Cross Metathesis Reactions

Cross Metathesis Reactions-29
For example, chiral molybdenium complex 10 catalyzes the desymmetrization of vinyl ethers to form dihydropyrans with moderate to good enantioselectivity (Eq. Further modification of heterocycles established through desymmetrizing RCM affords chiral and nonracemic acyclic structures containing carbon-carbon double bonds.For example, Schrock-type complex 11 catalyzes the cyclization of an allylborane, which undergoes oxidation to yield a chiral diol with very high stereoselectivity and moderate yield (Eq. The vast majority of olefin metathesis reactions are catalyzed by complexes of either molybdenum (Schrock type) or ruthenium (Grubbs type).

For example, chiral molybdenium complex 10 catalyzes the desymmetrization of vinyl ethers to form dihydropyrans with moderate to good enantioselectivity (Eq. Further modification of heterocycles established through desymmetrizing RCM affords chiral and nonracemic acyclic structures containing carbon-carbon double bonds.

Treatment with hydrogen peroxide and sodium hydroxide yields stereodefined allylic diols (Eq. Unsaturated lactams are a biochemically important class of heterocycles that can be prepared via ring-closing metathesis.

Catalyst 1 is effective in the preparation of five- or six-membered lactams, but crotonamides must be used as unsubstituted α,β-unsaturated amides coordinate to molybdenum, preventing reaction (Eq. Unsubstituted α,β-unsaturated esters can likewise coordinate to the metal center and prevent reaction.

It remains important to consider the substitution pattern of the alkene and the activity and functional-group compatibility of the catalyst.

Grubbs-type catalysts are problematic in cyclizations of amines due to coordination of the Lewis-basic nitrogen to ruthenium.

In RCM reactions, reactants are typically designed so that the desired cyclic alkene is accompanied by a small gaseous olefin such as ethylene or propene, the loss of which drives the reaction forward.

Cross Metathesis Reactions

Highly dilute conditions discourage intermolecular metathesis and thereby also promote RCM.

Intramolecular cycloaddition yields new metallacyclobutane 9, which undergoes cycloreversion to expel the metal carbene catalyst and generate the product cyclic alkene.

Because this mechanism relies on both [2 2] cycloadditions and retro-cycloadditions, in general each step is reversible, resulting in an equilibrium mixture of olefinic products.

Heterocycles containing a carbon-carbon double bond can also be prepared via RCM.

Although nitrogen- and oxygen-containing rings are the most common products, heterocycles containing phosphorus, silicon, boron, sulfur, and other elements have also been prepared.

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