High yielding preparation of dicarba-closo-dodecaboranes using a silver(I) mediated dehydrogenative alkyne-insertion reaction

Article abstract of DOI:10.1021/ic400928v

The synthesis of 1,2-dicarba-closo-dodecaboranes (ortho-carboranes) is often low yielding which is a critical issue given the increasing use of boron clusters in material science and medicinal chemistry. To address this barrier, a series of Cu, Ag, and Au salts were screened to identify compounds that would enhance the yields of ortho-caboranes produced when treating alkynes with B ₁₀H₁₂(CH₃CN)₂. Using a variety of functionalized ligands including mono- and polyfunctional internal and terminal alkynes, significant increases in yield were observed when AgNO₃ was used in catalytic amounts. AgNO₃ appears to prevent unwanted reduction/hydroboration of the alkyne prior to carborane formation, and the process is compatible with aryl, halo, hydroxy, nitrile, carbamate, and carbonyl functionalized alkynes.

Full text of DOI:10.1021/ic400928v

Article
pubs.acs.org/IC
High Yielding Preparation of Dicarba-closo-dodecaboranes Using a
Silver(I) Mediated Dehydrogenative Alkyne-Insertion Reaction
Antonio Toppino,† Afaf R. Genady,†,‡ Mohamed E. El-Zaria,†,‡ James Reeve,† Fargol Mostofian,†
Jeff Kent,† and John F. Valliant*,†
†Department of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario L8S 4M1, Canada
‡Department of Chemistry, Faculty of Science, University of Tanta, 31527Tanta, Egypt
S
* Supporting Information
ABSTRACT: The synthesis of 1,2-dicarba-closo-dodecaboranes (ortho-
carboranes) is often low yielding which is a critical issue given the increasing
use of boron clusters in material science and medicinal chemistry. To address
this barrier, a series of Cu, Ag, and Au salts were screened to identify
compounds that would enhance the yields of ortho-caboranes produced when
treating alkynes with B10H12(CH3CN)2. Using a variety of functionalized
ligands including mono- and polyfunctional internal and terminal alkynes,
significant increases in yield were observed when AgNO3 was used in catalytic
amounts. AgNO3 appears to prevent unwanted reduction/hydroboration of
the alkyne prior to carborane formation, and the process is compatible with
aryl, halo, hydroxy, nitrile, carbamate, and carbonyl functionalized alkynes.
INTRODUCTION
■
1,2-Dicarba-closo-dodecaboranes (ortho-carboranes) are poly-
hedral clusters of boron and carbon atoms that were first
reported in the early 1960s.1 There has been renewed interest
in carboranes for both materials2 and medicinal chemistry
applications3 which exploit the unique architecture and physical
and chemical properties of the cluster.4 Carboranes are being
used as hydrophobic pharmacophores for developing new
inorganic pharmaceuticals,5 molecular imaging probes, and
radiotherapeutics6 while for material science applications they
Figure 1. Methods for the preparation of 1,2-dicarba-closo-
dodecaboranes.
offer a platform for creating electronically tunable building
blocks.7
One of the longstanding challenges associated with carborane
chemistry is that the general method used to prepare them
often produces products in modest to very low yield.
Traditionally, carboranes are made by combining decaborane
(B10H14) with a Lewis base such as acetonitrile or
dialkylsulfides to create a reactive complex (B10H12L2) (Figure
1a)8 which is then treated with an alkyne to give the desired
product. Yields of carboranes prepared in this manner vary
significantly and can produce complex mixtures particularly for
hindered alkynes which require lengthy reaction times (24−48
h) and elevated temperatures.1,9 An alternative route is to
functionalize ortho-carborane directly. However, this approach
requires a strong base where the resulting anion is hindered,
necessitating the use of reactive electrophiles to produce
products in acceptable yield.10
utilizes ionic liquids (Figure 1b) and decaborane, which was
successful in increasing yields for reactions involving terminal
and internal alkynes where products could be produced in
under an hour. The most effective method employed a catalytic
amount of the ionic liquid 1-butyl-3-methyl-imidazolium
chloride (bmimCl) in toluene.
It has been reported that the yields of carboranes from less
basic acetylenes such as propargyl bromide are generally greater
than those from more basic acetylenes.12 This is somewhat
counterintuitive since less basic acetylenes introduce higher
activation enthalpies. The observed difference in yield is
believed to be due to the fact that less basic alkynes do not
readily undergo degradative hydroboration reactions which
compete with carborane formation. We hypothesized that it
may be possible to enhance carborane formation using
Sneddon and co-workers recently introduced an alternative
strategy for the preparation of carboranes where enhanced
yields and shorter reaction times were observed.11 The method
Received: April 15, 2013
Published: July 5, 2013
© 2013 American Chemical Society
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transition metals to mediate the basicity of alkynes thereby
preventing premature degradation. To test this concept, a
screening study was initiated using Lewis acids including a
library of group 11(1B) metal salts that are known to
coordinate to alkynes.13 From this study we found that
silver(I) significantly enhanced the yield of carboranes derived
from both terminal and internal alkynes.
strated improved yield over the uncatalyzed reaction. As a
comparison, reactions involving PdCl2 and AlCl3, which are
both reported to activate alkynes,15 were also run. The reaction
with Pd(II) showed no improvement in yield while the reaction
with AlCl3 increased the yield; however, isolation of the
product was not trivial and required extensive chromatography.
