Halogen source

Both electrophilic and nucleophilic reactions can generate halogenopur-ines with differences in regioselectivity dependent on substituents and on the nature of the substrate (anion, neutral molecule, or cation). In the neutral molecule nucleophilic displacements occur in the order 2 > 4 > 6 in the anion the imidazole ring may be sufficiently 7r-excessive for attack to occur at C-2, and the nucleophilic substitution order becomes 4 > 6 > 2. Strong electron donors are usually necessary to promote 2-halogenation by electrophilic halogen sources. 

Hydrozirconation of terminal alkynes R-C=CH (R= aryl, alkyl) with 1 affords terminally ( )-Zr-substituted alkenes with high efficiency and excellent stereochemical and regiochemical control (>98%). These alkenylzirconocene complexes are of particular interest for synthetic use [136, 143, 144]. Moreover, beside the electropositive halogen sources [145] and heteroatom electrophiles [3] used in the pioneering studies to directly cleave the Zr-C bond, ( )-vinyl-Zr complexes were recently transformed into a number of other trans-functionalized alkenes such as ( )-vinyl-sul-fides[146], vinylic selenol esters [147], vinyl-sulfones [148], vinyl-iodonium [149], vinyl-(R0)2P(0) [150], and vinilic tellurides [143]. 

Dichloro-2-N-octyl-isothiazolin-3-one and 2-methylthio-4-tert-butylamino-6-cyclopropylamino-S-triazine or N,N-dimethyl-N -phenyl-N -fluorodichloromethylthio)sulfamide Anthraquinones Free halogen sources with 5,5-dimethylhydantoin,... 

A variety of reagents can function as the electrophile E+ in the general mechanism. The most useful synthetic procedures for preparation of halides are based on the halogens, positive halogens sources, and diethyl azodicarboxylate. A 1 1 adduct formed from triphenylphosphine and bromine converts alcohols to bromides.15 The alcohol displaces bromide ion from the pentavalent adduct, giving an alkoxyphosphonium intermediate. The phosphonium ion intermediate then undergoes nucleophilic attack by bromide ion, forming triphenylphosphine oxide. 

Carboxylic acids can be converted to acyl chlorides and bromides by a combination of triphenylphosphine and a halogen source. Triphenylphosphine and carbon tetrachloride convert acids to the corresponding acyl chloride.100 Similarly, carboxylic acids react with the triphenyl phosphine-bromine adduct to give acyl bromides.101 Triphenviphosphine-iV-hromosuccinimide also generates acyl bromide in situ.102 All these reactions involve acyloxyphosphonium ions and are mechanistically analogous to the alcohol-to-halide conversions that are discussed in Section 3.1.2. 

Bromoamidation of cyclic olefins allowed the synthesis of bicyclic oxazolines. For instance, treatment of cyclohexene with A-bromoacctamidc as the halogen source and different nitriles at 0 °C, in the presence of SnCU or BF3 Et20 and water, led to oxazolines 128 through intermediate traw.v-bromoamides 127. The scope of the bromoamidation appears quite broad with regard to olefinic and nitrile components <06JA9644>. 

AS A FLAME RETARDANT. The zinc borate is an efficient synergist of organic halogen sources. In certain halogen-containing systems such as unsaturated polyester, epoxy (3), and rigid PVC, the zinc borate alone can outperform antimony oxide as shown by the Oxygen Index and UL-94 tests (Fig. 3, 4, and 5). 

See also Halides Salts Halogen sources, activation of,... 

The disproportionation of III into IV and V can be rationalized on the basis of the known reaction of phosphoranes with halogen sources giving a-haloalkyl-phosphonium salts. 

By this procednre excess 1,2-dibromotetrachloroethane acts as a halogen source, converting a small amount of telluride into the corresponding dibromide, which is then... 

The formation of an acyl halide involves the reaction of a carboxylic acid with a halogen source. The common halogen sources tire compounds like PX3, PXj, ClOCCOCl (oxalyl chloride), or SO.V2> where. Y is a halogen. The most commonly used acyl halides are the chlorides, and the simplistic reaction is RCOOH —> RCOCl. Figure 12-15 illustrates the mechanism using thionyl... 

A-Chlorohydroxamic esters (2) are universally synthesized from the parent hydroxamic esters (4) using the positive halogenation source, tert-butyl hypochlorite " . Reactions are... 

A wide variety of species can function as the electrophile E+ in the general mechanism. The most useful synthetic procedures for preparation of halides are based on the halogens, positive halogen sources, and diethyl azodicarboxylate. 

Sato and Narita provided an improved synthesis of various halopyrazines in which hydroxypyrazines 160 were activated with TMSCl to give silyl ethers 161 <99JHC783>. Subsequent treatment of 161 with the appropriate phosphorus-based halogen source provided halopyrazines 162 in 46-94% overall yield. This two-step process was accomplished without isolation of intermediate 161 and provides a milder, more convenient approach than the traditional heating of hydxoxypyrazines with PX directly. 

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