Substituent effect on benzylic lithiation of sulfides. Synthesis of diboronic acids derived from aryl-alkyl sulfides

Article abstract of DOI:10.1016/j.tet.2013.02.075

The relative benzylic deprotonation rate constants of aryl-benzyl sulfides have been measured and the obtained values were compared with the substituent constants using the Hammett equation and with deprotonation Gibbs energies calculated on B3LYP/aug-cc-pVDZ level. The deprotonation rate depends on the stabilization of the negative charge, which is spread over the benzene ring. The series of brominated alkyl-aryl sulfides was di-lithiated by Br/Li exchange using t-BuLi and the obtained organolithium compounds were converted into the respective diboronic acids. Lithiation of aryl-benzyl sulfides containing a bromine atom in the para or ortho positions occurs selectively, however for the meta derivative, the process is plagued by competitive benzylic deprotonation. This fact can be rationalized on the basis of the obtained relative deprotonation rate constants. The X-ray analysis of bis-(2- dihydroxyborylphenylthio)methane revealed the existence of three different structural motifs, which stabilize the structure by hydrogen bonding formation.

Full text of DOI:10.1016/j.tet.2013.02.075

Tetrahedron 69 (2013) 3159e3166
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Substituent effect on benzylic lithiation of sulfides. Synthesis
of diboronic acids derived from arylealkyl sulfides
a
a
,
a
ꢀa
*
Marek Da˛browski , Krzysztof Durka , Tomasz Klis , Janusz Serwatowski ,
b
ꢀ
Krzysztof Wozniak
a Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland
b Warsaw University, 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:
The relative benzylic deprotonation rate constants of arylebenzyl sulfides have been measured and the
obtained values were compared with the substituent constants using the Hammett equation and with
deprotonation Gibbs energies calculated on B3LYP/aug-cc-pVDZ level. The deprotonation rate depends
on the stabilization of the negative charge, which is spread over the benzene ring. The series of bro-
minated alkylearyl sulfides was di-lithiated by Br/Li exchange using t-BuLi and the obtained organo-
lithium compounds were converted into the respective diboronic acids. Lithiation of arylebenzyl sulfides
containing a bromine atom in the para or ortho positions occurs selectively, however for the meta de-
rivative, the process is plagued by competitive benzylic deprotonation. This fact can be rationalized on
the basis of the obtained relative deprotonation rate constants. The X-ray analysis of bis-(2-
dihydroxyborylphenylthio)methane revealed the existence of three different structural motifs, which
stabilize the structure by hydrogen bonding formation.
Received 25 September 2012
Received in revised form 8 February 2013
Accepted 22 February 2013
Available online 26 February 2013
Keywords:
Lithiation
Boronic acids
Sulfides
Hammett equation
Ab initio calculations
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Gibbs energy obtained using quantum-chemical calculations per-
formed on B3LYP/aug-cc-pVDZ level of theory.
Boronic acids are reactive intermediates, which take part in
many valuable reactions in organic synthesis.1 Recently there has
been significant interest in the application of diboronic acids as the
components of polymeric porous materials for gas absorption.2
Although the first results of these studies are promising, the
broad range of diboronic acids needs to be tested to obtain high
quality materials. In this work we describe an effective method to
prepare diboronic acids by di-lithiation of alkylearyl sulfides.
However, the stabilizing effect of sulfur may result in the lowering
of the lithiation selectivity as a result of competitive deprotonation
at the aliphatic position.3 Our recent studies revealed that this
process is especially important in the lithiation of arylebenzyl
sulfides,4 but can also become dominant in the lithiation of
arylebenzyl ethers containing an electronegative substituent in the
phenyl ring.5 Thus, we decided to precede the preparation of
diboronic acids with kinetic studies. The relative rate constants for
the benzylic deprotonation of substituted arylebenzyl sulfides
were correlated with the structure using the Hammett equation.
The obtained results were next compared with the deprotonation
2. Results and discussion
2.1. Kinetic experiments
In order to perform the kinetic studies we decided to apply the
protocol where the THF solution containing 1 equiv of benzyle-
phenyl sulfide BPS (used as the reference) and 1 equiv of
substituted benzylephenyl sulfide XBPS was treated at ꢀ78 ꢁC with
1 equiv of LDA in the presence of excess (CH3)3SiCl as the in situ
electrophile. This procedure consisted of two steps, which involved
benzylic deprotonation of the starting material followed by the
reaction of the obtained benzyllithium derivative with an electro-
phile. In order to apply this methodology successfully, the depro-
tonation should be the rate determining step and the formed
benzyllithiums cannot undergo any side reactions. Our initial ex-
periments revealed that addition of (2-chlorobenzyl)phenyl sulfide
to the solution of a-(phenylthio)benzyllithium followed by silyla-
tion with Me3SiCl afforded the mixture of silylated sulfides
(Scheme 1).
We believed that determination of the isotope effect could
provide some evidence concerning the stability of benzyllithiums
obtained after deprotonation. Thus, in the lithiation of 1, an
* Corresponding author. Tel.: þ48 22 2347575; fax: þ48 22 6282741; e-mail
ꢀ
address: ktom@ch.pw.edu.pl (T. Klis).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.02.075

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M. Da˛browski et al. / Tetrahedron 69 (2013) 3159e3166
Scheme 1. Equilibration between benzylephenyl sulfides. Reaction conditions: THF, ꢀ78 ꢁC, Me3SiCl quench after 0.5 h.
!
