Alkyl halides dehalogenation

Predict the major and minor products of dehydrohalogenation of alkyl halides, dehalogenation of dibromides, and dehydration of alcohols. 

Dehydrohalogenation of Alkyl Halides Dehalogenation of 12-Dihalides Dehydration of Alcohols Alkenes from Ethers... 

Cr(II) has been used to bring about dehalogenation of alkyl halides involving the production of alkyl radicals, and details have been provided in a substantive review (Castro 1998). The ease of reduction is generally iodides > bromides > chlorides, while tertiary halides are the most reactive and primary halides the least (Castro and Kray 1963, 1966). 

An alternative route to sulphones utilizes the reaction of the appropriate activated halide with sodium dithionite or sodium hydroxymethanesulphinite [6], This procedure is limited to the preparation of symmetrical dialkyl sulphones and, although as a one-step reaction from the alkyl halide it is superior to the two-step oxidative route from the dialkyl sulphides, the overall yields tend to be moderately low (the best yield of 62% for dibenzyl sulphoxide using sodium dithionite is obtained after 20 hours at 120°C). The mechanism proposed for the reaction of sodium hydroxymethanesulphinite is shown in Scheme 4.20. The reaction is promoted by the addition of base and the best yield is obtained using Aliquat in the presence of potassium carbonate. It is noteworthy, however, that a comparable yield can be obtained in the absence of the catalyst. The reaction of phenacyl halides with sodium hydroxy-methane sulphinite leads to reductive dehalogenation [7]. 

NH3 is generated above pH 4.7. In these reactions, the hydride complexes [(Cp Ir)2(p-H)(p-OH)(g-HCOO)]" (25) and [Cp Ir(bpy)(H)]" (31), which would be generated from the reactions of 24 and 28 with HCOO, would be key catalytic intermediates. The dehalogenation of the alkyl halides using 24 did not occur, most likely due to the bulkiness of the active catalyst 25 compared to 31 in SN2-type reactions (entry 5). 

Alkenes are obtained by the transformation of various functional groups, e.g. dehydration of alcohols (see Section 5.4.3), dehalogenation of alkyl halides (see Section 5.4.5) and dehalogenation or reduction of alkyl dihalides (see Section 5.4.5). These reactions are known as elimination reactions. An elimination reaction results when a proton and a leaving group are removed from adjacent carbon atoms, giving rise to a tt bond between the two carbon atoms. 

Lithium triethylborohydride (Super-Hydride) is a much more powerful reducing agent than lithium aluminium hydride. It is useful for the reductive dehalogenation of alkyl halides, but unlike lithium aluminium hydride does not affect aryl halides. It is available as solution in tetrahydrofuran in sealed containers under nitrogen. The solutions are flammable and moisture sensitive and should be handled with the same precautions as are taken with other organometallic reagents (see Section 4.2.47, p. 442). 

The reduction potentials for various alkyl halides range from +0.5 to +1.5 V therefore, when Fe° serves as an electron donor, the reaction is thermodynamically favorable. Because three reductants are present in the treatment system (Fe°, H2, and Fe2+), three possible pathways exist. Equation (13.9) represents the oxidation of Fe° by reduction of a halogenated compound. In the second pathway, the ferrous iron behaves as a reductant, as represented in Equation (13.10). This reaction is relatively slow because the ability to reduce a pollutant by ferrous iron is dependent on the speciation ferrous ions, which is determined by the ligands present in the system. The third possible pathway, Equation (13.11), is dehalogenation by hydrogen. This reaction does not occur easily without a catalyst. In addition, if hydrogen levels become too high, corrosion is inhibited (Matheson and Tratnyek, 1994) ... 

Mechanism of the radical chain dehalogenation of alkyl halides by tributyltin hydride. The various termination steps are not shown. 

Radical Dehalogenation of an Alkyl Halide with Tributyltin Hydride Section 21.7 Figure 21.3... 

In Chapter 4, we saw that Sml2 mediates the dehalogenation of a range of alkyl halides. These reactions proceed by reduction to alkyl radicals that are then... 

For a way of overcoming this problem, let s go back to the reaction we looked at a few pages ago, the dehalogenation of alkyl halides by Bu3SnH. The mechanism involves formation of an alkyl (carbon-centred) radical by abstraction of Br by Bu3Sn. This alkyl radical then just abstracted H from... 

Red-Al [sodium bis(2-methoxyethoxy)aluminium hydride] reduces aliphatic halides and aromatic halides to hydrocarbons. Reductive dehalogenation of alkyl halides is most commonly carried out with super hydride. Epoxide ring can also be opened by super hydride. 

With PCI3, Cu atoms yielded smaller amounts of product. Copper also proved useful in dehalogenation reactions with alkyl halides ... 

The reduction of saturated alkyl halides to alkanes, as represented in equation (1), is the most fundamental reaction of reductive dehalogenations of organic halides. The importance of these reductions has stimulated considerable investigation, and a number of successful approaches have been reported hitherto. Numerous reducing agents or reagent systems are available and many of them have been applied to practical organic synthesis with notable success. 

The order of ease of reductive dehalogenation of organic halides in the same type of structural environment is I > Br > Cl F. This order is parallel with the dissociation energy of carbon-halogen bonds (HsC—I 234 kJ mol- H3C—Br 293 kJ mol" H3C—Cl 351 kJ mol- H3C—F 452 kJ moL ) and is generally observed in the reduction of alkyl halides. Consequently, selective reduction of di- or polyhalides containing different halogen atoms is possible. Fluorides are often removed only with difficulty and examples of such reductions are comparatively limited. 

A classical method using Na- or Li-liquid ammonia (Birch reduction conditions) is effective for reductive dehalogenations of aryl and vinylic halides, but it is not always successfully applied to alkyl halides, although cyclopropyl halides and bridgehead halogens are exceptions.Under such conditions, the reactions are often accompanied by side reactions, such as elimination, the Wurtz coupling reaction, cyclization and reduction of carbonyl compounds. An example, a synthesis of pentaprismane (1), is shown in Scheme 4. ... 

A currently employed method for dehalogenation of alkyl halides is to use a Li- or Na-alcohol reagent system. This method is effective not only for simple alkyl halides but also for the reduction of a halogen atom attached to a bridgehead. Carbon-carbon unsaturated bonds are not affected under the conditions, as shown in Scheme 5. ... 

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