cis-bromination of alkenes cis-Bromination of Encapsulated Alkenes** Gunther Steinfeld, Vasile Lozan, and Berthold Kersting* Metalated container molecules are currently attracting much interest, since their properties are often different from those of their constituent components. [1–3] Several groups have already reported that such assemblies show a higher chemical reactivity than their unmodified analogues, [4–6] but so far it is unclear, whether they are also applicable in stereoselective transformations. [7,8] This led us to study the bromination of encapsulated alkene ligands in complexes of the type A (Scheme 1); we hoped that the binding pocket would exert an effect on the stereochemical course of the reaction. We report here the synthesis and structures of a series of dicobalt complexes of the type [(L Me )Co 2 (m-O 2 CR)] n+ bearing a,b- unsaturated carboxylate ligands (Table 1) and demonstrate the remarkable cis-bromination of the encapsulated sub- strates. The acrylato-bridged dicobalt( iii ) complex 3 was selected in orienting experiments (Scheme 2). Complex 3 can be readily prepared in high yields by the reaction of the m-Cl species 1 with sodium acrylate in methanol, followed by a two-electron oxidation of the intermediate Co II Co II species 2. The kinetically inert complex 3 was subsequently found to undergo the bromination reaction without interference by side reactions. Thus, reaction of 3 with a tenfold excess of Br 2 proceeded smoothly and produced complex 4, which was identified by IR and NMR spectroscopy, as the sole product. [9] The fact that the reduction of 4 with NaBH 4 and the reaction of 1 with sodium 2,3-dibromopropionate yield the same complex 5 (Scheme 2) is also in accord with the formulation of 4. The cinnamato-bridged dicobalt( iii ) complex 6, whose synthesis and structure were reported earlier, [10] was exam- ined next. In this case the bromination reaction was complete after 6 h at ambient temperature and yielded a single addition product 7 in nearly quantitative yield (Scheme 3). NMR- spectroscopic studies of 7 and a single-crystal X-ray structure determination of the reduced Co II Co II complex 8 revealed the presence of a bridging 2,3-dibromo-3-phenylpropionato ligand (threo dl pair). [9] The expected complex 12 of the erythro form of 2,3-dibromo-3-phenylpropionate, which was prepared for comparative purposes according to the route depicted in Scheme 4, is only produced in low yields (< 3%). Therefore, the bromination of the alkene encapsulated in 6 is a highly diastereoselective syn addition. Scheme 1. Structures of dicobalt complexes [(L Me )Co 2 (m-X)] n+ (X = bind- ing site). The cavity representation of the ligand (L Me ) 2 in A should not be confused with the one used for the calixarenes. Table 1: Synthesized complexes and selected analytical data. [a] Complex n ˜ (RCO 2  ) [cm 1 ] [b] E 1 , E 2 [V] [c] [(L Me )Co II 2 (m-Cl)] + 1 [(L Me )Co II 2 (m-O 2 CCH¼CH 2 )] + 2 1578, 1430 (1639) 0.22, 0.59 [(L Me )Co III 2 (m-O 2 CCH¼CH 2 )] 3+ 3 1519, 1428 (1635) 0.22, 0.60 [(L Me )Co III 2 (m-O 2 CCHBrCH 2 Br)] 3+ 4 1559, 1386 0.31, 0.70 [(L Me )Co II 2 (m-O 2 CCHBrCH 2 Br)] + 5 1627, 1394 0.30, 0.69 [(L Me )Co III 2 (m-O 2 CCH¼CHPh)] 3+ 6 1505, 1388 (1631) 0.20, 0.60 [(L Me )Co III 2 (m-threo-O 2 CCHBrCHBrPh)] 3+ 7 1560, 1384 0.32, 0.69 [(L Me )Co II 2 (m-threo-O 2 CCHBrCHBrPh)] + 8 1627, 1390 0.32, 0.70 PhCHBr CHBr CO 2 H(threo-dl pair) 9 PhCHBr CHBr CO 2 H(erythro-dl pair) 10 [(L Me )Co II 2 (m-erythro-O 2 CCHBrCHBrPh)] + 11 1623, 1393 0.30, 0.68 [(L Me )Co III 2 (m-erythro-O 2 CCHBrCHBrPh)] 3+ 12 1550, 1390 0.30, 0.68 [a] The complexes were isolated as ClO 4  or BPh 4  salts. [b] The values in parentheses refer to the IR band poisitions of the C ¼C stretches. [c] The redox potentials [E 1 (Co III,II /Co II,II ), E 2 (Co III,III /Co III,II )] were determined for the perchlo- rate salts in CH 3 CN and are referenced to the saturated calomel electrode (SCE). Co II Co II O O Br 2 Co III Co III O O O ONa O ONa Br Br 10 Br 2 NaBH 4 Co II Co II Cl Co III Co III O O Br Br Co II Co II O O Br Br 2 3 4 2 min (68 %) RT, 4d (78 %) (88 %) (43 %) (73 %) 1 5 + + 3+ 3+ + Scheme 2. Preparation of dicobalt complexes 2–5. Numbers in parentheses refer to yields of isolated products. [*] Priv.-Doz.Dr. B. Kersting, Dipl.-Chem. G. Steinfeld, Dr. V. Lozan Institut für Anorganische und Analytische Chemie Universität Freiburg Albertstrasse 21, 79104 Freiburg (Germany) Fax: (+ 49)761-203-5987 E-mail: berthold.kersting@ac.uni-freiburg.de [**] This work was supported by the Deutsche Forschungsgemeinschaft (Project No. KE 585/3-1). B. K. thanks Prof. Dr. H. Vahrenkamp for his support of this work. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie 2261 Angew. Chem. Int. Ed. 2003, 42, 2261 – 2263 DOI: 10.1002/anie.200351131 # 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim