Exhaustive chlorination of [B12H12]2- without chlorine gas and the use of [B12Cl12]2- as a supporting anion in catalytic hydrodefluorination of aliphatic C-F bonds

Article abstract of DOI:10.1021/ic200024u

The fully chlorinated closo-dodecaborate salt Cs₂[B ₁₂Cl₁₂] was prepared in high yield from Cs ₂[B₁₂H₁₂] and SO₂Cl₂ in acetonitrile at refluxing temperature. [Ph₃C]₂[B ₁₂Cl₁₂] was obtained by simple metathesis reactions. Catalytic hydrodefluorination of benzotrifluoride sp³ C-F bonds was accomplished using [Ph₃C]₂[B₁₂Cl₁₂] as a precatalyst and Et₃SiH as a stoichiometric reagent. Full consumption of the sp³ C-F bonds in p-FC₆H ₄CF₃ and C₆F₅CF₃ with a turnover number up to 2000 was achieved.

Full text of DOI:10.1021/ic200024u

COMMUNICATION
pubs.acs.org/IC
2
Exhaustive Chlorination of [B H ] without Chlorine Gas and the
-
1
Use of [B Cl ] as a Supporting Anion in Catalytic
2 12
2
-
12 12
Hydrodefluorination of Aliphatic C-F Bonds
Weixing Gu and Oleg V. Ozerov*
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77842, United States
S Supporting Information
b
ABSTRACT: The fully chlorinated closo-dodecaborate salt
Cs [B Cl ] was prepared in high yield from Cs [B H ]
2
12 12
2
12 12
and SO Cl in acetonitrile at refluxing temperature.
2
2
[
Ph C] [B Cl ] was obtained by simple metathesis reac-
3 2 12 12
3
tions. Catalytic hydrodefluorination of benzotrifluoride sp
C-F bonds was accomplished using [Ph C] [B Cl ] as a
3
2
12 12
precatalyst and Et SiH as a stoichiometric reagent. Full
Figure 1. Representation of the monocarba-closo-dodecaborate(-)
3
-
11 11 12 12
2-
3
[
HCB H ] anion (left) and closo-dodecaborate [B H ] dianion
consumption of the sp C-F bonds in p-FC H CF and
6
4
3
(right). Dots at vertices represent boron atoms; each vertex is capped
C F CF with a turnover number up to 2000 was achieved.
6
5
3
with a hydrogen atom.
Scheme 1. New Synthesis of the Perchlorinated Dodecabo-
2-
rate [B Cl ]
12 12
eakly coordinating anions (WCAs) are an important
concept in chemistry because they allow the use and study
W
1
-3
of highly reactive cations in condensed phases. Over the last 2
-
decades, halogenated carborane anions [HCB X ] (X = H,
1
1 11
Me, Hal) have been demonstrated to be an especially advanta-
geous WCA class owing to their low basicity and nucleophilicity
combined with a matchless stability and propensity for crystal-
-
15-18
[HCB X ] argued for comparably low basicity.
In this
11 11
4
,5
lization (Figure 1). However, the arduousness inherent in the
Communication, we present a new, simple, and chlorine gas-free
-
syntheses of both the parent [HCB H ] and its halogenated
11
11
method of synthesis of closo-dodecachlorododecaborate
2
12 12 12 12
-
2-
derivatives, and the consequently high cost, has so far prevented
truly widespread use of halogenated carboranes. In contrast, their
isoelectronic brethren, halogenated closo-dodecaborates
[
B Cl ] from [B H ] , as well as experiments demon-
2
-
strating the competence of [B Cl ] as a robust WCA in
catalytic hydrodefluorination (HDF) of sp C-F bonds
12
12
3
2
-
[
B X ] (X = Hal), are derived from the much more easily
12 12
2 6,7
(Scheme 1).
-
accessible parent [B H ] dianion, especially in view of
12
12
Syntheses of polychlorinated dodecaborates have previously
relied on the use of Cl as the chlorinating agent. [B H Cl ]
2
-
Knapp et al.’s recent report of a single-step, multigram synthesis
2
12
6
6
8
2-
from NaBH . Fluorination of [B H ] with F was thor-
4
12 12
2
was synthesized by the reaction of [H O] [B H ] with chlor-
3 2 12 12
19
9
oughly examined recently by Strauss et al. The 2- charge on
B X ] makes them an unconventional choice for WCAs,
ine in water at 0 °C. Selectivity in the partial chlorination of
2-
2
-
20
[
12 12
[B H ] has been analyzed computationally. Muetterties
12 12
2
-
and perhaps that is why they have rarely been considered for
et al. first reported the synthesis of [B Cl ] in 1964 by
12
12
9
-11
2
-
11
WCA applications despite having been known for decades.
