The influence of boronate groups on the selectivity of the Br-Li exchange in model dibromoaryl boronates

Article abstract of DOI:10.1002/ejoc.201300145

The selectivity of the Br/Li exchange reaction of 6-butyl-2-(2,5- dibromophenyl)-1,3,6,2-dioxazaborocane (2,5-Br₂C₆H ₃B(OCH₂CH₂)₂NBu) and the analogous anionic derivatives, 2,5-Br₂C₆H₃B(OiPr) ₃Li and 2,5-Br₂C₆H₃BF₃K, was investigated using nBuLi as the lithiating reagent. In the case of the former compound there was a slight preference for lithiation at the 5-position. For 2,5-Br₂C₆H₃B(OiPr)₃Li, the lithiation occured exclusively at C5, but for 2,5-Br₂C ₆H₃BF₃K, 2-lithiation was preferred. Calculations performed for the lithiation of ortho- and meta-brominated phenyl boronates revealed that ortho-lithiated aryl boronates are thermodynamically more stable, but that the Br/Li exchange is generally dictated by kinetics, which accounts for the variation of selectivity depending on the type of boronate group. In addition, the successful generation of the 2,5-dilithiophenyl boronate species 2,5-Li₂C₆H₃B(OCH ₂CH₂)₂NBu by a double Br/Li interconversion is reported. The Br/Li exchange in the related 6-butyl-2-[3-(2,5-dibromothienyl)]- 1,3,6,2-dioxazaborocane, 2,5-Br₂-3-ThB(OCH₂CH ₂)₂NBu, occured preferentially at the 5-position, but the product was readily transformed into the more stable 2-lithiated isomer. The use of 2 equiv. of nBuLi resulted in the efficient formation of the dilithiated species, 2,5-Li₂-3-ThB(OCH₂CH₂)₂NBu. The obtained lithiated aryl boronates were converted into functionalized arylboronic acids by treatment with selected electrophiles. The selectivity of the Br/Li exchange in model dibromoaryl boronates was studied in the context of the directing effect of boron-based groups. Theoretical calculations revealed that ortho-lithiated aryl boronates are thermodynamically more stable, but the Br/Li exchange is generally dictated by kinetics. The lithiated aryl and thienyl boronates were converted into functionalized boronic acids. Copyright

Full text of DOI:10.1002/ejoc.201300145

FULL PAPER
DOI: 10.1002/ejoc.201300145
The Influence of Boronate Groups on the Selectivity of the Br–Li Exchange in
Model Dibromoaryl Boronates
Krzysztof Durka,*[a] Sergiusz Lulin´ski,*[a] Jaromir Smetek,[a] Marek Dabrowski,[a]
˛
˛
[a]
[b]
´
Janusz Serwatowski, and Krzysztof Wozniak
Keywords: Lithiation / Boron / Metalation / Regioselectivity / Density functional calculations
The selectivity of the Br/Li exchange reaction of 6-butyl-2-
pending on the type of boronate group. In addition, the suc-
(2,5-dibromophenyl)-1,3,6,2-dioxazaborocane (2,5-Br2C6H3B- cessful generation of the 2,5-dilithiophenyl boronate species
(OCH2CH2)2NBu) and the analogous anionic derivatives,
2,5-Br2C6H3B(OiPr)3Li and 2,5-Br2C6H3BF3K, was investi-
gated using nBuLi as the lithiating reagent. In the case of the
former compound there was a slight preference for lithiation
at the 5-position. For 2,5-Br2C6H3B(OiPr)3Li, the lithiation oc-
cured exclusively at C5, but for 2,5-Br2C6H3BF3K, 2-lithiation
was preferred. Calculations performed for the lithiation of or-
tho- and meta-brominated phenyl boronates revealed that
ortho-lithiated aryl boronates are thermodynamically more
stable, but that the Br/Li exchange is generally dictated by
kinetics, which accounts for the variation of selectivity de-
2,5-Li2C6H3B(OCH2CH2)2NBu by a double Br/Li interconver-
sion is reported. The Br/Li exchange in the related 6-butyl-
2-[3-(2,5-dibromothienyl)]-1,3,6,2-dioxazaborocane, 2,5-Br2-
3-ThB(OCH2CH2)2NBu, occured preferentially at the 5-posi-
tion, but the product was readily transformed into the more
stable 2-lithiated isomer. The use of 2 equiv. of nBuLi re-
sulted in the efficient formation of the dilithiated species, 2,5-
Li2-3-ThB(OCH2CH2)2NBu. The obtained lithiated aryl
boronates were converted into functionalized arylboronic
acids by treatment with selected electrophiles.
cently, we have found that the directing effect of boronate
groups operates in the deprotonative lithiation of boronated
thiophenes.[4] This study investigates the directing effects on
halogen–lithium exchange with selected representative di-
bromoarenes, namely 1,4-dibromobenzene and 2,5-di-
bromothiophene containing neutral or anionic boron-based
groups, i.e., B(OCH2CH2)2NBu, B(OR)3Li, or BF3K. The
experimental work has been compared with the results of
theoretical calculations to shed light on the mechanistic de-
tails of the halogen–lithium exchange reactions studied.
Introduction
Halogen–lithium exchange is one of the fundamental
methods, and perhaps the most versatile method used to
generate organolithium compounds. Its important advan-
tage is that it is extremely rapid, even at very low tempera-
tures, which means that it can provide a direct access to
many unstable organolithium intermediates. Furthermore,
this method offers a convenient route to compounds that
are not accessible by directed deprotonative metalation re-
actions.[1] The halogen–lithium exchange reaction between
polyhalobenzenes and nBuLi has been investigated with
particular emphasis on the regiospecificity of the halo-
lithiobenzene formation. It is known that various functional
groups, such as alkoxy, amino, amido, fluoro, and CF3
groups, increase the reactivity of neighboring halogen
atoms in halogen–lithium exchange reactions.[2] The interest
of our group is focused on the development of selective ap-
proaches to functionalized aryl and heteroarylboronic acids
using lithiated aryl boronates as key intermediates.[3] Re-
Results and Discussion
Preliminary results on the selectivity of the low-tempera-
ture (–90 °C) halogen–lithium exchange reaction of 6-butyl-
2-(2,5-dibromophenyl)-1,3,6,2-dioxazaborocane 2,5-Br2C6-
H3B(OCH2CH2)2NBu (1a) using nBuLi as the lithiating
reagent showed that the reaction occurs preferentially at
C5, that is, the position more remote from the boronate
group (Scheme 1). This was proved by the subsequent reac-
tion with DMF and hydrolysis, which gave a 3:7 mixture of
5-bromo-2-formylphenylboronic acid (1c) and 2-bromo-5-
formylphenylboronic acid (1d).[5] We repeated this lithiation
at a slightly higher temperature (–80 °C), and then added
MeI. After aqueous work-up, we obtained a mixture of 5-
bromo-2-methylphenylboronic acid (1e) and 2-bromo-5-
methylphenylboronic acid (1f) in a ratio of 4:5. Hence, it
was confirmed that the borocanyl group has little impact
[a] Warsaw University of Technology, Faculty of Chemistry,
Department of Physical Chemistry,
Noakowskiego 3, 00-664 Warsaw, Poland
Fax: +48-22-6282741
E-mail: kdurka@gmail.com, serek@ch.pw.edu.pl
Homepage: lmt.ch.pw.edu.pl
[b] University of Warsaw, Department of Chemistry, Structural Re-
search Laboratory, Pasteura 1, 02-093 Warsaw, Poland
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300145.
