Synthesis, structure, and reactivity of closo-2,3,4,5,6,7,8,9,10,11-decahydroxy-1,12-bis(sulfonic acid)-1,12-dicarbadodecaborane(12)

Article abstract of DOI:10.1021/ja011917g

The reaction of closo-1,12-bis(lithio)-1,12-dicarbadodecaborane(12) (1,12-bis(lithio)-p-carborane) with SO₂ formed closo-1,12 -bis(lithiosulfinato)-p-carborane (10) in nearly quantitative yield. The latter was converted to closo-1,12-bis(sulfin

Full text of DOI:10.1021/ja011917g

J. Am. Chem. Soc. 2001, 123, 12791-12797
12791
Synthesis, Structure, and Reactivity of
closo-2,3,4,5,6,7,8,9,10,11-Decahydroxy-1,12-bis(sulfonic
acid)-1,12-dicarbadodecaborane(12)
Axel Herzog, Carolyn B. Knobler, and M. Frederick Hawthorne*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles,
405 Hilgard AVenue, Los Angeles, California 90095-1569
ReceiVed August 8, 2001
Abstract: The reaction of closo-1,12-bis(lithio)-1,12-dicarbadodecaborane(12) (1,12-bis(lithio)-p-carborane)
with SO2 formed closo-1,12-bis(lithiosulfinato)-p-carborane (10) in nearly quantitative yield. The latter was
converted to closo-1,12-bis(sulfinic acid)-p-carborane (13) via H+-exchange. The corresponding 1,12-bis-
(sulfonic acid) derivative of p-carborane (12) was obtained in high yield by treating 10 with SO2Cl2 and
subsequent AlCl3 mediated hydrolysis of the closo-1,12-bis(chlorosulfonyl)-p-carborane intermediate. The
exhaustive oxidation of 12 in hot aqueous H2O2 (30%) afforded B-decahydroxy-1,12-bis(sulfonic acid)-p-
carborane (15) in 40% yield. As a byproduct, closo-B-decahydroxy-1-sulfonic acid-p-carborane (14) was formed.
Both 14 and 15 were also obtained from the hydroxylation of 10 and 13. Compound 14 was obtained directly
in 88% yield by heating 1-sulfinic acid-p-carborane (17) in H2O2 (30%). Compound 17 was synthesized from
diphenylmethylsilyl-protected p-carborane by using the method employed in the synthesis of 13. The X-ray
structures of 15, its disodium salt, and its dipotassium salt are presented and discussed. Exhaustive methylation
of 15 with methyl triflate furnishes closo-B-decamethoxy-1,12-bis(methyl sulfonate)-p-carborane (20). The
characterization of closomer 20 also includes its crystal structure determination.
Introduction
Scheme 1
Recent research led to the discovery that the icosahedral
boranes Cs2[closo-B12H12] (1), Cs[closo-CB11H12] (2), and closo-
1,12-(HOCH2)2-1,12-C2B10H10 (3) form the B-perhydroxylated
species Cs2[closo-B12(OH)12] (Cs24), Cs[closo-1-H-1-CB11-
(OH)11] (Cs5), and closo-1,12-(H)2-1,12-C2B10(OH)10 (6) (Scheme
1) when heated in aqueous 30% H2O2.1,2 These icosahedral
species containing aromatic 26-electron cages represent a novel
class of so-called camouflaged polyhedral borane derivatives.3
Consequently, the question was raised whether the spherical
sheath of hydroxyl groups in 4-6 can serve as cores for the
synthesis of dendrimer-like derivatives1,3,4 (extended clo-
somers5,6). In the case of 4 the first step in this direction has
been successfully taken. Very recently it has been demonstrated
that 4 can be fully acetylated by using acetic acid anhydride or
completely benzoylated when benzoyl chloride was reacted with
[(n-Bu)4N]24.5 Furthermore, the dodecabenzyl ether derivative
of 4 was obtained by reacting [PPN]24 and benzyl chloride in
the presence of Hu¨nigs base.6 Subsequently, the closomer
[B12(OCH2Ph)12]2- could be reversibly oxidized in two, one-
electron steps to the corresponding neutral hyperclosomer.6
The reactivity of Cs5 has not yet been investigated, mainly
due to its availability in only relatively low yield combined with
the scarcity of the carborane precursor. Meanwhile, the utility
of 6 as a reagent is limited by its extremely poor solubility in
all common organic solvents as well as water. However, for
the following reasons we were motivated to further investigate
the chemistry of 6. Among the three B-perhydroxy derivatives
4-6, compound 6 can be obtained in the highest yield (80%).
More importantly, 6 contains two CH vertices which enhance
its availibility in organic syntheses. Hydrophilic substituents at
the 1- and 12-positions of 6 should improve the solubility of
the resulting B-decahydroxylated carborane and provide two
distinguishable reactive sites in a hypothetical extended closomer
structure.
Reported here are reactivity studies of 6 and the syntheses
of sulfinates and sulfonates of p-carborane as suitable precursor
compounds for conversion to the water-soluble 1-mono- and
1,12-bis(sulfonic acid) derivatives of closo-B-decahydroxy-p-
carborane. Structural features of closo-B-decahydroxy-p-carbo-
rane-1,12-bis(sulfonic acid) are compared to those found for 6.
(1) Peymann, T.; Herzog, A.; Knobler, C. B.; Hawthorne, M. F. Angew.
Chem., Int. Ed. Engl. 1999, 38, 1062-1064.
(2) Peymann, T.; Knobler, C. B.; Kahn, S. I.; Hawthorne, M. F. J. Am.
Chem. Soc. 2001, 123, 2182-2185.
(3) Rockwell, J. J.; Herzog, A.; Peymann, T.; Knobler, C. B.; Hawthorne,
M. F. Curr. Sci. 2000, 78, 405-409.
(4) Housecroft, C. E. Angew. Chem., Int. Ed. Engl. 1999, 38, 2717-
Results and Discussion
2719.
(5) Maderna, A.; Knobler, C. B.; Hawthorne, M. F. Angew. Chem., Int.
Ed. Engl. 2001, 40, 1662-1664.
(6) Peymann, T.; Knobler, C. B.; Kahn, S. I.; Hawthorne, M. F. Angew.
Chem., Int. Ed. Engl. 2001, 40, 1664-1667.
