Evidence from cross-metathesis reactions

The cross-metathesis of cyclopentene with imsymmetrical olefins is catalyzed by WOCl4/Bu4Sn or WOCl4/Et2AlCl. Herisson (1971) observed that the products of reaction with pent-2-ene consisted of three series of compounds Q M Q , Q M Q, and Q M Q (n = 1-4), where Q = ethylidene, = propylidene, and M represents n ring-opened units of cyclopentene M. These series were formed in the statistical ratio 1 2 1 even in the initial products. Similar results were obtained with cyclooctene, cycloocta-1,5-diene, and cyclododeca-l,5,9-triene in place of cyclopentene. It was these observations that led to the proposal of the metal carbene mechanism, since direct exchange between the double bonds of the reactant molecules would yield only the unsymmetrical series. The formation of the three series of compounds is accounted for in terms of reactions (l)-(6). 

A more stringent test is to react cyclooctene with a mixture of but-2-ene and oct-4-ene. If the metal carbene mechanism is correct, one may expect to find the C14 product of double cross-metathesis, i.e. MeCH=CH(CH2)6CH==CHPr, and an initial value of 4.0 for the product of the two ratios (C14/C12) and (C14/C16). The observed ratio is 4.05 0.05 for cis reactants and 4.11 0.09 for trans reactants (Katz 1977a). 

Cross-metathesis reactions between undeuterated and deuterated dienes, such as octa-1,7-diene (Grubbs 1976) and 2,2 -divinylbiphenyl (Katz 1976b) have also been studied as a means of testing the metal carbene mechanism. The initial proportions of ethene-tfo, -d2, and -d4 formed in these reactions support this mechanism. Likewise for the cross-metathesis between c/5,cw-deca-2,8-dienes 

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The Metal Carbene/Metallacyclobutane Mechanism 3.2 Evidence from cross-metathesis reactions... 

The chemoselectivity provided by the solid support in the cross-metathesis reaction is evident in ene-ene metathesis reactions, as both olefins are capable of a homodimerization reaction. The Mata laboratory has devoted significant effort toward the understanding and development of cross-metathesis reactions on a solid support. To probe the limits of the reaction, aliphatic, aryl, and acryloyl alkenes 8,11, and 12 were immobilized and reacted with a variety of soluble olefins 13-16 from different classifications based on homodimerization potential (Scheme 6.2). For example, the immobilized acrylate 8 was reacted with several soluble olefins to form the immobilized coupling product 9. Subsequent cleavage with TFA and esterification with diazomethane delivered the esters 10. Other immobilized alkenes explored in the same process were 11 and 12. Among other olefins, they were reacted with the soluble alkenes 13-16. Modest to good yields (shown in parentheses) were obtained, but, most importantly, no dimerization of the immobilized alkenes 8,11, and 12 was observed, and undesired homocoupling products of the alkenes 13-16 in solution were easily washed away in the workup step. 

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