Halogen species

The latter is an exttemely reactive species. Trifluoroacetate is a good leaving group and facilitates cleavage of the O—Br bond. The acyl hypohalites are also the active halogenating species in solutions of the hypohalous acids in carboxylic acids, where they exist in equilibrium. 

Hypobromite has also been used as a sulphoxide oxidant. In this case, as with hypochlorite, multi-halogenated species are usually found. Thus, treatment of ethyl methyl sulphone with a basic aqueous hypobromite solution at 45 °C yielded the product shown in equation (37) in high yield97. In the presence of a-carboxylate groups, C—C bond cleavage occurs96 (equation 38). 

In vanadium-dependent haloperoxidases, the metal center is coordinated to the imidazole system of a histidine residue, which is similarly responsible for creating hypochlorite or hypobromite as electrophilic halogenating species [274]. Remarkably, a representative of this enzyme class is capable of performing stereoselective incorporation of halides, as has been reported for the conversion of nerolidol to various snyderols. The overall reaction commences through a bromonium intermediate, which cyclizes in an intramolecular process the resulting carbocation can ultimately be trapped upon elimination to three snyderols (Scheme 9.37) [275]. 

Figure 12.3 The strong oxidant chloramine-T can react with iodide anion in aqueous solution to form a highly reactive mixed halogen species. 125IC1 then can modify tyrosine and histidine groups in proteins to form radiolabeled products.

Figure 12.5 IODO-GEN is a water-insoluble oxidizing agent that can react with 1251 - to form a highly reactive mixed halogen species, 125IC1. This intermediate can add radioactive iodine atoms to tyrosine or histidine side chain rings.

Figure 15.8 The Bingel reaction for the modification of fullerenes involves the in situ formation of a reactive halogen species in the presence of the strong base DBU. The cyclopropanation product can be used to create many bioconjugates.

Common "parent" phosphorus-halogen species include particularly the P(III) system PC13 and the P(V) species P(0)C13. Of course, the pentacoordinated species PC15 must also be considered for its availability as reagent precursor. These are species that often serve not only in a direct manner in carbon-phosphorus bond formation but also in the construction of partially or fully esterified species prior to carbon-phosphorus bond incorporation. 

Acetals have been used in the presence of Lewis acids, particularly zinc chloride and ferric chloride, for the addition of phosphorus-halogen species to prepare 1-alkoxyphosphonic dichlorides and dialkyl 1-alkoxyphosphonates (from phosphorus trichloride and dialkyl phosphinous chlorides, respectively).143-145 It should be noted that good yields of these types of products have also been reported in the absence of catalysts.146 147 Other types of substrates have also been used in these types of processes. These include acylals,148 amidals,149 orthoformates,150 and orthoacetates.151... 

Conjugated olefinic systems have also been used in reaction with phosphorus-halogen species. For example, methylphosphonic dichloride adds regioselectively to acrylic acid in the presence of phosphorus trichloride (used as solvent), to produce the acid chloride 3-phosphonopropionyl chloride (Equation 4.35).158... 

The lifetimes of halogen species in the atmosphere are given in Ref. [122], These values are reproduced in Table 8.8. The incredibly long lifetimes of FI 1 and F12 and their gradual diffusion into the stratosphere pose the problem. Even though use of these materials has virtually stopped today, their effects are likely to be felt for decades. 

The best sensitivity for 1,2-dibromoethane quantification is obtained by either electron capture detector (ECD) or Hall electrolytic conductivity detector (HECD) in the halide detection mode, since these detectors are relatively insensitive to nonhalogenated species and very sensitive to halogenated species. Another common detection device is a mass spectrometer (MS) connected to a GC. The GC/MS combination provides unequivocal identification of 1,2-dibromoethane in samples containing multiple components having similar GC elution characteristics (see Table 6-2). To date, GC equipped with either ECD or HECD has provided the greatest sensitivity for detecting... 

Haloperoxidases act as halide-transfer reagents in the presence of halide ions and hydrogen peroxide. In the first step, the halide ion is oxidized to a halonium-ion carrier, from which the positive halogen species is then transferred to the double bond. In an aqueous medium, the intermediary carbocation is trapped and racemic halohydrins are formed (Eq. 7). Selective examples of CPO-cata-lyzed formation of halohydrins are given in Table 9. In CPO-catalyzed reaction. 

The distribution of halogen species in aqueous solution depends on pH and equilibrium constants for the above reactions. Table III lists the distribution of species for each halogen at the three pH levels reported in this paper. Derivation of equations for calculation of halogen species concentration are presented in reference [1 ]. 

Sodium periodate or positive halogenanting species such as A-chlorosuccinimide and f-bntyl hypochlorite" are nsefnl oxidizing reagents. 

However, with the more reactive chlorine, chlorination can occur at either position, though the major product is the benzylic halide. Benzylic bromina-tion is also efficiently achieved by the use of N-bromosuccinimide as the halogenating species. 

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