Bis(trifluoromethanesulfonimide) (BSI): Acidity and application to hydrofunctionalization as a Br?nsted acid catalyst

Article abstract of DOI:10.1016/j.tet.2021.132037

A binaphthyl derivative, bearing bis(trifluoromethanesulfonimide) (BSI) moiety, was developed as a novel Br?nsted acid. Computational prediction of the pK_a value of BSI indicated its classification as a strong Br?nsted acid. BSI catalyzed the h

Full text of DOI:10.1016/j.tet.2021.132037

Tetrahedron 85 (2021) 132037
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Bis(trifluoromethanesulfonimide) (BSI): Acidity and application to
hydrofunctionalization as a Brønsted acid catalyst
Ryukichi Takagi*, Yuichiro Sakai, Duyen Thi Duong
Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8526, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A binaphthyl derivative, bearing bis(trifluoromethanesulfonimide) (BSI) moiety, was developed as a
novel Brønsted acid. Computational prediction of the pKa value of BSI indicated its classification as a
strong Brønsted acid. BSI catalyzed the hydroamination of alkenyl amines in hexafluoroisopropanol
(HFIP) with efficiency comparable to that of TsOH∙H2O, Tf2NH, and TfOH. The adjacent sulfonimide
groups on BSI were important for enhancing acidity. BSI additionally catalyzed the hydroalkoxylation of
alkenyl alcohols in HFIP with high efficiency.
Received 17 December 2020
Received in revised form
11 February 2021
Accepted 16 February 2021
Available online 5 March 2021
© 2021 Elsevier Ltd. All rights reserved.
Keywords:
Brønsted acid
Sulfonimide
Hydroamination
Hydroalkoxylation
HFIP
1. Introduction
latter strategy, chiral Brønsted acids bearing two strongly electron-
withdrawing groups, such as dicarboxylic acid [10], disulfonic acid
Brønsted acids have recently emerged as a significant tool in
metal-free catalysis for a range of organic synthetic reactions [1].
Owing to their acidity, they are principally used to activate basic
substrates bearing nitrogen-containing nucleophiles, including
aldimines, ketimines, and aziridines, while harsh reaction condi-
tions are required for the activation of less basic substrates, such as
alkenes and alkynes [2]. Therefore, although several asymmetric
hydrofunctionalization reactions of alkenyl derivatives catalyzed by
chiral Brønsted acids have been developed [3], only three examples
(hydroamination of alkenyl [4] or dienyl amines [5], and hydro-
alkoxylation of alkenyl alcohol [6]) have been reported to occur via
direct activation of the alkene moiety of alkenyl derivatives.
Thus far, two strategies have been applied to overcome acidity
limitations and create a strong chiral Brønsted acid [7]. The intro-
duction of strongly electron-withdrawing groups, such as N-triflyl
phosphoramide [7h], disulfonimide [7e,8], imidodiphosphate
[7d,9] and imidodiphosphorimidate [7a,6] onto the chiral binaph-
thol (BINOL) skeleton is one strategy. Another relies on stabilization
of the conjugate base of the Brønsted acid by intramolecular
hydrogen bonding from the adjacent functional group. Based on the
[7g,11], diphosphoric acid [12], tether-linked diphosphoric acid [13]
and conjugate-base-stabilized carboxylic acids [14] have been
developed. Terada and co-workers employed X-ray diffraction
analysis to reveal intramolecular hydrogen bonding between the
two phosphoric acid moieties in diphosphoric acid [13d]. This
suggests that intramolecular hydrogen bonding in a Brønsted acid
bearing two adjacent strongly electron-withdrawing groups could
play an important role in attaining high reactivity and stereo-
selectivity. We focused on bis(trifluoromethanesulfonyl)imide
(triflimide: Tf2NH), a super Brønsted acid [15], and designed a
BINOL-derived
Brønsted
acid
bearing
bis(trifluoro
methanesulfonimide) group at the 2,20-positions (BSI) 1 (Fig. 1). In
this report, the preparation method of 1 and its application to the
hydrofunctionalization of alkenyl amines and alcohols as
Brønsted acid are described.
a
2. Results and discussion
BSI (1) was synthesized by reaction of trifluoro
methanesulfonamide with the sulfonyl chloride intermediate,
which was prepared from chiral BINOL via SuzukiꢀMiyaura
coupling, NewmanꢀKwart rearrangement, oxidation, and chlori-
nation, according to List’s procedure (non-substituted sulfonimide
1a was synthesized from racemic-BINOL) (Scheme 1) [8b], [c],
* Corresponding author.
