Catalyzed reaction of triethylborane with pyrazole

Article abstract of DOI:10.1016/S0022-328X(99)00705-6

The reaction of triethylborane with pyrazole leading to 4,4,8,8-tetraethylpyrazabole was found to be catalyzed by carboxylic acids. 2,2′-Dimethylpropanoic (pivalic) or benzoic acids act as catalysts in this reaction. Intermediates occurring in the catalyt

Full text of DOI:10.1016/S0022-328X(99)00705-6

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Journal of Organometallic Chemistry 597 (2000) 190–195
Catalyzed reaction of triethylborane with pyrazole
Marek Da˛browski, Sergiusz Lulin´ski, Izabela Madura, Janusz Serwatowski *,
Janusz Zachara
Faculty of Chemistry, Warsaw Uni6ersity of Technology, Noakowskiego 3, PL-00-664 Warsaw, Poland
Received 29 June 1999; received in revised form 4 November 1999
Dedicated to Professor Stanis*law Pasynkiewicz on the occasion of his 70th birthday
Abstract
The reaction of triethylborane with pyrazole leading to 4,4,8,8-tetraethylpyrazabole was found to be catalyzed by carboxylic
acids. 2,2%-Dimethylpropanoic (pivalic) or benzoic acids act as catalysts in this reaction. Intermediates occurring in the catalytic
cycle have been isolated and characterized by NMR as well as IR spectroscopy, and in one case also by X-ray crystallography.
The mechanism of the reaction is proposed. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Triethylborane; Pyrazole; Catalysis
1. Introduction
Tetraalkylpyrazaboles are usually synthesized in the
reactions of pyrazole (pzH) and/or its derivatives with
trialkylboranes [1] or tetraalkyldiboranes 6 [2] (Scheme
1). Due to the high stability of the boron–carbon bond
Scheme 1.
these reactions require elevated temperatures (e.g. 130–
150°C, Eq. (1)):
130°C
2Et3B+2pzH ꢀꢀꢀꢁ TEP+2EtH
tonolysis is postulated [5]. Rothgery and Ko¨ster [6]
(1)
investigated the mechanism of the reactions of Et3B
with primary amines in the presence of (2,2%-di-
methyl)propanoyloxydiethylborane. They proved that
in this case the mechanism is complicated by the forma-
tion of a stable addition complex of Me3CꢀCOOBEt2
with amine, which can be isolated when the molar ratio
of reactants is 1:1. Decomposition of the complex oc-
curs only when an excess of amine is present and leads
to the formation of the corresponding aminodiethylbo-
rane and ammonium salt of 2,2%-dimethylpropanoic
acid. In conclusion, carboxylic acids can be used as
active catalysts for protonolysis of trialkylboranes. The
mechanism of catalysis depends on the type of pro-
tolytic reagent, however, the first step is always the
formation of acyloxydialkylborane.
xylene
It is well known that carboxylic acids readily cleave
BꢀC bonds in trialkylboranes. Brown et al. [3] and
Toporcer et al. [4] explained this unusual reactivity of
carboxylic acids towards BR3 compounds by the forma-
tion of an intermediate complex possessing a six-mem-
bered ring structure. Coordination of the boron atom
results in an increased charge density on the a-carbon
atom, which facilitates the elimination of an alkane
molecule. The product of the reaction is acyloxydialkyl-
borane. It is unstable in the presence of water, alcohols
and other ꢀOH reagents and readily undergoes pro-
tonolysis under their influence, to afford compounds of
the type R2BOY (Y=H, R, SO3H etc.) with recovery
of carboxylic acid. A simple mechanism of this pro-
We have found that pivalic and benzoic acids cata-
lyze also the known reaction of triethylborane with
pyrazole. In a previous paper [7] we described the
* Corresponding author. Fax: +48-22-628-2741.
E-mail address: serwat@ch.pw.edu.pl (J. Serwatowski)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00705-6