Reactivity of 6. Initially, it was found that 6 remains insoluble
and unchanged when heated in aqueous concentrated NaOH
solution or 6 M H2SO4. In formally viewing 6 as a polysac-
10.1021/ja011917g CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/01/2001

12792 J. Am. Chem. Soc., Vol. 123, No. 51, 2001
Herzog et al.
charide, characteristic functionalizations were attempted. How-
ever, methylation under neither phase transfer conditions with
dimethyl sulfate7 nor with lithium methyl sulfinyl methylide
and methyl iodide8 were successful. A suspension of 6 in liquid
ammonia in the presence of 2 equiv of dissolved sodium metal
did not react. Efforts to esterify 6 by treating it with neat
benzoic- or triflic anhydride at elevated temperatures failed as
did reactions of 6 with various carboxylic acid chlorides, with
or without an added base. A suspension of 6 in neat chloro
sulfonic acid yielded a clear yellow solution at its boiling
temperature but the resulting mixture of closo-carborane, as well
as boric acid derivatives, likely sulfates, was inseparable. Given
the chemical inertness of 6, we focused on the introduction of
substituents at the 1- and 12-vertices of p-carborane which would
be retained during the BH-hydroxylation reaction.
Scheme 2
Previously, the exhaustive hydroxylation of p-carborane by
H2O2 was achieved by using the poorly water-soluble diol 3 as
a precursor (Scheme 1). The loss of the -CH2OH functions
during the oxidation suggests their conversion to -COOH
groups and subsequent decarboxylation. As an alternative,
carboxylic acid functions were placed well away from the cage
carbon vertices using a substituent that is more robust toward
oxidation. closo-1,12-Bis(4-carboxyphenyl)-p-carborane9 (7) as
well as its disodium salt were reacted in excess H2O2 (30%) at
90 °C. These reactions proceeded vigorously and were com-
pleted within 3 h (11B NMR)sabout 4 times faster than found
for the B-perhydroxylation of 3. Again the complete loss of the
C-substituents occurred and 6 was formed in 35% yield (Scheme
1).
carborane with SO2.15 It was reported that 9 eliminates SO2
when heated in H2O.15
The synthesis of the p-carborane analogue of 8 started by
preparing closo-1,12-bis(lithio sulfinato)-p-carborane (10) by
reacting bis(lithio)-p-carborane with SO2. The reaction of 10
16
with SO2Cl2 furnished closo-1,12-bis(chlorosulfonyl)-p-car-
borane (11) in quantitative yield. Use of the AlCl3-mediated
hydrolysis of 11, analogous to the procedure employed in the
synthesis of 8,12 afforded the bis(sulfonic acid) closo-1,12-
(HO3S)2-1,12-C2B10H10 (12) in 95% yield. In parallel, the
corresponding bis(sulfinic acid) closo-1,12-(HO2S)2-1,12-
C2B10H10 (13) was obtained as an air-stable compound after
H+-cation exchange of 10 (Scheme 2).
As expected, the three species, 12, 13, and 10, are quite water-
soluble. Their reactions with 30% H2O2 were studied in the
given order with the assumption that the sulfinates would be
converted to the sulfonic acids under the hydroxylation reaction
conditions.17 Hydroxylation of 12 with excess 30% H2O2 at the
reflux temperature was completed within 5.5 h. Three products,
boric acid (20%, singlet in the 11B NMR spectrum at 20 ppm),
an unidentified borate (20%, singlet in the 11B NMR spectrum
at -4 ppm), and the B-perhydroxylated carborane derivative
(60%, broad singlet in the 11B NMR spectrum at -16 ppm),
formed. In contrast to the hydroxylation of 3 (6 precipitated
and could be isolated by filtration) the products from the
hydroxylation of 12 remained in solution. Their isolation
required the decomposition of all residual peroxide prior to the
drying step. It was found that a peroxide-free solution was
obtained by treatment of the reaction mixture with an excess
of concentrated HBr at 80 °C for 12 h. Furthermore, 11B NMR
spectra indicated that this procedure had no influence upon the
identity (chemical shift) nor the distribution (integration ratios)
of the products. Upon removal of all volatiles in vacuo the
products were partitioned in accord with their solubilities in
methanol. The methanol-insoluble residue was identified as
closo-B-decahydroxy-1-sulfonic acid-p-carborane (14, 15%)
whereas, to our surprise, the desired closo-B-decahydroxy-1,-
12-bis(sulfonic acid)-p-carborane (15) crystallized from the
methanol extract (45% yield)18 (Scheme 3). The boric acid
byproducts and minor amounts of incompletely hydroxylated
carborane remained dissolved in the methanol. Both 14 and 15
Reasoning that carbon-containing groups cannot withstand
the harsh oxidation conditions required for hydroxylation,
inorganic derivatives of p-carborane such as C-sulfonic acids
were pursued as suitable precursors for the BH-hydroxylation
reaction.
Synthesis of Sulfinic and Sulfonic Acid Derivatives of
p-Carborane and Their BH-Hydroxylation. Generally, sul-
fonic acids are known to exhibit excellent solubility properties
in aqueous media. The sulfur atom already possesses its highest
formal oxidation state and the sulfur-carbon bond is known to
be very stable.10 Although previous studies of the oxidation of
the SH-groups of 1,7- and 1,12-bis(thiol)-substituted m- and
p-carborane have been reported,11-13 1,12-sulfinic and sulfonic
acid derivatives of p-carborane had not yet been prepared. The
synthesis of closo-1,7-(HO3S)2-1,7-C2B10H10 (8),11 also the
subject of a patent,14 has been accomplished by a stepwise
oxidative conversion of the parent 1,7-bis(thiol) to the corre-
sponding closo-1,7-bis(chlorosulfenyl)- and closo-1,7-bis(chlo-
rosulfonyl)-m-carborane, respectively. With the assistance of
AlCl3, the latter compound was hydrolyzed to 8. Monosulfinic
acid derivatives of o-carborane including closo-1-HO2S-1,2-
C2B10H11 (9) have been prepared by reacting the lithiated
(7) (a) Merz, A. Angew. Chem., Int. Ed. Engl. 1973, 12, 846-847. (b)
West, E. S.; Holden, R. F. Org. Synth. 1953, 3, 800-803.