E-mail address: rtakagi@hiroshima-u.ac.jp (R. Takagi).
https://doi.org/10.1016/j.tet.2021.132037
0040-4020/© 2021 Elsevier Ltd. All rights reserved.

R. Takagi, Y. Sakai and D.T. Duong
Tetrahedron 85 (2021) 132037
[11c,16]. Although non-substituted sulfonimide 1a was deliques-
cent akin toTf2NH, 3,30-substituted sulfonimides 1b and c were not.
To evaluate the acidic strength of sulfonimide 1, the pKa value of
sulfonimide 1a in DMSO has been predicted by a computational
study using the direct method at the SMD/M06-2x/
6e311þþG(2df,2p)//B3LYP/6-31þG(d) level of theory (Fig. 2) [17].
The predicted pKa value (ꢀ5.26) of 1a suggested that sulfonimide 1
is one of the stronger acids among the known BINOL-derived
Brønsted acids, including disulfonic acid (BINSA), N-tri-
flylphosphoramide (NTPA), sulfuryl imides (JINGLE) and disul-
fonimide (DSI). The optimized structure of 1a and its conjugate
base indicated that the intramolecular hydrogen bonding between
the two sulfonimides plays an important role in enhancing acidity.
Having assessed the acidity of sulfonimides 1, their efficiency as
catalysts in the intramolecular hydroamination of alkenyl amine 2a
was evaluated (Table 1). The hydroamination reaction was con-
ducted in CH2Cl2 at 40 ꢁC for 7 days indicated that the acidity of
sulfonimides 1a and 1b was insufficient for the reaction (entries 1
and 2). The introduction of a CF3 group on the 3,30-substituents
enhanced the reactivity of sulfonimide 1c, and pyrrolidine 3a was
obtained in good yield (81% yield, entry 3). Such a reactivity
enhancement achieved by the introduction of an electron-
withdrawing group accords with the computational pKa predic-
tion by Li and Cheng [17]. When benzene was used as the solvent,
the chemical yield was slightly decreased (entry 4). In order to
improve the efficiency of the reaction, polyfluorinated alcohols
were used as the solvent due to their unique properties, including
their protic and non-nucleophilic character, excellent hydrogen
bonding donor ability, high ionizability beneficial for stabilizing
cationic species, and high polarity (entries 5e7) [18]. The hydro-
amination reaction proceeded smoothly in HFIP even at 20 ꢁC
(entry 5). Unfortunately, TFE was not effective in the reaction (entry
6). Conducting the reaction in HFIP at 60 ꢁC accelerated the
hydroamination, and alkenyl amine 2a was consumed within 12 h,
delivering pyrrolidine 3a in 73% yield (entry 7) [19]. Olefin-
isomerized product 4 was also obtained as a by-product. On these
hydroaminations, enantioselectivity was not observed. In the
chemistry of HFIP with Lewis or Brønsted acids, it is generally
admitted that the true active species as a catalyst is HFIP activated
with Lewis or Brønsted acids, and the activation mode is one of
reasons for the absence of asymmetric induction on the hydro-
amination [18c]. Thus, we focused only on the catalytic ability of
sulfonimide 1c as a Brønsted acid in further investigations.
Scheme 1. Synthesis of BSI (1).
Fig. 2. Computationally predicted pKa values of strong Brønsted acids in DMSO at the
SMD/M06-2x/6e311þþG(2df,2p)//B3LYP/6-31þG(d) level of theory.
result indicated that in addition to the acidity of the Brønsted acid,
the nucleophilicity of its conjugate base likewise affected the re-
action outcome. The poor hydroamination result obtained with
monosulfonimide 6 indicated the importance of adjacent sulfoni-
mide groups for the high reactivity of sulfonimide
1 [20].
Table 1
BSI catalyzed hydroamination of alkenyl amine 2a.
