Potassium (1-Organo-1H-1,2,3-triazol-4-yl)trifluoroborates from ethynyltrifluoroborate through a regioselective one-pot Cu-catalyzed azide-alkyne cycloaddition reaction

Article abstract of DOI:10.1002/ejoc.201300365

A novel series of 1,4-disubstituted 1,2,3-triazole-containing potassium trifluoroborates were prepared in good to excellent yields from the corresponding organohalides and potassium ethynyltrifluoroborate through a regioselective one-pot Cu-catalyzed azid

Full text of DOI:10.1002/ejoc.201300365

Job/Unit: O30365
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Date: 16-05-13 16:20:31
Pages: 6
SHORT COMMUNICATION
Click Chemistry
T. Kim, J. H. Song, K. H. Jeong, S. Lee,
J. Ham* ............................................ 1–6
Potassium (1-Organo-1H-1,2,3-triazol-4-
yl)trifluoroborates from Ethynyltrifluoro-
borate through a Regioselective One-Pot
The preparation of potassium (1-organo-
1H-1,2,3-triazol-4-yl)trifluoroborates from
potassium ethynyltrifluoroborate and alkyl
or aryl halides by employing a regioselec-
tive one-pot Cu-catalyzed azide–alkyne
cycloaddition (CuAAC) is elaborated. This
strategy allows unrestricted access to an
infinite variety of 1,4-disubstituted 1,2,3-
triazoles.
Cu-Catalyzed Azide–Alkyne Cycloaddition
Reaction
Keywords: Copper / Click chemistry / Bor-
on / Nitrogen heterocycles / Cross-coupling
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T. Kim, J. H. Song, K. H. Jeong, S. Lee, J. Ham
SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300365
Potassium (1-Organo-1H-1,2,3-triazol-4-yl)trifluoroborates from
Ethynyltrifluoroborate through a Regioselective One-Pot Cu-Catalyzed
Azide–Alkyne Cycloaddition Reaction
Taejung Kim,[a][‡] Jung Ho Song,[a][‡] Kyu Hyuk Jeong,[a] Seokjoon Lee,[b] and
Jungyeob Ham*[a]
Keywords: Copper / Click chemistry / Boron / Nitrogen heterocycles / Cross-coupling
A novel series of 1,4-disubstituted 1,2,3-triazole-containing
potassium trifluoroborates were prepared in good to excel-
lent yields from the corresponding organohalides and potas-
sium ethynyltrifluoroborate through a regioselective one-pot
Cu-catalyzed azide–alkyne cycloaddition (CuAAC) reaction.
Further Suzuki–Miyaura cross-coupling of these (organo-
1,2,3-triazol-4-yl)trifluoroborates with aryl and alkenyl brom-
ides by using a PdCl2(dppf)·CH2Cl2/TBAB [dppf = 1,1Ј-
bis(diphenylphosphanyl)ferrocene, TBAB = tetrabutylammo-
nium bromide] system under microwave irradiation was ex-
plored.
of a library of triazole compounds through a Pd-catalyzed
cross-coupling reaction. However, they could not perfectly
control the regioselectivity for 1,4- and 1,5-disubstituted
products containing pinacol boronate moieties unless a tri-
methylsilyl-substituted alkynyl boronate was used as the
starting material. More recently, Grob’s group at Novartis
Institutes published a report disclosing a regioselective
method for the CuAAC reaction by using N-methyl iminod-
iacetic acid (MIDA)-masked ethynylboronic acid and cross-
coupling reactions.[5] However, these previous methods for
the preparation of the target triazole compounds require
long reaction times, high reaction temperatures, and purifi-
cation by column chromatography to obtain the boron rea-
gents in satisfactory yields.
Over the past decade, potassium organotrifluoroborates
have become powerful synthetic building blocks for the for-
mation of new carbon–carbon bonds through Suzuki–
Miyaura cross-coupling, because these compounds are easy
to prepare and purify. They are also convenient to handle
because they are air- and moisture-stable crystalline solids.
Furthermore, the inertness of the trifluoroborate (-BF3)
group under reaction conditions for direct transformation
by using palladium or copper catalysts allows the prepara-
tion of hightly functionalized organotrifluoroborates.[6] Re-
cently, we demonstrated an efficient and rapid method to
prepare potassium (organo-1,2,3-triazol-1-aryl)trifluoro-
borates through a regioselective one-pot CuAAC reaction
from (azidoaryl)trifluoroborates generated in situ.[7] Sur-
prisingly, click reactions with potassium ethynyltrifluoro-
borate and cross-coupling reactions of these (1,2,3-triazol-
4-yl)trifluoroborates have yet to be reported, to the best our
knowledge (Scheme 1).