(8) (a) Senichiro, H. J. Biochem. 1964, 55, 205-208. (b) Blakeney, A.
B.; Stone, B. A. Carbohydr. Res. 1985, 140, 319-324.
(9) Colquhoun, H. M.; Lewis, D. F.; Herbertson, P. L.; Wade, K. Polymer
1997, 38, 4539-4546.
(10) Handbook of Chemistry and Physics, 71st ed.; Lide, D. R., Ed.;
CRC Press: Boca Raton, FL, 1990; pp 9-87.
(15) Zakharkin, L. I.; Zhigareva, G. G. IzV. Akad. Nauk SSSR, Ser. Khim.
1969, 3, 611-616.
(11) Semenuk, N. S.; Papetti, S.; Schroeder, H. Inorg. Chem. 1969, 8,
2441-2444.
(16) Hamada, T.; Yonemitsu, O. Synthesis 1986, 852-854.
(17) Muth, F. In Houben-Weyl Methoden der Organischen Chemie;
Mu¨ller, E., Ed.; G. Thieme Verlag: Stuttgart, New York, 1955; Vol. IX,
pp 299-342.
(12) Scott, R. N.; Hooks, H., Jr.; Siekhaus, J. F. Inorg. Chem. 1971, 10,
2358-2360.
(13) Zakharkin, L. I.; Zhigareva, G. G. Zh. Obshch. Khim. 1975, 45,
789-798.
(18) Upon standing in methanol another solid phase of 15 precipitates
over time. A swift workup to separate 14 from 15, as described herein, is
recommended.
(14) Scott, R. N.; Hooks, H., Jr. US Patent, Olin Corp. USA, 3919298
19751211, 1975; Chem. Abstr. 1984, 84, 74423.

Formation of B-Perhydroxylated Species
J. Am. Chem. Soc., Vol. 123, No. 51, 2001 12793
Scheme 3
Scheme 4
are quite soluble in H2O and were found to be stable in aqueous
base (NaOH).
Figure 1. ORTEP diagram of a portion of the structure of 15. The
ellipsoids represent a 30% probability level. Fragments of the neighbor-
ing icosahedra (B, C, and F) are shown to demonstrate intermolecular
O-H‚‚‚O interactions. Selected bond lengths [pm]: B-O 138.3(2)-
139.7(2); B-B 177.1(2)-185.0(2); B-C 172.9(2)-177.5(2); C(1)-
S(1) 181.9(5).
The hydroxylation of 13 as well as the workup procedure
was conducted according to the protocol described for the
oxidation of 12. While the reaction time for complete hydroxy-
lation of 13 was found to be essentially the same as that
observed for 12, more 14 (25%) was formed relative to 15
(35%).
The oxidation of 10 was completed within 2 h, but the yield
of hydroxylated derivative was only 30%. This result suggests
a dependence of the rates of hydroxylation and product
degradation upon pH.
explains the significant decrease in their water solubility
compared to 15.
Structural Characterization of 15. Crystals of 15 obtained
from methanol were suitable for an X-ray diffraction study
(Table 1). The structure, depicted in Figure 1, shows that the
counterions of the sulfonate functions of 15 are hydronium ions.
Diacid 15 crystallizes as a tetrahydrate in the triclinic space
group P-1, and is centrosymmetric. All ten hydroxyl hydrogen
atoms are involved in hydrogen bonds to water, other hydroxyl
group oxygen atoms (intermolecular), or two oxygen atoms of
the SO3--moieties (intramolecular). One type of water molecule
is bonded via its two hydrogen atoms to oxygen atoms of a
hydroxyl group and an SO3--oxygen, and it also forms hydrogen
bridges to another water molecule as well as to a hydroxyl
group. The other type of water molecule is bonded via its two
hydrogen atoms to oxygen atoms of a hydroxyl group and a
water molecule, and it is also hydrogen bonded to the hydronium
cation. Additionally, the cation displays hydrogen bridges to
an SO3--oxygen and a hydroxyl group.
Structural Characterization of the Disodium Salt of 15,
18. Single crystals of 18 were obtained from a dilute solution
of 15 in 2 M aqueous NaOH. Figure 2 shows the structure of
one of the four centrosymmetric dianions contained per unit
cell. The salt crystallizes in the monoclinic space group C2/c
with four molecules of water per dianion (Table 1). As found
in Na24,2 each sodium cation is octahedrally ligated by six
oxygen atoms. On a 2-fold axis, Na1A is coordinated by oxygen
atoms of two water molecules, two SO3--groups, and two
hydroxyl groups. Na2X, on a center of symmetry, interacts with
oxygen atoms of four hydroxyl groups and two SO3--moieties.
All ten hydroxyl hydrogen atoms are involved in hydrogen
bonds to water (two), other hydroxyl group oxygen atoms (two),
or oxygen atoms of the SO3--moieties (six). One water molecule
is bonded via its two hydrogen atoms to oxygen atoms of a
hydroxyl group and a water oxygen, and it also forms a
hydrogen bridge to a hydroxyl group. The other water molecule
is bonded via one of its two hydrogen atoms to an oxygen atom
of a SO3--group.
The superb water solubility of 14 encouraged the development
of a synthetic method that allows its preparation in higher yield.
Reaction of monolithiated closo-1-Ph2MeSi-1,12-C2B10H11 (16)
with SO2, subsequent desilylation with [(n-Bu)4N]F in THF at
50 °C, followed by flash chromatography with Et2O/methanol
yielded the (n-Bu)4N-salt of closo-1-sulfinic acid-p-carborane.
Cation exchange (H+) furnished the p-carborane analogue of
9, 17, as a colorless solid (Scheme 4). The sulfinic acid 17
slowly turns brown upon prolonged exposure to air. A two-
week old sample displayed a broadening of the resonance
comprising its 11B NMR spectrum and a mass spectrum (FAB,
negative mode) contained the parent peak of the closo-1-
sulfonate-p-carborane anion. However, the corresponding 13C
NMR spectrum did not show significant changes. Nevertheless,
it can be assumed that 17 decomposes slowly according to
3RSO2H f RSO2SR + RSO3H + H2O.19
Hydroxylation of a freshly prepared sample of 17 in H2O2
(30%) at the reflux temperature was completed after 3.5 h and
the 11B NMR spectrum of the crude reaction mixture indicated
the formation of 14 in approximately 90% yield, later confirmed
by mass balance after workup (HBr). Aside from B(OH)3 (3%)
and unidentified borate (3%) only small amounts of 6 were
isolated (5%). Astonishingly, the loss of SO2, observed during
the hydroxylation of 12 and 13 and more significantly in the
case of 9 in hot H2O,15 is negligible in this instance.