reacting [B H ] with excess Cl in an autoclave. Knapp
1
2
12
2
Our interest in WCAs stems from our studies on the use of
highly electrophilic main-group cation species for C-F bond
activation, where resistance of the anion to extreme levels of
et al. reported an improved method in 2009 that avoided the
high-pressure autoclave by passing chlorine gas through an
aqueous solution of Na B H at 100 °C for 40 h. In the wake
of our recent report on the chlorination of a monocarba-closo-
dodecaborate anion [HCB H ] with SbCl or SO Cl , we set
out to explore the potential utility of SO Cl as the chlorination
reagent in the synthesis of [B Cl ] . Compared to chlorine
gas, SO Cl is inexpensive, less hazardous to handle in the
8
2
12 12
12-14
21
Brønsted and Lewis acidity is key.
Halogenated carborane
-
anions performed very well, but the concerns over their cost and
lengthy syntheses remained. Hence, we were heartened to learn
11
11
5
2
2
2
2
8
,15-17
18
2
-
from the work of Knapp et al.
and of Reed et al. that
12
12
2
-
[
B Cl ] can be a WCA comparable to the halogenated
12
12
2 2
carboranes, albeit engendering considerably lower solubility.
Isolation of the highly reactive Me B Cl , [Et Si] [B Cl ],
laboratory under standard safety precautions, and more amen-
able to use on different reaction scales.
2
12 12
3
2
12 12
2
-
and H [B Cl ] demonstrated the robustness of [B Cl ]
while similar ν(CH ) frequencies in the C H (protonated
benzene) and ν(NH) in the R NH salts of [B Cl ] and
,
2
12 12
12 12
þ
6 7
Received: January 5, 2011
Published: March 01, 2011
2
þ
2-
3
12 12
r 2011 American Chemical Society
2726
dx.doi.org/10.1021/ic200024u
|Inorg. Chem. 2011, 50, 2726–2728

Inorganic Chemistry
COMMUNICATION
The direct reaction of 100 mg of Cs [B H ] with refluxing
neat SO Cl because a white suspension was observed through-
2
12 12
2
2
neat SO Cl for 24 h only led to a mixture of various partially
out the reaction. Converting Cs [B H ] to (Et NH) [B H ]
2
2
2 12 12 3 2 12 12
2-
11
chlorinated [B H12-xClx] , based on B NMR evidence
did not drastically improve the solubility in SO Cl , and the
2 2
1
2
(
Figure 2). The incomplete chlorination of Cs [B H ] by neat
reaction with [Et NH] [B H ] in neat SO Cl did not lead to
2
12 12
3 2 12 12 2 2
SO Cl is likely due to the low solubility of Cs [B H
Cl ] in
complete chlorination either. We noted that acetonitrile, in
which Cs [B H ] is more soluble, was successfully used as a
2
2
2
12 12-x
x
2
12 12
9
solvent for the fluorination of K [B H ] with F . Refluxing
2
12 12
2
Cs [B H ] in a 1:1 mixture of SO Cl and acetonitrile for 24 h
2
12 12
2
2
led to complete conversion to Cs [B Cl ] (Figure 2), which
2
12 12
22
was isolated upon recrystallization from water. We were able to
obtain 75-82% yield of Cs [B Cl ] starting with 0.1, 1, or 10 g
2
12 12
1
11
of Cs [B H ], and it was characterized by H and B NMR, IR
spectroscopy, and elemental analysis.
2
12 12
23
Following our original publication on the chlorination of
Cs[HCB H ] with SO Cl , we discovered that it was remark-
11
11
2
2
ably sensitive to the origin of the reagents and only worked well
20
with select and unpurified reagents. This may indicate that
some unknown impurities perform a catalytic role in that
reaction, but at present, we do not completely understand the
origin of the observed effects. In order to rule out the possibility
2
-
that the chlorination of [B H ] might also be catalyzed by
1
2 12
some impurities in the starting materials, the chlorination of
Cs [B H ] was performed with recrystallized Cs [B H ],
2
12 12
2
12 12
1
1
distilled acetonitrile, and distilled SO Cl and also with
2 2
24
Figure 2. Top: B NMR spectrum for the reaction of Cs [B H ]
2
12 12
11
protection from light. All of these variations led to complete
with refluxing SO
reaction of Cs
The x-axis scale is in ppm.