Introduction
Being one of the most important structural motifs for
the development of potentially bioactive molecules and new
materials, 1,4-disubstituted 1,2,3-triazoles are widely used
in various fields such as organic synthesis, medicinal chem-
istry, and materials science.[1] Ever since the Cu-catalyzed
azide–alkyne cycloaddition (CuAAC) reaction was reported
by Sharpless[2] and Meldal,[3] a number of mild and efficient
methods for the preparation of various 1,2,3-triazole com-
pounds have been disclosed. However, despite significant
recent improvements, the diversity of 1,2,3-triazoles that
can be obtained through the CuAAC reaction is limited by
the availability and high price of the terminal alkyne deriva-
tives from commercial sources. Moreover, some low-molec-
ular-weight alkynes are difficult to handle because of their
low boiling points. Therefore, new facile and efficient meth-
ods to obtain multifariously C4-functionalized triazole
compounds are still of synthetic interest.
In 2009, Harrity’s group reported the thermal [3+2] cy-
cloaddition reactions of various alkyl azides and alkynyl-
boronates[4] in the first literature report of the construction
[a] Natural Medicine Center, Korea Institute of Science and
Technology,
679 Saimdang-ro, Gangneung 210-340, South Korea
Fax: +82-33-650-3529
E-mail: ham0606@kist.re.kr
Homepage: gn.kist.re.kr
[b] Department of Basic Science, Kwandong University College of
Medicine,
522 Naegok-dong, Gangneung 210-701, South Korea
[‡] These authors contributed equally to this work
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300365.
2
www.eurjoc.org
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Regioselective One-Pot CuAAC Reaction of Ethynyltrifluoroborate
Table 1. Survey of one-pot CuAAC reaction conditions.[a]
Scheme 1. Reaction strategy for the development of various 1,4-
disubstituted 1,2,3-triazoles through potassium (1-organo-1H-
1,2,3-triazol-4-yl)trifluoroborates.
Entry
Temp.
[°C]
Additive
Time
[h]
Isolated yield [%]
In continuation of our studies on the development of
synthetic strategies for the direct functionalization of or-
ganotrifluoroborates, we found that potassium ethynyltri-
fluoroborate could be used as a starting material in the
CuAAC reaction instead of commercially available terminal
alkynes. Also, the resulting triazole compounds containing
trifluoroborate moieties could be utilized as coupling part-
ners in the Suzuki–Miyaura reaction. Herein, we report the
first one-pot preparation of potassium (1-organo-1H-1,2,3-
triazol-4-yl)trifluoroborates from potassium ethynyltrifluo-
roborate through a regioselective one-pot CuAAC reaction
and subsequent Suzuki–Miyaura cross-coupling reaction of
the trifluoroborates thus generated.
1a
2a
1[b,c]
2[c]
3
80
80
80
30
30
30
30
none
none
none
12
2
5
none
none
51
67
87
40
4
5
none
7
55
95
none
none
none
Na-ascorbate
Na-ascorbate
Na-ascorbate
2.5
0.5
2
6[d]
7[d,e]
96
93[f]
–
[g]
[a] All reactions were performed on a 0.1 mmol scale in 0.5 mL
of 30% aqueous [D6]DMSO in an NMR tube and then directly
monitored by 1H NMR spectroscopy. [b] Without H2O. [c] Without
Cs2CO3. [d] 20 mol-% of DMEDA was used as a ligand. [e] Reac-
tion was carried out by using 4-iodobenzene as a starting material
instead of benzyl bromide. [f] Isolated product was potassium (1-
phenyl-1H-1,2,3-triazol-4-yl)trifluoroborate (3a). [g] Other side
products were not observed.
Results and Discussion
Initially, we attempted to find optimized reaction condi-
tions for the formation of potassium (1-benzyl-1H-1,2,3-tri-
azol-4-yl)trifluoroborate (1a) through a regioselective one-
pot CuAAC reaction with potassium ethynyltrifluoroborate
(Table 1). In tests aimed at studying the effects of solvent
and base on reactivity (Table 1, entries 1–3), we found that
desired 1a could not be obtained if CuI was used as the
catalyst and NaN3 was used as the azide source at 80 °C;
instead, protodeboronated product 2a was easily generated
(Table 1, entries 1 and 2). However, if the reaction was per-
formed with 30% aqueous [D6]DMSO and Cs2CO3
(1 equiv.) at the same temperature for 5 h, 1a was obtained
in 51% isolated yield along with side product 2a in 40%
yield (Table 1, entry 3). Further optimization of the tem-
perature revealed that if the reaction was performed at
30 °C, only 1a was obtained in 55% yield (Table 1, entry 4).
