Alkene metathesis reactivity, structure

Carbenes are both reactive intermediates and ligands in catalysis. They occur as intermediates in the alkene metathesis reaction (Chapter 16) and the cyclopropanation of alkenes. As intermediates they carry hydrogen and carbon substituents and belong therefore to the class of Schrock carbenes. As ligands they contain nitrogen substituents and are clearly Fischer carbenes. They have received a great deal of attention in the last decade as ligands in catalytic metal complexes [58], but the structural motive was already explored in the early seventies [59],... 

Alkene metathesis has grown from a niche technique to a common component of the synthetic organic chemistry toolbox, driven in part by the development of more active catalyst systems, or those optimized for particular purposes. While the range of synthetic chemistry achieved has been exciting, the effects of structure on reactivity have not always been particularly clear, and rarely quantified. Understanding these relationships is important when designing new catalysts, reactions, and syntheses. Here, we examine what is known about the effect of structure on reactivity from two perspectives the catalyst, and the substrate. The initiation of the precatalyst determines the rate at which active catalyst enters the catalytic cycle the rate and selectivity of the alkene metathesis reaction is dependent on how the substrate and active catalyst Interact. The tools deployed in modern studies of mechanism and structure/activity relationships in alkene metathesis are discussed. 

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