Hypohalite addition

Addition of Hypohalous Acids and Hypohalites (Addition of Halogen, Oxygen)... 

Figure 10.9 Hypohalite addition across one of the weak, central double bonds of /3-carotene. The colour of an adsorbed material is lost when exposed to bleach because of hypohalite addition across a double bond, thereby decreasing the extent of conjugation, if not removing it altogether...

A variation of the general procedure discussed in this section consists of using acyl hypohalite addition to double bonds, according to Eq. (129). 

This section concludes with a reminder that, in addition to the hypohalous acids HOX and metal hypohalites M(OX) , various covalent (molecular) hypohalites are known. Hypochlorites are summarized in Table 17.22. All are volatile liquids or gases at room temperature and are discussed elsewhere (see Index). Organic hypohalites are unstable and rapidly expel HX or RX to form the corresponding aldehyde or ketone ... 

In addition to the applications indicated on p. 858. hypohalous acids are useful halogenating agents for Ixjth aromatic and aliphatic compounds. HOBr and HOI are usually generated in silii. The ease of aromatic halogenation increa.ses in the sequence OCl < OBr < Ol and is facilitated by salts of Pb or Ag. Another well-known reaction of hypohalites is their cleavage of methyl ketones to form carboxylates and haloform ... 

The traditional method of oxidizing hydrazine derivatives makes use of halogens or hypohalites as oxidizing agents. The techniques range from the preparation of l,l -azobis(l-cyclohexanenitrile) by the addition of bromine to an alcoholic hydrochloric acid solution of the corresponding hydrazine [89], through the use of bromine water [90, 91] to oxidations with sodium hypo-bromite [64] or sodium hypochlorite [92]. 

Boron trifluoride and boron trifluoride-diethyl ether complex can be used as a source of fluoride ions in the presence of hypobromites and hypochlorites, e.g. methyl hypobromitc, tert-butyl hypobromite, methyl hypochlorite in carbon tetrachloride at 25 C. The addition of bromine monofluoride" and chlorine monofluoride" to various alkenes is accompanied by the formation of the corresponding alkoxybromides and alkoxychlorides which hinder the isolation of the halofluorinated products.57 jV-Bromo- and A -chloro-substiluted alkyl- and arylamines. -amides, and -imides, A -chloro-A,-methylamine, A -bromo-A -methylamine, A -chloro-A, /V-dimethylamine, A-bromo-A.A-dimethylamine, ACV-dichloro-A -methylamine, V,fV-dibromo-,V-mcthylaminc, A -bromosuccinimide, -V-chlorosuccinimide, Af-bromoacct-amide, A.A -dichlorourethane, can be used in the reaction instead of the hypohalites. The reactions with various alkenes conducted in dichloromethane at room temperature in the presence of boron trifluoride-diethyl ether complex produce bromofluoro and chlorofluoro addition products in 40-80 % yield. However, the reactions are complicated by the addition of A -halo-succinimides and Af.A-dichlorourcthane to the C = C bonds.58... 

This alkoxyfluorination leads to alkoxy fluoro and difluoro compounds. The product distribution depends on the reaction conditions. Similarly, fluoro halides are obtained in addition to alkoxy halides using a combination of boron trifluoride and alkyl hypohalite.50... 

Alkyl and acyl hypohalites, when adding to carbon-carbon double bond, afford halohydrin ethers and esters, respectively.151 Regioselective and syn stereoselective addition of CF3OF, CF3CF2OF, and CF3COOF to stilbenes was reported.152-154 The stereochemistry was explained to originate from the formation and immediate... 

Other alkyl hypohalites usually add to carbon-carbon multiple bonds in a free-radical process.155-158 Ionic additions may be promoted by oxygen, BF3, or B(OMe)3.156-160 While the BF3-catalyzed reaction of alkyl hypochlorites and hypo-bromites gives mainly halofluorides,159 haloethers are formed in good yields but nonstereoselectively under other ionic conditions.156-158 160 In contrast, tert-BuOI reacts with alkenes in the presence of a catalytic amount of BF3 to produce 2-iodoethers.161 Since the addition is stereoselective, this suggests the participation of a symmetric iodonium ion intermediate without the involvement of carbocationic intermediates. 

