Methyl camouflage in the ten-vertex: Closo -dicarbaborane(10) series. Isolation of closo -1,6-R2C2B8Me8 (R = H and Me) and their monosubstituted analogues

Article abstract of DOI:10.1039/c8dt02586j

Reported are procedures leading to the first types of methyl camouflaged dicarbadecaboranes with fewer than eleven vertices. The compounds contain the closo-1,6-C₂B₈ scaffolding inside the egg-shaped hepta-decamethyl sheath, which im

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Methyl camouflage in the ten-vertex
closo-dicarbaborane(10) series. Isolation of
closo-1,6-R2C2B8Me8 (R = H and Me) and their
monosubstituted analogues†
Cite this: DOI: 10.1039/c8dt02586j
Mario Bakardjiev,a Oleg L. Tok,a Aleš Růžička,b Zdeňka Růžičková,b Josef Holub,a
a
Drahomír Hnyk, a Zbyněk Špalt,a Jindřich Fanfrlík c and Bohumil Štíbr
*
Received 25th June 2018,
Accepted 17th July 2018
Reported are procedures leading to the first types of methyl camouflaged dicarbadecaboranes with fewer
than eleven vertices. The compounds contain the closo-1,6-C2B8 scaffolding inside the egg-shaped
hepta – decamethyl sheath, which imparts unusually high air and solvolytic stability to all of these
compounds.
DOI: 10.1039/c8dt02586j
rsc.li/dalton
studies on reactions of dicarbaboranes closo-1,6-R2C2B8H8 (1)
(where R = H or Me) with already established methylation
Introduction
The chemistry of polyalkylated twelve-vertex carboranes, com- agents that lead to permethylation or, at least, to persubstitu-
pounds derived from closo [CB11H12]− and C2B10H12, were pio- tion on B-vertices.
neered by Hawthorne’s and Michl’s groups.1,2 This class of car-
boranes represents a new genre in organic chemistry, “spheri-
cal hydrocarbons” whose icosahedral core is sheathed by a
coating of alkyl substituents. In the dicarbaborane area perti-
Results and discussion
nent to this work, the reaction of 1,2-C2B10H12 (o-carborane) or
1,7-C2B10H12 (m-carborane) with excess MeI in the presence of
AlCl3 afforded octamethyl derivatives 1,2,3,6-H4C2B10Me8 and
1,2,3,7-H4C2B10Me8 in high yield;3 the two positively charged
boron positions adjacent to both carbons remain unmethyl-
ated. Complete B-methylation was, however, achieved via
methylation of p-carboranes 1,12-R2C2B10H10 (R = H, Me and
Br) with MeOTf/HOTf at reflux for 20 h to cleanly afford the
B-permethylated carboranes 1,12-R2C2B10Me10 in yields over
90%.4 These successful experiments, together with those
achieved in the methylation of arachno-6,9-C2B8H14,5 prompted
us to examine a similar methylation strategy in the ten-vertex
closo-1,6-C2B8H10 carborane series in which, except for electro-
philic halogenation,6 no work on B-substitution has so far
been reported. In this work we would like to present synthetic
Scheme 1 (left) demonstrates that the HOTf-catalyzed MeOTf
methylation of the parent closo-1,6-H2C2B8H8 (1a) dicarbabor-
ane in
a Pyrex tube leads at different temperatures
(110–175 °C) and varying periods of reaction time (48–120 h)
to persubstitution in all B-positions to generate a mixture of
the B-permethylated derivative closo-1,6-H2C2B8Me8 (2a) and
the triflate closo-1,6-H2C2B8Me7-8-OTf (2b). The mixture was
separated by LC on a silica gel support, using hexane and a
10% CH2Cl2-hexane mixture as the mobile phase. The yields of
2a and 2b varied in ranges 15–40% and 50–70%, respectively
(for approximate yields 5%, see Table 1 and for calculations
on the reaction mechanisms, see ref. 7). The table suggests
that raising temperatures over shorter reaction time seem
to lend support to the formation of the B-permethylated 2a
and 4a.
