Tungsten complexes allyl

The application of tungsten-<7r-allyl complexes provided yet another elegant and efficient access to paraconic acids (Scheme 29) [68]. The propargylic al-... 

Allyl complexes (pseudo-rotations, dynamic NMR studies, 1, 416 with tungsten carbonyls and isocyanides, 5, 688-689 rc-Allyl complexes with Cr, 5, 305 with Cr(II), 5, 300 with Cr(III), 5, 300 and cyclodextrins, 12, 789 in enyne carbometallation, 10, 328 with rhodium, 7, 220-221 (j-Allyl complexes, with iron, 6, 98 5-Allyldiisopinocampheylboranes, in asymmetric allylboration, 9, 198... 

The spectra of the tetra-T)3-allyl complexes of Mo, W, and Zr (Table XI) show significant differences. Whereas that of the zirconium complex has only two signals (one for the meso and one for the terminal carbon atoms), the spectra of the molybdenum and tungsten complexes consist of three... 

Several years ago transition metal mediated reactions in the area of C-glycoside synthesis were primarily limited to palladium and to a lesser extent nickel and manganese. Over the last few years several other metals, including chromium, molybdenum, tungsten, cobalt, and rhodium, have been utilized in C-glycoside synthesis. This section discusses the chemistry of palladium, which is divided into Stille-type couplings and 7T-allyl complexes. This is followed by considerations of the chemistry of chromium and the above-listed metals. A review by Frappa and Sinou entitled Transition Metal Catalysed Fimctionalization at the Anomeric Center of Carbohydrates appeared in early 1997 [55]. 

Mepz), Tp" = HB 3-Pr -4-Brpz)3. >=Quoted in <93CRV943> as ref. 435 (in press), tungsten pyrazolylborate 7r-allyl complexes (2D and H NOESY NMR studies). 

Keywords Allylic substitution, AUylation, Allylic alkylation, 7i-Allyl complexes. Palladium, Molybdenum, Tungsten, Iridium... 

LactonizattonT Lactone formation from propargylic alcohols and homologs in which the other propargylic position is substituted with a tungsten residue is readily achieved by treatment with triflic acid (0.25 equiv) in CH Clj at -40°. The tt-allyl complexes are demetallated on further treatment with CFjCOOH. 

When cr-allyl complexes of h -cyclopentadienyltricarbonylmolyb-denum (10) and tungsten (//) are irradiated with ultraviolet light, a (t-tt... 

Many allyl complexes of chromium group elements are known, particularly those possessing mixed cyclopentadienyl and carbonyl ligands. Chromium forms compounds of the composition [Cr(allyl)3], while molybdenum and tungsten, like metals of the second and third transition metal series of the preceding groups, coordinate a larger... 

The following metal compounds are used for the preparation of the catalysts oxides, metal carbonyls, halides, alkyl and allyl complexes, as well as molybdenum, tungsten, and rhenium sulfides. Oxides of iridium, osmium, ruthenium, rhodium, niobium, tantalum, lanthanum, tellurium, and tin are effective promoters, although their catalytic activity is considerably lower. Oxides of aluminum, silicon, titanium, manganese, zirconium as well as silicates and phosphates of these elements are utilized as supports. Also, mixtures of oxides are used. The best supports are those of alumina oxide and silica. 

However, the possibility of a hydride mechanism is not excluded. Irradiation of [6-27] may homolytically cleave the metal-carbon bond to form a 7r-allyl complex [6-28]. Hydride ion addition to the 7r-complex [6-29] gives an isopropyl complex, C5H5Fe(CO)3—CH(CH3)2 This process is designated as a n-a rearrangement. Analogous 7r-allyl and olefin complexes of molybdenum and of tungsten are known. 

Total syntheses of racemic protolichesterinic and roccellaric acids were achieved with the use of a tungsten-7i-ally complex in the key step (see Scheme) (Chen M-J, Liu R-S (1998) Efficient Total Syntheses of ( )-Protolichesterinic Acid and ( )-Roccellaric Acid via Tungsten-7t-allyl Complexes. Tetrahedron Letters 39 9465). 

Treatment of bis(pyridine) complexes of molybdenum and tungsten, M(f/ -allyl)Cl(CO)2(py)2 (M = Mo, W) with equimolar amounts of lithium amidinates Li[RC(NPh)2] (R = H, Me) afforded amidinato complexes of the type M(r -allyl)[RC(NPh)2](CO)2(py) (M = Mo, W). Reactions of the latter with acetonitrile, PEts, and P(OMe)3 have been investigated Free amidines react with M(r -allyl)Cl(CO)2(NCMe)2 according to Scheme 124 to give the corresponding bis(amidine) complexes. ... 

The report by Basset and co-workers on the metathesis of sulphur-containing alkenes using a tungsten alkylidene complex, mentioned previously for the acyclic cross-metathesis reaction (see Sect. 2.2), also contained early examples of ring-opening cross-metathesis of functionalised alkenes [20]. Allyl methyl sulphide was reacted with norbornene in the presence of the tungsten catalyst 5, to yield the desired ring-opened diene 35 (Eq. 29). 

If olehn metathesis is to be conducted in solution, solvents of low Lewis-basicity will generally give the best results (CH2CI2 > toluene > THF). As discussed above, metathesis is initiated by the formation of a jt-complex between the metal and the alkene. Hence, other nucleophiles will compete with the alkene for these coordination sites and in some systems even THF can lead to complete deactivation of the catalyst [786]. Tungsten-based catalysts which can even metathesize allyl thioethers have, however, been described [787]. 

---