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K. Durka, S. Lulin´ski et al.
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on the discrimination between the two lithiation sites upon
addition of nBuLi. Apparently, there is no preference for
the halogen–lithium exchange at C2 due to the precoordi-
nation of nBuLi by the oxygen atom of the adjacent boro-
canyl group.
Scheme 2. Lithiation of 2a and the regioselective formation of 5-
substituted-2-bromophenylboronic acids 1d, 1g, and 1h.
symmetric dimeric motifs typical of most boronic acids, in-
cluding diboronic acids.[8,9] Instead, 1g is assembled to form
hydrogen-bonded tetramers, which are further linked
through weaker lateral hydrogen bonds to produce chains
aligned parallel to the [100] direction (Figure 1, b).
Scheme 1. Lithiation of 1a and the formation of mixtures of 5-
bromo-2-substituted (1c, 1e) and 2-bromo-5-substituted (1d, 1f)
phenylboronic acids.
We also studied the reactivity of the related anionic ate
complex 2,5-Br2C6H3B(OiPr)3Li (2a) with nBuLi
(Scheme 2). The generation of 2a involved the in situ lithi-
ation/boronation[6] of 1,4-dibromobenzene with LDA/
B(OiPr)3 in THF at –70 °C. The resulting solution of 2a
contains 1 equiv. of iPr2NH, so it is necessary to use
2 equiv. of nBuLi in the subsequent reaction. The first
equiv. of nBuLi is consumed to regenerate LDA; only the
second equiv. is involved in the Br/Li exchange. The re-
sulting lithiate was quenched with selected electrophiles to
give the corresponding pure products 1d, 1g, and 1h in rea-
sonable yields. These results show that the lithiation occurs
with perfect selectivity for the C5 position remote from the
B(OiPr)3Li group. The relative deactivation of the 2-Br
atom in 2a towards halogen–lithium exchange can be due
to strongly electron-donating properties of the adjacent an-
ionic boronate functionality and/or steric congestion, which
makes the approach of nBuLi to the C2 position relatively
difficult.
The structure of 1g was confirmed by the X-ray analysis
(Figure 1). It is interesting to note that in one of two crys-
tallographically independent molecules 1g-A, the B(OH)2
group bound to the C3 atom adopts a rarely observed anti–
anti conformation.[7] Both of the molecular structures (i.e.,
1g-A and 1g-B) are stabilized by intramolecular OH···Br
hydrogen bonds (geometrical details are given in Table 1S
in the Supporting Information). The B(OH)2 groups (except
for the one at C9) are significantly twisted with respect to
the aromatic rings, with the torsion angles ranging from
Figure 1. a) Labeling of atoms and visualization of their atomic
displacement parameters (ADPs) at the 50% probability level in
molecules of 1g. b) Fragments of the crystal lattice showing the
hydrogen-bonding motifs. The arrows show the directions of propa-
18.5(1)° to 29.5(1)°. The crystal structure of 1g lacks centro- gation of the chains.
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Selectivity of the Br–Li Exchange in Model Dibromoaryl Boronates
We were interested comparing the behavior of 2a in the ported, but these products were always obtained as minor
halogen–lithium exchange with the reactivity of another an- components of inseparable mixtures, together with the iso-
ionic dibromophenyl boronate derivative: the trifluorobor- meric 1,3,5-triboronated benzenes.[16] To the best of our
ate salt 2,5-Br2C6H3BF3K (3a). Compound 3a was pre- knowledge, 1b-2,5Li2 is the first example of a boronated
pared by the standard route,[10] involving the reaction of dilithioarene species.
the appropriate arylboronic acid with KHF2. When 3a was
treated with nBuLi in THF at –80 °C, the 2-Br atom was
selectively exchanged with lithium as shown by the isolation
of 1c after reaction with DMF and hydrolysis (Scheme 3).
This points to a significant ortho-directing ability of tri-
fluoroborate group, which could be due to its smaller size
and the possible precomplexation of nBuLi by F···Li coor-
dination, which was recently invoked in the deprotonative
lithiation of BF3-complexed anilines and pyridines.[11]
Furthermore, the trifluoroborate group should be a much
weaker electron donor than trialkoxyborate, and hence it
should be much less effective in the electronic deactivation
of the neighboring 2-Br atom. More detailed theoretical
studies into the mechanism of the lithiation processes are
discussed below.
Scheme 4. Dilithiation of 1a and the formation of 2,5-disubstituted
phenylboronic acids 1i–k.
