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This process occurs via formation of a new alkylidene through one round of [2 2] cycloaddition and cycloreversion.Association and dissociation of a phosphine ligand also occurs in the case of Grubbs catalysts.Some examples are shown below (the red alkene indicates C-C bond formed through RCM).
Ring-closing metathesis, or RCM, is a widely used variation of olefin metathesis in organic chemistry for the synthesis of various unsaturated rings via the intramolecular metathesis of two terminal alkenes, which forms the cycloalkene as the E- or Z- isomers and volatile ethylene.
Tsuji describes the olefin metathesis reaction as “…potentially useful in organic synthesis” and addresses the need for the development of a more versatile catalyst to tolerate various functional groups.
The mechanism can be expanded to include the various competing equilibrium reactions as well as indicate where various side-products are formed along the reaction pathway, such as oligomers.
Although the reaction is still under thermodynamic control, an initial kinetic product, which may be dimerization or oligomerization of the starting material, is formed at the onset of the reaction as a result of higher catalyst reactivity.
Overall, it was shown that metal-catalyzed RCM reactions were very effective in C-C bond forming reactions, and would prove of great importance in organic synthesis, chemical biology, materials science, and various other fields to access a wide variety of unsaturated and highly functionalized cyclic analogues.
Initiation occurs through substitution of the catalyst’s alkene ligand with substrate.In macrocycles, the E-isomer is often obtained as a result of the thermodynamic bias in RCM reactions as E-isomers are more stable compared to Z-isomers.As a general trend, ruthenium NHC (N-heterocyclic carbene) catalysts favor E selectivity to form the trans isomer.This in part due to the steric clash between the substituents, which adopt a trans configuration as the most stable conformation in the metallacyclobutane intermediate, to form the E-isomer.The synthesis of stereopure Z- isomers were previously achieved via ring-closing alkyne metathesis.Increased catalyst activity also allows for the olefin products to reenter the catalytic cycle via non-terminal alkene addition onto the catalyst.Due to additional reactivity in strained olefins, an equilibrium distribution of products is observed; however, this equilibrium can be perturbed through a variety of techniques to overturn the product ratios in favor of the desired RCM product.Smaller rings, between 5 and 8 atoms, are more thermodynamically favored over medium to large rings due to lower ring strain.Ring strain arises from abnormal bond angles resulting in a higher heat of combustion relative to the linear counterpart.A kinetic product distribution could lead to mostly RCM products or may lead to oligomers and polymers, which are most often disfavored.With the advent of more reactive catalysts, equilibrium RCM is observed quite often which may lead to a greater product distribution.