The Lewis acid and conditions which showed positive impact
with phenylacetylene were applied to the internal alkyne
diphenylacetylene (Table 2) in a second series of screening
RESULTS AND DISCUSSION
■
Coinage metals have long been used to moderate the activity of
alkynes.14 Consequently, using phenyl acetylene as a model
system, a series of simple Lewis acids focusing mainly on
Au(III), Ag(I), and Cu(I) were screened for their ability to
promote carborane formation (Table 1). Initially, reactions
Table 2. Lewis Acid Screening Studies for an Internal
a
Alkyne
Table 1. Conditions, Reagents, and Results from Lewis Acid
a
Screening Experiments
entry
Lewis acid
solvent
temp (°C)
time
yield (%)
1
AgSbF6/AuCl3
CuCl
toluene
DCM
50
16 h
16 h
16 h
5 h
40
45
47
80
82
83
80
81
83
2
25
3
AgNO3
DCM
25
entry
Lewis acid
AuCl3
solvent temp (°C)
time
yield (%)
4
AgNO3, 2 equiv
AgNO3
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
100
100
100
100
100
100
100
120
120
1
DCM
25
16 h
16 h
4 days
4 days
2 days
16 h
2 days
16 h
2 days
16 h
16 h
16 h
2 days
16 h
16 h
16h
65
77
55
57
80
67
70
70
55
73
70
74
56
80
76
58
43
42
84
56
80
93
93
92
5
16 h
16 h
16 h
16 h
16 h
16 h
16 h
2 h
2
AuCl3
toluene
DCM
40
6
AgNO3
3
AgOTf
AgCl
25
7
AgNO3
4
DCM
25
8
AgNO3
5
CuCl
DCM
25
9
AgNO3
6
CuCl
toluene
DCM
45
av/std
AgNO3
81
2
7
AlCl3
25
no Lewis acid
(Bmim)Cl
41
34
8
AlCl3
toluene
DCM
45
9
CuI
25
a
Reactions involved a 2:1 ratio of diphenylacetylene and
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
av/std
CuI
toluene
DCM
45
B10H12(CH3CN)2 and 4 mol % of the Lewis acid compared to the
amount of the alkyne. The grey spheres represent B−H vertices.
CuBr
50
CuBr
toluene
DCM
50
AgI
40
AgNO3
AgNO3
AgSbF6
AgSbF6
PdCl2
DCM
50
tests. Without Lewis acid, the yield of 1,2-diphenylcarborane
was 41%. When silver nitrate was combined with diphenyla-
cetylene and B10H12(CH3CN)2 and the reaction heated for 16
h, the yield nearly doubled. This increase is significant
considering that hindered and internal alkynes typically
produce clusters in low yield but are nonetheless important
for the formation of boron-based bioactive compounds,
radiopharmaceuticals, and BNCT agents.4d,16
toluene
DCM
50
25
toluene
DCM
50
16 h
16h
25
AgSbF6/AuCl3
AgSbF6/AuCl3
AgNO3, 2 equiv
AgNO3
AgNO3
AgNO3
AgNO3
no Lewis acid
(Bmim)Cl
toluene
DCM
50
16 h
16 h
1.5 h
16 h
16 h
16 h
16 h
10 h
7 min
25
toluene
toluene
toluene
toluene
toluene
toluene
toluene
100
100
100
100
100
120
120
Further optimization focused on the silver nitrate system
because of the positive initial results and the availability,
solubility, and low-cost of the reagent. With phenylacetylene,
the reaction conditions were varied, and it was noted that when
the temperature was increased to 100 °C and reaction time
extended to 16 h, the isolated yield of 2 increased to 93%. This
exceeded both the yield with no Lewis acid (65−68%) and the
reaction run in ionic liquid (71%). There was no evidence of
residual silver in the product, and the 11B NMR and MS data
confirmed that the product was the closo- and not the
corresponding nido-carborane.
The silver mediated reaction was subsequently applied to a
series of functionalized alkynes that can be used to build up an
array of carborane-derived materials (Table 3). All reactions
were performed in triplicate and repeated independently by
different members of the laboratory to assess the consistency of
the methodology.
93
1
65−68
71
a
Reactions were performed using a 2:1 mol ratio of phenylacetylene to
B10H12(CH3CN)2 and 4 mol % of the Lewis acid with respect to the
amount of the alkyne. The grey spheres represent B−H vertices.
were performed in dichloromethane or toluene between room
temperature and 50 °C using 4 mol % of the Lewis acid with
respect to the amount of alkyne. As a control, reactions with
B10H12(CH3CN)2 alone were run under identical conditions.