An attempt at AlCl3-catalyzed methylation of 1a with neat
MeI (Pyrex tube, 80 °C, 72 h) yielded, however, only the iodo
compound closo-1,6-H2C2B8Me7-8-I (2c) (yield ∼75%) which
was isolated and purified, similarly as in the preceding case,
via LC chromatography. It is evident that the yields of the per-
methylated 2a are exacerbated by the formation of the 8-X sub-
stituted compounds 2b and 2c. As demonstrated on the reac-
tion between 2a and HOTf in MeOTf (170 °C, ∼50 h, yield of
2b 96%), these substituted compounds are likely to be formed
as a consequence of electrophilic H+-attack by HX evolved in
aInstitute of Inorganic Chemistry, Academy of Sciences of the Czech Republic,
Husinec-Řež 1001, Czech Republic
bDepartment of General and Inorganic Chemistry, Faculty of Chemical Technology,
University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic
cInstitute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences,
Flemingovo nám. 2, 166 10 Prague 6, Czech Republic
†Electronic supplementary information (ESI) available: NMR data and computed
11B NMR, cartesian geometries. CCDC 1573271 and 1573272. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c8dt02586j
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the triflate closo-1,6-Me2C2B8Me7-8-OTf (4b) (yield ∼75%)
which was separated via LC in a 10% dichloromethane/hexane
mixture. Nevertheless, the best yield of 4a (∼40%) was achieved
at 175 °C over a period of 48 h (see Table 1). Unfortunately, an
attempt at AlCl3-catalyzed methylation of 1b with neat MeI
(80 °C, 72 h) yielded an inseparable mixture of at least three
products, in which the prevailing iodo compound, closo-1,6-
Me2C2B8Me7-8-I (4c), could be identified by 11B NMR spec-
troscopy (see Fig. S16†).
The structures of 8-OTf and 8-I-derivatives 2b and 2c were
determined by an X-ray diffraction analysis (see Fig. 1 and 2).
Their crystallographic parameters thus extend those reported
previously for 1b 8 and 1,6-H2C2B8H7-8-Br6 derivatives of 1.
Both compounds adopt Archimedean antiprismatic geometry
with apical distances generally shorter than intra- and inter-
belt separations, the C–B bond vectors being generally shorter
than B–B bonds. The structures unambiguously confirm the
B8-substitution along with persubstitution in all other B- posi-
tions of the cluster. The crystals of 2a and 4a were not suitable
for crystallographic studies and their molecular geometries
were therefore optimized at the MP2/TZVP level (Fig. 3 and 4).
The optimization revealed that the comparable B–B and B–Me
bonding vectors are very similar to those found crystallographi-
cally for 2b and 2c. The computation has also led to a good
agreement between theoretical and experimental δ(11B) values
for 2a and 4a (max. deviation less than 3 ppm), for the individ-
ual values see ESI.†
Scheme 1 Synthesis of B-methyl derivatives of the closo-1,6-
H2C2B8H8 family.
Table 1 Approximate yields ( 5%) of permethylated (2a and 4a) and
triflato (2b and 4b) derivatives isolated
The HF/cc-pVTZ calculations of the electrostatic potential
(ESP) surface show that the parent 1a has hydridic BH vertices
which can form dihydrogen bonds,9 whereas the hydridic
B-bound hydrogens in 2a and 4a are replaced by methyl
groups of amphiphilic character.10 The hydrogen atoms of the
Subst.
