Tungsten catalysts alkene metathesis

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

Although the application of tungsten catalyst 5 to the cross-metathesis reaction of other alkenes has not been reported, Basset has demonstrated that to-un-saturated esters [18] and glycosides [21], as well as allyl phosphines [22], are tolerated as self-metathesis substrates. 

The report by Basset and co-workers on the metathesis of sulphur-containing alkenes using a tungsten alkylidene complex, mentioned previously for the acyclic cross-metathesis reaction (see Sect. 2.2), also contained early examples of ring-opening cross-metathesis of functionalised alkenes [20]. Allyl methyl sulphide was reacted with norbornene in the presence of the tungsten catalyst 5, to yield the desired ring-opened diene 35 (Eq. 29). 

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 general equation and mechanism for alkyne metathesis is depicted in Scheme 31. Alkyne metathesis is considerably less well developed in comparison to alkene metathesis. Garbyne complexes or carbyne complex precursors are among the most effective alkyne metathesis catalysts representative catalysts are depicted in Scheme 32. Tungsten carbyne complex 276 is one of the earliest alkyne metathesis catalysts, and has frequently been employed to initiate... 

The results of ab initio calculations on alkene metathesis catalysts M(OH)2(XH)(=CH2), MCl2(Y)(=CH2), and Mo(OH)2(NH)(=CHZ) are used to address the effects on the bonding caused by modifications in the metal, ligands, and alkylidene substituents. For instance the greater M =C bond polarity in W(OH)2(NH)(=CH2) versus Mo(OH)2(NH)(=CH2) correlates with the greater metathetical activity of the tungsten species.252... 

Schrock and Fischer type carbyne tungsten or molybdenum complexes are very interesting catalysts for alkene metathesis or alkyne polymerisation reactions. Within the first reaction steps they form carbene complexes and on these carbene complexes further metathesis or polymerisation occur. 

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. 

Fig. 12.12 Preparation of an oxo-tungsten complex, an active catalyst for metathesis of alkenes.

A related tungsten complex (R 0)2W(=NR)(=CHBu ) is an exceptionally active catalyst for alkene metathesis. Its activity is unchanged over periods of at least 1 day and the turnover is in hundreds of conversions per minute. A trigonal bipyramidal tungstenacyclobutane intermediate is again involved. 

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

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-... 

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

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