Halogen-bond-induced activation of a carbon-heteroatom bond

Article abstract of DOI:10.1002/anie.201101672

Benzhydryl bromide can be activated by novel halogen-bond donors and subsequently undergoes a Ritter-like reaction with acetonitrile (see scheme). Comparative experiments with non-iodinated reference compounds and tests with added acids indicate that halo

Full text of DOI:10.1002/anie.201101672

DOI: 10.1002/anie.201101672
Halogen Bonds
Halogen-Bond-Induced Activation of a Carbon–Heteroatom Bond**
Sebastian M. Walter, Florian Kniep, Eberhardt Herdtweck, and Stefan M. Huber*
Dedicated to Professor Robert Weiss on the occasion of his 70th birthday
Halogen bonds are attractive noncovalent interactions
between terminal halogen atoms in compounds of the type
R-X (X = Cl, Br, I) and Lewis bases LB.[1–3] Reasonably
strong halogen bonds are formed only when R is highly
electronegative, as for example in the case of polyfluorinated
alkyl or phenyl substituents (Scheme 1).[1,2] In these cases, a
Lately, the potential of thiourea derivatives[15] to act as
receptors for anions has been exploited increasingly for the
activation of substrates and for asymmetric induction in
hydrogen-bond-based organocatalytic reactions.[16,17] In these
cases, the catalyst binds to halides,[17b,d] alkoxides,[17a] carbox-
ylates,[17c] and other anions[17e] (which are usually liberated
during the reaction). Despite the various analogies with
hydrogen bonds, to the best of our knowledge no examples
are known in which organic[18,19] halogen-bond donors have
been used to activate organic substrates (apart from the
case[14] mentioned above). As part of our investigations
towards the use of halogen bonds in organic synthesis and
organocatalysis, we herein report on the first activation of a
carbon–heteroatom bond by novel (potentially bidentate)
halogen-bond donors.
Scheme 1. Commonly used polyfluorinated halogen-bond donors.
region of positive electrostatic potential is induced at the side
In view of the numerous examples of halogen-bond
adducts with halide salts, our aim was to activate carbon–
halogen bonds with strong XB donors. Coordination of the
latter to the lone pairs of the terminal halogen atom should
[4]
ꢀ
of the halogen atom opposite to the R X bond (“s hole”).
At the same time, the n!s* charge transfer of the Lewis base
with the halogen-bond donor R-X is also facilitated.[5] The
electronic nature of the halogen-bond interaction causes an
R–X···LB angle of approximately 1808 and thus high direc-
tionality.[2a,b]
ꢀ
lead at least to the weakening of the respective C X bond, if
not, in the extreme case, to its heterolytic cleavage. Addi-
tional driving force for the overall reaction (e.g. the sub-
stitution of the halide by an added nucleophile) could be
gained by the insolubility of the XB adduct formed by the XB
donor and the halide salt. In the broadest sense, the XB donor
would act as an organic Ag+ equivalent.
Although the interaction itself has been known for a long
time,[5a,b] it has received increased interest only since the early
1990s.[2a,b,6] Aside from basic research, mainly studies towards
the rational design of solids have been published,[2b,c,7] for
instance concerning liquid crystals[8] and conductive materi-
als.[9] In these investigations, strong halogen bonds (XBs) have
often been obtained with halides as Lewis bases.[2a,10] There
were also early indications for the occurrence of XBs in
solution,[11] which have recently been confirmed.[3c,12] XB-
based halide receptors, presented not long ago,[13] constitute a
first application of this interaction in solution. In addition,
Bolm et al. reported on the catalytic activity of XB donors
like 1 (Scheme 1) in the reduction of quinoline derivatives.[14]
We considered benzhydryl bromide[20] 4 (Table 1) to be an
ideal test substrate to realize this idea, since its comparably
+
ꢀ
labile C Br bond can, for instance, be activated by Ag
salts.[21] Deuterated acetonitrile as the solvent should ensure
good solubility, but should not exhibit a significant negative
effect on the formation of XB adducts.[3c] In the absence of
ꢀ
other reaction partners during the activation of the C Br
bond of 4, the solvent would also assume the role of a
nucleophile and would coordinate to the respective carbon
atom. Hydrolysis of the resulting nitrilium intermediate by
traces of water in the solvent would yield (deuterated) N-
benzhydryl acetamide 5 in this variant of the Ritter reac-
tion.[22] Without addition of an activating reagent, a solution
of the pure bromide 4 in wet acetonitrile reacted to give only
trace amounts of amide 5 after four days at room temperature
(Table 1, entry 1). We first tested monodentate XB donors 2
and 3 (Scheme 1), but neither of them was able to induce the
reaction (Table 1, entries 5 and 6).