M. Da˛browski et al. / Journal of Organometallic Chemistry 597 (2000) 190–195
191
2.2. System triethylborane–pyrazole–pi6alic acid
In the next step we investigated an interaction be-
tween the triethylborane–pyrazole complex and pivalic
acid. The reaction in the molar ratio 1:1 afforded the
addition complex of Me3CꢀCOOBEt2 with pyrazole (2)
[7] (Scheme 3).
Scheme 2.
Complex 2 has a dynamic character but it is stable
and can be stored at room temperature for several
weeks without decomposition. It is also stable for sev-
eral hours at 60°C in CDCl3 and decomposes rapidly
only above 100°C to yield tetraethylpyrazabole and
pivalic acid. This unexpected stability is probably
mostly due to the seven-membered ring structure. This
explanation can be supported by the fact that our
attempts to obtain the corresponding complex of type
2 with imidazole were unsuccessful. Considering the
great thermal stability of 2, it became clear to us that
the direct decomposition of the complex to TEP is not
important in view of the catalysis.
synthesis and characterization of the pyrazole complex
with 2,2-dimethylpropanoyloxydiethylborane. This
complex is the first relevant intermediate in the cata-
lytic cycle leading to the formation of 4,4,8,8-te-
traethylpyrazabole (TEP). In this paper we present the
results of our investigations on the mechanism of this
reaction.
2. Results and discussion
The addition of a catalytic amount of pivalic or
benzoic acids to the Et3B–pzH mixture in xylene de-
creases the reaction temperature from 130 to about
80°C. To explain the role of the acid, we have investi-
gated this reaction step-by-step and have tried to find
the stoichiometric behavior of all the reactants and
intermediates in the system. This is presented in several
of the following sections.
2.3. Reaction of 2 with pyrazole
Complex 2 reacts quite rapidly at room temperature
with an excess of pyrazole, to afford a colorless crys-
talline solid, only sparsely soluble in hexane. NMR
and IR data allowed one to propose the structure of
the (1-pyrazolyl)diethylborane-pyrazole adduct (3) for
this product. The reaction is reversible (Scheme 3).
Compounds of this type were reported by Yalpani et
al. [2] although it should be mentioned here that they
obtained complexes with a 1,5-cyclooctanediyl sub-
stituent on the boron atom.
As can be expected, the rapid proton exchange in
solution makes pyrazole rings of 3 equivalent. Com-
pound 3 is air stable but it slowly decomposes even at
room temperature and rapidly when melted (100°C), to
produce TEP and pzH. The decomposition of 3 is very
2.1. An equilibrium in the system
triethylborane–pyrazole
The reaction of triethylborane with pyrazole at am-
bient temperature gives a stable addition complex
Et3B·pzH (1), which shows no tendency to decompose
into the substrates or TEP under these conditions [7]
(Scheme 2). Nevertheless, we proved a significant dy-
namic character of this compound. The 11B-NMR
spectrum of the pure complex shows a signal at 0 ppm,
in the range typical for many other R3B-nitrogen
donor complexes [8]. However, when an excess of Et3B
was added, we did not observe the two signals of Et3B
and Et3B·pzH at 86 and 0 ppm, respectively. Instead,
only one broadened signal in the range 86 and 0 ppm
is present in the spectrum, and its chemical shift de-
pends on the Et3B:pzH molar ratio. The 1H-NMR
spectrum confirms the dynamic character of the com-
plex because two protons in the a-positions to nitrogen
atoms of the pyrazole ring are equivalent and give one
signal. This is possible only when the fast (in the NMR
time scale) exchange of the azole proton and Et3B
between molecules of the complex is assumed. The fact
of the dynamic character of the complex is very impor-
tant because the rate of the catalyzed reaction depends
mainly on the concentration of free Et3B in the system.
The faster the exchange process in the complex solu-
tion, the higher the ‘concentration’ of ‘free’ Et3B and
its ability to react with the catalyst molecule.
Scheme 3.

