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Carbenes chain mechanism

The understanding of the reaction mechanism is directly related to the role of the catalyst, i.e., the transition metal. It is universally accepted that olefin metathesis proceeds via the so-called metal carbene chain mechanism, first proposed by Herisson and Chauvin in 1971 [25]. The propagation reaction involves a transition metal carbene as the active species with a vacant coordination site at the transition metal. The olefin coordinates at this vacant site and subsequently a metalla-cyclobutane intermediate is formed. The metallacycle is unstable and cleaves in the opposite fashion to afford a new metal carbene complex and a new olefin. If this process is repeated often enough, eventually an equilibrium mixture of alkenes will be obtained. [Pg.333]

The metal-carbene chain-mechanism concept (Chauvin mechanism) has been strengthened by many studies of mechanistic details. The origin of the initial metal-carbene complex has received considerable attention, as has the metallocyclobutane-alkyUdene interconversion. Spectroscopic, kinetic, and... [Pg.100]

The elucidation of the mechanism for olefin metathesis reactions has provided one of the most challenging problems in organometallic chemistry. In Volume 1 Rooney and Stewart concluded that the carbene chain mechanism is now generally accepted for olefin metathesis reactions, but much remains to be learned about the formation and reactivity of metal-carbene intermediates, metallocycles, and especially the mechanistic aspects of chain initiations. Since that report, systems have been designed that begin to reveal the important mechanistic features of olefin metathesis. [Pg.101]

An analysis of the kinetics of the metal-carbene chain mechanism (66) shows that if rate constants are defined as in Eqs. (8) and (9) (here M is... [Pg.290]

Thus, although the sticky olefin hypothesis requires, as considered in Section II,D, an unacceptable account for the bonding in the four-membered ring intermediate, it is experimentally not easily distinguishable from the carbene-chain mechanism. However, the distribution of products formed in the double cross experiments should distinguish the two, as shown below, and so should the distribution of products in another version of the experiment that is essentially the reverse of the double cross. [Pg.293]

Now an analysis (66) of the kinetics of the metal-carbene chain mechanism shows that this product should be 4, and it also allows a precise definition of another notable feature of Fig. 1, the formation initially of more Cjz than Cig although less 2-butene was present than 4-octene. 2-Butene must be more reactive than 4-octene, presumably for steric reasons, and the ratios of the rate constants defined by Eqs. (15)... [Pg.295]

Consider now what the product r, x /-z would be if the conventional mechanism were correct, but the olefin displacement reaction were rate determining. The kinetics of a mechanistic scheme much like that in Scheme 4 shows (66) that ri x at zero time is a function of two ratios, that of the concentrations of butene and octene and that of their reactivities. However, for all reasonable values of these ratios, rj x rz is never greater than 2.94. Accordingly, since in the double cross experiment the product was determined to be about 4 at zero time, the conventional mechanism is excluded no matter which step is ratedetermining. The implication is that the carbene chain mechanism is correct. [Pg.296]

The kinetics of the olefin metathesis of 2-pentene by (pyri-dine)2Mo(NO)2Cl2 and organoaluminum halides have been measured (56) as first order in the metal and variable order in olefin (seemingly first order at high olefin concentration and up to order 1.7 at low olefin concentration) and were originally interpreted to support the conventional mechanism, but they now also seem in accord with the metal-carbene chain mechanism. [Pg.298]

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... [Pg.446]

The generally accepted mechanism is a chain mechanism, involving the intervention of a metal-carbene complex (126) and a four-membered ring containing... [Pg.1458]

From spectroscopic data, presented in the following, we conclude that the mechanism of polymerization is described by three series of intermediate states differing by the number of reactive radical or carbene chain ends these are the diradicals DR , the dicarbenes DC , and the asymmetric carbenes AC . Via a final chain termination reaction an additional series of reaction products is obtained. These are the stable oligomers SO with two unreactive chain ends. The schematic structures of the DR, DC, AC, and SO molecules are shown by example of the trimer in Table 2. The lengths of the dimer-, trimer-, tetramer-... units are characterized by the numbers n = 2, 3,4,... of the respective monomer molecules. The symbols and the schematic structures as well as the notation of the optical and the ESR absorption lines, are summarized in Table 2. [Pg.56]

