Self-cross metathesis

Self Cross-Metathesis of Labeled and Unlabeled 2-Butene... 

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

The cross metathesis of acrylic amides [71] and the self metathesis of two-electron-deficient alkenes [72] is possible using the precatalyst 56d. The performance of the three second-generation catalysts 56c,d (Table 3) and 71a (Scheme 16) in a domino RCM/CM of enynes and acrylates was recently compared by Grimaud et al. [73]. Enyne metathesis of 81 in the presence of methyl acrylate gives the desired product 82 only with phosphine-free 71a as a pre-... 

The reversible nature of cross metathesis is of synthetic importance because, by the use of a sufficiently active metathesis catalyst, it generally ensures the preferential formation of the most thermodynamically stable product. This results in the transformation of terminal olefins into internal ones, and we have seen that undesired self-metathesis products can be recycled by exposing them to a second CM process. 

This review focuses on the cross-metathesis reactions of functionalised alkenes catalysed by well-defined metal carbene complexes. The cross- and self-metath-esis reactions of unfunctionalised alkenes are of limited use to the synthetic organic chemist and therefore outside the scope of this review. Similarly, ill-defined multicomponent catalyst systems, which generally have very limited functional group tolerance, will only be included as a brief introduction to the subject area. 

Particularly noteworthy was the rhenium catalysed cross-metathesis of trans-hex-3-ene with vinyl acetate or a,p-unsaturated esters [4]. For example, crossmetathesis of methyl frans-crotonate with frans-hex-3-ene gave the desired cross-coupled product without any self-metathesis of the crotonate (Eq. 2). 

Using an equimolar quantity of allyl methyl sulphide and ds-pent-2-ene resulted in incomplete reaction of the allyl sulphide and some self-metathesis of the sulphide substrate. When an excess (4 equiv) of but-2-ene was used, however, the desired but-2-enyl sulphide was formed in a good yield at ambient temperature. In this case, the large quantities of unwanted hydrocarbon starting material and self-metathesis products were gaseous alkenes and therefore easily removed. Using a large excess of one alkene to improve the yield of the desired cross-metathesis product in this way is obviously only viable if this alkene is inexpensive and both it and its self-metathesis product are easily removed. 

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 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. 

Like styrene, acrylonitrile is a non-nucleophilic alkene which can stabilise the electron-rich molybdenum-carbon bond and therefore the cross-/self-metathe-sis selectivity was similarly dependent on the nucleophilicity of the second alkene [metallacycle 10 versus 12, see Scheme 2 (replace Ar with CN)]. A notable difference between the styrene and acrylonitrile cross-metathesis reactions is the reversal in stereochemistry observed, with the cis isomer dominating (3 1— 9 1) in the nitrile products. In general, the greater the steric bulk of the alkyl-substituted alkene, the higher the trans cis ratio in the product (Eq. 11). 

As expected, there was no formation of stilbenes or a dinitrile product and, more surprisingly, in all of the reactions reported only 5-7% of the allyltrimeth-ylsilane self-metathesis product was observed. It was proposed that this lack of allylsilane self-metathesis was due to the steric bulk of the TMS group reducing the reactivity of the Me3SiCH2 substituted alkylidene. In a more recent report by Blechert and co-workers it was noted that allyltrimethylsilane and its hydrocarbon equivalent (4,4-dimethylpent-l-ene) had comparable reactivities in the cross-metathesis reaction [28], further suggesting that the selectivity arises from steric rather than electronic effects. 

As with the allylsilane cross-metathesis reactions, significant quantities of allyl stannane self-metathesis were not detected in any of the reactions and the trans isomer predominated in the cross-metathesis products. Identical reactions were carried out using allyltributyl stannane, in place of allyltriphenyl stannane, but the yields of the cross-metathesis products were consistently lower and in many cases dropped below 25%. 

The examples listed in Table 3.21 illustrate the synthetic possibilities of cross metathesis. In many of the procedures reported, advantage is taken of the fact that some alkenes (e.g. acrylonitrile, styrenes) undergo slow self metathesis only. Interestingly, it is also possible to realize cross metathesis between alkenes and alkynes (Table 3.21, Entries 11-13), both in solution and on solid supports [927,928]. 

McNaughton, B. R. Bucholtz, K. M. Camaano-Moure, A. Miller, B. L. Self-selection in olefin cross metathesis The effect of remote functionality. Org. Lett. 2005, 7, 733-736. 

