Olefin Metathesis - Reaction Mechanism

Reaction Mechanism

Hérisson and Chauvin first proposed the widely accepted mechanism of transition metal alkene metathesis. The direct cycloaddition of two alkenes is formally symmetry forbidden and thus has a high activation energy. The Chauvin mechanism involves the cycloaddition of an alkene double bond to a transition metal alkylidene to form a metallacyclobutane intermediate. The metallacyclobutane produced can then cyclorevert to give either the original species or a new alkene and alkylidene. Interaction with the d-orbitals on the metal catalyst lowers the activation energy enough that the reaction can proceed rapidly at modest temperatures.

Like most chemical reactions, the metathesis pathway is driven by a thermodynamic imperative; that is, the final products are determined by the energetics of the possible products, with a distribution of products proportional to the exponential of their respective energy values. In olefin metathesis, however, this is especially relevant since all the possible products have similar energy values (all of them contain an olefin). Because of this the product mixture can be tuned by reaction conditions, such as gas pressure and substrate concentration. In some cases a given reaction can be run in either direction to near completion.

Cross metathesis and Ring-closing metathesis are often driven by the entropically favored evolution of ethylene or propylene, which are both gases. Because of this CM and RCM reactions often use alpha-olefins. The reverse reaction of CM of two alpha-olefins, ethenolysis, can be favored but requires high pressures of ethylene to increase ethylene concentration in solution. The reverse reaction of RCM, ring-opening metathesis, can likewise be favored by a large excess of an alpha-olefin, often styrene. Ring opening metathesis usually involves a strained alkene (often a norbornene) and the release of ring strain drives the reaction. Ring-closing metathesis, conversely, usually involves the formation of a five- or six-membered ring, which is energetically favorable; although these reactions tend to also evolve ethylene, as previously discussed. RCM has been used to close larger macrocycles, in which case the reaction may be kinetically controlled by running the reaction at extreme dilutions. The same substrates that undergo RCM can undergo acyclic diene metathesis, with ADMET favored at high concentrations. The Thorpe-Ingold effect may also be exploited to improve both reaction rates and product selectivity.

Cross-metathesis is synthetically equivalent to (and has replaced) a procedure of ozonolysis of an alkene to two ketone fragments followed by the reaction of one of them with a Wittig reagent.