Formation of dilithiated bis-(1H-pyrazol-1-yl)alkanes and their application in the synthesis of diboronic acids

Article abstract of DOI:10.1016/j.tetlet.2014.01.007

Bis-(1H-pyrazol-1-yl)alkanes were deprotonated at the pyrazole 5-positions on treatment with LDA in THF at low temperature. These dianions reacted with tert-butylisocyanate as the electrophile to install a tert-butylamide group at the pyrazole 5-position.

Full text of DOI:10.1016/j.tetlet.2014.01.007

Tetrahedron Letters 55 (2014) 1234–1238
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Formation of dilithiated bis-(1H-pyrazol-1-yl)alkanes
and their application in the synthesis of diboronic acids
a
a
a,
a
b
⇑
´
´
Krzysztof Durka , Agnieszka Górska , Tomasz Klis , Janusz Serwatowski , Krzysztof Wozniak
a Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland
b University of Warsaw, Faculty of Chemistry, Pasteura 1, 02-093 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Bis-(1H-pyrazol-1-yl)alkanes were deprotonated at the pyrazole 5-positions on treatment with LDA in
THF at low temperature. These dianions reacted with tert-butylisocyanate as the electrophile to install
a tert-butylamide group at the pyrazole 5-position. The obtained amides were next converted into the
respective diboronic acids by Br–Li exchange with t-BuLi in THF at low temperature, followed by the
use of triethyl borate as the electrophile. The X-ray analysis of the obtained diboronic acids revealed
the presence of a variety of structural motifs, which stabilize the structure by hydrogen bond formation.
The stabilization pattern differs greatly with a minor modification of the linker connecting the pyrazole
rings.
Received 8 October 2013
Revised 27 November 2013
Accepted 2 January 2014
Available online 9 January 2014
Keywords:
Lithiation
Boronic acid
Pyrazole
Ó 2014 Elsevier Ltd. All rights reserved.
Triethyl borate
Hydrogen bonding
Since the first synthesis of a boronic acid by Frankland in 1860,
this group of compounds has found many applications in various
areas of chemistry and biology.1 The steady rise in the usefulness
and popularity of boronic acids have necessitated the development
of new synthetic methods. The recent studies on the complexation
of boronic acids with diols have led to the development of a variety
of self-organization involving macrocycles, cages, capsules, and
porous covalent organic frameworks.2–7 These studies require the
use of a wide range of di-, tri-, or tetraboronic acids containing
the organic framework composed of substituted phenyl rings
connected by various linkers. However, nitrogen-heterocyclic
diboronic acids have not been studied as the components of
self-organized architectures. In this work we present an efficient
lithiation–boronation protocol for the synthesis of diboronic acids
containing a bis-(1H-pyrazol-1-yl)alkane framework. We antici-
pated that the presence of nitrogen atoms in these molecules
would result in the formation of solid-state networks held together
by hydrogen bonding of the –B(OH)2 groups and by Nꢀ ꢀ ꢀH–O or
N–B interactions.
