Transition-Metal-Free ipso-Trifluoromethylthiolation of Lithium Aryl Boronates

Article abstract of DOI:10.1021/acs.orglett.9b02236

A transition-metal-free direct trifluoromethylthiolation of the ipso-carbon of lithium aryl boronates with trifluoromethanesulfenate under mild conditions was described. In addition, late-stage site-selective C-H borylation/trifluoromethylation and C-Cl b

Full text of DOI:10.1021/acs.orglett.9b02236

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Transition-Metal-Free ipso-Trifluoromethylthiolation of Lithium Aryl
Boronates
Feng Shen,† Hanliang Zheng,‡ Xiao-Song Xue,‡ Long Lu,*,§ and Qilong Shen*,†
†Key Laboratory of Organofluorine Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic
Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China
‡State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tanjin 300071, China
§Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A transition-metal-free direct trifluoromethylth-
iolation of the ipso-carbon of lithium aryl boronates with
trifluoromethanesulfenate under mild conditions was de-
scribed. In addition, late-stage site-selective C−H boryla-
tion/trifluoromethylation and C−Cl borylation/trifluorome-
thylthiolation of biologically active molecules was developed.
Initial mechanistic study suggested that the Li+ cation plays a
vital role by coordinating to the oxygen atom of an aryl boronate complex and the oxygen of the reagent, thus allowing the aryl
group to directly attack the trifluoromethylthio group of the trifluoromethylthiolating reagent.
ecently, the reaction of an electrophile with a lithium aryl
stereoconfiguration (Figure 1C).6 Nevertheless, reaction of a
1
Rboronate complex, a type of so-called “ate complex” that lithium aryl boronate complex with an electrophile, wherein the
boronate acts as an aryl nucleophile to give an ipso-carbon
functionalized arene, has never been observed previously.
In 2013, our group initiated a project to develop a toolbox of
electrophilic fluoroalkylthiolating reagents as it was reported
that the fluoroalkylthiolating groups are highly lipophilic and
may improve significantly the drug molecule’s pharmacoki-
netics.7 Since then, a series of trifluoromethylthiolating (CF3S-
),8 difluoromethylthiolating (HCF2S-),9 and monofluorome-
thylthiolating (CH2FS-)10 reagents have been successfully
invented. During the investigation of the reactions of these
electrophilic reagents with lithium aryl boronate complexes, we
unexpectedly discovered that reaction of lithium aryl boronate
complexes with one of these reagents, trifluoromethanesulfenate
1a, exclusively gave ipso-carbon trifluoromethylthiolated arenes
in good yields (Figure 1D). The unprecedented reaction pattern
for the reaction of a lithium aryl boronate with reagent 1a
differed dramatically from those with other electrophiles, thus
prompting us to study the mechanism of the reaction. The
mechanistic studies disclosed that Li+ cation of the lithium aryl
boronate complex played a vital role by coordinating to the
oxygen atom of boronate and the oxygen of reagent 1a, thus
allowing the aryl group to directly attack the trifluoromethylthio
group of the reagent. Herein, we would like to present these
discoveries.