192
M. Da˛browski et al. / Journal of Organometallic Chemistry 597 (2000) 190–195
Table 1
,
Selected bond lengths (A) and angles (°) for 3
Bond lengths
BꢀN(11)
BꢀC(31)
1.615(2)
1.612(3)
1.346(2)
1.336(2)
1.334(2)
1.374(3)
1.361(3)
0.89(2)
BꢀN(21)
BꢀC(41)
N(21)ꢀN(22)
N(21)ꢀC(21)
N(22)ꢀC(23)
C(21)ꢀC(22)
C(22)ꢀC(23)
1.561(2)
1.609(3)
1.354(2)
1.349(2)
1.338(2)
1.363(3)
1.367(3)
N(11)ꢀN(12)
N(11)ꢀC(11)
N(12)ꢀC(13)
C(11)ꢀC(12)
C(12)ꢀC(13)
N(12)ꢀH(1)
Bond angles
N(21)ꢀBꢀN(11)
C(31)ꢀBꢀN(11)
C(41)ꢀBꢀN(11)
C(11)ꢀN(11)ꢀB
N(12)ꢀN(11)ꢀB
C(11)ꢀN(11)ꢀN(12) 105.6(2)
N(11)ꢀC(11)ꢀC(12) 110.4(2)
C(13)ꢀC(12)ꢀC(11) 105.4(2)
N(12)ꢀC(13)ꢀC(12) 107.9(2)
C(13)ꢀN(12)ꢀN(11) 110.6(2)
105.66(13) C(41)ꢀBꢀC(31)
114.5(2)
110.5(2)
110.81(14)
129.4(2)
122.06(13)
107.2(2)
107.8(2)
131.5(2)
N(21)ꢀBꢀC(31)
N(21)ꢀBꢀC(41)
C(21)ꢀN(21)ꢀB
122.92(14) N(22)ꢀN(21)ꢀB
C(21)ꢀN(21)ꢀN(22) 108.5(2)
N(21)ꢀC(21)ꢀC(22) 109.6(2)
C(21)ꢀC(22)ꢀC(23) 104.3(2)
N(22)ꢀC(23)ꢀC(22) 111.4(2)
C(23)ꢀN(22)ꢀN(21) 106.2(2)
Fig. 1. ORTEP plot of a single molecule of 3.
rapid in the presence of Et3B due to the stronger Lewis
acidity of the boron atom in the Et3B molecule com-
pared with Et2Bpz. In effect, the formation of 1 in this
system is preferred.
bound forming a dimer with a central 12-membered
ring. This is in accord with the result previously ob-
tained [2] for a similar species.
,
The N(12)···N(22% ) distance of 2.773(2) A and the
N(12)ꢀH(1)ꢀN(22%) angle value of 156(2)° are evidence
of the strong interactions (characteristic N···N hydro-
gen bonding distance is placed between 2.92 and 3.15
2.4. Molecular structure of 3
The molecular structure and atom-numbering scheme
of 3 are shown in Fig. 1 [9]. Selected bond lengths and
angles are presented in Table 1. The tetragonal boron
atom is surrounded by two ethyl groups and two
pyrazolyl ligands. The important structural detail to be
,
,
A) [11]. The distance N(12)ꢀH(1) is 0.89(2) A, so the
bond is asymmetric but its strength indicates a H-bond
with proton transfer as was found in the liquid.
2.4.1. Reaction of 2 with triethylborane
,
noted is the difference between BꢀN bonds (0.054 A).
Complex 2 reacts rapidly with triethylborane at room
temperature with evolution of ethane [7]. Product 4 was
obtained as an oily liquid after evaporation of the
solvent. Complex 4 was also formed when 2 was treated
This is a consequence of the non-equivalency of pyra-
zolyl ligands — the BꢀN bond is longer for contact
with the acidic pyrazole ring, pzH, and shorter with
anionic pz. These bond lengths seem to be standard
when compared with average distances obtained from
the Cambridge Crystallographic Data Base [10]: for
,
,
BꢀN(pzH) it is 1.614(20) A and for BꢀN(pz) 1.560(2) A.
In consideration of the larger negative charge on pz
and the presence of a free electron pair on N(22) a
short C(21)ꢀC(22) bond and elongation of other bonds
in the pz ring — particularly the N(21)ꢀC(21) bond are
observed. This is an effect of the free electron pair on
N(22) and also the preference of the resonance form
with a free electron pair on N(21), which is confirmed
by its slight pyramidalization; the pz ring and B show a
,
deviation from planarity of 0.053(2) A, on the contrary
the pzH ring is coplanar with B (deviation at 0.001(2)
,
A). However the free electron pair on N(22) is engaged
in very strong hydrogen bonding so the differences in
bond lengths and angles between pyrazole rings are not
very large. As is shown in Fig. 2, two molecules are
Fig. 2. ORTEP plot of 3-dimeric structure. The hydrogen atoms are
omitted for clarity (except NH).

