Debromination of 1,2-Bis(phenylseleno)benzene dibromide

Article abstract of DOI:10.1246/bcsj.20130098

The reaction of bis(phenylseleno)benzene dibromide 1 with tribromoborane afforded bromoselenonium tetrabromoborate 2, which has a weak CT interaction between the two selenium atoms. Using Ag reagents provided the first examples of 5-phenylselenanthrenium salts.

Full text of DOI:10.1246/bcsj.20130098

990
Bull. Chem. Soc. Jpn. Vol. 86, No. 8, 990-992 (2013)
Short Articles
lengths (Figure 1),27,28 and noncovalent interactions between
hypervalent chalcogen atoms in ortho-substituted benzenes
have been less investigated.32,33 We therefore became interested
in noncovalent interactions involving a hypervalent sele-
nium atom in an ortho-substituted benzene. We report herein
debromination reaction of 1,2-bis(phenylseleno)benzene dibro-
mide (1)32 to afford bromoselenonium salt, where a hypervalent
selenium atom can interact with a selenium atom at the ortho
position. Unexpected formation of selenanthrene cations bear-
ing hypervalent selenium atoms is also described.
Debromination of 1,2-
Bis(phenylseleno)benzene
Dibromide#
Masaichi Saito,*1 Masahiro Fujita,1
Yoshihiko Kanatomi,1 and Kazuya Ishimura2
Even using 2 equivalents of tribromoborane, only one bro-
mine atom was removed from bis(phenylseleno)benzene di-
bromide 1 to afford bromoselenonium tetrabromoborate 2
in 79% yield (Scheme 1),34 which was characterized by NMR
spectroscopy and elemental analysis. The yield of 2 increased
to 93% using an equivalent of tribromoborane. The molecular
structure of 2 was finally established by X-ray diffraction
analysis (Figure 2).35 The two phenyl groups located in the
ortho positions are nearly parallel (the angle is 9.12(16)°) with
the distance of approximately 3.5 ¡, which is in the normal
range for ³-³ stacking distances.36 The shortest distance
between the cationic selenium atom and a bromine atom in the
1Department of Chemistry, Graduate School of Science
and Engineering, Saitama University, Shimo-okubo,
Sakura-ku, Saitama, Saitama 338-8570
2Theoretical and Computational Chemistry Initiative,
Institute for Molecular Science, Myodaiji, Okazaki,
Aichi 444-8585
Received April 11, 2013
E-mail: masaichi@chem.saitama-u.ac.jp
¹
BBr4 moiety is 3.42 ¡, which is 0.3 ¡ shorter than the sum of
The reaction of bis(phenylseleno)benzene dibromide 1
with tribromoborane afforded bromoselenonium tetrabromo-
borate 2, which has a weak CT interaction between the two
selenium atoms. Using Ag reagents provided the first exam-
ples of 5-phenylselenanthrenium salts.
van der Waals radii (3.75 ¡).31 The Se-Br length of 2.3887(4) ¡
is slightly shorter than the corresponding lengths found in
the naphthalene and acenaphthene derivatives (2.4878(8)28 and
2.458(3) ¡,30 respectively). Most strikingly, the Se-Se distance
in 2 is 2.92 ¡,37 which is slightly longer than those found
in the naphthalene and acenaphthene derivatives (2.7628 and
2.80 ¡,30 respectively) but is much shorter than that found in 1
(3.20 ¡),32 and the sum of van der Waals radii (3.80 ¡).31 The
Se-Se-Br angle is 164°, which is slightly smaller than that
of the corresponding naphthalene and acenaphthene derivatives
(17228 and 171°,30 respectively). These geometric features sug-
gest that the two selenium and bromine atoms possess 3c-4e
hypervalent state.
Noncovalent interactions, such as hydrogen bonding, ³-³
stacking, van der Waals forces, and dipole-dipole interactions,
play important roles in thermodynamic stability, molecular
geometry, crystal packing, self-assembly, and many biological
activities.1-8 Among such interactions, the three-center-four-
electron bond, introduced by Rundle9 and Pimentel,10 is now
widely used to understand the electronic structures of hyper-
valent compounds, which have received considerable attention
for a few decades.11,12 Positioning large Group 16 and 17 atoms
in steric-congested environments causes noncovalent interac-
tions, involving hypervalent atoms. The two large heteroatoms
at the peri-positions in naphthalene13-28 and acenaphthene29,30
face such geometric constraints because the peri-distances in
naphthalene and acenaphthene are about 2.5 and 2.7 ¡, respec-
tively,29 which are slightly longer than the sum of the van der
Waals radii of two hydrogen atoms of 2.18 ¡ (Figure 1).31
Therefore, recently, naphthalene and acenaphthene derivatives
bearing large Group 16 and 17 atoms in hypervalent states at
the peri-positions have received considerable attention.13-30 In
contrast, the interatomic distances in ortho-substituted benzenes
can vary approximately 3 to 4 ¡, dependent on C-X bond
+
SePh
SePh
BBr3
−
BBr4
CH2Cl2/r. t.
SeBr2Ph
SeBrPh
1
2
Scheme 1. Reaction of 1,2-bis(phenylseleno)benzene dibro-
mide (1) with tribromoborane.
C(7)
Br
Se(1)
Se(2)
ca. 2.7 Å
ca. 2.5 Å
ca. 3 to 4 Å
Figure 1. The geometry of peri-substitution in naphthalenes
and acenaphthenes and ortho-substitution in benzenes.
Figure 2. ORTEP drawing of the cationic moiety of 2 (40%
probability).
Published on the web May 25, 2013; doi:10.1246/bcsj.20130098

