Alkene cross metathesis esters

On the other hand, our recent study on the highly efficient cross-metathesis of vinyltrialkoxy-and vinyltrisiloxy-silanes with various olefins, for example, with styrene [12] allyl eth [13] and esters [14] as well as octavinylsilsesquioxane [IS] with several olefins have opened a new opportunity for the use of alkene-cross-metathesis in the synthesis of unsaturated organosilicon compounds (see also Refs. [5] and [6]). In this p r new examples of the two reactions involving hetero(N,S,B)organic olefins have been overviewed. 

Unsaturated esters and silanes are not the only functionalised alkenes to have been employed as cross-metathesis substrates unsaturated alkyl chlorides [9], silylethers [10] and nitriles have all participated in metathesis reactions utilising... 

Although the application of tungsten catalyst 5 to the cross-metathesis reaction of other alkenes has not been reported, Basset has demonstrated that to-un-saturated esters [18] and glycosides [21], as well as allyl phosphines [22], are tolerated as self-metathesis substrates. 

The reaction tolerated a variety of functionality, including ester and ether groups on the alkyl-substituted alkene at least two carbons away from the double bond, and raefa-nitro or para-methoxy substituents on the styrene. As expected, cross-metathesis occurred selectively at the less hindered monosubsti-tuted double bond of dienes also containing a disubstituted alkene (Eq. 8). 

The two alkenes were so similar electronically and sterically, with the ester group too far away to have any affect on the double bond, that there was very little cross-/self-metathesis selectivity. An approximately statistical mixture of ester 13 and diester 14 was isolated. The high yield of the cross-metathesis product 13 obtained is due to the excess of the volatile hex-l-ene used, rather than a good cross-/self-metathesis selectivity. Although not as predominant as in the reactions involving styrene, trans alkenes were still the major products. 

The success of the cross-metathesis reactions involving styrene and acrylonitrile led to an investigation into the reactivity of other Ji-substituted terminal alkenes [27]. Vinylboranes, enones, dienes, enynes and a,p-unsaturated esters were tested, but all of these substrates failed to undergo the desired cross-metathesis reaction using the molybdenum catalyst. 

The first published report on the use of this catalyst for the cross-metathesis of functionalised acyclic alkenes was by Blechert and co-workers towards the end of 1996 [37]. This report was also noteworthy for its use of polymer-bound alkenes in the cross-metathesis reaction. Tritylpolystyrene-bound AT-Boc N-al-lylglycinol 18 was successfully cross-metathesised with both unfunctionalised alkenes and unsaturated esters (Eq. 17) (Table 1). 

It is noticeable that cross-metathesis with the unfunctionalised alkenes occurred in significantly higher yields over shorter reaction times and required a smaller excess of the soluble alkene. This was possibly due to the unfunctionalised alkenes, which are more nucleophilic than their ester containing counterparts, complementing the less nucleophilic/more carbon-metal bond stabilising allylglycinol 18. Comparable results were obtained from cross-metathesis reactions of the polymer-bound isomeric N-Boc C-allylglycinol with the same four alkenes. 

Ruthenium-catalyzed olefin cross-metathesis (ring-closing metathesis, RGM) between terminal alkenes and vinyl-boronic acid or esters has recently been developed for the synthesis of ( )-l-alkenylboron compounds from alkenes.459,460 The efficiency of protocol was proved in the synthesis of a key intermediate of epothilone 490 292 461 (Equation (84)). The vinyl boronate was given almost exclusively the trans-adduct. 

Cross-metathesis of conjugated electron-deficient alkenes such as a,ft-unsatur-ated esters, ketones, aldehydes, and amides often give high cross-product/dimer ratios due to the slow rate of dimerization of these substrates (Eq. 171). When this occurs, the cross-product is dominant even when the reactions are performed with a 1 1 stoichiometry of the reactants.330 When one of the alkene partners homodimerizes slowly, such as happens with electron-deficient and sterically hindered alkenes, the reaction is driven to the cross-product. With respect to the stereochemistry of the reaction, the E-isomer is obtained with electron-deficient alkenes (Eq. 171), and the E/Z ratio may also vary depending on the types of substituents present on the reactants. 

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. 

These developments were extended to multistep reactions on the solid support, for example, via immobilization of the alkene or the alkyne component as an ester. The immobilized alkyne 34 created the possibility for a cross metathesis with a soluble olefin however, as previously noted, undesired CM products generated from soluble alkenes can easily be removed by filtration and washing. After ene-yne cross metathesis to 35, the resin... 

Apart from halogen substituted alkenes, heteroatoms normally deactivate catalytic systems. However the synthetic utility of such reactions has encouraged further research in this field. Chlorine substitution at vinylic positions deactivates the double bond but halogen substituted alkenes in which the double bond is in an position undergo cross metathesis with internal alkenes. For example, 5-bromo-l-pentene undergoes cross metathesis with 2-pentene. Unsaturated compounds containing ester groups also react, e.g. methyl-9-octadecenoate is converted to 9-octadiene and dimethyl-9-octadecenedioate by the WCl —Sn(CH3)4 catalytic combination [15]. 

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