ANUJ JOSHI

Metathesis in the general sense is the formation of a product that has exchanged bonds between starting materials.

METATHESIS REACTIONS:

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

2005

Uses a molybdenumcatalyst

Shows greater reactivity than the first generation Grubbs catalyst, particularly with sterically demanding and electron deficient olefins

• Commercially available• Reasonably stable toward H2O, O2, and

minor impurities ease of handling• Lower reactivity vs. Molybdenum imido

alkylidene catalyst• High functional group tolerance

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.

J. AM. CHEM. SOC. 2007, 129, 7961-7968

Stereoselctive RCM

RCM of temporarily connected dienes

Fujimure, O.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118, 2499-2500.

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

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

The most effective selective CM reactions involve the reaction of a type I olefin with a less reactive type II or III olefin.