Finally, we decided to compare the reactivity of (2,5-di-
bromophenyl)boronates with that of their heterocyclic
counterpart 6-butyl-2-[3-(2,5-dibromothienyl)]-1,3,6,2-di-
oxazaborocane 4a. The synthesis of 4a is shown in
Scheme 5. The addition of 1 equiv. of nBuLi to the solution
of 4a in THF followed by derivatization with electrophiles
gave 5-bromo-2-substituted products 4c–d. This means that
the halogen–lithium exchange occurred at the position adja-
cent to the borocanyl group (Scheme 6), which is consistent
with our recent results on the thermodynamically con-
trolled deprotonative lithiation of 6-butyl-2-(3-thienyl)-
1,3,6,2-dioxazaborocane with LDA.[4] However, when we
performed the halogen–lithium exchange in the presence of
Scheme 3. Lithiation of 3a and formation of 5-bromo-2-formyl-
the internal electrophile B(OiPr)3,[17] we obtained 5-bromo-
phenylboronic acid (1c).
thiophene-2,4-diboronic acid (4e) as the major product
(Scheme 7). This indicates that the 5-Br atom remote from
the boronate is replaced by lithium more quickly, but in the
absence of an electrophile, the kinetic product (i.e., 4b-5Li)
undergoes rapid isomerization to the more stable isomer
We were also interested in whether it would be possible
to perform a double halogen–lithium exchange in 1a. The
parent 1,4-dilithiobenzene is accessible in high yield by the
treatment of 1,4-diiodobenzene with 2 equiv. of nBuLi un-
der mild conditions.[12] However, 1,4-dibromobenzene is
less reactive, and the completion of the second halogen–
lithium exchange is problematic.[12,13] The use of tBuLi gave
better conversions of dibromoarenes to the corresponding
dilithioarenes.[14] We found that the attempted double lithi-
ation of 1a using 2 equiv. of nBuLi was unsuccessful, as a
mixture of isomeric monolithiated intermediates 1b-2Li and
1b-5Li was generated. A DMF quench and hydrolysis gave
a mixture of 1c and 1d in a ratio of 3:7, and there was no
trace of the desired diformylated product. However, the use
of tBuLi (4.5 equiv.) resulted in the efficient formation of
2,5-dilithiophenyl boronate 1b-2,5Li2, as proved by its sub-
sequent reactions with electrophiles [i.e., DMF, CO2,
B(OiPr)3] leading to disubstituted phenylboronates 1i–k in
reasonable yields (Scheme 4).[15] It should be noted that the
synthesis of 1j and its analogs by the cobalt-catalyzed tri-
merization of borylated acetylenes has been previously re- 1,3,6,2-dioxazaborocane (4a).
Scheme 5. The synthesis of 6-butyl-2-[3-(2,5-dibromothienyl)]-
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(i.e., 4b-2Li) by the halogen-dance process,[18] which occurs Halogen–Lithium Exchange in Bromophenyl Boronates – a
effectively, due to relatively strong stabilization of α-thienyl Computational Approach
carbanions.[19]
We were interested to see whether the origin of the ob-
served selectivities was kinetic or thermodynamic in nature.
Our intention was to compare the results with those from
the deprotonation of 6-butyl-2-(3-thienyl)-1,3,6,2-dioxaza-
borocane with LDA, where the lithiation at the C2 position
adjacent to the borocanyl moiety was found to be thermo-
dynamically driven, but was kinetically less favored than the
competitive lithiation at C5.[4] Thus, we performed theoreti-
cal calculations on the halogen–lithium exchange involving
nBuLi, and, to simplify the calculations, the monobromin-
ated analogs of 1a–3a (2-Br, i.e., 1aЈ, 2aЈ, and 3aЈ; and 3-
Br, i.e., 1aЈЈ, 2aЈЈ, and 3aЈЈ) All calculations were performed
with the Gaussian09 program,[21] and the DFT method
(B3LYP functional)[22] was used with the 6-31+G(d)[23] ba-
sis set. Gibbs free energies were obtained from the fre-
quency calculations. As the experimental lithiations were
performed at temperatures around –80 °C, in all calcula-
tions, the temperature was set to exactly –80 °C. It is well
known that halogen–lithium exchange is hampered in non-
polar hydrocarbon solvents. Since the calculations for iso-
lated molecules resulted in high activation energies (above
130 kJmol–1) and positive product-to-substrate energy dif-
ferences,[24] the computations were performed with Thom-
asi’s polarized continuum model,[25] using the polarizable
conductor calculation model [SCRF(CPCM, solvent =
THF)]. With these parameters, the activation energies were
significantly lower, and products became more stable than
the corresponding substrates.
Scheme 6. Thermodynamic lithiation of 4a, and the regioselective
formation of 2-substituted 5-bromothiophene-3-boronic acids 4c–
d.
In the most common and frequently considered pathway,
the halogen–lithium exchange of aryl halides proceeds by
polar substitution of the nucleophile at the halogen atom.
Wittig suggested the formation of an intermediate ate com-
plex rather than SN2-like substitution.[26] Since the complex
is stabilized by electron-withdrawing groups, it was even
possible to isolate such an ate complex in the reaction of
pentafluorophenyllithium and pentafluorophenyl iodide.[27]
However, bromine is less likely than iodine to form hyperco-
ordinated structures, and in the presence of electron-donat-
ing boronate groups, such intermediates should be further
destabilized. This was also confirmed by calculations per-
formed for previously located transition-state structures
[1aЈ-2Li]‡ and [1aЈЈ-3Li]‡. These calculations showed that
the Br-centered ate complexes dissociated to the substrates.
Therefore, in our studies, we assumed that the activation
energy corresponding to SN2-like substitution is close to the
energy of ate-complex formation.
Scheme 7. Kinetic lithiation/in situ boronation of 4a.
The high susceptibility of 4a to halogen–lithium ex-
change was manifested by the generation of 6-butyl-2-
[3-(2,5-dilithiothienyl)]-1,3,6,2-dioxazaborocane 4b-2,5Li2,
which is consistent with the easy formation of the parent
2,5-dilithiothiophene from 2,5-dibromothiophene and
nBuLi.[20] Intermediate 4b-2,5Li2 was obtained in high yield
using 2.5 equiv. of nBuLi in THF/Et2O at –75 °C. Treat-
ment of 4b-2,5Li2 with selected electrophiles gave 2,5-disub-
stituted thiophene-3-boronic acids (4f–h; Scheme 8).
To choose the best model for studying the lithiation of
1a–3a, we first located the transition-state structures for the
halogen–lithium exchange of simple bromobenzene with
nBuLi in the presence of one, two, or three THF molecules
in the coordination sphere of lithium. Theoretical studies
on the mechanism of halogen–lithium exchange in PhBr
were carried out earlier by Ando.[28] In those studies, MeLi
was used as the model lithiating agent, and the activation
energy was found to be ca. 100 kJmol–1 (B3LYP/6-31G*
Scheme 8. Dilithiation of 4a and formation of 2,5-substituted thio-
phene-3-boronic acids and pinacolates 4f–h.
