Boron-Catalyzed Double Hydrofunctionalization Reactions of Unactivated Alkynes

Article abstract of DOI:10.1021/acscatal.8b00955

Tandem hydroalkoxylation/hydroallylation and hydroalkoxylation/hydrocyanation reactions of alkyl-substituted unactivated alkynes by catalytic systems based on B(C₆F₅)₃·nH₂O and silyl nucleophiles were developed.

Full text of DOI:10.1021/acscatal.8b00955

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Boron-Catalyzed Double Hydrofunctionalization
Reactions of Unactivated Alkynes
Masatoshi Shibuya, Masaki Okamoto, Shoji Fujita, Masanori Abe, and Yoshihiko Yamamoto
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00955 • Publication Date (Web): 06 Apr 2018
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ACS Catalysis
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Boron-Catalyzed Double Hydrofunctionalization Reactions of Unac-
tivated Alkynes
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Masatoshi Shibuya,* Masaki Okamoto, Shoji Fujita, Masanori Abe, and Yoshihiko Yamamoto
Department of Basic Medicinal Sciences, Graduate School of Pharmaceutical Sciences, Nagoya University, Furoꢀcho,
Chikusa, Nagoya 464ꢀ8601, Japan
ABSTRACT: Tandem hydroalkoxylation/hydroallylation and hydroalkoxylation/hydrocyanation reactions of alkylꢀsubstituted
unactivated alkynes by catalytic systems based on B(C6F5)3·nH2O and silyl nucleophiles were developed. The characteristic high
alkynophilicity of B(C6F5)3 enabled the selective activation of the unactivated alkynes in the presence of the reactive alkene of alꢀ
lylsilane. Moreover, the alkynes were electrophilically activated in the presence of cyanide in this catalytic system. Mechanistic
studies suggest that the alkynes are activated by the different catalytic species in the two reactions.
KEYWORDS: boron, tandem reaction, hydroalkoxylation, hydroallylation, hydrocyanation
Introduction
Tandem double hydrofunctionalization reaction of alkynes
enables the creation of structures having a tetrasubstituted
carbon center when two C–X bonds (X ≠ H) are regioselecꢀ
tively formed on the same internal carbon.1 Intramolecular
hydroalkoxylationꢀinitiated double hydrofunctionalization
reactions afford cyclic ethers having a tetraꢀsubstituted carbon
at the 2 position, which are powerful reactions for the rapid
syntheses of highly functionalized cyclic ethers. Such reacꢀ
tions can be established by the following tandem process: the
intramolecular hydroalkoxylation of alkynes, and the protonaꢀ
tion of the resultant 2ꢀalkylidene cyclic ethers to generate their
corresponding oxocarbenium intermediates, followed by the
addition of nucleophiles.2,3 To date, a number of transition
metal catalysts such as Pd, Rh, Ir, Au, and Cu have been reꢀ
ported to promote the intramolecular hydroalkoxylation of
unactivated alkynes to yield 2ꢀalkylidene cyclic ethers or their
corresponding isomers.2ꢀ4 Several studies have reported that 2ꢀ
alkoxyꢀ2ꢀalkylꢀsubstituted cyclic ethers were formed by a
double hydroalkoxylation (Figure 1a).2 The syntheses of 2ꢀ
Figure 1. (a) Previous studies. (b) This study.
alkylꢀ2ꢀarylꢀsubstituted cyclic ethers by the hydroalkoxylaꢀ
Based on this assumption, hydroxy alkyne 1a was treated with
triflic imide (Tf2NH) in the presence of allyltrimethylsilane (2)
(Scheme S1). This reaction unfortunately resulted in the forꢀ
tion/hydroarylation introducing electronꢀrich heteroaromatic
compounds such as indoles and pyrroles were also reported.3
However, reaction introducing other carbon nucleophiles in an
mation of the silyl ether, as Tf2NH protonated the allylsilane
intermolecular fashion has not been reported to the best of our
prior to activating alkyne 1a. This result suggests that discrimꢀ
ination between the alkyne and the reactive alkene of the alꢀ
knowledge. We, therefore, expect successful double hydroꢀ
functionalization reactions using more versatile nucleophiles
lylsilane is an issue in the development of our desired reaction.
