Disproportionation metathesis

It is to be hoped that measurements will be made in the near future which will put more substantial flesh on the skeleton of known bond enthalpy contributions in organo-transition metal compounds, so that a better understanding of the energetics of reactions such as olefin disproportionation (metathesis) and hydroformylation may be achieved. 

In the disproportionation (metathesis) of olefins, i.e. the reaction 2Ri-CH=CH-R2 a R1-CH=CH-R1 + R2-CH=CH-R2, a M0O3/ AI2O3 catalyst is often applied. Its preparation involves impregnation of Y-AI2O3 extrudates with an aqueous solution of ammonium heptamolybdate (AHM) or an ammoniacal solution of ammonium dimolybdate (ADM). 

Heterogeneous catalyst studies and factors that contributed to the discovery of the olefin disproportionation (metathesis) reaction are described. Also provided is a personal commentary on the developments of heterogeneous catalyst technology associated with this intriguing reaction and its commercialization. 

Interest in the olefin disproportionation (metathesis) reaction catalysed by transition-metal compounds has increased markedly during the present period and although a number of features of such reactions remain unexplained or are topics of controversy, sufficient evidence has now been obtained to suggest that carbene intermediates are involved in most catalytic systems. The exact nature of the catalytic sites remains obscure but the recent discovery of one-component metathesis catalysts may enable more rapid advances to be made in this direction. 

R. Banks, Discovery and Development of Olefin Disproportionation (Metathesis), American Chemical Society (1983). 

Disproportionation of Olefins. Disproportionation or the metathesis reaction offers an opportunity to convert surplus olefins to other desirable olefins. Phillips Petroleum and Institut Fransais du Petrc le have pioneered this technology for the dimerization of light olefins. The original metathesis reaction of Phillips Petroleum was intended to convert propylene to 2-butene and ethylene (58). The reverse reaction that converts 2-butene in the presence of excess ethylene to propylene has also been demonstrated (59). A commercial unit with a capacity of about 136,000 t/yr of propylene from ethylene via 2-butene has been in operation in the Gulf Coast since 1985 (60,61). In this process, ethylene is first dimerized to 2-butene foUowed by metathesis to yield propylene. Since this is a two-stage process, 2-butene can be produced from the first stage, if needed. In the dimerization step, about 95% purity of 2-butene is achieved at 90% ethylene conversion. 

The synthetic utility of the alkene metathesis reaction may in some cases be limited because of the formation of a mixture of products. The steps of the catalytic cycle are equilibrium processes, with the yields being determined by the thermodynamic equilibrium. The metathesis process generally tends to give complex mixtures of products. For example, pent-2-ene 8 disproportionates to give, at equilibrium, a statistical mixture of but-2-enes, pent-2-enes and hex-3-enes ... 

Table 8-5 indicates the wide variety of catalysts that can effect this type of disproportionation reaction, and Figure 8-7 is a flow diagram for the Phillips Co. triolefm process for the metathesis of propylene to produce 2-butene and ethylene. Anderson and Brown have discussed in depth this type of reaction and its general utilization. The utility with respect to propylene is to convert excess propylene to olefins of greater economic value. More discussion regarding olefin metathesis is noted in Chapter 9. 

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. 

Cyclopropanes in low yield were first noted in 1964 by Banks and Bailey (12) during the disproportionation of ethylene, but little significance was attached to that observation until recently, because such products had no obvious relevance to early mechanistic concepts based on pairwise rearrangements of bisolefin complexes. However, the subsequent adoption of carbenelike species as metathesis intermediates (4) provided a foundation for later development of cyclopropanation concepts. The notable results of Casey and Burkhardt (5) made an impact which seemed rather neatly to unify mechanistically the interconversion of cyclopropanes and metathesis olefins, although the reactions which they observed were stoichiometric rather than catalytic [see Eq. (4)]. Nevertheless, their work indicated a net redistribution of =CPh2 and =CH2 from (CO)5W=CPh2 and isobutylene, respectively, to form CH2=CPh2. Dissociation and transfer of CO yielded W(CO)6. Unfortunately, the fate of the isopropylidene moiety remained unknown. In 1976,... 

Alkane metathesis (AM shown for n-aUcanes in 6), which has also been referred to as disproportionation or redistribution of alkane chains, has great potential in the context of petroleum refining. 

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

Vlefin disproportionation or metathesis is a transition-metal catalyzed reaction involving the interchange of alkylidene units between olefins (Reaction 1) (1, 2, 3, 4). 

The reaction of berylliumorganic compounds with alcohols (method 4) or ketones (method 7) yields RBeOR, which usually are stable to further alcoholysis, but the action of the excess of alcohols on BeR2 (R = Me, Et, Pr ) at low temperatures gave Be(OR)2 (R = Me, Bu CEt3) [399], The dimeric Me-BeOMe easily disproportionates with the formation of BeMe2 and insoluble Be(OMe)2 [399], The metathesis of BeCl2 with lithium alkoxides (method 5) was proposed for the preparation of polymeric Be(ORn)2 and Be(OPri)2 [65] however, the IR spectra of the products presented in this publication indicate the presence of considerable amounts of admixtures. 

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... 

In 1967, Calderon, Chen, and Scott4) reported a homogeneous catalyst system comprised of tungsten hexachloride, ethanol, and ethylaluminum dichloride would disproportionate internal olefins. These authors used the term olefin metathesis to describe the reaction. At room temperature 2-pentene was transformed in one to three minutes into a mixture containing, at equilibrium, 25, 50, and 25 mole per cent of 2-butene, 2-pentene, and 3-hexene, respectively. Double-bond isomerization was not detected and a quantitative reaction selectivity was obtained. Additional reports by Calderon and cowor-... 

In 1984, the first very effective example of metathesis (disproportionation) of vinylsubstituted silicon compounds catalyzed by ruthenium complexes was reported [33]. It opened a new route of great synthetic importance and has allowed synthesis of a series of unsaturated silicon compounds according to the following equations, with the yield predominantly higher than 70%. Numerous reports on vinylsilane disproportionation (Eq. 21) [5, 33-38] and its co-dis-proportionation with olefins (Eq. 22) [37-44] have been published. 

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