[*] M. Sc. S. M. Walter,[+] Dipl.-Chem. F. Kniep,[+] Dr. E. Herdtweck,
Dr. S. M. Huber
Department Chemie, Technische Universitꢀt Mꢁnchen
Lichtenbergstrasse 4, 85747 Garching (Germany)
E-mail: stefan.m.huber@tum.de
Homepage: www.ch.tum.de/oc1/shuber/
[+] These authors contributed equally to this work.
[**] Our research is funded by the Fonds der Chemischen Industrie
(Liebig Scholarship to S.M.H.), the Deutsche Forschungsgemein-
schaft (DFG), and the Leonhart-Lorenz-Foundation. S.M.W. thanks
the TUM Graduate School. We thank Prof. Dr. Thorsten Bach and
his group for their great support. We also thank the referees for
helpful suggestions and comments.
Consequently we turned our attention to bidentate XB
donors, only few of which are presently known.[13] Instead of a
polyfluorinated backbone, we employed an alternative way to
ꢀ
enhance the electrophilicity of XB donors: the C I bonds in
compounds m-8 and p-8 (Scheme 2) are electrostatically
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101672.
activated by the respective imidazolium cores.[23]
Angew. Chem. Int. Ed. 2011, 50, 7187 –7191
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7187

Communications
Table 1: Reactions of benzhydryl bromide 4 with various activating
reagents in wet CD3CN.
recrystallization. The NMR spectra of both m-8 and p-8
show only one set of signals, indicating unhindered rotation of
the imidazolium substituents. The result of the X-ray struc-
tural analysis of p-8 is shown in Figure 1.[25] It represents the
first structural analysis of a 2-iodoimidazolium derivative as a
triflate salt. Particularly striking are the contacts of the XB
donor with the triflate counterions (2.838 ꢀ).[26] The respec-
tive distances are well below the sum of the van der Waals
radii (3.50 ꢀ),[27] which is already an indication of the high s*
acidity of p-8.
Entry
Activating
reagent
(Equiv.)[a]
Additive[b]
Yield [%][c]
1
2
3
4
5
6
7
8
–
–
–
–
–
py
–
py
–
–
–
–
–
py
–
py
–
py
–
–
py
–
ꢁ5
ꢁ5
12
7
25
ꢁ5
ꢁ5
ꢁ5
7
ꢁ5
85 [88][d]
85
HOTf
HOTf
HOTf
NBu4OTf
2
3
p-7
p-7
p-8
0.05
0.05
1.0
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.2
1.0
1.0
1.0
1.0
2.0
9
10
11
12
13
14
15
16
17
18
19
20
p-8
m-7
m-7
m-8
m-8
m-8
p-8’
p-9
12
7
28
80 [71][d]
75
97
54
49
–
–
10
[a] Equivalents of activating reagent (relative to 4). [b] Additive:
0.1 equivalents of pyridine (relative to 4); see text. [c] Yield of 5 after
96 h at room temperature according to 1H NMR analysis (see the
Supporting Information). [d] Yield of isolated product 5 in preparative
experiments with CH3CN.