M. Saito et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 8 (2013)
991
+
SePh
Se
AgX
X−
CH2Cl2
SeBr2Ph
Se
Ph
1
a: X=OTf
b: X=PF6
c: X=SbF6
d: X=BF4
3
Figure 3. Natural bond orbitals in 2. The lone pair on the
Se(2) atom (left) acts as a weak donor to the ·*(Se(1)-Br)
bond (right).
Scheme 2. Reaction of 1 with Ag salts.
Sb
C(13)
Se(1)
Figure 4. Contour map of electron density in the Se(1)-
Se(2)-C(7) plane with bond critical points (triangle) and
bond paths (heavy line) for 2. The contours [ea0¹3] are at 2l
(l = «8, «7, + , 0). Two Ph groups are omitted for clarity.
Se(2)
Figure 5. ORTEP drawing of 3c (40% probability).
+
To gain more insight into the structure of 2, geometric
optimization of the cationic moiety of 2 was performed at the
M06-2X39/6-311G(d)40,41 level of theory using the Gaussian
09 program,42 and the optimized geometry is consistent with
the structure found by X-ray diffraction analysis. In natural
population analysis,43 the charges of Se(1), Br, and Se(2)
are calculated to be +1.03, ¹0.15, and +0.54, respectively,
suggesting considerable cationic character of Se(1). The Se(2)
also possesses some cationic character. According to the
second-order perturbation analysis of the natural bond orbitals
(NBOs),43 weak donor-acceptor interactions are found. The
nature of the main charge-transfer is from the lone pair on the
Se(2) to the ·*(Se(1)-Br) bond depicted in Figure 3, and the
interaction energy is evaluated to be 23.2 kcal mol¹1. Similar
charge-transfer interactions were also observed in peri-sub-
stituted systems of naphthalene and acenaphthene.25,29 The
Se-Se Wiberg bond index (WBI)44 was calculated to be 0.19,
denoting weak covalent bonding character. The Atoms-in-
Molecule (AIM) analysis45,46 for 2 was also carried out and
a bond critical point was found between the two selenium
atoms (Figure 4). It is suggested that the 3c-4e type interaction
between the selenium and bromine atoms plays an important
role even in the ortho-substituted benzene.
Se
−BBr4
B or Ag reagent
2
3
1
SePh
+
4
Se
+
−X
+
Se
Ph
X=OTf, PF6,
SbF6, BF4
H
Scheme 3. Possible mechanism for the reactions of 1 with
B or Ag reagents.
slightly shorter than the sum of van der Waals radii (3.37 ¡).
The compounds 3a-3d are the first examples of 5-phenyl-
selenanthrenium salts, even though trialkyl- and triarylseleno-
nium salts have already been reported.51-54
The mechanism for the formation of 2 and 3 can be tenta-
tively understood as shown in Scheme 3. Debromination of
1 with B or Ag reagent affords intermediary dication 4, which
¹
reacts with soluble BBr4 anion to provide 2. However, when
Debromination reactions of 1 using Ag salts were next inves-
tigated. Reactions of 2 equivalents of AgOTf, AgSbF6, AgPF6,
and AgBF4 with 1 proceeded unexpectedly to provide 5-phen-
ylselenanthrenium salts 3a-3d instead of the bromoselenonium
salts (Scheme 2).34 Compounds 3 were characterized by NMR
spectroscopy and the molecular structures of 3a-3c were finally
established by X-ray diffraction analysis (Figure 5).35 All the
molecular structures are quite similar, and therefore only the
structure of SbF6 salt 3c is discussed. The central six-membered
ring has a boat conformation, similar to the parent selenanthe-
rene,47 leading to a butterfly conformation of the tricyclic frame-
work with the dihedral angle of 135°.48-50 The Se(2)-Se(1)-
C(13) angle is 73.3°, and the shortest Se(1)-F distance is 3.07 ¡,
using Ag salts, the formation of insoluble AgBr suppresses the
bromination of 4, and cyclization in 4 takes place to afford 3.
The involvement of dication 4 is still unclear because trapping
4 by amine was unsuccessful.55
In summary, bromoselenonium tetrabromoborate 2 was syn-
thesized by the reaction of bis(phenylseleno)benzene dibro-
mide 1 with tribromoborane. Even in the ortho-substituted
system, the two selenium atoms can interact through n-·*
charge-transfer. Using Ag salts afforded 3a-3d, the first exam-
ples of 5-phenylselenanthrenium salts.
This work was partially supported by Grant-in-Aids for
Challenging Exploratory Research (No. 23655029 for M.S.)

992
Bull. Chem. Soc. Jpn. Vol. 86, No. 8 (2013)
© 2013 The Chemical Society of Japan
30 F. R. Knight, K. S. A. Arachchige, R. A. M. Randall, M. Bühl,
A. M. Z. Slawin, J. D. Woollins, Dalton Trans. 2012, 41, 3154.
31 A. Bondi, J. Phys. Chem. 1964, 68, 441.
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan.
32 W. Nakanishi, S. Hayashi, J. Organomet. Chem. 2000, 611, 178.
33 W. Nakanishi, S. Hayashi, Y. Kusuyama, J. Chem. Soc., Perkin
Trans. 2 2002, 262.
Supporting Information
Experimental details for the preparation of compounds 2
and 3a-3d and the optimized geometry of 2. This material
is available free of charge on the web at: http://www.csj.jp/
journals/bcsj/.
34 Experimental details, see Supporting Information.
35 Crystallographic data have been deposited with Cambridge
Crystallographic Data Centre: Deposition numbers CCDC-932491
and 932492-4, for compounds 2 and 3a-3c, respectively. Copies of
the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge, CB2 1EZ, U.K.; Fax: +44 1223
336033; e-mail: deposit@ccdc.cam.ac.uk). Compound 3a cocrystal-
lized with one molecule of TfOH.
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