Tungsten //-complexes

There is no appreciable difference between the chromium, molybdenum, and tungsten homologs. The strong electron-donating power of the ONMe4 group can be pointed out. 

The potential values of the single oxidation step for the ferrocenyl carbyne-tungsten complexes are reported in Table 7-11. 

The tungsten complexes are slightly easier to oxidize than the corresponding chromium and molybdenum complexes. 

The tetrahedrally coordinated tungsten-carbyne complex (j/ -C5H5)Fe[t -C5H4-CsW(CO)2(j -C5H5)] also undergoes one-electron oxidation [E° = -1-0.64 V) in DME solution [34], 

Like the molybdenum complexes, a series of monoferrocenyl-, diferrocenyl- and triferrocenyl-phosphine pentacarbonyltungsten complexes have been prepared. Each ferrocenyl ligand undergoes reversible one-electron oxidation, whereas the tungsten moiety undergoes irreversible oxidation. The relevant potential values are reported in Table 7-12. 

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The only known trivalent tungsten complex is of the type M(I)2(W2Cl2). It is prepared by the reduction of strong hydrochloric acid solutions of K2WO4 with tin. If the reduction is not sufficient, a compound containing tetravalent tungsten, K2(WCl (OH)) [84238-10-0] is formed (57). 

Although distibenes, the antimony analogues of azo compounds, have never been isolated as free, monomeric molecules (130), a tungsten complex, tritungsten pentadecacarbonyl[p.2-Tj -diphenyldistibene] [82579-41-7] C2yH2Q025Sb2W2, has been prepared by the reductive dehalogenation of phenyldichlorostibine (131) ... 

As expected, the Sb—Sb bond distance in this complex is significandy shoitei than the Sb—Sb single bond distance. Chioinium and tungsten complexes of dialkyldistibines have also been isolated (132). 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

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],... 

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. 

Very recently, synthesis and structure of molybdenum and tungsten complexes of the relatively unhindered disilene Si2Me4 were reported. The x-ray structure of 84 shows a metallacyclosilane structure with W — Si = 2.606(2) A and Si —Si = 2.260(3) A. The W — Si bond length is within the range of various estimates of the Si and W covalent radii and the Si —Si distance falls midway between the expected values for a single (2.35 A) and a double bond (2.14 A) (Fig. 13). 

Tungsten complexes, 3, 973-1015 alkoxy carbonyl reactions, 2, 355 alkyl alkoxy reactions, 2, 358 amides... 

Another example of a [4S+1C] cycloaddition process is found in the reaction of alkenylcarbene complexes and lithium enolates derived from alkynyl methyl ketones. In Sect. 2.6.4.9 it was described how, in general, lithium enolates react with alkenylcarbene complexes to produce [3C+2S] cycloadducts. However, when the reaction is performed using lithium enolates derived from alkynyl methyl ketones and the temperature is raised to 65 °C, a new formal [4s+lcj cy-clopentenone derivative is formed [79] (Scheme 38). The mechanism proposed for this transformation supposes the formation of the [3C+2S] cycloadducts as depicted in Scheme 32 (see Sect. 2.6.4.9). This intermediate evolves through a retro-aldol-type reaction followed by an intramolecular Michael addition of the allyllithium to the ynone moiety to give the final cyclopentenone derivatives after hydrolysis. The role of the pentacarbonyltungsten fragment seems to be crucial for the outcome of this reaction, as experiments carried out with isolated intermediates in the absence of tungsten complexes do not afford the [4S+1C] cycloadducts (Scheme 38). 

Other reactions leading to azetidines include the dialkylation of chromium or tungsten complexes of aminocarbenes with 1,3-diiodopropane under phase-transfer conditions <96CL827> and the regio- and stereo-specific reaction of dimethylsulfoniumethoxy-carbonylmethylide with 2-substituted or 2,3-disubstituted N-arylsulfonylaziridines to afford S (R 7 H, = H) or 5 (R and R H) respectively, generally in useful yields <95JCS(P1)2605>. 

The amine substituent in the corresponding tungsten complex reduces AE to 21.3 kcal/mol, while the effect on AE is negligible. 

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