temperature, solvent, and electrophile, the lithiation of 1-alkylpy-
razoles followed by reaction with an electrophile affords 5-substi-
tuted products, mixtures of
a- and 5-substituted products, and
mixtures of 3- and 5-substituted products.9 However, there are
no reports on the formation of dilithiated bis-(1H-pyrazol-1-yl)alk-
anes. To gain an insight into the metalation mechanism, we per-
formed the lithiation–silylation of bis-pyrazole 1. Thus, a THF
solution containing one equivalent of 1 was added to a solution
of freshly prepared LDA (2 equiv) at ꢁ78 °C. The mixture was stir-
red at ꢁ78 °C for 1 h and next treated with two equivalents of Me3-
SiCl. The reaction afforded exclusively 2 which suggests that the
mechanism involves the formation of the dilithiated intermediate,
1-Li02 (Scheme 1). However, as was revealed by Balle et al., the reac-
tion of 1-methylpyrazole with n-BuLi in THF can occur under ki-
netic or thermodynamic control affording products functionalized
at the methyl group or at the pyrazole 5-position respectively.10
At the other site, the exocyclic
a-lithiated 1-methylpyrazole is
rather unstable and isomerization to the 5-position occurs. Stabil-
ity can be achieved when the 5-position on the pyrazole ring is
Lithiation reactions are some of most prevalent methods for
functionalizing C–H and C–halogen bonds in heterocycles.8
Pyrazole derivatives can be lithiated efficiently, however, lithiation
blocked as was demonstrated in the
ethylpyrazol-1-yl)methane.11
a-lithiation of bis(3,5-dim-
Taking these facts into account, we propose a reaction pathway
where 1 is lithiated first at the exocyclic -position, and the
initially formed 1-Li isomerizes into the thermodynamic
of different ring positions and exocyclic
a-positions compete, and
a
the regioselectivity often depends on the reaction conditions. It
has previously been revealed that depending on the time,
intermediate 1-Li0, which undergoes a second
a-deprotonation–
isomerization sequence with formation of 1-Li2 and 1-Li02, finally
affording 2 after the reaction with Me3SiCl. The formation of the
intermediate 1-Li in the first step was confirmed by the in situ
⇑
Corresponding author. Tel.: +48 22 2347575; fax: +48 22 6282741.
E-mail address: ktom@ch.pw.edu.pl (T. Klis´).
0040-4039/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2014.01.007

K. Durka et al. / Tetrahedron Letters 55 (2014) 1234–1238
1235
Li
SiMe3
Li
Li
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
LDA, -78 °C, THF
-78 °C
LDA, -78 °C, THF
2 Me3SiCl
-78 °C
Li
Li
N
N
Li
SiMe3
1-Li2
Scheme 1. The proposed mechanism for the formation of 2.
1
1-Li
1-Li2'
2
1-Li'
lithiation–silylation of 1 using LDA (2 equiv) and Me3SiCl (2 equiv).
This reaction resulted in the formation of 3 (Scheme 2).
We initially expected that the remaining equivalent of LDA
would lithiate 3 leading to the formation of other silylated prod-
ucts. However, the bulky –SiMe3 group retards further deprotona-
tion, which was confirmed by GC–MS analysis of the reaction
mixture and revealed the presence of only a small amount of 3a
(15% mol).
We have already shown that 1 is lithiated cleanly using LDA.
Regarding the synthesis of the novel class of diboronic acids
derived from bis-(1H-pyrazol-1-yl)alkanes, we attempted the
protocol involving the dilithiation of 1 using two equiv of LDA at
ꢁ78 °C (Scheme 3). The resulting dianion 1-Li02 was next treated
with two equivalents of B(OEt)3, however, after hydrolytic
work-up, we recovered the starting material. A similar result was
obtained after a lithiation–boronation sequence using 4. The con-
trol experiments involving the deprotonation of 4 followed by
addition of t-BuNCO and hydrolysis revealed that the reaction pro-
ceeds smoothly affording 5 as the sole product. Similarly, lithiation
of 4 followed by the addition of Me3SiCl as the electrophile re-
sulted in the selective formation of 6. The results obtained suggest
that the B–C bond at the pyrazole 5-position is easily hydrolyzed in
aqueous solution. On the other hand, the direct Br–Li exchange in 4
using n-BuLi or t-BuLi is not selective as deprotonation at the
5-position competes. In spite of this fact we decided to check the
possibility of synthesizing the respective diboronic acids from 5
by Br–Li exchange. We hoped that the B–C bond at the pyrazole
4-position would not undergo hydrolysis, affording stable dibo-
ronic acids. It should be stressed that the amide group itself is
known to be an efficient lithiation director.12 We assessed the
directing power of the amide groups by attempting ring lithiation
of didebromo-5, but found that metallation occurred only at the
methylene group. Thus, 5 was lithiated with t-BuLi (6 equiv) in
THF at ꢁ78 °C. Excess t-BuLi was required because the N–H bond
is easily deprotonated by alkyllithiums. Quenching the obtained
dianion with B(OEt)3 (4 equiv) and hydrolysis afforded the stable
diboronic acid 7 in almost quantitative yield (Scheme 4). It is note-
worthy that lithiation of compounds containing an amide group
can be problematic because of the possibility of N–H hydrogen
abstraction by the newly formed aryllithium derivative.13 This
problem was eliminated in the synthesis of 7 using the procedure
where the amide 5 was added to the excess of t-BuLi. We next
SiMe3
N
N
N
N
N
N
N
N
2 LDA, -78 °C
2 Me3SiCl in situ , THF
SiMe3
SiMe3
+
1
3 85%
3a 15%
Scheme 2. The in situ lithiation–silylation of 1.
