Metathesis Acyclic Alkenes

Instead ef the name metathesis, the term disproportionation is frequently applied to the reaction, and sometimes the term dismutation. For historical reasons the name disproportionation is most commonly used for the heterogeneously catalyzed reaction, while the homogeneously catalyzed reaction is usually designated as metathesis. The name disproportionation is correct in the case of the conversion of acyclic alkenes according to Eq. (1) however, this name is inadequate in most other situations, such as the reaction between two different alkenes, and reactions involving cycloalkenes. Similar objections apply to the name dismutation. The name metathesis is not subject to these limitations and, therefore, is preferred. 

The metathesis of acyclic alkenes substituted with other hydrocarbon groups, such as cycloalkyl, cycloalkenyl, or aryl groups, has also been observed. For instance, styrene is converted into ethene and 1,2-diphenyl-ethene (stilbene) (9, 9a). 

It has been suggested that these polymers are mainly linear, which may be a consequence of intermolecular metathesis reactions with traces of acyclic alkenes, or of other consecutive reactions 19-22). 

Mutual metathesis of a cyclic and an acyclic alkene provides still more possibilities in synthesizing organic compounds. For instance, cycloalkenes are cleaved by ethene into a,co-dienes. The reaction of 1,5-cyclooctadiene with ethene gives 1,5,9-decatriene (18) norbornene reacts with 2-butene to yield 1,3-dipropenylcyclopentane (30) ... 

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) ... 

Of course, even in the case of acyclic alkenes reaction enthalpy is not exactly zero, and therefore the product distribution is never completely statistically determined. Table V gives equilibrium data for the metathesis of some lower alkenes, where deviations of the reaction enthalpy from zero are relatively large. In this table the ratio of the contributions of the reaction enthalpy and the reaction entropy to the free enthalpy of the reaction, expressed as AHr/TASr, is given together with the equilibrium distribution. It can be seen that for the metathesis of the lower linear alkenes the equilibrium distribution is determined predominantly by the reaction entropy, whereas in the case of the lower branched alkenes the reaction enthalpy dominates. If the reaction enthalpy deviates substantially from zero, the influence of the temperature on the equilibrium distribution will be considerable, since the high temperature limit will always be a 2 1 1 distribution. Typical examples of the influence of the temperature are given in Tables VI and VII. 

H risson and Chauvin (88) examined the metathesis between acyclic alkenes and cycloalkenes (telomerization) in the presence of two other tungsten-based catalysts, namely WOCl4-Sn(n-C4He)4 and WOCI4-... 

Fig. 5. Mechanism of H risson and Chauvin for the mutual metathesis of a cyclic and an acyclic alkene.

The preferred kinetic model for the metathesis of acyclic alkenes is a Langmuir type model, with a rate-determining reaction between two adsorbed (complexed) molecules. For the metathesis of cycloalkenes, the kinetic model of Calderon as depicted in Fig. 4 agrees well with the experimental results. A scheme involving carbene complexes (Fig. 5) is less likely, which is consistent with the conclusion drawn from mechanistic considerations (Section III). However, Calderon s model might also fit the experimental data in the case of acyclic alkenes. If, for instance, the concentration of the dialkene complex is independent of the concentration of free alkene, the reaction will be first order with respect to the alkene. This has in fact been observed (Section IV.C.2) but, within certain limits, a first-order relationship can also be obtained from many hyperbolic models. Moreover, it seems unreasonable to assume that one single kinetic model could represent the experimental results of all systems under consideration. Clearly, further experimental work is needed to arrive at more definite conclusions. Especially, it is necessary to investigate whether conclusions derived for a particular system are valid for all catalyst systems. 

Acyclic alkadienes, metathesis of, 134 Acyclic alkenes, metathesis reaction of, 133, 134... 

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). 

The ring-opening cross-metathesis reaction is similar to the acyclic cross-metathesis reaction discussed above, except that one of the acyclic alkenes is replaced with a strained cyclic alkene (Scheme 5). 

In this case, the use of ethene as the acyclic alkene means that the diene 38 and polymeric compounds are the only possible products that can be formed from metathesis. 

Optimal yields were obtained by slow addition of the alkene substrates to a solution of the ruthenium vinylalkylidene and this allowed just two equivalents of the acyclic alkene to be used without significant formation of polymeric products. Unlike the acyclic cross-metathesis reactions, which generally favour the formation of tram products, the above ring-opening metathesis reactions yielded products in which the cis stereoisomer is predominant. Particularly noteworthy was the absence of significant amounts of products of type 31, formed from metathesis of one cyclic and two acyclic alkenes. In fact, considering the number of possible ring-opened products that could have been formed, these reactions showed remarkable selectivity (GC yields > 80%). 

Successful ring-opening cross-metathesis with symmetrical internal acyclic alkenes was, however, achieved by Blechert and Schneider [49]. Reaction of a variety of functionalised norbornene derivatives with fraws-hex-3-ene in the presence of the ruthenium vinylalkylidene catalyst 4 yielded the ring-opened products as predominantly trans-trans isomers (for example Eq. 33). 