Initial results showed increased yield for AuCl3, CuCl, and
AgNO3 both in dichloromethane and toluene. A mixed catalyst
system consisting of AgSbF6/AuCl3 in toluene also demon-
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and MS data of the isolated product were consistent with
literature values.
Table 3. Comparison of Conventional, Silver Nitrate
Mediated, and Ionic Liquid Reactions for the Preparation of
a
The preparation of the carborane 12 from propargyl bromide
in the presence of AgNO3 produced the desired compound in
89% yield which is comparable to yields following the
conventional method; however, the reaction was complete in
one-quarter the amount of time (30 min versus 2 h). The
reaction with propargyl bromide was one of the few examples
a Variety of Carborane Derivatives
yield (%)
compd
conventional method
Ag method
ionic liquid
2
5225a/279
93
81
77
84
88
89
84
84
52
59
1
2
1
2
1
7
4
3
6
4
7111c
4
41
34
6
6227
7428
67
that had a large standard deviation in yield (89
7). We
8
6511c
believe this to be due to the reactivity of the starting material
toward moisture as opposed to variability in the silver mediated
reaction, which was not observed for other alkynes. All
characterization data for the isolated product matched that in
the literature, and there was no evidence of hydrolysis or
reduction of 12; reactions which could potentially be promoted
by AgNO3. This is consistent with the chloropentyne derivative
6 which was also stable and showed no signs of silver halide
formation under the reported conditions.
10
12
14
16
18
22
9029
50
40, 5917
4
5523
3532
25
10
a
Select reactions were performed in head-to-head comparisons while
others were compared to literature yields. Reaction conditions and
ratios of reagents can be found in the Experimental Section and
Supporting Information.
Carborane carboxylic acid derivatives are useful synthons for
a variety of applications including the preparation of inorganic
analogues of pharmaceuticals.5c,18 The ethyl ester 14 was
successfully prepared from ethyl propiolate in greater than 80%
yield in 30 min which was a similar time employed for the
hydroxymethyl and bromomethyl derivatives 10 and 12. There
was no evidence of hydrolysis or decarboxylation based on the
mass spectrum of the product. The presence of a unique triplet
The yield of 6 prepared from 5-chloropentyne (Scheme 1)
increased by 15% over that from the conventional synthesis
Scheme 1
1
and quartet in the H NMR and a peak at 161.04 ppm in the
13C NMR spectrum confirmed the presence of the ester.
Focus shifted to alkynes which in our hands produced
carboranes in very low yield (Scheme 3). N-Pentylphthalimide
Scheme 3
method where the yield reached a maximum in 2 h rather than
16 h observed for the arylacetylene derivatives. This same trend
was also observed for 5-hexynenitrile where without the
addition of silver nitrate, the preparation of 8 required five
days to achieve 70% yield. One concern was that AgNO3 at the
elevated reaction temperature might cause degradation of the
nitrile. The melting point of the isolated product was 81−82
°C, and a CN stretch was observed in the IR (2250 cm−1)
which matched the associated literature values while the 13C
NMR showed a peak at 118.07 ppm which corresponded to the
chemical shift for the nitrile group observed in the spectrum of
an authentic standard of 8. The AgNO3 method also worked
effectively with propargyl alcohol (Scheme 2) where the yield
Scheme 2
15 is a useful precursor for the preparation of hetero-
difunctionalized carborane derivatives. Under standard con-
ditions the yield of 16 was 40% in our hands, and the reaction
mixture contained multiple impurities which made purification
challenging. With AgNO3, the reaction yield was greater than
80%, and purification was achieved by simple silica gel
chromatography with a single solvent mixture. A second
challenging construct was the alkyne 17 derived from phenyl
piperazine. Metal complexes of this class of compounds have
shown high affinity for α-adrenergic receptors; however,
preparation of the ligand is hampered by low yield of the
carborane. Without AgNO3, the yield of 18 was 4% after 24 h
while with the ionic liquid method the yield was 35%. With
AgNO3, the yield increased further to 52%.
of 10 increased by 21% for the same reaction with no Lewis
acid (88% versus 67%). The literature method for preparing 10
generally involved the use of a protected (acetylated) propargyl
alcohol prior to carborane formation, thus requiring a
subsequent deprotection step. With AgNO3 present, it was
possible to prepare the product in a single step in high yield
using a facile purification method. The melting point, NMR,
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Carborane amino acid derivatives19 are particularly useful for
the preparation of high affinity and metabolically resistant
peptides3b but can be difficult to synthesize in high yield.
Amino acid analogues like 20 and 22 (Scheme 4) which have
Scheme 4
orthogonal protecting groups can be used to prepare carborane
derived peptides via solid-phase synthesis. Using the conven-
tional synthetic approach, compound 22 was isolated in less
than 10% yield. When AgNO3 was used the yields remained
poor; however, when the amount of Lewis acid was increased 2-
fold, the yield of 22 rose to nearly 60% and isolation of the
product from impurities was much simpler. 11B NMR of the
products showed a typical pattern for substituted ortho-
carboranes, and there was no evidence of nido-carborane
formation. The MS spectra were consistent with the desired
products and show no evidence of silver being coordinated to
the ligands which was an initial concern given the potential to
form a coordination complex with the amino acid.