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Selectivity of the Br–Li Exchange in Model Dibromoaryl Boronates
level of theory, SCRF approach, THF as solvent, tempera- 1aЈ, 2aЈ, and 3aЈ were preceded by the formation of corre-
ture –90 °C). It was then concluded that coordination of sponding pre-complexes with nBuLi (i.e., pre-1aЈ-2Li, pre-
Me2O to the Li atom significantly decreased the activation 2aЈ-2Li, and pre-3aЈ-2Li). These intermediate prelithiated
energy (with two Me2O molecules, by as much as about complexes were similar in energy to the respective reactants.
45 kJmol–1), and slightly stabilized the lithiation product. The activation energies of the formation of the pre-com-
According to our results, the activation energy for the halo- plexes are 28.7, 10.6, and 9.1 kJmol–1 for 1aЈ, 2aЈ, and 3aЈ,
gen–lithium exchange of bromobenzene with nBuLi in the respectively, and are smaller than the activation energies of
presence of one molecule of THF amounts to 70.2 kJmol–1, the halogen–lithium exchange processes (46.2 kJmol–1 for
i.e., the value is slightly lower than that obtained by Ando 1aЈ, 70.3 kJmol–1 for 2aЈ, and 33.2 kJmol–1 for 3aЈ). This
(75.7 kJ/mol). The addition of the second THF molecule indicates that the halogen–lithium exchange is the rate-lim-
decreases the activation energy to 30.9 kJmol–1 iting step of the reaction. The reaction pathways and corre-
(53.5 kJmol–1 for Ando’s results), and finally, with three sponding transition-state structures are shown in Figures 2
molecules of THF, the activation energy becomes and 3.
36.8 kJmol–1. In turn, the Gibbs energies for the halogen–
The kinetically controlled regioselectivity of the halogen–
lithium exchange are equal to –16.0, –18.7, and lithium exchange in 1a and 2a is apparently dominated by
–10.0 kJmol–1 for models with one, two, and three THF the strong steric effect of the borocanyl and triisopropoxy-
molecules, respectively. The appropriate transition-state borate groups. The lithiated structures of 1aЈ-2Li and 2aЈ-
structures and reactions pathways are shown in Figure 2S 2Li are more stable than the corresponding isomeric C3-
in the Supporting Information.
lithiated derivatives (i.e., 1aЈЈ-3Li and 2aЈЈ-3Li) by about
The boronate groups can bring an alkyllithium reagent 10 kJmol–1. On the other hand, the formation of 3-Li iso-
close to the Br atom by precomplexation of the lithium mers is kinetically favored. The energy barriers are
atom with heteroatoms (O, F) bonded to the boron atom. 37.8 kJmol–1 for 1aЈЈ and 53.8 kJmol–1 for 2aЈЈ, while for
On the other hand, the subsequent halogen–lithium ex- 1aЈ and 2aЈ, they are 46.2 and 70.3 kJmol–1, respectively.
change can be sensitive to steric effects of the boronate However, the difference of only 8 kJmol–1 between the en-
groups. Based on our experience with bromobenzene, we ergy barriers of 1aЈ and 1aЈЈ indicates that both isomers
used nBuLi as the lithiating agent with a four-coordinate can be formed during the reaction. Indeed, the experiments
lithium atom (two THF molecules for lithiation at C2, and showed that halogen–lithium exchange in 1a followed by
three THF molecules in the case of C3-lithiation). The the addition of an electrophile led to the formation of two
halogen–lithium exchange reactions at the C2 position for regioisomers, whereas in the case of 2a, C3-lithiation was
Figure 2. Reaction pathways for the lithiation of a) 1aЈ; b) 1aЈЈ; c) 2aЈ; d) 2aЈЈ; e) 3aЈ; f) 3aЈЈ.
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Figure 3. Transition-state structures: a) [1aЈ-2Li]‡; b) [1aЈЈ-3Li]‡; c) [2aЈ-2Li]‡; d) [2aЈЈ-3Li]‡: e) [3aЈ-2Li]‡; f) [3aЈЈ-3Li]‡. Hydrogen atoms
are omitted for clarity.
observed exclusively. The situation was different for 3a. tion. However, the slight preference for lithiation at C5 in
Theoretical calculations showed that the BF3– group signifi- 1a suggests that the reaction is dictated by kinetics, and this
cantly stabilizes the aryllithium derivative and promotes the is strongly supported by transition-state calculations per-
metalation process (the energy difference between lithiated formed for halogen–lithium exchange reactions of related
derivatives 3aЈ-2Li and 3aЈЈ-3Li amounts to about monobrominated analogs of 1a. In the case of 2a, the lith-
15 kJmol–1). This is presumably due to the relatively small iation proceeds regioselectively at C5, apparently due to the
steric requirements of this group, and also the high elec- activation barrier for the remote lithiation being much
tronegativity of the fluorine atoms, which provides a good lower than that for the exchange of the 2-Br atom. Con-
stabilization of the transition state for 3aЈ-2Li. Therefore, versely, ortho-lithiation in 3a is strongly favored. This is
the activation barrier for the ortho-lithiation (33.2 kJmol–1) consistent with the slightly lower activation energy and the
is slightly lower than that on the pathway leading to 3aЈЈ- lower energy of formation of the ortho-lithiated phenyl tri-
3Li (37.1 kJmol–1). These findings are consistent with the fluoroborate. The reaction of the thiophene derivative 4a
experimental results, which showed that ortho-lithiation is with nBuLi revealed that lithiation at C5, i.e., at the posi-
strongly preferred for 3a.
tion remote from the borocanyl group is kinetically favored,
but that the resulting product 4b-5Li isomerizes rapidly in
the absence of an internal electrophile to give the more
stable (2-lithio-3-thienyl)boronate 4b-2Li. This shows a
close analogy with the behavior of the non-brominated ana-
log of 4a during deprotonative lithiation.[4] Studies into the
selectivity of halogen–lithium exchange in boronated di-
bromoarenes were complemented by the successful genera-
tion of boronated dilithiobenzene and dilithiothiophene
using excess tBuLi and nBuLi, respectively. These heterotri-
metallic reagents conveniently gave rise to 2,5-difunction-
alized phenyl- and thiophene-3-boronic acids or their pin-
acol esters. Currently, we are studying the metalation of
Conclusions
In conclusion, we have evaluated and compared the di-
recting properties of boron-based groups in halogen–lith-
ium exchange reactions using 1,4-dibromophenyl boronates
of three types as model substrates. Theoretical calculations
point to the increased thermodynamic stability of ortho-
lithiated phenyl boronates, which was recently also found
for related heterocyclic analogs, namely 2-lithio-3-thienyl
boronates.[4] This effect was attributed to the substantial
chelation effect involving intramolecular Li–O coordina-
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Selectivity of the Br–Li Exchange in Model Dibromoaryl Boronates
water (35 mL) at 0 °C. The mixture was stirred at room temperature
for 2 h, and the resulting slurry was filtered. The filtrate was con-
centrated to leave a solid, which was dried in vacuo at 90 °C. The
solid was extracted with boiling acetone (3ϫ 50 mL). Et2O
(200 mL) was added to the combined extracts to precipitate a solid,
which was filtered and dried in vacuo at 150 °C to give the title
other aryl and heteroaryl boronates. Specifically, our inter-
est will be focused on the observation of directing effects of
boronate-type groups in such systems.