ꢀ
ꢁ
ꢀ
ꢁ
c0x ꢀ cx
c0h ꢀ ch
intermolecular isotope effect was measured for the reaction of 1
and 1-d2 by the ratio of conversion of 1 to 2 and 1-d2 to 2-d1 upon
reaction with a deficient amount of LDA and an excess of Me3SiCl as
the in situ electrophile (Scheme 2). The integration of the 1H NMR
spectrum obtained after the hydrolytic workup of the reaction
mixture (CHbenzylic 4.00 ppm, ꢀSi(CH3)3 0.14 ppm) revealed that the
signals ratio corresponds with the selective formation of 2. In our
experiment, little deuterium was removed from 1-d2, rendering the
precise measurement of the amount of 2-d1. The non-measurable
concentration of 2-d1 in the reaction mixture suggests that the
intermolecular isotopic effect is very high, however, using 1H NMR
spectroscopy the quantitative evaluation is not possible. This result
is in agreement with the one-step mechanism of the lithiation via
a single transition state.6 The intermolecular isotope effect was next
examined using the sequential procedure where the equimolar
mixture of 1 and 1-d2 was reacted with a deficient amount of LDA at
ꢀ78 ꢁC. However, the quench with Me3SiCl was conducted after
5 min. Interestingly, in this case the amount of 2-d1 increased
significantly. The ratio of 2/2-d1¼10.3 can be rationalized on the
basis of the equilibration between 1-Li and 1-d2, which results with
the formation of 1-d1-Li and 1-d1. The fact that 2-d1 was not
detected in the reaction with Me3SiCl as the in situ electrophile
suggests that the equilibration process is slower than the quench
with Me3SiCl.
kx
kh
ch
cx
¼
(1)
The relatively large positive
r
value indicates that electrons are
moved toward the benzene ring in the transition state. This effect
corresponds with the stabilization of the benzylic carbanion
formed in the rate determining step. The rate of deprotonation is
a result of the competition between the inductive and conjugative
effect caused by the substituent. The highly electronegative fluo-
rine atom is the
s acceptor, however, it acts simultaneously as a p-
donor, and the resonance structure carrying the negative charge at
the carbon atom bonded to fluorine does not contribute much to
the electronic delocalization. Thus, the effect caused by a fluorine
atom in the para position is responsible for the lowering of the rate
of deprotonation in comparison to the derivative containing
a fluorine atom in the meta position where only
s polarization
works.10 The interplay between the acceptor properties of fluorine
and the lone pair/lone pair repulsion is evident for the ortho isomer,
for which rate of deprotonation is retarded by 50% in comparison to
the meta derivative and increased drastically in comparison to the
para-compound. On the other hand, the donation of
density from heavier halogens is much weaker (poor en overlap):
as a result the rate of deprotonation for para-halogenated de-
rivatives increases in the following order: kF<kCl<kBr. The effect of
p-electron
p
Scheme 2. Competitive reactions of benzylephenyl sulfide 1 and dideuterated benzylephenyl sulfide 1-d2 with LDA and Me3SiCl.
Once more, competition kinetic measurements were carried out
to determine the relative rate constants of the benzylic deproto-
nation. In order to calculate the relative rate constants between
XBPS and BPS (kx/kh), the second order kinetic equation was ap-
plied7 (Eq. 1) where cox and coh are the initial concentrations of
XBPS and BPS, respectively, and cx and ch are the remaining con-
centrations of XBPS and BPS. The calculated values of the relative
rate constants are collected in Table 1. The correlation between
pen overlap exhibited by the eOCH3 group in the para position is
especially high resulting in a deprotonation rate lower than that for
reference compound. On moving the eOCH3 group to the meta
position, the
pen interaction is impeded and only the weak neg-
ative inductive effect works. The eCF3 group is a potent
s
and
p
acceptor, however, it is much more effective as a long range elec-
tron withdrawing substituent.11 The effect of the eCF3 group on
deprotonation is highest for the para-isomer, whereas the rates of
deprotonation for the meta and ortho isomer are retarded by 40%
and 80%, respectively. In the case of the meta isomer only the
lg (kx/kh) and substituent constants (
the straight line (Fig. 1) with the gradient
s
)8 can be approximated with
r
¼3.9.9

M. Da˛browski et al. / Tetrahedron 69 (2013) 3159e3166
3161
Table 1
3. Computational studies
Relative rate constants kx/kh calculated with the amounts of two competing sub-
strates (x¼PhSCH2(XC6H4), h¼PhSCH2C6H5) after their simultaneous reaction with
LDA and Me3SiCl in situ in THF at ꢀ78 ꢁC and deprotonation enthalpies and Gibbs
energies [in kcal molꢀ1] calculated in the framework of the B3LYP density functional
Although the metalation reaction normally occurs ‘irreversibly’,
the knowledge of the equilibrium acidity of selected hydrogen
atoms can provide helpful information about substituent effects.
However, the experimental techniques, which usually involve gas-
phase methods, often suffer from inaccuracy and cannot be applied
to non-volatile substances.13 On the other hand quantum-chemical
computations give us tools for the comparison of the hydrogen
acidity within the same group of compounds. As shown by
Schlosser et al. acidity correlates with the calculated heterolytical
proton dissociation energy (deprotonation energies).14 Such an
approach has been recently successfully applied in the studies of
the substituent direction effects in the lithiation of boronated
thiophenes.15 It should be stressed that the values of deprotonation
energies do not correspond to the real energies of lithiation pro-
cesses and are only used for the comparison of acidities. The cal-
culations have been performed on B3LYP16/aug-cc-pVDZ17 level of
theory and all obtained values are deposited in Table 1. The corre-
lation of the obtained values with the lg kx/kh (Fig. 2) reflects the
general trend obtained using the Hammett equation: the increase
in deprotonation rate corresponds with the increase in acidity of
the benzylic hydrogen atoms (measured as deprotonation Gibbs
energy). However, in the series of meta substituted ABS’s the
deprotonation Gibbs energy value for a fluorine substituent de-
viates from the monotonic decrease observed for the other sub-
stituents. We are far from attributing these deviations to any
electronic effects because we realize that the extended calculations
including the other methods or basis sets should be performed in
order to justify the obtained deprotonation enthalpies.
