Synthesis, structural characterization, and reactivity studies of 5-CF 3SO3-B10H13

Article abstract of DOI:10.1021/ic500684b

In contrast to previous reactions carried out in cyclopentane solvent at room temperature that produced 6-TfO-B₁₀H₁₃ (TfO = CF ₃SO₃), the reaction of closo-B₁₀H ₁₀^2- with a large excess of trifluoromethanesulfonic acid in the ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate (bmimOTf) gave exclusively the previously unknown 5-TfO-B₁₀H ₁₃ isomer. Experimental and computational studies demonstrated that the difference in the products of the two reactions is a result of 6-TfO-B ₁₀H₁₃ isomerizing to 5-TfO-B₁₀H₁₃ above room temperature in bmimOTf solutions. Reactivity studies showed that 5-TfO-B₁₀H₁₃: (1) is deprotonated by reaction with 1,8-bis(dimethylamino)naphthalene to form the 5-TfO-B₁₀H ₁₂^1- anion; (2) reacts with alcohols to produce 6-RO-B₁₀H₁₃ boryl ethers (R = Me and 4-CH ₃O-C₆H₄); (3) undergoes olefin-hydroboration reactions to form 5-TfO-6,9-R₂-B₁₀H₁₁ derivatives; and (4) forms a 5-TfO-6,9-(Me₂S)₂-B ₁₀H₁₁ adduct at its Lewis acidic 6,9-borons upon reaction with dimethylsulfide. The 5-TfO-6,9-(Me₂S)₂-B ₁₀H₁₁ adduct was also found to undergo alkyne-insertion reactions to form a range of previously unreported triflate-substituted 4-TfO-ortho-carboranes (1-R-4-TfO-1,2-C₂B₁₀H₁₀) and reactions with triethylamine or ammonia to form the first TfO-substituted decaborate [R₃NH⁺]₂[2-TfO-B₁₀H ₉^2-], and [R₃NH⁺] ₂[1-TfO-B₁₀H₉^2-] (R = H, Et) salts.

Full text of DOI:10.1021/ic500684b

Article
pubs.acs.org/IC
Synthesis, Structural Characterization, and Reactivity Studies of
5‑CF3SO3‑B10H13
Emily R. Berkeley, William C. Ewing, Patrick J. Carroll, and Larry G. Sneddon*
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States
S
* Supporting Information
ABSTRACT: In contrast to previous reactions carried out in
cyclopentane solvent at room temperature that produced 6-TfO-
B10H13 (TfO = CF3SO3), the reaction of closo-B10H102− with a large
excess of trifluoromethanesulfonic acid in the ionic liquid 1-butyl-
3-methylimidazolium trifluoromethanesulfonate (bmimOTf) gave
exclusively the previously unknown 5-TfO-B10H13 isomer. Exper-
imental and computational studies demonstrated that the differ-
ence in the products of the two reactions is a result of 6-TfO-
B10H13 isomerizing to 5-TfO-B10H13 above room temperature in
bmimOTf solutions. Reactivity studies showed that 5-TfO-B10H13:
(1) is deprotonated by reaction with 1,8-bis(dimethylamino)-
1−
naphthalene to form the 5-TfO-B10H12 anion; (2) reacts with
alcohols to produce 6-RO-B10H13 boryl ethers (R = Me and
4-CH3O-C6H4); (3) undergoes olefin-hydroboration reactions to
form 5-TfO-6,9-R2-B10H11 derivatives; and (4) forms a 5-TfO-6,9-(Me2S)2-B10H11 adduct at its Lewis acidic 6,9-borons
upon reaction with dimethylsulfide. The 5-TfO-6,9-(Me2S)2-B10H11 adduct was also found to undergo alkyne-insertion
reactions to form a range of previously unreported triflate-substituted 4-TfO-ortho-carboranes (1-R-4-TfO-1,2-C2B10H10) and
reactions with triethylamine or ammonia to form the first TfO-substituted decaborate [R3NH+]2[2-TfO-B10H92−], and
[R3NH+]2[1-TfO-B10H92−] (R = H, Et) salts.
1
The previously reported synthesis of [Et3NH+]2[closo-B10H10
]
2−
studies confirmed the exclusive formation of the 6-R-B10H13
from the readily available and safe-to-handle [Et3NH+][BH4 ]
−
isomers. The hydroxyl-derivative 6-HO-B10H13 was similarly
salt makes the closo-B10H10 decaborate anion2 a potentially
2−
2−
synthesized from B10H10 upon reaction with sulfuric acid in
hexanes.5 Ewing demonstrated6 (eq 3) the high-yield syntheses
of the 6-X-B10H13 (X = Cl, Br, I) halo-decaboranes by the
attractive alternative to B10H14 as a starting material for the
syntheses of the larger-cage polyboranes that are needed for many
technological and/or medical applications.3 Several recently
reported studies have yielded significant advances toward
realizing this potential.4−7
2−
cage-opening reactions of closo-B10H10 with super acidic
mixtures of HX (X = Cl, Br) and an aluminum halide in the
ionic liquids bmimCl, bmimBr, and bmimI (bmim = 1-butyl-3-
methylimidazolium). It was then further shown7 that each
6-X-B10H13 could be converted to its 5-X-B10H13 isomer by a
base-catalyzed isomerization reaction.
2−
Hawthorne showed that closo-B10H10 undergoes a cage-
opening reaction to form 6-R-B10H13 (R = Ph, Cy, TfO (TfO =
CF3SO3)) compounds when reacted with benzene, cyclo-
hexane (eq 1), or triflate ion (eq 2) in the presence of tri-
fluoromethanesulfonic (triflic) acid (HOTf).4 Crystallographic
Even with these recent developments, new methods are still
needed to enable the efficient conversion of the closed-cage
2−
framework of closo-B10H10 to a wider range of polyboranes
and functionalized polyborane derivatives. In this Paper, we
demonstrate that, in contrast to the triflic acid reaction reported
2−
by Hawthorne, when the cage opening of closo-B10H10 with
triflic acid is carried out at 60 °C in a bmimOTf/HOTf solution,
only the 5-TfO-B10H13 isomer is formed. Reactivity studies of
5-TfO-B10H13 are also presented demonstrating that this
compound and its derivatives can be converted to a range of
useful polyboranes and carboranes.
Received: March 24, 2014
Published: May 1, 2014
© 2014 American Chemical Society
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Inorganic Chemistry
Article
Method D: Base Isomerization. 6-TfO-B10H13 (100 mg, 0.37 mmol)
was reacted with 3 μL (0.02 mmol) of triethylamine in toluene (5 mL) at
room temperature for 1 h. Toluene and triethylamine were vacuum-
evaporated to give 70 mg (0.26 mmol, 70%) of 1.
EXPERIMENTAL SECTION
■
General Procedures. Unless otherwise noted, all reactions and
manipulations were performed in dry glassware under nitrogen
atmospheres using the high-vacuum or inert-atmosphere techniques
described by Shriver and Drezdzon.8
[PSH+][5-TfO-B10H12−] (1−). A mixture of 1 (40 mg, 0.15 mmol) and
Proton Sponge (PS) (31.7 mg, 0.15 mmol) was reacted in CDCl3
(1 mL) at room temperature for 24 h. On the basis of NMR analysis, 1−
was produced in 95% yield.
Materials. Phenylacetylene, trifluoromethylphenylacetylene, 3-phe-
nyl-1-propyne, tert-butyl propiolate, cyclohexylacetylene, 1,8-nonadiyne,
1-octyne, propargyl acetate, and 4-bromo-1-butyne (Sigma-Aldrich) were
used as received. 6-TfO-B10H13,4 6,9-(Me2S)2-B10H12,9 5-I-B10H13,7 and
For 1−: yellow oil. 11B NMR (128.4 MHz, CH2Cl) (int, mult, JBH
=
Hz) 11.6 (1, d, br), 10.1 (1, s), −3.0 (1, d, 138), −9.0 (1, d, ∼117), −11.4
(1, d, 144), −12.9 (1, d, br), −14.4 (1, d, ∼159), −16.8 (1, d, 138),
−34.2 (1, d, 148), −49.3 (1, d, 149) ppm. DFT/GIAO-calculated 11B
NMR shifts (assign) 16.4 (B6), 15.4 (B5), 6.3 (B1), −5.8 (B8), −6.6
(B3), −9.3 (B10), −13.6 (B7), −13.9 (B9), −29.8 (B2), −47.5 (B4)
10
2−
[NH4+]2[closo-B10H10
]
were prepared as previously described. The
bmimOTf (Fluka) was azeotropically dried by refluxing with toluene,
followed by high-vacuum drying at 100 °C and then storage in a N2-filled
drybox. High-performance liquid chromatography (HPLC)- or American
Chemical Society (ACS)-grade hexanes, triethylamine, pentane, ether,
dichloromethane, dimethylsulfide, triflic acid, 1,8-bis(dimethylamino)-
naphthalene(Proton Sponge, PS), and silicagel (Fischer, 230−400 mesh)
were used as received. C6D6 (D, 99.5%), CDCl3 (D, 99.9%), and CD2Cl2
(D, 99.9%) (Cambridge Isotope) were used as received. Toluene (Fisher)
was dried over sodium prior to use.