could be readily generated in situ by mixing an aryl pinacol
boronate with an alkyl lithium at room temperature,2 has
attracted great interests. The reaction often occurs under mild
conditions in the absence of any transition metal catalyst, thus
circumventing the drawbacks commonly associated with
transition metal catalysts such as high cost, toxicity, and the
need of an extra removal step from the pharmaceutical
products.3
These studies have determined that the reaction pattern and
the structure of the product were versatile and highly dependent
on the substituents on the aryl group of the ate complex. For
instance, pioneer work from Aggarwal’s group showed that a
lithium aryl boronate complex with a meta-electron-donating
group could be activated by a suitable electrophile (NBS or
DDQ) to induce the stereospecific formation of a C(sp2)−
C(sp3) bond (Figure 1A).4 It was proposed that the reaction is
triggered by an initial electrophilic attack (SEAr) of the aryl
group by an electrophile, followed by subsequent 1,2-migration
of the alkyl group to the ipso-carbon of the dearomatized
intermediate with the simultaneous elimination of halide and the
boron moiety to give the final product. Alternatively, it was
found that in the presence of an appropriate acylating reagent,
lithium ortho- or para-hydrazinyl or para-phenoxide aryl
boronate underwent N−N bond or O−Bi bond cleavage with
concomitant 1,2-metalate rearrangement to give stereospecific
C(sp2)−C(sp3) bond formation (Figure 1B),5 whereas,
interestingly, with other substituents at the ortho- or para-
position, the lithium aryl boronate complex often acts as a
nucleophilic alkylating reagent to react with various of
electrophiles such as NIS, NBS, TCAA, DIAD, or Selectfluor
to give the functionalized alkane with inversion of the
Reaction of lithium biphenyl neopentylglycolato boronate
complex 2a with electrophilic trifluoromethylthiolating reagent
1a was initially chosen to probe the reaction pattern and
reactivity. It was found that treatment of boronate 2a with 1.5
Received: June 29, 2019
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.9b02236
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Letter
Scheme 1. Scope for the Reaction of Lithium Aryl Boronates
a
with Reagent 1a
a
Reaction conditions: lithium biphenylboronate (0.2 mmol), reagent
1a (0.4 mmol) in DMA/CH3CN (v/v = 1/1, 2.0 mL) at 60 °C for 24
b
Figure 1. Reaction of the lithium aryl boronate complex with an
electrophile.
h under an argon atmosphere. Isolated yields. Reagent 1a (0.6
c
mmol) was used. Yields were determined by 19F NMR spectroscopy
using PhCF3 as an internal standard.
equiv of reagent 1a in DMF or DMA gave, unexpectedly, the
direct ipso-C(sp2)-trifluoromethylthiolated product 3a in 10%
and 14% yield, respectively, after 24 h at room temperature,
whereas reactions in other common solvents such as CH3CN,
THF, CH2Cl2, or toluene did not occur at all. The yield of
compound 3a increased to 48% when the reaction was
conducted in DMA at 60 °C. Interestingly, using mixed
DMA/CH3CN (v/v = 50/50) as the solvent improved the
yield to 67%. In addition, the yield was further increased to 75%
when the amount of reagent 1a was increased to 2.0 equiv.
Under these conditions, we then further examined the effect of
other boronates and electrophilic trifluoromethylthiolating
reagents. It was found that reaction using of pinacol boronate
gave comparable yield, and using boronate derived from 1,3-diol
gave slightly lower yield, whereas reaction using boronate
derived from MeLi was ineffective and using boronate derived
from tBuLi occurred in 36% yield. On the other hand, reactions
of lithium aryl boronate 2a with several other electrophilic
trifluoromethylthiolating reagents 1b−1d8 did not take place at
all, indicating the unique and crucial role of reagent 1a in this
reaction (see Table S1 in the Supporting Information for
details). Notably, under these conditions, products for the
reaction of a lithium aryl boronate with other electrophiles such
as the C(sp2)−C(sp3) bond-forming compounds or function-
alized alkanes were not observed.
electron-deficient aryl groups (3f−3h, 3k−3l) reacted smoothly
after 24 h at 60 °C to give the corresponding trifluoromethylth-
iolation products in good yields (42−82%). Unlike previous
reported transition-metal-free coupling reactions of lithium aryl
boronates with electrophiles, in which the reaction pattern was
greatly affected by the position of the substituents on the aryl
group,1 the current reaction was not affected by the substituents
at all. Reactions of lithium aryl boronates with substituents at the
ortho-position (3r), meta-position (3i−3o), or para-position
(3a−3h, 3k−3o, 3q−3u) of the aryl group reacted smoothly to
give the corresponding products in good yields. It is worth
mentioning that reactions of lithium aryl boronate containing a
meta-electron-donating methoxy or dimethylamino group also
afforded the desired ipso-carbon-trifluoromethylthiolated prod-
uct in high yields (3i−3j). In these cases, the formation of sp2−
sp3 cross-coupling products was not observed. Gratifyingly, this
methodology was successfully applied to trifluoromethylthiola-
tion of lithium heteroaryl boronates to give the corresponding
trifluoromethylthiolated heteroarenes including indole (3w),
benzothiophene (3x), benzofuran (3y), furan (3z), and pyrazole
(3v) in high yields. Furthermore, many common functional
groups including fluorine, chlorine, bromine, and phenol
protected by tert-butyldimethylsilyl, dimethylamino, acetal,
ketal, morpholine, olefin, cyano, and trimethylsilyl were well-
tolerated.