2
Cl
2
for 24 h. Bottom: B NMR spectrum for the
2
-
conversion to the [B Cl ] product after 24 h at refluxing
12 12
22
2
[B12
H
12] with refluxing SO
2
2
Cl and acetonitrile for 8 h.
temperature.
Using literature cation-exchange procedures,
tained [Ph C] [B Cl ]. We have previously used trityl salts
8,17,18
we ob-
3
2
12 12
Scheme 2. Proposed Mechanism of HDF and the Reactions
with Benzotrifluorides under Study in This Work
-
of [B(C F ) ] and of halogenated carboranes as precatalysts for
6
5 4
12
the HDF of aliphatic C-F bonds with Et SiH. The proposed
3
HDF mechanism and the reactions under study in this work are
outlined in Scheme 2. The trityl salts are convenient to store and
are readily converted to triethylsilylium by reaction with excess
triethylsilane in the reaction mixture.
The results of the HDF reactions are summarized in Table 1.
In all reactions, o-dichlorobenzene was used as the solvent.
2
5
Et SiF and Et SiF were observed as the Si-F products. Along
3
2
2
with the Ar-CH HDF products, Friedel-Crafts products from
the attack on the aromatic rings of the solvent (or substrate and
product) were also observed by F NMR, as with other
catalysts. The HDF reactions of p-FC H CF worked well
3
19
12
6
4
3
using [Ph C] [B Cl ] as the precatalyst at 22 °C, with full
3
2
12 12
3
2
consumption of the sp C-F bonds, no conversion of the sp
2
6
C-F bond, and a turnover number (TON) of ca. 2000. To
2
-
further evaluate the competence of [B Cl ] , we performed
12
12
HDF reactions with the more challenging substrate C F CF
6
5
3
(
Table 1). At 22 °C, the reaction is fairly slow, with consumption
of only 6% of the C-F bonds in 24 h. However, the HDF re-
action accelerated dramatically at elevated temperature. Complete
Table 1. HDF Results (Ar-CF f Ar-CH )
3
3
a
b
b
c
no.
substrate
T (°C)
time (h)
catalyst mol %
Si-F convn %
C-F convn %
TON
1
2
3
4
1
2
2
2
22
22
50
80
0.5
24
24
1
0.049
0.053
0.052
0.052
87
1
>97
6
2040
110
82
90
96
1850
1920
>97
a
[B12Cl12] was used as a catalyst in these reactions. Catalyst loading was calculated based on the moles of [B12Cl12]2- versus the moles of C-
[
Ph C]
3 2
b
1
9
c
F bonds. Conversion was based on the disappearance of C-F bonds or the appearance of Si-F bonds by F NMR. TON was calculated as the
number of C-F bonds converted per each unit of [Ph C] B Cl .
3
2 12 12
2
727
dx.doi.org/10.1021/ic200024u |Inorg. Chem. 2011, 50, 2726–2728

Inorganic Chemistry
COMMUNICATION
3
or near-complete consumption of all sp C-F bonds with a
TON of ca. 2000 was achieved within 24 h at 50 °C and within 1 h
at 80 °C. The poor reactivity at room temperature may largely
reflect the much lower solubility of [Ph C] [B Cl ] compared
C. M.; Chen, C.-H.; Foxman, B. M.; Ozerov, O. V. J. Am. Chem. Soc.
2010, 132, 4946–4953.
(13) (a) Panisch, R.; Bolte, M.; Mueller, T. J. Am. Chem. Soc. 2006,
128, 9676–9682. (b) Klahn, M.; Fischer, C.; Spannenberg, A.; Rosenthal,
3
2
12 12
U.; Krossing, I. Tetrahedron Lett. 2007, 48, 8900–8903.
(14) Douvris, C.; Stoyanov, E. S.; Tham, F. S.; Reed, C. A. Chem.
Commun. 2007, 1145–1147.
with the carborane analogues. Nonetheless, these results illus-
trate that [B Cl ] can be used as a WCA in HDF catalysis,
which sustains thousands of turnovers, similarly to the results
that we reported with halogenated carboranes as supporting
2-
1
2
12
(
15) Derendorf, J.; Keβler, M.; Knapp, C.; R €u hle, M.; Schulz, C.
Dalton Trans. 2010, 39, 8671–8678.
16) Bolli, C.; Derendorf, J.; Keβler, M.; Knapp, C.; Scherer, H.;
1
2
anions.