Structural Characterization of 15 and Its Disodium and
Dipotassium Salts. The striking dissimilarity of the solubility
properties of 6 versus 15 prompted us to investigate the solid-
state structure of the latter compound. Additionally, X-ray
diffraction analyses were performed on the disodium salt and
the dipotassium salt of 15 (18 and 19, respectively), which
(19) (a) v. Braun, J.; Weissbach, K. Ber. 1930, 63, 2836-2847. (b)
Marvel, C. S.; Johnson, R. S. J. Org. Chem. 1948, 13, 822-829.

12794 J. Am. Chem. Soc., Vol. 123, No. 51, 2001
Herzog et al.
Table 1. Crystal Data, Data Collection, and Refinement Parametersa
15‚(H2O)6
C2H24B10O22S2
572.43
18‚(H2O)4
19‚(H2O)4
20
empirical formula
formula wt
T, K
C2H18B10Na2O20S2
580.36
C2H14B10K2O18S2
576.55
C14H36B10O16S2
632.65
298(2)
100(2)
100(2)
298(2)
crystal size, mm3
color, habit
crystal system
space group
cell dimensions:
a, Å
0.35 × 0.45 × 0.5
colorless parallelepiped
triclinic
0.02 × 0.1 × 0.6
colorless needle
monoclinic
C2/c
0.05 × 0.1 × 0.3
colorless parallelepiped
monoclinic
0.45 × 045 × 0.1
colorless plate
monoclinic
P21/c
P-1
C2/m
7.4735(14)
7.9376(15)
9.5097(18)
77.594(4)
73.677(3)
68.076(3)
498.40(16)
1
16.282(5)
7.343(2)
16.735(5)
90
102.579(5)
90
1952.9(10)
4
1.974
12.3129(18)
10.0702(13)
8.8975(12)
90
126.471(2)
90
887.2(2)
2
2.158
15.5395(15)
13.5678(14)
13.6715(14)
90
93.092(2)
90
2878.3(5)
4
1.460
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
F
calcd, g/cm3
1.907
0.378
294
abs coeff, mm-1
F(000)
0.419
1176
0.867
580
0.255
1320
θ range, deg
2.25 to 28.28
-9 e h e 9,
-10 e k e 10,
-12 e l e 8
3290
2.49 to 28.29
-21 e h e 20,
-7 e k e 9,
-21 e l e 22
6209
2.85 to 28.29
-16 e h e 14,
-13 e k e 10,
-10 e l e 11
2957
1.31 to 28.31
-14 e h e 20,
-18 e k e 12,
-17 e l e 17
18871
index ranges
no. of reflcns collected
no. of independent reflcns 2276 (Rint ) 0.0355)
2335 (Rint ) 0.0340)
1121 (Rint ) 0.0216)
6967 (Rint ) 0.0243)
completeness to θ max
abs corr
91.4%
96.5%
95.9%
97.4%
none
none
2335/3/193
SADABS
1121/0/99
SADABS
6967/0/438
no. of data/restraints/
2276/0/199
parameters
goodness-of-fit on F2
1.058
0.962
0.516
0.994
final R indices [I > 2σ(I)] R1 ) 0.0296, wR2 ) 0.0810 R1 ) 0.0342, wR2 ) 0.0779 R1 ) 0.0263, wR2 ) 0.0705 R1 ) 0.0794, wR2 ) 0.2118
R indices (all data)
R1 ) 0.0301, wR2 ) 0.0814 R1 ) 0.0594, wR2 ) 0.0852 R1 ) 0.0298, wR2 ) 0.0745 R1 ) 0.1005, wR2 ) 0.2386
extinction coeff
none
0.0000(3)
0.0028(10)
none
larg. diff peak, hole, eÅ-3 0.459 and -0.535
0.555 and -0.431
0.424 and -0.530
1.64 and -0.28
a Details in common: Mo KR radiation (λ ) 0.71073 Å), æ and ω scans, Bruker SMART CCD diffractometer. Refinement method: full-matrix
least-squares on F2. R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) {∑w(Fo - Fc2)2/∑w(Fo )2}1/2
.
2
2
Figure 3. ORTEP diagram of a portion of the structure of 19. Only
one of the two symmetrically coordinated potassium cations is shown.
Additional fragments from nearby cages are depicted to exhibit the
entire coordination sphere of the cation. The ellipsoids represent a 30%
probability level. Selected bond lengths [pm]: B-O 138.9(2)-
139.2(2); B-B 176.1(2)-184.1(2); B-C 172.3(3)-176.8(2); C(1)-
S(1) 182.9(2).
Figure 2. ORTEP diagram of a portion of the structure of 18. Half of
the symmetrical coordination sphere of each sodium cation (Na1X and
Na2X) is shown. Additional fragments of adjacent cages are included
to display relevant coordination sites and hydrogen bridges. The
ellipsoids represent a 30% probability level. Selected bond lengths [pm]:
B-O 137.7(3)-139.1(3); B-B 175.6(4)-183.6(4); B-C 174.0(3)-
178.7(4); C(1)-S(1) 182.4(2).
cations by seven hydroxyl oxygen atoms),2 the potassium cations
-
in 19 are surrounded by ten oxygen atoms, four from SO3
-
Structural Characterization of the Dipotassium Salt of 15,
19. Figure 3 shows the structure of one centrosymmetric dianion
of 19. The compound crystallizes in the monoclinic space group
C2/m with four molecules of water per dianion (Table 1).
Contrary to sparingly soluble K24 (coordination of the potassium
groups and six from hydroxyl groups. Only the remaining four
of the ten hydroxyl hydrogen atoms are involved in hydrogen
bonds to water or oxygen atoms of the SO3--moieties. One water
molecule bridges two hydroxyl groups via hydrogen bonding
of its hydrogen atoms to their oxygen atoms.

