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Decakis(dichloromethyl)-1 12-dicarba-closo-dodecaborane(12) Camouflage of an Icosahedral Carborane by Using Bulky Functional Substituents.

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Decakis(dichIoromethy1)-1,12-dicarba-cZosododecaborane(l2): Camouflage of an
Icosahedral Carborane by Using Bulky
Functional Substituents**
CI,/CCI,
UV/8 h
Wei Jiang, Carolyn B. Knobier, and
M. Frederick Hawthorne*
1
0 CH
Dedicated to Professor Walter Siebert
on the occasion of his 60th birthday
0 BMe
0 BCHClp
The icosahedral carboranes constitute a class of structurally
unique molecules with properties common to both organic and
polyhedral borane chemistry. Their exceptional thermal and
chemical stabilities and their ability to hold substituents in rigid
three-dimensional spatial orientations has made their derivatives desirable synthetic targets for modular chemistry."] In
contrast to the well-developed chemistry of the CH vertices of
the carboranes,['] substitution reactions that occur at BH vertices tend to be less straightforward and of limited utility due to
a lack of regiospecific control. Thus, electrophilic substitution
reactions at BH vertices typically lead to mixtures of product^.^^]
Under robust conditions, however, substitution of all available
BH vertices may occur to give a single final
Thus, we
have developed a synthetic procedure that leads to a class of
1 ,I 2-dicarba-closo-dodecaborane derivatives (para-carborane
derivatives), which are methylated at all boron vertices and
which also may have none, one, or both of the two CH vertices
similarly substituted.[51 Molecules of this class are structural
hybrids composed of an icosahedral carborane substructure
covered by an external sheath of hydrocarbon. Consequently,
the general reactivity of these molecules should resemble that of
aliphatic hydrocarbons, although the steric constraints placed
upon the alkyl groups present in these species would be expected
to control reactivity in a manner not exhibited by simple alkanes. Functionalization of the full array of alkyl groups on the
icosahedral framework will generate an outer sphere of molecular camouflage composed of reactive organic groups of great
importance to the further elaboration of carborane chemistry.
Depending upon the specific substituent groups introduced,
such compounds should be accessible for modular syntheses and
serve as precursors for other derivatives, as well. We here report
an entry into this chemistry. Beginning with decamethyl-1,I 2-dicarba-closo-dodecaborane(12) (l),we have synthesized a derivative in which all the methyl groups at the boron centers are
halogenated under radical-inducing conditions to afford decakis ( dichloromethyl ) - 1,12 - dicarba - closo - dodecaborane(l2)
(2), which has been structurally characterized by X-ray diffraction studies (see Fig. 1).
Chlorination of compound 1 (Scheme 1) in solution in carbon
tetrachloride by reacting 1 with chlorine under UV light for
eight hours at room temperature afforded 2 in 88 % yield. In the
'H NMR spectrum of 2, only two peaks were observed in a 1 :5
integrated ratio. The signal at 6 = 2.95 was assigned to the H
atoms attached to the C atoms of the icosahedral framework,
while the dichloromethyl groups attached to the B atoms gave
rise to a resonance at 6 = 1.55, which may be compared to the
resonances of the methyl groups at the boron atoms in 1
[*] Prof. M. F Hawthorne, Dr. W. Jiang, Dr. C. B. Knobler
Department of Chemistry and Biochemistry
University of California, Los Angeles
Los Angeles, CA 90095
Fax: Int. code +(310) 825-5490
e-mail: rnfh(a'chem.ucla.edu
[**I
Camouflaged Icosahedral Carboranes: Part 2. This work was supported by the
National Science Foundation (Grant CHE 9314037). Part 1: ref. [S].
2536
0 VCH
Verlagsgesellschafi mbH, 0-69451 Weinheim, 1996
2
Scheme 1. Chlorination of 1
(6 = 0.05[51). Similarly, in the 13C('H) NMR spectrum of 2,
only resonances arising from a single type of CH vertex and a
single type of dichloromethyl group were observed. A single,
relatively broad signal in the I l B NMR spectrum with no BH
coupling further confirmed the structure of 2. Mass spectral
analysis was consistent with a decamethyl para-carborane in
which twenty hydrogen atoms had been replaced with chlorine.
Reaction of 1with chlorine under these conditions is relatively rapid. Control experiments based upon ET mass spectra indicated that within thirty minutes the chlorination of 1 had
reached 80 % completion. Species 2 exhibited reduced solubility
in typical organic solvents compared to 1;it was only moderately soluble in carbon tetrachloride.
The radical-induced halogenation of methyl groups attached
to aromatic rings is a well-known organic procedure. Chlorination of methylated aromatic compounds can be carried out in
the presence of halogenation reagents such as N-chlorosuccinimide in a controlled way. However, this synthetic methodology
has not previously been explored with B-methyl groups attached
to a carborane cage, since halogenation of the BH vertices normally present is a facile competing reaction.['] In this study of the
halogenation reactions of 1, such reactions are not a problem
since all of the hydrogen atoms originally attached to boron
atoms in para-carborane have been replaced by methyl groups.
