Reductive coupling bromides

Reductive coupling reaction of fluonnated vinyl iodides or bromides has been used as a route to fluorinated dienes [246, 247, 248, 249, 250. Generally, the vinyl iodide is heated with copper metal in DMSO or DMF no 1 ntermediate perfluorovmy I-copper reagent is detected. Typical examples are shown m equations 163-165 [246, 247, 249. The X-ray crystal structure of perfluorotetracyclobutacyclooctatetraene, prepared via coupling of tetrafluoro-l,2-diiodocyclobutene with copper, is planar... 

The reductive coupling of allyl halides to 1,5-hexadiene at glassy C electrodes was catalyzed by tris(2, 2,-bipyridyl)cobalt(II) and tris(4,4 -dimethyl-2, 2/-bipyridyl)cobalt(II) in aqueous solutions of 0.1 M sodium dodecylsulfate (SDS) or 0.1 M cetyltrimethylammonium bromide (CTAB).48 An organocobalt(I) intermediate was observed by its separate voltammetric reduction peak in each system studied. This intermediate undergoes an internal redox reaction to form 1,5-hexadiene... 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Pletcher and associates [155, 159, 160] have studied the electrochemical reduction of alkyl bromides in the presence of a wide variety of macrocyclic Ni(II) complexes. Depending on the substrate, the mediator, and the reaction conditions, mixtures of the dimer and the disproportionation products of the alkyl radical intermediate were formed (cf. Section 18.4.1). The same group [161] reported that traces of metal ions (e.g., Cu2+) in the catholyte improved the current density and selectivity for several cathodic processes, and thus the conversion of trichloroacetic acid to chloroacetic acid. Electrochemical reductive coupling of organic halides was accompanied several times by hydrodehalogena-tion, especially when Ni complexes were used as mediators. In many of the reactions examined, dehalogenation of the substrate predominated over coupling [162-165]. 

The classical examples of these two routes are the conversion of 2,5-dimethyl-2,4-hexadiene (113) via the bisdibromocarbene adduct 114 into the terminally fully methylated bisallene 115 (Scheme 5.15) [43] and the reductive coupling of propargyl bromide (116). 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

A mixture of isomers was, however, obtained if the a,a -dibromoxylene was not symmetrical. This difficulty was overcome by using a reductive coupling of a substituted 3,4-dihydroisoquinoline (71) as its salt with an o-methoxycarbonyl-substituted benzyl bromide (73)129 [Eq. (60)]. 

The reductive coupling of phenacyl bromides using nickel carbonyl (68CC33) or an electrolytic procedure (81S625) provides excellent yields of 2,4-diarylfurans. Coupling of a-bromoaldimines with sodium or lithium provides mainly 3,4-disubstituted pyrroles, whereas isopropylmagnesium bromide yields mainly 2,4-disubstituted pyrroles (Scheme 77)... 

Reductive coupling benzyl ketones.1 Benzyl bromides and acyl chlorides undergo reductive coupling when treated with zinc (2 equiv.) and catalytic amounts of Pd[P(Cr,H5)3]4 or Cl2Pd[P(C6H5)3]2 to form benzyl ketones. The highest yields are obtained with aryl acyl chlorides. ArCHzZnBr is considered to be an intermediate. 

The complex formed on addition of cuprous iodide to a solution of a lithium dialkylamide in ether or tetrahydrofuran is effective in the reductive coupling of allylic halides to give 1,5-dienes with preservation of stereochemistry. This method has been used5 for the stereospecific synthesis of all-trans-squalene and (E,Z,Z,E) squalene from (E,E)- and (Z,JE)-farnesyl bromides, respectively. In an attempted synthesis of (3S)-squalene-2,3-epoxide, 4-[(4R)-2,2,5,5-tetramethyl-l,3-dioxolan-4-yl]butan-2-one (1) and the phosphonium iodide (2) were prepared.6 Unfortu-... 

Vitamin Bjoa has been used as catalyst precursor for the reductive coupling of 83 to 84 [202,227-229]. The electrochemical reduction at —1.5 V of the Co(III)-alkyl complex formed by oxidative addition of the bromide to the Co(I) species (formed at 0.9 V) probably leads to the anion, which may add to the double bond in competition with protonation. In DMF, reduction at —1.54 V gave the cyclized product 84 but with no trans cis selectivity (Scheme 24) [227]. 

In water, alkyl halides undergo reductive coupling with Mn-CuClj (30 1). In some cases cross-couplings are possible (e.g., 2-methylene-5-hexenolc acid from allyl bromide and bromomethacrylic acid). 

Reductive coupling [t, 166, before references]. The synthesis of octabromoful-valene (3) by the reductive coupling of hexabromocyclopentadiene (2) with cuprous bromide is reported by West and Kwitowski.7 The immediate precursor (2) was pre-... 

Reductive coupling of ketones. In the presence of trimethylsilyl bromide both saturated and conjugated ketones undergo coupling, at ipso and -positions, respectively. 

Dienes. Japanese chemists have prepared 1,5-dienes by reductive coupling of allylic haUdes with these complexes. For example, reaction of cyclohexylidenethyl bromide with the complex of lithium pyrrolidide and cuprous iodide in ether at 0° for 4 hr. gives the three products shown in equation I in essentially quantitative yield. The reaction is very sensitive, however, to the solvent and the temperature and also to the dialkylamine used for preparation of the catalyst. 

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