°C
Time (h)
Yield 2a or 4a (%)
Yield 2b or 4b (%)
1a
1a
1a
1b
1b
1b
110
130
175
110
130
175
120
90
48
120
90
48
20
25
40
15
15
40
70
65
50
70
75
40
the methylation reactions (X = OTf and I) at the most negative
B8 site6 of 2a:
1; 6-H2C2B8Me8 ð2aÞ þ HX ! CH4 þ 1; 6-H2C2B8Me7-8-X ð2b and 2cÞ
ð1Þ
It should be also noted that 2c can be also prepared in 81%
yield by MeI/AlCl3 methylation of the “pre-iodinated” closo-1,6-
H2C2B8H7-8-I (1c) derivative.6
Scheme 1 (right) also shows that the HOTf-catalyzed MeOTf
methylation of the C-dimethylated dicarbaborane closo-1,6-
Me2C2B8H8 (1b) (110 °C, 120 h) leads only to the octamethyl-
ated product closo-1,6-Me2C2B8Me6-2,3-H2 (3a, identified by
Fig. 1 ORTEP (30% probability level) representation of the molecular
structure of closo-1,6-H2C2B8Me7-8-OTf (2b). Selected bond lengths (Å):
11B NMR, see Fig. S16†), in which boron positions between O1–B8 1.473(3), C1–B2 1.594(4), C1–B4 1.606(4), B2–B3 1.888(4),
B4–B5 1.867(4), B5–B2 1.839(4), C6–B2 1.744(4), C6–B7 1.772(4), C6–
B10 1.621(4), B3–B7 1.808(4), B4–B8 1.829(4), B2–B9 1.810(4), B7–B8
1.826(4), B7–B10 1.728(4), B9–B10 1.728(4), B2–C2 1.569(4), B10–C10
1.571(4); angles (°): O1–B8–B4 112.3(2), O1–B8–B7 103.6(2), B2–C1–B3
both cluster carbon vertexes remain unmethylated, which is
probably due to combined sterical and electronic effects of the
CMe groups. However, under forcing conditions (15 h, 175 °C),
the methylation can be completed to form a mixture of the
72.8(2), B3–C1–B4 110.8(2), B2–B3–B4 89.9(2), B2–C6–B3 65.3(2), C6–
permethylated closo-1,6-Me2C2B8Me8 (4a) (yield ∼15%) with B7–B8 84.6(2), B9–C6–B7 97.4(2).
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Fig. 4 Molecular model of closo-1,6-Me2C2B8Me8 (4a) as obtained
from the MP2/TZVP optimization. B2–B3 is the longest vector in the
molecule, viz 1.886 Å. The other selected internal coordinates are (dis-
tances in Å, angles in °): C1–B2 1.599, C1–B4 1.620, B4–B5 1.851, B5–B2
1.841, C6–B2 1.750, C6–B7 1.799, C6–B10 1.625, B3–B7 1.796, B4–B8
1.837, B2–B9 1.797, B7–B8 1.853, B7–B10 1.705, B9–B10 1.705, B2–C1–
B3 72.2, B3–C1–B5 109.1, B2–B3–B4 89.5, B2–C6–B3 65.2, C6–B7–B8
87.1.
Fig. 2 ORTEP (40% probability level) representation of the molecular
structure of closo-1,6-H2C2B8Me7-8-I (2c). Selected bond lengths (Å):
I1–B8 2.176(3), C1–B2 1.596(4), C1–B4 1.615(4), B2–B3 1.896(4), B4–B5
1.879(4), B5–B2 1.851(4), C6–B2 1.746(4), C6–B7 1.774(4), C6–B10
1.620(4), B3–B7 1.807(4), B4–B8 1.828(4), B2–B9 1.806(4), B7–B8 1.821
(4), B7–B10 1.724(4), B9–B10 1.720(4), B2–C2 1.569(4), B10–C10 1.564
(3); angles (°): I1–B8–B4 119.3(2), I1–B8–B7 129.0(2), B2–C1–B3 72.9(2),
B3–C1–B5 111.1(2), B2–B3–B4 89.6(2), B2–C6–B3 65.7(2), C6–B7–B8
85.4(2), B9–C6–B7 96.4(2).
Fig. 5 Computed (HF/cc-pVTZ) electrostatic potential (ESP) surface for
(A) 1a, (B) 2a and (C) 4a. The color range of the ESP in kcal mol−1
12-vertex closo system.12 Conceivably, the whole 11B NMR
spectra of 2a and 4a are significantly paramagnetically
deshielded to high frequencies (i.e. downfield shifted) with
respect to that of 1a; for the computed 11B NMR spectrum of
the latter see ESI.†
The structures of all compounds isolated in this study are
also in excellent agreement with the results of multinuclear
11B, [11B-11B]-COSY,13 1H, and 13C NMR measurements that led
to complete assignments of individual cage BMe, BX and CH
Fig. 3 Molecular model of closo-1,6-H2C2B8Me8 (2a) as obtained from
the MP2/TZVP optimization. B2–B3 is the longest vector in the mole-
cule, viz 1.889 Å. The other selected internal coordinates are (distances
in Å, angles in °): C1–B2 1.593, C1–B4 1.610, B4–B5 1.854, B5–B2 1.843,
C6–B2 1.736, C6–B7 1.764, C6–B10 1.623, B3–B7 1.799, B4–B8 1.832,
B2–B9 1.800, B7–B8 1.845, B7–B10 1.707, B9–B10 1.707, B2–C1–B3
72.7, B3–C1–B5 110.2, B2–B3–B4 89.5, B2–C6–B3 65.9, C6–B7–B8
85.4.
