Tungsten alkenes

Other types of tungsten-alkene complexes that merit mention include the first group six jj -benzene complex, TpW(NO)(PMe3) ((( -CeHe), that contains a tt-bound benzene ligand activated toward Diels-Alder cycloaddition.Also, the jj -fiillerene complexes, mgr-W(CO)3(L-L)(jj -C6o) (65), have been prepared from W(CO)4(L-L) (L-L = a bidentate N- or P-donor ligand) in the presence of In some... 

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. 

The function of the tetraethyltin is to create vacant sites so that coordination of alkene molecules becomes possible, and to change the oxidation state of the tungsten atom from +6 to +4. Similar behavior of the aluminum compound in the system WCL-CgHsAlCb is not probable, because it has been demonstrated that WCle-AlClg is also an active catalyst (22, 44), which suggests that C2H5AICI2 functions as a Lewis acid. Vacant sites can be created by a Lewis acid as follows ... 

If the creation of vacant sites occurs in this way, it would be erroneous to conclude that the tungsten complex in its active form is not reduced, because reduction can also be accomplished by the reacting alkene molecules. 

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 ]. 

An obvious drawback in RCM-based synthesis of unsaturated macrocyclic natural compounds is the lack of control over the newly formed double bond. The products formed are usually obtained as mixture of ( /Z)-isomers with the (E)-isomer dominating in most cases. The best solution for this problem might be a sequence of RCAM followed by (E)- or (Z)-selective partial reduction. Until now, alkyne metathesis has remained in the shadow of alkene-based metathesis reactions. One of the reasons maybe the lack of commercially available catalysts for this type of reaction. When alkyne metathesis as a new synthetic tool was reviewed in early 1999 [184], there existed only a single report disclosed by Fiirstner s laboratory [185] on the RCAM-based conversion of functionalized diynes to triple-bonded 12- to 28-membered macrocycles with the concomitant expulsion of 2-butyne (cf Fig. 3a). These reactions were catalyzed by Schrock s tungsten-carbyne complex G. Since then, Furstner and coworkers have achieved a series of natural product syntheses, which seem to establish RCAM followed by partial reduction to (Z)- or (E)-cycloalkenes as a useful macrocyclization alternative to RCM. As work up to early 2000, including the development of alternative alkyne metathesis catalysts, is competently covered in Fiirstner s excellent review [2a], we will concentrate here only on the most recent natural product syntheses, which were all achieved by Fiirstner s team. 

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

When alkenes are treated with certain catalysts (most often tungsten, molybdenum, or rhenium complexes), they are converted to other alkenes in a reaction in which the alkylidene groups (R RX=) have become interchanged by a process schematically illustrated by the equation ... 

The unhindered phospha-alkene (175) has been characterised in solution as a complex involving ir-donation from the double bond to a tungsten carbonyl acceptor. This complex reacts with 1,3-dienes to give adducts e.g. (176), in which the phosphorus atom... 

Iwasawa and co-workers developed a facile method for the construction of polycyclic indole derivatives 190a and 190b by the tungsten(0)-catalyzed reaction of A-(2-(l-alkynyl)phenyl)imine 188 with the electron-rich alkenes 189a and 189b (Scheme 32).42b Photoirradiation of a mixture of imine 188 and ketene silyl acetal 189a with 10mol% of... 

W(CO)6 in toluene at room temperature gave the tricyclic adduct 190a in 94% yield after acidic workup. This reaction forms the tungsten-containing azomethine ylide 191, which undergoes the [3 + 2]-cycloaddition with 189a. The rhodium(n)-catalyzed cyclization of the ene-yne-aldimine 192 with alkene 193 into the cyclopropane 194 was reported by Uemura and Ohe (Scheme 32).42c... 

Diphenyl ditelluride adds to alkynes upon irradiation with a tungsten lamp in the absence of solvent to give 1,2-bis(phenyltelluro)alkenes (Equation (67)).191 The ( )-isomers are obtained as the major product through a radical chain mechanism. Functionalized internal alkynes such as dimethyl acetylenedicarboxylate and dihaloalkynes XCCX (X = C1 or Br) are able to participate in the reaction.192... 

The use of ill-defined catalysts for the cross-metathesis of allyl- and vinylsi-lanes has also received considerable attention, particularly within the past decade. Using certain ruthenium catalysts, allylsilanes were found to isomerise to the corresponding propenylsilanes prior to metathesis [5]. Using rhenium- or tungsten-based catalysts, however, successful cross-metathesis of allylsilanes with a variety of simple alkenes was achieved [6,7] (an example typical of the results reported is shown in Eq. 3). 

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. 

Prior to the first examples of the cross-metathesis of functionalised alkenes using these catalysts, however, was a report on the use of a lesser known tungsten complex 5 [18,19]. 

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). 

Tungsten-based catalysts including POMs show high efficiency of H202 utilization [17,18,78-100]. Ishii and coworkers [18] have reported effective H202-based epoxidation of alkenes catalyzed by H3PW12O40 combined with cetyl pyridinium chloride (CPC) as a phase-transfer agent ... 

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