Cross-metathesis, however, is usually a nonselective reaction. Transformation of two terminal alkenes in the presence of a metathesis catalyst, for instance, can give six possible products (three pairs of cis/trans isomers) since self-metathesis of each alkene and cross-metathesis occur in parallel. It has been observed, however, that terminal olefins when cross-metathesized with styrene yield trans-P-alkylstyrenes with high selectivity.5 A useful synthetic application of cross-metathesis is the cleavage of internal alkenes with ethylene called ethenolysis to yield terminal olefins ... 

Cross-metathesis enables the efficient preparation of acyclic alkenes and 1,3-dienes on insoluble supports (Figure 5.16). Unfortunately, some types of substrate show a high tendency to yield products of self-metathesis, i.e. symmetrical internal alkenes produced by dimerization of the resin-bound alkene. This is the case, for instance, with allylglycine and homoallylglycine derivatives. Dimerization of the resin-bound alkene can, however, be effectively suppressed by reducing the loading of the support [127]. 

Dimethylbut-l-ene (neohexene), which is inactive to self-metathesis, undergoes cross-metathesis with internal alkenes to high conversion when catalysed by WCl6/Et20/Bu4Sn153. 

Cross-metathesis of equimolar amounts of styrene with symmetrical alkenes occurs on Re207/Al2C>3 at 50 °C. 85-90% of the styrene is converted to 1-phenylalk-l-enes and only 10% to the self-metathesis product stilbene171. Likewise, the reaction of styrene with 0.5 equiv oct-l-ene catalysed by 8 gives >85% of the cross-metathesis product (>95% trans) and <4% of the self-metathesis product of oct-l-ene172. 

Where there is no spacer group between the C=C bond and the functional group, productive self-metathesis does not occur, but cross-metathesis reactions with other olefins are still possible. Recent impressive examples of this are the cross-metathesis reactions of acrylonitrile (equation 19). The reaction occurs with a wide variety of R groups. For 15 different compounds the yield of the new nitrile after 3 h at room temperature is 40-90%, with the cis isomer always strongly preferred (75-90%). Only minor amounts of RCH2CH=CHCH2R are formed, and no NCCH=CHCN182. The fact that acrylonitrile... 

The results of some cross-metathesis experiments for a series of nitriles CH2=CH(CH2) CN reacting with c -hept-3-ene are summarized in Table 4. No crossmetathesis occurs with acrylonitrile (n = 0). For n = 1, 2, 5, 8, 9 cross-metathesis products are formed in substantial amount, but for n = 3, 4 very little reaction occurs, an effect which is attributed to intramolecular coordination of the nitrile group to the metal centre in [Mt]=CH(CH2) CN (n = 3, 4), thereby reducing its metathesis activity or causing its destruction. With n > 5 the nitrile group has little influence on the reaction and its self-metathesis is preferred over that of hept-3-ene, whereas the reverse is true for n = 1,2. 

Metathesis of alkenes has been reviewed in terms of cross-metathesis, ring opening and closing, disproportionation, transmutation, and self-metathesis.34 A review on catalytic processes involving ft -carbon elimination has summarized recent progress on palladium-catalysed C-C bond cleavage in various cyclic and acyclic systems.35... 

Fig. 1 Olefin metathesis reactions (a) ring-opening metathesis (ROM) and ring-closing metathesis (RCM), (b) self-metathesis (SM), (c) cross metathesis (CM), (d) ring-opening metathesis polymerization (ROMP), and (e) acyclic diene metathesis (ADMET) polymerization...

Self-Metathesis and Cross-Metathesis of Unsaturated Organosilicon... 

Metathesis is a versatile reaction applicable to almost any olefinic substrate internal, terminal or cyclic alkenes, as well as dienes or polyenes. (Alkyne metathesis is a growing area, but will not be dealt with here.) The reaction is also known as olefin disproportionation or olefin transmutation, and involves the exchange of fragments between two double bonds. Cross metathesis (CM, Figure 1) is defined as the reaction of two discrete alkene molecules to form two new alkenes. Where the two starting alkene molecules are the same it is called self-metathesis. Ethenolysis is a specific type of cross metathesis where ethylene... 

Choi and co-workers [579] studied the reactivity of vinyl-terminated self-assembled monolayers (SAMs) of undec-lO-ene-1-thiol on gold (see Fig. 6.32) toward olefin cross-metathesis (CM). Vinyl groups on SAMs were successfully converted into a, P-unsaturated carbonyl groups by CM with acrylic acid, methyl acrylate, and acrylamide. Result shows that various useful functional groups can be introduced to SAMs on gold (and other solid surfaces) by olefin CM and suggests an alternative to the S3mthesis of desired molecules in solution [579]. 

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