Ruthenium carbene fragment

A benzannulation reaction yielding the naphthoquinone 61 could also be performed with the ruthenium carborane-stabilised carbene 60 and 1-hexyne [56] (Scheme 36). The ruthenium carbene unit can be regarded as an 18-electron fragment containing a formal Ru(II) centre coordinated to a dianionic six-electron-donor cobaltacarborane ligand. 

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

An alternative strategy for the functionalization of polyoxometalates relies on self-assembly processes. Up to now we failed to introduced a Schrock-type alkylidyne in the lacunar undodecaphosphotungstate. As the current trend in olefin metathesis reaction now favours ruthenium catalysts, such as the Grubbs s ones, we turn to ruthenium precursors and to more stable Lappert-type carbene fragments, stabilized in the a-position by nitrogen atoms. The reaction of [PWii039p- with the carbene precursor... 

In conclusion, the disappearance of the benzylidene fragment during the ATRP of methyl methacrylate could be explained by the reaction of the ruthenium benzylidene with the monomer, giving rise to highly unstable ruthenium ester-carbene complexes, and it is possible that these species then quickly decompose. In addition, the absence of [Ru=CH2] is also most probably indicative of the decomposition of these ruthenium carbene species, since [Ru=CH2] are presumed to be the propagating species in RCM and related ruthenium methylidene derivatives have a quite long lifetime in olefin metathesis. Until now, the exact nature of the inorganic decomposition products is not known. 

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. 

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. 

The terminal cross metathesis (CM) reaction, as depicted in Figure 3.5, is probably the most straightforward synthetic method to introduce complex molecular fragments or functional groups to a ROMP polymer chain end. The propagating ruthenium carbene complex typically reacts with an acyclic olefin in a CM reaction. The newly generated carbene complex is still metathesis-active, and can in principle undergo secondary metathesis reactions or initiate the polymerization of the residual monomer. 

In a sacrificial synthesis, cyclic monomers containing a cleavable group are incorporated into a block copolymer structure, as shown in Figure 3.7 [47]. A propagating ruthenium carbene complex is used as a macro initiator for the polymerization of the cleavable cyclic monomer to form a diblock copolymer. The polymer block composed of the cleavable monomer can be broken down into low molecular weight fragments (sacrificed), leaving just one functionality at the chain end of the first polymer block. 

Fig. 5 Typical half-sandwich ruthenium fragments used in the preparation of allenylidene complexes. Ancillary ligands include CO, mono- and bidentate phosphines or N-heterocyclic carbenes...

The fate of the benzylidene fragment in these ruthenium complexes is another matter of debate [20a, 25]. The thermal stability of benzylidene complexes 4-6 was tested at 85 °C, under conditions mimicking polymerisation of methyl methacrylate. As monitored by H NMR, complete disappearance of the benzylidene fragment of the mixed phosphine/Af-heterocyclic carbene complex 5 (R = Cy, R = (5j-CHMePh) was observed within 20 min, whereas the Grubbs complex, RuCl2(=CHPh)(PCy3)2, showed only 55 % decomposition, and the bis-A/ -heterocyclic carbene ruthenium complex 6 (R = Cy) 88 % decomposition over the same time interval (Figure 6). 

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