Heterodisubstituted 1,10-dicarba-closo-decaboranes from substituted nido-carborane precursors

Article abstract of DOI:10.1016/S0277-5387(99)00300-9

Mono- and heterodisubstituted 1,10-dicarba-closo-decaboranes 1 have been prepared from substituted nido-carboranes 3 and by carboxylation and arylation of 1-alkyl-p-carboranes. Thermal dehydrogenation followed by skeletal rearrangement of 3 furnished 1 in modest yields. Alkyl substituents tolerated the high temperature process whereas the 4-bromophenyl derivative 1d underwent partial disproportionation. The preparation of the nido-carboranes 3 was accomplished in three ways: by acetylene insertion to nonaborane 6, modified Plesek oxidation of dicarbaundecaborate anions 4 with Fe(III), and a new homogenous oxidation of 4 using gaseous SO₂. The newly developed homogenous deboronation with SO₂ appears to be more efficient than the classical Plesek oxidation especially for highly lypophilic carboranes. The overall yields for the preparation of substituted p-carboranes 1 using the three methods from 6 or 4 are about 8% and 14%, respectively. Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00300-9

Polyhedron 18 (1999) 3517–3526
www.elsevier.nl/locate/poly
Heterodisubstituted 1,10-dicarba-closo-decaboranes from substituted
nido-carborane precursors
1
*
ˇ
Zbynek Janousek , Piotr Kaszynski
Organic Materials Research Group, Department of Chemistry, Vanderbilt University, Box 1822 Station B, Nashville, TN 37235, USA
Received 26 April 1999; accepted 7 September 1999
Abstract
Mono- and heterodisubstituted 1,10-dicarba-closo-decaboranes 1 have been prepared from substituted nido-carboranes 3 and by
carboxylation and arylation of 1-alkyl-p-carboranes. Thermal dehydrogenation followed by skeletal rearrangement of 3 furnished 1 in
modest yields. Alkyl substituents tolerated the high temperature process whereas the 4-bromophenyl derivative 1d underwent partial
disproportionation. The preparation of the nido-carboranes 3 was accomplished in three ways: by acetylene insertion to nonaborane 6,
ˇ
modified Plesek oxidation of dicarbaundecaborate anions 4 with Fe(III), and a new homogenous oxidation of 4 using gaseous SO2. The
ˇ
newly developed homogenous deboronation with SO2 appears to be more efficient than the classical Plesek oxidation especially for highly
lypophilic carboranes. The overall yields for the preparation of substituted p-carboranes 1 using the three methods from 6 or 4 are about
8% and 14%, respectively.
1999 Elsevier Science Ltd. All rights reserved.
ˇ
Keywords: Substitued caboranes; Plesek oxidation; Synthesis
1. Introduction
10-Vertex p-carborane is obtained in a similar way to its
12-vertex analog by thermal rearrangement of the corre-
sponding o-carborane 2 [6]. The temperatures required for
the skeletal rearrangement of the 10-vertex carborane to 1
are in the range of 330–3508C [6], which are lower than
the thermal stability limit (|4508C) for 12-vertex alkylcar-
boranes [7]. Thus the thermal rearrangement of substituted
carboranes 2 (Scheme 1) appears to be a viable option for
the preparation of 10-vertex p-carborane derivatives in
contrast to the 12-vertex analogs, which require tempera-
tures above 6008C [8]. In fact, thermal isomerization of
C-methyl [6,9], C-phenyl [6] and C-C6H4F [10] 10-vertex
closo-carboranes has been reported to form the 1,10
isomers 1 almost quantitatively at 3508C.
Differentiation of carbon atoms in p-carboranes is an
important step in the preparation of compounds for materi-
als applications [1,2]. Recently [3], we developed an
efficient route to heterodisubstituted 12-vertex p-car-
boranes via silyl derivatives that can, in principle, be
extended to 10-vertex p-carborane (1a). Parent carborane
1a, however, is a volatile solid whose preparation [4,5] is
cumbersome and its heterodisubstituted derivatives might
be more conveniently synthesized using appropriately
functionalized precursors.
A key intermediate in the preparation of 10-vertex
closo-carboranes is the nido-carborane 3, which undergoes
thermal dehydrogenation to form the closo derivative 2
[4,11]. The reaction requires temperatures above 1808C for
the parent carborane [4,11] but is much lower for anions
[12] and aryl derivatives [13].
The nido precursor 3 has been prepared by the carefully
*Corresponding author. Tel.: 11-615-322-3458; fax: 11-615-322-
3458.
E-mail address: piotr@ctrvax.vanderbilt.edu (P. Kaszynski)
1Permanent address: Institute of Inorganic Chemistry, The Academy of
Sciences of the Czech Republic, 250 68 Rez near Prague, The Czech
Republic.
ˇ
controlled Plesek oxidation [11,14] of 11-vertex anion 4
[15–17] obtained either from o-carborane 5, or using the
recently described reaction of B9H13?L (6) with acetylenes
ˇ
[13,18]. The biphasic Plesek reaction works relatively well
for the parent nido-5,6-dicarbadecaborane (3a) but substi-
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00300-9
1999 Elsevier Science Ltd. All rights reserved.

ˇ
3518
Z. Janousek, P. Kaszynski / Polyhedron 18 (1999) 3517–3526
Scheme 1.
tution at the carbon atoms with lipophilic organic groups
dramatically diminishes the yields [14]. The nonaborane
reaction with two equivalents of acetylene is much less
sensitive to the substitution and reported yields of 3 are
50–80% based on 6 [13]. The preparation of C-substituted
nido-5,6-dicarbadecaboranes by alkyne insertion of
octaborane(12) is much less practical due to the limited
availability of the borane [19].
