Mechanism carbene

The A -fluoropyridinium salts produce 2-fluoropyridines on treatment with base. A carbene mechanism has been proposed [78] (equation 36). 

A solid-liquid phase-transfer technique is used to synthesize aryl difluoro-methyl sulfides and selenides thiophenols dissolved in an aromatic solvent are treated with solid sodium hydroxide in the presence of a catalytic amount of tris(3,6-dioxaheptyl)amine (TDA1) [49] This condensation proceeds by a carbene mechanism (equation 44)... 

Trifluoromethyl thiirane is formed by the action of tris(diethylamino)-phosphineon l-chloromethyl-2,2,2-trifluoroethyldisulfide [S2] (equation 73) Difluoromethyl phenyl selenide is prepared by treatment of lithium phenyl-selemde with chlorodifluoroniethane via a carbene mechanism [Si] (equation 44) Bis(2,2,2-trifluoroethyl)diselenide is formed in the reaction of 2,2,2-trifluoroethyl mesylate with lithium diselenide [84] (equation 74). 

Interest in this reaction was revived when the relevance of a carbene mechanism was realized, particularly following the demonstration (cf. SectionI,B) of a similar ring expansion of indene to 2-chloro-naphthalene by dichlorocarbene via the cyclopropane adduct. Indeed, at this time Nakazaki suggested that these reactions occurred by the addition of dichlorocarbene to the indolyl anion and subsequent rearrangement to the indolenine and, with loss of chloride ion, to the quinoline [Eq. (12)]. The preference of dichlorocarbene for... 

Contradictory to a carbene mechanism is the high selectivity which is typical for the metathesis reaction. In the case of carbene complexes, side reactions must be expected, such as addition and insertion [Eqs. (38) and (39)] ... 

Because in metathesis reactions with most catalyst systems a selectivity of nearly 100% is found, a carbene mechanism seems less likely. Banks and Bailey ( ) reported the formation of small quantities of C3-C6-alkenes, cyclopropane, and methylcyclopropane when ethene was passed over Mo(CO)6-A1203, which suggests reactions involving carbene complexes. However, similar results have not been reported elsewhere most probably the products found by Banks and Bailey were formed by side reactions, typical for their particular catalyst system. 

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. 

For didactic reasons we will not show mechanistic proposals that have been abandoned, because the carbene mechanism now7 is well established. It is referred to as the Chauvin-Herisson mechanism, as they were the first to propose the intermediacy of metal-alkylidene species [5], Their proposal was based on the observation (simplified for our purposes) that initially in the ringopening polymerisation of cyclopentene in the presence of 2-pentene a mixture of compounds was obtained rather than the single products if a pair-wise reaction of cyclopentene (or higher homologues) and 2-pentene would occur. Figure 16.3 shows the results. 

If cyclopentene would react pair-wise with 2-pentene, only one product would form, namely 2,7-decadiene, and a similar result for cyclodimers etc. of cyclopentene. If somehow, the alkylidene species would be transferred one by one, we would obtain a mixture of 2,7-nonadiene, 2,7-decadiene, and 2,7-undecadiene in a 1 2 1 ratio. The latter turned out to be the case, which led the authors to propose the participation of metal-carbene (metal alkylidene) intermediates [6], Via these intermediates the alkylidene parts of the alkenes are transferred one by one to an alkene. The mechanism is depicted in Figure 16.4. In the first step the reaction of two alkylidene precursors (ethylidene -bottom- and propylidene -top) with cyclopentene is shown. In the second step the orientation of the next 2-pentene determines whether nonadiene, decadiene or undecadiene is formed. It is clear that this leads to a statistical mixture, all rates being exactly equal, which need not be the case. Sometimes the results are indeed not the statistical mixture as some combinations of metal carbene complex and reacting alkene may be preferred, but it is still believed that a metal-carbene mechanism is involved. Deuterium labelling of alkenes by Gmbbs instead of differently substituted alkenes led to the same result as the experiments with the use of 2-pentene [7],... 

