Halides aryl, reductive coupling

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

Catalytic processes based on the use of electrogenerated nickel(O) bipyridine complexes have been a prominent theme in the laboratories of Nedelec, Perichon, and Troupel some of the more recent work has involved the following (1) cross-coupling of aryl halides with ethyl chloroacetate [143], with activated olefins [144], and with activated alkyl halides [145], (2) coupling of organic halides with carbon monoxide to form ketones [146], (3) coupling of a-chloroketones with aryl halides to give O -arylated ketones [147], and (4) formation of ketones via reduction of a mixture of a benzyl or alkyl halide with a metal carbonyl [148]. 

Ullmann reaction (4, 33-34). Semmelhack et al.1 recorded details of the coupling of aryl and vinyl halides with Ni(0) complexes, (COD)2Ni or Ni[P(C6H5)3]4. This reaction was used for the first synthesis of almusone (3), an antileukemic lignan obtained from the wood of Alnus japonica Steud. In the case of ort/io-substituted aryl iodides such as 1, reduction becomes a competing reaction and the yields are only moderate. In fact o,o-disubstituted aryl halides cannot be coupled under these conditions. Deliberate addition of a proton source increases formation of the reduction product. Acetic or trifluoroacetic acid are useful for this purpose. Added triphenyl-phosphine also promotes reduction. 

Kuroboshi, M. Waki, Y. Tanaka, H. Palladium-catalyzed tetrakis(dimethylamino)ethy-lene-promoted reductive coupling of aryl halides./. Org. Chem. 2003, 68, 3938-3942. Luo, F.-T. Jeevanandam, A. Basu, M. K. Efficient and high-turnover homocoupling reaction of aryl iodides by the use of palladacycle catalysts. A convenient way to prepare poly-p-phenylene. Tetrahedron Lett. 1998, 39, 7939-7942. 

Aminations of five-membered heterocyclic halides, such as furans and thiophenes, are limited. These substrates are particularly electron-rich. As a result, oxidative addition of the heteroaryl halide and reductive elimination of the amine are slower than for simple aryl halides (see Sections 4.7.1 and 4.7.3). In addition, the amine products can be air-sensitive and require special conditions for their isolation. Nevertheless, Watanabe has reported examples of successful couplings between diarylamines and bromothiophenes [126]. Triaryl-amines are important for materials applications because of their redox properties, and these particular triarylamines should be especially susceptible to electrochemical oxidation. Chart 1 shows the products formed from the amination of bromothiophenes and the associated yields. As can be seen, 3-bromothiophene reacted in higher yields than 2-bromothiophene, but the yields were more variable with substituted bromothiophenes. In some cases, acceptable yields for double additions to dibromothiophenes were achieved. These reactions all employed a third-generation catalyst (vide infra), containing a combination of Pd(OAc)2 and P(tBu)3. The yields for reactions of these substrates were much higher in the presence of this catalyst than they were in the presence of arylphosphine ligands. 

Nickel(O) triphenylphosphine species have been used to convert allyl halides to 1,5-hexadiene [332], for the reductive coupling of ethylene with aryl halides to give 1,1-diaryl-ethanes [333], and for the coupling of aryl halides and alkenes to prepare substituted olefins [334], In addition, l,2-bis[(di-2-propylphosphino)benzene]nickel(0) has been used for the reductive coupling of aryl halides [335], and l,2-bis[(diphenylphosphino)ethane]-nickel(O) has been employed to synthesize biphenyl from bromobenzene [336] and to prepare benzoic acid from bromobenzene in the presence of carbon dioxide [337]. 

Intermolecular reductive couplings between aromatic rings mostly involve either reactive aryl cr-radicals, for example, derived from aromatic halides by cleavage of the radical anions, or relatively stable 7r-radical anions derived from aromatic compounds activated by electron-withdrawing substituents. 

Intramolecular reductive coupling between the isoquinolinium systems (107) and an attached aromatic halide [Eq. (67)] [308], gives products similar to those obtained by the intramolecular coupling of aromatic halides with other aryl groups (Sec. V.B). 

Zhang, S Zhang, D., Llebeskind, L. S. Ambient Temperature, Ullmann-llke Reductive Coupling of Aryl, Heteroaryl, and Alkenyl Halides. J. Org. Chem. 1997, 62, 2312-2313. 

Palladium, in the form of palladium(II) acetate, has also been used to catalyze biaryl formation directly from aryl iodides (R3N 100 °C), especially P-NO2 and p-Cl derivatives. As usual, ortho substituents severely hinder this type of coupling. Related reductive couplings of aryl halides have been achieved using hydrazine and a Pd-Hg catalyst, electrochemically generated Pd° catalysts, or a palladium on carbon catalyst in the presence of aqueous sodium formate, sodium hydroxide and, crucially, a catalytic amount of a surfactant. The first two procedures look to be particularly selective and efficient, while the latter, rather different, method is not so efficient but does look amenable to large scale work. 

Terminal alkynes can be coupled directly to aryl and to vinyl halides in the presence of a palladium catalyst and a base. The mechanism of this reaction appears to involve oxidative addition of the sp halide to palladium(O), followed by alkynylation of the intermediate organopalladium halide and reductive elimination of the disubstituted alkyne (equations 17 and... 

Nickel. 12,355 13,197 It UUmann coupling. electrolysis of a DMF sola current, aryl halides underg Reduction of nitroam catalyst prepared by reduco nitroarenes are converted to Hydroindolone formnt alkaloids involves cyclizatit acid in refluxing isopropam MeOv... 

The 1,4-conjugate addition of triorganoindium to enones is promoted by a catalytic amount of Ni(COD)2 (Scheme 8.71) [100], Allylindium reagents can be used in Pd-catalyzed cross-coupling reaction with aryl halides (Scheme 8.72). The Pd-catalyzed allylic substitution of allyl carbonate produces 1,5-dienes (Scheme 8.73) [101]. The indium-mediated palladium-catalyzed Ullmann-type reductive coupling of aryl halides proceeds in aqueous media under air (Scheme 8.74) [102]. 

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