Exchange alkene metathesis

Exchange of alkylidene groups of alkenes—metathesis of olefins... 

The reaction in which two alkenes exchange their alkylidene fragments is called alkene metathesis. The synthesis of libraries in liquid- and solid-phase employing this kind of C-C bond formation has been reported. There are three types of me-... 

The trend of structural selectivity can be summarized as degenerate metathesis of terminal alkenes (exchange of methylene groups) > cross-metathesis of terminal and internal alkenes > metathesis of internal alkenes > productive metathesis of terminal alkenes (formation of internal alkene and ethylene).87 Since different catalyst systems exhibit different selectivities, a simple general picture accounting for all stereochemical phenomena of metathesis is not feasible. 

One of the most curious catalytic reactions of alkenes ever discovered is alkene metathesis or alkene dismutation, in which two alkenes exchange alkyli-dene groups, usually over a tungsten catalyst. The essence of the reaction is illustrated by a commercial process for converting excess propene to a mixture of ethene and butenes ... 

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... 

NMR studies of degenerate ligand exchange in generation I and generation II ruthenium alkylidene pro-catalysts for alkene metathesis... 

In the mid to late 1980s, transition-metal catalysts were developed that were particularly useful for carrying out olefin (alkene) metathesis reactions (Rouhi 2002). Metathesis is a reaction in which two molecules containing carbon-carbon double bonds exchange carbon atoms along with any groups attached to them ... 

Abstract The last half decade has been a period of unprecedented development for the range of transition-metal-catalysed alkylidene exchange reactions collectively known as alkene metathesis. These carbon-carbon bond forming processes have, in a relatively short time, evolved from relative obscurity into a major research area at the forefront of both modern organometallic and synthetic organic chemistry, driven by the rational design of ever more robust and powerful catalytic systems. The advent of modern well-defined catalysts has... 

Ring opening metathesis polymerization, which has been known since the discovery of the alkene metathesis reaction, has been given the acronym ROMP in recent years. In fact, the ROMP reaction was the first observation made in alkene metathesis chemistry, while the discovery of the exchange reaction in equation (1) actually occurred later. Acychc diene metathesis (ADMET) polymerization (equation 3) has only recently been shown to be a viable method for polymer synthesis, and it has been termed ADMET polymerization. ROMP reactions are driven by the release of ring strain from the monomer, while ADMET polymerization is driven by a shift in the equilibrium caused by the removal of one of the reaction products. 

Studies involving the specific exchange mechanisms," as well as theoretical calculations" " of alkene metathesis reactions, generally agree that the mechaiusm begins with jt-coordination of the alkene to the electrophihc metal as shown for a general diene in Scheme 5. 

The alkene metathesis reaction see Alkene Metathesis) exchanges alkylidene groups between different alkenes, and is catalyzed by a variety of high oxidation state, early transition metal species (equation 40). The reaction is of interest because it is the strongest bond in the alkene, the C=C bond, that is broken during the reaction. It is also commercially important in the Shell higher olefins process and in the polymerization of cycloalkenes. It is relevant to this article because carbenes are the key intermediates, and the best-known catalyst, (1), is a carbene complex. 

Alkene metathesis is a reaction catalysed by transition metals in which the carbon atoms that constitute the double bond of the alkene are exchanged with those of another alkene via a metal-carbene complex (Scheme 3.21). 

Carbamoyl complexes from metal carbonyls and amines 5.8.2.12.4 Carbanions reactions with alkene complexes 5.8.2.3,4 metal carbonyls 5.8.2.S.5 Carbene complexes by alkene metathesis 5.8.2.3.11 formation 5.8.2.8.5 Carbides alkali metal formation 5.10.2.1 bonding 5.10.2 formation 5.10.2 industrial uses 5.10.2 interstitial formation 5.10.2 Carbometallacycle formation 5.S.2.2.2 Carbometallacycles from n-allyl complexes 5.S.2.3.9 Carbon reaction with alkali metals 5.10.2.1 Carbon dioxide complexes formation 5.8.2.14.1 Carbon monoxide displacement by alkenes 5.8.2.3.1 Carbonyl complexes by ligand exchange 5.8.2.12.2 from carbon monoxide 5.8.2.12.1, 5.8.2.12.2... 

Another fundamental reaction of organic synthesis in which bond breaking and formation takes place is the metathesis reaction [1-3]. The alkene metathesis reaction [41] exchanges alkildene groups between different alkenes, and it is catalyzed by a variety of high-oxidation-state early transition-metal... 

The term of alkene (olefin) metathesis was introduced by Calderon and coworkers in 1967 [13] to describe a reaction in which 2-pentene is converted to 2-butene and 3-hexene in the presence of tungsten hexachloride, ethanol, and ethyl aluminum dichloride (Scheme 6.1). Nowadays, this term covers all reactions associated with the exchange of carbene (alkylidene) groups between alkenes, and the reaction has become a key process in polymer chemistry, as well as in fine and basic chemical synthesis, including petrochemistry [14, 15]. Alkene metathesis only takes place in the presence of an appropriate catalyst, and considerable research efforts have been devoted to design more active, selective, and stable catalysts [16-24]. In this field, computational chemistry has contributed considerably by bringing information that could not be derived from experimental studies. 

The alkyne metathesis reaction is a process that allows for the exchange of alkyli-dyne units between two alkynes (Scheme 6.18a) [69, 70]. This reaction has not been studied as much as alkene metathesis, despite a growing interest in this topic (Scheme 6.19). 

The first example in Table 6.6 involves an exchange reaction between two different alkenes (metathesis), and is elaborated upon in Scheme 6.55. This is a particularly important process that has gained value with the introduction of very specific catalysts that even allow rings to form with expulsion of small alkenes (i.e.. Ring Closing Metathesis [RCM]) using Grubbs catalysts. 

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