Carbon-tungsten double bonds

Conjugate hydride abstractions have also been used for the generation of carbon-metal double bonds. An interesting reaction sequence, in which a (thermally unstable) cationic, non-heteroatom-substituted tungsten carbene complex is prepared by conjugate hydride abstraction, is shown in Figure 3.9. 

Olefin metathesis occurs in the presence of a complex transition metal catalyst that contains a carbon-metal double bond. The metal is typically ruthenium (Ru), tungsten (W), or molybdenum (Mo). In a widely used catalyst, called Grubbs catalyst, the metal is Ru. 

Bryan, J. C., Mayer, J. M. Oxidative addition of carbon-oxygen and carbon-nitrogen double bonds to WCl2(PMePh2)4. Synthesis of tungsten metallaoxirane and tungsten oxo- and imido-alkylidene complexes. J. Am. Chem. Soc. 1990,112, 2298-2308. 

Geometrical parameters provide much more information than simple identification. The phosphorus-carbon distance of 1.686(6) A in this tungsten complex is consistent with there being a double bond, as it is close to those observed in open chain phosphaalkenes. But how much better it would have been if the parent compound could have been studied uncomplexed, so that the strain in this unusual ring system could have been investigated. The geometrical parameters would then have provided very useful information about the likely reactivity of the compound, and information about the changes on... 

The 772-phosphinocarbene complexes of tungsten show ambiphilic behavior. With Lewis acids such as MeS+, electrophilic attack occurs at the metal-carbon double bond affording the dicationic tunstaphosphathiabi-cyclo[1.1.0]butane complexes 102.96,97 On the other hand, nucleophiles such as trialkyl phosphines or the cyclopentadienyl anion C5H5 add at the carbe-nic center, affording phosphoranylidene complexes 10397 or tungstaphos-... 

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 ... 

The tungsten-carbon single, double, and triple bond lengths in this compound are 225.8, 194.2. and 178.5 pm, respectively, and the accompanying W—C—C bond angles are 124°. 150°. and 175 , all of which is quite consistent with tungsten-carbon bond orders of 1,2. and 3. 

In die last 10 years or so an exciting new strategy has emerged for the formation of carbon-carbon double bonds, namely olefin metathesis. This work grew out of the development of Ziegler-Natta catalysts for die polymerizarion of cyclic olefins. It was found that when 2-pentene was treated with a catalyst prepared from tungsten hexachloride and ethylaluminum dichloride, a mixture of 2-pentene, 2-butene, and 3-hexene was produced in minutes at room temperature (rt) ... 

Since silene is an unstable species, various transition metal-silene complexes coordinated by the silicon-carbon double bond have been reported. In 1970, Pannel reported the formation of silene by irradiation of an iron complex (Eq. 6) [8]. He obtained an iron-TMS complex that was apparently formed from silene and an iron-hydride complex generated from the starting iron complex by /3-hydrogen elimination [8]. Wrighton confirmed the existence of tungsten-and iron-silene complexes by examination of NMR spectra obtained at low temperature (Eqs. 7 and 8) [9]. 

Recently, norbornene was reported to undergo ring-opening polymerization in the presence of molybdenum, tungsten, and rhenium halide catalysts (33). In each of these studies (33-35), the catalysis was interpreted by mechanisms involving the scission of the carbon-carbon single bond adjacent to the double bond, e.g., Eq. (12). 

The molybdenum catalyst 2 has been used extensively for ADMET polymerization. This complex is easier to handle than the tungsten analog and is more tolerant of functionality. This complex has allowed the synthesis of polymers containing esters, carbonates, ethers, sulfides, aromatic amines, boronates, dichlorosilanes, siloxanes, acetals, and conjugated carbon-carbon double bonds [38-45]. Aldehydes, ketones, and protic functionahty are not tolerated. The molybdenum alkylidene will react with aldehydes and ketones, but not esters, in a Wittig fashion [64]. 

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