Both the carborane 5 and nonaborane 6 are conveniently
prepared from decaborane by alkyne insertion to B10H12?
L2 [17,20,21] and a reaction with Me2S/methanol [22],
respectively. Alternatively, 1,2-heterodisubstituted o-car-
boranes 5 are easily prepared using 1-(t-butyl)dimethyl-
silyl-o-carborane [23,24].
Overall, the preparation of 1,10-dicarba-closo-de-
caborane derivatives 1 relies on the net removal of two
boron atoms from and introduction of two carbon atoms to
decaborane followed by thermal dehydrogenation/re-
arrangement. Most of the individual steps in this synthesis
have been performed for the parent compounds or their
methyl or phenyl derivatives. There has been no attempt,
however, to develop a reliable synthetic procedure for
preparation of p-carboranes 1 disubstituted with aryl and
long chain alkyl groups using commercially available
precursors.
Here we focus on two aspects of the procedure: (a)
preparation of substituted nido-carborane 3 using a new
homogenous deboronation reaction of the nido anion 4 and
(b) conversion of 3 to p-carborane 1. We used n-hexyl and
n-pentyl alkyl groups and 4-bromophenyl as a function-
alized aryl substituent. Finally, we describe the synthesis
of carboxylic acids and the first example of Wade’s C-
arylation of the monoalkylated p-carborane as important
intermediates for liquid crystalline materials.
2. Results and discussion
2.1. Synthesis
Treatment of o-carborane 5b with methanolic KOH
gave a racemic mixture of the nido anion 4b in analogy to
the preparation of the parent anion [15] 4a and some of its
alkylated derivatives [16,17]. Unlike 4a, the hexyl deriva-
tive 4b is highly lipophilic, and it can be efficiently
extracted into hexanes as the corresponding acid 4b-H
upon protonation of its 1 M aqueous solutions. Use of
ether, rather than hexanes as solvent, increases the ef-
ficiency of extraction of the free acid, which can be easily
dissolved in hexanes after removing the ether. The treat-
ment of potassium salt 4b-K with NMe4Cl precipitates the
corresponding ammonium salt 4b-NMe4, which is com-
pletely soluble in benzene and ether, but only sparingly
soluble in hexanes.
2.1.1. nido-5,6-Carboranes
Oxidation of 4b-K with FeCl3 under Plesek conditions
ˇ
[11,14] was unsuccessful and only traces of the desired
nido product 3b were obtained. The relatively low pH of
FeCl3 aqueous solutions caused protonation of anion 4b to
4b-H, which quickly accumulated in the hexane layer
where it was prevented from further contact with the
oxidant. Increasing the pH of the solution by the addition
of potassium acetate proved helpful. A small increase in
the conversion rate was observed when 1 equivalent of
AcOK was used, but a 1:3 FeCl3 /AcOK ratio turned out to
be more practical. No further optimization of the reaction
conditions was attempted. The reaction took place over a
period of several hours, during which the anion 4b was
slowly extracted to the aqueous phase while the much less
acidic product 3b was accumulated in the hexane layer.
A short path vacuum distillation of the crude product
allowed for isolation of 3b in about 10%–16% yield,
Our synthesis is complemented with 2-D NMR spec-
troscopy of the products, which allows the complete
structural assignment of the 11B NMR signals.

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Z. Janousek, P. Kaszynski / Polyhedron 18 (1999) 3517–3526
3519
leaving behind a viscous oligomeric material, which was
presumably mostly a dimer similar to that reported for
oxidation of 3a under different conditions [25].
In an attempt to improve the yield of the nido-carborane
3b, we turned to homogenous deboronation reactions. In
particular, we focused on SO2 as the oxidant, inspired by
the observation that SOCl2 is reduced by polyhedral boron
hydrides [26].
The reaction of the free acid 4b-H with SO2 in hexanes
proved successful and a 16% yield of the desired nido
compounds was isolated by distillation. The analogous
reaction of the anhydrous salt 4b-NBu4 in benzene did not
work and only the unchanged starting material was ob-
served by TLC. This suggests that a proton, or more likely
a hydronium ion, is necessary for the reaction to occur.
Thus, the stoichiometry of the deboronation reaction can
be written as a reduction of S(IV) to elemental sulfur,
which is consistent with our observations of formation of a
yellow insoluble material.
Scheme 2.
Scheme 2) in carborane 4, while the removal of the more
hindered boron vertex (path b in Scheme 2) would result in
the 6-isomer. Complete isomerization of the 5-isomer 3b-I
to the 6-isomer 3b-II was observed upon treatment with a
base [27].
R-C2B9H11 2 H3O1 1 SO2 → R-C2B8H11 1 B(OH)3 1 Sx
(1)
Another route to nido-carboranes was explored in which
carboranes 3c and 3d were prepared from nonaborane 6
and appropriate acetylenes in about 50% yields according
to the literature method [13]. The isolated carborane 3d
was accompanied by about 30% of the closo derivative 2d.
