✍ Scribed by Rainer Stürmer
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Chlorinated arenes are cheap to manufacture and therefore play a vital role as intermediates in the chemical industry. However, in contrast to theirÐmuch more expensiveÐ brominated and iodinated counterparts, chloroarenes are quite unreactive in subsequent reactions. Classical functionalizations of the C À Cl bond in nonactivated arenes usually require harsh conditions, and side reactions may produce environmentally hazardous oxygenated chloroarenes. This leads to considerable problems in using these compounds as intermediates in the synthesis of more highly functionalized biologically active molecules, such as agrochemicals and pharmaceuticals.
Due to recent developments in catalysis, former problems might be overcome: The groups of S. L. Buchwald, G. C. Fu, J. F. Hartwig, and others reported significant improvements in CÀC, CÀN, and CÀO bond formation on chloroarenes. For all of these bond formations the right choice of catalyst is crucial for success. In other words: By ligand tuning, all three bond forming reactions can be realized through palladium catalysis.
The following account focuses on recent work, since the subject has been already reviewed. [1] Heck-and Suzuki-type couplings have been described by Fu et al. The reaction of chlorobenzene and styrene in refluxing dioxane in the presence of [Pd 2 (dba) 3 ] (dba dibenzylideneacetone) and the electron-rich, sterically hindered tri-tert-butylphosphane gives rise to trans-stilbene in 83 % yield [Eq. (a)]. Besides the choice of the ligandÐaryl phosphanes, tri-n-butylphosphane, and tricyclohexylphosphane show no conversionÐthe base is also crucial for Cl + 83 % (a) 1.5 mol % [Pd 2 (dba) 3 ]
6 mol % PtBu 3 Cs 2 CO 3 Recent work from Lipshutz et al. even shows the replacement of the rather expensive palladium with nickel, at least in CÀC bond formations. Chloroarenes are coupled with organozink compounds under nickel(0 0) catalysis.
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