methyl groups have positive ESP surface and the exo-skeletal units. Graphical inter-comparisons of the 11B NMR spectra of
carbon atoms have negative ESP surface (see Fig. 5). From the all compounds isolated are in Fig. S15 and S16.† The 11B and
viewpoint of electron transmission, the Me groups behave as 11B-{1H}-decoupled NMR spectra of all B-methylated isomers
weak electron acceptors, which might be surprising, as it is isolated are identical, exhibiting only singlet resonances,
opposed to toluene, xylene, hexamethylbenzene etc. However, which unambiguously points to persubstitution in all
the electron transmission just follows the established electro- B-vertexes. All the 11B NMR spectra of the Cs-symmetry (a
negativity concept that reflects the fact that exo-skeletal substi- slight gear effect of the Me groups) 2–4 derivatives, exhibiting
tuents are bonded via classical 2-centre 2-electron bonds.11 1 : 1 : 2 : 2 : 2 patterns of singlets, are in agreement with the
This agrees with the concept elaborated by Viñas et al. for a presence of the C1–C6–B10 plane.
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The 13C and 1H NMR spectra of compounds of type 2 and 4
are consistent with the presence of two different CH or CMe
and cluster units in positions 1 and 6 (reading upfield) and six
or seven, mostly well resolved, Me groups resonating in the
high-field area (see comparative Fig. S17 and S18†). Due to the
Me-substitution, all the 11B and 13C resonances in the spectra
of the B-permethylated compounds 2 are deshielded when
compared to those of the B-unsubstituted counterparts of
structure 1, while the corresponding 1H-resonances of cage CH
units are shielded. Similar features can be also traced in the
spectra of 8-X compounds with differences attributed to elec-
tronic effects induced by OTf and I substituents. The 19F NMR
spectra of the triflato compounds 2b and 4b are consistent
with the presence of one CF3 group (see Fig. S17 and S18†).
The spectra of the permethylated compounds 2a and 4a are
exemplified in Fig. 6 and 7 along with signal assignments, for
selection of other key NMR measurements see ESI (Fig. S1–
S14†).
Of extreme importance for further developments in the
field of B-clusters containing less than twelve cage vertices is
the question of their resistance to various cage-degradation
agents. According to our experience, the ten-vertex dicarbabor-
anes are most sensitive to the degradation with alcohols. In
order to estimate the effect of the Me-sheath, we have tested
the behaviour of the 1a/2a pair to ethanolysis in the presence
of a ten-fold excess of EtOH than required by eqn (2) and (3):
Fig. 7 190.2 MHz 11B, 600 MHz 1H, and 150.9 MHz 13C NMR spectra of
1,6-Me2C2B8Me8 (4a) (CDCl3).
1; 2-H2C2B8H8 þ 24EtOH ! 8BðOEtÞ3 þ C2H6 þ 14H2
ð2Þ
The progress of the reactions could be easily monitored by
integrated 11B NMR and a two-day treatment with EtOH had
no effect to 2a, while the unprotected 1a was completely
degraded to give B(OEt)3 over a period of ∼10 h.
1; 2-H2C2B8Me8 þ 16EtOH ! 8MeBðOEtÞ2 þ C2H6 þ 6H2 ð3Þ
Experimental
Materials and methods
All the reactions were carried out under argon atmosphere.
Dichloromethane and hexane were dried over CaH2 and
freshly distilled before use. Other conventional chemicals were
of reagent or analytical grade and were used as purchased.
1
NMR spectroscopy was performed at 400 and 600 Mz (for H),
inclusive of standard [11B-11B]-COSY13 experiments (all theore-
tical cross-peaks were observed) leading to complete assign-
ments of all resonances to individual cage B-vertexes.
