Ring-closing metathesis reaction cycle

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

Ring-closing metathesis (RCM) is an extremely powerful method for transforming acyclic dienes into unsaturated cyclic systems [1], The process is believed to proceed via the catalytic cycle outlined in Scheme 1, and the principal driving force for the reaction is the gain in entropy resulting from the loss of ethene. 

In 2003, Guillemine and Yao reported in two independent papers the synthesis of IL-supported catalysts for ring-closing metathesis, whereas a IL-supported palladium complex was found to catalyze the Heck reactions with good recyclability of up to ten cycles. ... 

A recent noteworthy example is the 14-electron metathesis catalyst 72 reported by Piers, which have no ancillary ligand to dissociate. " As expected, this catalyst initiates extremely rapidly, displaying initial rates of ring-closing metathesis (RCM) better than the bis-pyridine substituted Grubbs catalysts. Furthermore, reaction of the catalyst with 2.2 equivalents of ethylene at — 50°C in CD2CI2 allowed the observation and characterization of the metallocyclobutane 73 formed on expulsion of the vinyl phosphonimn salt (Equation (3.13)). " This represents the first observation of the generally accepted intermediate in the catalytic cycle. 

Complex molecules have also been synthesized by sequential use of the orga-nocatalytic a-amination of aldehydes by azodicarboxylates catalyzed by some of the systems shown above and by other reaction processes, such as the Passerini reaction [40], Homer-Wadsworth-Emmons olefination [41], Wittig olefination [42], and allylation reaction followed by a ring-closing metathesis [43], In addition, the combination of catalysts in cycle-specific organocascade processes [44] has allowed the synthesis of chiral complex molecules with good results. 

SCHEME 17.1 The ring-closing ene-ene metathesis reaction catalytic cycle. 

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