1
ppm. H{11B} NMR (400.1 MHz, CDCl3) (int, assign) 8.72 (1, PS+-
NHN), 7.69−7.97 (6, CH), 3.96 (1, BH), 3.19 (12, 4CH3), 2.86 (1,
BH), 2.80 (2, BH), 2.58 (1, BH), 2.37 (2, BH), 0.71 (1, BH), 0.14 (1,
BH, br), −2.47 (1, BH, br), −3.08 (1, BH, br) ppm (the remaining BH
resonance was not resolvable). ESI (neg) HRMS: m/z calc for
CH12B10O3F3S: 271.1390, found: 271.3192. IR (NaCl, cm−1): 3207
(m), 2961 (w), 2540 (s), 1463 (m), 1370 (s), 1264 (s), 1201 (s), 1157
(s), 1098 (m), 1031 (s), 980 (s), 912 (m), 859 (w), 831 (m), 767 (s),
733 (m), 638 (s).
Physical Measurements. 1H NMR spectra at 400.1 MHz and 11
B
NMR spectra at 128.4 MHz were obtained on a Bruker DMX 400
spectrometer. All 11B NMR chemical shifts are referenced to external
BF3·OEt2 (0.00 ppm), with a negative sign indicating an upfield shift. All
1H chemical shifts were measured relative to residual protons in the lock
solvents and are referenced to Me4Si (0.00 ppm). High- and low-
resolution mass spectra (HRMS and LRMS) using negative chemical
ionization (NCI) or electrospray ionization (ESI) techniques were
recorded on a Micromass Autospec Spectrometer or Waters GC-TOF
Premier, respectively. Infrared (IR) spectra were recorded on a
PerkinElmer Spectrum 100 FT-IR spectrometer using NaCl plates.
Elemental analyses were performed at the Microanalytical Laboratory,
UC Berkeley. Melting points (mp) were obtained on a standard melting-
point apparatus and are uncorrected.
6-CH3O-B10H13 (2). A mixture of 1 (100 mg, 0.37 mmol) and
methanol (30 mg, 0.93 mmol) was stirred at 70 °C in dichloroethane for
20 h. After the solvent was vacuum-evaporated, the remaining solid was
extracted with pentane. The pentane was vacuum-evaporated to give 2
1
(45.0 mg, 0.30 mmol, 80%) as a white solid. The 11B and H NMR
spectra of 2 were consistent with those previously reported.11
6-(4-CH3O-C6H4O)-B10H13 (3). A mixture of 1 (100 mg, 0.37 mmol)
and 4-methoxyphenol (50 mg, 0.40 mmol) was stirred at 70 °C in
dichloroethane for 20 h. After the solvent was vacuum-evaporated, the
remaining solid was extracted with pentane. The pentane was vacuum-
evaporated to give 3 as a white solid.
For 3: 69.1 mg, (0.31 mmol, 84%), mp 168−170 °C. 11B NMR (128.4
MHz, CH2Cl) (int, mult, JBH = Hz) 22.1 (1, s), 3.2 (3, d, 154), 1.4 (2, d,
155), −15.1 (2, d, 161), −33.8 (1, d, 147), −44.8 (1, d, 137) ppm.
1H{11B} NMR (400.1 MHz, CD2Cl2) (int, assign) 6.85−7.26 (4, C6H4),
3.85 (1, BH), 3.79 (3, CH3), 3.27 (2, BH), 3.19 (2, BH), 2.23 (2, BH),
0.27 (2, BH), −0.36 (2, BH, br), −1.77 (2, BH, br) ppm. NCI HRMS:
m/z calc for C7H20B10O2: 246.2394, found: 246.2402. IR (NaCl, cm−1):
3223 (s), 2835 (w), 2579 (w), 2259 (w), 1632 (w), 1605 (w), 1510 (s),
1441 (m), 1366 (m), 1298 (w), 1230 (s), 1180 (m), 1102 (m), 1032
(m), 825 (s), 733 (s), 459 (m).
5-TfO-B10H13 (1). Method A. Triflic acid (5.8 mL, 65.0 mmol) was
added dropwise to a rapidly stirring solution of [NH4 ]2[closo-B10H10
+
2−
]
(1.0 g, 6.5 mmol) in bmimOTf (3 mL, 8.05 mmol), and the solution was
heated at 60 °C for 12 h. The mixture was extracted with hexanes (∼300
mL) until 11B NMR analysis showed the extracts contained no product.
The extracts were filtered to remove any solids, and the solvent was
vacuum-evaporated to give an off-white viscous liquid. Two
recrystallizations from pentane produced a white solid.
For 1: 1.14 g (4.2 mmol, 64%), mp 28−29 °C. 11B NMR (128.4 MHz,
CDCl3) (int, mult, JBH = Hz) 12.3 (1, d, 154), 11.6 (2, d, br), 9.3 (1, s),
7.3 (1, d, 157), 2.4 (1, d, 153), −0.2 (1, d, 173), −4.8 (1, d, 161), −36.5
(1, d, 106), −37.2 (1, d, 113) ppm. Density functional theory/gauge-
independent atomic orbital (DFT/GIAO)-calculated 11B NMR shifts
(assign) 13.3 (B3), 12.4 (B1), 8.6 (B5), 7.7 (B9), 4.5 (B6), 3.1 (B10),
5-TfO-6,9-(C5H11)2-B10H11 (4) and 5-I-6,9-(C5H11)2-B10H11 (5). In
separate reactions, a mixture of 1 (150 mg, 0.55 mmol) and PtBr2 (20 mol %,
39.0 mg, 0.11 mmol) and a mixture 5-I-B10H13 (100 mg, 0.39 mmol) and
PtBr2 (20 mol %, 27.7 mg, 0.08 mmol) were reacted with 1-pentene
(3 mL, 27.3 mmol) at 55 °C for 2 d. The platinum bromide was filtered
off, and the solvent was vacuum-evaporated to give 4 and 5 as white
powders.
−1.1 (B8), −4.5 (B7), −39.7 (B4), −39.7 (B2) ppm. H{11B} NMR
1
(400.1 MHz, CDCl3) (int, assign) 4.1 (1, BH), 4.0 (2, BH), 3.8 (1, BH),
3.3 (1, BH), 3.2 (1, BH), 3.1 (1, BH), 1.2 (1, BH), 0.7 (1, BH), 0.4 (1,
BH), −1.5 (1, BH), −1.8 (1, BH), −2.0 (1, BH) ppm. NCI HRMS: m/z
calc for CH13B10O3F3S: 272.1482, found: 272.1468. Anal. Calcd: C:
4.44, H: 4.85; found: C: 4.54, H: 4.94. IR (NaCl, cm−1): 3584 (w), 2591
(s), 1901 (w), 1509 (m), 1400 (s), 1219 (s), 1147 (s), 1096 (s), 1037
(m), 995 (s), 953 (m), 875 (m), 622 (s). Crystals of 1 suitable for a
crystallographic determination were obtained by slow evaporation of a
hexanes solution.
For 4: 154.1 mg (0.37 mmol, 68%), mp 32−35 °C. 11B NMR (128.4 MHz,
C6D6) (int, mult, JBH = Hz) 25.7 (1, s), 23.0 (1, s), 8.5 (1, d, 125), 6.7
(1, s), 6.6 (1, d, ∼160), −0.47 (1, d, ∼173), −2.3 (1, d, ∼157), −6.9
(1, d, ∼129), −36.8 (1, d, 132), −37.5 (1, d, ∼136) ppm. DFT/GIAO-
calculated 11B NMR shifts (assign) 23.5 (B9), 18.9 (B6), 10.3 (B3), 9.5
(B5), 5.7 (B1), −0.4 (B10), −5.7 (B8), −9.9 (B7), −39.5 (B2), −40.6
1
(B4) ppm. H{11B} NMR (400.1 MHz, C6D6) (int, assign) 4.14 (1,
BH), 3.85 (1, BH), 3.31 (1, BH), 2.96 (1, BH), 2.82 (1, BH), 1.54 (16,
8CH2), 1.16 (6, 2CH3), 0.77 (1, BH, br), −1.17 (1, BH, br), −1.64 (BH,
br), −1.87 (1, BH, br) ppm (the remaining BH resonances were not
resolvable). NCI HRMS: m/z calc for C11H33B10O3F3S: 412.3033,
found: 412.3002. IR (NaCl, cm−1): 3210 (m), 2959 (s), 2929 (s), 2873
(m), 2861 (m), 2755 (s), 1924 (w), 1629 (w), 1466 (m), 1399 (s), 1247
(m), 1216 (s), 1147 (s), 1102 (m), 1072 (s), 974 (m), 916 (m), 850 (w),
720 (w), 626 (m), 508 (w).
