2.3- Dibromobutane, reduction

Cobalt(I) salen has been employed as a catalyst for the reduction of the following species benzal chloride [159] benzotrichloride [160] 1-bromobutane, 1-iodobutane, and 1,2-dibromobutane [161] iodoethane [162], benzyl chloride [163], and ethyl chloroacetate [164]. Rusling and coworkers have investigated the use of cobalt(I) salen, as well as vitamin Bi2s and cobalt(I) phthalocyanine, in both homogeneous phase and bicontin-uous microemulsions for the catalytic reduction of vicinal dibromides [165] and... 

Castro, C.E. and Belser, N.O. Biodehalogenation. Reductive dehalogenation of the biocides ethylene dibromide, l,2-dibromo-3-chloropropane, and 2,3-dibromobutane in soil, Environ. Sci. Technoi, 2(10) 779-783, 1968. 

Low temperature cyclic voltammetry is also able to demonstrate reduction of the individual rotamers of 2,3-dibromobutane [115]. At room temperature when there is fast bond rotation, reduction proceeds through the conformation with trans-periplanar arrangement of carbon-bromine bonds. At -120° C, a second peak at more negative potentials appears in the cyclic voltamogram, due to elimination from tlie staggered arrangement of carbon-bromine bonds. 

In the rings containing sulfur, reduction of sulfoxides of phenoxathiin and thianthrene can be performed in excellent yield with the aluminium chloride/sodium iodide or zinc dust/l,4-dibromobutane systems <1996CHEC-II(6)447>. 

In the synthesis of analogues of calicheamicin 71 and esperamicin Ajb, Moutel and Prandi employed the glycosyla-tion of a nitrone with a trichloroacetimidate as a key step - /3-N-O glycosidic bond formation. Preparation of the nitrone begins with the alkylation of the known alcohol 69 <1992CC1494> with 1,4-dibromobutane in the presence of sodium hydride. Subsequent aminoalkylation, amine protection with 9-fluorenylmethoxycarbonyl (Fmoc), and reduction with NaBHsCN were followed by nitrone 70 formation with 4-methoxybenzaldehyde (Scheme 8) <2001J(P1)305>. 

An alternate method for cyclizing ae./i-dihalobutanes is to use a controlled potential electrolytic reduction. 10 12 This method appears to be superior to the conventional reductive cyclization of 1,4-dihalobutanes with metals. Dibromides generally give better results than dichlorides in an aprotic solvent such as dimethylformamide or acetonitrile. Thus, a DC voltage of 1.8-3.0 V was applied for 6 hours to a solution of 1,4-dibromobutane (50 g) in dimethylformamide (1 L) in a cell consisting of a mercury cathode and a nichrome anode, to give cyclobutane and butane in 25 and 75 % yield, respectively.10,11 The experimental setup has been described in a detailed procedure.12... 

As described earlier, Wiberg145 reported the electroreduction of 1,4-dibromobutane at platinum and mercury-coated platinum giving evidence for a stepwise process, and a related study of the reduction of 1,4-dibromobutane at a reticulated vitreous carbon cathode in DMF containing TMAP was reported by Peters and collaborators158. 

In addition to the foregoing 1,4-difunctional compounds used for cyclizations, the following have been used to give pyridazines or theirreduced analogs , 4-hydroxy ketones, l,4-haloketones, l,4-dibromobutane, 2,5-dibromohexane, 1,4-halo-esters, or butadiene dioxide. Hexahydropyridazine results as one of the oxidation products of 1,4-diaminobutane and its 1-butyl analog is formed in the reduction of iV-nitroso-4-chloro-dibutylamine. ... 

Sodium arenetellurolates, prepared by reduction of diaryl ditelluriums with sodium borohydride, reacted with 1,4-dichlorobutane, 1,4-dibromobutane , and 1,5-dibromo-pentane forming aryl tetra- or pentamethylene telluronium halides. 

A number of other publications deal with the electrochemical behavior of a, oj-dibromoalkanes at mercury electrodes polarographic studies [85,86] electrolyses of 1,3-dibromopropane, 1,4-dibromobutane, and 1,5-dibromopentane [87,88] reduction of 1,10-dibromo- and 1,10-diiododecane [89] and electro syntheses of phenylcyclopropane and cyclopropanol [90]. In the presence of an arylalkene, reduction of 1,3-dibromopropane or 1,4-dibromobutane at nickel in DMF containing TBABr and with a sacrificial aluminum anode yields the corresponding cyclopentane or cyclohexane adduct [32] ... 

