Metathesis initiation step

O-Bond metathesis of the Ln-alkyl with the phosphine gives a Ln-phosphido complex. (This initiation step was observed to be faster when the hydride derivative [Cp 2bnH]2 was used.) Since the reactions were zero-order in substrate, the next... 

To summarize, experimental evidence has been advanced regarding hydride involvement in the initiation step of olefin metathesis with certain catalysts. One concept considers the source of the hydride to be external—that is, originating from a promoter or a cocatalyst. A second concept assumes a hydride being generated internally from the metathesizing olefin. It is quite possible that both concepts are operative. 

Note also that, in contrast to classical heterogeneous catalysts, the initiation step of [=SiORe(=CtBu)(=CHtBu)(CH2tBu)] is well defined and corresponds to the cross-metathesis of the alkene with the neopentyhdene ligand. In fact, in the metathesis of propene, 0.7 equiv of a 3 1 mixture of 3,3-dimethyl-l-butene and 4,4-dimethyl-2-pentene is formed (Figure 3.27) the nearly quantitative formation of cross-metathesis products is consistent with a real single-site catalyst. Moreover,... 

The reaction of PhSiHs with the Ni-Me precursors has been shown to resnlt in the formation of ohgosilanes (PhSiH) . The initial step of this reaction is beheved to go throngh a concerted a-bond metathesis (Kh/Kd 10), bnt mnltiple intermediates are formed in the later stages of the catalysis, some of which result from the reductive elimination see Reductive Elimination) of the indenyl ligand (Scheme 8). ... 

In any chain reaction, apart from initiation steps, the termination steps are also important. In metathesis there are many possibilities for termination reactions. Besides the reverse of the initiation step, the reaction between two carbene species is also a possibility (eq. (17)). The observation that, when using the Me4SnAVCl6 system, as well as methane traces of ethylene are also observed [26] is in agreement with this reaction. Further reactions which lead to loss of catalytic activity are (1) the destruction of the metallacyclobutane intermediate resulting in the formation of cyclopropanes or alkenes, and (2) the reaction of the metallacycle or metal carbene with impurities in the system or with the functional group in the case of a functionally substituted alkene (e. g., Wittig-type reactions of the metal carbene with carbonyl groups). 

The primary reason for the lower activities generally observed in Grubbs catalysts stems from the initiation step necessary to access the catalytic manifold for olefin metathesis. The mechanism by which these catalysts mediate metathesis is quite well understood,and the salient features are shown in Scheme 12. While the catalyst precursors 23 and 24 are 16-electron species, and therefore capable of taking up an olefmic substrate, uptake of olefin followed by metathesis via an 18-electron manifold via an associative mechanism accounts for less than 5% of the metathesis in a typical application. Instead, the bulk of the catalysis occurs from the 14-electron alkylidene, 25,... 

A general mechanism for this reaction, including the metathesis, initiation by either heat or light, the propagation cycle, and the termination steps in the presence or absence of LiCl, is outlined below. 

Fig. 27.5 Initial steps in the mechanism of alkene metathesis involving first and second generation Grubbs catalysts. Two possibilities for the formation of the metallocyclobutane intermediates are shown.

Supported Ta-Neopentylidene Complexes A silica-supported Ta-alkylidene [(=SiO) Ta(=CHCMe3)(CH2CMe3)p j] catalyst has been synthesized and well characterized [76, 77]. This Ta-alkylidene catalyst possesses a neopentyl ligand and an alkylidene group, and was found to be active for propane metathesis (TON=33). Since the precursor is not a hydride and a proposed carbene hydride is known to be the intermediate, this implies that Ta-neopentyl and neopentylidene organometaUic species are initially transformed into this key species (Scheme 2.10) [78-83]. Different initiation steps have been suggested. One involves the initial addition of the alkane into the alkylidene moiety to form multiple Ta-alkyl species, which can then decompose via an a-H abstraction to produce a Ta-alkylidene (Scheme 2.10a) [84, 85]. 

Scheme 8.4 Initial steps in the ruthenium alkylidene-catalyzed olefin metathesis reaction.

As previously discussed, the unfavorable equilibria associated with ligand dissociation during the initiation step of an olefin metathesis reaction have traditionally hindered the direct observation of metathesis-active ruthenacyclobutane intermediates [24]. Thus far, we have seen that the use of phosphonium alkylidene complexes, such as 22, can enable facile access to metallacycle formation by providing an alternative route for catalyst initiation. However, despite the utility of these trialkylphosphonium alkylidene catalysts, their preparation requires a multi-step synthetic route that requires the use of costly reagents [28]. In addition, the vinyl trialkylphosphonium salt generated following the reaction of 22 presents a less relevant model in comparison to the styrene (34) formed from the commercially available benzylidene catalysts. 

It is still unclear how the initiation step in alkene metathesis occurs and how the initial carbene forms. Commercial applications of metathesis include the triolefin process, in which propylene is converted to ethylene and butene, the neohexene process, in which the dimer of isobutylene, Me3CCH=CMe2, is metathesized with ethylene to give Me3CCH=CH2, an intermediate in the manufacture of synthetic musk, and a 1,5-hexadiene synthesis from 1,5-cy-clooctadiene and ethylene. Two other applications, SHOP and ROMP (Shell higher olefins process and ring-opening metathesis polymerization), are discussed in the next section. 

Initial metathesis of the substrate C=C bond gives MeCH=CR(OR) and a C=W carbene intermediate. This forms a metalacycle with the nearby alkyne and metathesis-like steps lead to product. 

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