Method B: Thermal Isomerization in bmimOTf. 6-TfO-B10H13
(50 mg, 0.19 mmol) was stirred in bmimOTf (1 mL) or bmimOTf/
HOTf at 55 °C for 36 h. The observed 5-TfO-B10H13 products were
extracted from the bmimOTf solutions with pentane, and the pentane
was then vacuum-evaporated to give 39 mg (0.15 mmol, 78%) and
34 mg (0.13 mmol, 68%) of 1, respectively.
Method C: Thermal Isomerization in Toluene. When 6-TfO-B10H13
(50 mg, 0.19 mmol) was stirred in toluene (5 mL) at 55 °C, no reaction
was observed after 30 h; however, when it was stirred at 85 °C for 36 h,
11B NMR analysis showed complete conversion to 1. The toluene was
vacuum-evaporated to give 46 mg (0.17 mmol, 92%) of 1.
For 5: 76.6 mg (0.29 mmol, 76%). 11B NMR (128.4 MHz, D2O) (int,
mult, JBH = Hz) 24.6 (2, s), 10.9 (1, d, 186), 9.3 (1, d, 169), 1.2 (1, d, 144),
−2.6 (1, d, 169), −4.2 (1, d, 173), −16.0 (1, d, 39), −34.7 (1, d, 163),
−36.0 (1, d, 161) ppm. 1H{11B} NMR (400.1 MHz, CD2Cl2) (int, assign)
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Inorganic Chemistry
Article
3.88 (1, BH), 3.25 (2, BH), 3.10 (1, BH), 2.92 (1, BH), 1.58 (8, 4CH2),
1.38 (8, 4CH2), 1.06 (1, BH), 0.92 (6, 2CH3), 0.72 (1, BH), −0.03 (1,
BH, br), −0.97 (1, BH, br), −1.21 (1, BH, br), −1.34 (1, BH, br) ppm
(the remaining BH resonances were not resolvable). IR (NaCl, cm−1):
2956 (s), 2925 (s), 2870 (m), 2552 (w), 1460 (m), 1377 (w), 1260 (m),
1088 (m), 800 (m). Crystals of 5 suitable for a crystallographic
determination were obtained by the slow evaporation of a hexanes
solution.
5-TfO-6,9-(Me2S)2-B10H11 (6). After a solution of 1 (500 mg,
1.8 mmol) and dimethylsulfide (3 mL, 4.1 mmol) in toluene (10 mL)
was stirred at room temperature for 3 h, a white precipitate formed. The
precipitate was collected via filtration, washed with toluene, and dried in
vacuo to give pure 6.
(B10) ppm. 1H{11B} NMR (400.1 MHz, CD2Cl2) (int, assign) 7.6−7.7
(4, C6H4), 4.15 (1, cage-CH), 3.08 (1, BH), 2.86 (1, BH), 2.80 (1, BH),
2.64 (1, BH), 2.55 (1, BH), 2.40 (1, BH), 2.27 (2, BH) ppm (the
remaining BH resonance was not resolvable). NCI HRMS: m/z calc for
C10H14B10O3F6S: 438.1498, found: 438.1496. IR (NaCl, cm−1): 3073
(m), 2597 (s), 1617 (m), 1411 (s), 1390 (s), 1219 (s), 1171 (s), 1135
(s), 1068 (s), 1020 (m), 989 (m), 924 (m), 848 (m), 616 (m). Crystals
of 8 suitable for a crystallographic determination were obtained from
slow evaporation of a pentane solution at −20 °C.
1-C6H11-4-TfO-1,2-C2B10H10 (9). 6 (200 mg, 0.51 mmol), cyclo-
hexylacetylene (3 equiv, 0.20 mL, 1.53 mmol), and toluene (10 mL).
For 9: colorless oil, 120.9 mg (0.32 mmol, 63%). 11B NMR (128.4
MHz, CD2Cl2) (int, mult, JBH = Hz) −0.7 (1, s), −4.5 (1, d, ∼175), −5.9
(1, d, ∼156), −11.0 (1, d, ∼164), −12.5 (3, d, 172), −16.4 (2, d, 173),
−19.1 (1, d, ∼147) ppm. DFT/GIAO-calculated 11B NMR shifts
(assign) −2.1 (B4), −3.6 (B7), −6.2 (B12), −10.9 (B8), −12.7 (B5),
For 6: 508 mg (1.3 mmol, 70%), mp 69−72 °C. 11B NMR (128.4
MHz, CDCl3) (int, mult, JBH = Hz) −6.0 (1, d, ∼122), −6.3 (1, s), −9.6
(1, d, 132), −22.9 (2, d, ∼149), −24.8 (1, d, br), −26.1 (1, d, ∼175),
−27.7 (1, d, ∼179), −40.6 (1, d, 147), −44.9 (1, d, 143) ppm. DFT/
GIAO-calculated 11B NMR shifts (assign) −2.8 (B2), −5.3 (B4), −5.8
(B5), −22.0 (B10), −22.3 (B7), −23.4 (B8), −23.8 (B9), −25.0 (B6),
−38.4 (B1), −44.8 (B3) ppm. 1H{11B} NMR (400.1 MHz, C6D6) (int,
assign) 3.25 (2, BH), 2.55 (1, BH), 2.44 (1, BH), 2.13 (2, BH), 1.79
(3, CH3), 1.66 (3, CH3), 1.65 (1, BH), 1.56 (3, CH3), 1.55 (3, CH3),
1.32 (1, BH), −0.14 (2, BH), −1.42 (1, BH) ppm. NCI HRMS: m/z calc
for C5H24B10O3F3S3: 395.1770, found: 395.1772. Anal. Calcd: C: 15.26,
H: 6.15; found: C: 15.72, H: 5.81%. IR (NaCl, cm−1): 3198 (w), 3022
(w), 2930 (w), 2536 (s), 1428 (m), 1382 (s), 1246 (m), 1206 (s), 1153
(s), 1073 (m), 1035 (m), 988 (s), 960 (s), 921 (m), 637 (m), 625 (m),
510 (m). Crystals of 6 suitable for a crystallographic determination were
obtained by the slow evaporation of a hexanes/dichloromethane
solution.
1
−14.7 (B3), −15.8 (B6), −17.6 (B9), −19.0 (B10) ppm. H NMR
(400.1 MHz, CD2Cl2) (int, assign) 3.79 (1, cage-CH), 2.36−0.88 (11,
C6H11) ppm. NCI HRMS: m/z calc for C9H21B10O3F3S: 376.2094,
found: 376.2131. IR (NaCl, cm−1): 2926 (s), 2853 (s), 2559 (s), 1449
(w), 1383 (m), 1247 (m), 1207 (s), 1153 (s), 1083 (w), 1040 (w), 1000
(w), 619 (w).
1-CH3(CH2)5-4-TfO-1,2-C2B10H10 (10). 6 (200 mg, 0.51 mmol),
1-octyne (3 equiv, 0.23 mL, 1.53 mmol), and toluene (10 mL).
For 10: colorless oil, 94.7 mg (0.26 mmol, 51%). 11B NMR (128.4
MHz, CD2Cl2) (int, mult, JBH = Hz) −1.0 (1, s), −4.0 (1, d, 159), −7.0
(1, d, 149), −12.5 (4, d, br), −14.8 (1, d, ∼140), −16.1 (1, d, ∼171),
−19.1 (1, d, ∼186) ppm. 1H NMR (400.1 MHz, CD2Cl2) (int, assign)
3.80 (1, cage-CH), 1.31 (10, 5CH2), 0.89 (3, CH3) ppm. NCI HRMS:
m/z calc for C8H21B10F3O3S: 364.2094, found: 364.2098. IR (NaCl,
cm−1): 3301 (s), 3025 (m), 2930 (s), 2861 (m), 2557 (s), 2116 (w),
1634 (m), 1383 (m), 1211 (m), 1153 (m), 1030 (m), 998 (m), 735 (m),
697 (w), 638 (m).
