Ruthenacyclobutane intermediates

If cycloalkene-yne 65 having an o -alkynyl substituent at an olefinic position in a cycloalkene is treated with a ruthenium catalyst, what kinds of products are produced. In this reaction, ruthenium mono-substituted carbene complex XVII is anticipated to be formed from a highly strained ruthenacyclobutane intermediate. If it then reacts with ethylene, triene 67 should be formed, but if XVII reacts with an alkene part intramolecularly, bicyclic compound 66 should be formed via ruthenacyclobutane (Scheme 23). 

It is well known that the reaction of the Grubbs complexes with alkyl vinyl ethers readily occurs at room temperature to yield the ruthenium complex with alkoxy-substituted carbene ligand, via a ruthenacyclobutane intermediate (Scheme 11) [28]. 

Ruthenacyclobutane Intermediates Derived from Phosphonium Alkylidene Complexes... 

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. 

Studies of catalyst decomposition in the presence of substrate have mostly focused on ethylene. In particular, it has been demonstrated that ethylene can induce the degradation of methylidene complex 19 to produce propylene as the main volatile organic byproduct [3, 39]. The proposed mechanism for this degradation involves the ruthenacyclobutane intermediate (20) undergoing a P-hydride elimination to form a ruthenium allyl-hydride species (21), which subsequently affords the propylene complex (22) upon reductive elimination (Scheme 11.8). 

A similar decomposition process has been observed for a second-generation ruthenacyclobutane intermediate, which will be discussed in Section 11.3.2 [40]. 

A combined experimental and computational study of substrate-induced decomposition pathways for alkene metathesis catalysts reveals the importance of / -hydride transfer from ruthenacyclobutane intermediates. Subsequent steps afford allyl hydride species, which then eliminate H2 to give catalytically inactive unsaturated complexes of the form [RuL(77 -alkene)Gl2]. ... 

Grubbs and Houk reported the computed energy barriers for the coupling of ethene with 59, which indicated a clear preference for the formation of the side-bound ruthenacyclobutane intermediate (Figure 4.5). The preference for the side-bound mechanism was attributed to a combination of steric and electronic factors. Since the alkylidene adopts a horizontal conformation in the bottom-bound transition states, steric repulsion of the alkylidene and... 

The calculated reaction energy for the formation of the ruthenacyclobutane intermediate (III-Dn4) from the u-complex 111-0 4 in the presence of III is endothermic (3.31 kcal/mol) for the Cp mode with activation energy of 31 kcal/mol (Figure 12.2). In contrast, the formation of Dn from the t-complex C is exothermic for both activation steps 1 (-11.95 kcal/mol) and 2 (-4.51 kcal/mol) with activation energies of 66 kcal/mol and 45 kcal/mol, respectively. The decomposition of the ruthenacyclobutane ( D, to Ej, ) is endothermic for both the heptylidene (15 kcal/ mol) and methylidene (ca. 23 kcal/mol) formation steps with activation energies of about 45 kcal/mol and 70 kcal/mol, respectively. The coordination of the 1-octene in the Cji mode has an about 2 kcal/mol-5 kcal/mol decrease in El for Dj, to Ej, ... 

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