Chemical shifts are given in ppm to high-frequency (low field)
of Ξ = 32.083971 MHz (nominally F3B·OEt2 in CDCl3) for 11B
(quoted
0.5 ppm), Ξ = 25.144 MHz for 13C (quoted
1
0.5 ppm), and Ξ = 100 MHz for H (quoted 0.05 ppm), Ξ is
defined as in ref. 14 and the solvent resonances were used as
internal secondary standards. The starting carboranes 1 and
1c were prepared according to the reported methods.6
Dimethylation of closo-1,6-C2B8H10 (1a) on carbon vertexes
A solution of 1a (120 mg, 1 mmol) in dry Et2O (ca. 10–20 ml)
was cooled to −78 °C and then treated dropwise with 2.5 M
LiBu (solution in hexane) (1 ml, 2.5 mmol) under stirring. The
Fig. 6 190.2 MHz 11B, 600 MHz 1H, and 150.9 MHz 13C NMR spectra of
1,6-H2C2B8Me8 (2a) (CDCl3).
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off-white slurry of the Li+ salt was stirred for additional 1 h a column packed with silica gel (∼2.5 × 20 cm) which was then
prior to dropwise addition of methyl triflate (MeOTf, m.w. eluted with 10% CH2Cl2 in hexane to collect the main fractions
164.1) (410 mg, 2.5 mmol) under cooling down in an dry-ice of RF (anal., hexane) ∼0.2 which was isolated via solvent evap-
bath. The mixture was then left stirring for additional 2 h at oration and identified by 11B NMR as compound 2c (269 mg,
room temperature. After adding 5% hydrochloric acid (10 ml) 78%). Further purification can be achieved by vacuum subli-
under repeated cooling and shaking, the Et2O layer was separ- mation at bath temperature ∼130 °C. For 2c: MS (ESI−): m/z
ated and evaporated to provide closo-1,6-Me2C2B8H8 (1b) (max.) calcd 345.16, found 345.12; for C9H23B8I (m.w. 344.67)
(140 mg, 93%) on sublimation. The purity was assessed by calcd 31.36% C, 6.73% H; found 30.47% C, 6.51% H. An analo-
11B NMR spectroscopy (see Fig. S6 and S7†).
gous experiment with 1b gave a mixture of at least three com-
pounds (350 mg, assessed by 11B NMR) that could not be sep-
arated via LC. (b) From 1c: A solution of the iodo carborane 1c
(51 mg, 0.21 mmol) in neat MeI (3 ml) was heated in a thick-
closo-1,6-R2C2B8Me8 (2a and 4a) and closo-1,6-R2C2B8Me7-8-OTf
(2b and 4b) (where R = H or Me)
A solution of carboranes 1a or 1b (reaction scale ∼1.5 mmol) walled reaction vessel equipped with a Teflon screw cap on
in neat MeOTf (10 ml) was treated with three drops of HOTf addition of anhydrous AlCl3 (ca. 20 mg, 0.2 mmol) at 80 °C for
and the mixture was heated at different temperatures for 72 hours. Further work-up as in the preceding experiment led
varying times in a thick-walled reaction vessel equipped with a to the isolation of 59 mg (81%) of 2c as a single product which
Teflon screw cap. In each experiment were the volatiles evapor- was identified by 11B NMR spectroscopy.
ated and the residue extracted with hexane and filtered
NMR test of resistance toward ethanolysis for closo-1,6-C2B8H10
(1a) and closo-1,6-H2C2B8Me8 (2a)
through a silica gel plug. The filtrate was then treated with
silica gel (∼5 g) and the hexane evaporated. The residual
material was mounted onto the top of a column packed with Compounds 1a (25 mg, 0.2 mmol) and 2a (25 mg, 0.1 mmol)
silica gel (∼2.5 × 20 cm) which was then eluted with hexane, were dissolved in ethanol (2 ml) In a NMR tube and both solu-
followed by a 15% CH2Cl2/hexane mixture to collect two main tions were monitored via integrated 11B NMR at ambient temp-
fractions of RF (anal., hexane) ∼0.6 and 0.10 which were identi- erature. After ∼10 h exposure the signals of 1a have completely
fied by 11B NMR and isolated via solvent evaporation as com- disappeared (see Fig. S15†) to form B(OEt)3, while those of 2a
pounds 2a, 2b, 4a, and 4b. Further purification can be did not change their intensity during 48 h (no formation of
achieved by vacuum sublimation at bath temperature ∼130 °C. B(OEt3) was detected).
The isolated yields of individual compounds are in Table 1.