Reaction of 6-TfO-B10H13 with Dimethylsulfide. The reaction of
4
6-TfO-B10H13 (80 mg, 0.30 mmol) and 2.7 mL (3.0 mmol) of
dimethylsulfide in toluene (5 mL) at room temperature for 24 h gave
6,9-(Me2S)2-B10H12 (20 mg, 0.08 mmol, 27%) along with ∼10%
unreacted 6-TfO-B10H13. The 11B spectrum of 6,9-(Me2S)2-B10H12 was
consistent with that previously reported.12
General Procedure for the Syntheses of 1-R-4-TfO-1,2-C2B10H10
(7−12). A 100 mL Schlenk flask equipped with a stir bar was charged
under N2 with 6, alkyne, and toluene. The flask was sealed and then
submerged in an oil bath heated at 70 °C. The stirred reaction was
monitored by 11B NMR until all of the 6 was consumed (12−18 h).
The solvent and unreacted alkyne were vacuum-evaporated, and the
resulting residue was extracted with pentane until all product was
removed. The extracts were combined, and the solvent was vacuum-
evaporated. All products were purified by silica-gel chromatography
using a 50/50 hexanes/CH2Cl2 eluent.
1-C6H5-4-TfO-1,2-C2B10H10 (7). 6 (200 mg, 0.51 mmol), phenyl-
acetylene (3 equiv 0.17 mL, 1.53 mmol), and toluene (10 mL).
For 7: colorless oil, 107.6 mg (0.29 mmol, 57%). 11B NMR (128.4
MHz, CD2Cl2) (int, mult, JBH = Hz) −0.6 (1, s), −3.6 (1, d, 155), −5.6
(1, d, 151), −11.1 (2, d, ∼193), −12.6 (3, d, ∼160), −16.4 (1, d, ∼177),
−18.2 (1, d, br) ppm. DFT/GIAO-calculated 11B NMR shifts (assign)
−1.1 (B4), −2.0 (B7), −5.2 (B12), −10.8 (B8), −11.7 (B5), −12.5
(B11), −15.2 (B6), −17.2 (B9), −17.3 (B3), −19.3 (B10) ppm.
1H{11B} NMR (400.1 MHz, CD2Cl2) (int, assign) 7.4−7.5 (5, C6H5),
3.99 (1, cage-CH), 3.10 (1, BH), 2.86 (1, BH), 2.67 (2, BH), 2.58 (1,
BH), 2.40 (1, BH), 2.27 (2, BH), 2.22 (1, BH) ppm. NCI HRMS: m/z
calc for C9H15B10O3F3S: 370.1625, found: 370.1631. IR (NaCl, cm−1):
3290 (m), 3027 (m), 2557 (s), 1603 (w), 1495 (m), 1445 (m), 1413
(m), 1213 (s), 1151 (m), 1030 (w), 997 (w), 937 (w), 815 (w), 758 (m),
731 (m), 693 (m), 618 (w).
1-CH3COOCH2-4-TfO-1,2-C2B10H10 (11). 6 (200 mg, 0.51 mmol),
propargyl acetate (3 equiv, 0.15 mL, 1.53 mmol), and toluene (10 mL).
For 11: colorless oil, 98.9 mg (0.27 mmol, 53%). 11B NMR (128.4
MHz, CD2Cl2) (int, mult, JBH = Hz) −0.8 (1, s), −3.1 (1, d, 156), −5.3
(1, d, 156), −12.0 (3, d, ∼131), −13.7 (2, d, ∼123), −15.5 (1, d, ∼128),
−18.7 (1, d, 139) ppm. DFT/GIAO-calculated 11B NMR shifts (assign)
−1.9 (B4), −4.8 (B8), −5.1 (B12), −9.9 (B7), −11.9 (B9), −14.2 (B3),
1
−14.7 (B5), −17.4 (B11), −18.3 (B6), −20.2 (B10) ppm. H{11B}
NMR (400.1 MHz, CD2Cl2) (int, assign) 4.76 (2, CH2, m), 4.05 (1,
cage-CH), 2.70 (1, BH), 2.60 (1, BH), 2.49 (1, BH), 2.38 (2, BH), 2.32
(1, BH), 2.16 (3, CH3) ppm (the remaining BH resonances were not
resolvable). NCI HRMS: m/z calc for C6H15B10O5F3S: 366.1523,
found: 366.1597. IR (NaCl, cm−1): 3067 (w), 2599 (m), 1758 (m),
1410 (m), 1367 (w), 1290 (s), 1148 (m), 1110 (w), 1054 (w), 1000 (w),
926 (w), 858 (w), 616 (w).
1-BrCH2CH2-4-TfO-1,2-C2B10H10 (12). 6 (200 mg, 0.51 mmol), 4-
bromo-1-butyne (3 equiv, 0.14 mL, 1.53 mmol), and toluene (10 mL).
For 12: colorless oil, 106.1 mg (0.27 mmol, 52%). 11B NMR (128.4
MHz, CD2Cl2) (int, mult, JBH = Hz) −1.0 (1, s), −3.3 (1, d, 155), −5.8
(1, d, 145), −11.4 (1, d, ∼135), −12.1 (2, d, ∼118), −12.9 (1, d, ∼152),
−14.7 (1, d, ∼163), −15.5 (1, d, ∼164), −18.7 (1, d, 164) ppm. 1H{11B}
NMR (400.1 MHz, CDCl3) (int, assign) 4.01 (1, cage-CH), 3.05 (2,
CH2), 2.90 (2, CH2), 2.72 (1, BH), 2.63 (1, BH), 2.53 (1, BH), 2.45 (1,
BH), 2.33 (1, BH), 2.28 (1, BH), 2.15 (2, BH) ppm (the remaining BH
resonance was not resolvable). NCI HRMS: m/z calc for
C5H14B10O3F3SBr: 400.0730, found: 400.0759. Anal. Calcd: C: 17.51,
H: 4.10; found: C: 17.89, H: 4.67%. IR (NaCl, cm−1): 3064 (w), 2928
(m), 2591 (s), 2259 (w), 1635 (w), 1407 (s), 1216 (s), 1150 (s), 1001
(m), 924 (m), 857 (w), 731 (w), 615 (m).
Reaction of 1 with 3-hexyne in bmimCl/Toluene. Following the
reported procedure,13 a mixture of 1 (150 mg, 0.56 mmol) and 300 mg
(3.7 mmol) of 3-hexyne in a biphasic toluene (5 mL)/bmimCl (100 mg,
0.57 mmol) was reacted at 120 °C for ∼7 min. The toluene layer was
removed and vacuum-evaporated to give 1,2-Et2-1,2-C2B10H10 (72.8 mg,
0.35 mmol, 65%). The 11B and 1H NMR spectra were consistent with
those previously reported.13 NCI HRMS: m/z calc C6B10H20: 202.2802;
found: 202.2819.
1-CF3C6H4-4-TfO-1,2-C2B10H10 (8). 6 (200 mg, 0.51 mmol),
trifluoromethylphenylacetylene (3 equiv, 0.23 mL, 1.53 mmol), and
toluene (10 mL).
For 8: colorless oil, 118.4 mg (0.27 mmol, 53%). 11B NMR (128.4
MHz, CD2Cl2) (int, mult, JBH = Hz) 0.9 (1, s), −1.7 (1, d, ∼176), −3.6
(1, d, br), −9.0 (1, d, ∼124), −11.0 (2, d, ∼139), −11.9 (2, d, ∼116),
−15.0 (1, d, ∼173), −17.0 (1, d, ∼175) ppm. DFT/GIAO-calculated
11B NMR shifts (assign) −0.9 (B4), −1.8 (B7), −4.2 (B12), −11.0 (B8),
−11.6 (B5), −12.6 (B11), −16.0 (B6), −17.0 (B3), −17.4 (B9), −19.5
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2−
1- and 2-TfO-B10H9 (16−19). In separate reactions, a 100 mL
RESULTS AND DISCUSSION
■
Schlenk flask equipped with a stir bar was charged under N2 with 6
(200 mg, 0.51 mmol). For one reaction, dichloromethane (5 mL) and
excess triethylamine (0.1 mL, 0.72 mmol) were added, and the solution
was stirred overnight. For the other reaction, excess liquid ammonia
(∼1 mL, condensed in at 77 K) was added, and then the stirred mixture
was allowed to very slowly warm to room temperature while the
flask was connected to a pressure-release bubbler. At the conclusion
of the reactions, vacuum-evaporation of the solvents and/or excess
amines gave a mixture of the ammonium salts of the 1-TfO-B10H92− and
In contrast to the finding4 that 6-TfO-B10H13 was formed by the
2−
room-temperature reaction of closo-B10H10 with triflic acid in
cyclopentane (eq 2), we observed that the 60 °C reaction of
+
[NH4 ]2[closo-B10H102−] with a large excess of triflic acid in the
ionic liquid bmimOTf gave exclusively the previously unknown
5-TfO-B10H13 (1) (eq 4). In a typical reaction, vigorous stirring
2-TfO-B10H9 anions. The [1-TfO-B10H92−] isomers were then sel-
2−
ectively precipitated from dichloromethane solutions at −78 °C.