Unlike the Fe(III) oxidation, the reaction with SO2 was
complicated by the formation of minor quantities of sulfur-
containing byproducts according to GCMS. These products
were not further characterized and a discussion on their
structure and formation will be presented elsewhere. Some
of the sulfur-containing species were removed from the
reaction mixture by aqueous workup and filtration through
a silica gel plug. A short-path vacuum distillation of the
resulting crude product typically gave about 15–18% of a
mixture of carboranes containing less than 5% of sulfur-
containing species, and the yield of the reaction was
independent of the 4b-H drying procedure. A pure mixture
of isomeric nido-carboranes 3b can be obtained by chro-
matographic separation.
In situ generation of SO2 from aqueous Na2SO3 and
sulfuric acid in a water/hexane system did not work well
and only traces of 3b were obtained. Other attempts using
SOCl2 or SO2Cl2 in hexanes or in hexane/water resulted
in relatively large amounts of sulfur- and chlorine-con-
taining carboranes as detected by GCMS.
The regiochemistry of the deboronation reactions was
rather unexpected. According to a recent literature report,
6-alkyl isomer (3-II) was formed in preference to its
5-isomer (3-I) in a ratio of about 3:1 under the Fe(III)
oxidation conditions [14]. In the present case, however,
oxidation of 4b either with SO2 or buffered FeCl3 pro-
duced the two isomeric products 3b-I and 3b-II in almost
equal proportions according to the GCMS and NMR
analyses. Surprisingly, however, the SO2 oxidation of a
rigorously dried 4b-H resulted in the preferential formation
(|3:1) of the apparently less thermodynamically stable
carborane 3b-I. This is consistent with the removal of the
boron vertex from the less hindered position (path a in
2.1.2. closo-1,10-Carboranes
The preparation of p-carborane 1 from nido-carborane 3
was accomplished in two steps: dehydrogenation to the
closo-carborane 2 and a high temperature skeletal re-
arrangement. The presence of the aryl substituent in 3d
permits its dehydrogenation at relatively low temperatures
and vacuum distillation of the crude reaction mixture at
about 2008C afforded the o-carborane 2d in 23% yield
based on nonaborane 6. Similar dehydrogenation of alkyl
nido-carboranes requires higher temperatures. Thus, heat-
ing neat alkylcarboranes 3b or 3c at 2608C for 1 h resulted
in dehydrogenation to 2b and 2c, respectively, and their
partial rearrangement to the corresponding meta isomers.
The resulting closo-carboranes were distilled from a
polymeric residue to avoid further degradation of the
product. The decomposition was particularly pronounced
in 3 obtained using SO2, and is presumably related to the
thermal instability of the sulfur-containing species catalyz-
ing product degradation.
To lower the temperature for the formation of 2 and to
limit the extent of decomposition of the substrate, the
nido-carborane 3c was converted to its sodium salt 3c-Na,
which was disproportionated to form the 2c and disodium
salt 3c-Na2 upon vacuum distillation at 1408C (Scheme 3).
The remaining disodium salt 3c-Na2 was oxidized with
anhydrous CuCl2 [5] to furnish an additional portion of the
ortho-carborane 2c, bringing the total yield to an accept-
able level of 23% based on 6. Thus, the use of the sodium
salts increased the yield of the dehydrogenation and also

ˇ
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Z. Janousek, P. Kaszynski / Polyhedron 18 (1999) 3517–3526
Table 1
NMR chemical shifts (d/ppm) and coupling constants (J/Hz) and their
assignments for 1-hexyl-1,2-C2B10H11 carborane 5b
Vertexa
d(11B)
1J(11B–1H)
d(1H)b
d
1c
–
–
–
2c
–
2.46e
2.08
2.36
2.29
2.70
2.78
2.90
3,6
4,5
7,11
8,10
12
213.15
212.12
213.87
29.95
26.36
22.95
152
142
161
150
150
147
Scheme 3.
improved the purity of the final carborane 1c as compared
with only the temperature-induced reaction.
9
a Assignments based on relative intensities and COSY correlation run
in benzene-d6.
The formation of the para isomers was accomplished by
thermolysis of the distilled alkyl-closo-carboranes in a
sealed tube at 3308C. The isolated carboranes 1b and 1c
were sufficiently pure (.97% by NMR and GCMS) for
subsequent transformations. Thermolysis of the bromo-
phenyl derivative closo-carborane at 3008C resulted in
complete rearrangement to the meta isomer with a minimal
loss of bromine (|2%). Increasing the temperature to
3508C resulted in complete rearrangement to 1d and about
10% disproportionation, forming debrominated and dib-
rominated products according to GCMS.
b Chemical shifts of directly bound hydrogens obtained from
1Hh11B(selective)j experiments.
c Skeletal carbon atoms: 13C NMR d 61.3 (C2), 75.5 (C1).
d Signals from the hexyl group: 1.55 (t, J58.6 Hz, C1H2), 1.13–1.19
(m, C2H2), 0.96–1.03 (m, C3H2C4H2), 0.80–0.84 (m C5H2), 0.87 (t,
J57.2 Hz, C6H2). 13C NMR d 14.14 (C6), 22.75(C5), 28.67 and 29.16
(C2C3), 31.40(C4), 37.89(C1); assignments based on a general trend and
additivity rules (O.A. Subbotin, T.V. Klimova, V.I. Stanko, Yu.A.