Arylebenzyl sulfide
kx/kh
D
H/kcal molꢀ1
D
G/kcal molꢀ1
PhSCH2(4-FC6H4)
PhSCH2(3-FC6H4)
PhSCH2(2-FC6H4)
1.7
18.8
9.5
389.9
386.4
387.5
387.6
387.2
386.4
385.7
386.3
387.1
384.4
386.3
386.2
383.4
382.2
398.9
392.9
393.8
394.7
397.8
401.0
382.1
378.7
379.8
379.8
379.5
378.7
378.0
378.6
379.4
376.7
378.6
378.5
375.7
374.6
390.9
385.1
385.9
386.8
389.8
392.4
PhSCH2(4-ClC6H4)
PhSCH2(3-ClC6H4)
PhSCH2(2-ClC6H4)
PhSCH2(4-BrC6H4)
PhSCH2(3-BrC6H4)
PhSCH2(2-BrC6H4)
PhSCH2(4-CF3C6H4)
PhSCH2(3-CF3C6H4)
PhSCH2(2-CF3C6H4)
PhSCH2(3-CNC6H4)
PhSCH2(2-CNC6H4)
PhSCH2(4-CH3OC6H4)
PhSCH2(3-CH3OC6H4)
PhSCH2(4-CH3C6H4)
PhSCH2(4-BNC6H4)a
PhSCH2(3-BNC6H4)a
PhSCH2(2-BNC6H4)a
13.4
36.3
23.7
15.2
64.3
22.1
100.5
58.2
20.2
143.3
324.0
0.05
1.1
0.3
0.5
0.1
y0b
a
BN¼6-butyl-(N-B)-1,3,6,2-dioxazaborocane.
b
The low conversion of PhSCH2(2-BNC6H4) resulted in a high uncertainty when
measuring the concentration precisely.
Fig. 1. The Hammett plot for benzylic deprotonation of a series of substituted ben-
zylephenyl sulfides (XBPS).
Fig. 2. A plot of the relative Gibbs energies for deprotonation of XPBS versus the
relative rate constant of deprotonation.
4. Synthesis of diboronic acids
electron withdrawing effect works, whereas the rate decrease for
the ortho isomer is probably due to the steric hindrance, which is
another important factor modulating deprotonations of eCF3
substituted arenes. The highest rates of deprotonation were ob-
served for the ABS containing a cyano group. This substituent ex-
hibits a significant electron withdrawing property, however, the
We next embarked on a study of di-lithiation via Br/Li exchange
of a series of sulfides composed of two bromobenzene fragments
linked via various carbonesulfur chains: (eCH2eSeCH2e,
eSeCH2eSe, eSe(CH2)3eSe, eSe(CH2)2eSe, eSeCH2e). Based
on our previous results we expected that hydrogen atoms bonded
to aliphatic carbon atoms in the studied compounds will be po-
tentially reactive toward deprotonation. Thus, lithiation/boronation
of 2aec was carried out in THF below ꢀ70 ꢁC using 4 equiv of t-BuLi.
The usual workup afforded the respective diboronic acids 3a and 3c
in high yields, whereas for 3b the yield was decreased to 67%
(Scheme 3). We attribute this result to competitive benzylic
deprotonation. This is in agreement with the previous kinetic
studies, which revealed that benzylic deprotonation is accelerated
when the bromine atom is placed in the meta position. Although
the benzylic positions in 2aeb are also susceptible to
conjugation of the
p orbital in this group with the p system of the
benzene allows the extension of the negative charge toward ni-
trogen leading to additional stabilization. Recently we have re-
ported the deactivating effect of the 6-butyl-(N-B)-1,3,6,2-
dioxazaborocane substituent on benzylic lithiation.12 The ob-
tained values of the relative deprotonation rate constants revealed
that this substituent can act as a
s donor, decreasing the rate of
benzylic deprotonation in comparison to the reference compound,
however, the steric hindrance seems to be the important factor
modulating the reaction rate (Table 1).

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M. Da˛browski et al. / Tetrahedron 69 (2013) 3159e3166
deprotonation, the Br/Li exchange occurs fast affording the re-
spective aryllithium compounds. However, when the lithiation/
borylation of 2b was performed in diethyl ether, the yield of 3b was
similar as that for 3a and 3c.
This approach using t-BuLi/B(OEt)3 in THF was thought to be
effective in the preparation of diboronic acids 9 and 11aec. We
expected that 8 and 10aec containing eCH2CH2e or eCH2CH2CH2e
linkers would give stable organo-dilithium intermediates. Thus,
Scheme 3. The synthesis of 3aec.
A similar protocol applied to 4aec resulted in formation of the
respective diboronic acids 5a and 5c in high yields, whereas 5b was
obtained in only 58% yield. The explanation of the yield decrease
relies on the competitive benzylic deprotonation as for the lith-
iation of 2b. The replacement of THF with diethyl ether resulted in
the quantitative formation of 5b (Scheme 4). We turned next our
attention to lithiation of 6aec (Scheme 5). As the methylene group
adjacent to the two sulfur atoms in these compounds is highly
susceptible to deprotonation, we decided to use diethyl ether in the
synthesis of diboronic acids 7aec. Thus, the reaction of 6a and 6b
with 4 equiv of t-BuLi, followed by treatment of the obtained
organolithium derivatives with B(OEt)3 afforded 7aeb in moderate
yields, which can be attributed to the competitive deprotonation of
the eCH2e group. However, the analogous reaction of 6c afforded
the respective boronic acid 7c in only 26% yield. This product was
contaminated with a significant amount of thiophenol (ca. 60%).
