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

Abstract

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.

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Introduction

The chemistry of polyalkylated twelve-vertex carboranes, compounds derived from closo [CB₁₁H₁₂]⁻ and C₂B₁₀H₁₂, were pioneered by Hawthorne's and Michl's groups.^1,2 This class of carboranes represents a new genre in organic chemistry, “spherical hydrocarbons” whose icosahedral core is sheathed by a coating of alkyl substituents. In the dicarbaborane area pertinent 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 unmethylated. 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 electrophilic halogenation,⁶ no work on B-substitution has so far been reported. In this work we would like to present synthetic studies on reactions of dicarbaboranes closo-1,6-R₂C₂B₈H₈ (1) (where R = H or Me) with already established methylation agents that lead to permethylation or, at least, to persubstitution on B-vertices.

Results and discussion

Scheme 1 (left) demonstrates that the HOTf-catalyzed MeOTf methylation of the parent closo-1,6-H₂C₂B₈H₈ (1a) dicarbaborane 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.

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

Subst.	°C	Time (h)	Yield 2a or 4a (%)	Yield 2b or 4b (%)
1a	110	120	20	70
1a	130	90	25	65
1a	175	48	40	50
1b	110	120	15	70
1b	130	90	15	75
1b	175	48	40	40

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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 permethylated 2a are exacerbated by the formation of the 8-X substituted compounds 2b and 2c. As demonstrated on the reaction 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 the methylation reactions (X = OTf and I) at the most negative B8 site⁶ of 2a:

{1,6-H₂C₂B₈Me₈ (2a) + HX → CH₄ + 1,6-H₂C₂B₈Me₇-8-X (2b and 2c) (1)

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 octamethylated product closo-1,6-Me₂C₂B₈Me₆-2,3-H₂ (3a, identified by ¹¹B NMR, see Fig. S16†), in which boron positions between 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 permethylated closo-1,6-Me₂C₂B₈Me₈ (4a) (yield ∼15%) with 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 spectroscopy (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 interbelt 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- positions 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 crystallographically 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 individual values see ESI.†

Fig. 1 ORTEP (30% probability level) representation of the molecular structure of closo-1,6-H₂C₂B₈Me₇-8-OTf (2b). Selected bond lengths (Å): 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 72.8(2), B3–C1–B4 110.8(2), B2–B3–B4 89.9(2), B2–C6–B3 65.3(2), C6–B7–B8 84.6(2), B9–C6–B7 97.4(2).

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. 3 Molecular model of closo-1,6-H₂C₂B₈Me₈ (2a) as obtained from the MP2/TZVP optimization. B2–B3 is the longest vector in the molecule, 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.

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 (distances 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.

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 methyl groups have positive ESP surface and the exo-skeletal carbon atoms have negative ESP surface (see Fig. 5). From the viewpoint of electron transmission, the Me groups behave as weak electron acceptors, which might be surprising, as it is opposed to toluene, xylene, hexamethylbenzene etc. However, the electron transmission just follows the established electronegativity concept that reflects the fact that exo-skeletal substituents are bonded via classical 2-centre 2-electron bonds.¹¹ This agrees with the concept elaborated by Viñas et al. for a 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.†

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.

The structures of all compounds isolated in this study are also in excellent agreement with the results of multinuclear ¹¹B, [¹¹B-¹¹B]-COSY,¹³¹H, and ¹³C NMR measurements that led to complete assignments of individual cage BMe, BX and CH units. Graphical inter-comparisons of the ¹¹B NMR spectra of all compounds isolated are in Fig. S15 and S16.† The ¹¹B and ¹¹B-{¹H}-decoupled NMR spectra of all B-methylated isomers isolated are identical, exhibiting only singlet resonances, which unambiguously points to persubstitution in all B-vertexes. All the ¹¹B NMR spectra of the C_s-symmetry (a slight gear effect of the Me groups) 2–4 derivatives, exhibiting 1 : 1 : 2 : 2 : 2 patterns of singlets, are in agreement with the presence of the C1–C6–B10 plane.

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 electronic 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†).

Fig. 6 190.2 MHz ¹¹B, 600 MHz ¹H, and 150.9 MHz ¹³C NMR spectra of 1,6-H₂C₂B₈Me₈ (2a) (CDCl₃).

