High yielding synthesis of carboranes under mild reaction conditions using a homogeneous silver(I) catalyst: Direct evidence of a bimetallic intermediate

Article abstract of DOI:10.1002/anie.201311012

Methods used to prepare functionalized carboranes generally require heating to high temperatures, and thus limits the range of derivatives which can be prepared directly from alkynes. We show here that by using a homogeneous silver(I) catalyst it is now possible to prepare carboranes in good to excellent yield at temperatures below 40 °C, including at room temperature. The process is general and provides an important new synthetic strategy for the preparation of functionalized boron clusters. Cluster around: Current methods to prepare carboranes require heating to high temperatures, thereby limiting the range of derivatives which can be prepared from functionalized alkynes. By using a Ag^I catalyst it is possible to prepare carboranes from substituted alkynes in good to excellent yields at temperatures below 40 °C, including room temperature. The approach provides an important new synthetic strategy for the preparation of functionalized boron clusters.

Full text of DOI:10.1002/anie.201311012

Angewandte
Chemie
DOI: 10.1002/anie.201311012
Cluster Compounds
High Yielding Synthesis of Carboranes Under Mild Reaction
Conditions Using a Homogeneous Silver(I) Catalyst: Direct Evidence
of a Bimetallic Intermediate**
Mohamed E. El-Zaria, Kunal Keskar, Afaf R. Genady, Joseph A. Ioppolo, James McNulty, and
John F. Valliant*
Abstract: Methods used to prepare functionalized carboranes
generally require heating to high temperatures, and thus limits
the range of derivatives which can be prepared directly from
alkynes. We show here that by using a homogeneous silver(I)
catalyst it is now possible to prepare carboranes in good to
excellent yield at temperatures below 408C, including at room
temperature. The process is general and provides an important
new synthetic strategy for the preparation of functionalized
boron clusters.
Recently our group reported that the addition of catalytic
silver nitrate to reactions involving alkynes and B10H12-
(CH3CN)2 significantly enhances the yield of carborane
formation.[4] In the presence of silver(I) salts there is evidence
of carborane formation at room temperature, however, to
achieve high yields and to solubilize the catalyst, reactions
had to be performed at temperatures of greater than 1008C.
We hypothesized that an appropriate homogeneous silver(I)
catalyst would similarly increase the yield of cluster formation
while reducing the temperature required to promote carbor-
ane formation, thus creating a milder alternative to existing
synthetic methods.
McNulty and co-workers recently reported the prepara-
tion of N,N-dialkyl(2-dialkylphosphanyl)benzamide ligands
which form crystalline complexes with silver(I). The silver
complexes have been shown to act as mild p acids, thus
effectively promoting the cycloaddition of azides onto
terminal alkynes and intramolecular hydroamination reac-
tions.[6] The catalysts are stable, well-defined nonsolvated
materials, and soluble in most organic solvents, thus making
them attractive homogeneous alternatives to AgNO3 for
potentially promoting the formation of carboranes. A pre-
liminary screening study using phenyl acetylene, B10H12-
(CH3CN)2, and the catalysts 1a and 1b, which differ in the
nature of the counter anion, was undertaken (Scheme 1). For
Carboranes are undergoing a renaissance because of their
unique structural and electronic properties,[1] and potential
use in creating new diagnostics, therapeutics, and electroni-
cally tunable materials.[2,3] A longstanding issue when prepar-
ing carboranes is that reactions generally require heating
between 80–1208C to achieve adequate yields and thus limits
the range of functionalized derivatives which can be prepared
directly from the corresponding alkynes (Figure 1).[5]
Figure 1. Summary of the main methods used to prepare 1,2-dicarba-
closo-dodecaboranes from substituted alkynes. L=CH3CN, L’=N,N-
diisopropyl-(2-di-tert-butylphosphanyl)benzamide.
Scheme 1. Preliminary screening reaction for catalysts 1a and 1b.