Figure 1. X-ray structural analysis of the XB donor p-8 (ellipsoids at
50% probability); selected bond lengths [ꢂ] and angles [8]: C1–I1
2.054(3), C1–N2 1.335(4), C1–N1 1.339(4), I1···O1 2.838(2); N1-C1-N2
107.9(2), C6-C5-N1-C1 62.7(4); x=center of inversion. Gray C,
white H, yellow-green F, cyan I, blue N, red O, yellow S.
Starting from the known[24] diimidazole p-6 (and the
analogously prepared m-6), we could obtain target com-
pounds m-8 and p-8 by iodination and subsequent methyl-
ation in very good yields (Scheme 2). The corresponding non-
iodinated compounds 7 were prepared by direct methylation
of 6. All saltlike products could be isolated by simple
Both p-8 and m-8 activate benzhydryl bromide (Table 1,
entries 11 and 16): according to 1H NMR analysis, addition of
p-8 resulted in 85% conversion of 4 to 5, while m-8 still gave
80% yield of 5. Side products or potential decomposition
products of the activating reagent were not evident in the
NMR spectra. The yields were confirmed in preparative
experiments[28] and the NMR spectra of the product agree
well with literature data on 5.[29] When 20 mol% of m-8 was
added (Table 1, entry 15), a conversion of 28% was obtained
according to NMR analysis; this points towards a stoichio-
metric effect of the activating reagent (including the back-
ground reaction). Maybe the XB donors 8 bind too strongly to
the liberated bromide and are not available for further
starting material 4. This assumption is supported by the fact
that the 13C NMR signal of the iodine-carrying carbon atoms
of m-8 undergoes a shift from d = 102.4 ppm to d = 110.2 ppm
in the course of the reaction (Figure 2). Comparative experi-
ments with m-8 indicate that this shift cannot be explained
even by complete complexation with 5; however, it is
approximately reproduced with an equimolar amount of
NBu4Br. Incidentally, this 13C NMR shift of m-8 already
points towards the contribution of halogen bonds in the
activation of 4 (see below).
Scheme 2. Synthesis of the activating reagents. a) MeOTf (4 equiv),
CH2Cl2; b) 1. nBuLi (2.6 equiv), THF, ꢀ788C; I2 (2.4 equiv), THF,
ꢀ788C; 2. MeOTf (4 equiv), CH2Cl2 (m-8, p-8) or 2. Me3OBF4
(2.5 equiv), CH2Cl2 (p-8’); c) 1. nBuLi (2.3 equiv), THF, ꢀ788C; CBr4
(2.0 equiv), THF, ꢀ788C; 2. MeOTf (5.6 equiv), CH2Cl2.
7188
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 7187 –7191

In theory, 8 could also induce hydrolysis of acetonitrile,
and the acetamide formed in this way over time could react
with 4. We can rule out this explanation, since even after
several weeks no acetamide was detectable in solutions of m-8
or p-8 in CD3CN (1H and 13C NMR spectra).
In further investigations regarding the mechanism of the
reaction we could show that the HBr formed during the
reaction is not active in an autocatalytic way. When one
equivalent of HBr was added at the start of the reaction (with
all other conditions maintained), only trace amounts of 5 had
been formed after 96 h. Likewise, coordination of the XB
donor to the HBr liberated during the reaction (and thus
ultimately the generation of HOTf) cannot explain the
activation of the substrate. Even in the (unrealistic) extreme
case in which one equivalent of HOTf was added at the start
of the reaction, 5 was generated in only 25% yield after 96 h
(Table 1, entry 5).
In order to assess the influence of further structural
variations of 8 on the effectiveness of the activation, we
prepared both the dibromo derivative (p-9, Scheme 2) and the
BF4 analogue (p-8’) of p-8. As a result of the less coordinating
nature of the counteranions (compared to triflate), the latter
should feature increased electrophilicity at the iodine centers.
As expected, p-9 exhibited a markedly reduced activation
potential (Table 1, entry 19), whereas the BF4 salt p-8’ was
more effective than the triflate compound p-8 (Table 1,
entry 18). Figure 3, in particular, suggests a significant kinetic
effect of the BF4 counteranions.