Ustynyuk, Russ. J. Gen. Chem. 49 (1979) 363).
e Signal from the skeletal C2H unit.
2.1.3. Functionalization of p-carboranes
resonances were correlated in 11B–11B COSY [30,31] and
1Hh11B(selective)j [32] experiments. 11B chemical shifts
observed for hexyl-o-carborane 5b (Table 1) and hexyl-
nido-carboranes 4b (Table 2) and 3b (Table 3) are in
agreement with general trends [33] and very similar to
those found in the analogous methyl derivatives [27].
As expected, removal of a m proton from the neutral
nido-carboranes 4b-H and 3b-II and the formation of the
corresponding anions 4b(2) and 3b-II(2) [27] results in a
substantial change in the chemical shifts. Deprotonation of
4b-H results in a 16 ppm shift downfield of the directly
affected boron atoms 9 and 11 and an upfield shift by 17
ppm for boron atom 10 (Fig. 1). Boron atoms 2 and 4
antipodal to B9 and B10, respectively, are substantially
shifted upfield by 25 ppm whereas B3, antipodal to B10 is
unaffected by the deprotonation. Resonances of the re-
Carboxylation of monoalkyl p-carboranes 1b and 1c
according to a general procedure [3,28] has led to the
corresponding carboxylic acids 1e and 1f, respectively, in
about 90% yield (Scheme 4). Wade’s arylation [29] of
1-pentyl-p-carborane (1c) provided an alternative route to
the bromophenyl derivative 1d and represents the first
example of such an arylation of a 10-vertex closo-car-
borane. The reaction proceeds in high yield with no
byproducts, unlike the synthesis of 1d from the nido
precursor 3d.
2.2. NMR analysis
The structures for the hexyl carboranes have been
confirmed by NMR spectroscopy and all 11B and 1H
Scheme 4.

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Z. Janousek, P. Kaszynski / Polyhedron 18 (1999) 3517–3526
3521
Table 2
NMR chemical shifts (d/ppm) and coupling constants (J/Hz) and their assignments for for 7(8)-hexyl-7,8-C2B9H12 (4b-H) 7(8)-hexyl-7,8-C2B9H11 NMe4
(4b-NMe4)
Vertexa
7-Hexyl-7,8-C2B9H12 (4b-H)
7-Hexyl-7,8-C2B9H11 NMe4 (4b-NMe4)
d(11B)
1J(11B–1H)
d(1H)b
d(11B)
1J(11B–1H)
d(1H)b
d
e
7c
8c
1
2
3
4
5
6
9
–
–
–
–
–
–
–
–
2.77f
2.25
3.43
1.92
3.14
2.68
2.77
1.77
2.39 (1.84)f
226.76
1 6.26
214.44
1 2.53
2 8.35
2 6.18
227.84
216.86
227.84
–
155
162
174
162
153
153
143
147
143
–
237.64 (238.57)
218.36 (219.42)
214.14 (215.00)
222.51 (223.33)
218.36 (219.42)
218.36 (218.76)
211.65 (212.67)
233.87 (234.69)
211.30 (212.29)
–
137
125
149
130
125
125
134
132/60
137
–
1.12 (0.42)
1.78 (1.13)
2.50 (1.68)
1.97 (1.24)
1.78 (1.13)
2.20 (1.44)
2.66 (1.94)
0.71 (0.00)
2.73 (1.94)
21.84 (22.62)
10
11
m-H
2.12
1.77
21.90
a Assignments based on relative intensities and COSY correlation run in benzene-d6 or in CDCl3 (in parenthesis).
b Chemical shifts of directly bound hydrogens obtained from 1Hh11B(selective)j experiments.
c Skeletal carbon atoms: 13C NMR d 4bH: 68.6 (C8), 87.6 (C7); 4bNMe4: 49.6 (C8), 62.0 (C7).
d Signals from the hexyl group: 1.46–1.54 and 1.60–1.68 (m, C1H2), 1.18–1.29 (m, C2H2), 1.06–1.16 (m, C3H2C4H2), 0.95–1.03 (m, C5H2), 0.91 (t,
J57.2 Hz, C6H2); 13C NMR d 14.23, 22.91, 29.29, 31.19, 31.74, 39.17.
e Signals from the hexyl group: 13C NMR d 14.39, 23.09, 30.12, 32.13, 32.32, 40.40, (NMe14 at 55.05).
f Signal from the skeletal C8H unit.
maining boron atoms 1, 5 and 6 are shifted upfield by
about 11 ppm.
proton and carbon atoms of the a-methylene group. The C1
carbon is shifted downfield by 11.24 ppm. Likewise in H
1
Deprotonation of 4b-H and the formation of a tetra-
methylammonium salt also affect the resonance of other
nuclei and the most significant effect is observed for the
cage atoms. The skeletal carbon atoms C7-R and C8-H are
significantly shielded and their resonances are shifted by
226 and 219 ppm, respectively. Similarly, the skeletal
C8-H proton in 4b-NMe4 is more shielded by 20.31 ppm
as compared to 4b-H.
NMR the set of diastereotopic hydrogens of the methylene
group adjacent to the cage is shifted downfield by 10.57
ppm.