[*] Dr. M. E. El-Zaria, K. Keskar, Dr. A. R. Genady, Dr. J. A. Ioppolo,
Dr. J. McNulty, Dr. J. F. Valliant
Department of Chemistry and Chemical Biology
McMaster University
1280 Main St W., Hamilton, Ont., L8S 4M1 Canada
E-mail: valliant@mcmaster.ca
the initial studies, reactions were performed in toluene and
the temperature was held constant at 408C to minimize the
overall reaction times prior to attempting reactions at room
temperature. Of the factors investigated, the ratio of reactants
had the largest impact on yield where the optimal condition
was a twofold excess of the alkyne to B10H12(CH3CN)2. With
respect to the amount of catalyst, a small increase in yield was
observed when 10 mol% of catalyst was employed instead of
5 mol% (70 versus 65% yield, respectively), whereas the
yield decreased to 55% when 20 mol% catalyst was
Dr. M. E. El-Zaria, Dr. A. R. Genady
Department of Chemistry, Faculty of Science
Tanta University, 31527 Tanta (Egypt)
[**] Financial support for this research was provided by the Natural
Sciences and Engineering Research Council (NSERC) of Canada.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201311012.
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employed. Varying the nature of the catalyst between the
acetate in 1a and nitrate in 1b had little impact on the yield.
The general utility of the catalyst under the most
promising conditions was evaluated subsequently by using
1b and a series of different alkynes for direct comparison to
reactions performed in the absence of catalyst (Table 1). In all
cases yields of the isolated products are reported. For
phenylacetylene the desired carborane formed in 85% yield
after 48 hours at 408C. The yield under identical reaction
within eight hours whereas the catalyst-free reaction was only
15% after 24 hours.
When repeated at room temperature, the yields of the
catalyzed reactions were below 50% after six hours for all
compounds tested. However after 48 hours (Table 1) five of
the alkynes tested generated carboranes in greater than 70%
yield, with the exception of phenylcarborane, diphenylcar-
borane, and the pthalimide-protected carborane which were
isolated in 58, 37, and 61% yield respectively. When the
control reactions were run in the absence of the catalyst,
yields of the products at room temperature were negligible
(< 2%). In general, at room temperature the catalyzed
reactions showed the largest increase in yield during the
first 48 hours. Further extension the reaction times lead to
smaller increases in the amount of product produced.
When AgNO3 is added quickly to large quantities of
alkynes the reactions can, in some instances, ignite. Caution
should therefore be taken when combining alkynes with silver
salts and particular attention paid to the order of addition of
reagents. A key advantage of using 1b is that there was no
evidence that the catalyst and alkyne reacted vigorously,
including when reactions are done at a larger scale. For
instance, when a 10-fold increase in reaction scale was run
using 5-chloropentyne and 1b the desired carborane was
readily isolated in 75% yield after 48 hours at room temper-
ature without issue. One additional advantage of using the
homogeneous catalyst as opposed to AgNO3 is that at the
completion of a reaction there was no evidence of silver metal
deposition, owing to the robust nature of the metal complex.[6]
In certain instances reactions did become heterogeneous,
however the desired products could be readily isolated by
filtration through Celite and simple silica gel chromatogra-
phy. To test the residual activity of the catalyst, after an initial
reaction period an additional aliquot of the borane and
propargyl bromide was added and the yield was 80%. A third
cycle produced the carborane in 82% yield, thus demonstrat-
ing that the catalyst remains active.
Table 1: Compounds, reaction conditions, and yields for catalyzed and
uncatalyzed reactions.
Entry
R
R1
Yield [%]
no catalyst[a] catalyst[b]
408C
catalyst[c]
RT
408C (t [h])
1
2
3
4
5
6
7
8
Ph
H
H
H
H
H
H
H
Ph
34Æ2.1
22Æ2.0
22Æ0.6
19Æ1.5
20Æ0.6
17Æ1.5
15Æ0.6
15Æ2.0
85Æ2.0 (48) 58Æ1.5
CH2Br
CH2OH
C3H6CN
C3H6Cl
R2CO2Me[d]
PhtNCH2
Ph
87Æ2.0 (2)
86Æ3.0 (2)
87Æ2.1 (2)
86Æ2.5 (2)
93Æ2.1 (8)
86Æ2.5 (2)
73Æ1.5 (8)
71Æ1.5
72Æ1.5
73Æ2.1
70Æ2.1
74Æ1.0
61Æ1.2
37Æ2.1
[a] Uncatalyzed reaction (n=3); reaction time=48 h for entry 1 and 24 h
for all other entries. [b] Catalyzed reaction (n=3) at 408C. [c] Catalyzed
reaction (n=3) at room temperature for 48 h (all entries). Yields of
uncatalyzed reactions at room temperature were <2%.