such as allyl or cyano group, which can be a foothold to lead
We
therefore
examined
the
use
of
tris(pentafluorophenyl)borane, B(C6F5)3,6 which is a unique
main group Lewis acid that is strongly acidic, stable, and steriꢀ
cally bulky.6ꢀ8 Hashmi and coworkers recently reported the
to diverse structures by further transformations (Figure 1b).
We recently reported the hydroalkoxylation/reduction using a
Brønsted acidꢀsilane catalytic system as a part of our interest
in the catalytic electrophilic activation of the unactivated alꢀ
kynes by Brønsted acid (Figure 1a).5 In this case, the formed
oxocarbenium species were reduced by Et3SiH to its correꢀ
sponding saturated cyclic ethers. We envisioned that if this
reaction is performed in the presence of an allylsilane instead
of the hydrosilane, the intramolecular hydroalkoxylaꢀ
tion/hydroallylation reaction will occur.
9
cyclization of enynes with B(C6F5)3, where the B(C6F5)3 prefꢀ
erentially activated the alkynes over alkenes. Although alꢀ
lylsilane 2 is more reactive than such alkenes, we expect that
B(C6F5)3 will selectively activate any unactivated alkynes in
the presence of 2. Thus, we herein report novel catalytic sysꢀ
tems based on the use of B(C6F5)3·nH2O10 and silyl nucleoꢀ
philes for the double hydrofunctionalization reaction
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Hydroalkoxyla-
(i.e., 44%), likely due to the hydrolysis of the ester. 4ꢀPentynꢀ
Page 2 of 8
Table
1.
Optimization
of
the
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5
6
7
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tion/Hydroallylation Reaction.
1ꢀol (1h), with no substituent on the linker between the hyꢀ
droxy group and the alkyne, afforded 3h in moderate yield.17
Secondary alcohol 1i efficiently cyclized to give 5ꢀalkylꢀ
substituted product 3i in 72% yield, although 3ꢀalkylꢀ
substituted product 3j was formed in a low yield (i.e., 9%).
Notably, enyne 1k selectively produced allylꢀsubstituted 3k in
68% yield without the production of propargylꢀsubstituted 3k’.
This result suggests that the alkyne was selectively activated
over alkene under the standard reaction conditions. Internal
alkyne 1l produced an inseparable mixture of 5ꢀexo cyclizaꢀ
tion product 3l and the 6ꢀendo cyclization product (i.e. 3m) in
a 19% combined yield, while the reaction of δꢀhydroxy alkyne
1m efficiently proceeded to give the desired product 3m in
70% yield.
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Table 2. Substrate Scope of the Hydroalkoxyla-
tion/Hydroallylation Reaction
a Recovered starting material.
of unactivated alkynes (Figure 1b).11,12 In addition, although
the waterꢀtolerance of B(C6F5)3 and the Brønsted acidity of
B(C6F5)3·H2O have been reported,10,12,13 our mechanistic invesꢀ
tigation uncovered the inherent catalytic behaviors of
B(C6F5)3·nH2O in the presence of silyl nucleophiles. The actiꢀ
vation modes of the alkynes in the two reactions examined
herein differ significantly.
Results and discussion
Initially, we treated hydroxy alkyne 1a with 120 mol %
B(C6F5)3·nH2O and 2.4 equiv allylsilane 2 in the presence of
H2O as a proton source in CH2Cl2 at 40 °C (Scheme S2). Theꢀ
se conditions allowed the hydroalkoxylation/hydroallylation
reaction to proceed to give the desired product 3a in a good
yield. Similarly, the use of 10 mol % B(C6F5)3·nH2O and 2.0
equiv allylsilane 2 also promoted the desired reaction; howevꢀ
er, the yield of 3a was not consistent in repeated experiments.