R3
R
R1
R4
R2
1. E+
N
N
N
N
N
N
N
N
N
N
N
N
2. H2O/H+
2 LDA, -78 °C
THF
(CH2)n
(CH2)n
(CH2)n
R4
R2
R3
R
R1
E+ = B(OEt)3
E+ = B(OEt)3
starting material recovered
starting material recovered
1 R = H, n = 1
4 R = Br, n = 1
4 R = Br, n = 1
4 R = Br, n = 1
8 R = Br, n = 2
9 R = Br, n = 3
10 R = Br, n = 8
1-Li2' R1 = H, R2 = Li
4-Li R1 = Br, R2 = Li
4-Li R1 = Br, R2 = Li
4-Li R1 = Br, R2 = Li
8-Li R1 = Br, R2 = Li
9-Li R1 = Br, R2 = Li
10-Li R1 = Br, R2 = Li
E+ = t-BuNCO
5
R3 = Br, R4= CONHt-Bu, 95%
E+ = Me3SiCl
6
R3 = Br, R4= SiMe3, 91%
E+ = t-BuNCO 11 R3 = Br, R4= CONHt-Bu, 97%
E+ = t-BuNCO 12 R3 = Br, R4= CONHt-Bu, 86%
E+ = t-BuNCO 13 R3 = Br, R4= CONHt-Bu, 89%
Scheme 3. Deprotonative dilithiation of bis-(1H-pyrazol-1-yl)alkanes by LDA.

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K. Durka et al. / Tetrahedron Letters 55 (2014) 1234–1238
B(OH)2
Br
Li
O
CONHt-Bu
CONLit-Bu
NHt-Bu
NHt-Bu
1. 4 B(OEt)3
2. H2O/H+
N
N
N
N
N
N
N
N
N
N
N
4 t-BuLi, -78 °C
(CH2)n
(CH2)n
(CH2)n
THF
N
CONHt-Bu
CONLit-Bu
O
B(OH)2
Br
Li
5
n = 1
7
96%
14 92%
15 87%
16
11 n = 2
12 n = 3
13 n = 8
Scheme 4. Synthesis of diboronic acids 7 and 14–16 by Br–Li exchange.
extended our study to the dilithiation of a series of compounds, 8,
9, and 10, composed of two pyrazole fragments separated by vari-
ous aliphatic linkers. Based on our previous results, we expected
that the hydrogen atoms at the pyrazole 5-position would be
potentially reactive toward deprotonation with LDA affording the
respective amides after reaction with t-BuNCO. Indeed, substrates
8, 9, and 10 were smoothly deprotonated by LDA in THF at
ꢁ78 °C to give the respective N-C5 dilithiated species, 8-Li, 9-Li,
and 10-Li. Electrophilic quenching of the dianions followed by
aqueous work-up afforded the respective amides 11, 12, and 13
in very good yields (Scheme 3).
Having prepared compounds 11–13, we next tried to convert
them into the respective diboronic acids. For this purpose we em-
ployed the lithiation–boronation protocol which was applied for 5.