Fig. 7 190.2 MHz ¹¹B, 600 MHz ¹H, and 150.9 MHz ¹³C NMR spectra of 1,6-Me₂C₂B₈Me₈ (4a) (CDCl₃).

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 dicarbaboranes 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):

1,2-H₂C₂B₈H₈ + 24EtOH → 8B(OEt)₃ + C₂H₆ + 14H₂ (2)

1,2-H₂C₂B₈Me₈ + 16EtOH → 8MeB(OEt)₂ + C₂H₆ + 6H₂ (3)

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.

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. NMR spectroscopy was performed at 400 and 600 Mz (for ¹H), inclusive of standard [¹¹B-¹¹B]-COSY¹³ experiments (all theoretical cross-peaks were observed) leading to complete assignments 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 ± 0.5 ppm), and Ξ = 100 MHz for ¹H (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 off-white slurry of the Li⁺ salt was stirred for additional 1 h prior to dropwise addition of methyl triflate (MeOTf, m.w. 164.1) (410 mg, 2.5 mmol) under cooling down in an dry-ice bath. The mixture was then left stirring for additional 2 h at room temperature. After adding 5% hydrochloric acid (10 ml) under repeated cooling and shaking, the Et₂O layer was separated and evaporated to provide closo-1,6-Me₂C₂B₈H₈ (1b) (140 mg, 93%) on sublimation. The purity was assessed by ¹¹B NMR spectroscopy (see Fig. S6 and S7†).

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) in neat MeOTf (10 ml) was treated with three drops of HOTf and the mixture was heated at different temperatures for varying times in a thick-walled reaction vessel equipped with a Teflon screw cap. In each experiment were the volatiles evaporated and the residue extracted with hexane and filtered 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 silica gel (∼2.5 × 20 cm) which was then eluted with hexane, followed by a 15% CH₂Cl₂/hexane mixture to collect two main fractions of R_F (anal., hexane) ∼0.6 and 0.10 which were identified by ¹¹B NMR and isolated via solvent evaporation as compounds 2a, 2b, 4a, and 4b. Further purification can be achieved by vacuum sublimation at bath temperature ∼130 °C. The isolated yields of individual compounds are in Table 1.

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 50.79% C, 11.05% H. For 2b MS (ESI⁻): m/z (max.) calcd 367.21, found 367.30; for C₁₀H₂₃B₈O₃SF₃ (m.w. 366.84) calcd 32.74% C, 6.32% H; found 32.30% C, 6.11% H. For 4a: MS (ESI⁻): m/z (max.) calcd 261.32, found 261.30; for C₁₂H₃₀B₈ (m.w. 260.85) calcd 55.25% C, 11.59% H; found 54.80% C, 11.05% H. For 4b MS (ESI⁻): m/z (max.) calcd 395.24, found 395.25; for C₁₂H₂₇B₈O₃SF₃ (m.w. 394.89) calcd 36.50% C, 6.89% H; found 36.10% C, 6.58% H.

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 (2 ml) was treated with HOTf (204 mg, 1.36 mmol) and heated at 170 °C for ∼50 h in a sealed glass ampoule. The volatiles were then evaporated, the residue dissolved in hexane and filtered through a silica gel plug. The hexane was then evaporated to give 60 mg (96%) of a white crystals which were identified as 2b as in the preceding experiment.

closo-1,6-H₂C₂B₈Me₇-8-I (2c)

(a) From1a: A solution of carborane 1a (122 mg, 1.0 mmol) in neat MeI (10 ml) was heated in a thick-walled reaction vessel equipped with a Teflon screw cap on addition of anhydrous AlCl₃ (ca. 14 mg, 0.14 mmol) at 80 °C for 72 hours. The volatiles were evaporated and the residue extracted with hexane. The extract was then treated with silica gel (∼5 g) and the hexane evaporated. The residual material was mounted atop of a column packed with silica gel (∼2.5 × 20 cm) which was then eluted with 10% CH₂Cl₂ in hexane to collect the main fractions of R_F (anal., hexane) ∼0.2 which was isolated via solvent evaporation and identified by ¹¹B NMR as compound 2c (269 mg, 78%). Further purification can be achieved by vacuum sublimation at bath temperature ∼130 °C. For 2c: MS (ESI⁻): m/z (max.) calcd 345.16, found 345.12; for C₉H₂₃B₈I (m.w. 344.67) calcd 31.36% C, 6.73% H; found 30.47% C, 6.51% H. An analogous experiment with 1b gave a mixture of at least three compounds (350 mg, assessed by ¹¹B NMR) that could not be separated via LC. (b) From1c: A solution of the iodo carborane 1c (51 mg, 0.21 mmol) in neat MeI (3 ml) was heated in a thick-walled reaction vessel equipped with a Teflon screw cap on addition of anhydrous AlCl₃ (ca. 20 mg, 0.2 mmol) at 80 °C for 72 hours. Further work-up as in the preceding experiment led to the isolation of 59 mg (81%) of 2c as a single product which was identified by ¹¹B NMR spectroscopy.

