Tungsten aryloxides

The formation of rings that contain a thioether linkage does not appear to be catalyzed efficiently by Ru, even when terminal olefins are present. On the other hand, molybdenum appears to work relatively well, as shown in Eqs. 30 [207] and 31 [208]. Under some conditions polymerization (ADMET) to give poly-thioethers is a possible alternative [26]. Aryloxide tungsten catalysts have also been employed successfully to prepare thioether derivatives [107,166,169]. Apparently the mismatch between a hard earlier metal center and a soft sulfur donor is what allows thioethers to be tolerated by molybdenum and tungsten. Similar arguments could be used to explain why cyclometalated aryloxycarbene complexes of tungsten have been successfully employed to prepare a variety of cyclic olefins such as the phosphine shown in Eq. 32 [107,193]. 

Macrocyclic 1,2-diazines were also investigated as ligands <2000AGE1968>. Especially interesting is the reaction of 1,2-diazines (pyridazine 26, phthalazine, and benzo[clcinnoline) with tungsten(ii)aryloxide complexes 33 as the N-N bond is cleaved (Equation 6) <2002CC2482>. 

The reaction must involve an alkoxide or aryloxide of a highly electropositive metal, i.e. the M—O bond must have high ionic character. Alkoxides of molybdenum and tungsten of formula M2 (OR)6, which have strong M—O a- and re-bonds will not, for example, react with alkyl halides to yield ether.261... 

In addition to the well-established chemistry of the M—O bond in M—O—C derivatives (4, 6, 7, 14, 28-32, 38, 39), the chemical reactivity of the C —O bond in aryloxide and related derivatives of mainly rhenium and tungsten (43) was recently highlighted by Mayer (164). Although the main focus of this con-... 

The dimethylphenylphosphonio- and triphenylphosphonio-ketenyl products have similar spectroscopic characteristics. Both contain a stereogenic centre at tungsten and display 1 1 1 NMR patterns for the Tp arms. Characteristic absorptions attributable to vco appear at ca. 1870 and 1710 cm i.e., at notably low frequencies, presumably reflecting the 7r-dative role of the aryloxide. ... 

The tungsten alkylidynes Tp W( = CH)(CO)2 and Tp W(=CPh)(CO)2 also arise from side reactions of phosphonium alkylidyne complexes with aryloxide nucleophiles from reaction of the quaternary phosphonium salts with adventitious hydroxide to give a dissociated alkyl anion or a product resulting from a 1,2-phenyl migration. At low temperature, phenyl migration is inhibited and only the terminal dissociation product is observed. At room temperature, both phenyl and C-H alkylidyne products form in an approximate 1 1 ratio. 

Undoubtedly the most active homogeneous catalyst systems are the well-defined metal alkylidene complexes synthesised in the laboratories of Schrock, Basset and Grubbs. First examples are the complexes I [29] and II [30]. The bulkiness of imido and aryloxide ligands probably slows down dimerization of these electron-deficient organometallic complexes to inactive complexes and prevents to some extent the coordination of the functional group to the tungsten atom [37]. 

A general mediod for the synthesis of aryloxy(chloro)neopentylidene-tungsten complexes involves the reaction of WCl4(OAr)2 (OAr = 2,6-disubstituted phenoxide) [20] with 1 equiv or 1.5 equiv of MgNp2(dioxane) (Np=CH2C(CH3)3) [11, 21] (Scheme 1). The reaction proceeds via a double (or triple) kylation of tungsten followed by an a-H abstraction and elimination of neopentane [22]. In the particular case of the 2,6-diphenyl-phenoxide complexes, the activation of m ortho C-H bond in one of the phenyl substituents of the aryloxide ligands leads to the elimination of HCl with the formation of a cyclometalated compound (Scheme 2) [23, 24]. 

Table 6.3 W-OAr distances for various aryloxide derivatives of tungsten...

The mononuclear aryloxide chemistry of these metals spans a wide range of formal oxidation states. This is particularly true for tungsten, where the aryloxide ligand has been coordinated to this metal centre in its highest oxidation state, e.g. [WIOCeHiMe-dle]," low valent carbonyl compounds, e.g. [NEt4][W(OPh)(CO)5]," and intermediate redox... 

---