Formation of B-Perhydroxylated Species
J. Am. Chem. Soc., Vol. 123, No. 51, 2001 12795
The structure of 15 illustrates its high affinity for the water
molecules and hydronium ions surrounding the icosahedron, thus
reflecting its propensity for solvation by water molecules in
solution. In 18 and 19, the hydroxylated icosahedra are linked
to a polymeric network via the alkali cations. Apparently, the
lattice energy of 19 exceeds by far the hydration enthalpy of
the 10-fold oxygen-coordinated potassium cation. Here, the lack
of participation of hydroxyl groups in this network results in
its low solubility in water as does the participation of all
hydroxyl groups in the network of 6.
The B-O bond lengths in 15 (138.3(2)-139.7(2) pm), 18
(137.7(3)-139.1(3) pm), and 19 (138.9(2)-139.2(2) pm) are
in the same range as those found in the solid-state structure of
6 (138.6(4)-140.3(4) pm). Furthermore, both the average
tropical (vertical) and meridional (horizontal) B-B bond
distances in 15, 18, and 19 are in the same range as in 6. It is
noteworthy that the meridional B-B bonds in all closo-B-
decahydroxy-p-carborane species are about 3.8% longer com-
pared to those of p-carborane (176.2(5) pm).20 The same
tendency and magnitude is observed in a comparison of the
corresponding C-B bonds. However, the tropical B-B bonds
in 6, 15, 18, and 19 are only 1.5 pm or less than 1% longer
than the comparable B-B bonds in p-carborane. This “oval-
ization” of the symmetrically substituted p-carborane cage has
recently been described for closo-1,12-bis(ethynyl)-p-carborane21
and can also be found in closo-1,12-(F3CS(O)2OCH2)2-1,12-
C2B10(CH3)10.22 In both of these cases ovalization was less
developed. A further distortion of the closo-cages of 15, 18,
and 19 originates from a slippage of the apical C-vertices relative
to the meridional five-membered boron rings, causing the C-B
bonds to differ in length by about 4.5 pm. Since the longer C-B
bonds are found opposite the charged oxygen of the sulfonate
group, the distortion may be the result of a trans-effect of
electronic and steric oxygen. Conspicuously, the average C-S
bond length for the carboranyl sulfonic acid derivatives (182.4-
(5) pm) exceeds those found for alkyl derivatives such as 2-(N-
morpholino)ethane sulfonic acid (178.9(7) pm)23 or sodium (4-
aminophenyl)methane sulfonate (179.5(3) pm).24 This finding
may explain the tendency of the carborane sulfonates to facilely
Figure 4. ORTEP diagram of the structure of 20. Shown is one of the
two noncrystallographically related molecules contained in the asym-
metric unit, both of which are centrosymmetric. The molecule shown
is not disordered whereas the second molecule of 20 is disordered with
the -SO2OCH3 moieties in two different orientations, at 82% and 18%
occupancy. The ellipsoids represent a 30% probability level. Selected
bond lengths [pm]: B-O 136.9(3)-138.4(3); B-B 177.0(4)-
185.2(4); B-C 174.5(3)-177.6(3); C(1A)-S(1) 178.5(5).
Scheme 5
it was found that 15 explodes violently when in contact with
neat acetic anhydride.
Attempts to benzylate 15 in anhydrous DMSO with solid
KOH and benzyl bromide26 resulted in its complete decomposi-
tion to B(OH)3. Similary, methylation of 15 under phase-transfer
conditions with dimethyl sulfate and [(n-Bu)4N]OH in a
benzene/water mixture7 did not occur and 15 was recovered
quantitatively. However, the reaction of the hexahydrate of 15
with neat methyl triflate in the presence of 2,6-di-tert-butyl-4-
methylpyridine27 led to the formation of closo-B-decamethoxy-
1,12-bis(methyl sulfonate)-p-carborane (20; 82% yield after
purification) (Scheme 5). Loss of sulfonate groups or other
degradation reactions were not observed. Compound 20 is
sparingly soluble in hexane, but soluble in polar organic
solvents. The solid did not display explosive properties.
Structural Characterization of 20. Crystals of 20 obtained
from acetone were suitable for an X-ray diffraction study (Table
1). The structure (Figure 4) confirmed that all hydroxyl groups
of 15 are methylated. The B-C (174.5(3)-177.6(3) pm) and
B-B bond lengths (177.0(4)-185.2(4) pm) in 20 do not differ
significantly from the comparable distances in 6, 15, 18, and
19; hence, the same cage distortions are observed. The B-O
bonds (136.9(3)-138.4(3) pm) in 20 are understandably about
1 pm shorter in 6, 15, 18, and 19. The C-S bond length in 20
(178.5(5) pm) is comparable with the C-S bond distance of
release SO2. In summary, the structural analyses of 15, 18, and
-
19 suggest only an insignificant electronic influence of the SO3
-
functions on the carborane scaffold and consequently on the
nucleophilicity of the B-hydroxyl oxygen atoms.
Reactivity Studies. It is imperative to note that two detona-
tions occurred when treating dry samples of 14, either upon
scratching with a glass rod or heating while evacuating the
containing vessel. Furthermore, 14 decomposes explosively
when heated to temperatures near 325 °C. Thus, 14 must be
categorized as a heat- and shock-sensitive substance. These
hazardous properties may be explained by a SO2 elimination
accompanied by a subsequent oxidation of the cage by SO2 in
its statu nascendi.
Similary, 15 exhibits an explosive decomposition point of
230 °C. Hence, as in the case of 14, 15 must be handled as a
heat- and potentially shock-sensitive25 compound. Furthermore,
(20) Davidson, M. G.; Hibbert, T. G.; Howard, J. A. K.; Mackinnon,
A.; Wade, K. Chem. Commun. 1996, 2285-2286.
(21) Batsanov, A. S.; Fox, M. A.; Howard, J. A. K.; MacBride, H. J.
A.; Wade, K. J. Organomet. Chem. 2000, 610, 20-24.
(22) Herzog, A.; Knobler, C. B.; Hawthorne, M. F.; Maderna, A.; Siebert,
W. J. Org. Chem. 1999, 64, 1045-1048.
(26) (a) Benedict, D. R.; Bianchi, T. A.; Cate, L. A. Synthesis 1979,
428-429. (b) Johnstone, R. A. W.; Rose, M. E. Tetrahedron 1979, 35,
2169-2173.