Interestingly, even in the presence of a large excess of halogenation reagent and under rather robust photochemical conditions,
great selectivity is exhibited in the degree of halogenation of the
methyl groups. Chlorination of 1 affords only dichlorination at
each methyl carbon atom, whereas bromination under a variety
of conditions has, as yet, been unsuccessful. Substitution of a
chlorine atom, however, activates the methyl carbon atom towards attack by a second chlorine atom and, under normal
circumstances, a third reaction of this sort will occur. Clearly,
steric constraints which arise on the crowded surface of the
carborane restrict this process to the formation of dichloromethyl
groups.
The synthesis of a mono(chloromethy1) derivative of 1 was
attempted by using a photochemical reaction which employed
only 1.3 equivalents of N-chlorosuccinimide. Analysis of the
product mixture by GC-MS reveaied that both a monochlorinated and a single dichlorinated product were formed, together
with unconverted starting material. This result suggests that the
chloromethyl group first formed is more susceptible to radical
attack than are any of the nine remaining methyl groups, since
otherwise, several isomeric bis(chloromethyl) derivatives of 1
would have been observed. We speculate that the perfluoro
derivative of 1 may be an accessible compound exhibiting enhanced acidity of the carborane CH vertices coupled with steric
protection at these sites by surrounding CF, groups.
A structural study was carried out on compound 2, and an
ORTEP representation of the structure is shown in Figure 1.[*I
In the solid state the dichloromethyl groups attached to the
0570-0833/96/352t-2536$15.00+ .2S/O
Angew,. Chem. Int. Ed. Engl. 1996. 35, No. 21
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(sealed capillary); 'HNMR (CDCI,): d = 2.95 (s, 2H, CH), 1.55 (hr S, 10H, BCHC12); "C{'H} NMR (CDCI, in CCI,): 6 =73.2 (CH), d = 4.6 (vbr, BCHCI,); "B
(mlr):calcd.
NMR (Et,O): 6 = -7.2 (lOB, BCHCI,); MS (El) for C,,B,,,H,,CI,,
973.3, found 973.2 ( M + ) .
Received: May 17, 1996 [Z9128IE]
German version: Angew. Chem. 1996,108, 2653-2655
CI40
Keywords: carboranes
*
chlorinations
- radical reactions
[l] a) V. I. Bregadze, Chem. Rev. 1992, 92, 209-223; h) V. V. Grushin, V. I. BreI
Organomet. Chem. Lib. 1988, 20, 1-68, and references
gadze, V. N. Kalinin, .
U
Fig. 1. The structure of 2 (ORTEP representation). Hydrogen atoms are omitted
for clarity. The molecule has a crystallographically imposed center of symmetry.
Selected bond lengths [A]: C1-B2 1.735(7), C2-B2 1.613(7), B2-B3 1.824(7),
C2-CI20 1.779(5), C2-Cl21 1.797(5).
boron centers adopt a highly symmetrical arrangement with
each group having an "equatorial" chlorine atom and an "axial" chlorine atom. Thus, each of the carborane CH vertices is
surrounded by a ring of five axial chlorine atoms, providing very
effective steric shielding; the remaining ten chlorine atoms form
two coplanar belts around the icosahedron. This arrangement
minimizes steric repulsions between the large chlorine atoms.
The C1 -C1 distance in 2 is fairly short [3.080(7) A] compared
to most para-carborane derivatives,[g1but longer than the corresponding distances found in other decakis-substituted derivatives. We have posited a sterically induced "roll-over'' effect on
the orbitals responsible for cluster bonding in decakis-substituted para-carborane derivatives,", leading to a displacement of
the substituents on the boron vertices towards the relatively
unhindered CH vertices, accompanied by a shortening of the
C-C crosscage distance. This effect is not particularly pronounced in 2, possibly due to the crowding around the CH
vertices by the axial chlorine atoms, but there is a significant, if
small, displacement of the boron substituents towards the C1C1 axis. The average angle defined by a carbon vertex, the
midpoint of the ClGCl axis, and the carbon atom of a
dichloromethyl group is 61.9(2)",whereas the value of the average carbon vertex-midpoint-boron vertex angle is 63.7(2)' in 2.
The successful radical chlorination of 1 leading to 2 represents
an important elaboration of this unique icosahedral cluster. The
new species reported here should undergo transformations to
form an unprecedented class of carborane-supported multifunctionalized compounds. The juxtaposition of organic-type reactivity onto cluster rigidity and the sheer density of functional
groups is expected to lead to novel properties including an increase in molecular dimensions. Exploratory reactivity studies
with thesemolecules is currently under active investigation.
