Cyclopentadiene derivatives

The substituted-cyclopentadiene derivatives analogs of [Cp2Zr(H)Cl]n (1), (MeCp)2Zr(H)Cl (2) [38], Cp CpZr(H)Cl (3) [39], and [(Me2Si)(C5H5)2Zr(H)Clj [40] are more soluble in non-polar organic solvents, and consequently they are more reactive. Thus, the direct competition between 1 and 2 for the hydrozirconation reaction of 1-hexene affords predominantly (86%) the product derived from the methylated derivative (Scheme 8-3) [41]. 

Pivalonitrile was also shown to be able to undergo a similar [2 + 2 + 2]-cyclotrimerization with a tantalla-cyclopentadiene derived from metallocyclization of 2equiv. of HC CBur. The resulting tantalum -pyridine... 

Sulfonamide derivatives of a-amino acids and the similar bissulfonamide derivatives of diamines can be used to prepare reactive Lewis acid complexes. Corey20 reported the Lewis acid (R,R)- or (5,5 )-complex 69, which can be employed at 10 mol% level to catalyze the Diels-Alder reaction of cyclopentadiene and imide. Reactions catalyzed by this complex give an endo.exo ratio of over 50 1, as well as a high ee (91%) at —78°C, and this can be further improved to 95% by carrying out the reaction at 90°C.20 The related aluminum complex 69b shows very similar reactivity at —78°C, with generally higher ee values, typically over 95%, for the reaction of cyclopentadiene derivatives with imide.20,21... 

As yet, there are not many examples of this. As a representative example, the reaction of zirconacyclopentadienes with aldehydes in the presence of A1C13 affords cyclopentadiene derivatives. In this case, the aluminum abstracts the oxygen atom from the aldehyde moiety (Eq. 2.31) [11]. Aluminacyclopentadiene might be the intermediate in this reaction. 

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). 

This also explains why in a thermal reaction a cyclopentadiene derivative shows only a singlet for the methyl group in PMR spectrum. 

Scheme 6.7 Generation and interception of the 1,2-cyclopentadiene derivative 25 by Balci and co-workers.

The treatment of 23 with methyllithium in the presence of furan gave rise to the tetracyclic product 26, which is obviously a [4 + 2]-cycloadduct of furan to the 1,2-cyclopentadiene derivative 25 [27]. The feature that the oxanorbornene system of 26 carries its saturated substituent in the endo-position is analogous to the [4 + 2]-cycloadducts of furan to all six-membered cyclic allenes (see Section 6.3). Balci et al. [36] also provided evidence for the generation of l-phenyl-l,2-cyclopentadiene. They postulated this species to be an intermediate in the reaction of l-phenyl-2-iodocydo-pentene with potassium tert-butoxide in benzene at 240 °C, which resulted in the formation of 1-phenyl- and 1,2-diphenylcyclopentene. Both products were considered as evidence in favor of the diradical nature rather than the allene structure of 1-phe-nyl-1,2 -cyclopentadiene. 

An analogous annulation reaction of an acetylenic ketone provides an access to a cyclopentadiene derivative. 

Yoon and Chae28 have described the DCA-induced photochemical conversion of the cyclopentadiene derivatives 63 into several products. However, only the awri-Bredt adduct 64 is different from those obtained by thermal activation. The experimental data collected have implicated a triplex intermediate 65 in the formation of 64. This triplex is the result of interaction between the diene, the non-conjugated alkene component and the sensitizer. While a mixture of cyclopentadienes was used, it is likely that the products 64 are formed exclusively from the 2-isomer 66. 

Some representative examples of the [3+2] annulatlon are listed in Table 1. Both cyclic and acyclic allenophiles participate in the reaction, a-Alkylidene ketones undergo annulation to provide access to spiro-fused systems, and acetylenic allenophiles react to form cyclopentadiene derivatives. The reactions of (E)- and (Z)-3-methy1-3-penten-2-one illustrate the stereochemical course of the annulation, which proceeds with a strong preference for the suprafacial addition of the allene to the two-carbon allenophile. The high stereoselectivity displayed by the reaction permits the stereocontrol led synthesis of a variety of mono- and polycyclic systems. 

CM of propynyl benzoate and 1,5-hexadiene is carried out to produce a cyclopentadiene derivative and a triene in 99% yield in a ratio of 1 to 1.2. The former compound is apparently derived from a (Z)-isomer of the triene by further metathesis (Equation (14)). ... 

