Frustrated Lewis Pair Catalyzed S-H Bond Borylation

Article abstract of DOI:10.1021/acs.organomet.7b00346

The frustrated Lewis pair (FLP) [NMe₂-C₆H₄-BH₂]₂ is shown to catalyze the dehydrogenative borylation of thiols. The scope of the reaction, the experimental and computational investigation of the mecha

Full text of DOI:10.1021/acs.organomet.7b00346

Article
pubs.acs.org/Organometallics
Frustrated Lewis Pair Catalyzed S−H Bond Borylation
́
Etienne Rochette, Hugo Boutin, and Frederic-Georges Fontaine*
́
́
Dep
G1V 0A6 Queb
́
artement de Chimie, Centre de Catalyse et Chimie Verte (C3 V) Universite
ec, Canada
́ ́ ́
Laval, 1045 Avenue de la Medecine, Quebec,
́
S
* Supporting Information
ABSTRACT: The frustrated Lewis pair (FLP) [NMe2-C6H4-
BH2]2 is shown to catalyze the dehydrogenative borylation of
thiols. The scope of the reaction, the experimental and
computational investigation of the mechanism, and the
application of the system to a one-pot Michael addition on
α−β unsaturated carbonyl leading to the β sulfido ketone are
reported.
formations.16 It was notably shown that these species act as
efficient catalysts for the metal-free dehydrogenative borylation
of heteroarenes through C−H bond activation.16c,17 The
catalytic cycle that was proposed relies on three separate
transformations. First, the FLP can activate the C−H bonds of
heteroarenes similarly to previous reports of H2 activation14,18
(Figure 1A). Then, the zwitterionic intermediate expels H2, the
INTRODUCTION
■
Boron-containing molecules are playing a predominant role in
the construction of molecular architectures and are ubiquitous
reagents in synthetic chemistry. The most notable examples are
alkyl- or aryl-boronates used as transmetalation agents in the
Suzuki−Miyaura palladium cross-coupling reaction.1 Although
compounds containing boron-heteroelement (B−E) bonds are
less common, they have been drawing attention as reagents in
metal-free transformations.2 For example, diboranes (B−B)
have been used in diboration and borylation reactions3 and the
B−N compounds have been used in the Strecker-type
aminative cyanation and in Mannich-type transformations.4
The Michael addition on α,-β unsaturated carbonyl compounds
can also be performed using B−N,5 B−S,6 B−P,7 and B−Se8
reagents. Similarly, the insertion of the diazo Me3SiCHN2 in a
B−S bond can lead to H−C(SR)(Bpin)(SiMe3).9
While many transition-metal catalyzed transformations have
been reported to generate B−E (E = O, N, S, and P) containing
species,10 it was recently demonstrated by Bertrand et al. that
many of these reagents can be prepared directly from
hydroboranes (B−H) and protic reagents (E−H) in the
absence of a catalyst.11 However, the preparation of B−S
containing compounds in good yields is challenging since the
uncatalyzed borylation of thiols require heating up at 120 °C
for up to 96 h. Therefore, catalysis is a requirement in the
preparation of thioboranes12 which are useful precursors for the
synthesis of a large array of sulfur containing bioactive
molecules.13
Although transition metal catalysts remain ubiquitous in the
construction of chemical bonds, a new paradigm in catalysis has
emerged with the discovery of frustrated Lewis pairs (FLPs).
FLPs have been shown to mediate the metal-free hydrogen
activation14 and acting as surrogates for transition metals in
many catalytic transformations.15 Several architectures have
been used as FLPs with amine-boranes being one of the most
active frameworks for bond activation and catalytic trans-
Figure 1. Catalytic cycle of the FLP mediated C−H bond borylation.
thermodynamic driving force of the process (Figure 1B).16c−e
As a last step, the latter intermediate undergoes sigma-bond
metathesis to generate the desired product (Figure 1C). To
extend our knowledge on this catalytic process, notably in
regard to the metathesis step and the deactivation of the C−H
borylation by protic sources, we investigated the activation of
E−H bonds (E = O, N, S, and Se) by simple amine−borane
Received: May 4, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00346
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[NMe2-C6H4-BH2]2 (1). We wish to report our conclusions
and our observation that this FLP framework acts as a highly
active catalyst for the borylation of thiols.
DISCUSSION
■
We first tested the reactivity of 1 with excess alcohol, amine, or
thiol (Scheme 1). The volatile tert-butyl derivatives were
Scheme 1. Stoichiometric Reactions between 1 and tert-
Butanol, tert-Butylamine, and tert-Butylthiol
Figure 2. DFT investigation of the borylation of tert-butylamine and
tert-butylthiol. ΔG in kcal·mol−1, ωB97XD/6-31+G**, SMD solvent =
chloroform.
similar reaction pathway to the catalytic borylation of
heteroarenes is observed. First, the activation of the E−H
bond is shown to be endergonic for the tert-butylthiol with an
intermediate at 1.8 kcal·mol−1 but exergonic for tert-butylamine
(ΔG = −9.6 kcal·mol−1). The release of H2, which has been
shown to be an important driving force in FLP-type
transformations,16c−e requires a ΔG⧧ of 26.7 kcal·mol−1 for
tert-butylamine but is more favorable for tert-butylthiol with a
barrier of 18.7 kcal·mol−1. Whereas a second S−H bond
activation is possible with the thiol (ΔG⧧ of 27.3 kcal·mol−1),
the second activation of an amine is more difficult with a ΔG⧧
of 32.3 kcal·mol−1, supporting the experimental observations.