In general, the silver mediated reaction works effectively
using an excess of B10H12(CH3CN)2 or the alkyne. The ratio
selected for each reaction, which was typically 2:1 alkyne:de-
caborane, was based on the one that required the simplest
purification protocol, which largely depended on the Rf of the
alkyne compared to that for the product. Reaction times varied
from 30 min to 16 h, and all reactions could easily be followed
by TLC. It should also be noted that when the reaction scale
for compounds 4 and 6 was increased by an order of
magnitude, the observed yields increased further (see
Supporting Information).
Figure 2. Interaction of Ag(I) with alkynes and a possible pathway for
the formation of ortho-carborane derivatives.
agreement with the literature for silver(I) interactions with
alkynes.21a When B10H12(CH3CN)2 was added the spectra
became much more complex, and there was significant overlap
with the Ag−alkyne complex. However, there was no evidence
of Ag complexes of the boron starting material or Ag−
carborane complexes of 8 indicating that the primary
interaction is likely between silver(I) and the alkyne. On the
basis of the literature and preliminary experimental evidence,
rather than simply catalyzing carborane formation, AgNO3 may
in fact be preventing unwanted side reactions from occurring
leaving the alkyne available to form the desired cluster. Clearly,
more detailed theoretical studies will be required to map out
changes in activation energy barriers akin to what was elegantly
done to probe the mechanism associated with the ionic liquid
promoted reactions.22 In the interim, AgNO3 can be used to
enhance the yield of carborane formation where our current
focus involves evaluating the methodology with an even
broader spectrum of substrates. Studies will include adding
various Ag(I) ligand complexes as opposed to the free metal to
determine if it is possible to increase yield further while
reducing reaction temperatures and times.
Having identified significant benefit to adding AgNO3,
attention shifted to proposing a possible reaction pathway
and potential basis for the observed yield enhancements. Hill et
al. demonstrated that the rate-determining step in carborane
CONCLUSION
formation is the addition of the alkyne to B10H12L2.20
A
■
A series of Cu, Ag, and Au salts were evaluated for their ability
to enhance the yields of carboranes when added to mixtures of
different alkynes and B10H12(CH3CN)2. Using a variety of
functionalized alkynes including mono- and polyfunctional
internal and terminal alkynes, significant increases in yield were
observed when AgNO3 was used in catalytic amounts. Products
were isolated using simple chromatographic methods and
structures confirmed spectroscopically and through comparison
to authentic standards.
competing reaction is hydroboration of the alkyne, which
prevents carborane formation. Strongly basic alkynes are more
susceptible to this side reaction (and related processes
associated with alkyne degradation); consequently, inhibiting
the pathway by moderating the reactivity of the alkyne would
lead to increased yield of the desired carborane products.
Under the current reaction conditions, which do not involve
the addition of base, the silver likely forms an intermediate π-
complex (Figure 2),21 decreasing Lewis basicity of the alkyne,
thereby preventing the unwanted hydroboration and related
alkyne degradation pathways. Following formation of the
carborane, which does not readily coordinate the metal, the
silver is liberated to coordinate to another alkyne, hence the
need to use only catalytic amounts of the metal. As an initial
probe of the reaction pathway, a mixture of 7 and AgNO3 were
combined at room temperature, and a mass spectrum was taken
after 5 min at room temperature and periodically after the
mixture was heated to 100 °C in toluene. The results showed
evidence of the Ag complex of 7 for the room temperature
samples and those during the initial heating periods which is in
EXPERIMENTAL SECTION
■
General. Unless otherwise stated, all chemical reagents were
purchased and used as received from Sigma-Aldrich without further
purification. Solvents were purchased from Caledon. Decaborane was
purchased and used as received from Katchem (Czech Republic). H-
Pra−OH·HCl and Fmoc-L-Pra−OH were purchased and used as
received from Advanced Chemtech. N-(4-Pentynyl)phthalimide,23
Boc-Pra-OMe,24 and B10H12(CH3CN)2 were synthesized according
8
to literature procedures. Reactions were monitored using Alugram Sil
G/UV254 thin layer chromatography (TLC) plates. Carborane-
containing species were visualized with 0.2% PdCl2 in hydrochloric
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acid (3.0 M) which, upon heating, gave dark brown spots. Column
chromatography was accomplished with Silica Gel 60 (EMD Chemical
Inc.) or ultrapure silica gel (Silicycle).
chloropentyne (58 mg, 0.57 mmol) were combined in the presence
of AgNO3 (5 mg, 0.03 mmol) in anhydrous toluene (3 mL) and
heated at 100 °C for 2 h. The solution was cooled to room
temperature, diluted with 10 mL of toluene, and filtered through
Celite. The resulting solution was concentrated under reduced
pressure and purified by silica gel chromatography (hexane:diethyl
ether 5:1, Rf = 0.25), (48 mg, 76%) of compound 4 as a white solid.