The formation of thiophenol can be reasonably explained by
assuming that the transformation of 6c-Li2 to 6c0-Li2 proceeds via
intramolecular eCH2e deprotonation (Scheme 6). Quenching of
6c0-Li2 with B(OEt)3 afforded 7c-B2, which is unstable and un-
lithiation of these compounds using 4 equiv of t-BuLi and sub-
sequent borylation with B(OEt)3 afforded the respective diboronic
acids 9 and 11aec in high yield.
5. X-ray studies
The formation of diboronic acid 7c was confirmed by single
crystal X-ray diffraction. To the best of our knowledge, this is the
first crystal structure of a diboronic acid, where aromatic rings
bearing eB(OH)2 groups are connected via a linker with a sulfur
atom. The obtained data revealed that 7c belongs to the monoclinic
P21/c space group. Views of the molecular structure together with
the atom labeling scheme are depicted in Fig. 3. The asymmetric
part of the unit cell contains two molecules of diboronic acid (7c(I)
and 7c(II)), which form a hydrogen-bonded R22ð8Þ dimer (7c(I)e
7c(II)), (O1eH27/O5 and O6eH32/O2 contacts). Additionally,
the anti-conformed hydroxyl groups are arranged in intramolecular
hydrogen interaction with sulfur atoms (O2eH28/S1 and
O5eH31/S3). The geometrical parameters of the hydrogen bonds
are given in the Supplementary data. In general, the molecular
geometries of 7c(I) and 7c(II) are very similar. The aromatic ring
planes within the same molecules are parallel to each other, and
dergoes the easy substitution at the
a-carbon atom repelling the
C6H5Sꢀ anion.18
Scheme 4. The synthesis of 5aec.
Scheme 5. The synthesis of 7aec, 9, and 11aec.

M. Da˛browski et al. / Tetrahedron 69 (2013) 3159e3166
3163
Scheme 6. The proposed mechanism of thiophenol formation on lithiation of 6c.
conformation, in 7c(II) molecule the syneanti conformations is
only observed for eB(OH)2 group bonded to C1.
For the second eB(OH)2 group (bonded to the C7 atom), the
residual density map reveals that hydrogen atoms are in disorder.
The molecules of 7c(I) and 7c(II) are also arranged in different
structural motifs. Hydrogen-bonded R22ð8Þ dimers are formed
between molecules of 7c(I) (O7eH33/O8). These 7c(I)e7c(I)
dimmers are joined to chains by lateral hydrogen interactions
(O8eH34/O7) within the R44ð8Þ motifs (Fig. 4). Such hydrogen
interaction is commonly observed for the structure of boronic and
diboronic acids.19 In contrast, the molecules of 7c(II) are mutually
displaceddeach molecule interacts with two neighbors via a single
hydrogen interaction (O3eH29A/O4, O4eH30A/O4) forming
chains with an alternating sequence of molecules (motif R33ð8Þ).
Both types of chain are parallel to the [101] direction and together
connect molecules of 7c(I) and 7c(II) into a 2-dimensional sheet
(Fig. 4). We also found that eB(OH)2 groups, which participate in
the dimeric 7c(I)e7c(II) interactions are almost coplanar with the
aromatic ring planes, while two other eB(OH)2 groups are signifi-
cantly twisted along the BeC bond with a dihedral angle close to
40ꢁ. The interactions between sheets are dominated by the weak
contacts, from which the CeH/S (dC3/S4¼3.569(3), symmetry
Fig. 3. Labeling of atoms and estimation of their atomic thermal motion as ADPs (50%
probability level) for 7c. Hydrogen interactions are shown as red dashed lines. Selected
ꢁ
ꢁ
bond lengths (A) and angles ( ): B1eO1 1.366(1), B1eO2 1.363(1), B1eC1 1.559(1),
O2eB1eO1 116.71(4), O2eB1eC1eC2 ꢀ42.96(11), B2eO4 1.382(1), B2eO3 1.353(1),
B2eC7 1.580(1), O4eB2eO3 118.31(5), O4eB2eC7eC8 5.48(12). Crystallographic data
for this structure has been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication number CCDC 893237.
operation: 1ꢀx, 1ꢀy, ꢀz) and CeH/
p
(dC11/C11¼3.56(2), symmetry
operation: 1ꢀx, 0.5ꢀy, 0.5ꢀz) should be mentioned.
Fig. 4. 2D hydrogen-bonded layer with the illustration of selected crystal motifs. Hydrogen bonds are shown as red, dashed lines. Aromatic and methylene hydrogen atoms are
omitted for clarity.
also remain parallel to the rings from the second independent
molecule. The main difference between 7c(I) and 7c(II) occurs in
the conformation of the eB(OH)2 groups, regarding proton position
within those groups. While, both eB(OH)2 groups in 7c(I) (bound to
C14 and C20 carbon atoms) adopt a well-established syneanti
6. Conclusions
We have developed an efficient method for the generation of
useful di-lithiated aryl-alkyl sulfides, which were successfully
employed in the synthesis of new diboronic acids. The selectivity of

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M. Da˛browski et al. / Tetrahedron 69 (2013) 3159e3166
the Br/Li exchange depends on the acidity of the hydrogen atoms in
the aliphatic position. For the hydrogen atoms in the benzylic po-
sition the acidity is strongly influenced by the substituent in the
(60 mL) at ꢀ70 ꢁC. The obtained yellow slurry was stirred for 0.5 h
at ꢀ70 ꢁC followed by the dropwise addition of B(OEt)3 (2.92 g,
0.02 mol). The mixture was stirred for 1 h and then quenched with
HClaq (10%, 50 mL). The resultant solution was extracted twice with
diethyl ether (2ꢂ50 mL) and the obtained organic phase was dried
over anhydrous MgSO4. The ethereal solution was filtered and the
solvent was evaporated under reduced pressure. Water (100 mL)
was added to the obtained precipitate. The crude product was
separated by filtration, washed with hexane (50 mL) and dried to
give 3a as a colorless powder; yield 2.60 g (86%), mp>200 ꢁC; nmax
(KBr) 3360 (br), 1594, 1510, 1354, 1263, 1115, 1020, 1006, 850, 821,
774, 724, 644, 625, 510 cmꢀ1; 1H NMR (400 MHz, acetone-d6þD2O):
benzene ring. The non-benzylic hydrogen atoms in the a position to
sulfur are rather unreactive toward deprotonation with t-BuLi in
THF at ꢀ78 ꢁC, unless they are flanked by two sulfur atoms. These
studies were complemented by kinetic experiments, which com-
pared the electronic and steric factors caused by substituents on the
benzylic deprotonation rate. Theoretical calculations supported the
observation that the benzylic deprotonation rate in the studied ABS
is strongly dependent on the acidity of the benzylic hydrogen. The
X-ray analysis of 7c revealed the existence of three different
structural motifs, which stabilize the structure by hydrogen
bonding formation.
