Alkylidyne ligand mechanism

Templeton and co-workers described a novel transformation of tf-acy ligands into alkylidyne ligands, which formally represents the abstraction of an oxygen atom [Eq. (9)] (48). The mechanism of this transformation is probably complex, and the yield of the alkylidyne complexes is relatively low. Nevertheless, this reaction deserves attention, since overall it repre-... 

The synthetic method leading to Nb-alkylidenes and Nb-alkylidynes was particularly successful, due to a quite remarkable difference in the reaction rate of 29 with ketones or aldehydes, vs the subsequent reaction of the alkylidene with ketones and aldehydes (see Scheme 37). The former reaction takes a few minutes at -40°C, while the latter one occurs in hours at room temperature.88 The reaction between 178 and benzaldehyde led to triphenylethylene and the niobyl derivative 184. Due to the difference in reaction rates between a and b in Scheme 37, it was found that the sequential addition of two different ketones or aldehydes to a THF solution of 29 produced a nonsymmetric olefin in a stepwise McMurry-type reaction.84 This is exemplified in the coupling shown in reaction c (Scheme 37). The proposed reaction pathway does not involve the intermediacy of a pinacolato ligand and therefore differs from the mechanism of the McMurry reaction and related reductive couplings at activated metal sites.89... 

The synthesis of alkylidyne complexes by y -addition of electrophiles to vinylidene and acetylide ligands is now well established (5,6). Pombeiro and co-workers synthesized several new rhenium alkylidyne complexes by protonation of the electron-rich vinylidene complexes 13 [Eq. (18)] (55). The mechanism of formation of the benzylcarbyne complex 14 (R = Ph)... 

The protonation of vinylidene ligands bound to electron-rich metal centers is well-known to give the corresponding alkylidyne complexes. The first kinetic study of such a process has been reported for the reaction shown in Eq. (26). The kinetic data were rationalized in terms of the overall mechanism shown in Scheme 10. For example, in the presence of excess NHEt3BPh4, reaction (26) obeys the rate law (27), which is a limiting form of the general rate law for Scheme 10. Initial... 

ABSTRACT. The reaction between Schrock-type W alkylidyne complexes and inorganic oxides leads to several surface complexes, at least one of them bearing an alkylidene ligand. In the case of W(CCMe3)Np3/Nb205, the high catalytic activity seems to be more related to the reaction of a reduced W- species with the olefin via a tT-allyl mechanism than to the presence of the alkylidene ligand itself. 

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