Addition reactions with alkenes to form cyclopropanes are the best studied reactions of carbene intermediates, both from the point of view of understanding carbene mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to that of... 

Nucleophilic fluoroalkylation in the anomeric position has been performed with trifluorochloroethylene, or with perfluorovinyl ethers." The compound obtained with trifluorochloroethylene is an irreversible inhibitor of a-glycosidases (Figure 6.44). " Trifluoromethylzinc bromide can be used to perform the difluor-omethylation of hydroxyls in various positions, probably through a carbenic mechanism. 

Path C is surely unlikely for several reasons, chief among them is that 23 is formed even at —45 °C, at which temperature the bicyclobutane would surely be stable. The real problem is to rule out the non-carbene mechanism, path A. Brinker cleverly does this by examining the dienes formed from 24 and 25 (Scheme 7.13). The products are nicely rationalized through intermediates 22a and 22b in which the direction of the initial addition is determined by the stabilizing or... 

This is a major route of decomposition of ethyl 2-furyldiazoacetate (%) (R = H, = C02Et) when heated in dichloromethane or methanol (74JOC2939). The same type of decomposition has been observed with other 2-furylcarbenes which were generated by decomposition of the sodium salts of tosylhydrazones at 3(X)°C (78JA7927). Thermolysis of the diazo compound 96 (R = R = H) in cyclooctane or styrene gave, besides the open-chain acetylene 97, products of intermolecular carbene insertion. This led the authors to favor the carbene mechanism of ring-opening (path A in Scheme 7). 

A new mechanism to interpret alkene formation in Fischer-Tropsch synthesis has been presented 499-501 There is a general agreement that hydrocarbon formation proceeds according to the modified carbene mechanism. Specifically, CO decomposes to form surface carbide and then undergoes hydrogenation to form surface methine (=CH), methylene (=CH2), methyl and, finally, methane. Linear hydrocarbons are formed in a stepwise polymerization of methylene species. When chain growth is terminated by p-hydride elimination [Eq. (3.61)], 1-alkenes may be formed,502 which is also called the alkyl mechanism ... 

This corresponds to the statistical distribution that is consistent with the carbene mechanism discussed below. A pairwise exchange was calculated to result in a 1 1.6 1 ratio of the three labeled ethylenes. The original 1 0 1 ratio of the three isotopomer 1,7-octadienes was found to be unchanged in the reaction mixture recovered at the end of the reaction. No isotope scrambling occurred, and isotope... 

However, rapid geometric isomerization consistent with the carbene mechanism obscures selectivity. 

An interesting suggestion regarding the possibility of a carbene mechanism was also raised but was disproved by NMR studies. In the polymerization of propylene with TiClj + (i3CH3)2A1I, only labeled methyls but no methylenes were observed. The formation of the latter could be expected if the carbene mechanism were operative. 

Photosantonic Acid. The formation of photosantonic acid (Formula 30) from lumisantonin poses problems of special interest. The ketene-carbene mechanism (Formulas 36-38) previously proposed (26,27) to account for this transformation fails to account for the formation of phenols in the irradiation of umbellulone (Formula 39) and the bicyclic... 

A further confirmation of the metal carbene mechanism is provided by enyne intramolecular metathesis reactions such as that depicted in equation 61. The C=C bond in the substrate becomes the single bond attaching the alkenyl group to the phenanthrene ring system634,635. 

In the presence of a solvent capable of reacting with a carbene center, products other than 2-fluoropyridines were formed. Indeed, these can become the major reaction products (87TL2705 89JOC1726), but not all products can be explained in terms of the carbene mechanism since in the 6-chloro-2-fluoropyridinium triflate (25 R = 6-C1, X = S03CF3) it is obvious that chloro is being replaced by fluoride (89JOC1726). 

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