Tungsten complexes with olefins

The orientation of the olefin in a -ir complex (Zeise s salt) is known [86] to be mainly defined by steric factors. In the study of tungsten complexes with n-donor two-center ligands, ketimines, and amines, we concluded that the electronic rather than steric factor exerts the crucial effect on the ligand orientation in the complex. 

Subsequently, other Ti-cyclobutane species were shown to be the reaction products of the Tebbe complex with olefins the analogous reaction with acetylenes gives metallacyclobutenes." Utilization of Ti-metaUacycles as initiators in metathesis provides the first example of a living metathesis polymerization system. Clear evidence of the intervention of metaUacarbenes and metallacyclobutanes in olefin metathesis was later furnished by Kress et al. through minute nuclear magnetic resonance (NMR) studies on norbomene polymerization with tungsten alkylidenes. 

The carbene mechanism of COER according to our hypothesis consists of forward (Z = O, X = Y = C) and backward (X = O, Y = Z = O) Wittig-like reactions. A transition metal-carbene complex reacts with a carbonyl compound generating an olefin and transition metal oxo-complex. Then the oxo-complex reacts with another olefin generating a new carbonyl compound and regenerating the transition metal-carbene complex. It is known that oxo-alkylidene complexes can be generated via oxidative addition reaction between some tungsten complexes with carbonyl compounds. 

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

The route to carbene initiation for systems catalyzed solely by transition metal salts (55, 54), or their combinations with Lewis acids such as A1C13 (55), is not well established. Nevertheless, some evidence suggests reduction of the metal by the olefinic substrate (55). Zero-valent (56) and hexavalent (57, 55) tungsten complexes that promote metathesis when activated by UV radiation are the least-understood metathesis systems. 

In marked contrast to the results of Gassman and Schrock, major differences were noted by Casey and co-workers in a series of studies utilizing phenylcarbene-substituted W(0) complexes in reactions with olefins. The H NMR spectra of new phenylcarbene tungsten and iron (69) complexes indicate a substantial positive charge residing on the carbene carbon, and as expected, these complexes readily form ylides on reaction with phosphines ... 

Mimoun and coworkersdescribed the first well-defined example of a d° metal aUtylperoxidic species 49 which epoxidized simple olefins with high selectivity. Several features of the epoxidation performed by 49 resemble those of the Halcon catalytic epoxidation process " . Novel tungsten complexes containing 2 -pyridyl alcoholate ligands like 50 have been synthesized and tested as catalysts in the epoxidation of cw-cyclooctene with TBHP in the absence of solvent . The system displayed modest catalytic activity (100% conversion in 60 h) but excellent product selectivity. 

Important evidence in support of the chain-reaction theory was the synthesis of carbene complexes, such as diphenylcarbenepentacarbonyl-tungsten(O) (4), and their reaction with olefins. The results in Eq. (12.16) demonstrate69 the scission of the carbon-carbon double bond of the alkene and the combination of the ethyl-idene fragment with the diphenylcarbene group of 4 ... 

In organic chemistry one surely thinks at once of the construction of cyclopropane derivatives from olefins and carbenes. Indeed, it has been shown that this also is possible with our complexes and with C=C double bonds that are electron-poor and arc either polarized or easily polarizable (77-81). As an example of this, I would like to cite the reaction of penta-carbonyl[methoxy (phenyl) carbene]chromium (0), -molybdenum (0), or -tungsten(0) with ethyl vinyl ether (79). One obtains the corresponding cyclopropane derivatives in this case, however, only when one removes... 

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