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trans-dibromo alkenes 26–30 with no tetrabromo derivatives detect- structurally diverse I(III) catalysts are part of ongoing studies in our
able. Only substrates bearing a terminal triple bond, such as phenyl group and will help to further extend the concept of iodine(III)-
acetylene and 1-hexyne, gave diastereomeric mixtures of 26 and 29 triggered halogen-induced reactions.
(65 : 35 and 94 : 6), respectively. It is noteworthy that the dibromina-
We thank S. Hoch for technical assistance. This work was funded
tion of both alkenes and alkynes proceeded chemoselectively with by a Liebig fellowship of the ‘‘Fonds der Chemischen Industrie’’.
no aromatic bromination being observed, even if highly activated
aryl moieties were present, such as in the 4-OMe phenyl ester 23.
Notes and references
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To make the method even more environmentally friendly, we
attempted to replace the electrophilic bromination agent NBS, which
(b) T. Kitamura and Y. Fujiwara, Org. Prep. Proced. Int., 1997, 29, 409;
(c) V. V. Zhdankin and P. J. Stang, Chem. Rev., 2002, 102, 2523;
produces succinimide as a waste product, by KBr/oxone.19,20 Under
these conditions the yields remained excellent while the reaction
times for the conversion of all substrates employed were significantly
shorter (down to 5 min). Although oxidative halogenations employ-
ing inorganic salts and oxone are literature-known,15c,21 our catalytic
procedure constitutes a significant improvement, as it avoids the
often needed harsh conditions. Even substrates bearing oxidatively
sensitive functionalities, like, e.g. hydroxy or aldehyde groups, were
smoothly transformed into their dibromo products (16 and 21) in
very good yields and selectivities. Also seemingly inert starting
materials, like, e.g. coumarin (35), which showed no reaction without
catalyst 7a at all (see ESI†), were easily converted into 22 using the
described protocol.
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The switch to dichlorinations22 was done by simply replacing
KBr by its corresponding chloride salt (KCl) affording the desired
trans dichlorocyclohexane (31) and 1,2-dichlorooctane (32) in 98%
and 91% yield, respectively (Fig. 2). Surprisingly, alcohol 33 was
obtained as a single diastereomer upon chlorination, in contrast
to the generation of its dibromo analog 16 (cf. Fig. 1). Compounds
bearing electron-deficient double bonds also reacted under these
conditions, providing the a-chlorinated products 34–36 in good
yields (71–88%).23 Application of alkynes produced the geminal
dichlorinated ketones 37 and 38. The corresponding dibromo
carbonyl compounds were found only in traces when KBr was
used, which may be due to the fact that chloride is less nucleo-
philic and thus less reactive compared to bromide.
ˇ
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In summary, we showed that subtle variations of the electronic
properties of the ortho substituents in iodobenzene pre-catalysts 4–7
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catalyst-dependent reactivity clearly correlates with the nucleophili-
city and thus with the strength of the trans effect exhibited by the
ortho substituent in the I(III) intermediates 8. In-depth studies of the
observed structure–reactivity relationships together with detailed
investigations of the alterations in the mechanisms addressed by
identified yet, further interactions of 8 and Br2 cannot be excluded.
13 H. Togo, M. Aoki, T. Kuramochi and M. Yokoyama, J. Chem. Soc.,
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16 For an enantioselective dibromination see: (a) A. K. El-Qisairi, H. A.
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Fig. 2 Oxidative chlorination of alkenes and alkynes. a The reactions were
carried out using alkene or alkyne (1 eq.), KCl (5.0 eq.), oxone (1.1 eq.),
and catalyst 7a (10 mol%) in DCM (0.2 M) at rt or 0 1C. b Isolated yields.
c d.r. determined by 1H NMR.
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