Experimental Section
1
compound (15.5 g, 91%) as a white powder, m.p. 253–254 °C. H
NMR ([D6]acetone, 400 MHz): δ = 7.64 (d, J = 2.5 Hz, 1 H, Ph),
7.28 (d, J = 8.5 Hz, 1 H, Ph), 7.14 (dd, 1 H, Ph) ppm. 13C{1H}
NMR ([D6]acetone, 100.6 MHz): δ = 137.5, 134.3, 130.9, 127.0,
120.8 ppm. 11B NMR ([D6]acetone, 64.16 MHz): δ = 1.5 (q, J =
50 Hz) ppm. 19F NMR ([D6]acetone, 376.48 MHz): δ = –141.2 (br.)
ppm. C6H3BBr2F3K (341.80): calcd. C 21.08, H 0.88; found C
21.28, H 0.85.
General Remarks: All reactions involving air- and moisture-sensi-
tive reagents were carried out under an argon atmosphere. Et2O
and THF were stored over sodium wire before use. NMR chemical
shifts are given relative to TMS, calibrated using known chemical
shifts of residual proton (1H) or carbon (13C) solvent resonances.
In the 13C NMR spectra, the resonances of boron-bound carbon
atoms were not observed in most cases, due to their broadening by
a quadrupolar boron nucleus.
(5-Bromo-2-formylphenyl)boronic Acid (1c): nBuLi (10.0 m in hex-
ane; 1.25 mL, 12.5 mmol) was added to a stirred solution of 3a
(1.71 g, 5 mmol) in THF (50 mL) at –78 °C. After stirring for
30 min at –78 °C, the mixture become brown. Then DMF (1 mL,
13 mmol) was added, and the mixture became colorless. The mix-
ture was stirred for 30 min, and then it was hydrolyzed with H2SO4
(1.5 m aq.; 20 mL). The organic phase was separated and concen-
trated under reduced pressure. The viscous residue was triturated
with hexane (5 mL) and water (5 mL) to give a yellow solid, which
was filtered and washed with CH2Cl2 (2ϫ 5 mL). Drying in vacuo
gave a crude product containing ca 5% of 1d. Recrystallization
from toluene/acetone (5:1) gave the title compound (0.70 g, 61%)
as a white powder, m.p. 161–162 °C. 1H NMR ([D6]acetone,
400 MHz): δ = 10.20 (s, 1 H, CHO), 7.95 (d, J = 1.5 Hz, 1 H, Ph),
7.87 (d, J = 8.5 Hz, 1 H, Ph), 7.81 [br., 2 H, B(OH)2], 7.79 (dd, 2
H, Ph) ppm. 13C{1H} NMR ([D6]acetone, 100.6 MHz): δ = 194.7,
139.7, 137.9, 133.6, 133.3, 131.4, 129.0 ppm. 11B NMR ([D6]acet-
one, 64.16 MHz): δ = 28 ppm. C7H6BBrO3 (228.84): calcd. C 36.74,
H 2.64; found C 38.78, H 3.14.
(2-Bromo-5-formylphenyl)boronic Acid (1d): A freshly prepared
solution of LDA [diisopropylamine (4.84 g, 4.4 mL, 30 mmol) and
nBuLi (10 m; 3 mL, 30 mmol) in THF (30 mL)] was added dropwise
to a solution of 1,4-dibromobenzene (7.05 g, 30 mmol) in THF
(70 mL) containing B(OiPr)3 (5.64 g, 7.15 mL, 30 mmol) at –75 °C.
The resulting white slurry was stirred for ca 30 min at –75 °C, and
then nBuLi (10 m, 6.1 mL, 61 mmol) was added dropwise. The mix-
ture was stirred for 1 h, and then it was quenched with DMF (7.3 g,
0.1 mol) followed by hydrolysis with H2SO4 (1.5 m aq., 50 mL). The
aqueous phase was separated and then extracted with Et2O (2ϫ
15 mL). The extracts were added to the organic phase, which was
concentrated under reduced pressure. The solid residue was filtered
and washed consecutively with water (2 ϫ 10 mL) and CH2Cl2
(5 mL). Drying in vacuo gave the title compound (4.3 g, 63%) as a
white powder, m.p. 122–124 °C. 1H NMR ([D6]acetone, 400 MHz):
δ = 10.02 (s, 1 H, CHO), 8.02 (d, J = 1.6 Hz, 1 H, Ph), 7.67 (2 H,
Ph) ppm. 13C{1H} NMR ([D6]acetone, 100.6 MHz): δ = 192.3,
136.9, 136.4, 135.6, 133.6, 133.4, 131.4 ppm. 11B NMR ([D6]acet-
one, 64.16 MHz): δ = 28 ppm. C7H6BBrO3 (228.84): calcd. C 36.74,
H 2.64; found C 36.78, H 2.74.
