Halides electroreductive cyclization

The electroreductive cyclization of some acetylenic halides in DMF has been reported by Moore and Peters. 6-Bromo-l-phenyl-l-hexyne (3) gave three polarographic waves at —2-35 V, —2-60 V and — 2-80 V (vs. S.C.E.). The first wave was correlated with C-Br fission ( -hexyl bromide was reduced at —2-29 V) while the two remaining waves corresponded to triple bond reduction (1-phenyl-1-hexyne gave waves at —2-65 V and —2-88 V). The electrolysis reaction mixture contained both five- and six-membered carbocycles as well as straight-chain reduction products... 

The tributyltin hydride-mediated carbon-carbon bond formation via radical addition and cyclization of alkyl halides with alkenes has often been a choice for construction of various organic molecules [1], However, the requirement for high-temperature initiators or photo initiation and the difficulties associated with purification of the products from tributyltin halides tend to limit the widespread use of these methods, despite the efforts to make the methods easier [Ic, 2], Recently, nickel-mediated radical additions and cyclizations have been introduced as promising alternatives to the tributyltin hydride methods. These are the nickel powder-acetic acid method for cyclization of haloamides to y-lactams, y -lactams and in-dolones, the borohydride exchange resin-nickel boride method for radical addition, nickel-catalyzed electroreductive cyclization and nickel-catalyzed Kharasch addition of polyhalo compounds. 

Electroreductive cyclization of acetylenic halides has been extensively studied by Peters, particularly in the case of 6-halo-l-phenyl-l-hexyne. A complicated scheme was proposed for the 6-chloro compound to explain the results. The origin of the cyclized products may be sought among others in the radical formed by direct reduction of the carbon-halogen bond. Another possibility is the reduction of the carbon-carbon triple bond (already recognized previously in the reductive cyclization of bis-propargylamines )... 

Other reactions studied include reduction of the triple bond of a-acetylenic esters and nitriles by tributyltin hydride in methanol electroreductive cyclization of acetylenic halides at a mercury cathode the trimerization and tetramerization of cyclo-octyne in the presence of various transition metals the kinetics of bromination of alka-l,3-diynes, of permanganate oxidation of acetylenedicarboxylic acid, and of iodination of propiolic acid the participation of the triple bond in reactions of various acetylenes of the general formula (225) and the trimerization of but-2-yne with tolyl-palladium chloride to give a [Pg.48]

In several examples the reductive halide-hydrogen exchange has been studied on a preparative scale. For example, the indirect electroreduction of 2-chloropyridine in DMF using anthracene as mediator gives pyridine in 83-86 % yield 2 . For the dehalogenation of 1-chlorohexane (80% yield), naphthalene is applied as redox catalyst. Similarly, 6-chloro-hexene yields 1-hexene (60%) and methylcyclopentane (25%), which is the product of the radical cyclization . The indirect electrochemical reduction of p- and y-bromocarboxylic esters forms coupling and elimination products besides the dehalogenated products... 

Hisaeda and coworkers subsequently reported radical (macro)cyclizations of oo-bromoalkyl acrylates 300 catalyzed by modified vitamin B12 301a under photo-lytic conditions (Fig. 71) [334], The active Co(I) catalyst is generated by electroreduction at platinum cathodes and subsequently substitutes the halide in 300. The resulting organocobalt(III) intermediate was characterized by UV and MS techniques in the dark. Photolysis triggers 6-endo, 10-endo, or 16-endo cyclizations affording the products 302 in 95, 15, and 43% yield, respectively. The reaction was inhibited by addition of the radical trap PBN 303 and the derived radical adducts were detected by ESR-spectroscopy. 

Electrochemical reduction of aryl halides in the presence of olefins (94), (equation 54) leads to the formation of arylated products (95). Electroreduction of several aralkyl halides at potentials ranging from -1.24 V to -1.54 V (see) gives products which involve dimerization, cyclization, and reduction to the arylalkanes. Carbanions and/or free radicals were again postulated as intermediates79. Aryl radicals generated from the electrochemical reduction of aryl halides have been added to carbon-carbon double bonds80,81. Electrochemical reduction of aryl halides in the presence of olefins leads to the formation of arylated products78. Preparative scale electrolyses were carried out in solvents such as acetonitrile, DMF and DMSO at constant potential or in liquid ammonia at constant current. The reaction is proposed to involve an S l mechanism. 

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