The high stability of 3, which is more stable by 12.8 kcal·mol−1
than the starting material, explains this lack of reactivity. The
summation of the angles around N of 360° for the minimized
structure of 3 confirms the planarity of the nitrogen atom and
the presence of a π bond with boron, which leads to reduced
Lewis acidity and to a detrimental effect in the metathesis step.
With this information in hand, we examined the possibility of
using 1 as a catalyst for the borylation of thiols using
thiophenol as a model substrate for optimization (Table 1).
As highlighted recently by Bertrand, the borylation of thiols
operates sluggishly at 80 °C, and only low yields of the desired
products were observed after 48 h (run 1). In the presence of
chosen to facilitate monitoring by NMR spectroscopy and to
ease purification. Unsurprisingly, 1 reacts rapidly with 2 equiv
1
of tert-butanol to give species 2 as evidenced by the H NMR
spectrum featuring a sharp singlet at 1.3 ppm integrating 18
protons for the tBu groups and by the 11B NMR shift at 27.3
ppm. However, only 1 equiv of tert-butylamine reacted with 1,
resulting in the formation of species 3. A resonance at 1.3 ppm
integrating for 9 protons was observed by 1H NMR
spectroscopy for the tBu moiety, and the broad signal at 5.0
ppm sharpened upon 11B decoupling, which is consistent with a
boron hydride. The 11B NMR spectrum showed a doublet (J =
96 Hz) at 36.5 ppm which became a singlet upon 1H
decoupling, confirming the presence of a B−H moiety (see
Figure S9). After 1 h at 80 °C, the reaction between 1 and tert-
butylthiol leads to product 4 where two thiolate moieties are on
the boron atom. The tBu signal (1.4 ppm) integrates for 18
1
protons in the H NMR spectrum, and the 11B NMR shift of
62.5 ppm is characteristic of such compound.19 It is possible to
1
observe in all reactions a H NMR resonance at 4.6 ppm
characteristic of H2.
HBpin (pin = pinacol) was then added to products 2−4 to
monitor the metathesis step and possible formation of tBu-E-
Bpin (E = O, NH, and S) which would enable catalysis. Species
2 and 3 were shown to be inert under such conditions, but 4
reacted within 1 h at 80 °C with HBpin to generate compound
5u. Although some precedents do exist,20 it is not surprising for
metathesis to be more challenging with alkoxy and amido
derivatives because the π overlap between the lone pair of the
heteroelements and the boron atom reduces the Lewis acidity
of boron and slows down metathesis. This observation also
rationalizes the lack of catalytic activity in the borylation of
heteroarenes in the presence of alcohols and protic amines.16c
Interestingly, the weak B−S π bond allows sulfido compounds
to undergo metathesis.
Table 1. Optimization of the Borylation of Thiophenol
Catalyzed by 1
catalyst loading
(mol %)
temperature
time
(h)
conversion
(%)
entry
(°C)
solvent
1
2
3
4
5
6
7
8
0
80
80
80
60
40
20
80
80
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
C6D6
48
2
31
0.5
2.5
2.5
2.5
2.5
2.5
2.5
>95
>95
>95
>95
>95
>95
>95
1
2
8
24
1
DFT calculations (ωB97XD/6-31+G**, SMD solvent =
chloroform level of theory) were performed in order to
rationalize this reactivity (Figure 2). According to DFT data, a
THF-d8
1
B
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0.5 mol % of 1, near complete conversion was observed after 2
h under identical conditions (run 2), confirming that the FLP is
acting as a catalyst for the borylation of thiols. It is possible to
reduce the reaction time to 1 h by increasing the catalyst
loading to 2.5 mol % (run 3). It is also possible to operate the
reaction at room temperature, but the rate of the reaction is
greatly reduced, taking about 24 h to reach completion (run 6).
As expected, it is possible to observe the rate of the reaction
increasing with higher temperature (runs 4 and 5). The nature
of the solvent (CDCl3, C6D6, and THF-d8) seems to have little
impact on the reaction rate (runs 3, 7, and 8).
compared to other alkylthiols and does not require higher
catalyst loading and higher temperature. The borylation of
benzylfuranethiol (5w) in the presence of 1 equiv of HBpin was
also possible under 2 h, suggesting that the activation of the S−
H bond occurs more rapidly than the activation of the C−H
bond since species 1 has been shown to activate the C−H
bonds of furans. Finally, we were able to expend this
transformation to the borylation of selenophenol (5x),
although the reaction took about 24 h to proceed to
completion with 10 mol % of catalyst at 80 °C.
In order to demonstrate the usefulness of the procedure in
metal-free synthesis, the one-pot borylation/Michael addition
was carried out under similar conditions than those previously
reported by Fernandez and Westcott.6 After mixing HBpin and
4-tolylthiol for 2 h and generating 5p, 4-phenyl-3-buten-2-one
was added to afford the β sulfido ketone. After an aqueous work
up and chromatography on silica gel, the final product was
isolated in 52% yield (Scheme 2).