The reaction was repeated three times affording an average yield of 77
1%. Mp: 50−51 °C. HRMS (EI) calcd for C5H17B10Cl: 222.1949.
Found: 222.1950 [M+]. The acquired characterization data was
consistent with literature values.27
1H, 13C, and 11B NMR spectra were recorded on Bruker AV700,
1
AV600, or AV200 spectrometers. H chemical shifts are reported in
ppm relative to the residual proton signal of the NMR solvents.
1
Coupling constants (J) are reported in Hertz (Hz). The H NMR
spectra of carboranes typically exhibit a broad signal between 3.00 and
−0.75 ppm arising from the protons attached to the boron atoms of
the cage, which were not reported separately in the ensuing
assignments. 13C chemical shifts are reported in ppm relative to the
carbon signal of the NMR solvents. 11B chemical shifts are reported in
ppm relative to an external standard of BF3·Et2O. Microwave reactions
were performed using a Biotage initiator microwave. Low resolution
mass spectra were obtained on a Waters/Micromass GCT-ToF
instrument using electron impact ionization or a Waters/Micromass
Quattro Ultima spectrometer using electrospray type ionization. High
resolution mass spectra were obtained on a Waters/Micromass Q-ToF
Ultimaglobal spectrometer. Infrared spectra were obtained on a Bruker
Tensor 27 FTIR spectrometer. Optical rotations were obtained using
an Atago Polax-2L polarimeter.
General Synthetic Procedure: Lewis Acid Screening (Meth-
od A). In a dried 5 mL Emry’s microwave vial, the Lewis acid was
combined with the alkyne and B10H12(CH3CN)2 in the indicated
amounts. The reaction vessel was sealed and kept under high vacuum
for several minutes prior to flushing with Ar. Anhydrous toluene was
added, and the resulting reaction mixture was stirred at 100 °C under
Ar for the indicated time. After cooling to room temperature the
solvent was removed under reduced pressure and the desired product
isolated by silica gel chromatography using the solvent system
indicated below.
General Synthetic Procedure: Conventional Method (Meth-
od B). In a dried 5 mL Emry’s microwave vial, B10H12(CH3CN)2 was
combined with the alkyne in the indicated amounts. The reaction
vessel was sealed and kept under high vacuum for few minutes prior to
flushing with Ar. Dry toluene was added, and the resulting mixture was
stirred at 100 °C under Ar for the indicated time. After cooling to
room temperature the solvent was removed under reduced pressure,
and the desired product was isolated by silica gel chromatography.
General Synthetic Procedure: Ionic Liquid (Method C). In a
dried 5 mL Emry’s microwave vial, 1-butyl-3-methylimidazolium
chloride and decaborane were combined in the indicated amounts.
The reaction vessel was sealed and kept under high vacuum for a few
minutes prior to flushing with Ar. The alkyne was dissolved in dry
toluene and added, and then the mixture was stirred at 100 °C under
Ar for the indicated time. After cooling to room temperature the
solvent was removed under reduced pressure, and the desired product
was isolated by silica gel chromatography.
1-Phenyl-1,2-dicarba-closo-dodecaborane (2). Method A.
B10H12(CH3CN)2 (53 mg, 0.26 mmol) and phenylacetylene (53 mg,
0.52 mmol) were combined in the presence of AgNO3 (4 mg, 0.02
mmol) in anhydrous toluene (3 mL) and heated at 100 °C for 16 h.
Purification by silica gel chromatography with hexane gave 53 mg
(93%) of compound 1 as a white solid. The reaction was repeated
three times affording an average yield of 93 1%. Mp: 66−67 °C.
HRMS (TOF ES) calcd for C8H15B10: 219.2181. Found: 219.2172
[M−]. The acquired characterization data was consistent with literature
values.25
1-(3-Cyanopropyl)-1,2-dicarba-closo-dodecaborane (8).
Method A. B10H12(CH3CN)2 (50 mg, 0.25 mmol) and 5-hexynenitrile
(46 mg, 0.49 mmol) were combined in the presence of AgNO3 (4 mg,
0.02 mmol) in anhydrous toluene (3 mL) and heated at 100 °C for 1.5
h. The solution was cooled to room temperature, diluted with 10 mL
of toluene, and filtered through Celite. The resulting solution was
concentrated under reduced pressure and purified by silica gel
chromatography (3:1 hexane:diethyl ether, Rf = 0.43) (45 mg, 87%) of
compound 3 as a white solid. The reaction was repeated three times
affording an average yield of 84 2%. Mp: 81−82 °C. HRMS (TOF
EI) calcd for C6H17B10N: 213.2303. Found: 213.2434 [M+]. The
acquired characterization data was consistent with literature values.28
1-(Hydroxymethyl)-1,2-dicarba-closo-dodecaborane (10).29
Method A. B10H12(CH3CN)2 (300 mg, 1.48 mmol) and propargyl
alcohol (42 mg, 0.75 mmol) were combined in the presence of AgNO3
(12 mg, 0.07 mmol) in anhydrous toluene (3 mL) and heated at 100
°C for 0.5 h. The solution was cooled to room temperature, diluted
with 10 mL of toluene, and filtered through Celite. The resulting
solution was concentrated under reduced pressure and purified by
silica gel chromatography (hexane:ethyl acetate 8:1, Rf = 0.30) and
gave 112 mg (0.64 mmol, 86%) of compound 7 as a white solid. The
reaction was repeated three times affording an average yield of 88
1
1%. Mp: 230−232 °C. H NMR (600 MHz, CDCl3) δ 4.10 (s, 2H,
CH2OH), 3.91 (s, 1H, CcarboraneH). 13C{1H} NMR (150 MHz, CDCl3)
δ 74.8, 65.1, 57.6. 11B{1H} NMR (192 MHz, CDCl3) δ −2.7, −4.9,
−9.0, −11.8, −13.2. FTIR (KBr, cm−1): ν 3059, 2959, 2593, 1342.
