Elimination from halides

Although an old method, the elimination of hydrogen halide from isolated halides is still occasionally recommended. An example is the formation of cholest-2-ene (108) from 3j5-chloro-5a-cholestane, followed by purification via the dibromide (ref. 185, p. 252). 

Such eliminations can usually be achieved under the conditions which work for sulfonates (e.g. bromides on alumina ), and the anti-coplanar arrangement appears to be preferred. A large difference in reaction rate between the epimeric 3-chloro-5a-cholestanes has been observed in DMSO-potassium /-butoxide.  

While the dehydrohalogenation of 3-halo-5a-steroids gives the A -olefin selectively, it has been shown that in the 5j5-series dehydrochlorination of 3j5-chloro compounds with quinoline gives a mixture of A - and A -olefins in a 45 55 ratio. 

Whereas simple olefins are not usually made by elimination from halides, conjugated systems are frequently obtained in this way. The cases of a- and j5-halo ketones and their vinylogues have already been covered. Allylic halides may also be eliminated to form dienes, for example, the 2,4-diene (109) 

An important application of the allylic bromination-dehydrobromination approach is the formation of the A -diene system e.g. 110) in vitamin D synthesis. Dehydrobromination of the A -7-bromo system in the presence of mercuric chloride gives a high yield of A -diene.  

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The preference for anf/-elimination from halides can be very strong. To see this is so, we must turn from open-chain compounds to cyclic compounds. In... 

Elimination from halides almost always gives a mixture of elimination and solvolysis products. The acid-catalysed elimination of water from alcohols provides a preparative alternative (reaction 5.29). The proto-nated alcohol 43 loses a water molecule to give the carbocation, which can v eliminate a proton to form the alkene. Even if some of the carbocation is trapped as the sulfate ester 44, this reaction is reversible, and the alkene can be distilled out of the reaction mixture to bring the reaction to completion. 

Effects of Substitution on Rates of Elimination from Halides... 

No examples of catalysis of unimolecular elimination from halides or esters have been reported. Fades and Stimson (1962) have shown that t-butyl chloride undergoes elimination in the gas-phase at a rate independent of the partial pressure of added sulphur hexafluoride, a substance known to accelerate certain decompositions (Bose and Hinshelwood, 1959). However, the pyrolysis of alcohols, first studied by Kistiakowsky and Schultz (1934) is accelerated by the presence of hydrogen halides (Maccoll and Stimson, 1960). The former authors showed that t-butyl alcohol decomposed homogeneously to yield isobutene and water, at a rate given by... 

In spite of the disadvantages, acid-catalysed dehydration of alcohols and base-induced eliminations from halides and sulfonates are used widely in the preparation of alkenes. Typical bases include alkali-metal hydroxides and alkoxides,... 

Other methods for preparing -chlorotoluene include cx-elimination from an organoteUurium(IV) halide (57), paHadium-cataly2ed decarbonylation of 4-methylben2oyl chloride (58), and desulfonylation of -toluenesulfonyl chloride cataly2ed by chlorine (59) or cblorotris(tripbenylpbospbine)rbodium (60). 

Ehminations of HX to give double bonds offer considerable scope for selectivity and choice of reaction conditions. The dehydration of alcohols is the most common example of this class and may be achieved directly or through intermediate derivatives. In most cases, such derivatives are transient species formed in situ, but sometimes e.g. sulfonates, certain other esters and halides) they are isolated and characterized. Eliminations from jS-substituted ketones are very facile. The dehydration of jS-hydroxy ketones has been covered in section V. 

Halobenzenes undergo nucleophilic aromatic substitution through either of two mechanisms. If the halobenzene has a strongly electron-withdrawing substituent in the ortho or para position, substitution occurs by addition of a nucleophile to the ring, followed by elimination of halide from the intermediate anion. If the halobenzene is not activated by an electron-withdrawing substituent, substitution can occur by elimination of HX to give a benzyne, followed by addition of a nucleophile. 

The ratio of products is not independent of the halide, however, being 68 percent elimination from the chloride and 56 percent from the iodide. The authors conclude that an ion-pair intermediate, which would retain the halide specificity, is responsible for at least part of the reaction. 

J. K. Kochi, D. M. Singleton u. L. J. Andrews, Alkenes from Halides and Epoxides by Reductive Eliminations with Cr(lI)-Complexes, Tetrahedron 24, 3503 (1968). 