M. Da˛browski et al. / Journal of Organometallic Chemistry 597 (2000) 190–195
193
TMS in 1H- and 13C-NMR spectra and relative to
Et2O·BF3 in 11B-NMR spectra. CDCl3 was the solvent
(NMR spectra). Molecular-weight determination was
performed cryoscopically in benzene. Triethylborane,
98%, was provided by MPI fu¨r Kohlenforschung, Mu¨l-
heim–Ruhr. Pyrazole (98%) and 2,2%-dimethyl-
propanoic acid (99%) were provided by Aldrich;
benzoic acid (pure) was purchased from POCh–Gli-
wice, Poland. (2,2%-Dimethyl)propanoyloxydiethyl-
borane was synthesized as described in the literature [5].
All reactions were carried out in an argon atmo-
sphere using standard Schlenk techniques. Pyrazole and
benzoic acid were dried in vacuo prior to use. Solvents
were dried with (Na,K)-benzophenone ketyl, distilled
,
under argon and stored over 4 A molecular sieves.
Other reagents were used without purification.
Scheme 4. The simplified pathway of the catalyzed reaction of
triethylborane with pyrazole. The reversibility of certain stages is
omitted for clarity.
3.1. Reaction of Et3B·pzH (1) with Me3CꢀCOOH
with Me3CꢀCOOBEt2. Complex
4 is stable and
A solution of Me3CCOOH (4.15 g, 40.7 mmol) in
heptane (15 ml) was quickly added to precooled (0°C)
Et3B·pzH (6.76 g, 40.7 mmol). After warming to 15°C
ethane evolution started. The solution was stirred at r.t.
for 1.5 h. During the next 1.5 h the temperature was
gradually raised to 50°C, to complete the reaction. The
total volume of ethane evolved was 0.82 NL (36.5
mmol, 90%). After evaporation of the solvent 9.50 g
(39.9 mmol, yield 98%) of a colorless, oily liquid was
obtained. 11B-NMR (96.24 MHz): l 7.1 [12].
decomposes rapidly only above 100°C to give TEP and
Me3CꢀCOOBEt2. It also decomposes readily when
treated with pzH, to give 3 and pivalic acid complexed
with pyrazole.
2.5. Mechanism of catalysis
The collected results indicate a much more complex
mechanism of catalysis than is postulated in the case of
other reactions of protolysis of trialkylboranes. The
simplified pathway of the catalyzed reaction is pre-
sented in Scheme 4. The rate of reaction is most proba-
bly limited mainly by the equilibrium between complex
1 and uncomplexed Et3B and pyrazole, which are active
reagents in the cycle. In fact, uncomplexed triethybo-
rane reacts with carboxylic acids rapidly below 0°C,
whereas, when it is complexed with pyrazole, imidazole
as well as quinuclidine, ethane evolution occurs only
above 15–20°C. In the next step, the Me3CꢀCOOBEt2
is immediately complexed with pyrazole, to give 2. An
analogy with the previously proposed [6] mechanism of
aminolysis appears. This intermediate is a key com-
pound in the system: it can interact either with pzH or
with Et3B, to yield 3 or 4, respectively. Complex 4 is
unstable in the presence of pzH and decomposes to 3.
Complex 3 is the most kinetically unstable intermediate
in the system so it readily undergoes dismutation, par-
ticularly in the presence of Et3B, to yield pzH and the
final product TEP (Scheme 4).
3.2. Preparation of 1-pyrazolyldiethylborane–pyrazole
adduct 3 in the reaction of 2 with pzH
Compound 2 (2.49 g, 10.5 mmol) was added to a
suspension of pzH (1.35 g, 19.8 mmol) in hexane (15
ml) over 2 min. The mixture was stirred for 30 min at
r.t. The precipitated white solid was filtered, washed
with hexane (4×5 ml), and dried in vacuo, yielding
0.72 g (3.5 mmol, 33%) 3. M.p. 95–102°C (dec.). Anal.
Calc. for C10H17BN4: C, 58.85; H, 8.40; N, 27.45.
Found: C, 57.66; H, 8.33; N, 27.53. 1H-NMR (300
MHz): l 13.8 (br, ꢀ1H, NH), 7.67 (br, 2H, CH), 7.62
(br, 2H, CH), 6.34 (t, 2H, CH), 0.79 (q, 4H, CH2), 0.64
(t, 6H, CH3). 11B-NMR (96.24 MHz, recrystallized
from hexane): l 2.6. IR (KBr, recrystallized from hex-
ane): 3300–2200 cm−1 (NꢀH…N).
3.3. Reaction of 2 with Et3B
Et3B (1.45 g, 2.2 ml, 14.8 mmol) was quickly added
to the solution of 2 (3.15 g, 14.7 mmol) in heptane (5
ml) at about −70°C. Ethane evolution began when the
temperature increased to ca. 10°C. After the next 10
min the temperature increased to 50°C. 0.30 NL (85%)
of ethane was evolved in the reaction. A yellowish
solution was obtained. 11B-NMR (96.24 MHz) analysis:
l=10.5 [12].
3. Experimental
1H-, 13C-, 11B-NMR as well as IR spectra were
recorded at room temperature (r.t.) (unless otherwise
noted). Chemical shifts are given in ppm relative to

