Elimination reactions carbenes formation

Reactions with alcohols proceed similarly, but as the products cannot eliminate any fragment, formation of alkoxy-carbenes is observed (Equation 1.14) ... 

A second type of elimination reaction for some halocarbons is the 1,1 elimination. For example, hydrolysis of the halogenated methanes or haloforms is thought to occur by proton abstraction and subsequent formation of a carbene that reacts with water or hydroxide to form carbon monoxide and water (Equations (9)-(ll)). 

The absence so far of unequivocal proof of carbene formation by a-elimination of hydrogen halide from organic halides and related reactions using hydroxide and alkoxide ions or weaker bases suggests that more detailed investigations of some reported carbenoid reactions may be justified. Thus carbenoid behaviour in the reaction between benzal chloride and potassium t-butoxide in the presence of olefins seems to be critically dependent upon the nature of the solvent (McElvain and Weyna, 1959 Cawley and Westheimer, 1960) and the reaction temperature (Hodgkins et al., 1964). 

In an interesting report, 1,2-dibromocyclohexane cleaves the Hg—Sn bond in Hg[Sn(CH2(Bu-f)3)] to form Hg, BrSn(CH2(Bu-f)3) and cyclohexene. Also noteworthy is the formation of carbene complexes (7 -arene)(CO)2Cr CR2 formed in the substitution/elimination reactions of stannyl complexes K[( -arene)(CO)2CrSnPh3] with activated C—X bond in organic dihalogeno derivatives R2CX2 or ionic halides [R2CX]Y . 

The oxidation state of the precursor ranges from W(0) to W(VI). but the oxidation state of the actual active site is very probably low. Tlie intriguing problem of the initial carbene formation has received acceptable explanations from Muetterties and Green s observations of a-elimination of hydrogen from W-CH) groups [9], Several reports of the formation of carbenoids by such reactions have been given in the literature (4.9). 

Carbene formation was mentioned in an earlier section. This elimination of HCl from 4-chlorophenol or elimination of other hydrogen halides from halophenols could have been inferred from earlier photochemical studies on this and other derivatives. Boule and his coworkers irradiated 4-chlorophenol under deoxygenated conditions and obtained the corresponding quinhydrone and the 2,4 -dihydroxy-5-chlorobiphenyl °. Other research demonstrated that its irradiation in neutral aqueous solutions gave the corresponding quinone " and also that de-aeration did not seem to affect the reaction. ... 

Another type of carbene complex is characterized by a high oxidation state of the central metal and an a-carbon atom that does not usually bear a hetero atom. It is called an alkylidene complex or Schrock-type complex, since R. R. Schrock first synthesized a tantalum complex of this type [14]. Formation of the tantalum carbon double bond is based on the a-elimination reaction of a neopentyl ligand as shown in eq.(2). 

In the less basic medium of aqueous acetone or aqueous dioxan containing hydroxide ion, a carbene mechanism has been suggested to explain the quantitative conversion of 4-nitrobenzyl chloride and 4-nitrobenzyl dimethyl-sulphonium ion into cis- and /ra/is-4,4 -dinitrostilbenes . In both cases, the rate of elimination follows first-order kinetics in both the substrate and hydroxide ion, and deuterium exchange of labelled substrates occurs more rapidly than stilbene formation. In this aqueous medium, a rate-determining ion-pair dissociation is unlikely. However, again the selectivity of the carbene is astonishing and it is difficult to explain the lack of insertion products with the reaction medium or the failure to isolate cyclopropane adducts when the elimination reactions are performed in the presence of added olefins. 