3. Discussion and conclusions
The two new methods for oxidation of substituted nido-
dicarbaundecaborates 4 with buffered Fe(III) or with SO2
produce the nido-carboranes 3 in similar modest yields of
The hexyl chain atoms are modestly deshielded in 4b-
NMe4 relative to 4b-H and the most affected are the
Table 3
NMR chemical shifts (d/ppm) and coupling constants (J/Hz) and their assignments for 6-and 5-Hexyl-5,6-C2B8H11 carboranes 3b
Vertexa
6-Hexyl-5,6-C2B8H11 (3b-I)
5-Hexyl-5,6-C2B8H11 (3b-II)
d(11B)
1J(11B–1H)
d(1H)b
d(11B)
1J(11B–1H)
d(1H)b
5c
–
–
–
–
4.85d
–
–
–
–
–
–
6c
6.60d
1
2
3
4
7
8
9
10
2.02
222.80
23.68
240.34
2.99
2.02
25.50
29.06
–
143
177
|140
150
144
143
|160
156
–
3.27
1.52
2.73
0.50
3.21
2.84
6.13
225.10
22.07
238.88
4.30
0.36
22.07
27.02
–
156
174
146
156
147
|160
146
|160
–
3.48
e
2.73
0.62
3.39
2.83
2.97
3.08
e
e
m-H (9,10)
m-H (8,9)
22.12
22.52
21.88
22.39
–
–
–
–
a Assignments based on relative intensities and COSY correlation run in CD3CN.
b Chemical shifts of directly bound hydrogens obtained from 1Hh11B(selective)j experiments.
c Skeletal carbon atoms.
d Signal from the skeletal CH unit.
e Not identified.

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Z. Janousek, P. Kaszynski / Polyhedron 18 (1999) 3517–3526
3522
Fig. 1. Stick representation of the 11B NMR chemical shifts for 7(8)-hexyl-7,8-dicarba-nido-undecaborane (4b-H) and its tetramethylammonium salt
4b-NMe4 recorded in benzene-d6.
ˇ
about 15% after distillation. The latter deboronation with
SO2 takes place under homogenous conditions which lends
itself to extension to highly lipophilic anions 4. Both
methods, however, are less efficient than the synthesis of 3
from nonaborane 6 and acetylenes (.25%).
The conversion of the alkyl nido-carboranes 3 to the
more chemically stable closo compounds is a low yield
process during which the reactants and products undergo
partial polymerization. The yield of the dehydrogenation
and the purity of the final product is improved by using the
dispropotionation of the nido-carborane sodium salt fol-
lowed by oxidative cage closure with CuCl2.
The high temperatures required for the skeletal re-
arrangement of the closo-carboranes were tolerated by
alkyl groups well but the 4-bromophenyl substituent
underwent partial (10%) disproportionation. Filtration of a
pentane solution of the closo-carboranes 2c through a short
silica gel plug immediately prior to the thermolysis was
found to increase the yield.
The overall yields for the preparation of substituted
para-carboranes are rather low. 1-Hexyl-p-carborane 1b
was obtained in about 8% overall yield based on 5b using
either the Fe(III) or the SO2 methods whereas the pentyl
analog 1c was prepared in 15% overall yield from
nonaborane 6, or 7% yield based on commercial technical
grade B10H14.
Plesek synthesis [14] and the modified anion dispropotio-
nation method can be as high as 45%. Assuming statistical
monoalkylation of 1a, the overall yield for preparation of
1b or 1c from either o-carborane 5a or nonaborane 6
should be around 20%. This is higher than the synthesis of
the monoalkyl derivatives from substituted precursors.
Monoarylation of carboranes [3,29,34] is generally more
efficient than monoalkylation and overall yields higher
than 20% should be expected for preparation of aryl
derivatives of 1a.
The largest loss of material occurs on the nido to closo
ring closure step and yields higher than 50% have not been
observed. This may be due to a reactivity difference
between the two isomers 5-alkyl and 6-alkyl-nido-car-
borane. It is possible that one of the isomers is less
chemically and thermally stable and/or is less prone to
form the closo cage and undergoes polymerization whereas
the other isomer is largely transformed into the closo
product. This hypothesis requires further experimental
work.
The procedure developed here for preparation of alkyl-
p-carborane 1 is simple and reliable but the overall yields
are lower than those obtained by statistical monoalkylation
of the parent p-carborane 1a. Arylation of 1c to form 1d is
another successful adaptation of reactions typical for 12-
vertex p-carborane. Alkylation, carboxylation, ethynylation
[35], and now arylation constitute a set of key synthetic
tools for preparation of liquid crystalline materials [36].
For comparison, the preparation of the parent p-car-
borane (1a) from o-carborane (5a) using the improved

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Z. Janousek, P. Kaszynski / Polyhedron 18 (1999) 3517–3526
3523
4. Experimental
29.7, 27.5, 26.3, 23.8, 23.1, 22.2, 1.1, 2.2, 3.1, 4.6,
6.4.