(23) Christensen, A. N.; Hazell, R. G.; Lehmann, M. S.; Nielsen, M.
Acta Chem. Scand. 1993, 47, 753-756.
(24) Lechat, J. R. Acta Crystallogr. Sec. B 1979, 35, 183-185.
(25) (a) Compound 15 did not detonate upon scratching. However, this
does not preclude the occurrence of such an event when dealing with this
species. (b) See preface of Experimental Section.
(27) (a) Arnarp, J.; Kenne, L.; Lindberg, B.; Lo¨nngren, J. Carbohydr.
Res. 1975, 44, C5. (b) Berry, J. M.; Hall, L. D. Carbohydr. Res. 1976, 47,
307-310. (c) Arnarp, J.; Lo¨nngren, J. Acta Chem. Scand. Ser. B 1978, 32,
465-467.

12796 J. Am. Chem. Soc., Vol. 123, No. 51, 2001
Herzog et al.
MHz, acetone): δ 15.8 (d, J ) 153 Hz). MS (FAB, negative mode):
271.05 (M - H+).
the triflate ester closo-1,12-(F3CS(O)2OCH2)2-1,12-C2B10(CH3)10
(179.6(7) pm).22 The C-O distances in 20 (141.1(3)-143.3(3)
pm) are in the typical range of organic methoxy derivatives.
closo-1,12-Bis(chlorosulfonyl)-1,12-dicarbadodecaborane(12) (11).
Neat SO2Cl2 (1.2 g, 9.0 mmol) was added to a suspension of closo-
1,12-bis(lithiosulfinato)-1,12-dicarbadodecaborane(12) (10) (see prepa-
ration of 13 above) (1.2 g, 4.2 mmol) in CH2Cl2 (30 mL) at 0 °C. All
volatiles were removed under reduced pressure after the reaction mixture
had stirred for 1 h at room temperature. Water was added to the
remaining residue and the product was extracted with CH2Cl2 (3 × 15
mL). The combined organic layers were washed with water and dried,
and the residue was recrystallized (CH2Cl2) to yield 11 as colorless
crystals (1.35 g, 94%). Mp 232 °C (lit.11 mp 182 °C). FT-IR (cm-1):
2650, 1394, 1182, 1085, 1026, 920, 777, 606, 563, 542. 1H NMR (500
MHz, CDCl3, 322K): δ 3.6-2.2 (BH).13C NMR (125.8 MHz, CDCl3,
322 K): δ 99.1 (Ccarborane).11B{H} NMR (160.5 MHz, CH2Cl2): δ -12.4
(d, J ) 147 Hz). HR-MS (EI): calcd 341.0382, found 341.0378.
closo-1,12-Bis(sulfonic acid)-1,12-dicarbadodecaborane(12) (12).
A mixture of 11 (1.2 g, 3.5 mmol) and AlCl3 (1.0 g, 7.5 mmol) in
toluene (30 mL) was heated for 5 h at 85 °C. The toluene was distilled
off and, at 0 °C, first water (30 mL) and then concentrated HCl (10
mL) were added to the residue. The mixture was filtered and dried
under reduced pressure. The remaining solid residue was redissolved
in water (5 mL), filtered through a syringe filter (0.45 µm), and proton
exchanged. The dried eluate was recrystallized from Et2O to yield 12
as colorless crystals (980 mg, 92%). Mp 219 °C. FT-IR (cm-1): 3448,
2784, 2632, 2391, 1792, 1295, 1079, 915, 853, 794, 738, 601, 438. 1H
NMR (500 MHz, D2O): δ 2.65-1.35 (BH).13C NMR (125.8 MHz,
D2O, 10% CD3OD): δ 101.7 (Ccarborane).11B{H} NMR (160.5 MHz,
H2O): δ -14.7 (d, J ) 150 Hz). MS (FAB, negative mode): 304.11
Conclusion
The exploratory study of closo-B-perhydroxyl-p-carborane
presented here demonstrates that its C-H vertices can be utilized
as platforms for substituents which control its solubility and
reactivity. By placing sulfonate groups at the 1 and 12 positions
of 6, the methanol-soluble species 15‚(H2O)6 was formed and
successfully converted to its permethylated derivative 20.
Extension of this research will proceed in several directions.
The scope of the reaction of 15 with alternative triflic acid
esters28 will be investigated to determine whether ether-
functionalized sulfonate diester analogues of 1525a can be
obtained. Additionally, the analogous alkylation of 1425b will
be examined. An attractive long-term goal is the synthesis of a
thiol species such as closo-1,12-(HS)2-1,12-C2B10(OR)10 and
closo-1-H-12-HS-1,12-C2B10(OR)10 (R ) alkyl, aryl) via the
reduction of the SO3R-functions. Such compounds could play
an important role in closomer chemistry.5,6 Furthermore, sulfonic
acid groups in 1425b and 1525 will be replaced by alternative
substituents such as phosphonate functions with the expectation
of obtaining less hazardous intermediates.
Experimental Section
(M - H+, 15%), 287.09 (M - H+ - [OH], 45%), 287.09 (M - H+
2[OH], 45%), 223.08 (M - H+ - [SO3H], 20%), 207.09 (M - H+
[SO3H] - [OH], 100%).
-
-
Caution! On the scale and under the conditions described here, no
explosions haVe occurred during the synthesis of the B-decahydroxy-
lated compounds. NeVertheless, this does not rule out the possibility
of such an eVent when dealing with carboranes and hydrogen peroxide.
Departure from the reported procedures is not recommended, and
extreme precautions should always be taken to ensure the identity and
purity of all reagents and the use of adequate shielding to contain
possible explosions.
Caution! The dry compounds closo-1,12-bis(sulfonic acid)-B-
decahydroxy-p-carborane (15) and more seVerely closo-1-sulfonic acid-
B-decahydroxy-p-carborane (14) are shock and heat sensitiVe. Extreme
caution is adVised when handling them. Mechanical and thermal stress
should be aVoided and protectiVe apparel and suitable shielding must
be employed.
p-Carborane was purchased from Katchem Ltd., Czech Republic
(http://www.katchem.cz) and Aldrich supplied the reagents n-BuLi, SO2,
SO2Cl2, anhydrous AlCl3, Ph2(CH3)SiCl, [(n-Bu)4N]F, and CF3SO3-
CH3. The H2O2 (30%), Na2CO3‚H2O (Fisher) was used as received.