Use of a symmetrical acyclic alkene limits the possible metathesis products to the desired diene (for example 45) and products formed from polymerisation of the cyclic substrate. Competing ROMP was suppressed in these reactions by using dilute conditions and a tenfold excess of hex-3-ene. By adding the cyclic substrate slowly to a solution of the catalyst and ris-hex-3-ene (which was significantly more reactive than the trans isomer), less than two equivalents of the acyclic alkene were used without causing a significant drop in the cross-metathesis yield. 

A subsequent publication by Blechert and co-workers demonstrated that the molybdenum alkylidene 3 and the ruthenium benzylidene 17 were also active catalysts for ring-opening cross-metathesis reactions [50]. Norbornene and 7-oxanorbornene derivatives underwent selective ring-opening cross-metathesis with a variety of terminal acyclic alkenes including acrylonitrile, an allylsilane, an allyl stannane and allyl cyanide (for example Eq. 34). 

As the representative examples in Scheme 6.11 illustrate, similar stragies may be applied to the corresponding alkenyl ethers (vs. styrenyl ethers) [26], The Zr-catalyzed kinetic resolution/Ru-catalyzed metathesis protocol thus delivers optically pure 2-substituted di-hydrofurans that cannot be accessed by resolution of the five-membered ring heterocycles (see Scheme 6.8). It should be noted, however, that the efficiency of the Zr-catalyzed resolution is strongly dependent, and not in a predictable manner, not only on the presence but the substitution of the acyclic alkene site of the diene substrate. The examples shown in Scheme 6.11 clearly illustrate this issue. 

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

In the steric course of olefin metathesis, acyclic and cyclic alkenes exhibit opposite behavior. The stereoselectivity of the transformation of most acyclic olefins is low and usually goes to equilibrium. In some systems there is a strong initial preference for retention of stereochemistry, that is for the transformation of cis to cis and that of trans to tram isomers.88 This means that geometric isomers of an olefin give different isomeric mixtures of the same product ... 

For the cleavage of alkenes from a support by metathesis, several strategies can be envisaged. In most of the examples reported to date, ring-closing metathesis of resin-bound dienes has been used to release either a cycloalkene or an acyclic alkene into solution (Figure 3.38, Table 3.44). Further metathesis of the products in solution occurs only to a small extent when the initially released products are internal alkenes, because these normally react more slowly with the catalytically active carbene complex than terminal alkenes. If, however, terminal alkenes are to be prepared, selfmetathesis of the product (to yield ethene and a symmetrically disubstituted ethene) is likely to become a serious side reaction. This side reaction can be suppressed by conducting the metathesis reaction in the presence of ethene [782,783]. 

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]. 

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... 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

Scheme 12.16 The co-metathesis of pent-2-ene and cyclopentene to yield mono acyclic alkenes (upper) and high molecular weight telomers and polymers (lower) double bond geometry is arbitrary.

Opening of a strained ring system and the subsequent coupling with an acyclic alkene results in the formation of diene products. Because of many metathesis pathways available in the systems containing a cyclic and a linear olefin, the... 

Metathesis is not limited to acyclic alkenes. Cyclic alkenes also undergo metathesis with the formation of polymers. Homogeneous catalysts are used in most cases. New polymers are produced in this way by Huls, CdF Chimie and Hercules. 

It was recognized early that efficient olefin cross metathesis could provide new methods for the synthesis of complex molecules. However, neither (la) nor (2a) were very effective at intermolecular cross metathesis owing to poor reaction selectivity (cross vs. intramolecular metathesis) and low E. Z ratios see (E) (Z) Isomers) The advent of more active and functional group tolerant olefin metathesis catalysts recently made cross metathesis a viable route for constructing a large variety of fimctionalized acyclic alkenes. 

The original catalysts reported for the metathesis of acyclic alkenes were related to alkene polymerization systems. Banks and Bailey reported in 1964 that the heterogeneous cobalt molybdate complexes would promote the metathesis reaction. Since that time a wide variety of catalysts that use Mo, W and Re as Ae active metal in combination with a variety of supports, promoters and activation conditions have been reported. These heterogeneous catalysts are the systems of choice for most industrial fine chemical applications. 

Related molybdenum catalysts appear to show even more functional group tolerance. To date, the major test of functional group compatibility has been in the synthesis of polymers however, it is anticipated that this activity will persist into acyclic metathesis. Later transition metals are active in the metathesis polymerization of highly functiondized cyclic alkenes. These catalyst systems, which appear to tolerate almost all functional groups, show very low activity for acyclic alkene metathesis. If these systems can be activated, the problems associated with the use of alkene metathesis in the synthesis of multifunctional organics will be solved. 

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). 

Cross-Metathesis Between a Cyclic and an Acyclic Alkene... 

The metathesis polymerization of acyclic alkenes is an equilibrium step propagation via, dimers, trimers, tetramers, etc. The scheme is ... 

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