Although a detailed reaction mechanism has yet to be de-
Scheme 2. Proposed catalytic cycle for 1,2,3-triazole formation
from potassium ethynyltrifluoroborate with R–N3 in the presence
of copper.
termined, we propose the following mechanism for the de- one-pot CuAAC reaction conditions not only curtailed the
sired and undesired reaction cycles on the basis of literature reaction time to 0.5 h, but also increased the yield of 1a to
precedence.[8] As shown in Scheme 2, the two reaction cy- 96% without contamination of side product 2a. Interest-
cles are probably initiated by nucleophilic attack of the acet- ingly, an attempt to prepare potassium (1-phenyl-1H-1,2,3-
ylide, generated with base, onto the copper catalyst (cata- triazol-4-yl)trifluoroborate (3a) from iodobenzene was also
lytic cycle A) and boron–copper transmetalation under successful under the same reaction conditions. However, the
heating conditions (catalytic cycle B). In the next step, R– reaction time was slightly extended to 2 h (Table 1, entry 7).
N3 is then activated by coordination to the C–CuLx com- These CuAAC conditions served as the basis for all of our
plex, which provides intermediates A1 and B1. These inter- other studies.
mediates subsequently undergo formation of copper metalla-
With the optimized conditions in hand, we examined re-
cycles A3 and B3. Finally, 1,2,3-triazoles A5 and B5 are actions of various alkyl bromides with potassium ethynyltri-
generated by protonation of copper–triazolide intermedi- fluoroborate, and the results are shown in Table 2. This
ates A4 and B4, and the copper catalyst is regenerated.
method tolerates various functional groups attached to the
Dramatically, when 20 mol-% of sodium ascorbate was benzyl bromide. The reactions with benzyl bromides that
used as an additive[9] and N,NЈ-dimethylethylenediamine contained either electron-donating groups or electron-with-
(DMEDA) was used as a ligand[10] (Table 1, entry 6), these drawing groups afforded the desired products in 79–96%
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T. Kim, J. H. Song, K. H. Jeong, S. Lee, J. Ham
SHORT COMMUNICATION
yields (Table 2, entries 1–11). Interestingly, when 4-bromo- moanisole instead of iodobenzene and 4-iodoanisole re-
methylbenzyl bromide was used as the starting material, sulted in a slight decrease in the yield to 87% in both cases
the reaction gave the desired dipotassium bis[(1,2,3-triazol- (Table 3, entries 1 and 4). Encouraged by these results, we
4-yl)trifluoroborate] product[11] in 83% yield (Table 2, en- proceeded with further screening, which revealed that the
try 12). Also, the regioselective one-pot CuAAC reactions use of trifluoromethyl-functionalized iodobenzene also pro-
of allyl bromide, α-bromo esters, and alkyl bromide pro- vided good yields of the desired products with m- and p-
ceeded easily to give the desired products in good yields iodobenzotrifluoride (Table 3, entries 6 and 7). Notably, the
(Table 2, entries 13–16).
reactions also tolerate free hydroxy groups (Table 3, en-
tries 9 and 10) and amino groups (Table 3, entry 13); the
corresponding desired products were observed in all cases.
Unfortunately, the regioselective one-pot CuAAC reaction
in some cases of o- or m-substituted aryl halides did not
proceed because of steric hindrance or because of the low
reactivity of the aryl azides (Table 3, entries 5, 8, 11, and
12).
Table 2. Regioselective one-pot CuAAC reactions with various
alkyl bromides and potassium ethynyltrifluoroborate.[a]
Table 3. Regioselective one-pot CuAAC reactions with various aryl
halides and potassium ethynyltrifluoroborate.[a]
[a] All reactions were performed on a 0.4 mmol scale in 2.0 mL of
1
30% aqueous DMSO and monitored by H NMR spectroscopy in
D2O. [b] Isolated yield. [c] Bromobenzene was used. [d] 4-Bromo-
anisole was used.
[a] All reactions were performed on a 0.4 mmol scale in 2.0 mL of
1
30% aqueous DMSO and monitored by H NMR spectroscopy in
With the synthesis of 1,4-disubstituted 1,2,3-triazoles ul-
timately in mind, the scope of the Suzuki–Miyaura cross-
coupling reaction of 1a was investigated with respect to
coupling conditions. We found that if the reaction was per-
formed at 150 °C for 40 min in the presence of PdCl2(dppf)·
D2O. [b] Isolated yield. [c] Reaction was performed with potassium
ethynyltrifluoroborate (2 equiv.), NaN3 (3 equiv.), CuI (20 mol-%),
Na-ascorbate and DMEDA (40 mol-%), and Cs2CO3 (2 equiv.)
based on 1,4-bis(bromomethyl)benzene.