AG 500W-X8 (Bio-Rad) was used as the cation-exchange resin. The
solvents THF and CH2Cl2 were dried over sodium metal and CaH2,
respectively, and distilled prior to use. All NMR spectra were recorded
with Bruker ARX 400 and 500 spectrometers. Infrared spectra were
obtained with a Nicolet Nexus 470 using KBr-pellets. Mass spectra
(FAB) were obtained on a VG ZAB-SE mass spectrometer.
closo-1,12-Bis(sulfinic acid)-1,12-dicarbadodecaborane(12) (13).
A solution of n-BuLi in hexanes (5.9 mL, 13.9 mmol, 2.34 M) was
added dropwise to a solution of p-carborane (1.0 g, 6.9 mmol) in THF
at 0 °C. After the suspension had stirred for 2 h at ambient temperature,
SO2 (1.0 g, 15.6 mmol) was condensed in the reaction mixture at -78
°C. The reaction mixture was allowed to warm to room temperature
and all volatiles were thoroughly removed under reduced pressure. The
remaining off-white solid was washed with ether and dried to yield
10, which was redissolved in H2O (5 mL), filtered through a syringe
filter (0.45 µm), and proton exchanged. The eluate was dried in vacuo
and the remaining solid was recrystallized from CH3CN to yield 13 as
colorless crystals (1.65 g, 88%). Mp 208 °C. FT-IR (cm-1): 3432 (br),
2627, 2441, 1792, 1322, 1080, 915, 875, 788, 732, 491, 442. 1H NMR
(400 MHz, [d6]-acetone): δ 9.92 (2H, SO2H), 3.5-1.6 (10H, BH).13C
NMR (100 MHz, [d6]-acetone): δ 98.7 (Ccarborane). 11B{H} NMR (160.5
closo-1-Diphenylmethylsilyl-1,12-dicarbadodecaborane(12) (16).
A solution of n-BuLi in hexanes (6.3 mL, 13.9 mmol, 2.2 M) was
added dropwise to a solution of p-carborane (2.0 g, 13.9 mmol) in THF
at 0 °C and the reaction mixture was stirred at room temperature for 2
h. Upon addition of solid Ph2MeSiCl (3.23 g, 13.9 mmol) at -18 °C
the reaction mixture was stirred at ambient temperature for 12 h. The
solvent was removed under reduced pressure and unreacted p-carborane
was sublimed with use of a dry ice cooled coldfinger. The sublimation
residue was dissolved in toluene (80 mL) and extracted with water.
The toluene was distilled from the organic phase and the dry residue
was extracted with Et2O (10 × 20 mL) leaving pure closo-1,12-bis-
(diphenylmethylsilyl)-p-carborane (21) as an undissolved solid. Com-
pound 16 was sublimed from the residue of the dried Et2O extract at
130 °C/10-5 Torr (2.5 g, 53%). Mp 138 °C. 1H NMR (400 MHz,
CDCl3): δ 7.60 (4H, m, Ph), 7.42 (6H, m, Ph), 2.83 (1H, s, Ccarborane
-
H), 3.00-1.55 (10H, BH), 0.68 (3H, s, CH3).13C NMR (100.6 MHz,
CDCl3): δ 135.5 (Ph), 132.7 (Ph), 130.9 (Ph), 128.0 (Ph), 73.6
(Ccarborane), 67.8 (Ccarborane), -3.0 (SiCH3).11B{H} NMR (160.5 MHz,
1
benzene): δ (d, J ) Hz). 21: Mp 200 °C. H NMR (500 MHz, [d8]-
toluene, 350 K): δ 7.52 (8H, m, Ph), 7.10 (12H, m, Ph), 3.00-1.85
(10H, BH), 0.50 (3H, s, CH3).13C NMR (125.8 MHz, [d8]-toluene, 350
K): δ 137.6 (Ph), 135.9 (Ph), 134.2 (Ph), 130.2 (Ph), 79.3 (Ccarborane),
-2.9 (SiCH3).29Si NMR (99.4 MHz, [d8]-toluene, 350 K): δ 9.42. 11B-
{H} NMR (160.5 MHz, toluene): δ -9.3 (d, J ) 150 Hz).
closo-1-Sulfinic Acid-1,12-dicarbadodecaborane(12) (17). A solu-
tion of n-BuLi in hexanes (2.67 mL, 5.9 mmol, 2.2 M) was added to
a solution of 16 (2.0 g, 5.9 mmol) in THF at 0 °C. After the suspension
had stirred for 2 h at ambient temperature, excess SO2 was condensed
in the reaction mixture at -78 °C. The reaction mixture was allowed
to warm to room temperature and all volatiles were removed under
reduced pressure. The remaining residue was resuspended in THF (20
mL) and a solution of [(n-Bu)4N]F in THF (6.5 mL, 6.5 mmol, 1 M)
was added. The resulting clear solution was stirred at 50 °C for 8 h
whereupon all volatiles were removed in vacuo. The resulting residue
was redissolved in Et2O (30 mL) and after extraction with H2O (2 ×
20 mL) flashed through a bed of silica with Et2O. The silica was eluted
with methanol and the eluate dried in vacuo. The obtained oily residue
was proton exchanged with methanol/H2O (1:4) and the solid residue
obtained upon removal of the solvents in vacuo was recrystallized from
(28) Stang P. J.; Hanack, M.; Subramanian, L. R. Synthesis 1982, 85-
126.

Formation of B-Perhydroxylated Species
J. Am. Chem. Soc., Vol. 123, No. 51, 2001 12797
CH3CN to yield 17 as colorless crystals (787 mg, 64%). Mp 76 °C.
a colorless powder. The dry compound can detonate when scratched
and for the purpose of storage and transfer, stock solutions of it in
H2O are recommended. Yield 812 mg (88%).