194
M. Da˛browski et al. / Journal of Organometallic Chemistry 597 (2000) 190–195
3.4. Reaction of 4 with pzH
bath at 45°C. Slow cooling (8 h) gave crystals suitable
for X-ray analysis. Crystal data, data collection and
refinement parameters for 3 are summarized in Table
2. A single crystal was measured on a Siemens P3
diffractometer at r.t. The unit cell was determined by
indexing of 31 well-centered reflections. Intensity data
were collected using graphite monochromated MoꢀKa
Complex 4 (2.08 g, 6.8 mmol) was added to a sus-
pension of pzH (1.92 g, 28.2 mmol) in heptane (10 ml).
After a few minutes a fine precipitate formed. The
mixture was stirred for 18 h at r.t. Then the slurry was
diluted with heptane (10 ml) and stirred for an addi-
tional 4 h. A white crystalline solid was filtered,
washed with hexane (3×15 ml), and dried in vacuo, to
afford 2.00 g (9.8 mmol, yield 72%) of crude 3. M.p.
98–105°C (dec.). Recrystallization from hexane (50 ml)
afforded 0.93 g (yield 33%) of the pure product. M.p.
99–102°C (dec.). 11B-NMR: (64.16 MHz): l 2.6.
,
radiation (u=0.71073 A). Two checked reflections
were measured every 70 reflections and they indicated
27% crystal decomposition during data collection. Af-
ter correction for the Lorentz-polarization effect and
crystal decomposition, the equivalent reflections were
averaged. The structure was solved by direct methods
using SHELXS-86 [13] program. Full-matrix least-
squares refinement method on F2 values was carried
out using the SHELXL-93 [14] program. Positional and
thermal parameters were followed by subsequent con-
version to anisotropic thermal coefficients for all non-
hydrogen atoms. The hydrogen atom positions were
located in the difference Fourier maps and were refined
isotropically.
3.5. Crystal structure determination of 3
A sample of 3 (about 0.3 g) was dissolved in hexane
(20 ml) at 60°C. The solution was placed in a water
Table 2
Crystal data and structure refinement for 3
Empirical formula
Formula weight
Crystal class
C10H17BN4
204.09
Monoclinic
P21/c
4. Supplementary material
Space group
Full tables of bond distances and angles, data collec-
tion, as well as solution and refinement, tables of
atomic coordinates and equivalent isotropic displace-
ment parameters, as well as anisotropic displacement
parameters for compound 3 are available (4 pages)
from the Cambridge Crystallographic Data Centre,
CCDC no. 138590. Copies of this information may be
obtained from The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (Fax: +44-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk).
Unit cell parameters
,
a (A)
9.4450(14)
7.7210(11)
17.132(2)
105.570(11)
1203.5(3)
4
1.126
440
0.070
,
b (A)
,
c (A)
i (°)
3
,
V (A )
Z
Dcalc (Mg m−3
F(000)
)
Absorption coefficient
(mm–1
)
,
Radiation
MoꢀKa (u=0.71073 A),
graphite-monochromated
293(2)
Temperature (K)
2q Range for data
collection
Acknowledgements
4.48–50.10
Index ranges
0BhB10, 0BkB9,
−20BlB19
2563
We gratefully acknowledge the support by Aldrich
Chemical Company, Milwaukee, WI, USA, through
continuous donation of chemicals and equipment.
We also thank the State Committee for Scientific Re-
search for financial support (grant no. d1 3 T 09A 017
13).
Reflections collected
Independent reflections
Reflections observed
Refinement method
Weighting scheme
1823 (Rint=2.21%)
1495 [I\2|(I)]
Full-matrix least-squares on F2
w=[|2(F2o)+(0.0221P)2
+0.4563P]−1
,
where P=(F2o+2Fc2)/3
1822/0/205
Data/restraints/
parameters
References
Final R indices
(obs. data)
R indices (all data)
R1=0.0382, wR2=0.0825
[1] S. Trofimenko, J. Am. Chem. Soc. 89 (1967) 3165.
[2] M. Yalpani, R. Boese, R. Ko¨ster, Chem. Ber. 123 (1990) 1285.
[3] H.C. Brown, N.C. He´bert, J. Organomet. Chem. 255 (1983)
135–141.
R1=0.0524,
wR2=0.0970
1.114
Goodness-of-fit on F2
Extinction coefficient
0.026(2)
+0.142 and −0.134
[4] H. Toporcer, R.E. Dessy, S.I.E. Green, J. Am. Chem. Soc. 87
(1965) 1236.
[5] R. Ko¨ster, H. Bellut, W. Fenzl, Liebigs Ann. Chem. (1974) 54.
[6] E. Rothgery, R. Ko¨ster, Liebigs Ann. Chem. (1974) 101.
Largest diff. peak and
−3
,
hole (e A
)

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[13] G.M. Sheldrick, Acta Crystallogr. Sect A 46 (1990) 467.
[14] G.M. Sheldrick, SHELXL-93: Program for the Refinement
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.