HRMS (TOF EI) calcd for C3H14B10O: 174.2050. Found: 174.2018
[M+].
Method B. B10H12(CH3CN)2 (150 mg, 0.74 mmol) and propargyl
alcohol (83 mg, 1.48 mmol) were combined in 3 mL of anhydrous
toluene and heated at 100 °C for 2 h. Purification by silica gel
chromatography with diethyl ether gave 86 mg (0.49 mmol, 66%) of
compound 7 as a white solid.
1-(Bromomethyl)-1,2-dicarba-closo-dodecaborane (12)..9,30
Method A. B10H12(CH3CN)2 (55 mg, 0.27 mmol) and propargyl
bromide (80 wt % in toluene, 76 mg, 0.52 mmol) were combined in
the presence of AgNO3 (4 mg, 0.02 mmol) in anhydrous toluene (3
mL) and heated at 100 °C for 0.5 h. The solution was cooled to room
temperature, diluted with 10 mL of toluene, and filtered through
Celite. The resulting solution was concentrated under reduced
pressure, purified by silica gel chromatography (hexane, Rf = 0.25),
and gave 61 mg (0.26 mmol, 96%) of compound 8 as a white solid.
The reaction was repeated three times affording an average yield of 89
1
7%. Mp: 30−31 °C. H NMR (600 MHz, CDCl3) δ 4.01 (s, 1H,
CcarboraneH), 3.97 (s, 2H, CH2Br). 13C{1H} NMR (150 MHz, CDCl3)
δ 71.2, 61.0, 32.2. 11B{1H} NMR (192 MHz, CDCl3) δ −2.4, −5.0,
−8.6, −10.5, −12.4, −12.9. FTIR (neat, cm−1): ν 3067, 2616, 1429.
HRMS (TOF EI) calcd for C3H13B10Br: 237.1191. Found: 237.1185
[M+].
1,2-Diphenyl-1,2-dicarba-closo-dodecaborane (4). Method A.
B10H12(CH3CN)2 (52 mg, 0.26 mmol) and diphenylacetylene (91 mg,
0.51 mmol) were combined in the presence of AgNO3 (4 mg, 0.02
mmol) in anhydrous toluene (3 mL) and heated at 100 °C for 16 h.
Purification by silica gel chromatography with hexane gave 63 mg
(82%) of compound 2 as a white solid. The reaction was repeated
three times affording an average yield of 81 2%. Mp: 148−149 °C.
HRMS (TOF EI) calcd for C14H20B10: 296.2615. Found: 296.2615
[M+]. The acquired characterization data was consistent with literature
values.1,26
Ethyl(1,2-dicarba-closo-dodecaborane)-1-carboxylate
(14).31 Method A. B10H12(CH3CN)2 (162 mg, 0.80 mmol) and ethyl
propriolate (39 mg, 0.40 mmol) were combined in the presence of
AgNO3 (7 mg, 0.04 mmol) in anhydrous toluene (3 mL) and heated
at 100 °C for 0.5 h. Purification by silica gel chromatography with
diethyl ether gave 64 mg (0.30 mmol, 74%) of compound 9 as a white
solid. The reaction was repeated three times affording an average yield
1
of 84 4%. Mp: 54−56 °C. H NMR (600 MHz, CDCl3) δ 4.29 (q,
2H, J = 6.0 Hz, CH2CH3), 4.08 (s, 1H, CcarboraneH), 1.32 (t, 3H,
1-(3-Chloropropyl)-1,2-dicarba-closo-dodecaborane (6).
Method A. B10H12(CH3CN)2 (58 mg, 0.29 mmol) and 5-
CH2CH3). 13C{1H} NMR (150 MHz, CDCl3) δ 161.0, 69.0, 64.9,
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56.9, 13.8. 11B{1H} NMR (192 MHz, CDCl3) δ −2.6, −8.9, −12.0,
−13.6. FTIR (KBr, cm−1): ν 3079, 2603, 2582, 1743. HRMS (TOF
EI) calcd for C5H16B10O2: 216.2156. Found: 216.2198 [M+].
