Angewandte
Chemie
Table 1: Synthesized complexes and selected analytical data.[a]
cis-bromination of alkenes
ꢀ
ꢀ1 [b]
1
2
[c]
Complex
n˜ (RCO2 ) [cm
]
E , E [V]
cis-Bromination of Encapsulated Alkenes**
Me
II
+
[
[
[
[
(L )Co (m-Cl)] 1
2
Me
II
+
(L )Co (m-O CCH¼CH )]
2
1578, 1430 (1639) 0.22, 0.59
1519, 1428 (1635) 0.22, 0.60
2
2
2
Me
III
3+
(L )Co (m-O CCH¼CH )]
3
Gunther Steinfeld, Vasile Lozan, and Berthold Kersting*
2
III
2
2
Me
3+
(L )Co (m-O CCHBrCH Br)]
4
1559, 1386
1627, 1394
0.31, 0.70
0.30, 0.69
2
2
2
Me
II
+
[
(L )Co (m-O CCHBrCH Br)]
5
6
2
2
2
[(LMe)Co
III
(m-O
CCH
CHPh)]
3+
1505, 1388 (1631) 0.20, 0.60
¼
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
2
2
(LMe)Co (m-threo-O CCHBrCHBrPh)]
III
3+
7
1560, 1384
1627, 1390
0.32, 0.69
0.32, 0.70
[
2
2
Me)Co
II
2
(m-threo-O
CCHBrCHBrPh)]
+
8
[(L
2
PhCHBrꢀCHBrꢀCO H (threo-dl pair) 9
2
already reported that such assemblies show a higher chemical
reactivity than their unmodified analogues,[ but so far it is
unclear, whether they are also applicable in stereoselective
PhCHBrꢀCHBrꢀCO H (erythro-dl pair) 10
2
4–6]
Me
II
+
[
(L )Co (m-erythro-O CCHBrCHBrPh)] 11
1623, 1393
0.30, 0.68
0.30, 0.68
2
2
[(LMe)Co (m-erythro-O CCHBrCHBrPh)] 12 1550, 1390
III
3+
2
2
[
7,8]
transformations.
This led us to study the bromination of
ꢀ
ꢀ
[
a] The complexes were isolated as ClO4 or BPh4 salts. [b] The values in
encapsulated alkene ligands in complexes of the type A
parentheses refer to the IR band poisitions of the C¼C stretches. [c] The redox
(
Scheme 1); we hoped that the binding pocket would exert an
1
III,II
II,II
2
III,III
III,II
potentials [E (Co /Co ), E (Co /Co )] were determined for the perchlo-
rate salts in CH CN and are referenced to the saturated calomel electrode
3
(SCE).
The fact that the reduction of 4 with NaBH and the reaction
4
of 1 with sodium 2,3-dibromopropionate yield the same
complex 5 (Scheme 2) is also in accord with the formulation
of 4.
+
+
3+
Scheme 1. Structures of dicobalt complexes [(LMe)Co (m-X)] (X=bind-
n+
2
Me 2ꢀ
ing site). The cavity representation of the ligand (L
)
in A should
O
ONa
88 %)
Br2
not be confused with the one used for the calixarenes.
Cl
O
O
O
O
(
2 min (68 %)
II
II
II
II
III
III
Co Co
Co Co
Co Co
effect on the stereochemical course of the reaction. We report
here the synthesis and structures of a series of dicobalt
1
2
3
Me
n+
10 Br2
RT, 4d (78 %)
complexes of the type [(L )Co (m-O CR)] bearing a,b-
2
2
unsaturated carboxylate ligands (Table 1) and demonstrate
the remarkable cis-bromination of the encapsulated sub-
strates.
+
3+
Br
Br
ONa
Br
Br
Br
Br
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
O
NaBH4
(43 %)
O
O
O
O
(
73 %)
II
II
III
III
Co Co
Co Co
5
4
II
II
two-electron oxidation of the intermediate Co Co 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 Br2
proceeded smoothly and produced complex 4, which was
Scheme 2. Preparation of dicobalt complexes 2–5. Numbers in parentheses
refer to yields of isolated products.
The cinnamato-bridged dicobalt(iii) complex 6, whose
synthesis and structure were reported earlier, was exam-
[
9]
[10]
identified by IR and NMR spectroscopy, as the sole product.
ined next. In this case the bromination reaction was complete
after 6h 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
[
*] 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
II
II
determination of the reduced Co Co complex 8 revealed the
presence of bridging 2,3-dibromo-3-phenylpropionato
a
[
9]
ligand (threo dl pair). 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.
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.
Angew. Chem. Int. Ed. 2003, 42, 2261 – 2263
DOI: 10.1002/anie.200351131
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2261