d
7.78 (4H, d, J¼8.0 Hz, Ar), 7.24 (4H, d, J¼8.0 Hz, Ar), 3.61 (4H, s,
eCH2e); 13C{1H} NMR (100.6 MHz, acetone-d6þD2O): 140.42,
134.21, 127.99, 35.2. Anal. Calcd for C14H16B2O4S: C, 55.69, H, 5.34.
Found: C, 55.76, H, 5.43.
7. Experimental section
7.1. General
7.2.2. Bis-(3-dihydroxyborylbenzyl)sulfide (3b). The product was
prepared as described for 3a starting with 2b (3.72 g, 0.01 mol) and
using diethyl ether (150 mL) as the solvent; yield 2.50 g (83%),
colorless powder, mp >190 ꢁC; nmax (KBr) 3296 (br), 1600, 1484,
1420, 1349, 1280, 1236, 1208, 1176, 1160, 1100, 1076, 1028, 808, 708,
1H and 13C NMR spectra were recorded at room temperature on
a Bruker 400 MHz spectrometer. Chemical shifts are given in parts
per million relative to TMS in 1H and 13C NMR spectra. IR spectra
were recorded on a Specord M80 spectrometer. Elemental analyzes
were run with a VARIO ELIII apparatus. All analyzes were carried
out at the Analytical Unit of the Chemistry Department, Warsaw
University of Technology. All chemicals were received from Aldrich.
THF and Et2O were stored over sodium prior to use. All reactions
were carried out under dry argon using standard Schlenk tech-
niques. Melting points were determined in Pyrex capillary tubes
with MeL-Temp apparatus.
Competition kinetics: (a) Pairs of substrates BPS and XBPS
(5 mmol each) were dissolved in tetrahydrofuran (30 mL) and
Me3SiCl (20 mmol) was added to the obtained solution. The ob-
tained mixture was cooled to ꢀ78 ꢁC (internal temperature,
acetone-dry ice bath) maintaining the stirring. (b) Preparation of
LDA solution: THF (30 mL) and diisopropylamine (5 mmol, 0.70 mL)
were placed into the Schlenk flask and the obtained solution was
cooled to ꢀ78 ꢁC (internal temperature). A commercial solution of
n-BuLi (5 mmol, 0.5 mL) in hexanes was added to diisopropylamine
solution and the reaction mixture was stirred for 0.5 h. The ob-
tained LDA was transferred rapidly via cannula to the stirred so-
lution of substrates. The reaction mixture has been stirred at ꢀ78 ꢁC
for 2 h. The cooling bath was removed and the reaction mixture was
heated to ambient temperature. 2-Bromobenzyloxy-2,6-
difluorobenzene (1.49 g, 5 mmol) as the internal reference was
added to the reaction mixture maintaining the stirring. The ob-
tained solution was poured onto HClaq (20%, 60 mL) and stirred for
15 min. The organic phase was extracted with diethyl ether
(3ꢂ30 mL) and the combined ether phases were dried over anhy-
drous MgSO4. The solvent was evaporated using water bath (ca.
50 ꢁC) and rotary vacuum pump. The obtained oil was analyzed by
1H NMR spectroscopy. The peak areas of substrates BPS (ch) and
XBPS (cx) relative to those of the internal standard ðcxo ¼ coxÞ were
listed and were used to calculate the rate ratios kx/kh. In each case it
was ascertain that the consumption of substrates BPS and XBPS was
counterbalanced by the formation of silylated products in corre-
sponding quantities.
656, 564, 452 cmꢀ1; 1H NMR (400 MHz, acetone-d6):
d 7.81 (2H, s,
Ar), 7.75e7.72 (2H, m, Ar), 7.37e7.34 (2H, m, Ar), 7.28 (2H, t,
J¼7.0 Hz, Ar), 3.66 (4H, s); 13C{1H} NMR (100.6 MHz, acetone-d6):
138.25, 135.60, 133.47, 131.66, 128.33, 36.53. Anal. Calcd for
C14H16B2O4S: C, 55.69, H, 5.34. Found: C, 55.79, H, 5.16.
7.2.3. Bis-(2-dihydroxyborylbenzyl)sulfide (3c). The product was
prepared as described for 3a starting with 2c (3.72 g, 0.01 mol);
yield 2.54 g (84%), colorless powder, mp>138 ꢁC; nmax (KBr) 3357
(br), 2954, 1587, 1564, 1464, 1423, 1341, 1253, 1145, 1060, 1014, 818,
761, 643, 589, 512, 494, 464, 417 cmꢀ1; 1H NMR (200 MHz, acetone-
d6):
d
7.58 (2H, d, J¼7.2 Hz, Ar), 7.30e7.10 (6H, m, Ar), 3.90 (4H, s);
13C{1H} NMR (100.6 MHz, acetone-d6): 143.81, 135.26, 130.24,
129.89, 126.75, 36.98. Anal. Calcd for C14H16B2O4S: C, 55.69, H, 5.34.
