Cross-metathesis with dienes

Unfortunately, this product was isolated as a mixture with diene 36, formed from cross-metathesis with a second equivalent of the allyl sulphide, and was contaminated with some polymeric residues. It is also important to note that an excess of the sulphide was required to suppress competing ROMP of the norbornene. A similar result was obtained for the reaction of allyl methyl sulphide with cyclop entene. 

Both catalysts 1 and 2 are effective in promoting cross-metathesis leading to various conjugated dienes from alkenes and alkynes. Chiral 2-(a-acetoxybenzyl)-1,3-butadiene is obtained from (/f)-3-acetoxy-3-phenylpropyne via cross-metathesis with ethylene. - Furthermore, the reaction of 1,6-diynes with alkenes is even more intriguing ... 

A chain mechanism for olefin metathesis explains product-time distributions in reactions between cyclo-octene and acyclic olefins. Even at the start of the reaction between cyclo-octene, trans-hut-2-ene, and trans-oct-4-ene in the presence of the catalyst, a significant amount of tetradeca-2,10-diene was found. " Tetradeca-1,9-diene was the principal product of metathesis reactions between cyclo-octene and hex-l-ene in the presence of tungsten catalysts. Ethylene and dec-5-ene formed by self-metathesis of the hex-l-ene also underwent cross-metathesis with the cyclo-octene to give deca-1,9-diene and octadeca-5,13-diene further reactions gave higher members of these homologous series. ... 

Marc L. Snapper of Boston College opened J. Org. Chem. 2008, 73, 3754) the strained cyclobutene 15 with ethylene to give the diene 16. Remarkably, cross metathesis with 17 delivered 18 with high regioselectivity, setting the stage for the preparation of the S-F -Isopiostane 19. 

The startingpoint for the preparation of 1 was the commercial aldehyde 6. Enantioselective allylation followed by silylation delivered 7, which on cross metathesis with methacrolein gave the diene aldehyde 8. Imine formation then completed the construction of 1. 

In the reaction system, the intramolecular [3+2] cycloaddition of mthenium-alkenyl carbene complex 123 is competing with the oligomerization of diene 121a by rathenium-catalyzed cross-metathesis. When diene 121a is treated with complex II or III, which are more reactive olefin metathesis catalysts than complex I, the oligomerization of diene 121a is superior to the intramolecular [3+2] cycloaddition (Scheme 5.24, entries 2 and 3). 

The significant potential of the ruthenium complex 65 was further underlined in the catalytic asymmetric ring-opening/cross metathesis of the cyclic alkene 70 (Scheme 44). This transformation is catalyzed by 5% mol of 65 at room temperature, in air, and with undistilled and nondegassed THF to deliver the corresponding diene 71 in 96% ee and 66% isolated yield. In standard conditions (distilled and degassed THF), the alkene 70 reacts in 75 min to give the diene in 95% ee and 76% yield, with only 0.5 mol % of catalyst. 

The report by Basset and co-workers on the metathesis of sulphur-containing alkenes using a tungsten alkylidene complex, mentioned previously for the acyclic cross-metathesis reaction (see Sect. 2.2), also contained early examples of ring-opening cross-metathesis of functionalised alkenes [20]. Allyl methyl sulphide was reacted with norbornene in the presence of the tungsten catalyst 5, to yield the desired ring-opened diene 35 (Eq. 29). 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

Since the alkene formed in this reaction can further react with other alkenes, many products should be formed in the cross-metathesis (CM). Therefore, in the early days, only ring-closing metathesis (RCM) of diene was investigated. It is known that the reaction is catalyzed by a transition metal. Pioneering work on olefin metathesis was undertaken by Villemin and Tsuji, who reported the synthesis of lactones using alkene metathesis ... 

Cross-metathesis of terminal alkyne 142 and cyclopentene gives cyclic compound 143 having a diene moiety [Eq. (6.114)]. ° Terminal ruthenium carbene generated from an alkyne and methylidene ruthenium carbene complex reacts with cyclopentene to afford two-carbon elongated cycloheptadiene 143 ... 

Acyclic dienes are the products in cross-metathesis of cycloalkenes and acyclic alkenes. With ethylene, a,co-dienes are formed ... 

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]. 

Cross-metathesis of two different alkenes 11 and 42 usually produces a mixture of products 6 and 15. However, depending on the functional groups R1 and R2, the cross-product 6 is obtained with high selectivity rather than the homoproduct 15 from 11 and 42. Some terminal alkenes, such as allylstannane [16], acrylonitrile [17,18] and allylsilane [19], undergo clean cross-metathesis to give cross-products 6 as the main product, rather than homoproducts 15. Cross-metathesis of the cyclic alkenes 43 with terminal alkenes 42 can be used for the synthesis of dienes 44. 

A very useful cross-metathesis is the reaction involving ethylene, which is called ethenolysis. Reaction of ethylene with internal alkenes produces the more useful terminal alkenes. Two terminal alkenes 45 and 42 are formed from the unsymmetric alkene 6 and ethylene. The symmetric alkenes 11 are converted to single terminal alkenes 45. The terminal dienes 46 are formed by ethenolysis of the cyclic alkenes 43. 

Cleavage of polymer-bound allyl esters with palladium catalysts provides general access to 7i-allyl complexes, which can react with a variety of nucleophiles. This has been used in the development of re-allyl-based linkers. Ene-yne cross metathesis and subsequent cleavage in the presence of different nucleophiles yields the corresponding functionalized dienes 93 [93] (Scheme 6.1.19). 

Tandem RCM and cross-metathesis reactions of allyl hexa-l,5-dien-3-yl ether with alkenes leads to 2-alkylidene 3,6-dihydro-27f-pyrans (Scheme 6) <07TL1417>. 

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... 

Cross metathesis of ethylene with internal alkenes provides a facile route to terminal alkenes. A number of processes have been described that use this transformation however, the only products, besides neohexene,that appear to be important are the a,o>-dienes that result from metathesis of cyclic alkenes with an excess of eAylene. This family of compounds should find a wide variety of applications. 

Ri r3 In an alkene metathesis two alkenes react with an appropriate catalyst to form two new alkenes. There are different types of alkene pj2 r4 metathesis reactions The intermolecular reaction is called cross metathesis (CM), whereas intramolecular metathesis is divided into ring-closing metathesis (RCM) and ring-opening metathesis (ROM). Also two polymerization versions of alkene metathesis exist metathesis polymerization of acyclic dienes and ring-opening metathesis polymerization (ROMP). 

Olefin metathesis is a unique carbon skeleton redistribution in which unsaturated carbon-carbon bonds are rearranged in the presence of metal carbene complexes. With the advent of efficient catalysts, this reaction has emerged as a powerful tool for the formation of C-C bonds in chemistry [1]. Olefin metathesis can be utilized in five types of reactions ring-closing metathesis (RCM), ring-opening metathesis (ROM), respective ringopening metathesis polymerization (ROMP), cross-metathesis (CM), and acyclic diene metathesis polymerization (ADMET). 

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