The unique reaction pattern for the reaction of lithium aryl
boronate with electrophilic trifluoromethylthiolating reagent 1a
prompted us to investigate the generality of the reaction, and the
results are summarized in Scheme 1. In general, lithium aryl
boronates with both electron-rich (3a−3e, 3i−3j, 3m−3u) and
To further expand the scope of the current site-specific
trifluoromethylthiolating protocol, we developed a C−H
activation borylation/trifluoromethylthiolation sequence. As
shown in Scheme 2, under the conditions developed by Hartwig
B
DOI: 10.1021/acs.orglett.9b02236
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To further evaluate the utility of this protocol, we tried to
synthesize Tiflorex, a marketed drug for the treatment of
anorexia nervosa, under the standard conditions.14 Aryl boronic
ester 10a could be obtained in 90% yield through lithium−
halogen exchange of 9, followed by addition of triisopropyl
borate and neopentyl glycol. Treatment of boronic ester 10a
Scheme 2. C−H Borylation/Trifluoromethylthiolation of
Arenes
n
with 1.0 equiv of BuLi generated in situ the corresponding
lithium aryl boronate 10. Without isolation, lithium aryl
boronate was allowed to react with reagent 1a under the
standard conditions to give compound 11 in 53% yield.
Hydrolysis of the acetal protecting group in compound 12,
followed by reductive amination, afforded Tiflorex in 89% yield
(Figure 2).
and co-workers for Ir-catalyzed highly selective C−H borylation
of arenes,11 a few 1,2- or 1,3-disubstituted arenes were
selectively borylated. Without isolation, the aryl boronates
were treated with nBuLi and reagent 1a to give the
corresponding trifluoromethylthiolated arenes 4a−d in good
yields.
Many drug molecules and agrochemicals contain a C−Cl
bond, and the trifluoromethylthio group is generally considered
as a pseudohalogen.12 We, therefore, envisaged whether the C−
Cl bond in these compounds can be converted to their
analogous trifluoromethylthiolated derivatives. For example,
Meclozine (5a), an antihistamine used to treat motion
sickness,13 contains a C−Cl bond that could be easily converted
into aryl boronic ester (5b) through palladium-catalyzed
coupling with diboronic acid, followed by in situ esterification
with neopentyl glycol. Treatment of the boronic neopentyl
Figure 2. Transition-metal-free preparation of Tiflorex.
We have now established a unique, general reaction between
lithium aryl boronates and electrophilic trifluoromethylthiolat-
ing reagent 1a, which differed greatly from reactions of lithium
aryl boronates with other electrophiles. Mechanistically, the
current reaction should proceed through a different pathway
other than those illustrated in Figure 1A−C. We next shifted our
investigation to explore the mechanism of the reaction.
First, to probe whether the reaction goes through a free radial
or single-electron transfer (SET) process, we studied the
reaction in the presence of 1.0 equiv of free radical inhibitor,
such as the radical trap (TEMPO) and 2,6-di-tert-butyl-4-
methylphenol or SET inhibitor 1,4-dinitrobenzene. It was found
that the yields of the reaction in the presence of these agents
decreased slightly. Yet, the reactions were not completely
inhibited (Figure 3a). In addition, reaction of lithium aryl
boronate, which contained an alkene moiety and can serve as a
radical cyclization probe under the standard conditions, afforded
n
glycol esters (5b) with BuLi generated in situ lithium aryl
boronate, which was further converted to the trifluoromethylth-
ioated arene (5) in 52% yield under standard reaction
conditions (eq 1).
To illustrate the applicability of this protocol, we applied it to
transition-metal-free late-stage trifluoromethylthiolation of
natural products or drug molecules including dihydrocholester-
ol, D-δ-tocopherol, and pterostilbene. As shown in eqs 2−4,
these reactions occurred smoothly to give the corresponding
products 6−8 in good yields.