Found: C, 55.70, H, 5.54.
7.2.4. (4-Dihydroxyborylphenyl)(4-dihydroxyborylbenzyl)sulfide
(5a). The product was prepared as described for 3a starting with 4a
(3.58 g, 0.01 mol); yield 2.51 g (87%), colorless powder, mp¼256 ꢁC;
nmax (KBr) 3363 (br), 1596, 1512, 1352, 1268, 1116, 1020, 1004, 848,
820, 772, 728, 648, 628, 512 cmꢀ1
d6þD2O):
;
1H NMR (400 MHz, acetone-
d
7.78 (2H, d, J¼8.0 Hz, Ar), 7.74 (2H, d, J¼8.4 Hz, Ar), 7.35
(2H, d, J¼8.0 Hz, Ar), 7.28 (2H, d, J¼8.4 Hz, Ar), 4.24 (2H, s, eCH2e);
13C{1H} NMR (100.6 MHz, acetone-d6þD2O): 140.35, 140.19, 135.42,
135.08, 128.74, 127.67, 37.60. Anal. Calcd for C13H14B2O4S: C, 54.23,
H, 4.90. Found: C, 54.12, H, 4.98.
7.2.5. (4-Dihydroxyborylphenyl)(3-dihydroxyborylbenzyl)sulfide
(5b). The product was prepared as described for 3a starting
with 4b (3.58 g, 0.01 mol) and using diethyl ether (120 mL) as
the solvent; yield 2.79 g (97%), colorless powder, mp¼239 ꢁC;
nmax (KBr) 3299 (br), 1653, 1596, 1544, 1488, 1350, 1180, 1108,
1036, 1012, 984, 816, 728, 704, 648, 628, 504, 450, 428 cmꢀ1
;
1H
NMR (400 MHz, acetone-d6þD2O):
d 7.89 (1H, s, Ar), 7.75e7.71
(3H, m, Ar), 7.44e7.42 (1H, m, Ar), 7.30e7.24 (3H, m, Ar), 4.23
(2H, s, eCH2e); 13C{1H} NMR (100.6 MHz, acetone-d6þD2O):
140.60, 137.21, 135.67, 135.52, 133.85, 131.58, 128.45, 127.57,
37.79. Anal. Calcd for C13H14B2O4S: C, 54.23, H, 4.90. Found: C,
53.90, H, 5.02.
Error estimation: on the basis of numerous repetitive experi-
ments we assume the relative rates kx/kh to be afflicted with an
average error of less than 10% unless they are less than 1.0.
7.2. Lithiation procedures
7.2.6. (4-Dihydroxyborylphenyl)(2-dihydroxyborylbenzyl)sulfide
(5c). The product was prepared as described for 3a starting with 4c
(3.58 g, 0.01 mol); yield 2.65 g (92%), colorless powder, mp¼242 ꢁC;
nmax (KBr) 3393 (br), 3061,1592,1340,1185,1098, 736, 688 cmꢀ1; 1H
7.2.1. Bis-(4-dihydroxyborylbenzyl)sulfide (3a). A solution of bis-(4-
bromobenzyl)sulfide 2a (3.7 g, 0.01 mol) in THF (20 mL) was added
to a stirred solution of t-BuLi (1.7 M, 24 mL, 0.04 mol) in THF

M. Da˛browski et al. / Tetrahedron 69 (2013) 3159e3166
3165
NMR (400 MHz, acetone-d6):
d
7.74 (2H, d, J¼8.0 Hz, Ar), 7.64 (1H,
13C{1H} NMR (100.6 MHz, acetone-d6þD2O): 139.81, 135.56, 127.61,
31.42. Anal. Calcd for C15H18B2O4S2: C, 51.76, H, 5.21. Found: C, 51.86,
H 5.31.
dd, J¼7.2, 1.6 Hz, Ar), 7.32e7.27 (3H, m, Ar), 7.24 (1H, td, J¼7.2,
1.6 Hz, Ar), 7.16 (1H, td, J¼7.2, 1.6 Hz, Ar), 4.53 (2H, s, eCH2e); 13C
{1H} NMR (100.6 MHz, acetone-d6): 142.75, 140.87, 135.31, 130.07,
129.99, 127.59, 126.93, 37.63. Anal. Calcd for C13H14B2O4S: C, 54.23,
H, 4.90. Found: C, 53.87, H, 5.02.
7.2.12. 1,3-[Bis-(3-dihydroxyborylphenylthio)]propane
(11b). The
product was prepared as described for 3a starting with 10b (4.18 g,
0.01 mol); yield 2.85 g (82%), colorless powder, mp>87 ꢁC; nmax
(KBr) 3403 (br), 3304 (br), 2928, 1584, 1564, 1472, 1406, 1344, 1256,
7.2.7. Bis-(4-dihydroxyborylphenylthio)methane (7a). A solution of
bis-(4-bromobenzylthio)methane 6a (3.9 g, 0.01 mol) in Et2O
(60 mL) was added dropwise to a stirred solution of t-BuLi (1.7 M,
24 mL, 0.04 mol) in Et2O (150 mL) at ꢀ70 ꢁC. The obtained yellowish
precipitate was stirred for 0.5 h at ꢀ70 ꢁC followed by the dropwise
addition of B(OEt)3 (2.92 g, 0.02 mol). The mixture was stirred for
1 h and then quenched with HClaq (10%, 50 mL). The organic phase
was separated and dried over anhydrous MgSO4. The ethereal so-
lution was filtered and the solvent was evaporated under reduced
pressure. Water (100 mL) was added to the obtained precipitate.