A comparison of catalytic efficiencies of several Brønsted acids
with that of sulfonimide 1c toward hydroamination is summarized
in Table 2. An analogous catalytic hydroamination reaction using
Ca(NTf2)2 in HFIP was reported by Gandon and Leboeuf [18c]. They
found out that the reaction can be promoted by Tf2NH. However,
details regarding the Brønsted-acid-catalyzed hydroamination
were not described. The hydroamination catalyzed by TsOH∙H2O,
Tf2NH, TfOH, and 1c afforded pyrrolidine 3a in good chemical yield
(entries 1e4). A small amount of 3-alkenyl amine 4a, generated by
acid-catalyzed olefin-isomerization, was also observed. When
phosphoric acid 5 was used as the Brønsted acid, olefin-isomerized
product, 3-alkenyl amine 4a, was formed as the major product. This
entry catalyst conditions
yield (%)a e.e. (%)b
1
2
3
4
5
6
7c
rac-1a
1b
1c
1c
1c
DCM, 40 ꢁC, 7 days
DCM, 40 ꢁC, 7 days
DCM, 40 ꢁC, 7 days
benzene, 40 ꢁC, 7 days
HFIP, 20 ꢁC, 4 days
TFE, 20 ꢁC, 4 days then 40 ꢁC, 3 days 25
HFIP, 60 ꢁC, 12 h
73
23
26
81
70
82
e
15
3
0
N.D.
0
1c
1c
N.D.
a
b
c
Isolated yield.
Determined by chiral stationary phase HPLC analysis.
Alkene 4a, which was generated from olefin-isomerization of 2a, was also ob-
tained: E-4a: 5.6% yield, Z-4a: 4.4% yield. Ts ¼ p-MeC6H4SO2. DCM ¼ CH2Cl2. HFIP ¼
Fig. 1. BSI (1).
(CF3)2CHOH. TFE ¼ CF3CH2OH, N.D. ¼ not determined.
2

R. Takagi, Y. Sakai and D.T. Duong
Tetrahedron 85 (2021) 132037
Considering the time profile of the yield of pyrrolidine 3a in the
hydroamination reaction of 2a, the order of Brønsted acid catalytic
efficiency was TsOH∙H2O, Tf2NH > TfOH z 1c >> 6 (Fig. 3).
Next, the scope of 1c-catalyzed hydroamination was examined
(Table 3). Based on the results listed in Table 1, hydroamination of
alkenyl amines 2 was performed at 60 ꢁC. The introduction of a
gem-methyl group on the alkenyl amine accelerated the reaction
rate (entry 1). The hydroamination of alkenyl amine containing an
internal olefin required a longer reaction time to consume the
starting material than that of terminal olefin derivatives (entries 2
and 3). As olefin-isomerization of alkenyl amine was a competing
reaction under hydroamination reaction conditions, pyrrolidine 3c
was generated as the major product in the case of alkenyl amine 2d
(entry 4) [21]. For phenyl-substituted alkenyl amines 2e and 2f,
cyclization occurred at the benzylic position, wherein the most
stable benzylic cation was generated, and pyrrolidine 3e or piper-
idine 7f was obtained, respectively (entries 5 and 6). However,
when the reaction was performed at 20 ꢁC with 2f, the chemical
yield of cyclization products 3f and 7f decreased and pyrrolidine 3f
was obtained as the major product. These data suggest that the
reaction rate of the cyclization at 60 ꢁC follows the order of 5-
exo > terminal olefin-isomerization > internal olefin isomerization
z 5-endo > 6-exo (for 2a-d) [22] and the benzylic cation inter-
mediate is generated from 2e,f as the stable cation intermediate at
that temperature.
Fig. 3. Time profile of the chemical yield of pyrrolidine 3a in the hydroamination of 2a.
chiral BINOL skeleton. A comparison of the catalytic efficiency of BSI
with that of monosulfonimide 6 indicated the importance of
adjacent sulfonimide groups for enhancing the acidity of BSI. BSI
also catalyzed the hydroalkoxylation of alkenyl alcohols in HFIP
with high efficiency. Advantages of BSI include its strong Brønsted
acidity, reusability, and ease of handling than TsOH∙H2O, Tf2NH,
and TfOH. Further studies aimed at developing a stereoselective
reaction using BSI are currently underway in our laboratory.