Deuteration of the ionic liquid s cation has not only been applied to obtain proton-free ionic liquids for H-NMR experiments but also as direct probe for the reactivity of 1,3-dialkylimidazolium based ionic liquids vs. Ir(0) nanoclusters [70]. After addition ofD2 to [BMIM][(CF3S02)2N] in the presence of an lr(0) nanocluster Finkeand coworkers found deuterium incorporation at the 2-H, 4-H, 5-H and 8-H positions of the imidazolium cation while the control experiment in the absence of the lr(0) cluster showed no D-incorportion. The authors concluded from their H-NMR experiments that a sequence of N-heterocylic carbene formation by oxidative addition (see Section 3.1.2 for more details) of the imidazolium cation, H/D scrambling atop the nanocluster surface, followed by the reductive elimination of a C-D bond takes place. From carefiil kinetic investigations the authors concluded that the co-ordinatively unsaturated nanocluster surface acts indeed as the true catalyst for the H/D-ejadiange reaction. 

Ionic liquid stability is known to be a function of temperature (for details see Section 3.1) but the presence of nucleophiles/bases and the water content also have to be considered. There is no doubt that, under the conditions of a catalytic reaction, temperature stability issues are more complicated than imder the conditions of a TGA experiment. The presence of the catalyst complex, the reactants and impurities in the system may well influence the thermal stability of the ionic liquid. Basic and nucleophilic counter-ions, reactants and metal complexes may not only lead to deprotonation of 1,3-dialkylimidazolium ions (to form carbene moieties that will undergo further consecutive reactions) but will also promote thermal dealkylation of the ionic liquid s cation. If basic reaction conditions are required for the catalysis only tetraalkylphosphonium ions can be recommended as the ionic liquid s cation at this point in time. Tetraalkylphosphonium cations have been recently shown to display reasonabe stability, even under strongly basic conditions [290]. In contrast, all nitrogen-based cations suffer to some extent from either carbene formation, Hofmann elimination or rapid dealkylation (with alkyl transfer onto the nucleophilic anion). 

But there are also shifts in paradigms that have clearly opened the field and will certainly inspire a lot of great science in the future. For example, ionic liquids have been perceived in the past to be notoriously unstable towards strong bases due to carbene formation, Hofmann-elimination or dealkylation [44]. Recent work by Clyburne s group [45] and QUILL/Belfast [46] has impressively demonstrated, however, that some ionic liquid structures can indeed be quite base stable and these ionic liquids have been successfully applied in organic reactions in contact with strongly basic reagents. 

Competitive a- and jS-dehydroffaiorination of 1,1-difluoroethane has been studied by chemical activation i and shock wave techniques olefin production via the former mechanism (carbene formation followed by a rapid 1 2-hydrogen shift) was own to contribute ca. 10% to the total elimination in the activation work and ca. 13% in the thermal system. Unimolecular reactions of chemically activated CHs CHF CDs, CHa CHF-CHg CHs, and (CH3)sCF have also been studied. Dehydrochlorination of hydrochlorofluoromethanes is considered later (p. 35). 

Carbene formation during vacuum-UV photolysis (7.6 eV) and radiolysis of cyclooctane by unimolecular H2 elimination from a single carbon atom (1,1-elimination) was shown to be the main reaction channel of excited cyclooctane (equation 6)". The carbene 6 is formed with an efficiency close to 100% in the long-wavelength photolysis of the sodium salt of the p-tosylhydrazone 7 as well as in the vacuum-UV photolysis and radiolysis of cyclooctane. This was deduced from the similar product distribution of both reactions to give cyclooctene (8), bicydo[3.3.0]octane (9) and bicyclo[5.1.0]octane (10) ... 

This is a group of cyclic polyethers which are used as phase transfer catalysts. These have been used for esterifications, saponifications, anhydride formation, oxidations, aromatic substitution reactions, elimination reactions, displacement reactions, generation of carbenes, alkylations etc. Some of the examples are as follows ... 

The cycloaddition of a conjugated it system to an electrophilic molecule by the formation of two new a bonds to an atom of the electrophile in a concerted manner is known as cheletropic addition reaction and its reverse process in which two a bonds are broken from the same atom of the adduct is known as cheletropic elimination reaction. In cheletropic elimination, the driving force is often from the entropic benefit of gaseous elimination of N2, CO, and SO2. For example, the cycloaddition of 1,3-butadiene and its derivatives with SO2 and of alkene with a carbene are cheletropic addition reactions. 

