The first N,N′-ditosyl-substituted cyclic boron cation, stabilized by neighboring-group participation by two sulfonyl groups, and the alternative, stabilized by polar solvents

Article abstract of DOI:10.1016/j.tetlet.2004.11.068

When 2-bromo-1,3-ditosyl-1,3,2-diazaborolidine was treated with AgSbF ₆, a novel cyclic boron cation was formed in CD₂Cl ₂, the ¹¹B NMR chemical shift of which appeared at 8.7 ppm. Ab initio calculations were consistent with the cationic boron center being stabilized by neighboring-group participation of the two sulfonyl functions. The reaction in CD₃NO₂ resulted in an alternate formation of a cyclic boron cation species (16.2 ppm), stabilized by coordination to the basic solvents.

Full text of DOI:10.1016/j.tetlet.2004.11.068

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 209–212
The first N,N0-ditosyl-substituted cyclic boron cation, stabilized
by neighboring-group participation by two sulfonyl groups,
and the alternative, stabilized by polar solvents
Syun-ichi Kiyooka,a,* Ryoji Fujiyama,a Md. Khabir Uddin,b Kazuki Goh,a
Yoshiya Nagano,b Mizue Fujiob and Yuho Tsunob
aDepartment of Material Science, Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan
bInstitute of Materials Chemistry and Engineering, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan
Accepted 15 November 2004
Available online 26 November 2004
Abstract—When 2-bromo-1,3-ditosyl-1,3,2-diazaborolidine was treated with AgSbF6, a novel cyclic boron cation was formed in
CD2Cl2, the 11B NMR chemical shift of which appeared at 8.7ppm. Ab initio calculations were consistent with the cationic boron
center being stabilized by neighboring-group participation of the two sulfonyl functions. The reaction in CD3NO2 resulted in an
alternate formation of a cyclic boron cation species (16.2ppm), stabilized by coordination to the basic solvents.
Ó 2004 Elsevier Ltd. All rights reserved.
Although a limited number of reports dealing with bor-
+
+
on cation species as catalysts in organic synthesis1 have
appeared, in regard to utilizing their extreme Lewis acid-
ity, our understanding of the electrophilicity of such
boron cations is not necessarily sufficient, even at the
present stage of development. We are continuing
our 11B NMR and ab initio calculation studies of boron
cations, involving the historical diphenylboron cation,
in order to clarify the fundamental properties of boron
cations in solution.2 Tetracoordinate and tricoordinate
boron cations by coordination to electrophiles, owing
to their inherent high Lewis acidity have been detected
and are classified as boronium and borenium ions,
respectively.3 While dicoordinate borinium ions (1)
would be expected to be stable if the boron atom is sp
hybridized, p-bonding, using two vacant p orbitals of
the boron with other heteroatoms, should certainly be
necessary for producing stable borinium ions, which
correspond to borylenes (2), as shown in Figure 1. Some
interesting borylenes involving transition metals, instead
of heteroatoms, have recently been reported.4 Taking
into account the fact that BN p-back-bonding is effi-
cient, bis(amino)boron cations (3) represent an adequate
system for stable borinium cations. Stable diamino bori-
R
B R
X
B
Y
1
2
+
N
N
+
B
N
N
B
cyclic boron cation
3
4
Figure 1.
nium ions can be isolated when the bulkiness of the ami-
no substituents is increased5 or extended conjugation is
allowed, such as –P@N–B–N@P–.6 However, the
expected stable borinium ions were not formed and only
tricoordinate boron (borenium ion) adducts were pro-
duced in the cyclic system 4, and no linear NBN skele-
tons with BN p-bonding were detected.7 For the first
time, we report here that the use of cyclic N,N0-ditos-
yl-diazaborolidines readily facilitate to the formation
of stable cyclic boron cations.
Keywords: Boron cation; Structure characterization; 11B NMR.
A precursor, 2-bromo-1,3-ditosyl-1,3,2-diazaborolidine
5, of cyclic boron cations was prepared according to
Coreyꢀs procedure.8 Preparation of 5: A solution of
*
Corresponding author. Tel.: +81 888448295; fax: +81 888448359;
e-mail: kiyooka@cc.kochi-u.ac.jp
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.11.068

