Hydrogen halide eliminations

TABLE 5. Survey of sulfonyl halide additions to unsaturated systems and hydrogen halide eliminations to unsaturated sulfones... 

As a consequence of facile homolytic cleavages, sulfonyl halides (I > Br > Cl F unsuitable) are able to add to unsaturated C—C systems. To prevent (or reduce) competing polymerizations, the additions of sulfonyl chlorides have been recommended to be carried out in the presence of copper(I/II) salts (Asscher-Vofsi reaction ). Comprehensive surveys have been published on the resulting j8-halogeno sulfones (or their vinyloguous compounds) as well as on their dehalogenation products (vinyl sulfones, 1-sulfonyl-l, 3-dienes, etc.). Table 5 reviews a series of sulfonyl halide additions and facile hydrogen halide eliminations. 

In other systems ( 5.8.4.2.2) hydrogen halide elimination often follows oxidative addition of Ge—H, and such addition may be the initial step in other fast reactions where only the elimination is observed. 

CHjCFCl. Fluence dependence studies indicate that competitive hydrogen halide elimination channels can be influenced by up>-pumping rates near the dissociation threshold... 

Hydrogen halide eliminations take place from 1,1- and 1,2-dihaloalkanes (equations 5 and 6), as w ell as from vinyl halides (equations 7 and 8). Dehydrohalogenation... 

Alkoxyacetylenes are obtained by alcohol and hydrogen halide elimination from dialkylacetals of a-chloroaldehydes, induced by NaNH, in liquid ammonia (equation 81)108. 4-Pentyn-l-ol is obtained in similar fashion (equation 82) . 

If the decomposition is a unimolecular process then a loose, four-centred transition state has to be assumed. The log 15 value, though slightly high, is not unreasonable considering other four-centred elimination reactions such as, for instance, the hydrogen halide eliminations. However, the activation energy, determined experimentally, is higher than expected for such a reaction. 

The gas-phase photochemistry of haiogenated ethenes has been studied in the UV and VUV [60, 61], as well as in the infrared, using multiple-photon-absorption excitation with powerful CO2 laser sources [62-66]. Also, sensitized decompositions, for example using electronically excited Hg( P) atoms, have also been reported [67-69]. The net gas-phase photochemistry of these systems usually involves hydrogen halide elimination via three-and/or four-center transition states, with some evidence for simple bond fission producing halogen atoms in the case of Hgf Pj) photosensitization [70]. 

By far the most important type of reaction displayed by halophosphines is nucleophilic substitution. This is pivotal to the preparation of many other three-coordinate compounds containing either solely P—C, P—O, P—N bonds, or mixed combinations. These reactions are often exothermic and frequently carried out at low temperatures. For the synthesis of phosphorus(III) compounds containing a P—O or P—N bond it is often necessary to add a base (triethylamine or pyridine are frequently used) to capture the hydrogen halide eliminated from these condensation reactions. In the case of P—C bond formation, a variety of routes are possible using various carbon-derived nucleophiles. 

Many cyclopropyl chlorides and bromides have been converted to alkoxycyclopropanes by treatment with a strong base, in most cases potassium rerf-butoxide, in an appropriate organic solvent (Table 13). Under such conditions, hydrogen halide elimination takes place, yielding strained cyclopropene intermediates, which are trapped by nucleophilic attack of the alkoxide. Overall, a simple substitution occurs when a bond is formed between the alkoxide group and the carbon atom to which the halide was attached. This is the case when l-chloro-5-methyl-exo-6-phenyl-3-oxabicyclo[3.1.0]hexan-2-one (1) was reacted with potassium /ert-butoxide l-/er/-butoxy-5-methyl-ent/o-6-phenyl-3-oxabicyclo[3.1.0]hexan-2-one (2) was isolated in 94% yield.If a C-O bond is established at the other olefinic carbon atom, a C H bond is concomitantly formed at the carbon atom, to which the halide was attached. The result is a double substitution which is discussed elsewhere (see Section 5.2.1.3 ). When the substrate contains more than one halogen atom, several elimination reactions usually take place. Thus, treatment of 1 -bromo-2-chloro-2-methylcyclopropane (3) with an excess of potassium /er/-butoxide gave l-ter/-butoxy-2-methylenecyclopropane (4) in 30% yield. 

Boehm, H.-P., Mair, G., Stohr, T., et al. (1984). Carbon as a catalyst in oxidation reactions and hydrogen halide elimination reactions. Fuel, 63, 1061-3. 

Hydrogen halide elimination is a useful method of obtaining RP=CR2-type compounds (6.582), some of which are very stable (6.583). 

The knowledge of amino-iminoboranes has increased significantly since the preparation of the first few examples of this type of compounds (see Boron Compounds 3rd Suppl. Vol. 3, 1988, p. 100), and two reviews have appeared about the chemistry of such species [1, 2]. Compounds of the general formula RN=B-NR R, which can be isolated at room temperature, are kinetically stabilized by steric protection. A survey is compiled in Table 4/9 together with the synthetic method employed and relevant NMR and v( BN) IR data. The sterically hindered amino-iminoboranes are prepared either by vacuum pyrolysis and FSi(CH3)3-elimina-tion from their fluorine-containing diaminoborane precursors in the gas phase, or by hydrogen halide elimination with bases from the corresponding diaminoborane derivatives in solution. 

The ease of hydrogen halide elimination can also be used for forming chloromaleic acid 69 from dichlorosuccinic acid 56. 

Kalume A, George L, Cunningham N, Reid SA (2013) Concerted and sequential pathways of proton-coupled electron transfer in hydrogen halide elimination. Chem Phys Lett 556 35-38. doi 10.1016/j.cplett.2012.11.053... 

The use of sodium hydrogen carbonate under solvent-free conditions and microwave irradiation is best method for N-alkylating pyrazoles. The yields are good and the method is devoid of side reactions like quatemization, isomerization and hydrogen halide elimination. Solvent-free conditions are the only ones that allow the preparation of 1-substituted pyrazoles from secondary halides (Almena et al., 2009). 

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