2H, J = 12 Hz, ArH), 7.41 (m, 2H, ArH), 7.33 (m, 2H, ArH), 5.82 (d,
1H, J = 7.86 Hz, NH), 4.48 (m, 1H, NHCHCO), 4.41 (m, 1H,
CHCH2O), 4.26 (m, 2H, CHCH2O), 2.80 (m, 2H, CH2CCH), 2.09
(m, 1H, CH2CCH), 1.53 (s, 9H, CH3). 13C{1H} NMR (150 MHz,
CDCl3) δ 169.4, 155.7, 143.9, 143.8, 141.3, 127.8, 127.1, 125.2, 120.0,
82.8, 78.6, 71.6, 67.2, 52.7, 47.2, 28.0, 23.0. FTIR (KBr, cm−1): ν 3300,
2978, 1717, 1506. HRMS (TOF ES) calcd for C24H25NO4Na:
414.1681. Found: 414.1676 [M + Na]+.
1-(3-Phthalimidopropyl)-1,2-dicarba-closo-dodecaborane
(16). Method A. B10H12(CH3CN)2 (51 mg, 0.25 mmol) and N-(4-
pentynyl)phthalimide (107 mg, 0.50 mmol) were combined in the
presence of AgNO3 (4 mg, 0.02 mmol) in anhydrous toluene (3 mL)
and heated at 100 °C for 16 h. Purification by recrystallization (slow
evaporation of hexane−dichloromethane at room temperature) gave
71 mg (85%) of compound 5 as a white solid (hexane:ethyl acetate
1:1, Rf = 0.46). The reaction was repeated three times affording an
average yield of 84 3%. Mp: 120−122 °C. HRMS (TOF EI) calcd
for C13H21B10NO2: 333.2525; Found: 333.2649 [M+]. The acquired
characterization data was consistent with literature values.23
Fmoc-L-Car-Ot-butyl (22). Method A. B10H12(CH3CN)2 (64 mg,
0.32 mmol) and Fmoc-Pra-OtButyl (62 mg, 0.16 mmol) were
combined in the presence of AgNO3 (3 mg, 0.02 mmol) in anhydrous
toluene (3 mL) and heated at 100 °C for 2 h. Purification by silica gel
chromatography (ethyl acetate:hexane; 1:2; Rf = 0.65) gave 47 mg
(59%) of compound 11 as a white solid. The reaction was repeated
three times affording an average yield of 59 4%. Mp: 93−95 °C.
1-[(4-(2-Methoxyphenyl)piperazin-1-yl)methyl]-1,2-dicarba-
closo-dodecaborane (18). Method A. B10H12(CH3CN)2 (52 mg,
0.26 mmol) and 1-(2-methoxyphenyl)-4-(prop-2-ynyl)piperazine (118
mg, 0.52 mmol) were combined in the presence of AgNO3 (4 mg, 0.02
mmol) in anhydrous toluene (3 mL) and heated at 100 °C for 16 h.
The solution was cooled to room temperature, diluted with 10 mL of
toluene, and filtered through Celite. The resulting solution was
concentrated under reduced pressure, purified by silica gel
chromatography (hexane:dichloromethane 4:1, Rf = 0.29), and gave
52 mg (0.15 mmol, 58%) of compound 6 as a white solid. The
reaction was repeated three times affording an average yield of 52
6%. Mp: 133−135 °C. HRMS (TOF CI) calcd for C14H28B10N2O:
348.3212, Found: 348.3200 [M+]. The acquired characterization data
was consistent with literature values.32
1
[α]26 −25°. H NMR (600 MHz, CDCl3) δ 7.77 (d, 2H, J = 6 Hz,
ArH), 7.58 (d, 2H, J = 6 Hz, ArH), 7.42 (m, 2H, ArH), 7.33 (m, 2H,
ArH), 5.23 (bs, 1H, NH), 4.50 (m, 1H, CHCH2O), 4.43 (m, 1H,
CHCH2O), 4.22 (m, 1H, CHCH2O), 4.21 (m, 1H, NHCHCO), 3.90
(s, 1H, CcarboraneH), 2.88 (m, 1H, CcarboraneCH2CHNH), 2.60 (m, 1H,
CcarboraneCH2CHNH), 1.47 (s, 9H, (CH3)3). 13C{1H} NMR (150
MHz, CDCl3) δ 169.0, 155.7, 143.6, 141.4, 127.9, 127.2, 124.9, 120.1,
83.9, 72.0, 67.2, 60.6, 54.2, 47.1, 39.6, 27.8. 11B{1H} NMR (192 MHz,
CDCl3) δ −2.06, −5.02, −9.17, −11.06, −12.79. FTIR (KBr, cm−1): ν
3326, 2584, 1716. HRMS (TOF ES) calcd for C24H36B10NO4:
510.3660. Found: 510.3655 [M + H]+.
Method B. B10H12(CH3CN)2 (50 mg, 0.25 mmol) and Fmoc-Pra-
OtButyl (194 mg, 0.50 mmol) were combined in anhydrous toluene (3
mL) and heated at 100 °C for 16 h. Purification by silica gel
chromatography (petroleum ether:diethyl ether; 3:2) gave 13 mg
(0.03 mmol, 10%) of compound 11 as a white solid.