The Puddephatt-Tipper team " have shown that reductive elimination involving the formation of cyclopropanes from platinacyclopropanes appears to involve a concerted process rather than the production of carbene-alkene intermediates (as does also the oxidative addition involving the reverse reaction, and the skeletal isomerization of platinacyclopropanes). They " have also proposed a similar concerted behavior for a reaction which could be looked upon either as a reductive elimination or a substitution, namely, the overall process in equation (46). 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

The running order in this chapter is essentially similar to that used in the preceding one. The main section, dealing with metal-carbon Group VIII triads in the order, iron, cobalt, and nickel, and is followed by a shorter section on carbene and carbyne complexes. Oxidative addition or reductive elimination reactions are included only when they lead directly to the formation or rupture of metal-carbon bonds. 

The importance of 0x0 ligands has been previously suspected by the numerous examples of oxygen containing activators. These reactions clearly demonstrated the crucial role of an 0x0 hgand. This activity could well be based on the double bond character of the M=0 Unkage. This may well facilitate carbene formation via a sequence of metal hydride reactions involving internal oxidative addition and reductive elimination. 

Certain imidazolinium salts, particularly derivatives with bulky N,N -sah-stituents and some triazolium salts could not be deprotonated with strong bases like NaH, but a thermally induced a-elimination reaction proved successful in these cases. This approach was attempted as early as 1960 by Wanzlick and Schikora who heated C2-trichloromethyl substituted imidazolidine but could only isolate the carbene dimer (see Scheme 1.2). Enders et al. successfully heated triazoline 67 under elimination of methanol from the C5 carbon atom and formation of triazol-5-ylidene 68 (Scheme l.lOa). ... 

On the way to further extension of the scope of Cp Co "-catalyzed C-H functionalization, Glorius and coworkers developed a condensation reaction of 2-arylpyridine derivatives and diazoesters to form unique polycyclic heteroaromatics having 6//-pyrido[2,l-a]isoquinolin-6-one skeletons (Scheme 10.15) [38]. The reaction is achieved by the combination of a bench-stable Co precatalyst [Cp Co(CO)l2] [39], a silver salt (AgSbFg), and an acetate source (KOAc) in trifluoroethanol. The in situ-formed Cp Co catalyst is proposed to play a dual role in this condensation reaction. First, it promotes formal carbene insertion into the ortho C-H bond through pyridine-directed C-H metalation, cobalt-carbene formation, carbene insertion into the aryl-Co bond, and protodemetalation. Second, it acts as a Lewis acid to facilitate nucleophilic attack of the pyridine moiety to the ester group, which eventually leads to the product through aromatization and elimination of methanol. The thus-synthesized polycyclic heteroarenes exhibit bright and color-tunable fluorescence in solution and in the solid state. 

It is notable that two different types of dienes have been produced depending on the structure of a,p-unsaturated substrates. Similar reaction mechanisms can be proposed oxidative addition-Pd carbene formation-migratory insertion affords intermediate E. Diene A is released with subsequent p-H elimination for the cyclic or linear substrates without hydrogen at the 8-position. Otherwise, complex E prefers to undergo rearrangement to give intermediate G for cyclic... 

In this reaction a combination of CuCl and O2 is used as oxidant The reaction is initiated by the oxidation of CuCl to Cu(ll) species by oxygen, which then oxidizes Pd(0) to Pd(n) species (Fig. 25). Subsequently, similar Pd carbene formation and p-hydride elimination take place to afford the olefin products. 

Weak base such as cesium carbonate has been utilized in this reaction to generate diazo compounds in situ from tosylhydrazones through the Bamford-Stevens reaction. The reaction is initiated by palladium-promoted decarboxylation of propargylic carbonate to form propargylpalladium complex A, which then tautomerizes to afford allenylpalladium intermediate B. Subsequently, the common carbene formation-migratory insertion-p-hydride elimination occurs to afford various vinylallenes (Fig. 30). 

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