Methylidene complex

The mechanistic investigations presented in this section have stimulated research directed to the development of advanced ruthenium precatalysts for olefin metathesis. It was pointed out by Grubbs et al. that the utility of a catalyst is determined by the ratio of catalysis to the rate of decomposition [31]. The decomposition of ruthenium methylidene complexes, which attribute to approximately 95% of the turnover, proceeds monomolecularly, which explains the commonly observed problem that slowly reacting substrates require high catalyst loadings [31]. This problem has been addressed by the development of a novel class of ruthenium precatalysts, the so-called second-generation catalysts. 

The methylidene complex [E] is the true catalyst, which as far as we can tell has the shortest half-life, and its reaction with an olefin produces ethylene as a by-product. 

Reaction of 3 with Ph3C+PF6" resulted in the formation of methylidene complex [(n-C5H5)Re(N0)(PPh3)(CH2)]+ PF6 (8) in 88-100% spectroscopic yields, as shown in Figure 11. Although 8 decomposes in solution slowly at -10 °C and rapidly at 25 °C (She decomposition is second order in 8), it can be isolated as an off-white powder (pure by H NMR) when the reaction is worked up at -23 °C. The methylidene H and 13C NMR chemical shifts are similar to those observed previously for carbene complexes [28]. However, the multiplicity of the H NMR spectrum indicates the two methylidene protons to be non-equivalent (Figure 11). Since no coalescence is.observed below the decomposition point of 8, a lower limit of AG >15 kcal/mol can be set for the rotational barrier about the rhenium-methylidene bond. 

Figure 11. Synthesis of first detectable electrophilic methylidene complex...

The methylidene complex forms crystalline, analytically pure adducts with pyridine and phosphines, as shown in Figure 12. These reactions establish the methylidene carbon as electrophilic. Relevant to a future mechanistic point, no reaction was observed between 8 and dimethyl ether. 

Figure 15. Reason for the nonobservability of the methylidene complex during the half-methylation experiment...

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. 

As noted above, titanocene-alkylidenes can be prepared using various methods and starting materials. Like the methylidene complex, higher alkylidene complexes are useful for the transformation of carbonyl compounds to highly substituted olefins. Ketones and aldehydes are converted into substituted allenes by treatment with titanocene-alkenylidenes prepared by olefin metathesis between titanocene-methylidene and substituted allenes (see Scheme 14.7) [17]. Titanocene-alkenylidene complexes can also be prepared from... 

Methylidene-rare earth complexes (27) have been synthesized and fully characterized.32 In these complexes, the methylene should be seen as a doubly charged negative ligand. Nevertheless, these methylidene complexes react as Schrock-type nucleophilic carbenes, (27) therefore being analogous to the Tebbe reagent. 

In the Tebbe reaction with pyridine iV-oxides and their benzo derivatives, regioselective attack at the cr-position occurs along with the expected deoxygenation (Equations 47 and 48) <2000AGE2529>. The presumed organome-tallic intermediate is the titanocene methylidene complex [Cp2Ti=CH2]. 

It is worthy to note here that the methylidene complex 11 is a poor initiator for olefin metathesis reactions at room temperature. Although this complex can undergo multiple catalytic turnovers, if it is intercepted by free phosphine ligand, it becomes incapable of reentering the metathesis catalytic cycle.32... 

The methylidene complex [ReCp (NO)(PPh3)CH2]+ is stable above 100 °C. Not surprisingly, (106) is less stable and decomposes to [(Re)(j) -C2H4)] and [(Re)(solvent)]+ via a rate-determining dimerization process. It can, however, be trapped by a variety of nucleophiles to give both... 

Elemental sulfur, selenium, or tellurium reacts with the M=C bond in the electron-rich methylidene complex 250 to give coordinated thio-, se-leno-, or telluroformaldehydes (251) (766). The electrophilic methylidene... 

Addition of Na2S to the methylidene complex 307 gives rise to a trinu-clear sulfonium salt (308) (108). The action of a protic nucleophile, like H2S, on the prochiral phosphinoketene ligand in 309 (R = Me, Ph) leads to five-membered chiral metallaheterocycles (310), which are organometallic derivates of y-thiolactones (197). 

In Section 10-2-2, we discussed the synthesis of Tebbe s reagent, a bridged Ti methylidene complex. This reagent converts a C=0 group to an alkene in a manner analogous to that of a phosphorus ylide (Wittig reagent).56 Conversion of a carbonyl to a terminal alkene may seem to be of limited utility until one realizes that, unlike phosphorus ylides that react only with aldehydes and ketones, Tebbe s... 

This procedure can be followed by phosphine exchange to give catalyst 6. Complex 6 can be stirred under an atmosphere of ethylene to give quantitative yield of the methylidene complex 7, which was the first isolable and metathesis-active metal methylidene complex reported in the literature [81] (Scheme 6.21). Complex 6 is the most widely used catalyst of this series, and is commonly referred to as fhe Grubbs catalyst or more recently as the first-generation Grubbs catalyst . 

It is notable that the mefhylidenes, especially the second-generation mefhyU-dene, are relatively unreactive towards metathesis. In fact, the second-generation methylidene complex polymerizes cyclooctadiene by ROMP 4 orders of magnitude slower than benzylidene 10 [101]. Thus, the unique properties of the methylidene complex discussed above are even more apparent for fhe second-generation catalyst series, due to the extremely slow phosphine dissociation of the methyUdene complexes. 

These results indicate fhat fhe methylidene complex is fhe botUeneck in the ADMET reaction with Grubbs catalysts, which suggests fhat performing fhe ADMET reaction on internal olefins may be beneficial. For example, ADMET wifh 2,10-dodecadiene versus 1,9-decadiene would completely avoid formation of the methylidene and removal of 2-butene would be fhe driving force of the reaction (Scheme 6.28). 

Again, the methylidene complex shows different behavior than the other carbenes. The decomposition of the methylidene is not affected by the presence of excess phosphine and is first order in catalyst. The decomposition in this case appears to be due to activation of C-H bonds in the L ligand. 

Scheme 6.31 Decomposition processes (a) general carbene (b) methylidene complex.

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