Method C. B10H14 (63 mg, 0.52 mmol) and 1-butyl-3-
methylimidazolium chloride (50 mg, 0.29 mmol) were combined,
and Fmoc-Pra-OtButyl (405 mg, 1.03 mmol) dissolved in anhydrous
toluene (3 mL) was added. The resulting mixture was heated at 100
°C for 16 h. Purification by silica gel chromatography (petroleum
ether:diethyl ether; 3:2) gave 65 mg (0.13 mmol, 25%) of compound
11 as a white solid.
MS Reaction Monitoring Experiments. 5-Hexynenitrile (0.5
mmol) was dissolved in toluene, giving a 1 M solution. Silver nitrate
(0.5 equiv) was added and the reaction mixture heated to 100 °C, and
samples were taken diluted by 1:1000 in MeCN. Spectra were
collected in positive mode at select time points. The process was
repeated in the presence of 0.5 mmol of B10H12(CH3CN)2.
Boc-L-Carb-OMe (20).33 Method A. B10H12(CH3CN)2 (100 mg,
0.50 mmol) and Boc-Pra-OMe (56 mg, 0.25 mmol) were combined in
the presence of AgNO3 (4 mg, 0.02 mmol) in anhydrous toluene (3
mL) and heated at 100 °C for 2.5 h. Purification by silica gel
chromatography (ethyl acetate:hexane; 1:3; Rf = 0.37) gave 50 mg
(58%) of compound 10 as a white solid. The reaction was repeated
three times affording an average yield of 60 4%. Mp: 129−130 °C.
1
[α]26 −14°. H NMR (600 MHz, CDCl3) δ 5.02 (bs, 1H, NH), 4.28
(bs, 1H, CH2CHNH), 3.99 (bs, 1H, CcarboraneH), 3.77 (s, 3H, OCH3),
2.93 (dd, 1H, J = 6 Hz, J = 18 Hz, CH2CHNH), 2.61 (m, 1H,
CH2CHNH), 1.45 (s, 9H, (CH3)3). 13C{1H} NMR (150 MHz,
CDCl3) δ 170.8, 155.1, 81.2, 71.7, 60.6, 53.1, 39.6, 28.2. 11B{1H}
NMR (192 MHz, CDCl3) δ −2.2, −5.0, −9.0, −10.4, −11.1, 12.9.
FTIR (KBr, cm−1): ν 3360, 3216, 3057, 2580, 1738, 1703. HRMS
(TOF ES) calcd for C11H27B10NO4Na: 368.2847. Found: 368.2847
[M + Na]+.
Method B. B10H12(CH3CN)2 (150 mg, 0.75 mmol) and Boc-Pra-
OMe (252 mg, 1.11 mmol) were combined in anhydrous toluene (3
mL) and heated at 100 °C for 16 h. Purification by silica gel
chromatography (ethyl acetate:hexane; 3:1) gave 98 mg (0.28 mmol,
38%) of compound 10 as a white solid.
Method C. B10H14 (61 mg, 0.50 mmol) and 1-butyl-3-
methylimidazolium chloride (50 mg, 0.29 mmol) were combined,
and Boc-Pra-OMe (228 mg, 1.00 mmol) dissolved in anhydrous
toluene (3 mL) was added. The resulting mixture was heated at 100
°C for 2 h. Purification by silica gel chromatography (ethyl
acetate:hexane; 3:1) gave 60 mg (0.17 mmol, 35%) of compound
10 as a white solid.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H, 13C, 11B, IR, and HRMS spectra for reported compounds,
additional experimental methods, and tables of reaction yields
and conditions. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: valliant@mcmaster.ca.
Notes
Fmoc-L-Pra-Ot-butyl (21). Following a literature procedure,34
Fmoc-L-propargylglycine (2.00 g, 5.96 mmol) and tert-butyl 2,2,2-
trichloroacetimidate (5.40 g, 24.7 mmol) were combined in a 50 mL
round-bottom flask and dissolved in a mixture of anhydrous
dichloromethane and tetrahydrofuran (4:1 v/v, 50 mL). The resulting
mixture was stirred at room temperature overnight under argon. The
solvent was subsequently evaporated under reduced pressure, the
residue dissolved in ethyl acetate (50 mL) and the solution extracted
with 10% sodium bicarbonate (2 × 100 mL), brine (1× 100 mL), and
water (1× 100 mL). The organic layer was dried using sodium sulfate
and evaporated under reduced pressure. The desired product was
isolated by silica gel chromatography (hexane:ethyl acetate; 3:1)
affording 1a (1.53 g, 65%) as a white solid. Mp: 57−58 °C. [α]26 −6°.
1H NMR (600 MHz, CDCl3) δ 7.77 (d, 2H, J = 12 Hz, ArH), 7.64 (d,
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the Natural Sciences and Engineering
Research Council (NSERC) of Canada for financial support of
this work.
REFERENCES
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