These reactions afforded the respective diboronic acids 14 and 15
in high yield. However, the reaction with 13 was unselective
affording the diboronic acid 16 accompanied by a significant
amount of an undefined side-product (probably the respective
monoboronic acid). The identity of 16 was based on the 1H NMR
spectrum of the crude product, as we were unable to purify it using
conventional methods. The numerous attempts to purify 16 by
recrystallization resulted in decomposition via B–C bond cleavage.
We next attempted to characterize the synthesized diboronic
acids by single-crystal X-ray diffraction. To the best of our knowl-
edge, there are no reports concerning the crystal structure of pyr-
azole-boronic acids. In the case of compounds 7 and 14, we
obtained crystals suitable for X-ray analysis. The obtained results
revealed that 7 and 14 crystallize in monoclinic P21/c and triclinic
P-1 space groups.14,15 Views of the molecular structure together
with the atom labeling scheme are included in Figure 1. In the case
of product 7, both –B(OH)2 groups adopt syn–anti conformations
and are almost coplanar with the pyrazole ring. The anti-oriented
–OH groups form intramolecular hydrogen bond interactions with
the carbonyl oxygen atoms. Additionally, the amide hydrogen
atoms are arranged in intramolecular N–Hꢀ ꢀ ꢀN interactions with
the pyrazole rings. The geometrical parameters of the hydrogen
bonds are given in Table 1 in the Supporting Information. In the
crystal lattice the adjacent molecules are linked via O–H. . .O inter-
actions within centrosymmetric R22(8) motifs, which lead to the
formation of [101] chains. Further propagation of chains is
Figure 1. Labelling of atoms and estimation of their atomic thermal motion as anisotropic displacement parameters (ADPs, 50% probability level) for compounds 7 and 14.
Hydrogen interactions are shown as red dashed lines. Crystallographic data for both structures have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication numbers CCDC 937991 (7) and 937992 (14).

K. Durka et al. / Tetrahedron Letters 55 (2014) 1234–1238
1237
through much weaker C–H. . .O interactions (Fig. 2). The structure
of 7 consists of one molecule of acetone per one molecule of dibo-
ronic acid. These solvent molecules do not participate in hydrogen
bonding interactions and therefore seem to be a little disordered.
The structure of product 14 is characterized by a longer linker
between the pyrazole rings (CH2CH2 unit) and the insertion of a
water solvent molecule instead of acetone. Despite these small
differences, 14 presents a completely different picture for the
supramolecular assembly. In contrast to 7, the –OH groups are
not arranged in intramolecular interactions. What is more, the
commonly found centrosymmetric motifs formed by the two –
B(OH)2 groups are not observed here.16 One of the –B(OH)2 groups
adopts a rare syn–syn conformation, and plays a double donor role
in a hydrogen-bond interaction with a water molecule. The syn-ori-
ented –OH group from the second –B(OH)2 moiety links two
neighboring molecules into a dimer through O–H. . .N interactions.
As shown in Figure 3, the connections between adjacent dimers are
formed between anti-oriented –OH groups and the carbonyl oxy-
gen atoms. It should be noted that only one nitrogen atom from
the –NH group is arranged in an intramolecular hydrogen bond
with the –OH group; the second forms a relatively weak intermo-
lecular hydrogen bond interaction [(dN. . .O = 3.104(2) Å] with an
adjacent molecule. Each water molecule is engaged in one O–
H. . .N interaction with the pyrazole nitrogen atom, one O–H. . .O
interaction with the carbonyl oxygen atom and also is the acceptor
of two hydrogen bonds from one –B(OH)2 group.
In conclusion, we have developed an efficient method for the
generation of synthetically useful dilithiated bis(1H-pyrazol-1-
yl)alkanes, which were successfully employed in the synthesis of
new functionalized diboronic acids containing t-BuNHCO groups
Figure 2. The pattern of hydrogen bonds in 7. Hydrogen O–H. . .O bonds are shown as red, dashed lines, and C–H. . .O bonds as green, dashed lines. Aromatic and aliphatic
hydrogen atoms are omitted for clarity.