If six electrons in a ring constitutes aromaticity, then furan is aromatic. But such aromaticity is only formal and has no meaning unless it can be shown that furan has properties modified beyond what can be expected from a molecule that can be variously regarded as a divinyl ether and a cyclopentadiene derivative as circumstances demand. No such demonstration exists, with the possible exception of the NMR ring current shielding... 

The compounds (Bu 0)3 W=CR are alkyne metathesis catalysts.280 281 Probable intermediates in these reactions are tungstacyclobutadienes and the latter are seen in the presence of Cl ligands.282 Addition of alkynes to the latter leads to the formation of substituted r/ -cyclopentadiene derivatives... 

The preparation of highly functionalized cyclopentadiene derivatives may by achieved by using oxalenes or their pyrylium salts (90 and 92, respectively), (see Scheme 14, where X = O). In this way, azalenes 26,51 29,8Z 88 32,51 or 3651 can be obtained by heating salts (90) with primary amines in dimethyl-formamide (Eq. 6). The reaction with sodium hydrogen sulfide gives thialenes... 

Desilylation or destannylation of the corresponding cyclopentadiene derivatives can be easily performed in addition to deprotonation, as shown schematically in Scheme 6. The possibility for desilylation indicates the pronounced reactivity of the silicon-carbon bond, which is described in more detail in Section III. Nucleophiles as well as... 

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... 

Asymmetric Diels-Alder reactions. The acrylate of (+ )-l is markedly superior to ( —)-menthyl acrylate in effecting asymmetric induction in Diels-Alder reaction with cyclopentadiene (Lewis acid catalysis, equation I). A chiral intermediate in Corey s prostaglandin synthesis was obtained by the reaction of (-1- )-8-phenylmenthyl acrylate with a cyclopentadiene derivative.1... 

There is, however, about a tenfold decrease in the rate constant when the ring size is increased by one methylene unit [29], and an ideal C1-C4 distance is responsible for the impressive reactivity of cyclopentadiene in comparison to larger cyclic homologues [16a]. Electron-donating substituents enhance the reactivity, and for 1,3-cyclopentadiene derivatives... 

The facile decarbonylation of pentacarbonyl complexes 23a and 23b results in tetra-carbonyl carbene complex intermediates 24a and 24b, respectively. Their annulation can produce benzene and/or cyclopentadiene derivatives 25 and/or 26. In the case of aryl acyla-mino complex 23a, the reaction course is shifted towards the benzannulation reaction (25a 26a = 84 16). With the vinyl carbene complex 23b, the benzannulation product 25b (25b 26b = 100 0) is produced exclusively. 

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. 

Roesky introduced bis(iminophosphorano)methanides to rare earth chemistry with a comprehensive study of trivalent rare earth bis(imino-phosphorano)methanide dichlorides by the synthesis of samarium (51), dysprosium (52), erbium (53), ytterbium (54), lutetium (55), and yttrium (56) derivatives.37 Complexes 51-56 were prepared from the corresponding anhydrous rare earth trichlorides and 7 in THF and 51 and 56 were further derivatised with two equivalents of potassium diphenylamide to produce 57 and 58, respectively.37 Additionally, treatment of 51, 53, and 56 with two equivalents of sodium cyclopentadienyl resulted in the formation of the bis(cyclopentadienly) derivatives 59-61.38 In 51-61 a metal-methanide bond was observed in the solid state, and for 56 this was shown to persist in solution by 13C NMR spectroscopy (8Ch 17.6 ppm, JYc = 3.6 2/py = 89.1 Hz). However, for 61 the NMR data suggested the yttrium-carbon bond was lost in solution. DFT calculations supported the presence of an yttrium-methanide contact in 56 with a calculated shared electron number (SEN) of 0.40 for the yttrium-carbon bond in a monomeric gas phase model of 56 for comparison, the yttrium-nitrogen bond SEN was calculated to be 0.41. 

Scheme 1.21. Diels-Alder addition of an enatiomerically pure cyclopentadiene derived from (.S, .S )-thrcitol to Cgo- Separation of the diastereoisomeric adducts 225 and 226, followed by cleavage of the acetal protecting group, afforded enantiomerically pure ketones which were configurationally assigned as (+)-(R,R)-227 and (—)-(S,S)-227.

Other synthetic routes to alkene-metal complexes reported in the literature involve bridge cleavage reactions of halogeno complexes (method F), facile intermolecular scrambling reactions involving metal-alkene and metal-PF3 complexes (18, 81) (method G) (Scheme 4), and the coupling together of two alkyne units to afford a metallo-cyclopentadiene derivative (method H). 

Amine elimination process can be used starting from cyclopentadiene derivatives to produce in good yield ansa metallocenes. 

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