HX Elimination Reactions

HFC-245eb can be prepared from hexafluoropropene (HFP) by the sequence shown in eq 20 in which the HFP is hydrogenated over a Pd/C catalyst to HFC-236ea. This is dehydrofluorinated over carbon to HFC-1225ye, which is then hyro-genated over a Cu/Pd/C catalyst to HFC-245eb [74], 

Haloalkenes can be prepared by dehydrohalogenating saturated hydrogen-containing polyhalocarbons using liquid alkali metal acid fluoride and/or alkali metal fluoride compositions [75], HCFC-133a can be converted to CF2=CHC1 using these catalyst systems as shown in eq 21. 

The substitution of chlorine with hydrogen by catalytic hydrodechlorination (HDC) is a well known reaction. Earlier work by Bitner et al. [76] and Gervasutti et al. [77] over Pd/C using CFC-114a showed that the reaction proceeded primarily to HFC-134a although a small amount of HCFC-124 was also obtained (eq 22). 

Conversion of the HCFC-124 to HFC-134a also occurred, but at much higher temperature. This has been recently studied and a proposed mechanism for the direct formation of HFC-134a involves a surface carbene intermediate, CF3CF  

The effect of catalyst support has also been reported to be have a pronounced effect. Kellner and Rao [82] reported 100% selectivity to HFC-134a from HCFC-124, using the unconventional support AIF3. Acid washing of the carbon to remove trace quantities of metal impurities was reported by Rao [83] to increase conversion of the CFC-114a and to significantly increase the HFC-134a HCFC-124 ratio. 

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Since polarizabilities of groups or atoms are also manifested in bond strengths, some form of Hirschfelder s rule can also be developed to treat these types of reactions (Laidler, 1987). For example, the activation energies for many molecular elimination reactions seem to be correlated with one-third of the sum of the dissociation energies of the bonds that are being broken. This empirical relationship seem to hold well for a large variety of HX elimination reactions (see Table XI). 

Alkali metal polyhydrogen fluoride melts are tunable, participatory solvents for halogen exchange, HF addition, and HX elimination reactions. High conversions with excellent selectivities to the desired fluorocarbon products are possible with the appropriate melt. The resulting alkali metal chlorides can be easily regenerated with anhydrous hydrogen fluoride in a separate step. 

Decomposition of the ketenes resembles the three-centered HX elimination reaction (Section 2.4.5) and a carbene is formed. The energy available to CO is Ex plus any El associated with the CO elimination process. Rearrangement... 

O-isopropylidene derivative (57) must exist in pyridine solution in a conformation which favors anhydro-ring formation rather than elimination. Considerable degradation occurred when the 5-iodo derivative (63) was treated with silver fluoride in pyridine (36). The products, which were isolated in small yield, were identified as thymine and l-[2-(5-methylfuryl)]-thymine (65). This same compound (65) was formed in high yield when the 5 -mesylate 64 was treated with potassium tert-hx Xy -ate in dimethyl sulfoxide (16). The formation of 65 from 63 or 64 clearly involves the rearrangement of an intermediate 2, 4 -diene. In a different approach to the problem of introducing terminal unsaturation into pento-furanoid nucleosides, Robins and co-workers (32,37) have employed mild base catalyzed E2 elimination reactions. Thus, treatment of the 5 -tosylate (59) with potassium tert-butylate in tert-butyl alcohol afforded a high yield of the 4 -ene (60) (37). This reaction may proceed via the 2,5 ... 

The two most common elimination reactions arc dehydroUalogenalion—the loss of HX from an alkyl halide—and dehydration—(he loss of water from an alcohol. Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base such as potassium hydroxide. For example, bromocvclohexane yields cyclohexene when treated with KOH in ethanol solution. 

We said at the beginning of this chapter that two kinds of reactions can happen when a nucleophile/Lewis base reacts with an alkyl halide. The nucleophile can either substitute for the halide by reaction at carbon or cause elimination of HX by reaction at a neighboring hydrogen ... 