Figure 3. The pattern of hydrogen bonds in 14. Hydrogen O–H. . .O bonds are shown as red, dashed lines, and C–H. . .O bonds as green, dashed lines. Aromatic and aliphatic
hydrogen atoms are omitted for clarity.

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K. Durka et al. / Tetrahedron Letters 55 (2014) 1234–1238
at each pyrazole 5-position and B(OH)2 groups at each pyrazole
4-position. This was achieved by combination of selective
dilithiation by deprotonation and Br–Li exchange. This was com-
plemented by studies on the selectivity of the lithiation of bis-
(1H-pyrazol-1-yl)methane. These studies revealed the essential
reactivity of the –CH2– group toward deprotonation by LDA
affording the kinetic product, which isomerizes into the thermody-
namic dilithio-pyrazole derivative. However, the introduction of
the –B(OH)2 at the pyrazole 5-positions in 1 and 4 afforded prod-
ucts which were unstable in aqueous solution. The X-ray analysis
of two of the obtained diboronic acids, 7 and 14, revealed unique
supramolecular assemblies which differed significantly with a
small change in the structure of the linker connecting the two pyr-
azole rings. The participation of the water molecule in hydrogen
bond formation with the hydrogen atoms from the –B(OH)2 is
noteworthy. The obtained diboronic acids are being tested as
components of covalent organic frameworks in reactions with
diols, and the results will be presented elsewhere.
Typical procedure for the synthesis of diboronic acids: Bis-(4-bro-
mo-5-t-butylamido-1H-pyrazol-1-yl)methane (5): A solution of
bis-(4-bromo-1H-pyrazol-1-yl)methane (4) (3.05 g, 10 mmol) in
THF (20 mL) was added to a stirred solution of freshly prepared
LDA (20 mmol) in THF (30 mL) at –78 °C. The resultant colorless
solution was stirred for 1 h to give a colorless precipitate. The elec-
trophile, t-BuNCO (2.0 g, 20 mmol) was then added to the stirred
mixture to give a colorless solution which was stirred for 1 h and
then hydrolyzed with H2O (100 mL). Dilute aq H2SO4 was added
until the pH was slightly acidic. Et2O (50 mL) was next added
and the mixture stirred for 10 min. The organic phase was sepa-
rated and the aq phase extracted with Et2O (20 mL). The combined
organic solutions were dried over MgSO4 and evaporated to give a
solid which was recrystallized from hexane (20 mL) to give 5 as
colorless crystals; yield 4.79 g (95%), mp = 152–154 °C; 1H NMR
(200 MHz, CDCl3): d 7.52 (2H, s, ArH), 7.21 (2H, br s, NH), 6.86
(2H, s, –CH2–), 1.45 (18H, s, –CH3); 13C{1H} NMR (100.6 MHz,
CDCl3): 157.26, 140.69, 135.44, 94.41, 63.43, 52.47, 28.65. Anal.
Calcd for C17H24Br2N6O2: C, 40.49, H, 4.80, N, 16.67. Found: C,
40.36, H, 4.63, N, 16.71.
C17H28B2N6O6: C, 47.04, H, 6.50, N, 19.36 Found: C, 47.02, H, 6.48,
N, 19.00.
Acknowledgments
This work was supported by the Narodowe Centrum Nauki,
(National Science Centre) in the framework of project 2011/03/B/
ST5/02755, and by the European Union in the framework of the
European Social Fund through the Warsaw University of Technol-
ogy Development Programme, realized by the Center for Advanced
Studies. The support of Aldrich Chemical Co., Milwaukee, WI,
U.S.A., through the donation of chemicals and equipment is grate-
fully acknowledged.
Supplementary data
Supplementary data (experimental procedures and spectro-
scopic data for the new compounds, selected single-crystal XRD
data for 7 and 14) associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.01.
007.
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