Benzyl halides dimerization

The cross-coupling reactions of benzylic halides and acyl halides produced the expected ketones in good to high yields(39,47). The present method can supplement the corresponding Grignard reaction, which does not work well since benzylmagnesium halides undergo dimerization readily. 

Carbonvlation of Benzyl Halides. Several organometallic reactions involving anionic species in an aqueous-organic two-phase reaction system have been effectively promoted by phase transfer catalysts(34). These include reactions of cobalt and iron complexes. A favorite model reaction is the carbonylation of benzyl halides using the cobalt tetracarbonyl anion catalyst. Numerous examples have appeared in the literature(35) on the preparation of phenylacetic acid using aqueous sodium hydroxide as the base and trialkylammonium salts (Equation 1). These reactions occur at low pressures of carbon monoxide and mild reaction temperatures. Early work on the carbonylation of alkyl halides required the use of sodium amalgam to generate the cobalt tetracarbonyl anion from the cobalt dimer(36). 

The initially formed radical R disproportionates (path a),dimerizes (path b), reacts with active cathodes (path c), rearranges (path d), adds to double bonds (path e) or is reduced to an anion (path f). Products of radical origin (a—e) occur mainly in the reduction of alkyl iodides, benzyl halides and in some cases of alkyl bromides. 

Electrochemical reduction of nickel(I)(salen) in the presence of benzal chloride at a carbon electrode in DMF leads to a variety of monomeric and dimeric products171. Cyclic voltammetric studies using benzylic halides showed that carbanions can bring about auto-catalytic processes by acting as electron-transfer catalysts (Scheme 16)172. 

On treatment with LAH, benzylic halides and related compounds are reduced to the corresponding hydrocarbon (equations 42 and 43), - although in some cases coupled, dimeric products can also be obtained. Selectivity between halides is possible, and in some examples LiH has been added to the reaction mixture. ... 

A-Benzyl-2-methylaziridine with phenol and dimethylformamide at 60° formed (2S,55)-dimethyM,4-dibenzylpiperazine (1666), 1-cyclohexylaziridine with phenylmagnesium bromide gave 1,4-dicyclohexylpiperazine (1667), and some 1-alkylaziridines with alkyl, alkenyl, or benzyl halides, as well as with dialkylchloro-ethylamines, have been found to give nearly quantitative yields of the corresponding 1,1,4-trialkylpiperazinium halides (1668). Dimerization of dimethyl l-(p-methoxyphenyl)aziridine-2,3-dicarboxylate gave two isomers of 2,3,5,6-... 

Aromatic and benzylic halides can also be dimerized under Barbier conditions. 2-Bromofluorene dimerizes in a 52% yield with the magnesium-cuprous chloride combination [66]. 

Considering the formation of C02" on CdS-DMF, we successfully applied the CdS-DMF photocatalysis to the fixation of CO2 into benzophenone (BP), acetophenone (AP), and benzyl halides (BnCl and BtiBr) (Table 1). Four substrates gave benzilic acid, atrolactic acid, and phenylacetic acid as respective fixation products, with dimerized and hydrogenated products. Considering the mechanism proposed above, C02 formed through the same route as described therein should participate in the fixation reaction. 

Benzylic and allylic halides dimerize (homo-couple) readily in the presence of a number of metal species. Synthetically useful yields of homo-coupled products have been obtained using chlorotris(tri-phenylphosphine)cobalt(I), nickel(O) complexes generated in situ, ° Te " species, VCb/LAH, CrCh/LAH and TiCb or TiCl4/LAH. Representative examples are given in equations (37) and (38). Treatment of 8-bromocrotonates with zinc(O) in DMSO leads to dimerization. ... 

Allyl and benzyl halides are reactive, but often the yield of organomagnesiums is low, dimerization of the organic portion being the main reaction. For this reason, allyl halides frequently are used simultaneously with Mg metal and the substrate with which the intermediate organomagnesium species is to react " ... 

Benzyl halides also form dimers in a coupling side reaction high dilution of benzyl chloride, however, leads to high yields of the organomagnesium-halide reagent . Depending on the substitution, other benzyl halides give more or less dimerization e.g.. 

A number of benzylic halides and onium salts under the influence of base undergo an apparent a-elimination to yield dimeric olefins, viz. 

The free radicals generated by treating t-butyl, allyl, or benzyl halides with three equivalents of chromous chloride couple to form symmetrical dimers. Alternatively, a careful choice of reaction conditions enables good yields of cross-coupled products to be prepared, e.g. (2) - (3). °... 

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. 

Analytical data on the soluble products isolated from chloroform are in excellent agreement with the composition 1 Ni+2 1 monoalkylated ligand 1 I or Br. The magnetic moment of this methylated complex was found to be 1.89 Bohr magnetons per nickel (II). The molar conductivities of the methylated and benzylated complexes in methanol at 25° C. are 75.4 and 68.4 ohm-1, respectively. These values approximate those expected for uni-univalent electrolytes in this solvent. The formulation of these alkylated compounds as dimeric electrolytes (structure VII) does not appear to be totally consistent with their physical properties. One or both halide ions may be bound to the metal ion. These results lead to the easily understood generalization that terminal sulfur atoms alkylate more readily than bridged mercaptide groups. 

Alkyl halides with an electron-withdrawing group on the halogen-bearing carbon can be dimerized to olefins by treatment with bases. Z may be nitro, aryl, etc. It is likely that in most cases the mechanism451 involves nucleophilic substitution followed by elimination452 (illustrated for benzyl chloride) ... 

Coupling of alkyl halides (cf. 8,111).1 (-Alkyl, benzylic, and allylic halides form symmetrical dimers in about 85-95% yield when treated with CrCI,.5 The dimerization involves monoalkylchromium(lll) complexes followed by a radical reaction. Crosscoupling of these same halides is also possible, but yields are lower (40-90%). 

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