NMR test of resistance toward ethanolysis for closo-1,6-C₂B₈H₁₀ (1a) and closo-1,6-H₂C₂B₈Me₈ (2a)

Compounds 1a (25 mg, 0.2 mmol) and 2a (25 mg, 0.1 mmol) were dissolved in ethanol (2 ml) In a NMR tube and both solutions were monitored via integrated ¹¹B NMR at ambient temperature. After ∼10 h exposure the signals of 1a have completely disappeared (see Fig. S15†) to form B(OEt)₃, while those of 2a did not change their intensity during 48 h (no formation of B(OEt₃) was detected).

Computational details

Magnetic shielding was calculated using the GIAO-MP2 method incorporated into Gaussian09¹⁵ utilizing the IGLO-II basis with the MP2/TZVP geometry and frozen core electrons. Electrostatic potentials were computed at the HF/cc-pVDZ level using Gaussian09 and Molekel4.3¹⁶ programs. It has recently been shown that this basis set size is sufficient for these purposes.¹⁷

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) were collected at 150(2)K with a Bruker D8-Venture diffractometer equipped with Mo (Mo/K_α radiation; λ = 0.71073 Å) microfocus X-ray (IμS) source, Photon CMOS detector and Oxford Cryosystems cooling device was used for data collection. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were corrected 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 (Bruker AXS) system.¹⁸ Hydrogen atoms were mostly localized on a difference Fourier map, however to ensure uniformity of treatment of crystal, all hydrogen were recalculated into idealized positions (riding model) and assigned temperature factors H_iso(H) = 1.2U_eq (pivot atom) or of 1.5U_eq (methyl). H atoms in methyl groups were placed with C–H distances of 0.96 while the hydrogen atoms of the C–H in the carborane cage were assigned according to the maxima on the difference Fourier map. R_int = ∑|F_o² − F_o,mean²|/∑F_o², S = [∑(w(F_o² − F_c²)²)/(N_diffrs − N_params)]^½ for all data, R(F) = ∑||F_o| − |F_c||/∑|F_o|for observed data, wR(F²) = [∑(w(F_o² − F_c²)²)/(∑w(F_o²)²)]^½ for all data. Crystallographic data for structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC no. 1573271 and 1573272† for 2b and 2c, respectively. 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) ų, D_c = 1.216 g cm⁻³, μ = 0.193 mm⁻¹, T_min/T_max = 0.3997/0.7455; −12 ≤ h ≤ 13, −14 ≤ k ≤ 16, −19 ≤ l ≤ 19; 17 134 reflections measured (θ_max = 27.12°), 4409 independent (R_int = 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 = 4, V = 1646.59(15) ų, D_c = 1.390 g cm⁻³, μ = 1.919 mm⁻¹, T_min/T_max = 0.5122/0.745; −9 ≤ h ≤ 9, −17 ≤ k ≤ 17, −20 ≤ l ≤ 20; 28 878 reflections measured (θ_max = 27.12°), 3635 independent (R_int = 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.

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 substituents 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 otherwise moderately stable compounds 1 by creating a protective 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 solution after a 48 h exposure to solvolysis, while the unprotected 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 positive expectations for smaller-cage B-chemistry that was practically abandoned for air-instability as one of the reasons; one might therefore expect new developments in the area of highly alkylated compounds with less than twelve vertexes in the near future. Moreover, Me-substituted derivatives thus far isolated can be used as “Me-labels” for designed syntheses in specific areas of carborane chemistry, for example in metal-insertion, cluster-rearrangement or cluster-degradation reactions; relevant experiments are being in progress in our laboratories.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the Grant Agency of the Czech Republic (project no. 16-01618S).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR data and computed ¹¹B NMR, cartesian geometries. CCDC 1573271 and 1573272. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt02586j

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