Alkene Metathesis with ester

Ethers, esters, amides and imidazolidines containing an epithio group are said to be effective in enhancing the antiwear and extreme pressure peiformance of lubricants. Other uses of thiiranes are as follows fuel gas odorant (2-methylthiirane), improvement of antistatic and wetting properties of fibers and films [poly(ethyleneglycol) ethers of 2-hydroxymethyl thiirane], inhibition of alkene metathesis (2-methylthiirane), stabilizers for poly(thiirane) (halogen adducts of thiiranes), enhancement of respiration of tobacco leaves (thiirane), tobacco additives to reduce nicotine and to reduce phenol levels in smoke [2-(methoxymethyl)thiirane], stabilizers for trichloroethylene and 1,1,1-trichloroethane (2-methylthiirane, 2-hydroxymethylthiirane) and stabilizers for organic compounds (0,0-dialkyldithiophosphate esters of 2-mercaptomethylthiirane). The product of the reaction of aniline with thiirane is reported to be useful in the flotation of zinc sulfide. 

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. 

The yields from aldehyde alkylidenation is somewhat lower due to the reductive dimerization of aldehydes with low-valent Ti. Alkylidenation of esters is possible by the reaction of 1,1 -dibromoalkane. TiCU and Zn in the presence of TMEDA to give (Z) vinyl ethers [60], Cyclic vinyl ethers are prepared from unsaturated esters in two steps. The first step is formation of the acyclic enol ethers using a stoichiometric amount of the Ti reagent, and the second step is ring-closing alkene metathesis catalysed by Mo complex 19. Thus the benzofiiran moiety of sophora compound I (199, R = H) was synthesized by the carbonyl alkenation of ester in 197 with the Ti reagent prepared in situ, and the subsequent catalytic RCM of the resulting enol ether 198 catalysed by 19 [61]. 

The Tebbe reagent reacts with some alkenes. The tricyclo[5.3.0.0] ring 207 was obtained nearly quantitatively by domino alkene metathesis and carbonyl alkenation of the norbomene-type ester 205 with the Tebbe reagent. This interesting reaction to give the intermediate 206 can be explained by the kinetic preference of the Tebbe reagent for the strained double bond over the ester. Alkenation of the ester in 206 produces 207. Capnellene (208) has been synthesized by applying this reaction as a key reaction [65],... 

The photochemical reactions of arenecarboxylic acid esters with alkenes has received recent attention by Cantrell. - For example, irradiation of 2,3-dimethyl-2-butene and methyl benzoate gave a mixture of alkoxyoxetane (56), carbonyl-alkene metathesis product (57) and ketone (58), resulting from alkoxy radical allylic hydrogen abstraction and radical recombination. Such alkoxyoxetane photoproducts are... 

The metathesis of unsaturated fatty-acid esters has received considerable attention. Aspects of the metathesis of fatty-acid esters and related compounds were recently reviewed by Boelhouwer.With the WCl6/Me4Sn system, both self-metathesis and co-metathesis (with symmetric alkenes) of unsaturated esters occur, provided that the double bond and the ester group are separated by at least one methylene group.As an example, conversion of C8-C=C-C7-COOC of 50% for self-metathesis and 70% for co-metathesis... 

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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