For [Et3NH+]2[1-TfO-B10H92−] (16). 72.2 mg (0.15 mmol, 30%). 11
B
NMR (128.4 MHz, D2O) (int, mult, JBH = Hz) 19.7 (1, s), −7.8 (1, d,
138), −32.7 (8, d, ∼130) ppm. DFT/GIAO-calculated 11B NMR shifts
(assign) 20.8 (B1), −6.6 (B10), −29.1 (B5), −29.5 (B3), −29.9 (B4),
−30.4 (B2), −33.0 (B9), −33.4 (B7), −35.0 (B6), −35.8 (B8) ppm.
DFT/GIAO-calculated shifts assuming TfO rotation: 20.8 (B1), −6.6
(B10), −29.7 (B2,3,4,5), −34.3 (B6,7,8,9) ppm. 1H{11B} NMR (400.1
MHz, D2O) (int, assign) 4.95 (2, NH), 3.16 (12, 6CH2), 1.34 (18,
6CH3) ppm. Anal. Calcd: C: 33.18, H: 8.78, N: 5.95; found: C: 31.22, H:
8.50, N: 5.49%.
+
of [NH4 ]2[closo-B10H102−] in bmimOTf at 60 °C for 12 h,
followed by extraction with hexanes and subsequent recrystal-
lization from pentane, gave crystalline 1 in 64% yield.
The 11B NMR spectra of 1 (Figure 1) were consistent with a C1
cage-symmetry, displaying eight separate doublet resonances,
For [Et3NH+]2[2-TfO-B10H92−] (17). 154.0 mg (0.33 mmol, 64%). 11
B
NMR (128.4 MHz, CD3CN) (int, mult, JBH = Hz) −1.1 (1, s), −2.2 (1,
d, 140), −5.2 (1, d, 149), −23.5 (1, d, ∼167), −24.8 (1, d, ∼169), −28.7
(2, d, ∼140), −31.4 (3, d, ∼105) ppm. DFT/GIAO-calculated 11B NMR
shifts (assign) 1.5 (B2), −0.5 (B10), −10.6 (B1), −25.0 (B7/B8), −26.2
(B5), −28.2 (B3), −31.8 (B6), −31.9 (B9), −34.1 (B4) ppm. 1H{11B}
NMR (400.1 MHz, D2O) (int, assign) 5.39 (2, NH), 3.11 (12, 6CH2),
1.20 (18, 6CH3) ppm. ESI HRMS: m/z calc for H9C1B10F3SO3:
266.1229, found: 266.0788. Anal. Calcd: C: 33.18, H: 8.78, N: 5.95;
found: C: 31.73, H: 8.56, N: 5.56%.
+
For [NH4 ]2[1-TfO-B10H92−] (18). 60 mg (0.20 mmol, 40%). The 11
B
NMR spectrum was identical to that observed for 16. 1H NMR (400.1
MHz, D2O) (int, assign) 7.1 (8, 2NH4 ) ppm. ESI LRMS: m/z calc for
+
H9B10O3SCF3: 268.1155, found: 268.1163.
+
For [NH4 ]2[2-TfO-B10H92−] (19). 79 mg (0.26 mmol, 52%). The 11
B
NMR spectrum was identical to that observed for 17. H{11B} NMR
(400.1 MHz, D2O) (int, assign) 7.1 (8, 2NH4 ) ppm.
1
+
Collection and Reduction of the Data. Crystallographic data
and structure refinement information are summarized in Supporting
Information, Table S1. X-ray intensity data for 1 (Penn3379), 5
(Penn3396), 6 (Penn3411), 8 (Penn3432), and 18 (Penn3403) were
collected on a Bruker APEXII CCD area detector. Preliminary indexing
was performed from a series of 36 rotation frames (0.5°) with exposures
of 10 s. Rotation frames were integrated using SAINT,14 producing a
listing of unaveraged F2 and σ(F2) values that were then passed to the
SHELXTL program package15 for further processing and structure
solution. All reflections were used during refinement. Non-hydrogen
atoms were refined anisotropically; cage hydrogen atoms were refined
isotropically, and all other hydrogen atoms were refined using a riding
model.
Figure 1. 11B{1H} (top) and 11B (bottom) NMR spectra of 1.
Computational Methods. DFT calculations were performed using
the Gaussian 09 package.16 Unless specifically stated, all ground state,
transition state, and intermediate geometries, as well as their electronic
and free energies, were obtained using the B3LYP/6-311G(d) level
without constraints for all H, C, B, O, F, and S atoms. The B3LYP/SDD
pseudopotential was used for the I atom in 6. The 11B NMR chemical
shifts were calculated at the B3LYP/6-311G(d) level using the GIAO
option within Gaussian and are referenced to BF3·O(C2H5)2 using an
absolute shielding constant of 102.24 ppm. Harmonic vibrational
analyses were carried out on the optimized geometries at the same level
to establish the nature of stationary points. Transition states were
located using the QST2 option in Gaussian 09 and were verified to
contain only one imaginary frequency. Continuity of the reaction
pathway was confirmed by intrinsic reaction coordinate (IRC) cal-
culations and/or visualization of the imaginary frequencies. All optimized
Cartesian coordinates are given in the Supporting Information in
Tables S2−S19. Calculated relative electronic energies for selected
structures are given in Supporting Information, Table S20.
with the doublet at 11.6 ppm having intensity-two, and one
singlet at 9.3 ppm arising from the TfO-substituted boron. The
agreement of the observed spectra with the chemical shifts
predicted by DFT/GIAO calculations (B3LYP/6-311G(d)) for
5-TfO-B10H13 allowed the assignment of the resonances
indicated in the Experimental Section. The 1H NMR spectrum
of 1 (Figure S1, Supporting Information) was again indicative of
C1 symmetry, with four separate bridging-hydrogen resonances
in the same 1-downfield/3-upfield pattern (1 at 0.4 ppm and
3 above 0.0 ppm) previously observed in the spectra of the 5-X-
B10H13 halogenated derivatives.6,7 As shown in Figure 2, a single-
crystal X-ray diffraction study confirmed the 5-TfO-B10H13
structure of 1.
The B5−O1 distance (1.447(4) Å) in 1 is slightly longer than
that found in 6-TfO-B10H13 (1.433(4) Å)4 but is significantly
longer than those found in 5-RO-B10H13 (R = Me (1.370(3) Å),
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Figure 2. Crystallographically determined structure of 1. Selected
distances (Å) and angles (deg): B5−O1, 1.447(4); B5−B6, 1.797(5);
B6−B7, 1.790(5); B7−B8, 1.982(5); B8−B9, 1.778(5); B9−B10,
1.784(5); B5−B10, 1.970(4); B2−B6, 1.730(4); B4−B9, 1.744(5);
B4−B10, 1.787(4); B3−B8, 1.752(4); B1−B5, 1.750(4); B2−B5,
1.784(5); B2−B7, 1.796(5); O1−S1, 1.538(2); O2−S1, 1.425(2);
O3−S1, 1.417(2); B5−O1−S1, 125.44(19); O1−B5−B6, 119.1(2);
O1−B5−B10, 118.1(2); B2−B6−B5, 60.75(18); B7−B6−B5,
101.1(2); B8−B9−B10, 104.9(2); B6−B5−B10, 118.2(2); B6−B7−
B8, 117.0(2).
ClC2H4OC2H4 (1.3604(19) Å), and CH3CCCH2
(1.3828(17) Å)).11 These differences are consistent with less
O1 → B5 π-bonding in 1, as is expected for the strongly electron-
withdrawing triflate group.
Investigations into the reason for the formation of the different
isomers observed in the cyclopentane (eq 1) and bmimOTf (eq 4)
reactions revealed that 6-TfO-B10H13 readily isomerized to its
5-TfO-B10H13 isomer when heated above room temperature in
either bmimOTf or toluene solvents (eq 5). For example, when a
Figure 3. DFT-calculated relative electronic energies (kcal/mol) for the
thermal isomerization pathway of 6-TfO to 5-TfO.