The scope of the reaction is illustrated in Table 2. Catalysis
proceeds smoothly to completion with several thiophenols,
Table 2. Scope of the Catalytic Borylation of Thiols and
a
1
Time Required for Full H NMR Conversion
Scheme 2. One-Pot Michael Addition of Thiophenol on 4-
Phenyl-3-buten-2-one through Catalytic Borylation
The mechanism of the borylation reaction was studied both
experimentally and using DFT (Figure 3). Since two B−H
bonds are present on the catalyst, one or two S−H activations
can occur before the transfer of the thiolate to HBpin by
metathesis. Monitoring of the reaction by 11B NMR confirmed
the resting state proposed by DFT for the substrates studied. In
a
Standard conditions (unless otherwise indicated): 2.5 mol % 1, 60
b
c
°C, CDCl3, 1.1 equiv of HBpin. After vacuum distillation. After
recrystallization in hexane. Conditions: 10 mol % 1, 80 °C, CDCl3,
1.1 equiv of HBPin. Conditions: 10 mol % 1, 80 °C, CDCl3, 2 equiv
d
e
of HBpin. Isolated yields are in parentheses.
using 2.5 mol % of 1 in CDCl3 at 60 °C for 4 h. Interestingly,
the reaction proceeds with reagents that were not purified
beforehand and thus contain residual moisture. The presence of
pinBOBpin suggests that HBpin is hydrolyzed before catalyst
deactivation. Similar to the unsubstituted thiophenol (5a), 2-,
3-, and 4-substituted fluoro (5b−d), chloro (5e−g), bromo
(5h−j), methoxy (5k−m), and methyl (5n−p) substituted
thiophenols were all completely borylated under 4 h at 60 °C,
with the exception of 2-bromo-thiophenol (5h) that proved to
be more challenging and required 16 h to get to complete
conversion. Pentafluorophenylthiol (5q) was fully converted
under 4 h, but it required a larger catalyst loading and a higher
temperature. Since 2,6-dimethylphenylthiol (5r) also took
longer time (24 h) and required 2 equiv of HBpin, it can be
assumed that steric hindrance around the sulfur atom reduces
the rate of the reaction. Less acidic alkanethiols such as decane
(5s), cyclohexyl (5t), tert-butyl (5u), and benzylthiols (5v) are
also less reactive than thiophenols, requiring between 20 and 24
h to get full conversion to the borylated analogues. Surprisingly,
the borylation of bulky tert-butylthiol is relatively easy
Figure 3. Proposed mechanism for the FLP catalyzed borylation of
thiol. ΔG in kcal·mol−1, ωB97XD/6-31+G**, SMD solvent =
chloroform.
C
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distilled. N,N-Dimethylaniline, n-BuLi, TMEDA, B(OMe)3, LiAlH4,
and TMSBr were purchased from Sigma-Aldrich and used as received.
[NMe2-C6H4-BH2]2 was synthesized according to previously reported
procedures.16d HBpin was purchased from Sigma-Aldrich and used as
received, synthesized according to reported procedure,21 or graciously
given by BASF for the larger scale reactions (containing NEt3 as
stabilizer). The source of HBpin did not influence the reaction rate,
but when synthesized HBpin was used, residual dichloromethane was
observed by NMR spectroscopy. Thiophenol (precursor for 5a) was
bought from Alfa Aesar. Precursors for 5b−g, 5i, 5k, 5m, 5p, 5q, 5t,
and 5v were bought from Oakwood, and precursors for 5h, 5j, 5o, 5r,
5s, 5u, 5w, and 5x were bought from Sigma-Aldrich. All thiols were
used without further purification.
Stoichiometric Experiments. NMe2-C6H4-B(OtBu)2 (2). First,
10.0 mg (1.0 equiv) of [NMe2-C6H4-BH2]2 was dissolved in CDCl3
and placed in a J. Young NMR tube after which 5.0 equiv (36.0 μL) of
tBuOH was added. The reaction was left at room temperature for 30
min, and the volatiles were removed under vacuum. CDCl3 was then
added, and the product NMe2-C6H4-B(OtBu)2 was characterized using
multinuclear NMR spectroscopy. 1H NMR (500 MHz, CDCl3) δ
7.22−7.17 (m, 2H), 6.86−6.80 (m, 2H), 2.86 (s, 6H), 1.31 (s, 18H).
13C{1H} NMR (126 MHz, CDCl3) δ 154.5 (s), 132.6 (s), 128.4 (s),
119.5 (s), 115.5 (s), 73.8 (s), 43.7 (s), 30.5 (s). The carbon linked
directly to boron was not observed. 11B{1H} NMR (160 MHz,
CDCl3) δ 27.3 (s).
the case of thiophenol and decanethiol (simplified by ethyl thiol
for the DFT calculations) the resting state of the catalyst was
shown to be the double activation product, as supported by the
presence of singlets at 62.3 and 59.8 ppm, respectively, by 11B
NMR. DFT data support this observation, with the double
activation products of ethanethiol and thiophenol being more
stable than the mono addition products by 8.0 and 7.2 kcal·
mol−1, respectively. Since the TS’s of the S−H activation are
alike in cycles A and B, it is likely that both substrates undergo
the latter catalytic cycle. However, in the case of tert-butylthiol,
although we know from our early experiments that a double
activation can occur, the resting state of the catalyst was
identified as the mono activation product, characterized as a
doublet (J = 146 Hz) at 60.0 ppm by 11B NMR, becoming a
1
singlet upon H decoupling. In the latter system, cycle A is
more likely responsible for the catalytic activity, which is also
consistent with DFT where the mono addition adduct is more
stable by 0.6 kcal·mol−1. Since the transition barriers are overall
lower in cycle A than B, this result explains the unexpected
difference in activity between tert-butylthiol and decanethiol,
where the former reagent undergoes borylation faster, even if it
possesses more steric bulk that the latter compound. Although
we were not able to locate the transition state for the metathesis
step with thiophenol, this transformation was shown to be rate-
determining with EtSH and tBuSH with ΔG⧧ of 30.5 and 25.9
kcal·mol−1, which is consistent with the observed reaction rates.