The crude product was separated by filtration, washed with hexane
(2ꢂ50 mL) and dried to give 7a as a colorless powder; yield 2.14 g
(67%), mp>260 ꢁC; nmax (KBr) 3221 (br), 2516, 2262, 1596, 1472,
1168, 1136, 1092, 1028, 880, 812, 792, 724, 700, 588, 518 cmꢀ1
;
1H
NMR (400 MHz, acetone-d6):
d
7.84 (2H, s, Ar), 7.66 (2H, dt, J¼7.6,
1.0 Hz, Ar), 7.38 (2H, dq, J¼7.6, 1.0, 0.6 Hz, Ar), 7.26 (2H, t, J¼7.6 Hz,
Ar), 3.09 (4H, t, J¼7.2 Hz, SeCH2e), 1.89 (2H, q, J¼7.2 Hz, CeCH2eC);
13C{1H} NMR (100.6 MHz, acetone-d6): 136.03, 135.52, 132.52,
131.56, 128.99, 32.36. Anal. Calcd for C15H18B2O4S2: C, 51.76, H, 5.21.
Found: C, 51.78, H, 5.24.
7.2.13. 1,3-[Bis-(2-dihydroxyborylphenylthio)]propane
(11c). The
product was prepared as described for 3a starting with 10c (4.18 g,
0.01 mol); yield 2.75 g (79%), colorless powder, mp¼177e178 ꢁC;
nmax (KBr) 3362 (br), 2956, 2920, 1584, 1560, 1464, 1428, 1340, 1252,
1393, 1197, 1116, 1020, 1004, 812, 725 cmꢀ1
;
1H NMR (400 MHz,
1141, 1056, 1012, 816, 760, 644, 588, 508, 492, 460, 416 cmꢀ1
;
1H
acetone-d6):
d
7.81 (4H, d, J¼8.4 Hz, Ar), 7.38 (4H, d, J¼8.4 Hz, Ar),
NMR (400 MHz, acetone-d6): d
7.73 (2H, dd, J¼8.0, 1.6 Hz, Ar), 7.42
4.61 (2H, s, eCH2e); 13C{1H} NMR (100.6 MHz, acetone-d6): 138.70,
135.57, 128.59, 37.47. Anal. Calcd for C13H14B2O4S2: C, 48.79, H, 4.41.
Found: C, 48.76, H, 4.50.
(2H, dd, J¼8.0,1.2 Hz, Ar), 7.33 (2H, td, J¼8.0,1.6 Hz, Ar), 7.25 (2H, td,
J¼8.0, 1.2 Hz, Ar), 3.03 (4H, t, J¼7.2 Hz, SeCH2e), 1.83 (2H, q,
J¼7.2 Hz, CeCH2eC); 13C{1H} NMR (100.6 MHz, acetone-d6):
139.88, 135.89, 133.11, 131.09, 127.45, 35.27. Anal. Calcd for
C15H18B2O4S2: C, 51.76, H, 5.21. Found: C, 51.58, H, 5.03.
7.2.8. Bis-(3-dihydroxyborylphenylthio)methane (7b). The product
was prepared as described for 7a starting with 6b (3.9 g, 0.01 mol);
yield 2.26 g (71%), colorless powder, mp>248 ꢁC; nmax (KBr) 3358
(br), 1585, 1564, 1474, 1410, 1345, 1255, 1163, 1135, 1094, 1026, 882,
X-ray data: The single crystal X-ray measurements were per-
formed on a Kuma KM4CCD
k-axis diffractometer with graphite-
ꢁ
monochromated MoKa radiation (
l
¼0.71073 A) and equipped with
814, 794, 724, 702, 586, 514 cmꢀ1
;
1H NMR (400 MHz, acetone-
an Oxford Cryosystems nitrogen gas-flow apparatus. The crystal was
positioned at 50 mm from the KM4CCD camera. The data were cor-
rected for Lorentz and polarization effects. Data reduction and
analysis were carried out with the Oxford Diffraction Ltd. suit of
programs.20 All structures were solved by direct methods using
SHELXS-97 and refined using SHELXL-97.21 All non-hydrogen atoms
were refined anisotropically. All hydrogen atoms were placed in
d6þD2O):
d 7.92 (2H, s, Ar), 7.74e7.72 (2H, m, Ar), 7.50e7.47 (2H, m,
Ar), 7.30 (2H, t, J¼7.6 Hz, Ar), 4.53 (2H, s, eCH2e); 13C{1H} NMR
(100.6 MHz, acetone-d6þD2O): 140.44, 140.26, 135.42, 135.10,
128.78, 127.77, 37.68. Anal. Calcd for C13H14B2O4S2: C, 48.79, H, 4.41.
Found: C, 48.62, H, 4.50.
ꢁ
ꢁ
7.2.9. Bis-(2-dihydroxyborylphenylthio)methane (7c). The product
was prepared as described for 7a starting with 6c (3.9 g, 0.01 mol);
yield 0.83 g (26%), colorless powder, mp¼114e115 ꢁC; nmax (KBr)
3362 (br), 1584, 1560, 1464, 1428, 1340, 1252, 1141, 1056, 1012, 816,
760, 644, 588, 508, 492, 460, 416 cmꢀ1; 1H NMR (400 MHz, acetone-
calculated positions with CeH distance of 0.95 A (phenyl), 0.99 A
(methylene) and OeH distance of 0.84 A. They were visible in dif-
ꢁ
ference maps and they were included in the refinement in riding-
motion approximation with Uiso (phenyl H)¼1.2 Ueq (C), Uiso (meth-
ylene H)¼1.5 Ueq (C) andUiso (hydroxyl H)¼1.5 Ueq (O). C26H19B4O8S4;
molecular weight, 320.00a.u.; T¼100(2) K; monoclinic space group,
d6þD2O): 7.66 (2H, dd, J¼7.2,1.6 Hz, Ar), 7.53 (2H, dd, J¼7.2, 0.8 Hz,
d
ꢁ
ꢁ
Ar), 7.35 (2H, td, J¼7.6, 1.6 Hz, Ar), 7.26 (2H, td, J¼7.2, 0.8 Hz, Ar),
4.46 (2H, s, eCH2e); 13C{1H} NMR (100.6 MHz, acetone-d6þD2O):
139.12, 135.36, 132.74, 130.84, 127.64, 43.10. Anal. Calcd for
C13H14B2O4S2: C, 48.79, H, 4.41. Found: C, 48.71, H, 4.52.