FT-IR (cm-1): 3335 (br), 2615, 1772, 1280, 1141, 1088, 1000, 906,
1
881, 838, 771, 726, 646, 589. H NMR (500 MHz, [d6]-acetone): δ
9.94 (s, SO2H), 3.54 (1H, s, CH), 2.90-1.55 (10H, BH).13C NMR
(125.8 MHz, [d6]-acetone): δ 98.5 (Ccarborane), 63.7 (Ccarborane).11B{H}
NMR (160.5 MHz, acetone): δ -12.5 (d, J ) 156 Hz). MS (FAB,
negative mode): 207.11 (M - H+, 100%), 223.10 (M + [O] + H+,
85%).
closo-2,3,4,5,6,7,8,9,10,11-Decamethoxy-1,12-bis(sulfonic acid meth-
yl ester)-1,12-dicarbadodecaborane(12) (20). A mixture of the
tetrahydrate of 15 (100 mg, 0.2 mmol), 2,6-di-tert-butyl-4-methyl-
pyridine (493 mg, 2.4 mmol), and methyl triflate (5 mL, 44.2 mmol)
was heated to reflux for 5 h. All volatiles were removed under reduced
pressure. The remaining residue was dissolved in CH2Cl2 and an
aqueous solution of Na2CO3 monohydrate (325 mg, 2.62 mmol) was
added dropwise. The organic layer was separated, washed with H2O
(2 × 15 mL), and dried over Na2SO4. After the solvent was removed
in vacuo, the pyridine was sublimed at 50 °C (0.1 mm) with use of a
dry ice cooled coldfinger. The remaining off-white solid was recrystal-
lized from acetone/Et2O to give 20 as colorless crystals (101 mg, 80%).
Mp 320-325 °C dec. FT-IR (cm-1): 2950, 2855, 1461, 1384, 1284,
1185, 1004, 967, 781, 577. 1H NMR (500 MHz, [d6]-acetone): δ 4.11
(6H, s, SOCH3), 3.82 (30H, s, BOCH3).13C NMR (125.8 MHz, [d6]-
acetone): δ 67.8 (br, s, Ccarborane), 59.8 (s, SOCH3), 54.8 (s, BOCH3).
11B{H} NMR (160.5 MHz, acetone): δ -14.3 (s). HR-MS (EI): calcd
632.2446 and 633.2420, obsd 632.2457 and 633.2428.
X-ray Crystallography. All atoms were located by use of statistical
methods. All non-hydrogen atoms were included with anisotropic
displacement parameters. For 15, 18, and 19, position parameters for
all hydrogen atoms were refined. All hydrogen atoms for 20 were kept
in calculated positions. The isotropic displacement parameters for
hydrogen atoms were based on the values for the attached atoms.
Scattering factors for H were obtained from Stewart et al.29 and for
other atoms were taken from.30
closo-2,3,4,5,6,7,8,9,10,11-Decahydroxy-1,12-bis(sulfonic acid)-1,-
12-dicarbadodecaborane(12) (15) and closo-2,3,4,5,6,7,8,9,10,11-
Decahydroxy-1-sulfonic Acid-1,12-dicarbadodecaborane(12) (14).
Aqueous 30% H2O2 (30 mL) was added to either of the diacids 12 and
13 (3.5 mmol), which were placed in a 100 mL single-neck flask
equipped with a reflux condenser seated in a Teflon sleeve. The solution
was refluxed for 5 h, after which time samples were taken for 11B NMR
monitoring. For this purpose the reaction mixture was cooled to room
temperature and then refluxed for a longer period, if required. Upon
completion of the oxidation, concentrated HBr (20 mL) was added to
the mixture and after being stirred for 2 h at room temperature the
mixture was kept at 80 °C for 8 h. Evolving bromine was occasionally
removed by a gentle steam of nitrogen. A negative KI/Starch test of a
dried specimen indicates completion of the redox process. Thereafter,
all volatiles were removed in vacuo and the remaining residue was
triturated in methanol. The suspension was centrifuged and the
supernatant solution was separated by using a pipet to withdraw the
supernatant. This procedure was repeated five times. The extraction
residue was dissolved in water, filtered, and dried to afford 14 as a
colorless solid. The dry compound can detonate when scratched and
for the purpose of storage and transfer, stock solutions of it in H2O
are recommended. Diacid 15 was obtained as colorless crystals from
the methanol washings by slow evaporation.18 15: Mp 230 °C
(explosive dec). FT-IR (cm-1): 3480 (br), 3220 (br), 2399 (br), 1642,
1300 (br), 1153 (br), 1049, 968, 734, 704, 630, 600, 535. 13C NMR
(125.8 MHz, CD3OD): δ 62.0 (Ccarborane). 11B{H} NMR (160.5 MHz,
MeOH): δ -16.0 (s). Negative-FAB: 462.91 (100%, M - H+), 382.97
(40%, M - SO3H+). 14: Mp 325 °C (explosive dec). FT-IR (cm-1):
3268 (br), 2450 (br), 1645, 1306 (br), 1290 (br), 1044, 704, 618, 608,
533.1H NMR (500 MHz, D2O): δ 2.77 (s, CH). 13C NMR (125.8 MHz,
D2O, 10% CD3OD): δ 59.9 (Ccarborane), 48.1 (Ccarborane). 11B{H} NMR
(160.5 MHz, H2O): δ -13.9 (s), -14.8 (s). MS (FAB, negative
mode): 462.91 (100%, M - H+), 382.97 (40%, M - SO3H+).
closo-2,3,4,5,6,7,8,9,10,11-Decahydroxy-1-sulfonic Acid-1,12-Di-
carbadodecaborane(12) (14). An aqueous solution of 30% H2O2 (20
mL) was added to freshly prepared p-carborane-1-sulfinic acid (17)
(500 mg, 2.4 mmol) placed in a 100 mL single neck flask. Instantly a
slightly exothermic reaction of brief duration occurred. The solution
was refluxed for 3.5 h and the 11B NMR monitored from this point on.
After workup according to the previous protocol, 14 was obtained as
Acknowledgment. We wish to gratefully acknowledge the
support of this research by the National Science Foundation
(NSF) (Grants CHE-9730006 and CHE-98713320) and equip-
ment grants CHE-9871332 and CHE-9974928.
Supporting Information Available: Tables listing atomic
coordinates, temperature factors, bond lengths and angles, and
torsion angles and details of the refinement of the X-ray
crystallographic data (PDF); crystallographic CIF file. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA011917G
(29) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J. Chem. Phys.
1965, 42, 3175-3183.
(30) Ibers, J. A.; Hamilton, W. C. The International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, 1974; Vol. IV.