It should be noted that the FLP-catalyzed transformations are
favored over the uncatalyzed systems by 15−20 kcal·mol−1.
NMe2-C6H4-BHNHtBu (3). First, 10.0 mg (1.0 equiv) of [NMe2-
C6H4-BH2]2 was dissolved in CDCl3 and placed in a J. Young NMR
tube after which 5.0 equiv (39.5 μL) of tBuNH2 was added. The
reaction was heated at 80 °C for 16 h, and the volatiles were removed
under vacuum. CDCl3 was then added, and the product NMe2-C6H4-
B(H)(NHtBu) was characterized using multinuclear NMR spectros-
1
copy. H NMR (500 MHz, CDCl3) δ 7.48 (dd, J = 7.5, 1.5 Hz, 1H),
CONCLUSION
7.32−7.27 (m, 1H), 7.03−6.98 (m, 2H), 4.97 (very broad, 1H), 2.78
(s, 6H), 1.32 (s, 9H). 1H{11B} NMR (500 MHz, CDCl3) δ 7.48 (dd, J
= 7.5, 1.5 Hz, 1H), 7.32−7.27 (m, 1H), 7.03−6.98 (m, 2H), 4.97 (s,
1H), 2.78 (s, 6H), 1.32 (s, 9H). The N−H signal was not observed.
13C{1H} NMR (126 MHz, CDCl3) δ 158.3 (s), 137.8 (s), 129.9 (s),
121.7 (s), 116.4 (s), 45.5 (s), 32.3 (s), 28.4 (s). The carbon linked
directly to boron was not observed. 11B{1H} NMR (160 MHz,
CDCl3) δ 36.5 (s). 11B NMR (160 MHz, CDCl3) δ 36.5 (d, J = 96
Hz).
■
In this contribution, we demonstrate that ambiphilic amine-
boranes can act as efficient metal-free catalysts for the
borylation of thiols. In presence of amines and alcohols, the
E−H activation leads to stable products that prevent catalysis
because of a strong π bond between boron and the lone pair of
nitrogen and the oxygen, respectively. The proposed borylation
mechanism relies on a FLP type transformation, in which the
release of H2 is an important driving force. This process can be
applied to one-pot metal-free Michael additions directly from
thiols and boranes. This chemistry is another step toward the
development of useful metal-free catalysts and we hope it will
help in the design of C−C, C−N, C−O, or C−S bond-forming
processes which would be major contributions to FLP
chemistry.
NMe2-C6H4-B(StBu)2 (4). First, 10.0 mg (1.0 equiv) of [NMe2-C6H4-
BH2]2 was dissolved in CDCl3 and placed in a J. Young NMR tube,
after which 5.0 equiv (42.0 μL) of tBuSH was added. The reaction was
heated at 80 °C for 1 h, and the volatiles were removed under vacuum.
CDCl3 was added, and the product NMe2-C6H4-B(StBu)2 was
1
characterized using multinuclear NMR spectroscopy. H NMR (500
MHz, CDCl3) δ 7.21 (ddd, J = 8.2, 7.3, 1.8 Hz, 1H), 7.15 (dd, J = 7.3,
1.7 Hz, 1H), 6.81−6.76 (m, 2H), 2.94 (s, 6H), 1.40 (s, 18H). 13C{1H}
NMR (126 MHz, CDCl3) δ 153.5 (s), 133.0 (s), 129.3 (s), 118.1 (s),
115.1 (s), 48.0 (s), 43.7 (s), 32.8 (s). The carbon linked directly to
boron was not observed. 11B{1H} NMR (160 MHz, CDCl3) δ 62.5
(s).
Catalytic Experiments. All catalytic experiments were carried out
in CDCl3 in standard NMR tubes for experiments at 60 °C and in J.
Young tubes for experiments at 80 °C. Compounds 5a, 5l, 5p, 5u, and
5w were also prepared starting with 500 mg of the respective thiols
and isolated either by distillation or recrystallization. As previously
reported,12 thioboranes exhibit significant air and moisture sensitivity,
and no satisfactory EA and HRMS could be obtained.
EXPERIMENTAL SECTION
■
General Procedures. Unless specified otherwise, manipulations
were carried out under a nitrogen atmosphere using standard glovebox
and Schlenk techniques. NMR spectra were recorded on an Agilent
Technologies NMR spectrometer at 500.0 MHz (1H), 470.59 (19F),
125.758 MHz (13C), and 160.46 MHz (11B) and on a Varian Inova
NMR AS400 spectrometer at 400.0 MHz (1H), 376.50 (19F), and
1
100.580 MHz (13C). H NMR and 13C{1H} NMR chemical shifts are
referenced to residual protons or carbons in deuterated solvent.
11B{1H} was calibrated using an external reference of BF3.Et2O. 19F
NMR was calibrated using CFCl3 as external standard. Multiplicities
are reported as singlet (s), doublet (d), triplet (t), quadruplet (q), and
multiplet (m). Chemical shifts are reported in ppm. Coupling
constants are reported in Hz. Mass Spectroscopy analyses were
carried out on an Agilent Technologies 6210 LC Time of Flight Mass
Spectrometer.