5-TfO-B10H13. In the DFT-optimized geometry of 6-TfO-B10H13
(6-TfO in Figure 3 and Supporting Information, Figure S2), the
triflate is bound at its O1 oxygen to B6, but the group has sufficient
range of motion that one of its other oxygens can interact with B5.
In TS1, the incoming triflate O2 atom approaches B5 (O2···B5,
3.46 Å), pushing the exo B5-hydrogen toward an endo cage
position, with the B6−O1 length increasing slightly from the
1.43 Å in 6-TfO to 1.45 Å in TS1. As O2 continues to approach
B5, the system decreases in energy until arriving at INT1, which is
24.0 kcal/mol higher than 6-TfO. As shown in Supporting
Information, Figure S2 (bottom), in INT1 the triflate O1−S−O2
unit (104.9°) spans the B5−B6 edge, forming a five-membered
ring, with the increased B6−O1 distance (1.76 Å) now close to the
decreased length of B5−O2 (1.77 Å). The former B5 exo-
hydrogen is endo at B5, and the hydrogen originally bridging the
B6−B5 edge has moved to the endo position on B6. The hydrogen
originally bridging the B6−B7 edge is endo at B7. In TS2, the
B5−O2 length has decreased to 1.48 Å as the B6−O1 length
increased to 2.37 Å, with TS2 being only slightly higher in energy
than INT1. As O1 loses all contact with B6, the B6 endo-hydrogen
moves to the exo position, while the B7 and B5 endo-hydrogens
move to bridging positions on the B6−B7 and B6−B5 edges. This
process, along with a shortening of the B5−O2 bond to 1.44 Å,
results in the formation of 5-TfO′, which is then converted to
the 5-TfO optimized structure of 1 (Supporting Information,
Figure S2) through a rotation about the B5−O2 bond, falling
−0.8 kcal/mol relative to 6-TfO. This DFT analysis suggests
that the facile 6-TfO- to 5-TfO-B10H13 isomerization is aided
by the unique ability of the triflate group to adopt a bidentate
configuration spanning the B6−B5 edge that facilitates the
migration of the TfO group to the B5 position.
pure sample of 6-TfO-B10H13 (prepared as described pre-
viously4) was heated at 55 °C in bmimOTf in either the presence
or absence of added triflic acid, isomerization to 1 was observed
within 36 h, providing 78 and 68% isolated yields, respectively.
While 6-TfO-B10H13 proved to be unreactive in toluene at 55 °C,
at 85 °C complete reaction was observed by 11B NMR in 36 h to
give a 92% isolated yield of 1 following workup. These
observations are consistent with DFT calculations using a variety
of basis sets that showed the isomerization of 6-TfO-B10H13 to
5-TfO-B10H13 to be thermodynamically favorable (B3LYP/
6-311G(d), −0.8 kcal/mol; B3LYP/6-311G(d,p), −3.3 kcal/
mol; B3LYP/6-311+G(3df,2p), −3.4 kcal/mol, see Supporting
Information, Table S20). Thus, both the experimental and
computational studies suggest that, as in the cyclopentane
reactions, the 6-TfO-B10H13 isomer is the likely initial product in
the bmimTfO reactions, but because of the higher temperature,
efficient isomerization to the energetically favored 5-TfO-B10H13
isomer readily occurs.
As shown in Figure 3, DFT (B3LYP/6-311G(d)) analysis
identified a unique pathway for the conversion of 6-TfO-B10H13 to
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While the neutral 6-X-B10H13 (X = Cl, Br, I) haloboranes have
not been observed to thermally isomerize, they have been shown
to convert at room temperature to their 5-X-B10H13 isomers in
the presence of catalytic amounts of base.7 It was similarly found
that the reaction of 6-TfO-B10H13 with 5 mol % triethylamine in
toluene solution at room temperature resulted in isomerization
to 1 (70% isolated yield) in only ∼1 h (eq 6).
Figure 5. 11B{1H} NMR spectrum of 1−. DFT/GIAO-calculated 11B
NMR shifts (assign): 16.4 (B6), 15.4 (B5), 6.3 (B1), −5.8 (B8), −6.6
(B3), −9.3 (B10), −13.6 (B7), −13.9 (B9), −29.8 (B2), −47.5 (B4)
ppm.
This result is consistent with the previously proposed
mechanistic pathway for the base-catalyzed isomerization
of the halo-decaboranes involving initial formation of the
TfO-Substitution Reactions: 6-RO-B10H13 (R = Me (2),
4-CH3O-C6H4O (3)). Nucleophilic attack of alcohols on 5-X-
B10H13 haloboranes was previously shown to result in halogen
substitution by an alkoxide to yield 6-RO-B10H13 ethers.11
Likewise, when 1 was reacted with methanol, clean conversion to
6-MeO-B10H13 (2) was observed (eq 7) with the 80% isolated
1−
1−
6-X-B10H12 anion, which then isomerizes to 5-X-B10H12
7
with subsequent reprotonation to form 5-X-B10H13. DFT
1−
calculations confirmed that the 5-TfO-B10H12 anion is of
lower energy (−1.35 kcal/mol) than the 6-TfO-B10H121− anion
(Supporting Information, Table S20).
Reactivity Studies of 5-TfO-B10H13. The efficient syntheses
of 1 described above allowed a systematic investigation of the
chemistry of this potentially important starting material. 1 has a
number of reactive sites including: (1) Brønsted acidic bridging-
hydrogens; (2) a triflate group that could be substituted by
metathesis reactions; (3) B6−H and B9−H groups capable of
hydroborating olefins; and (4) Lewis acidic borons (B6 and B9)
that can coordinate to Lewis bases. Initial examples of these types
of reactions are presented in the following sections.
Deprotonation: 5-TfO-B10H12 (1−). When 1 was reacted
1−
yield of 2 being significantly higher than that from the analogous
reaction of methanol with 5-Br-B10H13 (54% yield). While the
5-X-B10H13 haloboranes had been found to be unreactive toward
phenols, 1 readily reacted with para-methoxyphenol at 70 °C to
give 6-(4-CH3O-C6H4O)-B10H13 (3) in an 84% isolated yield.
The 11B NMR spectra of 3 were similar to those of 2, with a Cs
symmetric pattern (Supporting Information, Figure S4) and with
the substituted B6 boron appearing in its predicted downfield
position at 22.1 ppm. Again consistent with Cs cage symmetry, the
1H NMR spectrum exhibited two separate intensity-two bridge-
hydrogen resonances (Supporting Information, Figure S5).
The formation of the 6-substituted 6-RO-B10H13 derivatives
starting from 5-TfO-B10H13 is consistent with the observed
formation of 6-RO-B10H13 products in the reaction of alcohols
with 5-X-B10H13 (X = Cl, Br, I). DFT studies of the MeOH/5-Cl-
B10H13 reaction11 indicated that this reaction proceeds by initial
nucleophilic attack of the alcohol at B6 to form a transition state
in which the B5−B6 bridging hydrogen moves to an endo
position on B5. Then as HCl is eliminated from B5, the B6
hydrogen moves to the B5−B6 bridging position to produce
6-MeO-B10H13. A similar pathway with HOTf elimination at B5
can likewise account for formation of 2 and 3 in the MeOH/
5-TfO-B10H13 and 4-CH3O-C6H4OH/5-TfO-B10H13 reactions.
Alkene Hydroborations: 5-X-6,9-(C5H11)2-B10H11 (X = TfO
(4), I (5)). Platinum bromide has previously been shown to catalyze
olefin hydroborations by both B10H14 and its 6-R-B10H12 and 6-X-
B10H13 (X = Cl, I) derivatives to form 6,9-R2-B10H12,17 6,9-R,R′-
with 1 equiv of the strong Brønsted base Proton Sponge,
deprotonation to form the 5-TfO-B10H121− (1−) anion resulted.
Depending upon which bridging hydrogen is removed, four
different structures are possible for this anion (Supporting
Information, Figure S3). DFT calculations indicated the
structure in Figure 4, where the hydrogen bridging the B5−B6
Figure 4. DFT-optimized geometry of 1−.
edge in 1 was removed, to be lowest in energy (Supporting
Information, Table S20), and the GIAO-calculated 11B shifts
for this structure are in excellent agreement with the exper-
imentally observed spectrum (Figure 5). This structure is
8
B10H12,18 and 6-X-9-R-B10H12 alkyl-derivatives, respectively.
likewise consistent with the structure previously confirmed7 for
Likewise, when 1 or its 5-I-B10H13 analogue was stirred in neat
1-pentene at 55 °C for 2 d, 5-TfO-6,9-(C5H11)2-B10H11 (4) and
1−
5-Cl-B10H12
.
