Cyclopentadiene, addition

Figure 10.11 Schematic representation of the VB-structure of a l,4,ll,14,15,30-hexahydro[60]fullerene derivative involving a cyclopentadiene substructure (cyclopentadiene addition pattern).

An irreversible tt-ct rearrangement has been observed to occur with the treatment of titanocene dicloride (8) with dimethyl sulfoxide (17). The o complex (9) has been isolated, however, treatment of the reaction mixture with benzene produces the titanium complex of dimethyl sulfoxide (10), and treatment of the reaction mixture with maleic anhydride produces the cyclopentadiene addition product (11). Monitoring the reaction mixture by proton NMR spectroscopy demonstrates the ir-a change in the bonding of the cyclopentadienyl ligands. 

At the other extreme are cycloadditions (Chapter 35) such as the Diels-Alder reaction we examined a few pages back. Not only do two reagents become one product but a veiy precise orientation is required in the transition state usually meaning a large negative AS. Diels-Alder reactions usually have AS of about -120 to -160 J mol-1 K-1. The classic cyclopentadiene addition to maleic anhydride has AS = -144 J moT K-1. 

The enantiomerically pure (—)-2-norbornenone s, a key intermediate for (+)-bre-feldin A19 is prepared via a cyclopentadiene addition to the acrylate Id [a 97 3 mixture of (S)-endo- and (R)-exo-adducts was obtained19] or to Is. Replacement of the ether oxygen in the neopentyl side chain by a methylene group (e.g.. It, lu) decreases the diastereoselectivity20,... 

Derivatives of (S)- or ( )-l-phenylethylamine have been chosen as enantionierically pure auxiliaries for chiral acyclic bidentates 17. In the titanium(IV) chloride mediated cyclopentadiene addition to this species of acrylamides the enrfo-adducts are obtained predominantly (endojexo 97 3) but with poor diastereoselectivity [d.r. (5S,5 S)/(5R,5 S) 78 22]74. 

The Lewis acid mediated cyclopentadiene addition to cycloalkenone 12. derived from a chiral sulfoxide, has been reported38. Virtually complete diastereoselection and formation of a separable endoiexo mixture in good yield was observed. In contrast, the quinone89 15 undergoes exclusive endo addition to the less substituted double bond with lower diastereoselection due to a greater distance between the reaction site and the stereogenic center. 

Cyclopentadiene addition to an acrylate gives two diastereomers endo and exo), each of which has two enantiomers. In the presence of a chiral auxiliary, these four stereoisomers are all... 

The reactivity difference along endo and exo reaction paths has been shown by Berson et to depend clearly, although slightly, on the solvent polarity in the case of cyclopentadiene addition to some acrylic esters on increasing the polarity of the solvent, as defined by the Z parameter " , the logarithm of the endolexo ratio in the adduct increases in a roughly linear manner (Fig. 1). 

An other correlation has been found between log endolexo) in the cyclopentadiene addition to ring-substituted rrun.v-cinnamic acids and Hammett-type substituent constants the low absolute value of the slope (—0.30)"" indicates very similar substituent effects on the two reaction paths. For the reaction of 2-substituted anthracenes with maleic anhydride" the influence of substituents on log (synlanti) is even weaker"". 

A convenient way of storing and purifying the strained olefin bicyclo[2,2,0]hex-l(4)-ene is by formation of a platinum complex (776), whose structure has been confirmed by 2f-ray analysis. The olefin is readily liberated from (776) by carbon disulphide and undergoes the expected Diels-Alder reaction with cyclopentadiene. Addition of ethanol to (776) is accompanied by opening of the strained internal bond to give the metallocycle (777). ... 

Aldrin is obtained from the Diels-Alder addition product of cyclopentadiene and vinyl chloride by dehydrochlorination followed by condensation with hexachlorocyclopenta-diene. 

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

On the basis of the studies described in the preceding chapters, we anticipated that chelation is a requirement for efficient Lewis-acid catalysis. This notion was confirmed by an investigation of the coordination behaviour of dienophiles 4.11 and 4.12 (Scheme 4.4). In contrast to 4.10, these compounds failed to reveal a significant shift in the UV absorption band maxima in the presence of concentrations up to one molar of copper(ir)nitrate in water. Also the rate of the reaction of these dienophiles with cyclopentadiene was not significantly increased upon addition of copper(II)nitrate or y tterbium(III)triflate. 

Fortunately, in the presence of excess copper(II)nitrate, the elimination reaction is an order of magnitude slower than the desired Diels-Alder reaction with cyclopentadiene, so that upon addition of an excess of cyclopentadiene and copper(II)nitrate, 4.51 is converted smoothly into copper complex 4.53. Removal of the copper ions by treatment with an aqueous EDTA solution afforded in 71% yield crude Diels-Alder adduct 4.54. Catalysis of the Diels-Alder reaction by nickel(II)nitrate is also... 

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

Cyclodienes. These are polychlorinated cycHc hydrocarbons with endomethylene-bridged stmctures, prepared by the Diels-Alder diene reaction. The development of these insecticides resulted from the discovery in 1945 of chlordane, the chlorinated adduct of hexachlorocyclopentadiene and cyclopentadiene (qv). The addition of two Cl atoms across the double bond of the ftve-membered ring forms the two isomers of chlordane [12789-03-6] or l,2,4,5,6,7,8,8-octachloro-2,3,3t ,4,7,7t -hexahydro-4,7-methano-lJT-indene, QL-trans (mp 106.5°C) and pt-tis (32) (mp 104.5°C). The p-isomerhas signiftcantiy greater insecticidal activity. Technical chlordane is an amber Hquid (bp 175°C/267 Pa, vp 1.3 mPa at 25°C) which is soluble in water to about 9 fig/L. It has rat LD qS of 335, 430 (oral) and 840, 690 (dermal) mg/kg. Technical chlordane contains about 60% of the isomers and 10—20% of heptachlor. It has been used extensively as a soil insecticide for termite control and as a household insecticide. 

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