Allyl halides have been reduced with electrogenerated tris(bipyridine)cobalt(I) to afford 1,5-hexadiene [369,370]. Some of the earliest work with cobalt(I) salen involved its use for the catalytic reduction of bromoethane [371], bromobenzene [371], and /er/-butyl bromide and chloride [372]. More recently. Fry and coworkers examined the cobalt(I) salen-cata-lyzed reductions of benzal chloride [373-375] and of benzotrichloride [376], and the catalytic reductions of 1-bromobutane [377,378], 1-iodobutane [378], 1,2-dibromobutane [378], benzyl and 4-(trifluoromethyl)benzyl chlorides [379], iodoethane [380], diphenyl disulfide [381], 1,8-diiodooctane [382], and 3-chloro-2,4-pentanedione [383] have been investigated. 

Rusling and coworkers have carried out extensive studies of the use of electrogenerated cobalt(I) complexes (including cobalt(I) salen, vitamin Bi2s, and cobalt(I) phthalo-cyanine) as catalysts both in homogeneous phase and in bicontinuous microemulsions [384] for the reductions of 1,2-dibromoethane and 1,2-dibromobutane [385], the debromi-nation of alkyl vicinal dibromides [386], the dechlorination of DDT [387], the reductions of 1-bromobutane, 1-bromododecane, and ran5-l,2-dibromocyclohexane [388,389], and the reduction of benzyl bromide [390]. 

Other uses of cobalt(I) catalysts include the reductive intramolecular cyclization of bromocyclohexenones to form bicyclic ketones [391] and the radical cyclization of bro-moacetals [392,393]. Krautler and coworkers [394] found that 1,4-dibromobutane interacts with electrogenerated cob(I)alamin to afford a tetramethylene-l,4-di = Co -cobalamin species. In a recent study of the reactions of cobalt(I) tetraphenyl porphyrin with benzyl chloride or 1-chlorobutane, Zheng and coworkers [395] reported that alkyl radicals are transferred from the cobalt center to a nitrogen of a pyrrole ring, leading to formation of an A-alkyl cobalt porphyrin complex. 

Some papers have appeared that deal with the use of electrodes whose surfaces are modified with materials suitable for the catalytic reduction of halogenated organic compounds. Kerr and coworkers [408] employed a platinum electrode coated with poly-/7-nitrostyrene for the catalytic reduction of l,2-dibromo-l,2-diphenylethane. Catalytic reduction of 1,2-dibromo-l,2-diphenylethane, 1,2-dibromophenylethane, and 1,2-dibromopropane has been achieved with an electrode coated with covalently immobilized cobalt(II) or copper(II) tetraphenylporphyrin [409]. Carbon electrodes modified with /nc50-tetra(/7-aminophenyl)porphyrinatoiron(III) can be used for the catalytic reduction of benzyl bromide, triphenylmethyl bromide, and hexachloroethane when the surface-bound porphyrin is in the Fe(T) state [410]. Metal phthalocyanine-containing films on pyrolytic graphite have been utilized for the catalytic reduction of P anj -1,2-dibromocyclohexane and trichloroacetic acid [411], and copper and nickel phthalocyanines adsorbed onto carbon promote the catalytic reduction of 1,2-dibromobutane, n-<7/ 5-l,2-dibromocyclohexane, and trichloroacetic acid in bicontinuous microemulsions [412]. When carbon electrodes coated with anodically polymerized films of nickel(Il) salen are cathodically polarized to generate nickel(I) sites, it is possible to carry out the catalytic reduction of iodoethane and 2-iodopropane [29] and the reductive intramolecular cyclizations of 1,3-dibromopropane and of 1,4-dibromo- and 1,4-diiodobutane [413]. A volume edited by Murray [414] contains a valuable set of review chapters by experts in the field of chemically modified electrodes. 

Various other reagents can be used for this reductive debromination. 1,2-Dipropylcyclopropane was prepared from 4,6-dibromononane using chromium(II) perchlorate in dimethylformami-de/water (yield 93%), lithium amalgam in tetrahydrofuran (75%), lithium biphenylide in te-trahydrofuran (78%), potassium-sodium alloy in tetrahydrofuran (68%), zinc dust and zinc(II) chloride in propan-2-ol/water (95%) and alkyllithiums in tetrahydrofuran (BuLi 16%, i-BuLi 18%, t-BuLi 47%). Ring closure of 1,3-dibromobutane to methylcyclopropane was achieved by treatment with zero-valent copper, which was obtained from reaction of lithium naphthalen-ide and copper(I) iodide/tributylphosphane in tetrahydrofuran (yield 91%) ... 