The palladium(O) complex undergoes first an oxydative addition of the aryl halide. Then a substitution reaction of the halide anion by the amine occurs at the metal. The resulting amino-complex would lose the imine with simultaneous formation of an hydropalladium. A reductive elimination from this 18-electrons complex would give the aromatic hydrocarbon and regenerate at the same time the initial catalyst. 

Carbenes from Halides by a-Elimination. The a-elimination of hydrogen halide induced by strong base (Scheme 10.8, Entry 4) is restricted to reactants that do not have (3-hydrogens, because dehydrohalogenation by (3-elimination dominates when it can occur. The classic example of this method of carbene generation is the generation of dichlorocarbene by base-catalyzed decomposition of chloroform.152... 

Section D illustrates formation of carbenes from halides by a-elimination. The carbene precursors are formed either by deprotonation (Entries 14 and 17) or halogen-metal exchange (Entries 15 and 16). The carbene additions can take place at low temperature. Entry 17 is an example of generation of dichlorocarbene from chloroform under phase transfer conditions. 

As reported in Scheme 1 the process involves a series of steps. The alkylpalladium species 1 forms through oxidative addition of the aromatic iodide to palladium(O) followed by noibomene insertion (4-7). The ready generation of complex 2 (8-11) from 1 is due to the unfavourable stereochemistry preventing P-hydrogen elimination from 1 (12). Complex 2 further reacts with alkyl halides RX to form palladium(IV) complex 3 (13-15). Migration of the R group to the... 

As an alternative method, poly(cyclodiborazane)s were prepared by the reaction of bis(silylimine)s with chlorodialkylboranes or with methyl dialkylborinates (scheme 18).32 This reaction proceeds via the condensation between V-silylimine and boron halide, eliminating trimethylsilyl halide followed by dimerization. However, the isolated polymer became insoluble after several hours of exposure under air, which resulted from the cross-linking reactions of unreacted trimethylsilyl groups to form trimerized hydrobenzamide derivatives. 

Transformations through 1,2-addition to a formal PN double bond within the delocalized rc-electron system have been reported for the benzo-l,3,2-diazaphospholes 5 which are readily produced by thermally induced depolymerization of tetramers 6 [13] (Scheme 2). The monomers react further with mono- or difunctional acyl chlorides to give 2-chloro-l,3,2-diazaphospholenes with exocyclic amide functionalities at one nitrogen atom [34], Similar reactions of 6 with methyl triflate were found to proceed even at room temperature to give l-methyl-3-alkyl-benzo-l,3,2-diazaphospholenium triflates [35, 36], The reported butyl halide elimination from NHP precursor 13 to generate 1,3,2-diazaphosphole 14 upon heating to 250°C and the subsequent amine addition to furnish 15 (Scheme 5) illustrates another example of the reversibility of addition-elimination reactions [37],... 

A mechanistic picture which reconciles the experimental results is given in Scheme 24. It is assumed that both the heteroatom and the double bond of the allyl halide compete for an electrophilic metal carbene. Heteroatom attack yields a metalated ylide 129, which may go on to ylide 131 by demetalation and/or to allylmetal complex 130. Symmetry-allowed [2,3] rearrangement of 131 accounts for product 132, and metal elimination from 130 gives rise to products 132 and 133, corresponding to [2,3] and [1,2] rearrangement, respectively, as well as haloacetate (if R3 = CHc ). 

Later we found that diperester 5 is a much better precursor for the preparation of 2.26 But the easiest way of generating 2 is the thermal HC1 elimination from 3-chlorocyclopropene (6).27,28 This reaction is astonishing and is probably due to the resonance stabilization of 2 in the singlet ground state, as the thermal elimination of HC1 (in the absence of a base) does not work with ordinary halides. It should be added that 2 has been detected in interstellar clouds and it is claimed that 2 may be one of the most abundant hydrocarbons in interstellar space. 

A more general method for preparing carbenes often involves the a elimination of halides from carbanions.1-57 PAC can be used to examine the rates and energetics of the reverse reactions, the complexation of halides with carbenes (Fig. 5).58 Plots of A//com versus the proton affinities (PA) of the halides are linear for the two carbenes studied. Although the slopes of the plots are similar, complexation of the halides with phenylchlorocarbene is more exothermic than phenylfluorocarbene. This indicates that fluoro substitution stabilizes the carbene relative to the carbanion more than chloro substitution. The rate of complexation of carbenes with salts has also been examined by nanosecond absorption spectroscopy.59... 

Starting from 41 or 42, organometallic methodology will lead to carbenoids, which are expected to eliminate metal halides and generate the corresponding carbenes these can be detected by appropriate trapping reactions. 

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