Computational details
For 2a: MS (ESI−): m/z (max.) calcd 233.28, found 233.25;
for C10H26B8 (m.w. 232.80) calcd 51.59% C, 11.26% H; found Magnetic shielding was calculated using the GIAO-MP2 method
50.79% C, 11.05% H. For 2b MS (ESI−): m/z (max.) calcd incorporated into Gaussian0915 utilizing the IGLO-II basis
367.21, found 367.30; for C10H23B8O3SF3 (m.w. 366.84) calcd with the MP2/TZVP geometry and frozen core electrons.
32.74% C, 6.32% H; found 32.30% C, 6.11% H. For 4a: MS Electrostatic potentials were computed at the HF/cc-pVDZ level
(ESI−): m/z (max.) calcd 261.32, found 261.30; for C12H30B8 using Gaussian09 and Molekel4.316 programs. It has recently
(m.w. 260.85) calcd 55.25% C, 11.59% H; found 54.80% C, been shown that this basis set size is sufficient for these
11.05% H. For 4b MS (ESI−): m/z (max.) calcd 395.24, found purposes.17
395.25; for C12H27B8O3SF3 (m.w. 394.89) calcd 36.50% C,
6.89% H; found 36.10% C, 6.58% H.
X-ray crystallography
The X-ray data for the triflato and iodo derivatives 2b and 2c
(colourless crystals by slow evaporation of a hexane solution)
closo-1,6-H2C2B8Me7-8-OTf (2b) from closo-1,6-H2C2B8Me8 (2a)
A solution of compound 2a (40 mg, 0.17 mmol) in MeOTf were collected at 150(2)K with a Bruker D8-Venture diffract-
(2 ml) was treated with HOTf (204 mg, 1.36 mmol) and heated ometer equipped with Mo (Mo/Kα radiation; λ = 0.71073 Å)
at 170 °C for ∼50 h in a sealed glass ampoule. The volatiles microfocus X-ray (IµS) source, Photon CMOS detector and
were then evaporated, the residue dissolved in hexane and fil- Oxford Cryosystems cooling device was used for data collec-
tered through a silica gel plug. The hexane was then evapor- tion. The frames were integrated with the Bruker SAINT soft-
ated to give 60 mg (96%) of a white crystals which were identi- ware package using a narrow-frame algorithm. Data were cor-
fied as 2b as in the preceding experiment.
rected for absorption effects using the Multi-Scan method
(SADABS). Obtained data were treated by XT-version 2014/5
and SHELXL-2014/7 software implemented in APEX3 v2016.5-0
closo-1,6-H2C2B8Me7-8-I (2c)
(a) From 1a: A solution of carborane 1a (122 mg, 1.0 mmol) in (Bruker AXS) system.18 Hydrogen atoms were mostly localized
neat MeI (10 ml) was heated in a thick-walled reaction vessel on a difference Fourier map, however to ensure uniformity of
equipped with a Teflon screw cap on addition of anhydrous treatment of crystal, all hydrogen were recalculated into ideal-
AlCl3 (ca. 14 mg, 0.14 mmol) at 80 °C for 72 hours. The vola- ized positions (riding model) and assigned temperature
tiles were evaporated and the residue extracted with hexane. factors Hiso(H) = 1.2Ueq (pivot atom) or of 1.5Ueq (methyl).
The extract was then treated with silica gel (∼5 g) and the H atoms in methyl groups were placed with C–H distances of
hexane evaporated. The residual material was mounted atop of 0.96 while the hydrogen atoms of the C–H in the carborane
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cage were assigned according to the maxima on the difference areas of carborane chemistry, for example in metal-insertion,
2
2
2
Fourier map. Rint = ∑|Fo − Fo,mean2|/∑Fo , S = [∑(w(Fo
−
cluster-rearrangement or cluster-degradation reactions; rele-
1
2
2 2
Fc ) )/(Ndiffrs − Nparams)] for all data, R(F) = ∑||Fo| − |Fc||/ vant experiments are being in progress in our laboratories.
1
2
2 2
2 2
2
∑|Fo|for observed data, wR(F2) = [∑(w(Fo − Fc ) )/(∑w(Fo ) )]
for all data. Crystallographic data for structural analysis have
been deposited with the Cambridge Crystallographic Data Centre, Conflicts of interest
CCDC no. 1573271 and 1573272† for 2b and 2c, respectively.
There are no conflicts to declare.
