Cyclopentadienes, 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). ... 

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