Enantioselective Catalyst Systems from Copper(II) Triflate and BINOL–Silanediol

Article abstract of DOI:10.1002/chem.201801304

Silanediol and copper catalysis are merged, for the first time, to create an enhanced Lewis acid catalyst system for enantioselective heterocycle functionalization. The promise of this silanediol and copper catalyst combination is demonstrated in the enantioselective addition of indoles to alkylidene malonates to give rise to the desirable adducts in excellent yield and high enantiomeric excess. From these studies, 1,1′-bi-2-naphthol (BINOL)-based silanediols emerge as one-of-a-kind cocatalysts. Their potential role in the reaction pathway is also discussed.

Full text of DOI:10.1002/chem.201801304

A Journal of
Accepted Article
Title: Enantioselective Catalyst Systems from Copper(II) Triflate and
BINOL-Silanediol
Authors: Yong Guan, Jonathan W Attard, Michael David Visco,
Thomas James Fisher, and Anita Elaine Mattson
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.201801304
Link to VoR: http://dx.doi.org/10.1002/chem.201801304
Supported by

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COMMUNICATION
Enantioselective Catalyst Systems from Copper(II) Triflate and
BINOL-Silanediol
Yong Guan[a],**, Jonathan W. Attard[a],**, Michael D. Visco[b], Thomas J. Fisher[c], and Anita E. Mattson*[a]
Abstract: Silanediol and copper catalysis are merged, for the first
time, to create an enhanced Lewis acid catalyst system for
enantioselective heterocycle functionalization. The promise of this
silanediol and copper catalyst combination is demonstrated in the
enantioselective addition of indoles to alkylidene malonates to give
rise to the desirable adducts in excellent yield and high enantiomeric
excess. From our studies, BINOL-based silanediols emerged as
one-of-a-kind cocatalysts for the process and their potential role in
the reaction pathway is discussed.
This Work:
Prior Work:
1
1
R
R
=
Cu(OTf)2
= Cl or TfO
B: Catalyst Activation
A: Substrate Activation
OH
OH
=
Si
The interesting properties of the silanediol functional group
have enticed investigators to pursue its study in a number of
applications, such as materials, medicine, molecular recognition,
and catalysis.[1-4] In the context of enantioselective catalysis, the
hydrogen-bonding and anion-binding abilities of silanediols have
significant value.[5],[6] For instance, enantioselective silanediol
1
Enantiopure
Silanediol
Figure 1. Expanding the role of silanediols in catalysis: (A) Substrate
activation (B) Catalyst activation
catalysis has emerged as
a promising platform for the
functionalization of nitrogen- and oxygen-based heterocycles.
These reactions plausibly benefit from a silanediol anion-binding
mode of action; the silanediol may directly activate an ionic
substrate through hydrogen-bonding interactions with the
anionic component.[6,7] Notably, silanediols can offer reactivity
patterns that are inaccessible with other types of catalysts. For
instance, VANOL- and BINOL-based silanediols are, to date,
distinct in their ability to control the stereochemical outcome of
reactions of 4-silyloxybenzopyrylium triflates.[6a]
It is well known that metals catalyze the addition of indoles
to arylidene and alkylidene malonates.[11] In the case of
arylidene malonates, the reactions can be rendered highly
enantioselective in the presence of an appropriate chiral ligand,
Table 1. Identification of reaction conditions for silanediol and copper
cocatalysis in the reaction of 2a and 3a
O
O
O
O
The unique abilities of silanediols in enantioselective
catalysis enticed us to stop limiting their application to the direct
activation of substrates (A, Figure 1). We reasoned that
silanediols could work together with transition metal catalysts,
like Cu(OTf)2, thereby generating an enhanced catalyst system
(B) for the discovery of useful reactions.[8-10] Herein, we
communicate, for the first time, a silanediol and copper catalyst
system for the enantiocontrolled addition of indoles to alkylidene
malonates.
MeO
OMe
X mol % 1
X mol % TM
MeO
OMe
NH
+
*
toluene, – 28°C
NH
2a
3a
4a
Entry[a]
X mol % 1
X mol % TM
20 mol % Cu(OTf)
Yield[b]
ee[c]
72
9
1
2
30
30
92
2
20 mol % Sc(OTf)3
95
[a]
Dr. Y. Guan, Mr. J. Attard, Prof. A. Mattson
Department of Chemistry and Biochemistry
Worcester Polytechnic Institute
