Molybdenum 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]. 

The dihalides react with alcohols leading to the formation of an enormous range of alkoxides and even aryloxides [Mo(NO)(Me2pzb)X(OR)] as detailed in Table 7, The crystal structure of [Mo(NO)(Me2pzb)a(OPr1)] has been determined,125 and the molybdenum shows normal octahedral six-coordination, albeit with a very short Mo-O bond. 

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... 

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