Thus, following extensive optimization of the reaction condiꢀ
tions, a reproducible protocol was established (Table 1). More
specifically, a solution of 10 mol % B(C6F5)3·nH2O and 0.5
equiv H2O in 1,2ꢀdichloroethane (DCE) was premixed at 60
°C for 3 h, prior to the slow addition of a solution containing
1a and allylsilane 2 in DCE over 2 h. After complete addition
of 1a and 2, the reaction mixture was worked up to give the
desired allylꢀsubstituted cyclic ether 3a in 82% yield (entry 1).
Interestingly, we found that the premixing of B(C6F5)3·nH2O
and H2O was essential to ensure reproducible results (entry 2).
a 20 mol % B(C6F5)3·nH2O. b Yield determined by 1H NMR.
Upon screening a range of silyl nucleophiles, we found that
the hydroalkoxylation/hydrocyanation reaction was also proꢀ
moted by the use of trimethylsilyl cyanide (TMSCN, 5) as a
nucleophile. It should be noted that this reaction does not reꢀ
quire the slow addition technique. Thus, as outlined in Table
3, 4,4ꢀdisubstituted products 6a and 6b, 4ꢀmonosubstituted
products 6cꢀ6f, as well as 6ꢀmembered product 6m were proꢀ
vided in high yields (76–95%). The reaction of 4ꢀpentynꢀ1ꢀol
(1h) efficiently afforded the desired product in 81% NMR
yield. Interestingly, the results in the reactions of 1i, 1j, 1k and
internal alkynes 1l and 1n are significantly different from
those observed in the hydroalkoxylation/hydroallylation reacꢀ
tion (also see Table 2). More specifically, secondary alcohol 1i
afforded 5ꢀalkylꢀsubstituted product 6i in a low yield, while
the corresponding product 3i was produced in a high yield. On
the other hand, 3ꢀalkylꢀsubstituted product 6j was formed effiꢀ
ciently in 82% yield, while the corresponding product 3j was
obtained in a low yield. In the reaction of enyne 1k, 13% of
propargylꢀsubstituted 3k’ was produced together with allylꢀ
substituted 6k in 54% yield as an inseparable mixture. Furꢀ
thermore, internal alkynes 1l and 1n selectively gave 5ꢀexo
To account for this observation,
a 1:5 mixture of
B(C6F5)3·nH2O and H2O was monitored by 19F NMR spectrosꢀ
copy. However, a significant change in the signals was not
observed.14,15 We therefore assumed that premixing was reꢀ
quired to ensure the dissolution of H2O in DCE. Slow addition
of the reagents was essential to suppress the undesired dimeriꢀ
zation reaction (entry 3). An increase in the quantity of H2O to
2.2 equiv had a detrimental effect (entries 4 and 5), and 2.0
equiv of allylsilane 2 was optimal (entries 6 and 7).
With the optimized reaction conditions in hand, we moved on
to investigate the substrate scope of the hydroalkoxylaꢀ
tion/hydroallylation reaction (Table 2). In addition to the forꢀ
mation of 4,4-disubstituted product 3b from 1b, a variety of 4ꢀ
monosubstituted products 3c–3f were successfully obtained
from 1c–1f.16 Benzoate 3g was obtained in a moderate yield
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cyclization products 6l and 6n in high yields (85% and 83%,
ing the addition of 4 equiv TMSCN (5). This species was deꢀ
termined as H+[NCB(C6F5)3]− based on its similarities with the
spectrum of [K+(18ꢀcrownꢀ6)][NCB(C6F5)3]–.18 In addition, a
characteristic shift of the CN stretching vibrational band from
2191 (TMSCN) to 2252 cm–1 was observed in the analysis of
the 1:4 solution of B(C6F5)3·nH2O:TMSCN (5) by IR specꢀ
troscopy,19 and highꢀresolution mass spectrometry confirmed
the expected molecular ion mass (Calcd. for C19BF15N [M–
H]−: 537.9890; found 537.9891).
