Communications
DOI: 10.1002/anie.200801751
Reaction Intermediates
Observation of an Isolated Intermediate of the Nucleophilic Aromatic
Substition Reaction by Infrared Spectroscopy**
Hayato Hasegawa, Kenta Mizuse, Masaki Hachiya, Yoshiyuki Matsuda, Naohiko Mikami, and
Asuka Fujii*
Nucleophilic aromatic substitution (Scheme 1) is a major
reaction of electron-deficient aromatic compounds. s Com-
plexes are generally presumed to play the role of the
intermediate or the transition state in this reaction,[1,2] and it
is, therefore, important to understand the structures and
inherent stability of the s complexes.
Intracluster substitution reactions have only been observed in
the cases of halobenzenes. The cluster cations formed from
hexafluorobenzene (C6F6) and polar molecules are very
attractive targets to probe the intermediate structures in the
reaction because there are no ortho/meta/para isomers and
only one type of structure for the s complex is expected.
In this study, we report the first experimental observation
of a stable s complex for a system in which aromatic
substitution actually occurs. We employed the efficient
direct ionization of C6F6 by coherent vacuum ultraviolet
(VUV) light,[16] and confirmed the high substitution reactivity
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of the C6F6 /NH3 system to produce C6F5NH2 . We also
measured the infrared spectrum of the (C6F6-NH3)+ cluster
cation and showed that the cluster cation forms an inter-
mediate s complex.
Scheme 1. General mechanism of the nucleophilic substitution reac-
tion. X=leaving group, Y=electron-withdrawing group, Nu=nucleo-
phile.
Figure 1 shows the mass spectrum of the ions produced by
the one-photon ionization of the C6F6/NH3/He mixture by
VUV light. The most intense signal at m/z 186 corresponds to
Extensive studies have shown that nucleophilic aromatic
substitution reactions occur upon ionization of the gas-phase
clusters formed from halogen-substituted benzene and polar
solvent molecules.[3–10] In such systems, ionization causes the
aromatic compounds to become highly electron deficient,
thus ensuring the high reaction efficiencies. Recently, several
s-complex-type structures, such as H+C6H6, H+C6H5F, (C6H6-
NH3)+, and [CH3OC6H3(NO2)3]À have been observed in the
gas phase by spectroscopy.[11–15] However, the elimination step
is unfavorable in these cases because of their poor leaving
group (X = H). As a result, the high yields of the s complexes
sacrificed their ability to act as reaction intermediates. Such
intermediate structures have not so far been reported for
systems in which the substitution reactions can actually occur.
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C6F6 . The signal at m/z 183 is uniquely assigned to the
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substitution reaction product C6F5NH2 . The intensity of this
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signal is comparable to that of C6F6 , and it clearly demon-
strates that, upon ionization, the reaction efficiently proceeds
in the C6F6/NH3 system with the elimination of HF.
[*] H. Hasegawa, K. Mizuse, M. Hachiya, Dr. Y. Matsuda,
Prof. N. Mikami, Prof. A. Fujii
Department of Chemistry
Graduate School of Science
Figure 1. Mass spectrum of the ions produced by the one-photon
Tohoku University
ionization of the C6F6/NH3 mixture with VUV light.
Sendai 980-8578 (Japan)
Fax: (+81)22-795-6785
E-mail: asukafujii@mail.tains.tohoku.ac.jp
The signal at m/z 203 is attributed to the (C6F6-NH3)+
cluster cation. The IR spectrum of the cluster cation is shown
in Figure 2a. When this spectrum was measured, the mass
resolution of the second quadrupole mass spectrometer was
reduced to Dm ꢀ 10 to give sufficient fragment intensity, and
[**] We are grateful to Dr. Toshihiko Maeyama for helpful discussions.
This study was supported by a Grant-in-Aid for Scientific Research
(Japan): Project No. 19056001 from MEXT and Nos. 19205001,
20550005, 20750002, and 20·5015 from the JSPS. K.M. is supported
by JSPS Research Fellowships for Young Scientists. Most of the
calculations were performed at the Research Center for Computa-
tional Science, Okazaki (Japan).
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either or both the C6F6 and C6F5NH2 fragment ions were
monitored when recording the spectrum. Three bands are
seen in the NH stretching region of the IR spectrum, with the
two bands at the higher frequencies nearly degenerate. These
three bands clearly result from the symmetric and degener-
Supporting information for this article is available on the WWW
6008
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 6008 –6010