P21/c; unit cell dimensions, a¼24.446(2) A, b¼5.073(1) A,
3
ꢁ ꢁ
c¼24.734(2)
A,
b
¼106.84(1)ꢁ,
V¼2935.9(3)
A ;
Z¼8;
dcalcd¼1.448 g cmꢀ3; absorption coefficient,
m
¼0.372 mmꢀ1; F(000)¼
1328; crystal size, 0.10 mmꢂ0.10 mmꢂ0.07 mm;
q range for data
collection: 2.05ꢁe28.65ꢁ; index ranges: ꢀ31<h<32, ꢀ6<k<6,
ꢀ33<l<32; reflections collected: 23,191/unique: 3611 (Rint¼0.0324);
absorption correctiondmulti-scan; refinement methoddfull-ma-
trix least-squares on F2; goodness-of-fit on F2, GooF¼1.010; data/
7.2.10. 1,2-[Bis-(3-dihydroxyborylphenylthio)]ethane (9). The prod-
uct was prepared as described for 3a starting with 8 (4.04 g,
0.01 mol); yield 2.74 g (82%), colorless powder, mp>200 ꢁC; nmax
(KBr) 3265 (br), 1592, 1560, 1480, 1432, 1400, 1341, 1192, 1120, 1035,
restraints/parameters 3611/5/392; final
R1¼0.0571, wR2¼0.1116;
wR2¼0.1642; largest diffraction peak and hole: 0.52 and ꢀ0.50 e A
R
indices (I>2
indices (all data): R1¼0.1393,
ꢁꢀ3
s(I)):
777, 704 cmꢀ1
;
1H NMR (400 MHz, acetone-d6):
d
7.86 (2H, d,
R
J¼1.6 Hz, Ar), 7.70 (2H, dt, J¼7.2, 1.6 Hz, Ar), 7.36 (2H, dq, J¼7.2, 1.6,
1.2 Hz, Ar), 7.28 (2H, t, J¼7.2 Hz, Ar), 3.14 (4H, s, eCH2CH2e); 13C{1H}
NMR (100.6 MHz, acetone-d6): 136.24, 135.28, 133.00, 132.12,
129.17, 33.77. Anal. Calcd for C14H16B2O4S2: C, 50.34, H, 4.83. Found:
C, 50.21, H, 4.91.
.
Computational details: All geometryoptimizationsand frequency
calculations were carried out with GAUSSIAN0922 suite of programs
and Becke-style three parameter density functional method using
the LeeeYangeParr correlation functional (B3LYP).14 The aug-cc-
pVDZ15 basis sets were used to calculate the optimal geometries. The
minima were confirmed by vibrational frequencycalculations within
harmonic approximation (no imaginary frequencies). In optimiza-
tion processes no symmetry constraints were applied.
7.2.11. 1,3-[Bis-(4-dihydroxyborylphenylthio)]propane
(11a). The
product was prepared as described for 3a starting with 10a (4.18 g,
0.01 mol); yield 2.99 g (86%), colorless powder, mp>156 ꢁC; nmax
(KBr) 3300 (br), 2912, 2414, 1592, 1544, 1496, 1352, 1184, 1102, 1028,
Deprotonation energies were calculated according to the
scheme:
1008, 820, 732, 644, 628, 512, 496, 444 cmꢀ1 1H NMR (400 MHz,
;
acetone-d6þD2O):
d
7.76 (4H, d, J¼8.4 Hz, Ar), 7.28 (4H, d, J¼8.4 Hz,
AH ¼ Aꢀ þ Aþ;
D
E ¼ EAH ꢀ EA
Ar), 3.14 (4H, t, J¼7.2 Hz, SeCH2e), 1.93 (2H, q, J¼7.2 Hz, CeCH2eC);

3166
M. Da˛browski et al. / Tetrahedron 69 (2013) 3159e3166
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To establish the outcome of the quantum-chemical method we
have performed additional calculation for PhSCH2(2-FC6H4),
PhSCH2(3-FC6H4), and PhSCH2(4-FC6H4) at the MP223/6-31þG(d)24
and MP2/cc-pVTZ levels of theory, obtaining similar values of tꢀh1e
5. Durka, K.; Gontarczyk, K.; Klis, T.; Serwatowski, J.; Wozniak, K. Appl. Organomet.
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,
,
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380.2 kJ molꢀ1 for MP2/6-31þG(d) method and 381.1 kJ molꢀ1
377.4 kJ molꢀ1, and 378.9 kJ molꢀ1 for MP2/cc-pVTZ].
9. The approximation of the kinetic data with first, third and fractional (1.5) order
equations did not show the linear fit to lg (kx/kh)¼rs
.
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Acknowledgements
This work was supported by the ‘Iuventus Plus’ program of the
Polish Ministry of Science and Higher Education in the framework
of project 0111/IP3/2011/71. The X-ray measurements of compound
7c were undertaken at the Crystallographic Unit of the Physical
Chemistry Laboratory, Chemistry Department, University of War-
saw. The authors thank the Interdisciplinary Centre for Mathe-
matical and Computational Modeling in Warsaw for providing
computational facilities (G33-14). Support from Aldrich Chemical
Company, Milwaukee, WI, USA, through the donation of chemicals
and equipment is gratefully acknowledged.
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