Equilibria and stereoselectivity

The olefin metathesis reaction clearly provides a route for the establishment of equilibrium between all species formed by exchange of alkylidene moieties. Thermodynamic data may therefore be used to predict the ultimate equilibrium position alternatively, experimental equilibrium concentrations may be used to provide new thermodynamic data and to check existing data. 

In the simplest case, such as eqns (20) or (21), the system contains a single equilibrium and the heat of reaction is very small. 

In the more general case there will be a number of interlocking equilibria, as in system (22), and the route by which equilibrium is ultimately reached will depend on the relative rate constants of the individual components of the mechanism. 

With some catalysts the horizontal equilibria of (22) are established much more quickly than the vertical cis/trans equilibria and, in the initial stages, such metatheses may be very stereoselective. With other catalysts or with other substrates, the reverse is often true and there is little stereoselectivity. More complex still are the reactions involving more than two types of alkylidene moiety, derived from two or more reactant alkenes. Reactions of acyclic olefins are discussed in detail in Ch. 5-9. 

Cycloalkenes offer a wider range of possible equilibria, since the products may consist of rings or chains in a variety of sizes, and with different proportions and distributions of cis and trans double bonds. Consideration of such equilibria may be simplified by regarding the monomer as the first member of the cyclic oligomeric series see (23). 

---