Method A: Used for 5a−p and 5u−w. In a glovebox, 400 μL of a
solution containing 2.5 mg/mL (1.0 mg/400 μL) catalyst [NMe2-
C6H4-BH2]2 in CDCl3 (0.0038 mmol, 2.5 mol %) and 24 μL of HBpin
(0.165 mmol, 1.1 equiv) were added to an NMR tube. The substrate
(0.150 mmol, 1.0 equiv) was subsequently added. Liquid substrates
were added with a micropipette outside the glovebox, and solid
substrates were weighed and added inside the glovebox. The reaction
Materials. Solvents were purified by distillation over Na/
benzophenone. C6D6 was dried over Na/K alloy and distilled, THF-
d8 was dried on molecular sieves, and CDCl3 was dried over P2O5 and
1
was left in an oil bath at 60 °C, and H NMR spectra were taken
periodically until complete conversion was observed.
D
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Method B: Used for 5q, 5s, and 5t. The procedure is the same as
that in method A with the exception that a solution containing 10 mg/
mL catalyst (increasing the catalyst loading at 10 mol %) and a
temperature of 80 °C were used.
Method C: Used for 5r and 5x. The procedure is the same that as
in method B with the exception that the quantity of HBpin was
doubled (48 μL, 0.330 mmol, 2.2 equiv).
Phenylsulfur Pinacolborane (5a). First, 30.0 mg (2.5 mol %) of
[NMe2-C6H4-BH2]2 was placed in a Schlenk tube and dissolved in
about 4 mL of toluene. Then, 500 mg of thiophenol (1 equiv) was
added followed by the addition of 725 μL (1.1 equiv) of HBpin. The
reaction was then heated at 60 °C for about 2 h, after which the
solution was evaporated to dryness. The reaction was followed by the
release of H2 causing effervescence. The residual oil was distilled under
reduced pressure (boiling point of 75 °C at 1 mbar); 832 mg (78%
yield) of the title compound was obtained. 1H NMR (400 MHz,
CDCl3) δ 7.53−7.47 (m, 2H), 7.31−7.19 (m, 3H), 1.31 (s, 12H).
13C{1H} NMR (101 MHz, CDCl3) δ 135.5 (s, 1C), 131.6 (s, 1C),
129.9 (s, 1C), 129.9 (s, 1C), 85.2 (s, 2C), 24.5 (s, 4C). 11B{1H} NMR
(160 MHz, CDCl3) δ 32.9.
para-Bromophenylsulfur Pinacolborane (5j). 1H NMR (500
MHz, CDCl3) δ 7.41−7.35 (m, 4H), 1.31 (s, 12H). 13C{1H} NMR
(126 MHz, CDCl3) δ 136.3 (s, 1C), 134.6 (s, 2C), 132.4 (s, 2C),
131.7 (s, 2C), 85.5 (s, 2C), 24.5 (s, 4C). 11B{1H} NMR (160 MHz,
CDCl3) δ 32.6.
1
ortho-Methoxyphenylsulfur Pinacolborane (5k). H NMR (400
MHz, CDCl3) δ 7.53 (dd, J = 8.25, 1.67 Hz, 2H), 7.25 (ddd, J = 8.25,
7.47, 1.95 Hz, 2H), 3.85 (s, 3H), 1.28 (s, 12H). 13C{1H} NMR (126
MHz, CDCl3) 158.5 (s, 1C), 135.3 (s, 1C), 128.7 (s, 1C), 120.8 (s,
1C), 117.7 (s, 1C), 110.9 (s, 1C), 85.0 (s, 2C), 55.7 (s, 1C), 24.5 (s,
4C). 11B{1H} NMR (160 MHz, CDCl3) δ 32.7.
meta-Methoxyphenylsulfur Pinacolborane (5l). First, 23.7 mg
(2.5 mol %) of [NMe2-C6H4-BH2]2 was placed in a Schlenk tube and
dissolved in about 4 mL of toluene. Then, 500 mg of meta-
methoxythiophenol (1 equiv) was added, followed by the addition of
570 μL (1.1 equiv) of HBpin. The reaction was then heated at 60 °C
for about 4 h, after which the solution was evaporated to dryness. The
reaction can be followed by the release of H2 causing effervescence.
The residual oil was distilled under reduced pressure (boiling point of
85 °C at 1 mbar). 761 mg (80% yield) of the title compound was
ortho-Fluorophenylsulfur Pinacolborane (5b). 1H NMR (400
MHz, CDCl3) δ 7.53 (m, 1H), 7.24 (m, 1H), 7.06 (m, 2H), 1.29 (s,
12H). 13C{1H} NMR (126 MHz, CDCl3) δ 161.9 (d, J = 246.4 Hz,
1C), 135.8 (s, 1C), 129.2 (d, J = 7.66 Hz, 1C), 124.2 (d, J = 3.83 Hz,
1C), 116.8 (d, J = 18.6 Hz, 1C), 115.7 (d, J = 23.3 Hz, 1C), 85.5 (s,
2C), 24.4 (s, 4C). 11B{1H} NMR (160 MHz, CDCl3) δ 32.4. 19F
NMR (376 MHz, CDCl3) δ −106.5 (td, J = 7.7, 5.3 Hz, 1F).
meta-Fluorophenylsulfur Pinacolborane (5c). 1H NMR (400
MHz, CDCl3) δ 7.32−7.13 (m, 3H), 6.97−6.86 (m, 1H), 1.31 (s,
12H). 13C{1H} NMR (126 MHz, CDCl3) δ 162.3 (d, J = 247.2 Hz,
1C), 131.9 (d, J = 8.4 Hz, 1C), 129.7 (d, J = 8.5 Hz, 1C), 128.5 (d, J =
3.0 Hz, 1C), 119.8 (d, J = 23.2 Hz, 1C), 113.8 (d, J = 21.1 Hz, 1C),
85.5 (s, 2C), 24.5 (s, 4C). 11B{1H} NMR (160 MHz, CDCl3) δ 32.6.
