Alkyl radical addition to (η 6 -arene)tricarbonylmangenese complexes was investigated with SmI 2 , zinc, or indium as the radical initiator. Among them, the indium-mediated reactions of (η 6 -arene)tricarbonylmangenese cations with various alkyl iodides in aqueous media afforded the corresponding radical addition-reduction products in moderate to excellent yields.Complexation of an arene ring with a Cr(CO) 3 or Mn-(CO) 3 + moiety significantly reduces the electron density of the arene ring, thus allowing nucleophilic addition to occur. 1 The reduction of the electron density of the phenyl ring also encourages the addition of nucleophilic radicals. 2 For example, Merlic and co-workers demonstrated that ketyl radical addition to Cr(CO) 3 -complexed benzene is at least 100 000 times faster than attack on free benzene. 3 However, only a few examples were reported on radical addition to arene-metal complexes, most of which dealt with ketyl radical addition to (η 6 -arene)Cr(CO) 3 complexes. 3-5 Recently, we reported the first examples of radical addition to (η 6 -arene)tricarbonylmangenese cations (1) by reaction with alkylmercury halides. 5c The alkyl radical added to the phenyl ring in 1 to generate the corresponding 17-valence electron intermediate, which was presumably further reduced by alkylmercury halide, leading to the formation of the stable 18-valence-electron product 2 and to the regeneration of an alkyl radical. 6 In this reaction the alkylmercury halide served not only as the radical precursor but also as the reducing agent. This radical chain process worked well for tert-butylmercury halide, and an excellent yield of the product 2a was achieved. However, with isopropylmercury halide the chain process was less efficient, while almost no reaction could be observed for isobutyl-or n-butyl-mercury halide. Moreover, the high toxicity of alkylmercury halides also limited the application of the above method in organic synthesis. To develop general and convenient methods to conduct the radical reactions, we carried out the following investigation. We report here that alkyl radical addition to (η 6 -arene)Mn(CO) 3 + complexes can be successfully carried out in aqueous media by reaction with indium(0) and alkyl iodides. Results and DiscussionAs discussed above and also in the literature, 2,5c successful radical addition to arene-Mn(CO) 3 + complexes would require both a radical precursor and a reducing agent. A metal reductant and an alkyl halide would be an ideal combination to meet this requirement. Thus, we chose manganese complex 1a as the model substrate and isopropyl iodide as the radical source to explore this possibility (eq 1). The results are presented in Table 1.Treatment of an alkyl iodide with SmI 2 to generate an alkyl radical is well documented. 7 Thus, we first tested the SmI 2 /i-PrI system. The reaction of 1a with .(1) For reviews, see: (a) Kane-Maguire, L. A. P.; Honig, E. P.; Sweigart, D. A.
Reactions of fluoroarene-Cr(CO)3 complexes with SmIz and carbonyl compounds in TB[F/HMPA at -40--60 4c afforded the corresponding radical aromatic substitution products in high yield. Compared to the corresponding chloroarene-Cr(CO)3 complexes, fluoroarene-Cr(CO)o complexes showed a higher effiaency and slightly lower regioselectivity . Keywords radical substitution, arenetricarbonylchromiium complexComplexation of an arene ring by a transition metal dramatically alters its electronic nature, resulting in the remarkable enhancement in reactivity of the arene ring toward nucleophilic addition. ' While nucleophilic addition to arene-metal complexes has been widely studied and utilized in organic synthesis, fewer works2-' have been reported on the radical addition to arene-metal complexes and the chemistry is far from being well established. Schmalz et al . first discovered the intramolecular ketyl or azaketyl radical addition and substitution reactions of methoxylated arene-Cr( CO 13 complexes to give the corresponding demethoxylated products. Merlic and co-workers tested the competition between intramolecular addition to a Cr( C0)3 complexed arene and intramolecualr addition to a non-complexes arene using PhCH(CD2I)Ph-Cr(C0)3 as the model and observed a 2 : 1 preference for radical addition to the non-complexed arene.3 In contrast, they estimated from competition experiments that intermolecular ketyl radical addition to benzenetricarbonylchmium is faster than that attack on benzene by a factor of at least 100, OOO . Later, we successfully carried out the intermolecular radical aromatic substitutions on chloroarene-Cr (CO 13 complexes by the reaction with SmI2 and carbonyl compounds.' To further explore the scope of the intermolecular radical substitutions, we here choose fluoroarene-Cr( C0)3 complexes to substitute for chloroarene-Cr ( CO)3 complexes in reactions with SmI2 and carbonyl compounds.Reaction of fluorobenzene-Cr( CO), (1) with acetone ( 2 equiv.) and SmI2 ( 4 equiv.) in THF/HMPA at -40--60 t underwent smoothly to afford the corresponding substitution product 2a in almost quantitative yield. Other ketones and even aldehydes showed similar behavior and the results are listed in Scheme 1. With ketones, excellent yields of substitution products 2 were achieved, while the yields with aldehydes were lowered. The reactions were very clean and no byproduct could be detected except that a small amount of starting material 1 was recovered in some cases. While dechlorination always occurred in some extent in the reaction of chlorobenzene-Cr( C O )~, ' there was no defluorination product observed in the reaction of 1. Compared with chlombenzene-Cr(CO)3, the fluom-analog 1 apparently showed a higher efficiency in the radical substitution reactions .'In order to gain more information on the reaction pattern of fluoroarene-Cr(C0)3 complexes, we chose 0 -, rnand p-fluomtoluene-Cr( CO)J ( o , m , p-3)' and complex 4* bearing an electron-withdrawing group as the model substrates and subjected them to the reactions wit...
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