The 1H, 13C and 11B NMR spectra were obtained in
CDCl3 on Bruker instruments operating at 400.1 MHz,
75.5 MHz, and 128.4 MHz, respectively and referenced to
the solvent (1H and 13C) or to B(OMe)3 (18.1 ppm) unless
specified otherwise. Two dimensional spectra were per-
formed on a Varian XL-500 instrument and coupling
constants 1J(11B–1H) are taken from resolution-enhanced
11B spectra with digital resolution of 68 Hz. IR spectra of
neat samples were recorded in NaCl using an ATI Mateson
instrument. Mass spectrometry was performed using Hew-
lett–Packard 5890 instrument (GCMS) using a 15 m
methylsilicone column. Elemental analysis was provided
by Atlantic Microlab, Norcross, Georgia.
The neat distilled nido compounds (0.96 g) were heated
at 2508C under an atmosphere of dry nitrogen for 1 h and
vacuum distilled to give 0.25 g of closo-carboranes
[(GCMS, rt 9.1 min, m/z 184–193 (max at m/e 191, 38),
169–176 (max at m/e 174, 45), 154–163 (max at m/e 159,
35), 57 (100), 43 (32), 41 (41)]. Alternatively, a crude
mixture of the nido-carboranes (2.2 g) was dissolved in
dry ether (20 ml) and added to a suspension of NaH (0.50
g, 60% in mineral oil, twice washed with ether) in ether (5
ml). The mixture was stirred for 1 h at ambient tempera-
ture and the yellow supernatant liquid was transferred to a
dry flask. The remaining excess NaH was washed with
ether and the collected ethereal solutions were evaporated
to dryness. The resulting viscous oil was short path
distilled (1558C/0.5 torr) to yield 0.56 g of a colorless oil
and a glassy residue, which was stirred overnight with
anhydrous CuCl2 (2.0 g) in dry methylene chloride (20
ml). The resulting dark mixture was passed through a silica
gel plug and washed well with hexanes. The filtrate was
evaporated and the residue distilled to furnish an additional
0.31 g of a mixture of pentyl-o-carboranes 2c.
4.1. 1-Hexyl-1,10-dicarba-closo-decaborane (1b)
A crude mixture of nido isomers 3b obtained according
to Method A (0.97 g) was heated (Wood’s metal bath) in a
flask equipped with a Liebig condenser under dry nitrogen
atmosphere at 2608C for 1 h. The resulting yellow oil was
transferred to a heavy glass tube with aid of a small
amount of pentane. The tube was sealed and heated at
3308C for 10 h and the resulting yellow product was
short-path distilled (858C/0.15 torr) to give 0.34 g (35%
The closo-carboranes (0.87 g) were dissolved in pentane
and passed through a small silica gel plug (a pipette). The
resulting colorless solution was evaporated in a heavy wall
glass tube which was filled with nitrogen, sealed and
heated at 3308C for 12 h. The resulting yellowish product
was short-path distilled (1408C/10 torr) to give 0.56 g
1
yield or 8% yield based on 5b) of a colorless oil: H NMR
d 0.93 (t, J57.0 Hz, 3H), 1.25–1.47 (m, 4H), 1.48–1.54
(m, 2H), 1.6 (br d, J5159 Hz, 4H), 1.90-1.98 (m, 2H), 2.4
(br d, J5159 Hz, 4H), 3.18 (t, J58.2 Hz, 2H), 6.69 (br s,
1H); 13C NMR d 14.11, 22.65, 29.32, 31.71, 31.85, 35.16,
93.6 (br), 125.8 (br); 11B NMR d 214.4 (d, J5165 Hz,
4B), 212.2 (d, J5163 Hz, 4B); IR 3113, 2956, 2930,
2860, 2597, 1465, 1103 cm21; EIMS, m/e 205 (M, 1),
166–177 (max at m/e 175, 27), 153–164 (max at m/e 161,
86), 71 (66), 57 (40), 43 (100).
1
(65% yield) of a colorless oil: H NMR d 0.93 (t, J57.0
Hz, 3H), 1.25–1.47 (m, 4H), 1.48–1.54 (m, 2H), 1.6 (br d,
J5159 Hz, 4H), 1.90–1.98 (m, 2H), 2.4 (br d, J5159 Hz,
4H), 3.18 (t, J58.2 Hz, 2H), 6.69 (br s, 1H); 13C NMR d
14.03, 22.52, 31.53, 31.79, 35.11, 93.6 (br), 125.4 (br); 11B
NMR d 214.4 (d, J5165 Hz, 4B), 212.2 (d, J5163 Hz,
4B); IR (neat) 2958, 2932, 2862, 2594 cm21; EIMS, m/e
205 (M, 1), 166–177 (max at m/e 175, 27), 153–164 (max
at m/e 161, 86), 71 (66), 57 (40), 43 (100). Anal. Calc. for
C7H20B8: C, 44.08; H, 10.57. Found: C, 44.93; H, 10.35.
Thermolysis of nido-carboranes 3b obtained according
to Method B gave 6% of carborane 1b based on 5b.
4.2. 1-Pentyl-1,10-dicarba-closo-decaborane (1c)
4.3. 1-(4-Bromophenyl)-10-pentyl-1,10-dicarba-closo-
decaborane (1d)
A solution of nonaborane 6 (3.45 g, 20 mmol) and
1-heptyne (3.85 g, 40 mmol) in dry toluene (80 ml) was
refluxed for 30 min under an atmosphere of dry nitrogen.