Cyclic ketene acetals, which have utility as co-polymers with functional groups capable of cross-linking, etc., have been prepared by the elimination of HX from 2-halomethyl-l,3-dioxolanes. Milder conditions are used under phase-transfer conditions, compared with traditional procedures, which require a strong base and high temperatures. Solid liquid elimination reactions frequently use potassium f-butoxide [27], but acceptable yields have been achieved with potassium hydroxide and without loss of any chiral centres. The added dimension of sonication reduces reaction times and improves the yields [28, 29]. Microwave irradiation has also been used in the synthesis of methyleneacetals and dithioacetals [30] and yields are superior to those obtained with sonofication. 

Much evidence supports the conclusion that the elimination of the group HX from alkyl halides by bases is a trans elimination reaction. This means that the atoms H and X leave from the opposite site of the incipient double bond. It is mostly explained by assuming that the electrons which are left by the leaving proton and which will form the double bond prefer to attack the leaving group X from the rear (50). The transition state for the elimination, if it is concerted, is most stable if H, X, and the carbon atoms 1 and 2 lie on one plane, which in most molecules is best realized in the trans position (51). ... 

Molecular reactions are generally more difficult to treat because of the complexity of the possible transition states. The most widely studied complex molecular reaction class is HX elimination from halogenated hydrocarbons. These reactions proceed primarily via the formation of polar, four-centered tight transition states, and examples include... 

Bis-iV-alkylated complexes of Me2-9 and Me2-ll, as well as the tetra-methylated Ni(II)cyclam (NinTMC) derivatives, have been synthesized by the deprotonation of secondary amines followed by alkylation (34, 47,48). When EtI or other alkyl halide with /3-hydrogen was added to the deprotonated Ni(II) complex of cyclam or 11, HX elimination occurred instead of SN2 reaction. Therefore, ethylene gas was produced instead of -ethylated complex formation when EtI was added to the deprotonated complex of cyclam or 11. However, in the case of 8, bis- -ethylated Ni(II) complex was isolated. This may be because HX elimination is slower than SN2 reaction. The - -alkylated Ni(II) complexes of 9 (Me2-9 and Et2 9) and Me2-ll were stable against ligand dissociation in acidic aqueous solutions. The -alkylated complexes were dealkylated when the complexes were heated in aqueous solutions (34, 47). 

The formation of isocyanates has also been observed in reactions outlined in equations (21)-(23). It has been shown that the reactions of equation (21) most probably proceed via coordination of the amine function followed by rearrangement with elimination of HX. The reaction with PPh3 is interesting because of the initial formation of a nitrene species via N—O bond cleavage. Formation of a nitrene intermediate as a crucial step in the formation of the NCO ligand has also been proposed for the reaction with diazomethane. 

The opposite of an addition to a double bond is a 1,2-elimination reaction. In solution, where the reaction is promoted by solvent or by base, the most common eliminations (and those to which we shall limit our discussion) are those that involve loss of HX, although loss of X2 from 1,2-dihalides and similar reactions are also well known. The mechanisms of eliminations of HX are of three main types (1) The Ex (elimination, first-order), shown in Equation 7.22, which is the reverse of the AdE2 reaction and which has the same first, and rate-determining,... 

Many convenient methods for the introduction of carbon-carbon double bonds into a saturated carbon chain involve the removal of two atoms or groups from adjacent carbon atoms. Usually, but not invariably, one of these groups is hydrogen (i.e. the removal of HX). Two main types of elimination reactions are recognised - heterolytic processes in solution and pyrolytic reactions in the gas phase. A detailed discussion of the mechanisms of these reactions may be found in all standard and advanced textbooks in each of the reactions discussed below the probable mechanism is noted in relation to the aim of obtaining good yields of regio- or stereoisomerically pure compounds. 