2-(2,5-Diformylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1i):
tBuLi (1.7 m in pentane; 26.5 mL, 45 mmol) was added to THF
(50 mL) at –70 °C, and the resulting yellow solution was cooled to
–90 °C. Then a solution of 1a (4.04 g, 10 mmol) in THF (10 mL)
was added while the temperature was kept below –85 °C. The re-
sulting mixture was stirred for 20 min at –85 °C, then it was cooled
to –90 °C, and DMF (4 mL, 50 mmol) was added. The resulting
white slurry was stirred for 30 min and hydrolyzed with H2SO4
(1.5 m aq.; 20 mL). The organic phase was separated, and the sol-
vents were evaporated under reduced pressure. The viscous residue
was treated with pinacol (1.34 g, 12 mmol) in Et2O (10 mL). The
solvent was evaporated to give a crude solid product, which was
filtered and washed with water (2ϫ 5 mL). It was purified by
recrystallization from hexane (15 mL) to give the title compound
(1.54 g, 59%) as crystals, m.p. 98–99 °C. 1H NMR (CDCl3,
400 MHz): δ = 10.67 (s, 1 H, CHO), 10.13 (s, 1 H, CHO), 8.39 (s,
1 H, Ph), 8.10 (d, J = 8.0 Hz, 1 H, Ph), 8.07 (d, 1 H, Ph), 1.41 (s,
12 H, CH3) ppm. 13C{1H} NMR (CDCl3, 100.6 MHz): δ = 193.9,
191.7, 145.0, 138.4, 137.7, 131.0, 128.1, 84.9, 24.9 ppm. 11B NMR
(CDCl3, 64.16 MHz): δ = 31 ppm. C14H17BO4 (260.09): calcd. C
64.65, H 6.59; found C 64.35, H 6.51.
(4-Bromo-1,3-phenylene)diboronic Acid (1g): This compound was
prepared as described for 1d using an excess of B(OEt)3 (7.5 g,
32 mmol) as the electrophile. The product (5.5 g, 74%) was isolated
as a white powder, m.p. Ͼ 300 °C. 1H NMR ([D6]acetone,
400 MHz): δ = 8.00 (d, J = 1.5 Hz, 1 H, Ph), 7.71 (dd, J = 8.0 Hz,
1 H, Ph), 7.50 (d, 1 H, Ph), 7.34 [br., 2 H, B(OH)2], 7.31 [br., 2 H,
B(OH)2] ppm. 13C{1H} NMR ([D6]acetone, 100.6 MHz): δ = 141.4,
137.2, 131.8, 128.7 ppm. 11B NMR ([D6]acetone, 64.16 MHz): δ =
28 ppm. C6H7B2BrO4 (244.64): calcd. C 29.46, H 2.88; found C
29.84, H 3.00.
(2-Bromo-5-carboxyphenyl)boronic Acid (1h): A solution of lithiate
2b-5Li was prepared as described for 1d. It was cooled to –100 °C
and then carboxylated by passing through a stream of dried gas-
eous CO2 with rapid stirring. The resulting white slurry was left to
warm up to ca 0 °C to evaporate the excess of CO2, and then a
careful hydrolysis was undertaken with H2SO4 (2 m aq.; 50 mL).
The work-up was performed as described for 1d to give the title
1
compound (5.95 g, 81%) as a white powder, m.p. 230–232 °C. H
NMR ([D6]acetone, 400 MHz): δ = 11.35 (br., 1 H, CO2H), 8.15
(d, J = 2.0 Hz, 1 H, Ph), 7.87 (dd, J = 8.0 Hz, 1 H, Ph), 7.66 (d, 1
H, Ph), 7.57 [br., 2 H, B(OH)2] ppm. 13C{1H} NMR ([D6]acetone,
100.6 MHz): δ = 167.9, 136.0, 132.8, 132.1, 131.6, 129.7 ppm. 11B
Benzene-1,2,4-triboronic Tris(pinacolate) (1j): This compound was
prepared as described for 1i using an excess of B(OiPr)3 as the
electrophile. The crude benzene-1,2,4-triboronic acid was treated
with pinacol (3.6 g, 30 mmol) in acetone (15 mL). The solvent was
removed in vacuo, and the crude product was washed with water
(2ϫ 5 mL) and recrystallized from hexane to give the title com-
pound (3.53 g, 77%) as a white powder, m.p. 214–215 °C. 1H NMR
(CDCl3, 400 MHz): δ = 8.08 (s, 1 H, Ph), 7.79 (dd, J = 7.0, 1.5 Hz,
NMR ([D6]acetone, 64.16 MHz):
δ = 29 ppm. C7H6BBrO4
(244.84): calcd. C 34.34, H 2.47; found C 34.46, H 2.57.
Potassium (2,5-Dibromophenyl)trifluoroborate (3a): A solution of
(2,5-dibromophenyl)boronic acid (14.0 g, 0.05 mol) in MeOH
(50 mL) was treated with a solution of KHF2 (13.5 g, 0.17 mol) in
Eur. J. Org. Chem. 2013, 3023–3032
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3029

K. Durka, S. Lulin´ski et al.
NMR ([D6]acetone, 64.16 MHz): δ = 27 ppm. C5H4BBrO3S
FULL PAPER
1 H, Ph), 7.61 (dd, J = 1.0 Hz, 1 H, Ph), 1.36 (s, 24 H, Pin), 1.33
(s, 12 H, Pin) ppm. 13C{1H} NMR (CDCl3, 100.6 MHz): δ = 139.5,
135.3, 132.4, 83.85, 83.78, 83.66, 24.92, 24.87, 24.84 ppm. 11B
NMR (CDCl3, 64.16 MHz): δ = 31 ppm. C24H39B3O6 (456.00):
calcd. C 63.21, H 8.62; found C 62.39, H 8.49.
(234.87): calcd. C 25.57, H 1.72; found C 25.31, H 1.71.
5-Bromothiophene-2,3-diboronic Acid (4d): A solution containing
lithiate 4b-2Li (obtained as described for 4c) was quenched with a
solution of B(OMe)3 (1.2 g, 11.5 mmol) in Et2O (5 mL). Work-up
was carried out as described for 4c. The title compound (1.8 g,
(2,5-Dicarboxyphenyl)boronic Acid (1k): A mixture containing lithi-
ate 1b-2,5Li2 was prepared from 1a (2.46 g, 6.0 mmol) as described
for 1i. It was cooled to –110 °C, and then it was carboxylated by
passing through a stream of dried gaseous CO2 with rapid stirring.
The resulting white slurry was left to warm up to ca 0 °C to evapo-
rate the excess of CO2. Then it was subjected to careful hydrolysis
with H2SO4 (2 m aq.; 30 mL). The work-up was performed as de-
scribed for 1i to give the title compound (0.85 g, 69%) as a white
powder M.p. Ͼ 300 °C (dec). 1H NMR ([D6]acetone + D2O,
400 MHz): δ = 8.12 (b, 1 H, Ph), 7.94 (d, J = 8.0 Hz, 1 H, Ph), 7.82
(1 H, Ph) ppm. 13C{1H} NMR ([D6]acetone + D2O, 100.6 MHz): δ
= 171.3, 168.4, 146.2, 138.7, 133.8, 131.7, 129.7, 127.1 ppm. 11B
NMR ([D6]acetone + D2O, 64.16 MHz): δ = 28 ppm. C8H7BO6
(209.95): calcd. C 45.77, H 3.36; found C 45.73, H 3.61.