Finally, the scope of hydroalkoxylation catalyzed by 1c was
examined (Table 4). These reactions likewise proceeded via the
most stable benzylic cation intermediate.
3. Conclusion
In conclusion, we have developed a novel Brønsted acid con-
taining bis(trifluoromethanesulfonimide) (BSI) functionality.
Computational prediction of the pKa of BSI placed it in the category
of strong Brønsted acids. The hydroamination reaction of alkenyl
amine in HFIP was catalyzed by BSI with comparable efficiency to
that of TsOH∙H2O, Tf2NH, and TfOH. However, enantioselectivity in
the hydroamination was not observed, although BSI contained a
4. Experimental section
General Information. Internal references for 1H NMR spectra
were 0.0 ppm (Me4Si) for CDCl3 and CD3OD (3.31 ppm). Chemical
shifts for 13C NMR spectra were referenced to CDCl3 (77.0 ppm) and
CD3OD (49.0 ppm). Chemical shift for 19F NMR spectra were re-
ported on the basis on CF3CO2H (ꢀ76.0 ppm) as an external stan-
dard. High resolution mass spectral (HRMS) data were recorded
with a LTQ Orbitrap trap mass spectrometer using electrospray
ionization (ESI) method. Optical rotations were measured on a
digital polarimeter with a 0.1 dm cell at room temperature. All
reactions involving air- and moisture-sensitive reagents were car-
ried out under N2. All reactions were monitored by analytical thin-
layer chromatography (TLC), which was visualized by ultraviolet
light (254 nm).
Table 2
Comparison of catalytic efficiency of several Brønsted acids and 1c.
Typical procedure for preparation of BSI. According to the
literature procedure, the corresponding sulfonyl chloride was pre-
pared by N-chlorosuccinimide (NCS) and the corresponding thio-
carbamoyl derivative (for 1a) [8c] or thionyl chloride and the
corresponding sulfuric acid derivative (for 1b and 1c) [8b]. A
mixture of the corresponding sulfonyl chloride (0.27 mmol), tri-
fluoromethanesulfonamide (1.0 mmol), and K2CO3 (1.17 mmol) in
MeCN (1.3 mL) was stirred at 70 ꢁC for 24 h. The reaction mixture
was quenched with 1 M HCl, and extracted by EtOAc. The organic
layer was washed with H2O and brine, dried over Na2SO4, and
evaporated. The crude product was purified by column chroma-
tography on silica gel, dissolved in CH2Cl2 and washed with 6 M
HCl. Removing the solvent in vacuo yielded the desired product. BSI
was dried in vacuo at 50 ꢁC for 3 h before using.
yield (%)b
Entry
catalyst
conv (%)a
3a
E-4a
Z-4a
1
2
3
4
5
6
TsOH∙H2O
96
94
97
86
81
48
89
88
85
82
7
3.9
3.6
1
2.5
61
9.4
3.2
2.3
0.6
1.8
6
Tf2NH
TfOH
1c
5
6
BSI-1a. Purified by column chromatography (EtOAc/
33
3.3
hexane ¼ 20:1), 93% yield. White solid. m. p. 186e189 ꢁC.
Determined by1H NMR measurement of the crude product using 1,3,5-
a
D
[
a
]
ꢀ35.7 (c 1.25, MeOH). IR (KBr)
n 391, 3250, 1638, 1321,
8.30 (J ¼ 8.7 Hz, 2 H), 8.09
23
trimethoxybenzene as an internal standard.
b
Isolated yield.
1192 cmꢀ1. 1H NMR (500 MHz, CD3OD)
d
3

R. Takagi, Y. Sakai and D.T. Duong
Tetrahedron 85 (2021) 132037
Table 3
Scope of hydroamination catalyzed by 1c.
a
Isolated yield.
b
A mixture of Eꢀ and Z-N-toluenesulfonyl 4-hexen-1-amine was also obtained in 8% yield.
c
A mixture of Eꢀ and Z-isomer (E:Z ¼ 5.3:1) was used as the substrate.
d When the reaction was carried out at 20 ꢁC for 4 h, a mixture of pyrrolidine derivative and piperidine derivative was obtained (pyrrolidine 3f: 20% yield, piperidine 7f: 5%
yield).