Figure 2. Signals of the iodine-carrying carbon atoms of m-8 (13C NMR
spectra of solutions in CD3CN): a) m-8, b) m-8 and 5 (1:1), c) reaction
solution after four days (see Table 1, entry 16), d) m-8 and tetrabutyl-
ammonium bromide (1:1).
A critical aspect of investigations concerning the activa-
tion potential of XB donors is the risk that traces of acid
(which might be hard to exclude) are the actual active
reagent. As a consequence, experimental data could be
misinterpreted. To rule out this alternative, a series of
comparative kinetic measurements were performed (see
also Table 1 and Figure 3). To begin with, addition of
The mono-iodinated imidazolium salt 10 (Scheme 3),
which we synthesized for the purpose of comparison,
generated 5 in only 49% yield after 96 h even when two
Scheme 3. Mono-iodinated XB donor 10 along with further substrates
11 (no activation by 8) and 12 (after 96 h 40% conversion to the
amide by m-8).
Figure 3. Yield-versus-time profile of selected reactions from Table 1,
including the activation of 4 by p-8 and appropriate reference experi-
ments (y axis: yield of 5 based on 1H NMR spectroscopy). For further
plots see the Supporting Information.
equivalents of the activating reagent were used (Table 1,
entry 20). Although further studies regarding this matter are
required, this already seems to indicate a bidentate coordi-
nation of XB donors 8.[31]
Finally we tested the reactivity of benzyl bromide (11) and
a-methylbenzyl bromide (12) in preliminary experiments
(Scheme 3).[32] Under the reaction conditions corresponding
to those specified in Table 1, no conversion of 11 was found
with m-8 or p-8 after 96 h. In the case of 12 with m-8, however,
formation of the respective amide in about 40% yield was
observed after the same amount of time.
In summary, we could show that benzhydryl bromide 4
can be activated by the novel XB donors 8. Comparative
experiments with non-iodinated reference compounds and
tests with added acids provide indications that halogen bonds
are probably the basis for this effect. The successful con-
version of substrate 12 also seems to point towards a broader
applicability of this activation.
5 mol% of HOTf led to a measurable, but comparably low
yield of 5 (Table 1, entry 3). Moreover, this effect could be
suppressed by addition of 10 mol% of pyridine (Table 1,
entry 4), whereas m-8 and p-8 gave almost identical yields of 5
with the same additive (Table 1, entries 12 and 17).[30] Two
equivalents of NBu4OTf also did not induce any reaction
(Table 1, entry 6).
In addition, the non-iodinated compounds m-7 and p-7
activate 4 to a significantly lower extent (Table 1, entries 9
and 13; comparative experiments with pyridine as additive
have also been performed in these cases). Since 7 and 8 differ
only by the iodine substituents, these results are a strong
indication that the iodine centers are causally related to the
activation. If one also considers the NMR shifts reported
above (Figure 2), the activation at hand can very likely be
ascribed to halogen bonding.
Angew. Chem. Int. Ed. 2011, 50, 7187 –7191
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7189

Communications
Received: March 8, 2011
Revised: April 14, 2011
[12] P. Metrangolo, W. Panzeri, F. Recupero, G. Resnati, J. Fluorine
Chem. 2002, 114, 27.
Published online: June 29, 2011
[13] a) A. Mele, P. Metrangolo, H. Neukirch, T. Pilati, G. Resnati, J.
Am. Chem. Soc. 2005, 127, 14972; b) M. G. Sarwar, B. Dragisic,
S. Sagoo, M. S. Taylor, Angew. Chem. 2010, 122, 1718; Angew.
Keywords: alkylation · anions · halogen bonds ·
.
´
Chem. Int. Ed. 2010, 49, 1674; c) E. Dimitrijevic, P. Kvak, M. S.
imidazolium salts · solvolysis
Taylor, Chem. Commun. 2010, 46, 9025; d) during the prepara-
tion of this manuscript, a further XB-receptor was published: A.