[d] R2CO2Me=methyl 4,7,10,13,16,19,22,25,28-nonaoxahentriacont-30-
yn-1-oate. Pht=phthalimide. Additional details and results can be found
in the Supporting Information. n = number of repeats.
conditions for the noncatalyzed reaction was 34%. Propargyl
bromide and alcohol (entries 2 and 3) were converted into the
corresponding carboranes in 87 and 86% yield, respectively,
after only two hours at 408C. The yields in the absence of
catalyst after 24 hours were 22% in both cases. The catalyst
worked equally well for 5-hexynenitrile and 5-chloropentyne,
thus producing the functionalized carboranes (entries 4 and 5)
in 87 and 86% yields, respectively, which was over four times
the yield observed in the absence of the catalyst. The catalyst
also increased the yield of carboranes bearing a bifunctional
PEG linker (entry 6), which is often used in bioconjugate
chemistry as a spacer between targeting vectors and lipophilic
prosthetic groups like carboranes.[7] The desired PEG-car-
borane was isolated in over 90% yield after eight hours.
The catalyst was also effective for alkynes which in our
hands gave low and/or variable yields of carboranes when
using conventional methods. The pthalimide (Table 1,
entry 7) for instance was isolated in 86% yield where the
yield of the catalyst-free control experiment was only 15%.
Internal alkynes are also known to undergo low-yielding
reactions with B10H12(CH3CN)2. The silver(I) catalyst was
successful in producing 1,2-diphenylcarborane in 73% yield
The ability to perform reactions at significantly reduced
temperatures creates opportunities to prepare carboranes
from thermally sensitive alkynes which would degrade or
undergo unwanted side reactions if subjected to the conven-
tional synthetic methods shown in Figure 1. For example, the
tert-butyl ester of propiolic acid (3, Scheme 2) cannot be
converted into a carborane directly because at elevated
temperatures the product undergoes thermolysis with rapid
loss of CO2 and isobutene to generate ortho-carborane.[8]
With the homogeneous catalyst, the compound 4 was
produced in high yield (86%) at 408C where a crystal
structure of the closo-carborane was obtained by recrystalli-
zation from hexanes (Scheme 2).
2-Ethynylpyridine (5) is a similarly difficult starting
material from which to prepare carboranes. Several attempts
have been made to optimize the yield, which is typically 30%,
of 1-(2’-pyridyl)-1,2-dicarba-closo-dodecaborane.[9] One
method produced the product in 45% yield but required
the use of a solvent-free thermolysis procedure at 1008C in
a sealed tube.[10] At 408C and room temperature in the
presence of 1b the desired product 6 was isolated in 88 and
63% yield, respectively (Scheme 3). By using a lower reaction
2
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previously, the proposed mechanism likely involves formation
of a comparable intermediate. The coordination to silver
decreases the Lewis basicity of the alkyne, thereby preventing
unwanted hydroboration and related alkyne degradation
pathways, while also lowering the activation energy barrier
to carborane formation.[4] To test the proposed mechanism,
silver phenylacetylide was combined with B10H12(CH3CN)2,
and should, according to the proposed mechanism, generate
the desired product in comparable yield and time to that for
the silver catalyzed reaction with phenylacetylene. Somewhat
surprisingly the reaction (Scheme 4) was sluggish and
required 48 hours to generate the desired product, which
was isolated in a lower than expected yield both at room
temperature (21%) and 408C (55%).
