Olefins by metathesis

PrODUCHON of propylene and Cj J-C20 OLEFINS BY METATHESIS. ECONOMIC DATA (France ccmdinons, mid-1986)... 

Scheme 2.191 Synthesis of fluoro and difluoro olefins by metathesis of carbonyl compounds with fluoroalkyl phosphoranes (SBAH = sodium bis(methoxyethoxy)aluminum hydride) [12d].

Production of propylene and C, olefins by metathesis. Economic data... 

Stereospecific ring-opening polymerization of cyclic olefins by metathesis catalysts... 

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. 

Olefin cross metathesis starts to compete with traditional C=C bondforming reactions such as the Wittig reaction and its modifications, as illustrated by the increasing use of electron-deficient conjugated alkenes for the ( )-selective construction of enals and enoates. 

Imide-terminated telechelics are also synthesized by metathesis depolymerization, and it is found that phthalimide-substituted olefins allow for productive depolymerization when only one methylene spacer separates the nitrogen atom and the olefin (Fig. 8.21). This combination of steric hindrance around the nitrogen lone pair and decreased electron donation from resonance prevents the negative neighboring group effect. However, secondary acyclic amines are unable to produce telechelics through metathesis depolymerization because of unfavorable catalyst-amine interactions. 

The cometathesis product contains 10-15% of the wanted C10-C13 - /-olefins, which are separated from the undesired olefins by fractional distillation. The first runnings and the tailing olefins are recycled into the metathesis reactor so that at least all of the ethylene oligomers are transformed into the wanted C10-C,3 n-olefins, with the olefins of course having internal double bonds. 

However, the generation of cyclopropane derivatives was also shown to implicate an alternate route not requiring a metallocyclobutane transition state (14). In addition, a metathesis catalyst successfully converted certain cyclopropanes to metathesis-related olefins by way of a carbene elimination process (15-17), according to Eq. (3). 

An alternative approach has been formulated by Katz and co-workers (28). They consider the latent cis specificity in reactions of c/ s-olefins with metathesis catalysts to be quite high, based on their results with a catalyst having very low Lewis acidity. They further propose that the frequently observed lack of stereospecificity is mainly the consequence of cationic processes inherent in the metathesis step which mask stereospecificity. 

The organotitanium compounds produced by desulfurization of the diphenyl thioacetals of aldehydes 28 with the titanocene(II) species Cp2Ti[P(OEt)3]2 29 react with carbon—carbon double bonds to form the olefin metathesis-type products. Thioacetals 28 may be transformed into terminal olefins by desulfurization with 29 under an ethene atmosphere (Scheme 14.15) [27]. This reaction is believed to proceed through a titanacyclobutane intermediate, formed by cycloaddition of the titanocene-alkylidene with ethene. 

Scheme 5.13 Cleavage of an octenediol linker by olefin cross-metathesis to yield NPG63.69...

Several other observations suggest that nucleophilic carbene complexes, similarly to, e.g., sulfur ylides, can cyclopropanate acceptor-substituted olefins by an addition-elimination mechanism. If, e.g., acceptor-substituted olefins are added to a mixture of a simple alkene and the metathesis catalyst PhWCl3/AlCl3, the metathesis reaction is quenched and small amounts of acceptor-substituted cyclopropanes can be isolated [34]. 

Much more challenging is the targetted introduction of carbon substituents at terminal olefins by means of cross metathesis. Because of the mild reaction conditions under which alkene metathesis proceeds, cross metathesis could become an extremely valuable tool for the synthetic chemist if the critical parameters for productive cross metathesis between different, functionalized olefins were understood. 

Kamau, S. D. Hodge, R Hall, A. J. Dad, S. Ben-Haida, A. Cyclodepolymerization of olefin-containing polymers to give macrocychc oligomers by metathesis and the entropically-driven ROMP of the olefin-containing macocyclic esters. Polymer 2007,48, 6808-6822. 

Metathesis reactions are now widely used in natural product synthesis. Novel retrosynthetic analyses were developed because a carbon-carbon single bond can be formed after hydrogenation of a double bond constructed by metathesis. Although many types of metathesis are now known, the reaction is classified by olefin, enyne, and alkyne metatheses in this chapter. 

It is worth noting that 6,7-dihydro-2(3//)-oxepinone is an unusual lactone because it can be polymerized by two distinct mechanisms ROP of the cyclic esters by aluminum alkoxides, and the ring-opening metathesis polymerization (ROMP) of endocyclic olefins by the Schrock s catalyst (Fig. 28) [121]. 

S.S. Salim, R.K. Bellingham, V. Satcharoen, R.C.D. Brown, Synthesis of heterocyclic and carbocyclic fluoro-olefins by ring-closing metathesis, Org. Lett. 5 (2003) 3403-3406. 

The polymerization of cyclic, strained olefins by transition metal alkylidenes of general formula L M = CRR (L = ligand, R, R = H, alkyl, aryl) yields polymers formed via ring-opening that contain unsaturated double bonds within each repetitive unit. Since the mechanism is based on repetitive metathesis steps, this polymerization reaction is known as ring-opening metathesis polymerization (ROMP) (Scheme 1). 

Scheme 19 Synthesis of 3-(Aryl)-alkenyl-fS-lactams by olefin cross-Metathesis on solid support...

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