Promoters olefins

Tb allium (ITT) ttifluoroacetate promotes olefin cyclization reactions and intramolecular coupling reactions (32,33). 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

Chen and co-workers have studied the role of coke deposition in the conversion of methanol to olefins over SAPO-34 [111]. They found that the coke formed from oxygenates promoted olefin formation while the coke formed from olefins had only a deactivating effect The yield of olefins during the MTO reaction was found to go through a maximum as a function of both time and amount of coke. Coke was found to reduce the DME dilfusivity, which enhances the formation of olefins, particularly ethylene. The ethylene to propylene ratio increased with intracrystal-line coke content, regardless of the nature of the coke. 

The overall reaction of olefins, hydrogen, and carbon monoxide can be complex since not only are both linear and branched aldehydes (and hence alcohols) formed, but the same catalysts promote olefin isomerization. The products formed may be summarized as follows ... 

Eq. (25)],241 and an acid-promoted olefin cyclizat.ion [Eq. (26)].242-244 Finally, the photocycloaddition of thiobenzophenone with propiolic acids produced the 1-phenyl compounds 59.245... 

Numerous side reactions take place in the reaction zone, some of which are beneficial, but many of which are harmful (4). Some catalysts act to promote olefin isomerization to some extent as well as alkylation. In aviation and motor alkylate production, where more highly branched products are desirable, this isomerization reaction is of considerable benefit. This is especially true when the isomerization of butylene-1 to butylene-2 precedes the alkylation reaction. Harmful side reactions include hydrogen transfer and polymerization. The production of propane from propylene and of normal butane from butylene occurs by hydrogen transfer. Polymerization is very harmful because it not only produces the tar which spends the catalyst, but also reduces the yield of valuable products. 

The catalyst is based on high levels of a ZSM-5 type zeolite which has been doped with a combination of phosphorus, magnesium and calcium. This type of formulation has been used to produce ethylene and propylene from methanol and is known to promote olefin formation from a wide variety of feeds. ... 

Homogeneous polyaUcene catalysis has progressed to the point where metals not generally associated with coordination polymerization can now be made to promote olefin chain growth and metals long ago associated with polyalkene catalysis have been given new life. 

Fischer-type complexes containing an alkoxy group on the carbene carbon, which has been shown to promote olefin isomerization. 

There are a number of differences between the tungsten and molybdenum complexes. The main generality drawn from the data is that the tungsten complexes promote olefin metathesis quite a bit faster than the molybdenum complexes, but the tungsten complexes are less tolerant of functionality. The tungstacyclobutane is more stable than the molybdacyclobutane. Several tungstacyclobutanes have... 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

While cationic catalysts 16 are an effective and important class of catalysts, their electrophilicity still hampers their ability to tolerate polar functions. The less electrophilic neutral 17-19 and zwitterionic 20 nickel catalysts have been examined in an effort to enhance compatibility with polar monomers and co-monomers. Again, bulky ligands are necessary not only to promote olefin insertion, but also to discourage ligand-redistribution reactions resulting in deactivated bis-ligand complexes and decomposition products. For a typical neutral Ni(ii) system, such as 19, Brookhart has shown that the mechanism involves (i) associative displacement of the PR3 ligand by ethylene to... 

A variety of additives have been developed to suppress the migration or isomerization of olefins during an olefin metathesis process. In Ru-catalyzed olefin metathesis, it has been proposed that ruthenium hydride species, formed in situ during a metathesis reaction due to catalyst decomposition, are largely responsible for the isomerization process. For example, Grubbs and coworkers [57] have isolated the decomposition product 100, and have shown that it can promote olefin isomerization (Scheme 12.30). 

Other ruthenium catalysts have also been studied. In one study, ((IPrH2) (PCy3)(Cl)2Ru=CHPh) promoted ADMET faster than [Ru]2, although it too promoted olefin isomerization [28b]. The initiation rate of [Ru]3 was reported to be 1/30 that of [Ru]l, but the propagation rate was found to be four times faster [16]. The activity of [Ru]4 has even been shown to surpass that of Schrock s molybdenum-based catalysts [39]. 

---