Carbon tetrahalides

I. Halides from Alcohols by Triphenylphosphine-Carbon Tetrahalide... 

A halogenating system related to the preceding case is formed by the reaction of triphenylphosphine with molecular bromine or chlorine. The system is not as sensitive to moisture as the phosphine-carbon tetrahalide system (see preceding section), but it suffers from the disadvantage that hydrohalic acids are produced as the reaction proceeds. Nevertheless, sensitive compounds can be successfully halogenated by the system, as exemplified by the preparation of cinnamyl bromide from the alcohol. 

Like tellurophene the fused compound forms 1,1-addition products with halogens (73BSF2468), but further bromination of such a 1,1-dibromo species gave 1,1,2-tribromobenzo[/>]tellurophene, which was able to be reduced to 2-bromobenzo[b]tellurophene. The 3-chloro and -bromo compounds were isolated following reaction of triphenylphosphine and the appropriate carbon tetrahalide with 2,3-dihydro-3-oxobenzo[b]telluro-phene (80BSB763). 

Sulfonylaziridine 243 was halogenated in carbon tetrahalides in the presence of KOH as base [86] (Scheme 5.61). Although other examples of electrophile trapping of sulfonyl- and phosphonyl-stabilized metalated aziridines exist, the reactions were not stereoselective [87]. 

I2 or with a carbon tetrahalide. " Carboxylic acids, esters, and amides have been... 

The advantages of the carbon tetrahalide-organophosphine-alcohol reaction to prepare halides are simplicity of experimental procedure good yields relatively mild, essentially neutral reaction conditions absence of allylic rearrangements. The reaction proceeds with inversion of configuration and is a useful simple device for converting optically active alcohols to chiral halides in high optical purity.12-22... 

The first isolable product in the reaction of triphenylphosphine and carbon tetrachloride is the salt (73), which reacts rapidly with further phosphine to give the stable phosphorane (74).62 In contrast, tris-t-butylphosphine reacts with germanium and tin tetrahalides to form the salts (75) 3 compounds of the latter type have long been postulated as arising from the reactions of phosphines with carbon tetrahalides but so far have defied detection. 

Although the use of N,N-dimethylformamide permits satisfactory chlorination or bromination of primary positions at room temperature, side reactions can occur between carbon tetrahalides, triphenylphosphine, and JV,N-dimethylformamide that can, in some cases, result in low yields of halogenated products (see also, Section II, 2c p. 230). 

Often compounds of CH2X2 type are called methylene halides, e.g. methylene chloride (CH2CI2), CHX3 type compounds are called haloforms, e.g. chloroform (CHCI3), and CX4 type compounds are called carbon tetrahalides, e.g. carbon tetrachloride (CCI4). Methylene chloride (dichlor-omethane, DCM), chloroform and carbon tetrachloride are extensively used in organic synthesis as nonpolar solvents. 

A number of other methods exist for the a halogenation of carboxylic acids or their derivatives.134 The acids or their chlorides or anhydrides can be a chlorinated by treatment with CuCl in polar inert solvents (e.g., sulfolane).135 Acyl halides can be a brominated or chlorinated by use of N-bromo- or N-chlorosuccinimide and HBr or HC1.136 The latter is an ionic, not a free-radical halogenation (see 4-2). Direct iodination of carboxylic acids has been achieved with L-Cu(II) acetate in HO Ac.137 Acyl chlorides can be a iodinated with L and a trace of HI.138 Carboxylic esters can be a halogenated by conversion to their enolate ions with lithium N-isopropylcyclohexylamide in THF and treatment of this solution at - 78° with I2138 or with a carbon tetrahalide.139 Carboxylic acids, esters, and amides have been a fluorinated at -78°C with F2 diluted in Ni.,4°... 

Palmer and Miller118,130 examined the reaction of carbon atoms with CC1, CBr, and Cl in diffusion flames of alkali metals with haloforms and carbon tetrahalides. They assumed that the reaction was... 

A variety of phosphorus-coutaining reagents have been employed in sugar chemistry in the direct SN2 replacement of hydroxy groups mainly by halide ions [35], Most of these reagents used a combination of triphenylphosphine with an electrophilic halogen source such as /V-halosuccinimide [36,37], carbon tetrahalides [38-41], and more recently, tri-haloimidazole or iodine and imidazole [42-45] or related reagents [46J. The Mitsunobu... 

Arnold, R. T. Kulenovic, S. T. Carbanion halo-genations with carbon tetrahalides. a-Halo esters./. Org. Chem. 1978, 43, 3687-3689. 

The second group of the reactions resulting in the formation of phosphorylated ketenes is based on the specific properties of trivalent phosphorus compounds (2 -- ).The Phosphines bearing an activated hydrogen atom in oC -position readily react with carbon tetrahalides... 

Phosphorous ylides 44 react with carbon tetrahalides (CC14, CBr4) by the replacement of the hydrogen atoms on the a-carbon with halogen atoms to form halo ylides 45 and 46, according to Kolodyazhnyi and Golokhov122 (equation 81). They stated that halogena-... 

The nucleophilic nature of these radicals allows addition to carbon-nitrogen multiple bonds, and in selected cases this has been demonstrated. Suitable substrates for such additions include protonated pyridines, thiazoles and imidazoles (equation 183), and nitroalkyl anions359. Yamamoto and coworkers have also described cobalt-mediated multiple additions of carbon tetrahalides, benzyl bromides or a-bromo ketones to isocyanides, which are postulated to occur through the corresponding alkylcobalt(III) complexes360. 

Some of the halomethanes have acquired common names that are not clearly related to their structures. A compound of formula CH2X2 (a methylene group with two halogens) is called a methylene halide, a compound of formula CHX3 is called a haloform, and a compound of formula CX4 is called a carbon tetrahalide. 

A molecular dipole moment is the vector sum of the individual bond dipole moments. Molecular dipole moments are not easy to predict because they depend on the bond angles and other factors that vary with the specific molecule. Table 6-1 lists the experimentally measured dipole moments of the halogenated methanes. Notice how the four symmetrically oriented polar bonds of the carbon tetrahalides cancel to give a molecular dipole moment of zero. 

Carbon tetrahalides are good electron acceptors and react with electrons to give trihalomethyl radicals and halide ions. Amines are known to react with carbon tetrahalides by an electron-transfer mechanism (Wyrzykowska et al., 1978 Iwasaki et al., 1978). Some careful product studies have been made of the reaction between carbazole and carbon tetrachloride (Zelent and Durocher, 1981). 

The reaction with carbon tetrahalides [213] probably involves free-radical attack e.g. by CBra) rather than electrophilic attack on the enol, but is still subject to selection of the reaction site leading to the more stable conjugated product (c/. p. 159). The reaction with methyl chloride (and a base) has been described only in a patent [214], and no mechanistic details are available. It is surprising that so unreactive an electrophile should attack the enol ether. There is no clear reason for chloromethylation favouring C(4), beyond the suggestion [21 ] that the reagent may first form a complex with the 3-acetoxy group. 

The unsaturation of the carbonyl group provides the possibili of the carbon expanding its coordination shell from 3 to 4. thereby lowering the activation energy. Carbon tetrahalide cannot follow a similar path, but the halides of silicon and phosi orus can employ id orbitals to expand their octets ... 

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