Phosphine complexes decomposition pathways

Olefin metathesis is one of the most important reaction in organic synthesis [44], Complexes of Ru are extremely useful for this transformation, especially so-called Grubbs catalysts. The introduction of NHCs in Ru metathesis catalysts a decade ago ( second generation Grubbs catalysts) resulted in enhanced activity and lifetime, hence overall improved catalytic performance [45, 46]. However, compared to the archetypal phosphine-based Ru metathesis catalyst 24 (Fig. 13.3), Ru-NHC complexes such as 25 display specific reactivity patterns and as a consequence, are prone to additional decomposition pathways as well as non NHC-specific pathways [47]. 

Evidence for a major mode of catalyst deactivation in this system came from the observation of phosphonium cations (HPR3) in the reaction mixture, which could form through the pro to nation of free PR3 by the acidic dihydride complex. It is not known which species decomposes to release free PR3, but the decomposition pathway is exacerbated by the subsequent reactivity in which protonation of phosphine removes a proton from the metal dihydride, effectively removing a second metal species from the cycle. 

The resulting extraordinary stability of NHC-metal complexes has been utilized in many challenging applications. However, an increasing number of publications report that the metal-carbene bond is not inert [30-38]. For example, the migratory insertion of an NHC into a ruthenium-carbon double bond [30], the reductive elimination of alkylimidazolium salts from NHC alkyl complexes [37] or the ligand substitution of NHC ligands by phosphines [36,38] was described. In addition, the formation of palladium black is frequently observed in applications of palladium NHC complexes, also pointing at decomposition pathways. 

Organometallic compounds with a 17-electron configuration are often labile toward associative ligand exchange. Radical chain mechanisms are well established for phosphine substitution on metal carbonyl hydrides (Scheme 23), the 17-electron chain carrier being in most cases non hydridic. This mechanism, however, was also shown to operate for OsH2(CO)4 via the 17-electron hydride complex OsH(CO)4 [137]. Thus, phosphine addition to the radical prevails over the dimerization, which indeed occurs in the absence of phosphine [33] (section 6.5.7), and over other possible decomposition pathways. The second step of the chain propagation process in Scheme 23, for this osmium system, is another example of atom transfer to a hydride radical (section 6.5.6). 

The decomposition of the deuterium-labeled analog of 19, RuCl2(=CD2) (PCy3)2, led to the observation of a broad signal at 2.5 ppm in the H-NMR spectrum, suggesting that the decomposition of these methylidene complexes may involve the activation of phosphine C-H bonds [31]. This presumed phosphine activation could be a step in the primary decomposition pathway - a unimolecular process that includes the attack of PCyg on the methylidene - or in a secondary decomposition route. For instance, it is conceivable that some secondary decomposition of methylidene complexes may occur via a bimolecular process. 

The degree to which this decomposition pathway is operating in complex 19 under ethylene is unclear. In addition, the unimolecular decomposition route involving phosphine attack on the methylidene carbon seems to also be abundant when a reaction is conducted in the presence of ethylene [2]. Interestingly, studies... 

Grubbs and co-workers have studied the analogous nickel complexes in the presence and absence of excess phosphine and have found that di are three decomposition pathways, one for each of the different intermediates, 14e, 16e, and 18e, that can be formed (Eq. 7.36). 

Phosphine substitution on the phosphonium carbene was found to affect the initiation rate. Phosphine bulk helps stabilize the carbene complexes with respect to decomposition and kinetic deactivation by dimerization pathways [41]. All the complexes were synthesized through the trichloride intermediate 29 to prevent the decomposition of the complexes bearing the less bulky phosphine groups. The active, 14-electron complexes were then generated via the addition of B(C5F5)3 to abstract the chloride Hgand (Scheme 9.3). In solution, precatalysts bearing bulky phosphines were all monomeric, while the mixed phosphine cases tended to reversibly dimerize in solution. An illustrative dimerization is shown for catalyst 30, which possesses intermediate steric bulk at the phosphonium moiety. 

Notably the methylidene complexes (5) and (6a) appear to decompose by a different pathway than Ru alkylidenes and benzylidenes. The decomposition of both of them exhibit first-order kinetics (all other complexes have second-order kinetic) and is not inhibited by addition of free phosphines [10]. Moreover, using deuterium-labeled complexes it has been shown that phosphine takes part as a participant in this process in contrast to other alkylidenes (Scheme 2) [4],... 

An interesting example of contrast is provided by the thermal decomposition reaction of two sets of dialkyl complexes. Yamamotogroup has found that the decomposition of various [R2Pd(phosphine)2] complexes are first order in reactant and only slightly retarded by free phosphine, suggesting that the major pathway is ... 

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