The first step was achieved by three alternative procedures (i) the action of aluminum telluride on 1,4-dihalobutane at temperatures ranging from 125° to 175° this method is very tedious and the yield is poor (ii) the reaction between amorphous tellurium and 1,4-diiodobutane at 130°-140°, which gives a good yield (iii) the reaction between sodium telluride made in situ and 1,4-dibromobutane this is the most reliable method. The subsequent reduction of 1,1-dihalotetrahydrotellurophene (22) has been carried out using sulfur dioxide. By a similar procedure 3,3 -bistetrahydrotellurophene (24) has been prepared. 8,69... 

Dihalobutanes are used to prepare all three saturated heteroatom systems. Arsolanes are prepared by reductive cyclization of intermediate iodoarsines (Scheme 32). Arsolane synthesis by this method is reported to be superior over that with the Grignard reagent obtained from 1,4-dibromobutane <77JCS(D)704>. These arsolanes are also prepared by condensation of disodium arsenide with l,4-4ichlorobutane <70ZAAC(377)278>. 

Even more attractive is however to react the nitrile with 1,4-dibromobutane and carbon dioxide in a double Grignard reaction. The thiazolidine ring is in this case as well opened reductively to a ketone intermediate, which is epimerised and spontaneously forms the benzyl-protected dehydrobiotin. Hydrogenation and deprotection have already been discussed for Gerecke s route. 

A familiar example of a stereoselective reaction would be the formation of a higher yield of f rans-2-butene than cis-2-butene in an E2 reaction, no matter whether the starting material is (R)- or (S)-2-bromobutane. The addition of bromine to trans-2-butene to produce mcso-2,3-dibromobutane or the addition of bromine to the cis isomer to produce an equimolar mixture of the two enantiomers of 2,3-dibromobutane is a stereospecific reaction. Note that a reaction that gives only one of a pair of eirantiomers is not necessarily stereospecific. A yeast-mediated reduction of 3-chloropropiophenone gives (S)-3-chloro-l henylpropan-l-ol, with no evidence for formation of the R enantiomer. Because the reactant cannot exist as stereoisomers, it is not possible for stereoisomerically different reactants to give stereoisomerically different products, and the reaction can only be considered stereoselective, not stereospecific. 

Microelectrode voltammetry was used to study the mechanism of the catalysis by Co(II)(salen) of the reduction of butyl iodide, butyl bromide, and 1,2-dibromobutane in DMF and the influence of electrolytes on these reactions. The voltammetric characteristics can only be understood in terms of mechanisms that include additional steps to those proposed previously in the literature. ... 

The racemic acid was reduced somewhat more readily than the meso acid in acidic solution and with greater difficulty in neutral and weakly alkaline solutions. The difference in the half-wave potentials amounted to 0.05-0.07 V and the half-wave potentials of the esters differed even less. It is interesting that the reduction products from the diastereoisomers differ in the case of the acids and that the same product is always obtained in the case of the esters (further details below). McKeon [84] found much greater differences in the half-wave potentials of the meso and racemic compounds, amounting to 0.2-0.3 V, in the case of 1,4-dihydroxy-and l,4-dinitroxy-2,3-dibromobutanes. The latter give two waves, in the first of which the dihydroxy compound is formed in the second, as in the single wave of a dihydroxy compound, the respective unsaturated compound is evidently formed ... 

This behavior is explained by the fact that the monoanion of the racemic acid, which exists in weakly acidic solutions, is evidently capable of forming (through a hydrogen bond) a seven-mem-bered ring with the bromine atoms in the trans position. In trans-elimination this favours the formation of the isomer,i.e., maleic acid. The analogous cyclic monoanion of the meso acid is energetically favorable, since the bromine atoms are in the cis position, i.e., in the position with maximum repulsion. It is difficult to find an explanation for the fact that the reduction is not stereospecific in very acidic and alkaline solutions, as in esters of the meso- and racemic acids which are always reduced to esters of fumaric acid). It should be noted that chemical reduction ftodide — iodine) of meso- and racemic 2,3-dibromobutanes leads, respectively, to trans- and cis-butenes. 

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