anuj joshi - web hostingweb.uvic.ca/~djberg/chem423/anuj_presentation.pdfolefin metathesis • 1950s...
TRANSCRIPT
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ANUJ JOSHI
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Metathesis in the general sense is the formation of a product that has exchanged bonds between starting materials.
METATHESIS REACTIONS:
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OlefinMetathesis• 1950s– Inthepresenceofvariousorganometallocompounds, olefinmetathesis
(wheretheRgroupsofolefinswereswappedwiththeother)occurred
• 1970s– Chauvinandcolleaguesproposedmechanismthatseemedtofit– Likeadance
• 1980s– SchrockandGrubbssynthesized thefirstmetallo-carbenecompoundsthatwereshowntocatalyzeolefinmetathesis
• Threetypesofolefinmetathesis– Ringclosingmetathesis(RCM)– Ringopeningmetathesis(ROM)– Crossmetathesis(CM)
Image from Greco, G.E. Nobel Chemistry in the Laboratory. J. Chem. Ed. 2007,84(12), 1996
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2005
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Uses a molybdenumcatalyst
Shows greater reactivity than the first generation Grubbs catalyst, particularly with sterically demanding and electron deficient olefins
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• Commercially available• Reasonably stable toward H2O, O2, and
minor impurities ease of handling• Lower reactivity vs. Molybdenum imido
alkylidene catalyst• High functional group tolerance
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Increased initiation would permit lower catalyst loadings and reaction temperatures and improve efficiency.
Addition of phosphine scavengers (HCl, CuCl).However, these additives limit catalyst lifetime and functional group tolerance.
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J. AM. CHEM. SOC. 2007, 129, 7961-7968
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Stereoselctive RCM
RCM of temporarily connected dienes
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Fujimure, O.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118, 2499-2500.
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Experiments done by Chatterjee et al showed that the various olefin metathesis substrates could be classified by their relative reactivity
Type I: formed homodimers rapidlyType II: formed homodimers slowlyType III: didn’t form homodimers, participate in CMType IV: spectator olefin, no CM reaction
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Reactingtwoolefinsfromdifferentgroupscouldyieldstereospecific,easilypredictableproductsingoodyield
Reactivityofolefinsdependedonthingssuchassterics aswellasdeactivatingelectron-withdrawinggroups
Chatterjee, A. K. A General Model for Selectivity in Olefin Metathesis. J. Am. Chem. Soc. 2003,125, 11360-11370.
Type I Type II Type III Type IV
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The most effective selective CM reactions involve the reaction of a type I olefin with a less reactive type II or III olefin.