210
S. Kiyooka et al. / Tetrahedron Letters 46 (2005) 209–212
bis(N-p-toluenesulfonyl)-ethylenediamine
(37mg,
allowed to stand for 15min.9 An interesting up-field
shift, from 26.6 to 8.7ppm (from a BF3ÆOEt2 standard),
in 11B NMR spectra (CD2Cl2) was observed (Scheme 1).
This is the first observation of a cyclic boron cation
without coordination by solvent and/or base, as de-
picted in Figure 2. The optimized geometry of the cyclic
boron cation 6 at the B3LYP/6-31+G(d) level indicates
that the cationic boron center is stabilized by intramo-
lecular coordination with two oxygen atoms of both
the tosyl substituents where the B–O bond length is
0.1mmol) in anhydrous CH2Cl2 (1mL) was treated with
1equiv of 1M BBr3 in CH2Cl2 (100lL, 0.1mmol) at rt
under Ar. After stirring for 30min, the solution was
evaporated in vacuo in order to eliminate HBr com-
pletely.9 Ionization of 2-chloro-1,3-dimethyl-1,3,2-diaza-
borolidine by Lewis acids in the presence of a
coordinative base is known to result in the correspond-
ing 1,3-dimethyl-1,3,2-diazaborolidinium ion as bore-
nium.7 Based on this observation, it is possible that
the ditosyl homolog 5 would also ionize in the presence
of BBr3 or AlBr3 but such a reaction was not observed.
The abstraction of the bromine atom from 5 can be re-
garded as being retarded by the ditosyl substituents due
to their greater electron-withdrawing character. The use
of AgSbF6, which contains a SbX6 anion to be effective
as a counter ion of boron cations was considered.2c
Procedure for ionizing 5 to 6: The AgSbF6 salt (49mg,
0.15mmol) in dry CD2Cl2 was added to a solution of
5 (0.1mmol) in an NMR tube at rt under Ar, resulting
in the precipitation of AgBr. The reaction mixture was
˚
1.50A, as shown in Figure 3. In conclusion, this boron
cation exists as a tetracoordinate species of boronium
ion. The 11B chemical shift was calculated to be
7.5ppm using the GIAO-HF/6-311+G(2d,p) method10
on the basis of the optimized geometry. The agreement
between the calculated and found values suggests that
the actual structure of 6 may be close to the theoretical
structure. On the other hand, treatment of 5 with
AgSbF6 in CD3NO2 led to the formation of different
species of cyclic boron cations (7 and 70). The procedure
for 7 and 70: A solution of compound 5 (37mg,
Ts
Ts
NH
N
CH2Cl2
+
+
BBr3
B
Br
2HBr
rt under Ar
NH
Ts
N
1 equiv
Ts
5
Me
O
S
11B chemical shifts
O
N
+
N
AgSbF6
-
5
B
S
found: 8.7 ppm
calcd: 7.5 ppm
SbF6
CD2Cl2
O
rt under Ar
O
6
Me
Me
O
S
O
N
+
N
CD3
AgSbF6
5
B
S
O
N
CD3NO2
O
O
-
rt under Ar
SbF6
O
11B chemical shifts
found: 16.2 ppm
7
Me
Me
calcd as the mean of 7 and 7':
O
17.1 ppm
S
O
calcd for 7: 27.1 ppm
calcd for 7': 7.0 ppm
N
O2NCD3
O2NCD3
O
+
B
S
N
-
SbF6
O
7'
Me
Ts
11B chemical shifts
N
AgClO4
5
B
OClO3
found: 20.8 ppm
calcd: 24.8 ppm
CD3NO2
N
rt under Ar
Ts
8
Scheme 1. Cyclic boron cations stabilized by neighboring groups and by polar solvents.