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2
3
4
5
6
7
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respectively).
Table 3. Substrate Scope of the Hydroalkoxyla-
tion/Hydrocyanation Reactiona
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(a)
ortho
para
meta
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B(C6F5)3—nH2O
+1 equiv 2
+3 equiv 2
+5 equiv 2
0.0
ꢀ130.0
ꢀ140.0
ꢀ150.0
ꢀ160.0
ꢀ170
a After a solution of 10 mol % B(C6F5)3·nH2O and 2.0 equiv
H2O in DCE was premixed at 60 °C for 3 h, alkyne 1 and 2.0
equiv TMSCN were added, and the reaction mixture was stirred at
60 °C. b Yield determined by 1H NMR. c A small amount of impuꢀ
rity could not be removed.
(b)
B(C6F5)3—nH2O
+1 equiv 5
Thus, upon comparison of the substrate scopes of the hydroalꢀ
koxylation/hydroallylation
and
hydroalkoxylaꢀ
+2 equiv 5
tion/hydrocyanation reactions, notable differences were obꢀ
served as indicated above. In addition, when alkene 7 was
subjected to the standard reaction conditions, interesting reꢀ
sults were obtained (Scheme 1). Although no reaction was
observed under standard conditions of the hydroalkoxylaꢀ
tion/hydroallylation reaction, hydroalkoxylation of 7 was sucꢀ
cessful under standard conditions of the hydroalkoxylaꢀ
tion/hydrocyanation reaction to give 8 in 79% yield. These
results therefore indicate that the active species of these two
reactions must differ from one another.
+3 equiv 5
+4 equiv 5
0.0
ꢀ130.0
ꢀ140.0
ꢀ150.0
ꢀ160.0
ꢀ170
Figure 2. 19F NMR spectra of the reaction of B(C6F5)3·nH2O
with (a) allylsilane 2 and (b) TMSCN (5).
Scheme 1. Hydroalkoxylation of Alkene 7
To gain insight into the catalytically active species involved in
these transformations, we monitored the reaction of
B(C6F5)3·nH2O and allylsilane 2 by 19F NMR spectroscopy
(Figure 2a). Downfield shifts of all three peaks, particularly
the paraꢀF signal, were observed depending on the amount of
allylsilane 2 until 5 equiv of 2 was added. Interestingly, the
final spectrum was comparable with that of anhydrous
B(C6F5).18 We also monitored the reaction of B(C6F5)3·nH2O
with TMSCN (5)(Figure 2b). In this case, after multiple speꢀ
cies were observed, they converged to a single species followꢀ
Scheme 2. Plausible Reaction Mechanism
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Page 4 of 8
Science Research from AMED. S.F. express his thanks for the
Sasakawa Scientific Research Grant from the Japan Science Sociꢀ
ety.
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Scheme 3. Hydroalkoxylation/Hydrocyanation After in situ
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Catalyzed Multistep Reactions of Indoles with Alkynyl Alcohols. Chem. -
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Reddy, V. V. N.; Raju, P. V. K. ThorpeꢀIngold Effect in Copper(II)ꢀ
Catalyzed Formal HydroalkoxylationꢀHydroarylation Reaction of Alꢀ
kynols with Indoles. Tetrahedron Lett. 2009, 50, 6576ꢀ6579. (c) Patil, N.
T.; Singh, V.; Konala, A.; Mutyala, A. K. Gold(I)ꢀCatalyzed Oneꢀpot
Reaction between 2ꢀAlkynylanilines and Alkynols Leading to the Forꢀ
mation of Cꢀ3ꢀSubstituted Indoles: A Case of Formal Carboamination of
Alkynes. Tetrahedron Lett. 2010, 51, 1493ꢀ1496.
(4) (a) Utimoto, K. Palladium Catalyzed Synthesis of Heterocycles.