Caballero, N. G. White, P. D. Beer, Angew. Chem. 2011, 123,
1885; Angew. Chem. Int. Ed. 2011, 50, 1845.
[1] “Halogen Bonding: Fundamentals and Applications”: Structure
and Bonding, Vol. 126 (Eds.: P. Metrangolo, G. Resnati),
Springer, Berlin, 2008.
[2] Recent reviews: a) P. Metrangolo, H. Neukirch, T. Pilati, G.
Resnati, Acc. Chem. Res. 2005, 38, 386; b) P. Metrangolo, F.
Meyer, T. Pilati, G. Resnati, G. Terraneo, Angew. Chem. 2008,
120, 6206; Angew. Chem. Int. Ed. 2008, 47, 6114; c) M. Four-
miguꢁ, Curr. Opin. Solid State Mater. Sci. 2009, 13, 36; d) A. C.
Legon, Phys. Chem. Chem. Phys. 2010, 12, 7736.
[14] A. Bruckmann, M. A. Pena, C. Bolm, Synlett 2008, 900.
[15] Recently a chiral triazolium derivative was presented as a novel
organocatalyst, the action of which is based on its receptorlike
properties: K. Ohmatsu, M. Kiyokawa, T. Ooi, J. Am. Chem.
Soc. 2011, 133, 1307.
[16] Review: Z. Zhang, P. R. Schreiner, Chem. Soc. Rev. 2009, 38,
1187.
[17] Selected publications: a) M. Kotke, P. R. Schreiner, Tetrahedron
2006, 62, 434; b) I. T. Raheem, P. S. Thiara, E. A. Peterson, E. N.
Jacobsen, J. Am. Chem. Soc. 2007, 129, 13404; c) C. K. De, E. G.
Klauber, D. Seidel, J. Am. Chem. Soc. 2009, 131, 17060; d) A. R.
Brown, W.-H. Kuo, E. N. Jacobsen, J. Am. Chem. Soc. 2010, 132,
9286; e) G. E. Veitch, E. N. Jacobsen, Angew. Chem. 2010, 122,
7490; Angew. Chem. Int. Ed. 2010, 49, 7332.
[18] ICl3-catalyzed ring-opening polymerization of l-lactide: O.
Coulembier, F. Meyer, P. Dubois, Polym. Chem. 2010, 1, 434.
[19] Recent reviews concerning the use of elemental iodine in
synthesis or catalysis: a) H. Togo, S. Iida, Synlett 2006, 2159;
b) M. Jereb, D. Vrazic, M. Zupan, Tetrahedron 2011, 67, 1355.
The active species in these reactions is discussed to be HI
(amongst others), which is formed during the reaction (see
Ref. [19a]).
[20] For kinetic studies or for the determination of nucleophilicity
parameters, see, for example, a) O. T. Benfey, E. D. Hughes,
C. K. Ingold, J. Chem. Soc. 1952, 2488; b) T. B. Phan, C. Nolte, S.
Kobayashi, A. R. Ofial, H. Mayr, J. Am. Chem. Soc. 2009, 131,
11392; c) N. Streidl, B. Denegri, O. Kronja, H. Mayr, Acc. Chem.
Res. 2010, 43, 1537, and references therein.
[21] Examples: a) J. Cast, T. S. Stevens, J. Chem. Soc. 1953, 4180;
b) G. W. H. Cheeseman, J. Chem. Soc. 1957, 115; for an
enantioselective alkylation of benzhydryl bromide in the pres-
ence of a thiourea derivative, see also Ref. [17d].
[3] Selected recent publications: a) P. Auffinger, F. A. Hays, E.
Westhof, P. S. Ho, Proc. Natl. Acad. Sci. USA 2004, 101, 16789;
b) R. Cabot, C. A. Hunter, Chem. Commun. 2009, 2005; c) M. G.
Sarwar, B. Dragisic, L. J. Salsberg, C. Gouliaras, M. S. Taylor, J.