Two different reaction mechanisms have been postulated for the topochemical polymerization of diacetylenes involving diradical or carbene chain ends The first mechanism leads to a butatriene structure (I), the latter to the acetylene structure (II) of polydiacetylene chains. [Pg.126]

Three further computational explorations of carbene reaction mechanisms have been reported. DFT study of the Reimer-Tiemann reaction (formal reaction between CCl2 and a phenoxide ion) using either potassium or sodium hydroxide as base has revealed that the active carbenic species is an alkaline carbenoid form rather than its free form as suggested earlier. DFT study of the reaction of CHF with dioxygen has confirmed fhaf fhe firsf step involves formation of the planar HFCO2 adduct. The initial steps of 2,5-dimethylfuran thermal decomposition have been computationally identified as scission of the C—H bond in the methyl side chain and subsequent formation of ft- and a-carbenes (39) and (40) via [3,2]-H and [2,3]-methyl shifts, respectively (Scheme 6). Once generated, carbenes (39) and (40) are believed to follow diverse fragmentation pathways. ... [Pg.209]

Crystal structures of ethylmagnesium bromide Crystal structure of tetrameric phenyllithium etherate Representation of tt bonding in alkene-transition-metal complexes Mechanisms for addition of singlet and triplet carbenes to alkenes Frontier orbital interpretation of radical substituent effects Chain mechanism for radical addition reactions mediated by trialkylstannyl radicals... [Pg.818]

Another possibility is that carbene species are generated via the dissociative adsorption of ethylene onto two adjacent chromium sites [71]. A second ethylene molecule then forms an alkyl chain bridge between the two chromium sites this can subsequently propagate via either the Cossee or the Green-Rooney mechanism. [Pg.27]

Co-feeding of alcohols effects an increased rate of hydrocarbon formation, as shown in early experiments of Emmett and coworkers1"1 using 14C-labeled alcohols. These experiments were carried out in order to support the hydroxyl-carbene mechanism favored at that time. Their experiments were confirmed by Shi and Davis23 for Co catalysts and co-feeding of ethanol. Furthermore, in their study, the argument that ethanol may be dehydrated to ethene, incorporated, and followed by subsequent chain growth via CH2 insertion could be excluded, as co-fed ethanol incorporated much faster than ethene. [Pg.206]

The seemingly plausible Scheme shown in 4 is inconsistent with the results of the 13C0 labeling study as are most schemes which do not involve CO insertion for the chain propagation. We believe that ethylene arises from the same sequence of steps as the other hydrocarbon products. The role of the second metal center in the reduction cannot be described. We believe that the iron-iron bond is cleaved early in the reaction since the reduction in the presence of PBu3 produced the unsubstituted species, LiCpFe(C0)2. While there is too little information currently available to assess the importance of Scheme 3, our results on reduction in this iron system are not consistent with the normal CO insertion mechanism or with carbene oligomerization. We suggest Scheme 3 until further research can be accomplished. [Pg.273]


See other pages where Carbenes chain mechanism is mentioned: [Pg.288]    [Pg.289]    [Pg.295]    [Pg.298]    [Pg.224]    [Pg.288]    [Pg.289]    [Pg.295]    [Pg.298]    [Pg.224]    [Pg.449]    [Pg.98]    [Pg.446]    [Pg.1331]    [Pg.1154]    [Pg.438]    [Pg.264]    [Pg.101]    [Pg.1215]    [Pg.305]    [Pg.3]    [Pg.607]    [Pg.353]    [Pg.14]    [Pg.376]    [Pg.64]    [Pg.363]    [Pg.24]    [Pg.480]    [Pg.182]    [Pg.29]    [Pg.183]    [Pg.271]    [Pg.214]    [Pg.18]   
See also in sourсe #XX -- [ Pg.333 ]




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