(
In summary, we report a new, straightforward, chlorine gas-
free method for synthesis of the valuable [B Cl ] dianion. It
is also shown that [B Cl ] can be used as a WCA for C-F
bond activation mediated by electrophilic silylium species. The
B Cl ] -based catalysis showed longevity to comparable to
that of carborane-based catalysis in the HDF reactions.
Schulz, C.; Warneke, J. Angew. Chem., Int. Ed. 2010, 49, 3536–3538.
(17) Kessler, M.; Knapp, C.; Sagawe, V.; Scherer, H.; Uzun, R. Inorg.
Chem. 2010, 49, 5223–5230.
2
-
12
12
2
-
12
12
(
18) Avelar, A.; Tham, F. S.; Reed, C. A. Angew. Chem., Int. Ed. 2009,
48, 3491–3493.
19) Sivaev, I. B.; Bregadze, V. I.; Sj €o berg, S. Collect. Czech. Chem.
Commun. 2002, 67, 679–727.
20) Lep ꢀs ík, M.; Srnec, M.; Hnyk, D.; Gr €u ner, B.; Ple ꢀs ek, J.; Havlas,
Z.; Rulí ꢀs ek, L. Collect. Czech. Chem. Commun. 2009, 74, 1–27.
21) Gu, W.; McCulloch, B. J.; Reibenspies, J. H.; Ozerov, O. V.
2
-
[
12 12
(
(
’
ASSOCIATED CONTENT
(
S
Supporting Information. Full synthetic procedures,
spectroscopic methods, and HDF reaction details. This material
is available free of charge via the Internet at http://pubs.acs.org.
b
Chem. Commun. 2010, 45, 2820-2822. An Addition/Correction is now
available at http://www.rsc.org/suppdata/CC/c0/c001555e/addition.
htm.
(
22) Synthetic details for Cs
12] (1.00 g, 2.45 mmol) in acetonitrile (30 mL) in a Schlenk
flask was slowly added SO Cl (30 mL, 370 mmol). The mixture soon
2
[B12Cl12]. 1 g scale: To a suspension of
2
Cs [B12H
’
AUTHOR INFORMATION
2
2
Corresponding Author
became a clear solution and was heated to reflux for 8 h. The removal of
all volatiles gave a white solid, which was treated with 0.2 mL of a 10%
NaOH aqueous solution and then recrystallized from hot water with 1.5
g of CsCl (8.9 mmol) to give Cs [B Cl ]. Yield: 1.64 g (82%). 10 g
*E-mail: ozerov@mail.chem.tamu.edu.
2
12 12
scale: To a suspension of Cs
acetonitrile (300 mL) in a three-necked flask was slowly added SO
300 mL, 3.71 mol). The mixture soon became a clear solution and was
2
[B12
H
12] (9.98 g, 24.3 mmol) in
’
ACKNOWLEDGMENT
2
Cl
2
We are grateful for support of this research by the Department
(
of Energy, Office of Basic Energy Sciences (Grant DE-FG02-
heated to reflux. After 8 h, an aliquot was taken from the reaction
0
6ER15815), the Alfred P. Sloan Foundation, the Dreyfus
mixture. The removal of all volatiles gave a white solid, which was
1
1
Foundation, and the Research Corporation. We thank Dr.
Yohannes Rezenom and Vanessa Santiago of the Laboratory
for Biological Mass Spectrometry at Texas A&M University for
the mass spectrometry analyses. We are also grateful to Loren P.
Press for checking some of the chlorination reactions.
redissolved in CD CN for B NMR study, which showed that conver-
3
sion was only ca. 95% complete. After 24 h, all volatiles were removed
under vacuum to give a white solid, which was redissolved in CD
3
CN for
1
1
B NMR study. The resulting white powder was recrystallized from hot
1
1
2
water to give Cs [B12Cl12]. Yield: 16.03 g (81%). B NMR (128 MHz,
CD CN): δ -13.0 (s). (See a full description and data in the Supporting
3
Information).
’
REFERENCES
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(
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perturbation of the thermal gradients within the reaction apparatus.
2
090.
(
(
(
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2 2
(25) Et SiF presumably forms via redistribution of the substituents
(
on silicon under Lewis acid catalysis.
(26) We calculate TON based on the amount of the [B12Cl12
2
-
]
(
counterion. It is not clear whether both cations associated with it can be
catalytically engaged simultaneously at all times. If they are, then our
TON values should be divided by 2.
(
(
(
2
(
8
(
(
3
2
728
dx.doi.org/10.1021/ic200024u |Inorg. Chem. 2011, 50, 2726–2728