(d, J ¼ 7.6 Hz, 2 H), 7.95 (d, J ¼ 8.7 Hz, 2 H), 7.51 (t, J ¼ 7.6 Hz, 2 H),
7.23 (t, J ¼ 7.6 Hz, 2 H), 7.02 (d, J ¼ 7.6 Hz, 2 H). 13C NMR
127.7 (x2), 120.8 (q, J ¼ 323 Hz). 19F NMR (376 MHz, CD3OD)
d
ꢀ79.15. HRMS (ESIꢀ) m/z calcd for [M-2H þ Na]2ꢀ C34H20O8N2
-
(125 MHz,CD3OD)
d
140.5, 135.2, 134.7, 133.7, 129.1, 129.0, 128.3,
F6NaS4 848.99099, Found 848.99127.
128.1, 127.4, 124.9, 120.8 (q, J ¼ 320 Hz). 19F NMR (470 MHz, CD3OD)
BSI-3a. Purified by column chromatography (CH2Cl2/
D
d
ꢀ79.65. HRMS (APClþ) m/z calcd for C22H14O8N2F6NaS4 [MþNa]þ
MeOH ¼ 6:1), 83% yield. White solid. m. p.141e143 ꢁC. [
a] 23 48.5 (c
698.94110, found 698.94294.
1.27, CHCl3). IR (KBr) n
3468, 1621, 1311, 1280, 1185 cmꢀ1. 1H NMR
Sulfonyl chloride for BSI-1b. White solid. m. p. 177e180 ꢁC,
(400 MHz, CD3OD)
d
8.25 (s, 4 H), 7.96 (d, J ¼ 8.0 Hz, 2 H), 7.91 (s,
D
[
a
]
85.9 (c 1.07, CHCl3), IR (KBr)
n
1379, 1181 cmꢀ1
.
1H NMR
2 H), 7.88 (s, 2 H), 7.58 (t, J ¼ 8.0 Hz, 2 H), 7.35 (t, J ¼ 8.0 Hz, 2 H), 7.13
23
(400 MHz, CDCl3)
d
8.04 (s, 2 H), 7.98 (d, J ¼ 7.7 Hz, 2 H), 7.71 (t,
(d, J ¼ 8.0 Hz, 2 H). 13C NMR (100 MHz, CD3OD)
d 146.0, 139.4, 138.3,
J ¼ 7.7 Hz, 2 H), 7.64-7.60 (m, 4 H), 7.52-7.44 (m, 8 H), 7.20 (d,
136.6, 134.5, 134.2, 133.5, 132.8, 132.0, 131.4, 130.7, 130.0, 129.4,
J ¼ 7.7 Hz, 2 H). 13C NMR (100 MHz,CDCl3)
d
140.0, 139.0, 137.6,
128.5, 126.6, 125.2 (q, J ¼ 273 Hz), 121.4, 121.0 (q, J ¼ 320 Hz). 19
F
137.2, 135.0, 134.4, 131.6, 131.0, 130.5, 130.1, 128.5, 128.3, 128.1 (x2),
127.5, 127.3. HRMS (ESIþ) m/z calcd for C32H20O4Cl2NaS2 [MþNa]þ
625.00072, found 625.00720.
NMR (376 MHz, CD3OD)
d
ꢀ62.1, ꢀ78.3. HRMS (ESIꢀ) m/z calcd for
C
38H16O8N2F18S4 [M ꢀ 2H]2ꢀ 548.97565, found 548.97662.
The following compounds (6 [23], 2a [24], 2b [25], 2c [26], 2d
BSI-2b. Purified by column chromatography (EtOAc), 93% yield.
[24], 2e [27], 8b [28], 8c29) were prepared according to the known
D
White solid. m. p.104e105 ꢁC. [
a
]
23 55.9 (c 1.26, CHCl3). IR (KBr)
n
procedures.