1
72%) was obtained as a white powder, m.p. 114–115 °C. H NMR
([D6]acetone, 400 MHz): δ = 8.33 [s, 2 H, B(OH)2], 8.24 [s, 2 H,
B(OH)2], 7.57 (s, 1 H, Th) ppm. 13C{1H} NMR ([D6]acetone,
100.6 MHz): δ = 139.5, 116.6 ppm. 11B NMR ([D6]acetone,
64.16 MHz): δ = 27 ppm. C4H5B2BrO4S·H2O (268.69): calcd. C
17.88, H 2.63; found C 18.26, H 2.71.
5-Bromothiophene-2,4-diboronic Acid (4e): A solution of 4a (4.11 g,
10 mmol) and B(OiPr)3 (2.1 g, 11.2 mmol) in THF (25 mL) was
added to a stirred solution of nBuLi (1.6 m in hexane; 6.5 mL,
10.4 mmol) at –85 °C. After ca 15 min stirring at ca –80 °C (in-
ternal temperature), work-up was carried out as described for 4c
to give crude product 4e containing ca 20% of isomeric compound
4d. It was purified by the addition of Et2O/CH2Cl2/hexane (1:1:1,
30 mL). The resulting suspension was stirred for 30 min and then
filtered to give 4e (1.7 g, 66%) as a white powder, m.p. Ͼ 400 °C.
1H NMR ([D6]acetone, 400 MHz): δ = 7.68 (s, 1 H, Th), 7.45 [s, 2
H, B(OH)2], 7.18 [s, 2 H, B(OH)2] ppm. 13C{1H} NMR ([D6]-
acetone, 100.6 MHz): δ = 143.1, 125.3 ppm. 11B NMR ([D6]-
acetone, 64.16 MHz): δ = 27 ppm. C4H5B2BrO4S·H2O (268.69):
calcd. C 17.88, H 2.63; found C 18.30, H 2.66.
6-Butyl-2-[3-(2,5-dibromothienyl)]-1,3,6,2-dioxazaborocane (4a): A
solution of thiophene-3-boronic diethyl ester (36 g, 0.2 mol) in
CH2Cl2 was cooled to –60 °C, and then a solution of Br2 (64 g,
0.2 mol) in CH2Cl2 was added dropwise. The temperature was kept
below –50 °C during the addition. The resulting yellow solution
was warmed up to 0 °C to complete the reaction. Then it was co-
oled to –60 °C and diluted with Et2O (100 mL), and then Et3N
(42 g, 0.42 mol) was added dropwise. The resulting thick slurry was
diluted with hexane (100 mL) and filtered under an argon atmo-
sphere. The white precipitate of the ammonium salt by-product
(Et3NHBr) was washed with Et2O. The yellow filtrate was concen-
trated to give an oily residue, which was subjected to fractional
vacuum distillation to give crude 2,5-dibromothiophene-3-boronic
diethyl ester (55 g, 0.161 mol), b.p. 135–140 °C. This material was
dissolved in Et2O, and a solution of N-butyldiethanolamine (27 g,
0.168 mol) in Et2O (80 mL) was added in one portion. The forma-
tion of a white crystalline precipitate was observed after a few min-
utes. The resulting slurry was cooled to –50 °C and filtered. The
solid was washed with Et2O (2ϫ 50 mL) and dried in vacuo to give
the final product (56 g, 68%), m.p. 86–88 °C. 1H NMR (CDCl3,
400 MHz): δ = 7.01 (s, 1 H, Th), 4.08 (m, 4 H, CH2O), 3.11 (m, 2
2,5-Dicarboxythiophene-3-boronic Acid (4f): A solution of 4a
(4.11 g, 10 mmol) in THF (20 mL) was added to a stirred solution
of nBuLi (10 m; 2.2 mL, 22 mmol) in THF (30 mL) at –80 °C. After
ca 30 min stirring at ca –75 °C (internal temperature), the solution
of lithiate 4b-2,5Li2 was cooled to –110 °C and then carboxylated
by passing through a stream of dried gaseous CO2 with rapid stir-
ring. The resulting white slurry was left to warm up to ca 0 °C to
evaporate the excess of CO2, and then careful hydrolysis with
H2SO4 (2 m aq.; 15 mL) was undertaken. Further work-up was car-
ried out as described for 4c to give the title compound (2.0 g, 92%)
as a white powder, m.p. Ͼ 320 °C (dec). 1H NMR ([D6]acetone,
400 MHz): δ = 8.08 (s, 1 H, Th) ppm. 13C{1H} NMR ([D6]DMSO,
100.6 MHz): δ = 166.9, 163.1, 144.9, 141.7, 139.1 ppm. 11B NMR
([D6]DMSO, 64.16 MHz): δ = 28 ppm. C6H5BO6S (215.98): calcd.
C 33.37, H 2.33; found C 33.69, H 2.66.
H, CH2N), 3.04 (m,
NCH2CH2CH2CH3), 1.54 (m, 2 H, NCH2CH2CH2CH3), 1.22 (m,
H, NCH2CH2CH2CH3), 0.87 (t, 7.0 Hz, H,
2
H, CH2N), 2.57 (m,
2
H,
2,5-Diformylthiophene-3-boronic Acid (4g): A solution containing
dilithiate 4b-2,5Li2 (obtained as described for 4f) was quenched
with a solution of DMF (2.2 g, 30 mmol) in Et2O (10 mL). Further
work-up was carried out as described for 4c to give the title com-
pound (1.6 g, 87%) as a pale pink powder, m.p. 148–150 °C (dec).
1H NMR (400 MHz, [D6]acetone): δ = 10.46 (s, 1 H, CHO), 10.08
(s, 1 H, CHO), 8.29 (s, 1 H, Th), 8.08 [br, 2 H, B(OH)2] ppm. 13C
NMR ([D6]acetone, 100.6 MHz): δ = 187.6, 185.3, 156.0, 148.8,
143.9 ppm. 11B NMR ([D6]acetone, 64.16 MHz): δ = 27 ppm.
