Ruthenium carbene complexes propagating species

In a formal sense, complexes 1 represent pre-catalysts that convert in the first turn of the catalytic cycle (vide infra) into ruthenium methylidene species of type 3 which are believed to be the actual propagating species in solution (Schemes 2,4). The ease of formation of 3 strongly depends on the electronic properties of the original carbene moiety in 1. In addition to complexes la-c with R1=CH=CPh2, ruthenium carbenes with Rx=aryl (e.g. Id, Scheme 3) constitute another class of excellent metathesis pre-catalysts, which afford the methylidene complex 3 after an even shorter induction period [5]. In contrast, any kind of electron-withdrawing (e.g. -COOR) or electron-donating substitu-... 

Since the vinylcarbenes la-c and the aryl substituted carbene (pre)catalyst Id, in the first turn of the catalytic cycle, both afford methylidene complex 3 as the propagating species in solution, their application profiles are essentially identical. Differences in the rate of initiation are relevant in polymerization reactions, but are of minor importance for RCM to which this chapter is confined. Moreover, the close relationship between 1 and the ruthenium allenylidene complexes 2 mentioned above suggests that the scope and limitations of these latter catalysts will also be quite similar. Although this aspect merits further investigations, the data compiled in Table 1 clearly support this view. 

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. 

---