S. Kiyooka et al. / Tetrahedron Letters 46 (2005) 209–212
211
SO2R
9a
9b
R =
R =
Me
Ph
N
B
Br
N
Ph
SO2R
9
O
R
O
S
11B chemical shifts
Ph
Ph
N
+
AgSbF6
-
9
9
B
S
SbF6
10a 8.7 ppm
CD2Cl2
N
O
rt under Ar
10b
9.0 ppm
O
R
10
O
R
S
11B chemical shifts
Ph
Ph
O
N
+
AgSbF6
(CD3NO2)1~2
B
CD3NO2
N
O
11a 16.1 ppm
11b 16.2 ppm
S
-
rt under Ar
SbF6
O
R
11
Scheme 2. Chiral cyclic boron cations of the (R,R)-stien system.
ordinate species is inferred by analogy with the compa-
rable 11B chemical shifts of the neutral perchlorate 8,
derived from 5 and AgClO4 (Scheme 1). As mentioned
above, two types of N,N0-ditosyl-substituted cyclic bor-
on cations can be formed in both nonpolar and polar
aprotic solvent systems. The procedures were then
applied to the chiral congeners 9, prepared from
bis(N-p-toluenesulfonyl)-(R,R)-1,2-diphenylethylenedi-
amine, (R,R)-stien. The expected corresponding cyclic
boron cations were observed, as shown in Scheme 2.
Figure 2. Observed peaks in 11B NMR spectra of the starting borane 5
and the cyclic boron cation 6.
1.61Å
1.46Å
86°
A preliminary experimental result on the utilization of
these chiral cyclic boron cations in a Lewis acid cata-
lyzed asymmetric aldol reaction is as follows. The effec-
tiveness of precursor 9a to 10a and 10b was examined in
advance for the reaction. The neutral chiral borane 9a
underwent an asymmetric aldol reaction of hydrocin-
namaldehyde with a trimethylsilylketene acetal, derived
from S-phenyl thioacetate, in nitroethane to give the
corresponding aldol with 40% ee (S) but in only 10%
yield, which is corresponding to the loading quantity
(10mol%) of the catalyst. Under similar conditions the
two different chiral boron cations, 10a and 10b, were
examined in the reaction with expectations based on
the development of new chiral catalysts. Unfortunately,
these reactions did not proceed at all. These results can
be assigned significance in terms of the sufficient, coordi-
native saturation of the boron cations in being stabilized
by intramolecular and/or intermolecular interactions
with electrophiles, where such tight complexes no longer
exchange the present ligands with an external substrate
aldehyde. Thus, for the purpose of the effective use of
the chiral boran cations as catalysts for asymmetric
reactions the species must have more labile ligands,
which are exchangeable with external substrates in solu-
tion, where ample time is allowed for activation of the
substrates.12
1.50Å
115°
125°
95°
Figure 3. Optimized structure of 6 [B3LYP/6-31+G* basis sets].
0.1mmol) in dry CH2Cl2 (1mL) was treated with 1equiv
of 1M BBr3 in CH2Cl2 (100lL, 0.1mmol) at rt under
Ar. After stirring for 30min, the solution was evapo-
rated in vacuo in order to eliminate HBr completely.11
The 11B NMR signal of the species in CD3NO2 ap-
peared at 16.2ppm, which is 11.1ppm higher than that
(27.3ppm) of the starting 5 in the solvent. Ab initio calcu-
lations of the 11B chemical shift were performed using
two models corresponding to mono-solvent and di-sol-
vent coordinate species. The calculated 11B chemical
shifts were 27.1 and 7.0ppm, respectively. The preferen-
tial structure for the cyclic boron cation in CD3NO2
should be an equilibrium mixture of the mono-solvent
coordinate 7 and the di-solvent coordinate 70 in compari-
son of their simply averaged value (17.1ppm) with the
observed one (Scheme 1). The existence of 7 as a trico-
In conclusion novel cyclic boron cation 6 formed by
treatment with AgSbF6 in an aprotic nonpolar solvent,

212
S. Kiyooka et al. / Tetrahedron Letters 46 (2005) 209–212
7. Narula, C. K.; No¨th, H. Inorg. Chem. 1984, 23, 4147–
4152.
CD2Cl2, was found to be stabilized by intramolecular
coordination to the oxygen atoms at the both sides of
the boron cationic center. On the other hand, in an
aprotic polar solvent, CD3NO2, an equilibrium mixture
of different cyclic boron cation species, 7 and 70,was de-
tected. The latter can be attributed to stabilization of the
nitromethane solvated cations by electron donor O-
atoms of the solvent. The corresponding chiral cyclic
boron cations, 10 and 11, could also be prepared.
8. Corey, E. J.; Yu, C.-M.; Kim, S. S. J. Am. Chem. Soc.
1989, 111, 5495–5496.
9. Compound 5: 11B NMR (160.35MHz, CD2Cl2, ppm): d
1
26.5ppm. H NMR (500MHz, CD2Cl2, ppm): d 2.44 (s,
6H, p-Me), 3.72 (br s, 4H), 7.35 (d, J = 8.6Hz, 4H, Ar),
7.99 (d, J = 8.6Hz, 4H, Ar). 13C NMR (125.77MHz,
CD2Cl2, ppm): d 21.7, 46.7, 127.6, 130.2, 136.1, 145.4. 6:
11B NMR (160.35MHz, CD2Cl2, ppm): d 8.7ppm. 1H
NMR (500MHz, CD2Cl2, ppm): d 2.46 (s, 6H, p-Me), 3.17
(br s, 4H), 7.44 (br s, 4H, Ar), 7.82 (br s, 4H, Ar). 13C
NMR (125.77MHz, CD2Cl2, ppm): d 21.8, 43.7, 128.1,
130.9, 147.0.
Method of calculation: The calculations were performed
with a Gaussian 98 rev. A.11, using the B3LYP methods
with 3-6-31+G* basis sets for geometry optimizations.13
10. (a) London, F. J. Phys. Radium (Paris) 1937, 8, 397; (b)
Ditchfield, R. Mol. Phys. 1974, 24, 789; (c) Wolinski, K.;
Hilton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112, 8251–
8260.
Acknowledgements
11. Compound 7: 11B NMR (160.35MHz, CD3NO2, ppm):
d 16.16ppm. 1H NMR (500MHz, CD3NO2, ppm): d
2.50 (s, 6H, p-Me), 3.37 (br s, 4H), 7.53 (d, J = 8.0Hz,
4H, Ar), 7.85 (d, J = 8.0Hz, 4H, Ar). 13C NMR
(125.77MHz, CD3NO2, ppm): d 21.9, 44.1, 129.6, 131.8,
134.6, 148.6.
This work was supported by a Grant-in-Aid for Scien-
tific Research from the Japan Society for the Promotion
of Science.
12. The complex, reported by Corey,1a appears to be one of
the structures that satisfy this need
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