Alkene metathesis precatalysts initiation

Typical alkene metathesis precatalysts take the form displayed in Figure 2.3, consisting of a ruthenium(II) center, a carbene with substituent R, two anionic ligands X (typically chloride), a nondissociating l and L (typically a trialkylphosphine or NHC), and a dissociating ligand, which is most often either a phosphine or a chelating alkoxyarene. While the nature of X, L,, and R all influence the initiation rate and mechanism, it is the nature of L and X that detemiine the catalytic activity of the active species itself complexes G2, M2, and GH2 all produce the same active species, albeit via different mechanisms and at different rates. 

Catalysts continue to be developed for particular alkene metathesis applications, such as stereoselective cross metathesis. These precatalysts are tasked with selective metathesis and turnover, but must maintain Z-selectivity throughout the reaction. New ruthenium(II) species featuring a Ru-C bond have been recruited for this purpose. In a short time, reactivity gains and improved initiation rates have been achieved in this new area by manipulation of the X-type ligand. 

The development of new precatalysts for alkene metathesis has been a highly active and creative area of research. The number of new precatalysts being produced continues at a dizzying pace. However, few meet the needs of stability, efficiency, and activity required for a useful catalyst. That said, there are still many useful precatalysts available to choose from. Precatalyst selection depends on many considerations, but one must take into account the initiation rate, either as a pure measure of the efficiency of conversion to an active form of the catalyst or to help determine initial precatalyst loading. It is important to note, however, that a fast initiator does not always finish the race. This is because initiation alone does not determine how well the catalyst is matched to the reactivity of the reactants in a given application. However, initiation rates can help guide catalyst selection and will narrow the field to a few select precatalysts to screen for a desired application. 

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. 

This chapter will focus almost exclusively on alkene metathesis catalyzed by well-defined homogeneous ruthenium-catalyst systems and is divided into three sections (1) a discussion of how precatalyst structure affects the rate and mechanism of initiation in two key series of metathesis precatalysts (2) a discussion of how substrate structure, both close to and remote from the alkene termini, affects the rate and selectivity of alkene metathesis in synthetic chemistry and (3) the tools that have been used by experimental and theoretical chemists to study alkene metathesis reactions. In each case, the discussion will be focused on the specific topics interested readers are referred to a recent article which covers a wider range of the mechanistic aspects of alkene metathesis with ruthenium complexes, albeit in less depth. 

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