Pure Appl. Chem. 1983, 55, 1845–1852. (b) Wilckens, K.; Uhlemann, M.;
Czekelius, C. GoldꢀCatalyzed endoꢀCyclizations of 1,4ꢀDiynes to Sevenꢀ
Membered Ring Heterocycles. Chem. - Eur. J. 2009, 15, 13323ꢀ13326. (c)
Seo, S. Y.; Yu, X. H.; Marks, T. J. Intramolecular Hydroalkoxylaꢀ
tion/Cyclization of Alkynyl Alcohols Mediated by Lanthanide Catalysts.
Scope and Reaction Mechanism. J. Am. Chem. Soc. 2009, 131, 263ꢀ276.
(d) Pouy, M. J.; Delp, S. A.; Uddin, J.; Ramdeen, V. M.; Cochrane, N. A.;
Fortman, G. C.; Gunnoe, T. B.; Cundari, T. R.; Sabat, M.; Myers, W. H.
Intramolecular Hydroalkoxylation and Hydroamination of Alkynes Cataꢀ
lyzed by Cu(I) Complexes Supported by NꢀHeterocyclic Carbene Ligands.
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(5) Shibuya, M.; Fujita, S.; Abe, M.; Yamamoto, Y. Brønsted Acꢀ
id/Silane Catalytic System for Intramolecular Hydroalkoxylation and
Hydroamination of Unactivated Alkynes. ACS Catal. 2017, 7, 2848ꢀ2852.
(6) (a) Piers, W. E.; Chivers, T. Pentafluorophenylboranes: from Obꢀ
scurity to Applications. Chem. Soc. Rev. 1997, 26, 345ꢀ354. (b) Erker, G.
Tris(pentafluorophenyl)borane: A Special Boron Lewis Acid for Special
Reactions. Dalton Trans. 2005, 1883ꢀ1890.
(7) B(C6F5)3 has been also demonstrated to play a key role in Frustrated
Lewis pair (FLP) chemistry, see; Stephan, D. W.; Erker, G. Frustrated
Lewis Pair Chemistry: Development and Perspectives. Angew. Chem. Int.
Ed. 2015, 54, 6400ꢀ6441.
(8) FLPꢀmediated intramolecular hydroamination/hydrogenation reacꢀ
tion of alkynes has been reported, see: Mahdi, T.; Stephan, D. W. Stoichiꢀ
ometric and Catalytic Interꢀ and Intramolecular Hydroamination of Terꢀ
minal Alkynes by Frustrated Lewis Pairs. Chem. - Eur. J. 2015, 21,
11134ꢀ11142.
(9) Hansmann, M. M.; Melen, R. L.; Rudolph, M.; Rominger, F.;
Wadepohl, H.; Stephan, D. W.; Hashmi, A. S. K. Cyclopropanaꢀ
tion/Carboboration Reactions of Enynes with B(C6F5)3. J. Am. Chem. Soc.
2015, 137, 15469ꢀ15477.
(10) B(C6F5)3 rapidly forms hydrates B(C6F5)3·nH2O under air, see:
Bergquist, C.; Bridgewater, B. M.; Harlan, C. J.; Norton, J. R.; Friesner, R.
A.; Parkin, G. Aqua, Alcohol, and Acetonitrile Adducts of
Tris(perfluorophenyl)borane: Evaluation of Brønsted Acidity and Ligand
Lability with Experimental and Computational Methods. J. Am. Chem.
Soc. 2000, 122, 10581ꢀ10590.
