Tungsten complexes studies

Epoxidation systems based on molybdenum and tungsten catalysts have been extensively studied for more than 40 years. The typical catalysts - MoVI-oxo or WVI-oxo species - do, however, behave rather differently, depending on whether anionic or neutral complexes are employed. Whereas the anionic catalysts, especially the use of tungstates under phase-transfer conditions, are able to activate aqueous hydrogen peroxide efficiently for the formation of epoxides, neutral molybdenum or tungsten complexes do react with hydrogen peroxide, but better selectivities are often achieved with organic hydroperoxides (e.g., TBHP) as terminal oxidants [44, 45],... 

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... 

Mechanistic studies (108) have suggested that the formation of the binu-clear complex (71) on acidification of the product of the reaction between methyllithium and W(CO)s[C(OEt)Me] proceeds via the intermediate formation of dimethylcarbene- and vinylidene-tungsten complexes. These then cycloadd and rearrange as shown ... 

More attention has been paid to tungsten complexes and Raubenheimer and coworkers502 studied the reactions of carbene complexes such as [W(CO)s C(OEt)Ph) with a variety of electrophiles to give coordinated thioaldehydes. Fischer s group has reported many studies on the behavior of pentacarbonyltungsten-coordinated thiobenzaldehyde [(CO)sW S=C(Ph)H ] with vinyl ethers503 (equation 153) and alkynes504. 

Studies on the base-catalyzed substitution of halide for methoxide in complexes rans-[M(NH)X(dppe)2]+ (M = Mo or W) have shown (for X = F or Cl) that the imido group in the tungsten complexes is some 1000-fold less acidic than in the molybdenum ones (188). 

An unusual reaction of W111 with nitriles has been reported by Schrock et al. to provide metal a nitrido tungsten complex [46], Treatment of 98 with acetonitrile or benzonitrile provides the tungsten carbyne 99, along with the nitrido complex 100 (Eq. (37)). Additional studies resulted in the identification of reaction conditions that... 

Cycloaddition reactions of a,/3-unsaturated chromium and tungsten complexes have been studied to a great extent and have been reviewed.3 -6 Our report on cycloaddition of (l-alkynyl)carbene complexes is restricted to a short abstract and an update including more recent results. A most remarkable feature of [4+2] cycloadditions of 1,3 dienes to C=C bonds of (l-alkynyl)carbene complexes, e.g., li, is that such reactions proceed under very mild conditions, compared to those for reactions of propargylic esters, e.g., 41. Thus, formation of a Diels-Alder adduct, e.g., a norbornadiene derivative 42, can be achieved at 25°C via carbene complexes instead of at 190°C via the direct route (Scheme 15).68 Ligand disengagement from compound 40 can be achieved in various ways, e.g., by formation of an ester 43 through oxidation of the Cr=C bond, or by formation of an allyl silane 4369 or a stannane.70 71... 

The last series of tungsten complexes that have been electrochemically studied belong to the 3,5-dimethylpyrazolyl-borato complexes of the type shown in Scheme 7-7. Also in this case, they exhibit the one-electron oxidation of the ferrocenyl centre and the one-electron reduction of the molybdenum fragment [55], The relevant redox potentials are summarized in Table 7-13. 

The photochemical cis-trans isomerization of bisphosphine-substituted carbyne complexes [Eq. (57), Section IV,B], which was studied in our group, was postulated to proceed via pentacoordinate metal ketenyl intermediates (97). Support for this postulate was found in the low-temperature characterization of ketenyl complex 208 on irradiation of complexes 207 in the presence of PMe, [Eq. (164)]. In the presence of phenylacetylene the ketenyl alkyne tungsten complex W(/ -PhCCO)CI(PhC2H)(CO)(PMe3)2 forms at low temperatures. 

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