Alkoxyl radicals can be generated by a variety of methods including peroxide reduction, nitrite ester photolysis, hypohalite thermolysis, and fragmentation of epoxyalkyl radicals (for additional examples of alkoxyl radical generation, see Section 4.2.S.2). Hypohalites are excellent halogen atom donors to carbon-centered radicals, and a recent example of this type of cyclization from the work of Kraus is illustrated in Scheme 43.182 Oxidation of the hemiketal (57) presumably forms an intermediate hypoiodite, which spontaneously cyclizes to (58) by an atom transfer mechanism. Unfortunately, the direct application of the Barton method for the generation of alkoxyl radicals fails because the intermediate pyridine-thione carbonates are sensitive to hydrolytic reactions. However, in a very important recent development, Beckwith and Hay have shown that alkoxyl radicals are formed from N-alkoxypyridinethiones.183 Al-... 

The synthesis of chlorine (I) and bromine (I) trifluoromethanesulfonates (triflates) was reported by DesMarteau. Stability and reactivity of these materials are similar to those of perfluoroalkyl hypohalites. Both compounds readily react at low temperature with a variety of fluoroolefins. Based on NMR analysis of the products of adding CF3S020X to pure cis- or trans-isomers of 1, 2-difluoroethylene, it was concluded that the reaction proceeds as syn-addition [35]. This statement was later criticized [18], since the assignment of stereoisomers was found to be incorrect. According to [18], addition of CF3S020X to haloolefins, as well as reactions of ClF, BrF and IF proceed as anti-addition via cyclic halonium cationic intermediates. 

The addition of silver or mercury oxide precipitates the metal halide, leaving behind the aqueous solution, which is very reactive and is used for bleaching and sterilizing purposes. Solutions of hypohalites can be made by introducing the appropriate halogen into basic solutions (equation 113), but these will also contain some halide anions. To obtain pure hypochlorites, it is necessary to carefully neutralize pure hypochlorous acid. The most stable hypochlorites are those of Li, Ca, Sr, and Ba, although some others are also known but are not as stable, or are not sufficiently pure. 

In addition to the existence of ionic hypohalites, molecular hypohalites are also known such as CIONO2 and CIOCIO3 with the ClO-group. 

The latter possibility obeys the peculiar synergistic effect of the C-2 alkoxy substituent of I that makes these dihydropyrans unique. Without it, nothing of this occurs. These substrates have been used for the detection of positive charge development at C-6, among other things, in additions of alkyl hypohalites and some diazocarbonyl compounds to the double bond, and in the interesting fragmentation of norcarane derivatives illustrated in Scheme 38.2. ... 

The hypohalite oxidations are easy to carry out. The methyl ketones, pure or dissolved in dioxane, are added to cooled or warm solutions of hypohalites in water. The reverse order of addition has also been used [736], An exothermic reaction ensues, and the haloform starts forming a heavy organic layer. The excess hypohalite is destroyed by sodium bisulfite, the heavy layer is separated, the residual haloform is removed by steam distillation, and the aqueous solution of the alkaline salt of the carboxylic acid is treated with sulfuric or hydrochloric acid (equation 420). 

The oxidation of a methyl ketone to a carboxylic acid with one less carbon by oxidants other than hypohalites is exemplified by the oxidation of 2-acetylfluorene with sodium dichromate. In addition to the methyl keto group, the methylene group is also oxidized (equation 432) [626]. 

This reaction is naturally strongly dependent on pH the addition of alkali favours the formation of halide and hypohalite. 

Alkoxy radicals for ring expansion can be generated from alcohols by oxidative methods such as hypohalite thermolysis/photolysis [19a] and lead tetraacetate oxidation [19b], or peroxide reduction [19c]. The recent development of the hyper-valent organoiodine reagent (diacetoxyiodo)benzene (DIB) provides another way for efficient generation of alkoxy radicals (Scheme 11) [19d]. Additional oxidative methods to prepare cyclopropyloxy radicals include reaction of tertiary cyclopropanols or their silyl ether derivatives with various reagents such as manganese(III) tris(pyridine-2-carboxylate) [Mn(pic)3] [20a], Fe(III) salts [20b], and vanadyl ace-tylacetate [20c] (Scheme 12). 

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