Am. Chem. Soc. 2010, 132, 1646; d) N. L. Kilah, M. D. Wise, C. J.
Serpell, A. L. Thompson, N. G. White, K. E. Christensen, P. D.
Beer, J. Am. Chem. Soc. 2010, 132, 11893; e) L. A. Hardegger, B.
Kuhn, B. Spinnler, L. Anselm, R. Ecabert, M. Stihle, B. Gsell, R.
Thoma, J. Diez, J. Benz, J.-M. Plancher, G. Hartmann, D. W.
Banner, W. Haap, F. Diederich, Angew. Chem. 2011, 123, 329;
Angew. Chem. Int. Ed. 2011, 50, 314.
[4] Definition: P. Politzer, J. S. Murray, T. Clark, Phys. Chem. Chem.
Phys. 2010, 12, 7748, and references therein.
[5] Selected publications: a) H. A. Bent, Chem. Rev. 1968, 68, 587;
b) O. Hassel, Angew. Chem. 1970, 82, 821; c) R. Weiss, M.
Rechinger, F. Hampel, Angew. Chem. 1994, 106, 901; Angew.
Chem. Int. Ed. Engl. 1994, 33, 893; d) R. Weiss, O. Schwab, F.
Hampel, Chem. Eur. J. 1999, 5, 968; e) A. Karpfen, Theor. Chem.
Acc. 2003, 110, 1.
[6] Selected publications: a) V. R. Pedireddi, D. S. Reddy, B. S.
Goud, D. C. Craug, A. D. Rae, G. R. Desiraju, J. Chem. Soc.
Perkin Trans. 2 1994, 2353; b) A. C. Legon, Chem. Eur. J. 1998, 4,
1890; c) A. C. Legon, Angew. Chem. 1999, 111, 2850; Angew.
Chem. Int. Ed. 1999, 38, 2686; d) A. Farina, S. V. Meille, M. T.
Messina, P. Metrangolo, G. Resnati, G. Vecchio, Angew. Chem.
1999, 111, 2585; Angew. Chem. Int. Ed. 1999, 38, 2433; e) E.
Corradi, S. V. Meille, M. T. Messina, P. Metrangolo, G. Resnati,
Angew. Chem. 2000, 112, 1852; Angew. Chem. Int. Ed. 2000, 39,
1782.
[7] Reviews: a) P. Metrangolo, G. Resnati, Chem. Eur. J. 2001, 7,
2511; b) K. Rissanen, CrystEngComm 2008, 10, 1107; c) L.
Brammer, G. M. Espallargas, S. Libri, CrystEngComm 2008, 10,
1712; d) R. Bertani, P. Sgarbossa, A. Venzo, F. Lelj, M. Amati, G.
Resnati, T. Pilati, P. Metrangolo, G. Terraneo, Coord. Chem. Rev.
2010, 254, 677.
[8] a) H. L. Nguyen, P. N. Horton, M. B. Hursthouse, A. C. Legon,
D. W. Bruce, J. Am. Chem. Soc. 2004, 126, 16; b) D. W. Bruce, P.
Metrangolo, F. Meyer, T. Pilati, C. Praesang, G. Resnati, G.
Terraneo, S. G. Wainwright, A. C. Whitwood, Chem. Eur. J. 2010,
16, 9511.
[22] Ritter-like reaction of benzhydryl bromide in acetonitrile with
Fe(ClO4)3: B. Kumar, H. Kumar, N. Singh, Indian J. Chem. Sect.
B 1991, 30, 460.
[23] Similar halogenated imidazolium compounds have been used
previously as XB donors, see Refs. [3d,13d] and a) N. Kuhn, T.
Kratz, G. Henkel, J. Chem. Soc. Chem. Commun. 1993, 1778;
b) A. J. Arduengo III, M. Tamm, C. J. Calabrese, J. Am. Chem.
Soc. 1994, 116, 3625; c) N. Kuhn, A. Abu-Rayyan, K. Eichele, S.
