Ruthenium carbene precatalysts

The ruthenium carbene precatalysts are separated into six major families, each of which will be discussed in turn. Every effort has been made to make comparisons both within and across families, but quantitative comparisons are difficult to make in some cases due to a lack of kinetic data. In some cases, conversion data for a particular reaction have been given, though the reader must pay close attention to the conditions used to evaluate particular catalysts because they are rarely identical. In some cases, rate constants have been corrected (by us) to a common temperature using the Eyring equation in these cases, we will note this clearly. 

The overall metathesis activity of this class of ruthenium-carbene catalysts is determined by the relative magnitudes of several rate constants (i) the rate constant of phosphine dissociation (fej), which dictates the rate at which the precatalyst complex enters the catalytic cycle (ii) the ratio of k i/k2. which dictates the rate of catalyst deactivation (by re-coordination of phosphine) versus catalytic turnover (by coordination of olefmic substrate and subsequent steps) and (iii) the rate constant of metallacyclobutane formation (k ), which dictates the rate of carbon-carbon bond formation. 

A relatively new class of alkene metathesis precatalysts has emerged that contains the highly conjugated indenylidene fragment (Chapter 14). The ruthenium carbene is geminally disubstituted and easily prepared. Representative examples of these complexes (32-36) are shown in Figure 9.1. 

The highly conjugated ruthenium indenylidene precatalysts are a unique class of complexes discovered through an alternative synthesis of the carbene moiety (Chapter 14). A wide variety of precatalysts are available with different dissociable L2-type ligands. Recent mechanistic studies have provided initiation rate data that support a dissociative mechanism, with one exception. These initiators have been modified in the aromatic moieties of the H2lMes group, and they show promise in RCM of hindered dienes. 

Mass spectrometry (MS) studies have played a key role in the study of metathesis reactions, particularly in the hands of Chen and coworkers, who have identified intermediates in the catalytic cycle,and probed the energetics of their reactions, using electrospray MS techniques. Species such as 14e ruthenium carbene complexes can be detected by MS in the presence of different alkene substrates, the different carbene products (from CM or ROMP, for example) can be detected. Further, the fragments into which any proposed species can be broken by successively higher lens potentials can be used to check the species structure. In successive and more advanced studies, interpretation of data from the energy-resolved, coUision-induced dissociation cross-section measurements allowed the construction of potential energy surfaces for some steps of the metathesis reaction.Metathesis precatalysts were typically custom-made species, modified with ionic tags, to facilitate detection by MS. 

Ruthenium Precatalysts with N-Heterocyclic Carbene Ligands.238... 

Abstract For many years after its discovery, olefin metathesis was hardly used as a synthetic tool. This situation changed when well-defined and stable carbene complexes of molybdenum and ruthenium were discovered as efficient precatalysts in the early 1990s. In particular, the high activity and selectivity in ring-closure reactions stimulated further research in this area and led to numerous applications in organic synthesis. Today, olefin metathesis is one of the... 

Ruthenium Precatalysts with JV-Heterocydic Carbene Ligands... 

The search for even more active and recyclable ruthenium-based metathesis catalysts has recently led to the development of phosphine-free complexes by combining the concept of ligation with N-heterocyclic carbenes and benzyli-denes bearing a coordinating isopropoxy ligand. The latter was exemplified for Hoveyda s monophosphine complex 13 in Scheme 5 [12]. Pioneering studies in this field have been conducted by the groups of Hoveyda [49a] and Blechert [49b], who described the phosphine-free precatalyst 71a. Compound 71a is prepared either from 56d [49a] or from 13 [49b], as illustrated in Scheme 16. 

The hydroxy-binaphthyl functionalised saturated imidazolium salt is readily available from 1-amino-I -hydroxy-binaphthyl in a reaction with a ( oc-protected mesitylamine aldehyde [86] (see Figure 4.24). The resulting Schiff base is reduced to the diamine by Na(OAc)3BH. Subsequent deprotection and ring closure reaction with triethyl orthoformate yields the corresponding hydroxy-binaphthyl functionalised saturated imidazolium salt. Reaction with silver(I) carbonate and subsequent carbene transfer to the ruthenium(II) precursor yields the asymmetric olefin metathesis precatalyst. 

Enantioselective cyclopropanation is currently being explored. The ruthenium complex shown previously in Figure 7 also reacts with EDA and styrene to afford a transxis ratio of 4 1, with 46% ee of the trans isomer. The cis isomer is nearly racemic (<10% ee). The use of four-substituted stjrrene derivatives dramatically increases the diastereoisomeric excess of the trans isomer, with 4-fluorostyrene giving an 11 1 ratio, with 50% ee (74). Conversely, as shown in Figure 21, the porphyrin-like [RuCl(PNNP)]+ precatalyst reacts with EDA/ styrene to afford the cis isomer at a ratio of 10 1, with an enantiomeric excess of 87% (76). These types of ruthenium complexes have also been described as epoxi-dation catalysts above clearly there are mechanistic similarities between the oxo-and carbene- intermediates, which could help elucidate the reasons behind such variable enantioselectivity. Other ruthenium complexes that catalyze cyclopropanation include CpRu(II) catalysts, arene ruthenium complexes, and ruthenium-salen complexes. Cp Ru(cod)Cl is also known to catalyze the related reaction of diazo compounds with alkynes, affording the corresponding 1,3-diene (Figure 22) (77). 

Figure 2.2 Selected examples of A/-heterocyclic carbene (NHQ ligands used for ruthenium-based alkene metathesis precatalysts.

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. 

Figure 3.1 gives a generalized graphical overview of all reported syntheses of telechelic polymers via ROMP. Any polymerization within this scheme will start at the far left with either a commercially available ruthenium initiator or precatalyst (initiation box), which is either used as is or needs to be functionally derivatized (initiation box, middle). Alternatively, Mo, W, Ta, or Ti carbene initiators are used (initiation box, top). Subsequently, a monomer... 

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