Tungsten complexes selectivity

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],... 

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. 

Phosphinidenes (R-P), short-lived, reactive intermediates that are isoelec-tronic with nitrenes and carbenes, are accessible by fragmentation of phos-phiranes, phosphol-3-enes, triazaphospholenes, and oxazaphospholenes and can be characterized by trapping reactions. The metal-complexed species [R-P-M, e.g., M = W(CO)5] are more stable (but also not isolable) and react more selectively. As a model example we describe the transformation of (3,4-dimethyl-l-phenylphosphole)(pentacarbonyl)tungsten 4 with dimethyl acetylenedicarboxylate to the 7-phosphanorbomadiene derivative 5.11 The (phosphinidene)(pentacarbonyl)tungsten complex 6 is generated from the latter by a [4+ 1]-cycloreversion and trapped with diphenylacetylene via a [2+1]-cycloaddition to furnish the triphenyl-2-phosphirene complex 7 (Scheme l).12... 

Volatile tungsten(IV) 0x0 alkoxide//3-diketonate complexes of structural type W(0)(/1-diketonate)(OR)3 (R = t-Pr, f-Bu /3-diketonate = acac, 32, hfac, 33) are excellent precursors for the low-pressure CVD of electrochromic tungsten oxide films, as shown by Chisholm and coworkers. While with 32b and 33b the produced films were transparent and blue, 32a and 33a led to pale yellow layers which darkened to pale green-blue when exposed to air for several days. With the exception of 33a, all other tungsten complexes formed good-quality films with respect to adhesion and surface coverage however, no surface selectivity for quartz, pyrex or ITO was observed. 

In the Diels-Alder reaction, the typical dienophile will display lower stereoselectivity with acyclic dienes than with cyclic dienes. This is the case for the reactions of both methyl ciotonate and the prope-nyl tungsten complex (52b) with Danishefsky s diene (Scheme 9). Carbene complex (52b) gives a 58 42 selectivity in favor of the exo adduct (66), while methyl crotonate gives a similar selectivity of 66 34 in favor of the exo adduct (63) (64 and 65 are both derived from elimination of methanol from the endo adduct). This example reveals that the Diels-Alder reactions of caibene complexes are viable with functionalized and highly electron rich dienes. Furthermore, although the reaction of the carbene complex (52b) with Danishefsky s diene is not stereoselective, it does occur with a much greater rate and efficiency than that for the organic ester for which it can serve as synthon. 

Dichloroiodo)benzene is commonly used as a reagent for the chlorination or oxidation of various transition metal complexes. Numerous examples of oxidative chlorination of various molybdenum and tungsten complexes with PhICl2 have been reported in the literature [67-74]. In a representative example, oxidative decarbonylation of the cyclopentadienyl complexes 83 with one equivalent of PhICh selectively yields the respective molybdenum(IV) or tungsten(IV) complexes 84 in excellent yields (Scheme 3.31) [73]. 

Molybdenum and tungsten complexes as models for oxygen atom transfer enzymes have been deployed in the full catalytic cycle from Scheme 4.3 predominantly in the early days of this field of research. A selection of the respective determined Michaelis-Menten parameters were expertly reviewed by Holm et al. Since in some cases both forms of model complexes (M and M mimicking the fully reduced or fully oxidized active sites, respectively) are isolable and available in a sufficient amount, the isolated half-reactions are much more often investigated than the whole catalytic cycle. This means that either the reduced form of the enzyme model is oxidized by an oxygen donor substrate like TMAO or the oxidized form is reduced by an oxygen acceptor substrate like triphenylphosphine (PhgP). The observed kinetic behaviour is in some cases described to be of a saturation type. An observation which... 

Similar reactions proceed in the presence of molybdenum and tungsten complexes as catalysts. These catalysts are more reactive than chromium catalysts but the reaction selectivities are low. The induction period is usually one hour. The induction period is thought to be caused by the ligand exchange reaction of an aryl group and diene with the obvious formation of an intermediate as shown in eq. 

---