Schwarz, M. Steimann, Inorg. Chim. Acta 2004, 357, 1799;
d) C. J. Serpell, N. L. Kilah, P. J. Costa, V. Fꢁlix, P. D. Beer,
Angew. Chem. 2010, 122, 5450; Angew. Chem. Int. Ed. 2010, 49,
5322.
[24] H.-J. Cristau, P. P. Cellier, J.-F. Spindler, M. Taillefer, Chem. Eur.
J. 2004, 10, 5607.
[25] Single-crystal X-ray structure analysis of compound p-8: color-
less prism, [(C14H14I2N4)2+], 2[(CF3O3S)ꢀ], Mr = 790.25; mono-
clinic, space group C2/c (no. 15), a = 15.9775(4), b = 6.9921(2),
c = 23.0804(6) ꢀ, b = 100.6533(10)8, V= 2534.01(12) ꢀ3, Z = 4,
l(MoKa) = 0.71073 ꢀ, m = 2.731 mmꢀ1, 1calcd = 2.071 gcmꢀ3, T=
123(1) K, F(000) = 1512, crystal dimensions 0.41 ꢂ 0.51 ꢂ
0.56 mm; R1 = 0.0231 (2307, Io > 2s(Io)), wR2 = 0.0519 (all
2308 data), GOF = 1.422, 164 parameters, D1max/min = 0.42/
ꢀ0.56 eꢀꢀ3. CCDC 815890 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
[9] a) H. M. Yamamoto, J.-I. Yamaura, R. Kato, J. Am. Chem. Soc.
1998, 120, 5905; b) M. Fourmiguꢁ, P. Batail, Chem. Rev. 2004,
104, 5379.
[10] Selected publications: a) R. Weiss, M. Rechinger, F. Hampel, A.
Wolski, Angew. Chem. 1995, 107, 483; Angew. Chem. Int. Ed.
Engl. 1995, 34, 441; b) S. Triguero, R. Llusar, V. Polo, M.
Fourmiguꢁ, Cryst. Growth Des. 2008, 8, 2241; c) P. Metrangolo,
T. Pilati, G. Terraneo, S. Biella, G. Resnati, CrystEngComm
2009, 11, 1187.
[11] T. Di Paolo, C. Sandorfy, Can. J. Chem. 1974, 52, 3612.
7190
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 7187 –7191

charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
70% yield (as a mixture of salts with a counterion distribution of
90% triflate and 10% bromide).
[29] T. Maki, K. Ishihara, H. Yamamoto, Org. Lett. 2006, 8, 1431.
[30] Pyridine itself also reacts slowly with benzhydryl bromide to give
the corresponding pyridinium salt (see, for example, Y. Pocker, J.
Chem. Soc. 1959, 3944), which we could also observe in the NMR
spectra.
[31] Orientating DFT calculations indicate that bidentate coordina-
tion of m-8 and p-8 to a single halogen center is feasible (see the
Supporting Information).
[26] Shortest known (noncovalent) iodine–oxygen contact: 2.76 ꢀ
(L. Ouahab, F. Setifi, S. Golhen, T. Imakubo, R. Lescouꢃzec, F.
Lloret, M. Julve, R. Swietlik, C. R. Chim. 2005, 8, 1286).
Coordination of triflate to an IIII center: I–O = 2.89 ꢀ (P. J.
Stang, K. Chen, A. M. Arif, J. Am. Chem. Soc. 1995, 117, 8793).
[27] A. Bondi, J. Phys. Chem. 1964, 68, 441. If one considers “polar
flattening” (S. C. Nyburg, C. H. Faerman, Acta Crystallogr. Sect.
BA 1985, 41, 274), a value of 3.30 ꢀ is obtained.
[32] On the basis of NMR experiments we have also obtained
indications that 8 coordinates to other types of substrates
(carbonyl compounds, imines) as well.
[28] The activating reagent m-8 or p-8 could be re-isolated from the
reaction solutions of the preparative-scale experiments in about
Angew. Chem. Int. Ed. 2011, 50, 7187 –7191
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7191