19F NMR (376 MHz, CDCl3) δ −112.7 to −112.8 (m, 1F).
para-Fluorophenylsulfur Pinacolborane (5d). 1H NMR (400
MHz, CDCl3) δ 7.46−7.38 (m, 2H), 7.00−6.92 (m, 2H), 1.29 (s,
12H). 13C{1H} NMR (126 MHz, CDCl3) δ 162.0 (d, J = 246.1 Hz,
1C), 134.8 (d, J = 8.08 Hz, 2C), 124.6 (d, J = 3.51 Hz, 1C), 115.7 (d, J
= 21.9 Hz, 2C), 85.4 (s, 2C), 24.5 (s, 4C). 11B{1H} NMR (160 MHz,
CDCl3) δ 32.8. 19F NMR (376 MHz, CDCl3) δ −115.6 (ddd, J = 13.8,
8.58, 5.08 Hz, 1F).
1
obtained. H NMR (400 MHz, CDCl3) δ 7.20−7.15 (m, 1H), 7.09−
7.06 (m, 2H), 6.78 (ddd, J = 8.2, 2.3, 1.3 Hz, 1H), 3.79 (s, 3H), 1.31
(s, 12H). 13C{1H} (126 MHz, CDCl3) δ 159.4 (s, 1C), 130.7 (s, 1C),
129.4 (s, 1C), 125.3 (s, 1C), 118.2 (s, 1C), 112.9 (s, 1C), 85.3 (s, 2C),
55.2 (s, 1C), 24.5 (s, 4C). 11B{1H} (160 MHz, CDCl3) δ 32.8.
1
para-Methoxyphenylsulfur Pinacolborane (5m). H NMR (400
MHz, CDCl3) δ 7.38 (d, J = 8.9 Hz, 2H), 6.82 (d, J = 8.8 Hz, 2H),
3.78 (s, 3H), 1.29 (s, 12H). 13C{1H} NMR (126 MHz, CDCl3) δ
158.8 (s, 1C), 134.5 (s, 2C), 119.9 (s, 1C), 114.4 (s, 2C), 85.2 (s, 2C),
55.2 (s, 1C), 24.5 (s, 4C). 11B{1H} NMR (160 MHz, CDCl3) δ 33.0.
ortho-Methylphenylsulfur Pinacolborane (5n). 1H NMR (400
MHz, CDCl3) δ 7.59−7.50 (m, 1H), 7.23−7.08 (m, 3H), 2.43 (s, 3H),
1.29 (s, 12H). 13C{1H} NMR (126 MHz, CDCl3) δ 140.5 (s,1C),
134.9 (s, 1C), 130.2 (s, 1C), 128.9 (s, 1C), 127.4 (s, 1C), 126.1 (s,
1C), 85.2 (s, 2C), 24.5 (s, 4C), 21.6 (s, 1C). 11B{1H} NMR (160
MHz, CDCl3) δ 32.7.
meta-Methylphenylsulfur Pinacolborane (5o). 1H NMR (400
MHz, CDCl3) δ 7.37−7.28 (m, 2H), 7.16 (dd, J = 8.5, 7.4 Hz, 1H),
7.08−7.02 (m, 1H), 2.33 (s, 3H), 1.31 (s, 12H). 13C{1H} NMR (126
MHz, CDCl3) δ 138.3 (s,1C), 133.7 (s, 1C), 130.1 (s, 1C), 128.5 (s,
1C), 127.6 (s, 1C), 85.2 (s, 2C), 24.5 (s, 4C), 21.3 (s, 1C). The
carbon bonded to the sulfur atom could not be assigned. 11B {1H}
NMR (160 MHz, CDCl3) δ 32.9.
ortho-Chlorophenylsulfur Pinacolborane (5e). 1H NMR (500
MHz, CDCl3) δ 7.69−7.65 (m, 1H), 7.43−7.39 (m, 1H), 7.23−7.16
(m, 2H), 1.31 (s, 12H). 13C{1H} NMR (126 MHz, CDCl3) δ 137.3 (s,
1C), 135.8 (s, 1C), 129.8 (s, 1C), 129.2 (s, 1C), 128.5 (s, 1C), 126.8
(s, 1C), 85.5 (s, 2C), 24.5 (s, 4C). 11B{1H} NMR (160 MHz, CDCl3)
δ 32.4.
para-Methylphenylsulfur Pinacolborane (5p). First, 26.8 mg (2.5
mol %) of [NMe2-C6H4-BH2]2 was placed in a Schlenk tube and
dissolved in about 4 mL of toluene. Then, 500 mg of 4-
methylthiophenol (1 equiv) was added, followed by 645 μL of
HBpin (1.1 equiv). The reaction was then heated at 60 °C for 2 h,
after which the solution was evaporated to dryness. The reaction was
followed by the release of H2 causing effervescence. The residual white
solid was dissolved in hot hexane; the mixture was filtered and placed
at −35 °C overnight. The next morning, an appreciable amount of
white crystals had formed. The supernatant was removed and crystals
dried under vacuum; 525 mg (52% yield) of the title compound was
meta-Chlorophenylsulfur Pinacolborane (5f). 1H NMR (500
MHz, CDCl3) δ 7.54−7.49 (m, 1H), 7.39 (dt, J = 6.7, 1.9 Hz, 2H),
7.23−7.17 (m, 2H), 1.32 (s, 12H). 13C{1H} NMR (126 MHz, CDCl3)
δ 134.1 (s,1C), 132.7 (s, 1C), 131.7 (s, 1C), 131.1 (s, 1C), 129.6 (s,
1C), 127.0 (s, 1C), 85.5 (s, 2C), 24.5 (s, 4C). 11B{1H} NMR (160
MHz, CDCl3) δ 32.7.