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5-I-6,9-(C5H11)2-B10H11 (5) were produced in 68 and 76%
isolated yields, respectively (eq 8).
While 4 was isolated as an oil, 5 was obtained as a crystalline
solid. A structural determination of 5 (Figure 7) confirmed anti-
Markovnikov hydroboration of 1-pentene by the B6−H and
B9−H groups, with the observed B6−C1 and B9−C6 distances
(both 1.571(3) Å) being similar to those previously reported
for 6-alkyldecaboranes.18,20 Both the B5−I1 (2.180(2) Å) and
B5−B6 (1.816(3) Å) distances in 5 are elongated relative to
those found6 in 5-I-B10H13 (B5−I, 2.166(5); B5−B6, 1.788(7) Å).
Formation of Lewis Acid−Lewis Base Adducts: 5-TfO-6,9-
(Me2S)2-B10H11 (6). Decaborane is known to react with Lewis
bases, such as dialkylsulfides or acetonitrile, to form adducts at its
Lewis acidic B6 and B9 borons. Such arachno-6,9-L2B10H12
compounds are key starting materials for carborane syntheses.21
However, decaborane derivatives with substitution at one
of the B6 or B9 boron positions exhibit lower Lewis acidities
than the parent decaborane, and as a result, neither the 6-X-
B10H13 nor 6-R-B10H13 derivatives form adducts at B9 upon
reaction with bases. We likewise did not observe B9 adduct
formation when 6-TfO-B10H13 was reacted with dimethylsulfide.
On the other hand, when 1 was reacted with a large excess of
dimethylsulfide in toluene at room temperature for 3 h, the
diadduct 6 precipitated as a white solid and was isolated in 70%
yield (eq 9).
The 11B NMR spectra for both compounds were similar and in
excellent agreement with their DFT/GIAO-calculated values, with
the triflate-substituted B5 in 4 appearing at 6.7 ppm and the two
singlets arising from alkyl-substituted B6 and B9 farther downfield
at 25.7 and 23.0 ppm (Supporting Information, Figure S6).
In the 11B NMR spectrum of 5, the alkyl-substituted B6 and B9
resonances again appear downfield as broad overlapped peaks
(centered at 24.6 ppm), but the iodinated B5 resonance appears
upfield (−16.0 ppm), which is similar to its position in 5-I-B10H13
(−13.6 ppm19). As can be seen in Figure 6, the B5 resonance in 5
The 11B NMR spectra of 6 (Supporting Information, Figure S9)
are consistent with its predicted C1 symmetry, displaying eight
doublets (with the doublet at −22.9 ppm of intensity-two) along
with a singlet resonance, at −6.3 ppm, for the triflate-substituted
B5 that overlaps the lowest-field doublet. The DFT/GIAO
calculations for 6 are again in good agreement with the
experimental 11B NMR data. The 1H{11B} NMR spectrum of 6
(Supporting Information, Figure S10) also indicated C1 cage
symmetry, with two bridging-hydrogen resonances and four
separate methyl resonances.
Figure 6. 11B{1H} (top) and 11B (bottom) NMR spectra of 5.
also shows clear doublet fine-structure, with a coupling constant
value (J = 39 Hz) consistent with the coupling of B5 to the
bridging hydrogen at the B5−B6 edge. The 1H{11B} NMR spectra
of both compounds show four separate bridge-hydrogen
resonances (Supporting Information, Figures S7 and S8).
The single-crystal X-ray determination shown in Figure 8
confirmed 6 to have a cage structure similar to those observed for
other arachno-6,9-L2B10H12 derivatives,22 with the two bridging
Figure 7. Crystallographically determined structure of 5. Selected distances (Å) and angles (deg): I1−B5, 2.180(2); B6−C1, 1.571(3); B9−C6,
1.571(3); B5−B6, 1.816(3); B6−B7, 1.800(3); B7−B8, 1.960(4); B8−B9, 1.804(3); B9−B10, 1.800(3); B5−B10, 1.977(3); B4−B9, 1.736(3); B2−B6,
1.736(3); B2−B5, 1.795(3); B2−B7, 1.796(3); B3−B7, 1.755(3); B1−B10, 1.764(3); B1−B5, 1.750(4); B4−B10, 1.790(3); B6−B5−I1, 118.76(14);
B2−B5−B6, 57.48(12); B4−B10−B9, 57.83(13); B7−B6−B5, 102.33(15); B10−B9−B8, 103.67(16); C2−C1−B6, 112.59(16); C6−B9−B4,
129.98(19); C1−B6−B2, 130.18(18); B10−B5−I1, 115.70(12).
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Figure 8. Crystallographically determined structure of 6. Selected
distances (Å) and angles (deg): B5−O1, 1.497(2); B6−S1, 1.9143(19);
B9−S2, 1.917(2); B5−B6, 1.829(3); B6−B7, 1.883(3); B7−B8,
1.863(3); B8−B9, 1.866(3); B9−B10, 1.850(3); B5−B10, 1.864(3);
B4−B9, 1.743(3); B2−B6, 1.746(3); B1−B4, 1.753(3); B1−B5,
1.767(3); B2−B5, 1.757(3); B3−B7, 1.778(3); S3−O2, 1.412(2);
S3−O1, 1.5117(14); S3−O3, 1.4067(18); B5−O1−S3, 127.90(12);
O1−B5−B6, 122.96(14); O1−B5−B10, 118.87(15); C2−S1−B6,
102.97(10); C1−S1−B6, 105.43(10); C4−S2−B9, 102.66(10); C3−
S2−B9, 104.71(10); B4−B9−S2, 106.40(12); B10−B9−B8,
105.11(13); B5−B6−B7, 103.88(13); B2−B6−S1, 107.95(12).
Figure 9. Crystallographically determined structure of 8. Selected
distances (Å) and angles (deg): B4−O1, 1.451(3); C1−C13, 1.506(3);
C1−C2, 1.671(4); C1−B6, 1.696(4); C1−B4, 1.722(4); C1−B5,
1.726(4); C1−B3, 1.750(4); C2−B6, 1.702(4); C2−B11, 1.714(4);
C2−B7, 1.721(4); C2−B3, 1.729(4); B3−B8, 1.769(4); B3−B7,
1.771(4); B3−B4, 1.782(4); B4−B9, 1.767(4); B4−B8, 1.772(4);
B4−B5, 1.777(4); B5−B6, 1.759(4); B5−B10, 1.776(4); B5−B9,
1.781(4); B6−B10, 1.745(4); B6−B11, 1.746(4); B7−B12, 1.774(5);
B7−B11, 1.776(5); B7−B8, 1.784(4); B8−B12, 1.783(4); B8−B9,
1.794(4); B9−B10, 1.788(4); B9−B12, 1.791(5); B10−B11, 1.775(5);
B10−B12, 1.785(5); B11−B12, 1.768(5); S1−O1, 1.5375(17); S1−O2,
1.407(2); Si−O3, 1.403(2); B4−O1−S1, 130.45(16); C13−C1−C2,
119.9(2); C13−C1−B4, 122.0(2); O1−B4−1, 116.0(2); O1−B4−B3,
114.1(2).
hydrogens spanning the B5−B10 and B7−B8 edges. In 6, the
O1−B5 distance (1.497(2) Å) is significantly longer and the
S3−O1 (1.5117(14) Å) distance is shorter than their cor-
responding distances in 1 (1.447(4) and 1.538(2) Å, respectively),
suggesting less O1 → B5 π-bonding with the more electron-rich 6
arachno framework than with the 1 nido cage. The biggest effect
of the B5-bonded TfO group on the cage framework appears
to be on the B5−B6 distance (1.829(3) Å), which is consider-
ably shortened relative to the B6−B7 (1.883(3) Å), B9−B8
(1.866(3) Å), and B9−B10 (1.850(3) Å) distances.
Alkyne Reactions with 5-TfO-6,9-(Me2S)2-B10H11: Syntheses
of Triflate-Functionalized ortho-Carboranes (7−12). Deca-
borane and its 6,9-(R2S)2-B10H12 derivatives are key compounds
along the synthetic pathway to the C2B10 series of carboranes.21
These carboranes have many technological and/or medical
applications,2 but in many cases their use requires vertex-specific
functionalization. While straightforward methods are now
available to C-substituted carboranes,21 selective routes to
B-functionalized compounds are less well-developed. One of
the most common B-substituted carborane derivatives is the
B-halocarboranes, and a number of methods have been
developed that allow substitution at different boron sites.21a
The triflate group is often used as a pseudohalogen in many types
of organic palladium-coupling and nucleophilic substitution reac-
tions, and recent studies23 of C-substituted triflated carboranes
also suggest a similar utility for polyborane cluster derivatization.