The solvent was removed under reduced pressure and the
yellow oily residue was diluted with hexanes (25 ml) and
passed through a silica gel plug (100 ml) which was
washed with hexanes (250 ml). The hexane washings were
evaporated and the oily residue (3.0 g) was short-path
vacuum distilled (1558C/0.5 torr) to give 0.96 g of a
colorless oil and 1.48 g of a dark viscous residue. The
distilled mixture contained almost equal proportions of the
5-pentyl and 6-pentyl carboranes: GCMS, rt 10.7 and 11.2
min, m/z 183–194 (max at m/e 192, 43), 167–180 (max at
m/e 174, 48), 154–163 (max at m/e 159, 54), 57 (100), 43
(25), 41 (44); 11B NMR d 240.8, 239.6, 225.6, 223.3,
4.3.1. Method A
A solution of 1-(4-bromophenyl)-1-heptyne (1.56 g, 6.2
mmol) and nonaborane 6 (0.54 g, 3.1 mmol) in dry toluene
(30 ml) was refluxed for 30 min. The solvent was
evaporated and the viscous residue was treated with
hexanes and passed through a SiO2 plug which was
washed with hexanes (100 ml). Evaporation of the hexane
eluent left 0.64 g of a yellowish oily mixture of car-
boranes. The crude mixture (0.64 g) was short-path
distilled. The first fraction (0.19 g), collected up to 1508C,
was discarded and a second (0.25 g) was collected up to
2208C/0.1 torr. The carboranes were heated at 3508C using

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Z. Janousek, P. Kaszynski / Polyhedron 18 (1999) 3517–3526
3524
Wood’s metal bath under an atmosphere of dry nitrogen
for 8 h, cooled down, dissolved in hexanes, and filtered
through a silica gel plug. The hexane washings were
evaporated and the oily residue was short-path distilled
(2008C/0.1 torr) to give 0.21 g of 85% pure product which
was purified using chromatography (silica and hexanes).
1.36–1.44 (m, 4H), 1.50–1.57 (m, 2H), 1.91–1.99 (m,
2H), 3.23 (t, J58.3 Hz, 2H), 11.7 (br s, 1H); 13C NMR d
14.09, 22.62, 29.24, 31.65, 31.86, 35.23, 105.1 (br), 130.0
(br), 171.33; 11B NMR d 211.5 (d, J5159 Hz); IR 3012,
2943, 2924, 2860, 2599, 1714, 1432, 1307, 1209 cm21
.
Anal. Calc. for C9H20B8O2: C, 43.46; H, 8.91. Found: C,
43.47; H, 8.95.
4.3.2. Method B
4.5. 10-Pentyl-1,10-dicarba-closo-decaborane-1-
carboxylic acid (1f)
1-Pentyl-1,10-dicarba-closo-decaborane (1c, 0.51 g, 2.7
mmol) was dissolved in dry DME (15 ml) and treated with
BuLi (2.3 M, 1.2 ml, 2.8 mmol) at 2788C. The mixture
was allowed to reach room temperature, stirred for 20 min,
and dry CuI (0.73 g) was added in one portion. The
resulting black solution was stirred for 15 min and dry
pyridine (1.6 ml) was added, followed by 4-bromo-1-
iodobenzene (3.0 g, 10.6 mmol). The resulting mixture
was stirred, gently refluxed overnight, cooled, and poured
into dilute hydrochloric acid. The product was extracted
with hexanes (33), dried and passed through a silica gel
plug. The colorless hexane washings were evaporated. The
resulting semicrystalline material (3.6 g) was recrystallized
twice from a small amount of pentane yielding starting
p-bromoiodobenzene. The mother liquors were evaporated
and the remaining bromoiodobenzene was sublimed off
(1258C/10 torr). The product (0.81 g, 87% yield) was
The acid was obtained according to the procedure
described for 1e: subl. 1208C/0.3 torr; mp 102–1038C; H
1
NMR d 0.97 (t, J57.0 Hz, 3H), 1.42–1.54 (m, 4H),
1.91–1.99 (m, 2H), 3.22 (t, J58.3 Hz, 2H), 11.5 (br s,
1H); 13C NMR d 14.02, 22.49, 31.56, 31.71, 35.18, 104.9
(br), 129.9 (br), 171.35; 11B NMR d 211.5 (d, J5160
Hz). Anal. Calc. for C8H20B8O2: C, 40.94; H, 8.59.
Found: C, 41.46; H, 8.70.
4.6. 5-Hexyl- and 6-Hexyl-5,6-dicarba-nido-decaborane
(3b)
4.6.1. Method A (FeCl3 )
Potassium acetate (47.0 g, 480 mmol) was added to a
mixture of FeCl3?6H2O (44.0 g, 160 mmol) in water (150
ml) and hexane (100 ml) followed by 1 M solution of
4b-K (20 ml, 10 mmol), prepared according to a general
procedure, added over a period of 30 min. The resulting
red reaction mixture was stirred overnight and then reflux-
ed for 4 h, cooled and filtered through Celite. The hexane
layer was separated, dried, and solvent removed, leaving
0.50 g of a colorless oil. The aqueous layer was extracted
with ether (50 ml). Removal of the ether gave a red oil (4
g) which was extracted with hexanes yielding additional
0.12 g of the crude product. The combined crude products
were short-path distilled (125–1508C/0.1 torr) to give
0.40–0.60 g of colorless oil of 3b (10%–15% yield based
on carborane 5b).