Preparation of H+[NCB(C6F5)3]–
9
On the basis of the above experimental observations, plausible
reaction mechanisms are shown in Scheme 2. In the hydroalꢀ
koxylation/hydroallylation reaction, although the effect of the
dehydration of B(C6F5)3·nH2O by allylsilane 2 was unclear,
B(C6F5)3 behaved as a Lewis acid catalyst to activate the alꢀ
kynes, which promote the hydroalkoxylation reaction followed
by the protonation and the allylation. In the hydroalkoxylaꢀ
tion/hydrocyanation reaction, H+[NCB(C6F5)3]− generated
from B(C6F5)3·nH2O and TMSCN (5) behaved as a Brønsted
acid catalyst to activate the alkynes and promote the hydroalꢀ
koxylation/hydrocyanation reaction, which is consistent with
the fact that Brønsted acids such as TfOH and HI promote the
intramolecular hydroalkoxylation of alkenes (Scheme 1).20,21
Finally, in order to verify the possibility of H+[NCB(C6F5)3]−
as an actual species, we conducted the following control exꢀ
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periment (Scheme 3). After premixing
a solution of
B(C6F5)3·nH2O and H2O in DCE at 60 °C for 3 h, TMSCN (5)
was added. 19F NMR analysis of the resultant solution showed
the generation of H+[NCB(C6F5)3]− (Figure S6). After that, 1a
was added to the mixture. We confirmed the desired reaction
to proceed with the almost same efficiency.
In conclusion, we successfully developed B(C6F5)3·nH2Oꢀ
allylsilane 2 and B(C6F5)3·nH2Oꢀsilyl cyanide 5 catalytic sysꢀ
tems for the tandem hydroalkoxylation/hydroallylation and
hydroalkoxylation/hydrocyanation reactions of unactivated
alkynes. These reactions enabled the rapid synthesis of highly
functionalized cyclic ethers bearing allylꢀ or cyanoꢀsubstituted
tetraꢀsubstituted carbon centers at the 2 position.22 The mechaꢀ
nistic investigations suggest that the alkynes were activated by
B(C6F5)3 as a Lewis acid catalyst in the presence of allylsilane
2, whereas in the presence of TMSCN (5), they were activated
by H+[NCB(C6F5)3]− as a Brønsted acid catalyst. These results
demonstrate the potency of the catalytic system using
B(C6F5)3·nH2O and silyl nucleophiles. Further efforts to exꢀ
pand the utility of the catalytic system are underway.
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: mꢀshibu@ps.nagoyaꢀu.ac.jp.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS
Publications
website.
Control experiments, experimental details, characterization of
new compounds, and NMR spectra (PDF)
ACKNOWLEDGMENT
(11) (a) Imamura, K.; Yoshikawa, E.; Gevorgyan, V.; Sudo, T.; Asao,
N.; Yamamoto, Y. Lewis AcidꢀMediated Intramolecular Addition of Silyl
Enol Ethers to Internal Unactivated Alkynes. Can. J. Chem. 2001, 79,
This work was supported by JSPS KAKENHI (No. JP16K08162),
and the Platform Project for Supporting Drug Discovery and Life
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1624ꢀ1631. (b) Rubin, M.; Gevorgyan, V. B(C6F5)3ꢀCatalyzed Allylation
(16) (a) Schmitt, A.; Reissig, H. U., On the Stereoselectivity of γꢀ
Lactol Substitutions with Allylꢀ and Propargylsilanes ꢀ Synthesis of Diꢀ
substituted Tetrahydrofuran Derivatives. Eur. J. Org. Chem. 2000, 3893ꢀ
3901. (b)Schmitt, A.; Reissig, H. U. Lewis acidꢀPromoted Reactions of γꢀ
lactols with Silyl Enol Ethers ꢀ Stereoselective Formation of Functionalꢀ
ized Tetrahydrofuran Derivatives. Eur. J. Org. Chem. 2001, 1169ꢀ1174.
(17) Kaneti, J.; Kirby, A. J.; Koedjikov, A. H.; Pojarlieff, I. G. Thorpeꢀ
Ingold Effects in Cyclizations to FiveꢀMembered and SixꢀMembered
Rings Containing Planar Segments. The Rearrangement of N(1)ꢀAlkylꢀ
substituted Dihydroorotic Acids to Hydantoinacetic Acids in Base. Org.
Biomol. Chem. 2004, 2, 1098ꢀ1103.
(18) Vei, I. C.; Pascu, S. I.; Green, M. L. H.; Green, J. C.; Schilling, R.
E.; Anderson, G. D. W.; Rees, L. H. Synthesis and Study of New Binucleꢀ
ar Compounds Containing Bridging (µꢀCN)B(C6F5)3 and (µꢀNC)B(C6F5)3
Systems. Dalton Trans. 2003, 2550ꢀ2557.