para-Chlorophenylsulfur Pinacolborane (5g). 1H NMR (400
MHz, CDCl3) δ 7.45−7.38 (m, 2H), 7.27−7.19 (m, 2H), 1.30 (s,
12H). 13C{1H} NMR (101 MHz, CDCl3) δ 134.3 (s, 2C), 132.9 (s,
1C), 128.8 (s, 2C), 128.2 (s, 1C), 85.4 (s, 2C), 24.5 (s, 4C). 11B{1H}
NMR (160 MHz, CDCl3) δ 32.7.
1
obtained. H NMR (400 MHz, CDCl3) δ 7.40−7.34 (m, 2H), 7.13−
7.05 (m, 2H), 2.33 (s, 3H), 1.31 (s, 12H). 13C{1H} NMR (101 MHz,
CDCl3) δ 136.6 (s,1C), 133.0 (s, 2C), 129.5 (s, 2C), 125.9 (s, 1C),
85.2 (s, 2C), 24.5 (s, 4C), 21.1 (s, 1C). 11B{1H} NMR (160 MHz,
CDCl3) δ 32.9.
ortho-Bromophenylsulfur Pinacolborane (5h). 1H NMR (400
MHz, CDCl3) δ 7.69 (dd, J = 7.8, 1.7 Hz, 1H), 7.59 (dd, J = 8.0, 1.5
Hz, 1H), 7.27−7.22 (m, 1H), 7.11 (m, 1H), 1.30 (s, 12H). 13C{1H}
NMR (126 MHz, CDCl3) δ 135.6, 133.1, 131.4, 128.6, 127.5, 85.5,
24.5. 11B{1H} NMR (160 MHz, CDCl3) δ 32.6.
Pentafluorophenylsulfur Pinacolborane (5q). 1H NMR (400
MHz, CDCl3) δ 1.28 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3)
δ 148.6−145.0 (m, 2C), 142.2−139.6 (m, 1C), 139.1−136.4 (m, 2C),
86.4 (s, 2C), 24.3 (s, 4C). The quaternary carbon bonded to the sulfur
atom could not be assigned. 11B {1H} NMR (160 MHz, CDCl3) δ
31.6. 19F NMR (376 MHz, CDCl3) δ −131.54 to −131.67 (m, 2F),
−154.20 (tt, J = 21.1, 1.9 Hz, 1F), −161.70 to −161.97 (m, 2F).
2,6-Dimethylphenylsulfur Pinacolborane (5r). 1H NMR (500
MHz, CDCl3) δ 7.11 (s, 3H), 2.47 (s, 6H), 1.27 (s, 12H). 13C {1H}
NMR (101 MHz, CDCl3) δ 142.1 (s, 1C), 128.5 (s, 1 or 2C), 127.8 (s,
meta-Bromophenylsulfur Pinacolborane (5i). 1H NMR (500
MHz, CDCl3) δ 7.67 (t, J = 1.8 Hz, 1H), 7.43 (ddd, J = 7.9, 1.8,
1.0 Hz, 1H), 7.36 (ddd, J = 8.0, 2.0, 1.0 Hz, 1H), 7.13 (t, J = 7.9 Hz,
1H), 1.32 (s, 12H). 13C{1H} NMR (126 MHz, CDCl3) δ 135.5 (s,
1C), 131.6 (s, 1C), 129.9 (s, 1C), 129.9 (s, 1C), 122.2 (s, 1C), 85.2 (s,
2C), 24.5 (s, 4C). The quaternary carbon bonded to the sulfur atom
could not be assigned. 11B{1H} NMR (160 MHz, CDCl3) δ 32.6.
E
DOI: 10.1021/acs.organomet.7b00346
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2C), 127.4 (s, 1 or 2C), 85.0 (s, 2C), 24.5 (s, 4C), 22.8 (s, 2C).
11B{1H} NMR (160 MHz, CDCl3) δ 32.4.
Computational Details. Unless specified otherwise, all the
calculations were performed on the full structures of the reported
compounds. Calculations were performed with the GAUSSIAN 09
suite of programs.22 The ωB97XD functional23 was qualified as
promising by Grimme24 and was used to accurately describe the
mechanism of FLP mediated hydrogenation of alkynes25 and was thus
used in combination with the 6-31++G** basis set for all atoms.26 The
transition states were located and confirmed by frequency calculations
(single imaginary frequency). The stationary points were characterized
as minima by full vibration frequencies calculations (no imaginary
frequency). All geometry optimizations were carried out without any
symmetry constraints at the ωB97XD/6-31G** level of theory. The
energies were then refined by single point calculations to include
solvent effects using the SMD solvation model27 and chloroform as
solvent at the ωB97XD/6-31+G** level of theory.28
Decanesulfur Pinacolborane (5s). 1H NMR (400 MHz, CDCl3) δ
2.64 (t, J = 7.3 Hz, 2H), 1.63−1.53 (m, 2H), 1.43−1.14 (m, 26H),
0.87 (t, J = 6.9 Hz, 3H). 13C{1H} NMR (126 MHz, CDCl3) δ 84.6 (s,
2C), 32.4 (s, 1C), 31.9 (s, 1C), 29.6 (s, 1C), 29.5 (s, 1C), 29.3 (s, 1C),
29.1 (s, 1C), 28.5 (s, 1C), 26.6 (s, 1C), 24.5 (s, 4C), 22.7 (s, 1C), 14.1
(s, 1C). 11B{1H} NMR (160 MHz, CDCl3) δ 33.5.