Figure 3. Mechanistic investigation.
C
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Organic Letters
Letter
the ipso-C(sp2)-trifluoromethylthiolated product 3r in 54%
yield. In this case, the radical cyclization product was not
observed (Figure 3b). These results are against a free radical or
SET pathway.
coordinates to the two oxygen atoms of both reagents was 8.2
kcal/mol lower than the transition state in which the lithium
cation coordinates to the oxygen atom of the aryl boronate and
the fluorine atom of reagent 1a. Consequently, the attack of the
aryl group of the boronate to the trifluoromethylthio group of
reagent 1a is much easier than the attack of the butyl group of
the boronate.
In summary, an unprecedented transition-metal-free direct
ipso-C(sp2)-trifluoromethylthiolation of lithium aryl boronate
complexes was developed. This reaction represents a new
reaction pattern for the reaction of a lithium aryl boronate with
an electrophile. Moreover, the current protocol is a comple-
mentary method for the preparation of trifluoromethylthiolated
arenes.16 With this protocol, late-stage site-specific C−H and
C−Cl trifluoromethylthiolation of biologically active molecules
was realized. Furthermore, the protocol was successfully applied
to the synthesis of Tiflorex, a marketed drug for the treatment of
anorexia nervosa. Mechanistic study indicates that the lithium
cation plays a vital role by coordinating to both oxygen atoms of
boronate and the trifluoromethylthiolating reagent at the same
time, thus promoting the attack of the aryl group toward to the
trifluoromethylthio group. Studies of lithium aryl boronate
complexes with other electrophilic fluoroalkylating reagents
developed in our laboratory are currently underway.
Next, an intramolecular competition experiment of lithium
diaryl boronate 13 with two different aryl groups showed that
trifluoromethylthiolated thiophene 14 was obtained exclusively
in 70% yield, along with 4-methoxyphenylbronic neopentyl
glycol ester, which was detected by GC-MS (Figure 3c). If the
reaction proceeds via a homolytic cleavage of the aromatic
carbon−boron bond to form an aryl radical, which then attacks
reagent 1a to afford the product, we would observe two
trifluoromethylthiolated products because the bond strength of
both carbon−boron bonds in lithium diaryl boronate 13 is
similar. The fact that the reaction was highly selectively to afford
compound 14 supports that the reaction did not proceed via a
free radical pathway.
To get more insight into the reaction mechanism, we studied
the kinetics of the reaction. It was found that the reaction was
first-order in reagent 1a and aryl boronate complex 2i (see
Supporting Information for details), which indicated that both
the lithium aryl boronate and the reagent were involved in the
rate-determining step of the reaction.
Finally, to probe the effect of the lithium cation on the
reaction, we studied the reaction in the presence of different
amounts of 12-C-4. It was found that the yields decreased
significantly with the increase of the amount of 12-C-4 (Figure
4). These results suggest that the lithium cation played a vital
role in the formation of the desired product.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b02236.
Synthesis, analytical data, computational details, and
NMR data of 2a, 2b, 3a−3z, 4a−4d, and 5−14 (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: lulong@sioc.ac.cn.
*E-mail: shenql@sioc.ac.cn.
ORCID
Figure 4. Reaction of lithium aryl boronates with reagent 1a in the
presence of 12-C-4.
Xiao-Song Xue: 0000-0003-4541-8702
Qilong Shen: 0000-0001-5622-153X
Notes
We proposed that the reaction likely proceeds via a transition
state in which the lithium cation coordinates to both the oxygen
atom of the aryl boronate complex and the oxygen atom of
reagent 1a at the same time, as shown in Figure 1d. The aryl
group then attacks the trifluoromethylthio group to give the
product. To gain more support for this mechanistic assumption,
DFT calculations were conducted.15 As shown in Figure 5, it was
found that the transition state in which the lithium cation
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the financial support from
National Natural Science Foundation of China (21625206,
21632009, 21572258, 21572259, 21421002) and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(XDB20000000).
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