stereochemistries of Lewis acid catalyzed reactions of chiral acrylates to dienes. This force field has been used in the study of several reactions involving bulky substituents. 

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