Next, having obtained the optimal reaction conditions CH2Cl2 [10 mol-%, dppf = 1,1Ј-bis(diphenylphosphanyl)-
(Table 1, entry 7), we examined the regioselective one-pot ferrocene] and tetrabutylammonium bromide (TBAB) as an
CuAAC reaction of aryl halides with potassium ethynyltri- additive[12] under microwave irradiation of 80 W, the best
fluoroborate (Table 3). In a test of the different reactivities isolated yield was obtained under the optimized reaction
of o-, m-, and p-iodoanisole, the yields of the corresponding conditions (see the Supporting Information).
products increased in the order ortho Ͻ meta Ͻ para
Finally, we examined the Suzuki–Miyaura cross-coupling
(Table 3, entries 2–4). The use of bromobenzene and 4-bro- reactions of 1a and 3a with different aryl and alkenyl brom-
4
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Regioselective One-Pot CuAAC Reaction of Ethynyltrifluoroborate
halide (0.48 mmol, 1.2 equiv.) under atmospheric conditions. The
reaction mixture was stirred in an oil bath at 30 °C. Upon comple-
tion of the reaction, the solvent was removed in vacuo at 60–70 °C.
The residual product was dissolved in dry acetone (3ϫ 3 mL), and
the insoluble salts were removed by filtration through Celite. The
solvent was concentrated on a rotary evaporator. The addition of
Et2O led to the precipitation of the product. The product was fil-
tered and dried in vacuo to afford the desired pure product.
ides under the optimized coupling conditions, and the reac-
tion results are summarized in Table 4. As expected, the
coupling reactions with various functionalized aryl brom-
ides gave the desired products in good yields (Table 4, en-
tries 1–5). In the reaction with 1,2,2-triphenylvinyl bromide
as a sterically bulky coupling partner alkene, the product
was isolated in a reasonable yield of 57% (Table 4, entry 6).
Moderate yields were also obtained for the coupling reac-
tions of 3a to yield 4g and 4h (Table 4, entries 7 and 8).
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures and analytical and spectro-
scopic data for new compounds.
Table 4. Microwave-assisted C–C bond-forming cross-coupling.[a]
Acknowledgments
This work was supported by the KIST institutional program (Pro-
ject No. 2Z03840). We are grateful to Professor Gary A. Molander
(University of Pennsylvania) for his scientific advice.
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[a] All reactions were performed on a 0.3 mmol scale by using a
microwave reactor for 40 min at 150 °C. [b] Isolated yield.
Conclusions
We have successfully prepared potassium (1-organo-1H-
1,2,3-triazol-4-yl)trifluoroborates from ethynyltrifluorobor-
ate through a perfectly regioselective one-pot CuAAC reac-
tion and have shown that these products can be cross-cou-
pled under Pd-catalyzed Suzuki–Miyaura cross-coupling re-
action conditions. This method allows convenient access to
versatile triazole-containing trifluoroborate reagents, a class
that can be difficult to obtain by other means. Further de-
velopment of these 1,2,3-triazoletrifluoroborates and their
applications are currently under investigation by our group.
Experimental Section
General Procedure for the Synthesis of Potassium (1-Organo-1H-
1,2,3-triazol-4-yl)trifluoroborates: To a solution of potassium eth-
ynyltrifluoroborate (0.4 mmol), sodium azide (0.6 mmol,
1.5 equiv.), CuI (0.04 mmol, 10 mol-%), sodium ascorbate
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(0.08 mmol,
20 mol-%),
N,NЈ-dimethylethylenediamine
(0.08 mmol, 20 mol-%), and Cs2CO3 (0.4 mmol, 1 equiv.) in 30%
aq. DMSO (2.0 mL) was added the corresponding alkyl or aryl
Eur. J. Org. Chem. 0000, 0–0
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T. Kim, J. H. Song, K. H. Jeong, S. Lee, J. Ham
SHORT COMMUNICATION
[6]
cross-coupling reaction under the optimized conditions
(Table 4) [Eq. (1)] and bromodeboronation by using tetrabut-
ylammonium tribromide (TBABr3) [Eq. (2)][13] as shown below.
The structures of reaction products 1l-a and 1l-b were deter-
mined by 1H NMR, 13C NMR, and IR spectroscopy and mass
spectrometry (see the Supporting Information).
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[11] For determination of the dipotassium bis[(1,2,3-triazol-4-yl)tri-
fluoroborate] 1l, we performed a Pd-catalyzed Suzuki–Miyaura
Received: March 11, 2013
Published Online: ᭿
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