Ruthenium-based olefin metathesis mechanisms

MECHANISM OF RUTHENIUM BASED OLEFIN METATHESIS CATALYSTS... 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

In order to improve the catalytic performance of ruthenium-based metathesis complexes, extensive theoretical [9,10] and experimental [11,12] studies were conducted. The generally accepted mechanism involves the formation of a highly active and unstable 14-electron complex through the dissociation of one of the phosphine ligands (Scheme 11.1) [13-17]. This actual catalytic species allows the binding of incoming olefin to form the Chauvin metallacyclobutane [18] intermediate and eventually the reaction products. 

In the year 2005, Nobel Prize in Chemistry was awarded to Yves Chauvin, Robert H. Grubbs, and Riehard R. Schrock for the development of the metathesis method in organie synthesis [41-43], It is a general view that, this reaction revolutionized different fields of synthetic chemistry. Chauvin proposed the carbene mechanism to explain this reaction [44] and Schrock prepared the first well-defined highly active metathesis catalysts [45], while, Grubbs developed ruthenium based active and air stable catalysts, which are tolerant to various functional groups in the presence of olefins [46]. 

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