Cyclopentadiene derivatives acidity

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... 

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... 

Several authors have employed Burton and Pepper s scheme to calculate propagation rate constants for systems displaying the rather frequent behaviour of fast initiation-limited yields. Ykeda et al. and Yamamoto et al. have applied this principle to the polymerisation of styrene derivatives by sulphuric acid, and Kohjiyaet al. to the system cyclopentadiene-perchloric acid We wish to caution about the uncritical usage of this procedure, since, as has been shown above, it can lead to gross underestimates. In fact, it should only be used where there is sufficient mechanistic evidence to justify its application. 

Alternatively, the 5,6-double bond in the initial adduct can be elaborated via Diels-Alder reactions with cyclopentadiene derivatives and this provides an entry to the tetracy-clo[5.2.1.0 0 ]decane series. Hence reaction of the diethyl azodicarboxylate adduct with hexachlorocyclopentadiene gave the adduct 13 which was elaborated to give endo,syn-l,7,8,9,10,10-hexachlorotetracyclo[5.2.1.0 0 ]dec-8-ene (14). Similar syntheses yield derivatives with a variety of substituents at CIO and in this series it was found that sulfuric acid hydrolysis of the carbamates gave a better yield than sodium hydroxide in methanol. The... 

The strong destabilization of sp centers by fluorine is impressively demonstrated by the acidities of different fluorine-containing cyclopentadiene derivatives (Scheme 2.38) - despite the strong, cumulative negative inductive (—4) effect of five fluorine atoms, the acidity of pentafluorocyclopentadiene [90] is similar to that of cyclopentadiene. The reason for this unexpected behavior is that deprotonation would create one more fluorinated sp center. In contrast, the expected behavior is observed for the pentakis (trifluoromethyl) derivative, which is more acidic by 16 orders of magnitude [91]. 

Through a net [2 + 2+1] cycloaddition, iminocyclopentadi-enes can be obtained through the initial formation of a zircona-cyclopentadiene derivative (from two alkynes and a low valent zirconocene component), and subsequent reaction with an aryl isocyanate and BF3-Et20 (eq 65). Quite remarkably, no other Lewis acid investigated provided this cycloaddition only EtAlCL led to a productive reaction, although different adducts were obtained. 

Cyclobutanes from the addition of 1-chloro-l-fluoroethylene to chlorotrifluoro- or tetrafluoro-ethylene, of 2-fluoropropene to tetrafluoroethylene, and of 1-chloro-2,2-difluoroethylene to vinyl fluoride or propene have been claimed to be anaesthetics. Diels-Alder addition of cyclopentadiene to acids of the type, trans-RpCHiCH COaH (Rf = CHaF, CHFa, CFs, C2F5, or n-CsFv) and a number of related esters and other derivatives, at 25 °C, results in predominant formation of adduct witii an endo fluoroalkyl group. The addition of cyclopentadiene to the 1,1-difluoroethylenes, CFalCFa, CFa CF CFs, CFa CCla, CFa C(CF3)2, CFaiCCl-CFaCl, and CFa.CFH has been recorded and the fiee-radical bromina-tion of the resulting norbomenes studied. Buta-1,3-diene and trifluorovinyl-sulphur pentafluoride yield essentially a mbcture of cis- and rmns-cyclobutanes (116), with little or no cyclohexene formation the olefin resembles perfluoropropene which yields ca. 5% of cyclohexene, in this respect. Perfluoroindene, which adds... 

Early-transition-metal amides are extremely reactive toward protic adds and are therefore useful precursors to a variety of M-X complexes (in which X is an anionic ligand less basic than an amide) by reaction of the amido complex with H-X (Equation 4.13). Common exchange processes that favor elimination of amine involve the reaction with alcohols, amides, amidines, and guanidines. However, weakly acidic hydrocarbons, such as cyclopentadiene derivatives, also react to form Cp complexes (Equation 4.14). ... 

The cyclopentadiene derivatives CpCuPBua and Cp2Ni react with a-bromo-phenylacetic acids to give the cyclopentadienyl acids (19). ... 

Other reactions studied include reduction of the triple bond of a-acetylenic esters and nitriles by tributyltin hydride in methanol electroreductive cyclization of acetylenic halides at a mercury cathode the trimerization and tetramerization of cyclo-octyne in the presence of various transition metals the kinetics of bromination of alka-l,3-diynes, of permanganate oxidation of acetylenedicarboxylic acid, and of iodination of propiolic acid the participation of the triple bond in reactions of various acetylenes of the general formula (225) and the trimerization of but-2-yne with tolyl-palladium chloride to give a [Pg.48]

Zirconacyclopentene 1-5 could also be applied in synthetic reaction (Scheme 1.8). For example, in the presence of CuCl and iodine, oxidative demetallation of 1-5 gives cyclobutene derivatives [34]. Reaction of 1-5 with acid chloride gives tri-subslimted cyclopentadiene derivatives [35]. This research group also reported zirconocene-mediated cyclization of alkyne, ethylene, and two molecules of aldehyde toward synthesis of 2-alkenyl tetrahydrofuran [36]. 

Halogenation. Halogens and halogen acids add readily to the unsaturated carbon linkages of the cyclopentadiene molecule. By such additions a series of halogenated derivatives range, in the case of the chloride, from 3-chlotocydopentene to tetrachlorocyclopentane. Of all the possible chloto derivatives of CPD, only hexachlorocyclopentadiene [77-47-4] ever reached commercial status. It was used as an insecticide, but this use has been discontinued because of its toxicity (see Chlorocarbons and chlorohydrocarbons, toxic aromatics). It can be prepared by a Hquid phase chlorination of CPD below 50°C (29). 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

A key transformation in Corey s prostaglandin synthesis is a Diels-Alder reaction between a 5-(alkoxymethyl)-l,3-cyclopenta-diene and a ketene equivalent such as 2-chloroacrylonitrile (16). As we have already witnessed in Scheme 3, it is possible to bring about a smooth [4+2] cycloaddition reaction between 5-substituted cyclopentadiene 15 and 2-chloroacrylonitrile (16) to give racemic 14 as a mixture of epimeric chloronitriles. Under these conditions, the diastereomeric chloronitriles are both produced in racemic form because one enantiotopic face of dienophile 16 will participate in a Diels-Alder reaction with the same facility as the other enantiotopic face. In subsequent work, Corey s group demonstrated that racemic hydroxy acid 11, derived in three steps from racemic 14 (see Scheme 3), could be resolved in a classical fashion with (+)-ephe-... 

---