3434, 1620, 1308, 1190 cmꢀ1. 1H NMR (400 MHz, CD3OD)
d
7.86 (d,
(4E)-Methyl-N-(5-phenylpent-4-en-1-yl)
mide (2f). A mixture of 2a (51.0 mg, 0.23 mmol), Hoveyda-Grubbs
catalyst 2nd generation (20.0 mg, 24.3 mol), CH2Cl2 (4.0 mL) and
styrene (90 l, 0.78 mmol) was refluxed for 15 h [30]. The reaction
mixture was evaporated to obtain the crude product. The crude
benzenesulfona-
J ¼ 8.0 Hz, 2 H), 7.78 (s, 2 H), 7.68 (d, J ¼ 6.1 Hz, 4 H), 7.50 (t,
J ¼ 8.0 Hz, 2 H), 7.37-7.29 (m, 6 H), 7.23 (t, J ¼ 8.0 Hz, 2 H), 7.15 (d,
m
J ¼ 8.0 Hz, 2 H). 13C NMR (100 MHz, CD3OD)
d
143.2, 139.7, 139.3,
m
138.9, 134.8, 133.8, 133.7, 131.8 (x2), 130.2, 129.3, 128.3, 128.0 (x2),
4

R. Takagi, Y. Sakai and D.T. Duong
Tetrahedron 85 (2021) 132037
Table 4
Scope of hydroalkoxylation catalyzed by 1c.
a
Isolated yield.
b
A mixture of Eꢀ and Z-isomer (E:Z ¼ 5.3:1) was used as the substrate.
c
A mixture of Eꢀ and Z-isomer (E:Z ¼ 22:1) was used as the substrate.
product was purified by column chromatography on silica gel
(CH2Cl2) to give 2f (36.6 mg, 51%) as a white solid. The spectral data
of the purified product 2f are in the agreement with the published
data [31].
in Table 3, entries 3 and 4.
Computational methods. The pKa value of sulfonimide 1a in
DMSO has been predicted by the computational study using the
direct method following X. Li, and J.-P. Cheng’s method [18].
4-Phenylpent-4-en-1-ol (8a). 3-Phenyl-3-butenyl-1-cyanide
was prepared according to known procedure [32]. Diisobutylalu-
minum hydride (DIBAL-H) (Ca. 1.0 M in hexane, 2.4 mL, 2.4 mmol)
was added dropwise to 3-phenyl-3-butenyl-1-cyanide (240 mg,
1.5 mmol) in CH2Cl2 (4.4 mL) at ꢀ78 ꢁC. After the reaction mixture
was stirred at the temperature for 2 h, the mixture was quenched
by sat. NH4Cl at ꢀ78 ꢁC. The reaction mixture was extracted with
EtOAc, dried over Na2SO4, filtered, and concentrated in vacuo.
NaBH4 (66.0 mg, 1.7 mmol) was added slowly to the obtained crude
product in MeOH (4.4 mL) at r. t., and the reaction mixture was
stirred overnight. The mixture was quenched by sat. NH4Cl,
extracted with EtOAc, dried over Na2SO4, filtered, and concentrated
in vacuo. The crude product was purified by column chromatog-
raphy (SiO2, EtOAc/hexane ¼ 15:1) to give 8a (50 mg, 20% for 2
steps) as a colorless oil. The spectral data of the purified product 8a
are in the agreement with the published data [33].
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
5. Acknowledgements
MS, and optical rotation measurements were made at the Nat-
ural Science Center for Basic Research and Development (N-BARD),
Hiroshima University.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2021.132037.
Typical procedure for hydrofunctionalization. Alkenylamine
4-methyl-N-(pent-4-en-1-yl)benzenesulfonamide 2a (41.0 mg,
0.17 mmol) was charged in a crew-cap vial equipped with a stir bar.
References
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and the tube was sealed. The reaction mixture was stirred at 60 ꢁC
and the progress of the reaction was monitored by thin layer
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reaction was quenched with solid NaHCO3 and filtered. The filtrate
was evaporated, and the crude product was purified by column
chromatography (SiO2, EtOAc/hexane ¼ 15:1) to give hydro-
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7c [38], 7e [2a], 9a [39], 9b [40], 9c41) are in the agreement with the
published data. Compounds 3c and 7c were obtained as a mixture
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6