Scheme 2. Reaction of tert-butyl propiolate with B10H12(CH3CN)2 under
thermal and catalyzed reaction conditions and the single-crystal X-ray
structure of 4. Thermal ellipsoids are shown at 50% probability.[16]
Scheme 4. Reaction of silver phenylacetylide with B10H12(CH3CN)2 in
the presence and absence of a silver(I) catalyst.
Scheme 3. Synthesis of carboranes 6 and 9. TBAF=tetra-n-butylammo-
nium fluoride, THF=tetrahydrofuran.
When silver phenylacetylide was combined with B10H12-
(CH3CN)2 and catalyst 1b the reaction proceeded much more
rapidly and there was clear evidence of product formation
after two hours at 408C. After 48 hours the yield exceeded
80%. These yields and rates were comparable to head-to-
head reactions run using phenylacetylene in the presence of
the catalyst. The same trend was observed when 1b was
replaced with AgNO3, although the yields were lower since
the latter catalyst requires higher temperatures than does 1b.
These results suggest that the reaction mechanism may
involve a bimetallic species (Figure 2) comparable to the
mechanism recently proposed for copper(I)-catalyzed Huis-
gen cycloaddition reaction uncovered using elegant crossover
experiments with an isotopically enriched copper source.[15] In
support of the proposed mechanism, we were able to obtain
direct evidence for the formation of a bimetallic complex. A
mass spectrum of 1-hexyne in the presence of 1b exhibited
peaks corresponding to the bimetallic complex A (Figure 2)
with the molecular formula C48H81N2P2O2Ag2, displaying the
expected isotopic distribution pattern (m/z 993 to 998)
associated with the natural isotopic abundance of silver
temperature there was significant reduction in the number of
observed side products compared to the uncatalyzed reaction,
and it greatly facilitated isolation of 6.
The preparation of ortho-carborane is nontrivial and is
typically done using a hazardous procedure involving gaseous
acetylene heated to 908C over 24 hours.[5b] Other methods
have been reported and involve heating functionalized
carboranes with permanganate or decarboxylation of
esters.[8,11] One alternative and potentially safer route is one
using trimethylsilylacetylene (7; bp = 538C), which is readily
available and easy to handle. Unfortunately conventional
reaction conditions do not generate the corresponding
carborane from this particular alkyne. When the reaction
was performed in the presence of 1b, trimethylsilyl-ortho-
carborane (8: 47%) and ortho-carborane (9: 45%) were
isolated at 408C. Treatment of 8 with TBAF[12] at low
temperature generated the desired product in high yield,
thus providing a convenient and safe method for producing
ortho-carborane (Scheme 3). The compound 8 is also a useful
starting material for preparing monofunctionalized boron
cluster compounds.[13]
(
107Ag,109Ag) and other relevant (12C/13C) isotopes (Figure 2).
In terms of the mechanism, for terminal alkynes the
current hypothesis is that the labile amide group in 1b is
replaced by the alkyne, thus forming an intermediate
p complex which ultimately forms the s-bonded silver
acetylide.[14] For catalysis by AgNO3, which we reported
This mechanism also offers an explanation as to why for
internal alkynes, which cannot form the s-Ag–alkyne com-
plex, the observed yield enhancement is not as significant as
for terminal alkynes and that significantly longer reaction
times are needed.
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Received: December 19, 2013
Published online: && &&, &&&&
Keywords: alkynes · cluster compounds ·
.
homogeneous catalysis · silver · structure elucidation
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Communications
Cluster Compounds
M. E. El-Zaria, K. Keskar, A. R. Genady,
J. A. Ioppolo, J. McNulty,
J. F. Valliant*
&&&& — &&&&
High Yielding Synthesis of Carboranes
Under Mild Reaction Conditions Using
a Homogeneous Silver(I) Catalyst: Direct
Evidence of a Bimetallic Intermediate
Cluster around: Current methods to pre-
pare carboranes require heating to high
temperatures, thereby limiting the range
of derivatives which can be prepared from
functionalized alkynes. By using a AgI
catalyst it is possible to prepare carbor-
anes from substituted alkynes in good to
excellent yields at temperatures below
408C, including room temperature. The
approach provides an important new
synthetic strategy for the preparation of
functionalized boron clusters.
6
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