Bisalkoxide complexes

When bisalkoxide complexes are first obtained from [Re(C-t-Bu)(CH-t-Bu)Cl2]x. exclusively the syn rotamer is observed (eq 16). When a solution of the syn rotamer is heated, a mixture of syn and anti rotamers is obtained, the ratio varying with the steric bulk and electronic nature of the ligands. The Ha resonance for the anti rotamer is found downfield of that for the syn rotamer and has a relatively high value for... 

The anti isomers have been observed in the solution for bisalkoxide complexes when the photolysis of syn isomers was conducted at low temperatures [57]. syn Isomers are the norm for MAP species, although anti isomers have now been observed through the aforementioned photolysis of the syn species. 

The catalysts for these reactions are derived from both molybdenum and ruthenium, although products with higher enantiomeric excess have been obtained to date with molybdenum catalysts. As noted above, this molybdenum catalyst is based on the structure of Schrock s original bisalkoxide catalyst, but with an optically active bisnaphtholate in place of the two tertiary fluoroalkoxides. These catalysts can be used as isolated complexes or they can be generated in situ from the imido carbene molybdenum bistriflate DME complex and the diolate, as shown in Equation 21.16, or from the bispyrrolyl complex, as shown in Equation 21.17. ... 

The rates of interconversion of the syn and anti isomers were found to be one to two orders of magnitude faster for W MAP complexes than for Mo MAP complexes. Little is known about the rates of interconversion of syn-and anti-protons for MAP species [2c], although the rates of syn/anti interconversions in bisalkoxide Mo imido alkylidene complexes have been found to vary over approximately six orders of magnitude [57]. It is not surprising that the sterically demanding NArj jg52 ligand would destabilize the syn isomer for steric reasons and lead to mixtures that contain both syn and anti species, as observed. 

Until recently, the vast majority of high-oxidation state Mo and W alkylidene complexes active for olefin metathesis were bisalkoxides, bisaryloxides, or biden-tate variations of aryloxides (biphenolates and binaphtholates). Calculations have suggested that catalysts that contain a stereogenic metal center, especially MAP complexes, are more efficient for electronic reasons [7]. 

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