Metathesis of alkenes

Recently, a few examples of the metathesis of alkenes carrying functional groups have been reported. According to a patent, acrylonitrile reacts with propene to crotononitrile (cis and trans) and ethene 10) ... 

Because one might expect steric hindrance to be important, it is worth mentioning that the metathesis of alkenes branched at the double bond has been reported. Thus, isobutene gives (small) quantities of 2,3-dimethy 1-2-butene and ethene (16, 17) ... 

In Section III.D this reaction will be discussed further, particularly in connection with the mechanism of the metathesis of alkenes. 

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

There are two plausible reactions which lead to the observed products of the metathesis of alkenes. The first possibility involves cleavage of a carbon-carbon single bond adjacent to the double bond the second involves cleavage of the double bond itself. The following transalkylation... 

It can be concluded that the metathesis reaction of acyclic alkenes and cycloalkenes proceeds via the rupture and formation of carbon-carbon double bonds, i.e. that the metathesis of alkenes is a true transalkylidenation reaction. 

A few publications have appeared concerning the metathesis of alkynes so far only heterogeneous systems with acyclic alkynes have been reported (31-33). From experiments with [l-14C]2-hexyne this reaction was found to be analogous to the metathesis of alkenes, because it turned out to be a transalkylidynation reaction (33) ... 

From the above, a parallel appears to exist between the metathesis of alkenes and alkynes. Both reactions result in a redistribution of, respectively, alkylidene and alkylidyne groups. Moreover, the results obtained so far suggest that in both cases the reaction might proceed via a metallocyclic intermediate. 

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

The reaction is called metathesis of alkenes. In the example shown above, 2-pentene (either cis, trans, or a cis-trans mixture) is converted to a mixture of 50%... 

Metal alkylidene complexes find application in the metathesis of alkenes, the cvclopropanation of alkenes (Grubbs, Schrock), Wittig type reactions, and the McMurry reaction. In suitable complexes a-elimination can occur twice yielding alkylidync complexes. See Figure 2.21 for an example with tungsten. 

Metal allenylidene complexes (M=C=C=CR2) are organometallic species having a double bond betv een a metal and a carbon, such as metal carbenes (M=CR2), metal vinylidenes (M=C=CR2), and other metal cumulenylidenes like M=C=C= C=CR2 [1]. These metal-carbon double bonds are reactive enough to be employed for many organic transformations, both catalytically and stoichiometrically [1, 2]. Especially, the metathesis of alkenes via metal carbenes may be one ofthe most useful reactions in the field of recent organic synthesis [3], vhile metal vinylidenes are also revealed to be the important species in many organic syntheses such as alkyne polymerization and cycloaromatization [4, 5]. 

The few known examples indicate that more severe reaction conditions are usually required than in the metathesis of alkenes. 

With the discovery by Grubbs of ruthenium carbene complexes such as Cl2(PCy3)2Ru=CHR, which mediate olefin metathesis under mild reaction conditions and which are compatible with a broad range of functional groups [111], the application of olefin metathesis to solid-phase synthesis became a realistic approach for the preparation of alkenes. Both ring-closing metathesis and cross-metathesis of alkenes and alkynes bound to insoluble supports have been realized (Figure 5.12). 

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... 

Scheme 1.12 Metathesis of alkenes with organometallic catalysis.

Transition-metal-catalysed metathesis of alkenes (Scheme 1.12) is more removed from conventional organic chemistry than the above Michael-like reaction, and its investigation has been a major challenge (see Chapter 12). The novelty and enormous value of these reactions have been recognised by the award of the 2005 Nobel Prize for Chemistry to Chauvin, Schrock and Grubbs for their seminal investigations in this area [28]. 

Product distribution analysis, and kinetics determined by classical and advanced NMR techniques the transition-metal-catalysed metathesis of alkenes... 

Although the quasi-cyclobutane mechanism accounts fully for the products of the metathesis of alkenes at equilibrium, see above, some workers were uncomfortable with the unconventional nature of the bonding in the proposed intermediate. An alternative mechanism which brings about the same net alkylidene exchange (lower pathway in Scheme 12.14)... 

Metathesis of alkene 6 to give the new alkenes 11 and 15 is explanined by the following mechanism. The first step is [2+2] cycloaddition between metal carbene 5 and alkene 6 to generate the metallacyclobutane 7 as an intermediate. The real catalyst 8 is generated by retrocycloaddition of the metallacyclobutane 7. Reaction of 8 with alkene 6 generates the metallacyclobutanes 9 and 10 as intermediates. The intermediate 10 is a nonproductive intermediate, which reproduces 6 and 8, while 9 is a productive intermediate and yields the new alkene 11 and the real catalyst 12. Cycloaddition of 12 to alkene 6 produces the productive intermediate 14, from which the new alkene 15 and the active catalytic species 8 are formed. The intermediate 13 is a nonproductive one. 

Metathesis of alkenes is essentially a class of reactions where an interchange of C atoms between pairs of double bonds takes place. A few representative examples are shown by the reactions listed in Fig. 7.10. The industrial use of metathesis reactions so far has been limited mainly to exchange metathesis (Fig. 7.10, top, backward reaction) as in the SHOP process, and ring-open metathesis polymerization (ROMP). As already mentioned (Section 7.5), Vas-tenamer is a polymer made by Hulls by ROMP from cyclooctene. Similarly, the polymer from norborene by ROMP is manufactured by CdF Chemie and is sold by the trade name of Norsorex . 

Schrock RR, Czekelius C. Recent advances in the syntheses 136. and applications of molybdenum and tungsten alkylidene and alkylidyne catalysts for the metathesis of alkenes and alkynes. 

Metathesis of Alkenes (Alkene or Olefin Metathesis) Alkene metathesis... 

Tungsten hexachloride reacts with dry silica gel to form a surface complex with average composition ( i02)WCl4. Van Roosmalen and associates reported that with tetramethyltin the complex yields a solid catalyst for the metathesis of alkenes with the same activity as the WOQ4-SnMe4 system, but is less active for the metathesis of methyl oleate. The supported complex can be used several times without additional SnMe4. ... 

A reaction which is relatively new is the so-called a-bond metathesis [39,40]. Tlie word metathesis is used because of the now well-known metathesis of alkenes and alkynes. It is a concerted 2+2 reaction immediately followed by its retrograde reaction giving metathesis. The transition state is strongly polarized, such that in the reaction of M-R with H-H (Fig. 4.34), the transition state contains a [FI...H...R]"M unit with large negative charges at the terminal groups. 

Although the Ti complexes served to help elucidate the true nature of metathesis catalysts, they were not useful in a practical sense because they were not as active as other catalysts and the oxophilic nature of Ti prevented their use in metathesis of alkenes containing oxygen groups. 

Tungsten hexacarbonyl W(CO)e catalyzes a wide range of organic reactions, such as the metathesis of alkenes. 

Scheme 17.7 Ring closing metathesis of alkenes under [bmi]PF ...

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