Prevention of Undesirable Isomerization during Olefin Metathesis S.
Most ring-closing metathesis reactions are carried out at fairly high dilution of the substrate (10 - 50 mM) with catalyst loadings of 5 - 10 mol % and at slightly elevated temperatures (25 - 110 ºC). Molybdenum catalyst 1 exhibits extreme sensitivity to air and water such that use of a glovebox is ideal. On the other hand, ruthenium catalysts are more stable in air and Schlenck tubes are typically used. Standard workup involves concentration of the reaction mixture, aqueous extraction, and purification via silica gel chromatography, recrystallization, or distillation. Because the standard procedure can leave behind traces of ruthenium, more rigorous workup procedures have been developed that use additional ligands, supercritical fluids, and mesoporous silicates to decrease ruthenium concentrations to extremely low levels.
The Enyne-Metathesis Reaction 4.
Chapter 3 details the incorporation of mesoionic carbenes (MICs) into ruthenium metathesis catalysts. The activity of these catalysts in several metathesis assays was measured and correlated to their initiation rates. The protonolysis of a Ru-MIC bond and the incorporation of this reaction into an acid-activated catalyst are also described.
In year two, we have used this reaction development tostop first generation catalysis (one paper published) and to expand thereaction scope in enyne metathesis, publishing three new methodology papers.
Jan 19, 2011 Alkyne Metathesis Reaction Mechanism.
Chapter 4 focuses on addressing the selectivity challenges associated with olefin metathesis, particularly during RCM macrocyclization reactions where E/Z mixtures are typically obtained.
Olefin Metathesis Grubbs Reaction.
Although initial examples of ring-closing metathesis used poorly defined metal catalysts, subsequent development of Schrock-type molybdenum catalysts such as 1 and Grubbs-type ruthenium catalysts such as 2 - 6 greatly expanded the scope and utility of RCM (Eq. 2). In general, molybdenum catalysts display high activity but are unstable toward air or water; ruthenium catalysts are less active but exhibit good selectivity and functional-group compatibility. RCM has been employed extensively in organic synthesis to establish both saturated and unsaturated rings; the reaction can be used to form carbocycles or heterocycles.
Covering the complete breadth of the olefin metathesis reaction.
The vast majority of olefin metathesis reactions are catalyzed by complexes of either molybdenum (Schrock type) or ruthenium (Grubbs type). Molybdenum catalyst 1 was developed before the Grubbs-type catalysts and is highly active, but sensitivity of this catalyst to air and water limits its applicability. Ruthenium catalysts 2 and 3 are less active and cannot be recycled, but exhibit better functional-group tolerance than the rather indiscriminate catalyst 1.
Olefin Metathesis, Grubbs Reaction - Organic chemistry
and and (2001) Mechanism and Activity of Ruthenium Olefin Metathesis Catalysts. Journal of the American Chemical Society, 123 (27). pp. 6543-6554. ISSN 0002-7863.
Olefin Metathesis Grubbs Reaction ..
Olefin metathesis involves the exchange of two alkylidene groups to generate two new olefins from one or more starting alkenes. Cleavage of the carbon-carbon double bond is accompanied by the formation of two new carbon-carbon double bonds. This reaction was first observed in 1931, investigated by Du Pont and other manufacturers in the 1950's, and finally defined by Calderon in 1967. Partly due to its relevance to petrochemical industry, olefin metathesis reactions have been investigated extensively. Four general classes of reactions have emerged: cross metathesis, an intermolecular reaction of two alkenes; ring-opening metathesis polymerization (ROMP), in which a cyclic alkene opens to form a polyolefin; ring-opening metathesis (ROM), the opening of a cyclic alkene to form a diene; and ring-closing metathesis (RCM), in which reaction of a diene affords a cyclic alkene and a small olefinic byproduct. RCM is the focus of this article (Eq. 1).