Experimental Procedure
Large Sulfur -Nitrogen Heterocycles:
Preparation of the Sulfur Imides S,NH (n = 8,
9, 11) and Structures of S,NH and S9NH**
Ralf Steudel," Klaus Bergemann, Jiirgen Buschmann,
and Peter Luger
A considerable number of sulfur homocycles S, has been prepared as pure materials (n = 6-13, 15, 18,20) and most of these
have been characterized by X-ray crystal structure analyses.[']
[*I
Standard Schlenk line techniques were employed for all manipulations. Carbon
tetrachloride was freshly distilled from calcium hydride under nitrogen prior to use
and degassed by three freeze-pump-thaw cycles. All reactions were carried out under
a N, atmosphere. Photolysis reactions were carried out using a Hanovia high-pressure mercury vapor lamp.
2: Compound 1 (0.10 g, 0.35 mmol) was dissolved in carbon tetrachloride (25 mL)
in a tube-type quartz photoreactor equipped with a gas bubbler connected to a
chlorine gas cylinder. The solution was photolyzed at room temperature for 8 h.
Solvent was removed in vacuo, and the crude product was extracted with hexane.
After filtration, the solid was recrystallized from a solvent mixture of carbon tetrachloride and hexane to afford 2 as a white solid (0.30 g, 88 %). 290 "C (decomp)
Anpew. Chem. Int. Ed. Enpl. 19%,35. No. 2i
therein.
[2] R. N. Grimes, Carborunes, Academic Press, New York, 1970, pp. 54-180, and
references therein.
[3] J. PleSek, Z. PlzBk, J. Stuchlik, S. Heimauek, CON.Czech. Chem. Commun. 1981,
46, 1748- 1763.
[4] R. R. Srivastava, D. S. Wilbur, Abstracts of Papers, 208th National Meeting of
the American Chemical Society, American Chemical Society, Washington
D. C., 1994, p. 208.
[S] W. Jiang, C. B. Knobler,'M. D. Mortimer, M. F. Hawthorne, Angew. Chem.
1995,107, 1470-1473; Angew. Chem. lnt. Ed. Engl. 1995,34, 1332-1334.
[6] a) A. D. N. Vaz, G. Schoellmanu, J. Org. Chem. 1984,49,1286;b) R. S. Neale,
R. G. Schepers, M. R. Walsh, ibid. 1964,29,3390; c) K . Ohkata, Y. Ohyama, K.
Akiba, Heterocycles 1994, 37, 859.
[7] L. I. Zakharkin, V. I. Stanko, A. I. Klimova, lzv. Akad. Nauk SSSR Ser. Khim.
1964, 771.
M , = 1157.8, mono[8] Crystallographic data for 2.2(C,H8): C,,B,,H,,CI,,,
clinic, space group P2,/c, a =10.589(7), b =10.626(7), c =19.662(13) A, /3 =
92.68(2)", V = 2 2 1 0 A 3 , Z = 2 , p r a , e d = 1 . 7 4 g c m ~T=2
3 , S0 C , y =1 2 .7 cm - ' .
Data were collected on a Huber diffractometer constructed by Prof. C. E.
Strouse of this department, by using Mo,, radiation (A = 0.7107 A) to a maximum 28 = 60", giving 6444 unique reflections, of which 3098 reflections with
1>3u(Z) were retained for structure analysis. The data were corrected for
Lorentz and polarization effects and for secondary extinction and absorption.
The CI atoms were located by using direct methods (SHELX86). The crystal
contains two molecules of toluene per carborane molecule. All non-hydrogen
atoms were refined anisotropically. All solvent hydrogen atoms were placed in
calculated positions, C-H =1.0 A. The benzene ring of the toluene molecule
has been treated as a rigid group, angle =120", C-C =3.395A. All other H
atoms were located and were included but parameters were not refined. The final
discrepancy index was R = 0.049, R , = 0.062. The largest peak on a final difference electron density map was 0.57 e k ' . Crystallographic data (excluding
structure factors) for the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-179.104. Copies of the data can be obtained free of charge on
application to The Director, CCDC, 12 Union Road, Cambridge CB2 lEZ, UK
(fax: Int. code +(1223) 336-033; e-mail: teched@chemcrys.cam.ac.uk).
[9] W. Jiang, C . B. Knobler, C. E. Curtis, M. D. Mortimer, M. F. Hawthorne, Znorg. Chem. 1995, 34, 3491.
Prof. Dr. R. Steudel, Dr. K. Bergemann
Iustitut fur Anorganische und Analytische Chemie
der Techuischen Uuiversitat, Sekr. C 2
Strasse des 17. Juni 135, D-I0623 Berlin (Germany)
Fax: Int. code +(30)31 42 65 19
e-mail: steudel@wap0204.chem.tu-berlin.de
Prof. Dr. P. Luger, Dr. J. Buschmann
Institut fur Kristallographie der Freien Universitat
Takustrasse 6, D-14195 Berlin (Germany)
I**] Sulfur Compounds, Part 196. This work has been supported by the Deutsche
Forschungsgerneinschaft and the Fonds der Chemischen Industrie. Part 195:
see ref. [l].
0 VCH Verla.gszesellschaft mbH, D-69451 Weinheim, 1996
0570-083319613521-25378 15.OO-t ,2510
2531
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using, carborane, dicarba, substituents, dodecaborane, closs, camouflage, decakis, icosahedral, function, dichloromethane, bulka
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