1
collected at 1908C/0.5 torr as a colorless viscous oil: H
NMR d 0.97 (t, J57.1 Hz, 3H), 1.40–1.57 (m, 4H),
1.91–1.99 (m, 2H), 3.19 (t, J58.3 Hz, 2H), 7.56 (d,
J58.4 Hz, 2H), 7.65 (d, J58.4 Hz, 2H); 13C NMR d
14.03, 22.51, 31.57, 31.77, 34.53, 114.9 (br) 121.4 (br),
122.55, 130.55, 131.43, 137.30; 11B NMR d 212.0 (d,
J5159 Hz); IR (neat) 2956, 2931, 2860, 2593, 1493, 1465,
1069, 1012, 817 cm21; EIMS, m/e 340–350 (cluster with
max at m/e 345, 100), 57 (10), 43 (29), 41 (32). Anal.
Calc. for C13H15B8Br: C, 45.17; H, 6.71. Found: C, 45.76;
H, 6.67.
4.4. 10-Hexyl-1,10-dicarba-closo-decaborane-1-
carboxylic acid (1e)
4.6.2. Method B (SO2 )
1-Hexyl-1,10-dicarba-closo-decaborane (0.51 g, 2.5
mmol) was dissolved in dry THF (15 ml), treated with
BuLi (1.6 1.M, 1.8 ml, 2.9 mmol) at 2788C. The mixture
was allowed to reach room temperature and stirred for 20
min and cooled down. Then carbon dioxide generated from
dry ice was bubbled through the stirred solution for 1 h.
The solvent was removed, the resulting white solid was
washed with hexanes, dissolved in water and acidified with
conc. HCl. The resulting product was extracted with ether
hexane 1:1 mixture (3310 ml), the organic layer was
dried (Na2SO4) and the solvent evaporated to yield 0.60 g
of crude product. Sublimation (100–1108C/0.15 torr)
followed by recrystallization of the product (0.59 g) from
pentane (2788C) gave 0.53 g (86% yield) of white acid
1e: mp 83–858C; 1H NMR d 0.94 (t, J57.0 Hz, 3H),
A 1 M solution of 4b-K (20 ml, 10 mmol) prepared
according to a general procedure [15] was acidified with
10% HCl and the oily product was extracted with ether.
The organic layer was dried, solvent evaporated, and the
oily residue was dried under vacuum overnight leaving 4.4
g of colorless viscous oil 4b-H (For more rigorous drying
the material was dissolved in hexanes, evaporated, and
1
drying was continued for 12 h at 458C): H NMR d (300
MHz, C6D6) 0.90 (t, J57.1 Hz, 3H), 0.96–1.04 (m, 2H),
1.06–1.16 (m, 4H), 1.18–1.30 (m, 2H), 1.45–1.55 (m,
1H), 1.58–1.68 (m, 1H), 2.74 (br, 1H); 13C NMR d
(C6D6) 14.23, 22.91, 29.29, 31.19, 31.74, 39.17, 68.7 (br),
87.6 (br); EIMS, m/z 214–219 (max at m/z 216, 13),
180–188 (max at m/z 185, 30), 167–175 (max at m/z 171,
34), 71 (40), 57 (28), 43 (100).

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Z. Janousek, P. Kaszynski / Polyhedron 18 (1999) 3517–3526
3525
The resulting carborane 4b-H (4.4 g) was dissolved in
Acknowledgements
hexanes (120 ml) and a slow stream of SO2, generated
from dry Na2SO3 (80 g) and 50% H2SO4 (80 ml), was
passed through the stirred hexane solution over a period of
8 h. The reaction was initially somewhat exothermic, the
initial red color changed to yellow, and the mixture
become silty. After stirring at rt for the total of 8 h the
mixture was filtered through a short silica plug and stirred
with water (50 ml) for 2 h. The hexane layer was
separated, dried (MgSO4), solvents evaporated, and the
residue (0.76 g) vacuum distilled (1508C/0.4 torr) to yield
0.67 g (16% yield based on 5b) a crude mixture of isomers
3b contaminated with ,5% of sulfur-containing species
(GCMS).
This project was supported in part by an NSF CAREER
grant (DMR-9703002). One of the authors (Z.J.) was
partially supported by the Grant Agency of the Czech
Republic (No. 203/97/0060). We are grateful to Dr. James
G. Carver of Redstone Arsenal for a generous gift of
hexylcarborane and decaborane, Dr. Jiri Fusek of ASCR
ˇ
for providing 2D NMR spectra, and Dr. Jaromil Plesek for
helpful discussions.
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A mixture of 4-bromo-1-iodobenzene (14.15 g, 50
mmol), 1-heptyne (4.81 g, 50 mmol), (Ph3P)2PdCl2 (100
mg), CuI (50 mg) and triethylamine (100 ml) was stirred
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(1208C/0.5 torr) the main fraction of 11.16 g (89% yield)
was collected at 1308C/0.15 torr as a colorless oil: 1H
NMR (300 MHz) d 0.92 (t, J57.1 Hz, 3H), 1.30–1.47 (m,
4H), 1.55–1.65 (m, 2H), 2.38 (t, J57.8 Hz, 2H), 7.24 (d,
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