However, B-triflated ortho-carboranes have not been available
for these applications. The availability of 1 and 6 suggested the
possibility of employing these compounds as starting materials
for the syntheses of such derivatives.
found to give high yields of B-alkyl derivatives13b,4b 3- or 4-R-1,
2-C2B10H10. Unfortunately, this method proved unsuitable for
reactions with 1. While alkyne insertion was achieved, the triflate
substituent was not retained with, for example, the reaction of 1
and 3-hexyne, producing only 1,2-Et2C2B10H10.
The more traditional method for carborane synthesis has
employed the reaction of decaborane and an alkyne in the
presence of a Lewis base, such as a dialkylsulfide,21 with the initial
step in this reaction proposed to involve the formation of a 6,
9-(R2S)2-B10H12 complex. Dissociation of one Me2S from this
adduct then enables alkyne association to form a 6-(RCCH)-
9-(Me2S)-B10H12 intermediate. The loss of the second
dimethylsulfide and H2 allows alkyne insertion into the open
face of the decaborane cage to form the ortho-carborane
structure.24 The report25 that acetylene insertion into arachno-
2-Br-6,9-(Et2S)2-B10H11 produced a mixture of B-bromonated
C2B10-carboranes suggested that 6 could be employed as a
precursor to B-triflated ortho-carboranes.
We recently reported that ortho-carboranes can be efficiently
synthesized by the reaction of decaborane with alkynes in biphasic
ionic liquid/toluene solutions.13 Analogous biphasic ionic liquid/
toluene reactions of 6-R-B10H13 derivatives with alkynes were also
6 was found to react with a variety of terminal alkynes to give
the corresponding 1-R-4-TfO-1,2-C2B10H10 ortho-carborane
derivatives (eq 10). In these reactions, 6 was stirred with 3
equiv of the corresponding alkyne in toluene at 70 °C until 11
B
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Figure 11. 11B{1H} NMR spectrum of (top) 1-TfO-B10H92−; (bottom)
2−
2-TfO-B10H9
.
2-C2B10H10, should be possible depending upon the alkyne
orientation during insertion (Figure 10). However, in all cases,
both the NMR data and TLC analyses of the reactions with 6
indicated only one product was formed. In the 2-R-4-X-1,
2-C2B10H10 isomer, the H-substituted carbon is adjacent to
the B-OTf, while in the 1-R-4-X-1,2-C2B10H10 isomer the
R-substituted carbon is adjacent to the B-OTf. The greater
substituent−triflate steric interactions would appear to disfavor
the 1-R-4-X-1,2-C2B10H10 isomer where the two groups are
adjacent and, indeed, the DFT-calculated energies for the two
isomers (Supporting Information, Table S20, R = Me) indicated
that the 2-R-4-X-1,2-C2B10H10 isomer is lowest in energy.
Nevertheless, the structural determination confirmed that 8 is
the 1-R-4-X-1,2-C2B10H10 isomer. This suggests that 7−12 are
kinetic products that could possibly result from an initial
sterically favorable step where alkyne coordination occurs at its
less-hindered CH carbon on the unsubstituted B6−B7−B8−B9
side of the cage before final insertion into the cage framework.
Cage-Closing Reactions of 6: Syntheses of closo-1-TfO-
Figure 10. Possible carborane products depending upon alkyne
orientation during insertion.
NMR analysis indicated the reaction was complete. After
evaporation of the solvent, the remaining oily residues were
extracted with hexanes to give the crude ortho-carborane
products. Purification by silica-gel column chromatography
using 50/50 hexanes/CH2Cl2 eluent gave the pure carboranes
in 51−63% isolated yields. All products were obtained as clear
oils at room temperature.
The 11B NMR spectra of all 1-R-4-TfO-1,2-C2B10H10
derivatives were quite similar, with the singlet resonance arising
from the TfO-substituted B4 vertex in the 0.9 to −1.0 ppm
2−
2−
1
range. Representative 11B and H NMR spectra are shown in
B10H9 (16, 18) and closo-2-TfO-B10H9 (17, 19) Salts. The
previous report26 that the reactions of 5-X-6,9-(Me2S)2-B10H11
with liquid ammonia produce the halo-substituted closo-2-X-
Supporting Information, Figures S11 and S12 for 11. DFT/
GIAO calculations allowed the assignment of the resonances for
7−12 given in the Experimental Section.
As shown in Figure 9, the structure of 8 was confirmed by a
single-crystal X-ray diffraction study of crystals grown at −20 °C
in pentane (Figure 9). The bond lengths in 8 are nearly identical
to those found in a nontriflated carborane with a similar organic
substituent (1-(3-HCCC6H4)-1,2-C2B10H11).13a The B4−O1
distance (1.451(3) Å) of 8 is similar to that observed for 1
(1.447(4) Å).
2−
B10H9 (X = F, Br, I) anions suggested that similar reactions
with 6 could provide the first route to TfO-substituted
decaborates.
The reaction of 6 with excess triethylamine produced a
mixture of the [Et3NH+]2[1-TfO-B10H92−] (16) and
[Et3NH+]2[2-TfO-B10H92−] (17) isomers in 30 and 64% yields,
while the analogous reaction with liquid ammonia gave
+
+
2−
[NH4 ]2[1-TfO-B10H92−] (18) and [NH4 ]2[2-TfO-B10H9
]
(19) in 40 and 52% yields (eq 11).
It is perhaps surprising that only one product was observed in
each of the reactions of 6 with a terminal alkyne. On the basis of
the previously proposed pathway24 for alkyne insertion, two
different isomers, 1-R-4-X-1,2-C2B10H10 and 2-R-4-X-1,
In both reactions, the two products could be separated by
the selective precipitation of the less-soluble 16 and 18 salts. The
11B NMR patterns and chemical shifts for both isomers are in
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Information, Figure S13, the 11B NMR spectrum of the 2-TfO-
B10H92− anion (17 and 19) exhibited seven resonances (with the
peak at −28.7 ppm of intensity-two and the −31.4 ppm peak of
intensity-three), with the triflate-substituted B2 singlet resonance
appearing at −1.1 ppm.
+
As shown in Figure 12, the structure of [NH4 ]2[1-TfO-
B10H92−] (18) was confirmed by a single-crystal X-ray
determination. The B1−O1 bond distance (1.487(3) Å) is
longer than that observed for 1 (1.447(4) Å). The cage distances
are similar to those in the previously reported28 structure of
[1-I-B10H92−], indicating the substituent has little effect on cage-
bonding.
In summary, ionic liquid/triflic acid cage-opening reactions
2−
of closo-B10H10 now provide a convenient synthetic route to
5-TfO-B10H13. As illustrated by the initial reactivity studies
reported herein, this compound and its derivatives should now
prove to be valuable new starting materials for the construction
of more complex B-substituted polyboranes and carboranes.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures of DFT-optimized geometries, a table of crystallographic
data, tables of the coordinates and energies of DFT-optimized
1
structures, 11B and H NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 12. Crystallographically determined structure of 18. Selected
distances (Å) and angles (deg): B1−O1, 1.487(4); B1−B2, 1.677(5);
B1−B5, 1.681(5); B1−B4, 1.683(5); B1−B3, 1.683(4); B2−B9,
1.804(5); B2−B6, 1.818(5); B2−B5, 1.840(5); B2−B3, 1.848(5); B3−
B6, 1.803(5); B3−B7, 1.805(5); B3−B4, 1.844(4); B4−B8, 1.817(5);
B4−B7, 1.823(4); B4−B5, 1.848(5); B5−B8, 1.812(5); B5−B9,
1.813(5); B6−B10, 1.702(5); B6−B7, 1.836(5); B6−B9, 1.837(5);
B7−B10, 1.708(5); B7−B8, 1.844(5); B8−B10, 1.702(5); B8−B9,
1.844(5); B9−B10, 1.702(5); S1−O3, 1.4317(2); S1−O2, 1.416(2);
S1−O1, 1.505(2); B1−O1−S1, 129.27(19); O1−B1−B2, 132.2(3);
O1−B1−B4, 125.6(3); O1−B1−B3, 126.6(3); O1−B1−B5, 131.6(3).
AUTHOR INFORMATION
Corresponding Author
*Email: lsneddon@sas.upenn.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Science Foundation for
support of this project. We also thank the National Science
Foundation for an instrumentation grant (CHE-0840438) used
for the purchase of the X-ray diffractometer employed in these
studies.
excellent agreement with their DFT/GIAO-calculated values.
27
2−
Consistent with the 11B NMR spectra of 1-HO-B10H9
and
other 1-substituted derivatives,2 the spectra of 16 and 18 showed
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