(19) Nagata, T.; Matsubara, H.; Kiyokawa, K.; Minakata, S. Catalytic
Activation of 1ꢀCyanoꢀ3,3ꢀdimethylꢀ3ꢀ(1H)ꢀ1,2ꢀbenziodoxole with
B(C6F5)3 Enabling the Electrophilic Cyanation of Silyl Enol Ethers. Org.
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(20) (a) Coulombel, L.; Duñach, E. Triflic AcidꢀCatalysed Cyclisation
of Unsaturated Alcohols. Green Chem. 2004, 6, 499ꢀ501. (b) Fujita, S.;
Abe, M.; Shibuya, M.; Yamamoto, Y. Intramolecular Hydroalkoxylation
of Unactivated Alkenes Using SilaneꢀIodine Catalytic System. Org. Lett.
2015, 17, 3822ꢀ3825.
(21) B(C6F5)3·nH2Odid not promote the hydroalkoxylation of alkene 7
(Scheme S6).
(22) To illustrate the versatility of the allylꢀ and cyanoꢀsubstituted cyꢀ
clic ethers, several transformation of 3a and 6a are shown in Scheme S7.
of Secondary Benzyl Acetates with Allylsilanes. Org. Lett. 2001, 3, 2705ꢀ
2707. (c) Rajagopal, G.; Kim, S. S. Synthesis of αꢀAryl Nitriles through
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Tetrahedron 2009, 65, 4351ꢀ4355. (d) Dryzhakov, M.; Hellal, M.; Wolf,
E.; Falk, F. C.; Moran, J. NitroꢀAssisted Brønsted Acid Catalysis: Appliꢀ
cation to a Challenging Catalytic Azidation. J. Am. Chem. Soc. 2015, 137,
9555ꢀ9558.
(12) (a) Parks, D. J.; Blackwell, J. M.; Piers, W. E. Studies on the
Mechanism of B(C6F5)3ꢀCatalyzed Hydrosilation of Carbonyl Functions. J.
Org. Chem. 2000, 65, 3090ꢀ3098. (b) Berkefeld, A.; Piers, W. E.; Parvez,
M. Tandem Frustrated Lewis Pair/Tris(pentafluorophenyl)boraneꢀ
Catalyzed Deoxygenative Hydrosilylation of Carbon Dioxide. J. Am.
Chem. Soc. 2010, 132, 10660ꢀ10661. (c) Gyomore, A.; Bakos, M.; Foldes,
T.; Papai, I.; Domjan, A.; Soós, T. MoistureꢀTolerant Frustrated Lewis
Pair Catalyst for Hydrogenation of Aldehydes and Ketones. ACS Catal.
2015, 5, 5366ꢀ5372. (d) Scott, D. J.; Simmons, T. R.; Lawrence, E. J.;
Wildgoose, G. G.; Fuchter, M. J.; Ashley, A. E. Facile Protocol for Waterꢀ
Tolerant "Frustrated Lewis Pair"ꢀCatalyzed Hydrogenation. ACS Catal.
2015, 5, 5540ꢀ5544. (e) Fasano, V.; Radcliffe, J. E.; Ingleson, M. J.
B(C6F5)3ꢀCatalyzed Reductive Amination using Hydrosilanes. ACS Catal.
2016, 6, 1793ꢀ1798.
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H. Tris(pentafluorophenyl)boron as an Efficient, AirꢀStable, and Water
Tolerant Lewis Acid Catalyst. Bull. Chem. Soc. Jpn 1995, 68, 1721ꢀ1730.
(14) The small shift of the signals was observed, indicating the increase
of higher hydrates of B(C6F5)3 (Figure S3).
(15) The desired reaction also efficiently proceeded using 1.0 equiv
2,6ꢀdimethylphenol as a proton source instead of H2O. Under the condiꢀ
tions, the premixing was not required (Scheme S3).
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