Cyclohexylsulfur Pinacolborane (5t). 1H NMR (400 MHz,
CDCl3) δ 3.16−3.04 (m, 1H), 2.00−1.93 (m, 2H), 1.76−1.68 (m,
3H), 1.46−1.28 (m, 5H), 1.27 (s, 12H). 13C{1H} NMR (126 MHz,
CDCl3) δ 84.4 (s, 2C), 44.1 (s, 1C), 40.4 (s, 1C), 36.4 (s, 1C), 26.2 (s,
1C), 25.5 (s, 1C), 24.9 (s, 1C), 24.5 (s, 4C). 11B{1H} NMR (160
MHz, CDCl3, borosilicate tube) δ 33.4.
tert-Butylsulfur Pinacolborane (5u). First, 36.8 mg (2.5 mol %) of
[NMe2-C6H4-BH2]2 was placed in a Schlenk tube and dissolved in
about 4 mL of toluene. Then, 500 mg of tert-butythiol (1 equiv) was
added, followed by the addition of 885 μL (1.1 equiv) of HBpin. The
reaction was then heated at 60 °C for about 20 h, after which the
solution was evaporated to dryness under reduced pressure. The
reaction was followed by the release of H2 causing effervescence. The
residual colorless oil was distilled under reduced pressure (boiling
point of 35 °C at a pressure of 1 mbar); 1.164 g (76% yield) of the title
compound was obtained. 1H NMR (400 MHz, CDCl3) δ 1.46 (s, 9H),
1.28 (s, 12H). 13C {1H} NMR (126 MHz, CDCl3) δ 84.1 (s, 2C), 43.8
(s, 1C), 33.5 (s, 3C), 24.5 (s, 4C). 11B{1H} NMR (160 MHz, CDCl3)
δ 33.1.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00346.
■
S
Cartesian coordinates of the optimized structures with
their associated free enthalpy and Gibbs free energies
(XYZ)
NMR spectra of the compounds, details on the
determination of the catalyst resting state (PDF)
1
Benzylsulfur Pinacolborane (5v). H NMR (400 MHz, CDCl3) δ
7.38−7.35 (m, 2H), 7.33−7.28 (m, 2H), 7.26−7.22 (m, 1H), 3.92 (s,
2H), 1.32 (s, 12H). 13C{1H} NMR (126 MHz, CDCl3) δ 140.5 (s,
1C), 128.6 (s, 2C), 128.4 (s, 2C), 126.8 (s, 1C), 85.1 (s, 2C), 30.7 (s,
1C), 24.6 (s, 4C). 11B{1H} NMR (160 MHz, CDCl3) δ 33.5.
AUTHOR INFORMATION
Corresponding Author
*E-mail: frederic.fontaine@chm.ulaval.ca.
■
Furfurylsulfur Pinacolborane (5w). First, 29.0 mg (2.5 mol %) of
[NMe2-C6H4-BH2]2 was placed in a Schlenk tube and dissolved in
about 4 mL of toluene. Then, 500 mg of 2-furanmethanethiol (1
equiv) was added, followed by the addition of 700 μL (1.1 equiv) of
HBpin. The reaction was then heated at 60 °C for about 2 h, after
which the solution was evaporated to dryness. The reaction was
followed by the release of H2 causing effervescence. The residual
colorless oil was distilled under reduced pressure (boiling point 60 °C
at about 1 mbar); 978 mg (93% yield) of the title compound was
ORCID
Frederic-Georges Fontaine: 0000-0003-3385-0258
́
́
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Sciences and
Engineering Research Council (NSERC) of Canada and the
Centre de Catalyse et Chimie Verte (Quebec). We acknowl-
edge BASF for the gift of HBpin. E.R. acknowledges NSERC,
FRQNT and Vanier for scholarships. We acknowledge Calcul
Canada for computation time. We acknowledge S. A. Westcott
for helpful discussions.
1
obtained. H NMR (500 MHz, CDCl3) δ 7.31 (s, 1H), 6.28 (s, 1H),
6.18 (s, 1H), 3.90 (s, 2H), 1.30 (s, 12H). 13C{1H} NMR (126 MHz,
CDCl3) δ 153.4 (s, 1C), 141.6 (s, 1C), 110.3 (s, 1C), 106.7 (s, 1C),
85.1 (s, 2C), 24.5 (s, 4C), 22.9 (s, 1C). 11B{1H} NMR (160 MHz,
CDCl3) δ 33.4.
Phenylselenyl Pinacolborane (5x). 1H NMR (500 MHz, CDCl3) δ
7.62−7.56 (m, 2H), 7.26−7.19 (m, 3H), 1.32 (s, 12H). 13C{1H} NMR
(101 MHz, CDCl3) δ 134.4 (s, 2C), 129.2 (s, 2C), 128.9 (s, 1C),
126.7 (s, 1C), 85.6 (s, 2C), 24.6 (s, 4C). 11B{1H} NMR (160 MHz,
CDCl3) δ 33.8.
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DOI: 10.1021/acs.organomet.7b00346
Organometallics XXXX, XXX, XXX−XXX