Evaluation of Molybdenum and Tungsten Metathesis Catalysts for Homogeneous Tandem Alkane Metathesis

Over 40 molybdenum and tungsten imido alkylidene mono(alkoxide) mono(pyrrolide) (MAP) or bis(alkoxide) olefin metathesis catalysts were examined in combination with Ir-based pincer-type catalysts for the metathesis of n-octane. The imido group, alkoxide, and metal in the metathesis catalysts were all found to be important variables. The best catalyst was W(NAr)(CHR)(OSiPh3)(2) (Ar = 2,6-diisopropylphenyl), which performed about twice as well as the only previously employed catalyst, Mo(NAr)(CHR)[OCMe(CF3)(2)](2). Product yields decreased at temperatures greater than 125 degrees C, most likely because of the instability of the metathesis catalysts at such temperatures. POCOP Ir catalysts gave higher yields than PCP Ir catalysts, although the latter exhibited some selectivity for formation of tetradecane. Eight catalysts were synthesized in situ through addition of alcohols to bis(2,5-dimethylpyrrolide) complexes; in situ catalysts were shown to perform approximately as well as the isolated complexes, which suggests that 2,5-dimethylpyrrole is not detrimental to the alkane metathesis process and that potential catalysts can be screened more conveniently in this way.