The mechanism for the generation of quinone methide 58 by reductive elimination of 55 has been investigated.106 Single-electron reduction by 55 by pulse radiolysis in water gives the semiquinone radical anion 56, whose decay was monitored by transient absorption spectroscopy. This radical anion partitions between disproportionation to 60 and elimination to form the radical 58. Disproportionation dominates at pH 7, but as the pH is lowered to 3 the competing elimination reaction to form the quinone methide radical 58 is observed for X = -OMe and -OPh. It was proposed that the product yields are controlled by the position of the equilibrium for protonation of 56 and that 56 undergoes mainly disproportionation, while the semiquinone radical 57 - undergoes mainly elimination of HX (Scheme 28). The quinone methide 59 is then formed by the one-electron reduction of 58. 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

The first two steps of this mechanism are the same as the elimination reaction. Both reactions are carried out under acidic conditions. The difference is that halide ion serve as good nucleophiles and are present in high concentration. The elimination reaction is carried out using concentrated sulphuric acid and only weak nucleophiles are present (i.e. water) in low concentration. Thus, some elimination may occur and although the reaction of alcohols with HX produces mainly alkyl halide, some alkene byproduct is usually present. 

The elimination of HX which occurs upon UY irradiation of <5-haloketones, such as the <5-X-valerophenones, is the result of a photoreaction characteristic of the 0=0 group the reaction proceeds via loss of X from the biradical formed by y-hydrogen abstraction256. A comparable HX elimination does not occur with y-haloketones, but does to some extent with an -iodoketone. 

Alkynes can be prepared from dihaloalkanes by elimination of two molecules of HX. This reaction requires very strongly basic conditions so potassium hydroxide at elevated temperatures or the stronger base sodium amide (NaNH2) is commonly employed. Examples are provided by the following equations ... 

You will meet rearrangements in several chapters later in the book especially Chapter 37. Another common fate of cations, and something that may also happen instead of an intended S l or S>j2 reaction, is an elimination reaction where an alkene is formed by the nucleophile acting as a base to remove HX instead of adding to the molecule, an elimination reaction (El)... 

Here are two deceptively similar elimination reactions. The leaving group changes and the reaction conditions are very different but the overall process is elimination of HX to produce one of two alkenes. 

Most syntheses make the side-chain alkene by an elimination reaction so the first disconnection is an FGI adding HX back into the alkene. The last C-C bond-forming operation in most syntheses is an intramolecular aldol reaction to make the enone so that can be disconnected next. It is the starting material for the aldol, a simple monocyclic diketone, which is usually made by a fragmentation reaction because this is a good way to set up the stereochemistry. 

Theoretically, it is possible for the process of olefin coordination and insertion to continue as in Ziegler-Natta polymerization (Chapter 52) but with palladium the metal is expelled from the molecule by a p-hydride elimination reaction and the product is an alkene. For the whole process to be catalytic, a palladium(O) complex must be regenerated from the palladium(ll) product of P-hydride elimination. This occurs in the presence of base which removes HX from the palladium(II) species. 

In presence of dienes (Scheme 1), heterodienes, and heterotrienes (Scheme 2) rapid cycloadditions take place which prevent other reaction modes of the silylenes (insertions, polymerization, or HX-elimination-polymerization). Although a concerted [l+4]-cycloaddition is symmetry-allowed a stepwise mechanism (Scheme 3) via a three-membered intermediate is prefered or at least partly be competing to account for the formation of double-bond isomers. Ususally the formal [4+l]-cycloadducts (allylsilane-type) are the main products while the isomers with vinylsilane-units are side-products (< 30 %). Exceptions are... 

Phosphido-bridged dimers [Pd2(/tr-PR2)2X2(PR2H)2] can be obtained by reaction of secondary phosphine complexes [PdX2(PR2H)2] with a base. HX elimination is easier in the order PPh2H > PPhEtH > PEt2H. 

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