Crystallographic data for 2b: C10H23B8F3O3S, M = 366.82,
orthorhombic, P212121, a = 10.2153(5), b = 12.7603(7), c =
15.3713(9) Å, β = 90°, Z = 4, V = 2003.65(19) Å3, Dc = 1.216
g cm−3, μ = 0.193 mm−1, Tmin/Tmax = 0.3997/0.7455; −12 ≤ h ≤
13, −14 ≤ k ≤ 16, −19 ≤ l ≤ 19; 17 134 reflections measured
(θmax = 27.12°), 4409 independent (Rint = 0.0291), 4098 with I >
2σ(I), 241 parameters, S = 1.046, R1 (obs. data) = 0.0394, wR2
(all data) = 0.1050; max., min. residual electron density =
0.185, −0.306 e Å−3. Flack parameter 0.02(2). Crystallographic
data for 2c: C9H23B8I, M = 344.65, monoclinic, P21/n, a =
7.7088(4), b = 13.5358(7), c = 16.1895(9) Å, β = 102.909(2) °, Z =
Acknowledgements
The work was supported by the Grant Agency of the Czech
Republic (project no. 16-01618S).
Notes and references
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/
Tmax = 0.5122/0.745; −9 ≤ h ≤ 9, −17 ≤ k ≤ 17, −20 ≤ l ≤ 20;
28 878 reflections measured (θmax = 27.12°), 3635 independent
(Rint = 0.0263), 3116 with I > 2σ(I), 178 parameters, S = 1.182,
R1 (obs. data) = 0.0257, wR2 (all data) = 0.0811; max., min.
residual electron density = 0.472, −1.252 e Å−3
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Conclusions
The work is the initial venture into the area of permethylated
chemistry of closed cages with fewer than eleven vertices. It
was demonstrated that all B-positions in the closo-1,6-
R2C2B8H8 (1) framework can be furnished with methyl substi-
tuents via electrophilic reactions with MeOTf and/or MeI
reagents. In contrast to its 12-vertex 1,7-R2C2B10H10 analogue,3
all the B-positions, inclusive of those between carbon vertexes,
can be methylated. The reactions result in permethylated
derivatives of 1a which can be, in fact, viewed as rigid, egg-
shaped hydrocarbons with a carborane core inside (see Fig. 5).
It should be underlined that such structural arrangements
impart an exceptionally high air and solvolytic stability to the
9 J. Fanfrlík, M. Lepšík, D. Hořínek, Z. Havlas and P. Hobza,
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otherwise moderately stable compounds 1 by creating a protec- 10 (a) C. Esterhuysen, A. Hesselmann and T. Clark,
tive methyl sheath around the most sensitive boron section of
the molecule.5 For example, stability tests have proved that
compound 2a can be quantitatively recovered from ethanolic
ChemPhysChem, 2017, 18, 772–784; (b) J. Fanfrlík,
A. Pecina, J. Řezáč, R. Sedlák, D. Hnyk, M. Lepšík and
P. Hobza, Phys. Chem. Chem. Phys., 2017, 19, 18194–18200.
solution after a 48 h exposure to solvolysis, while the unpro- 11 In contrast, in the closo-1,6-C2B8 skeleton the C1 and C6
tected 1a is decomposed to B(OEt)3 over ∼10 h ethanolysis, as
assessed by 11B NMR measurements. Furthermore, from
experience we know that solid 2a is stable in air for at least two
weeks without any noticeable degradation to B(OH)3. This
straightforward enhancement of stability actually brings posi-
tive expectations for smaller-cage B-chemistry that was practi-
cally abandoned for air-instability as one of the reasons; one
might therefore expect new developments in the area of highly
atoms participate in five multicenter bondings of 3c2e and
4c2e types, which results in in their positive charges as
opposed to classical electronegativity complex. See:
P. Melichar, D. Hnyk and J. Fanfrlík, Phys. Chem. Chem.
Phys., 2018, 20, 4666–4675. Positive charges of endo-
carbons in closo systems were also verified experimentally:
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alkylated compounds with less than twelve vertexes in the near 12 F. Teixidor, G. Barberà, A. Vaca, R. Kivekäs, R. Sillanpää,
future. Moreover, Me-substituted derivatives thus far isolated
can be used as “Me-labels” for designed syntheses in specific
J. Oliva and C. Viñas, J. Am. Chem. Soc., 2005, 127,
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