C6H5BO4S·0.5H2O (192.99): calcd. C 37.34, H 3.13; found C 37.48,
H 3.04.
2
J
=
3
NCH2CH2CH2CH3) ppm. 13C NMR (CDCl3, 100.6 MHz): δ =
136.7, 115.0, 109.4, 62.8, 58.5, 57.6, 26.8, 20.2, 13.7 ppm. 11B NMR
(CDCl3, 64.16 MHz): δ = 11 ppm. C12H18BBr2NO2S (410.96):
calcd. C 35.07, H 4.41, N 3.41; found C 35.40, H 4.64, N 3.48.
(5-Bromo-2-formyl)thiophene-3-boronic Acid (4c): A solution of 4a
(4.12 g, 10 mmol) in THF (15 mL) was added to a stirred solution
of nBuLi (10 m; 1.0 mL, 10 mmol) in THF (15 mL) at –85 °C. After
ca 15 min stirring at ca. –80 °C (internal temperature), the mixture
containing lithiate 4b-2Li was quenched with DMF (1.1 g,
15 mmol) in Et2O (5 mL). The mixture was stirred for 30 min, and
then it was hydrolyzed with sulfuric acid (1.5 m aq.; 10 mL). The
aqueous phase was separated, then it was extracted with diethyl
ether (2ϫ 15 mL). The extracts were added to the organic phase,
which was concentrated under reduced pressure. The crude product
was washed with water (3ϫ 10 mL) and CH2Cl2 (10 mL) to give
the title compound (1.5 g, 64%) as a pale yellow powder, m.p.
2-[2,5-Bis(tert-butylcarboxamido-3-thienyl)]-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (4h): A solution containing dilithiate 4b-2,5Li2
(obtained as described for the synthesis of 4f) was quenched with
a solution of tBuNCO (2.2 g, 22 mmol) in Et2O (20 mL) at –90 °C.
The mixture was stirred for 30 min at –75 °C and then hydrolyzed
with H2SO4 (2 m aq.; 15 mL). The aqueous phase was separated
and then extracted with diethyl ether (2ϫ 15 mL). The extracts
were added to the organic phase, which was concentrated under
reduced pressure to give a viscous residue. Pinacol (1.2 g, 10 mmol)
and hexane (15 mL) were added, and the mixture was stirred for
1
Ͼ 155 °C (dec). H NMR (400 MHz, [D6]acetone): δ = 10.21 (s, 1
H, CHO), 8.00 [br, 2 H, B(OH)2], 7.56 (s, 1 H, Th) ppm. 13C NMR
([D6]acetone, 100.6 MHz): δ = 185.5, 152.8, 139.3, 122.8 ppm. 11B
3030
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 3023–3032

Selectivity of the Br–Li Exchange in Model Dibromoaryl Boronates
1 h at 40 °C. The resulting white suspension was filtered. The crude
product was washed with water and hexane (10 mL), dried, and
recrystallized from hexane/CH2Cl2 (1:1, 20 mL) to give 4h (2.6 g,
64%) as a white powder, m.p. 248–250 °C. 1H NMR (400 MHz,
[D6]acetone): δ = 8.61 (br., 1 H, NH), 7.77 (s, 1 H, Th), 7.42 (br.,
1 H, NH), 1.43 (s, 9 H, tBu), 1.42 (s, 9 H, tBu), 1.40 (s, 12 H, Pin)
ppm. 13C NMR (CDCl3, 100.6 MHz): δ = 161.4, 161.2, 156.7,
144.6, 135.4, 86.1, 52.4, 52.3, 28.9, 28.8, 25.1 ppm. 11B NMR
(CDCl3, 64.16 MHz): δ = 26 ppm. C20H33BN2O4S (408.37): calcd.
C 58.82, H 8.15, N 6.86; found C 58.79, H 7.87, N 6.80.
03/B/ST5/02755. The authors thank the Interdisciplinary Centre
for Mathematical and Computational Modelling in Warsaw for
providing computational facilities (G33-14). Support from the Ald-
rich Chemical Co., Milwaukee, WI, U.S.A., through continuous do-
nation of chemicals and equipment is gratefully acknowledged.
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Sansano, M. Yus, Tetrahedron 2003, 59, 9255–9303.
X-Ray Data: Suitable crystals were obtained by slow evaporation
of an acetone solution of 1g. Single crystal X-ray measurements of
1g were performed with graphite-monochromated Mo-Kα radiation
using a Kuma KM4CCD κ-axis diffractometer equipped with an
Oxford Cryosystems nitrogen gas-flow apparatus. Data reduction
and analysis were carried out with the Oxford Diffraction Ltd. suite
of programs.[29] All structures were solved by direct methods using
SHELXS-97 and refined using SHELXL-97.[30] The refinement
was based on F2 for all reflections except those with very negative
F2. All non-hydrogen atoms were refined anisotropically.
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1g: C6H7B2BrO4; molecular weight, 244.64; T = 100(2) K; mono-
clinic space group Cc; unit cell dimensions, a = 28.918(3), b =
3.770(1), c = 16.676(2) Å, α = 90, β = 109.88(1), γ = 90°, V =
1709.7(3) Å3; Z = 4; dcalc = 1.901 gcm–3; absorption coefficient μ
= 4.781 mm–1; F(000) = 960; crystal size, 0.15 ϫ 0.12 ϫ 0.02 mm;
τ range for data collection: 2.29–33.93°; index ranges: –39 Ͻ h Ͻ
39, –5 Ͻ k Ͻ 5, –22 Ͻ l Ͻ 22; reflections collected: 8066; unique:
3746 (Rint = 0.0615); absorption correction – multi-scan; refine-
ment method – full-matrix least-squares on F2; goodness-of-fit on
F2, GooF = 1.062; data/restraints/parameters 3746/2/235; final R
indices [I Ͼ 2σ(I)]: R1 = 0.0410, wR2 = 0.0660; R indices (all data):
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CCDC-917887 contains the supplementary crystallographic data
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Acknowledgments
This work was supported by the Narodowe Centrum Nauki
(National Science Centre) in the framework of project DEC-2011/
Eur. J. Org. Chem. 2013, 3023–3032
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3031

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Received: January 28, 2013
Published Online: April 3, 2013
3032
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Eur. J. Org. Chem. 2013, 3023–3032