100 Institute Road, Worcester, MA 01609
E-mail: aemattson@wpi.edu
**authors contributed equally
Dr. Michael D. Visco
Celgene
86 Morris Avenue
Summit, NJ
Dr. Thomas J. Fisher
3
4
30
20
30
0
20 mol % In(OTf)3
20 mol % Zn(OTf)2
20 mol % CuOTf
20 mol %Cu(OTf)2
0 mol % Cu(OTf)2
20 mol % CuSO4
20 mol % CuCl
88
0
10
--
5
10
15
0
31
0
[b]
[c]
6
7
30
30
30
30
30
--
8
0
--
The Goodyear Tire and Rubber Company
200 Innovation Way
Akron,OH
9
0
--
Supporting information for this article is given via a link at the end
of the document.
10
11
20 mol % CuI
0
--
10 mol % HOTf
0
--
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10.1002/chem.201801304
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Substituents at the 7-position of indole were also well accepted.
For example, 7-methylindole afforded 4h in 96% yield and 75%
ee (entry 7).
12
13
20
10
20
20 mol % Cu(OTf)2
10 mol %Cu(OTf)2
20 mol % Cu(OTf)2
93
70
99
73
70
75
Having identified a workable copper and silanediol catalyst
system for the introduction of indole to 2a, further probing of the
influence of 2 and 3 on the outcome of the reaction revealed the
enantiomeric excess was highly dependent on substrate
structure (Table 2).12 The substituent on the ester had a rather
dramatic effect on enantiocontrol, the trend being smaller
14[d]
[a] See supporting information for details on the experimental procedure; [b]
Yield determined by 1H NMR spectroscopy; [c] enantiomeric excess
determined by HPLC; [d] 0.5 equiv. 1,1,1-trifluoroisopropanol (TFIP) added.
such as trisoxazolines.[11e] Attaining high levels of enantiocontrol
in the addition of indole to alkylidene malonates, on the other
hand, remains a significant challenge. With this in mind, we
setout to explore silanediol and transition metal catalyst systems
as a solution for the enantioselective addition of indole (3a) to
cyclohexylidene malonate (2a, Table 1).
Table 2. Affect of substrate structure on reaction
O
O
O
O
20 mol % 1
20 mol % Cu(OTf)2
MeO
OMe
NH
NH
Our studies began with an examination of the influence of
30 mol % of BINOL-based silanediol 1 and 20 mol % of a wide
selection of transition metals, a few of which are listed in Table
1, in the addition of indole (3a) to cyclohexylidene malonate (2a).
Only the combination of 1 and Cu(OTf)2 gave rise to a high yield
of 4a with high levels of enantiocontrol (92% yield, 72% ee, entry
1). Other transition metals tested could generate high yields of
desired product but low levels of enantiocontrol (e.g. Sc(OTf)3,
In(OTf)3, entries 2 and 3). CuOTf also proved inferior to its
Cu(OTf)2 counterpart (entry 5). Control experiments revealed the
important roles of both the silanediol and Cu(OTf)2 in achieving
excellent yields and stereocontrol: 20 mol % of Cu(OTf)2 alone,
with no silanediol added, resulted in a dramatic drop in the yield
of 4a (15%, entry 6). No product was observed when 30 mol %
of the silanediol was tested as the sole catalyst of the reaction
(entry 7). Copper sources other than Cu(OTf)2, such as CuSO4,
CuCl, and CuI, were also evaluated but they did not yield
product, suggesting the triflate counterion is essential to the
success of the reaction (entries 8 – 10). Triflic acid was ruled out
as the co-catalyst when control experiments resulted in no
reaction (entry 11). The optimization of the copper to silanediol
ratio resulted in an improvement in the enantiomeric ratio: 20
mol % of each silanediol and Cu(OTf)2 was identified as the best
combination in terms of yield and stereocontrol (93% yield, 73%
ee, entry 12). The catalyst loadings could be dropped to 10 mol
% with no significant drop in enantiomeric excess, but the yield
was slightly lower (70% yield, 70% ee, entry 13). Finally, a
screening of additives enabled us pinpoint that 0.5 equiv of
1,1,1-trifluoroisopropanol (TFIP) gave a small boost in the
enantiomeric excess (75% ee, entry 14).
MeO
OMe
R1
+
*
TFIP (50 mol %)
toluene, – 28°C
R1
R2
R2
2
3
4
Entry[a]
R1
R2
Prod
uct
Yield(%)[
ee(%)[c]
b]
1
2
Cy
Cy
Cy
Cy
Cy
Cy
Cy
Ph
H
4a
4b
4c
4d
4e
4f
98
96
75
77
68
71
72
86
75
49
52
30
49
58
30
5-Me
5-tBu
5-Ph
3
92
4
92
5
5-(2-naphthyl)
66
6
5-(3,5)-CF3Ph
58
7
7-Me
H
4g
4h
4i
95
8
99d
16d,e
99d
50d
99d,e
92d
9
p-MeO-Ph
p-NO2-Ph
o-tolyl
H
Having identified a workable copper and silanediol catalyst
system for the introduction of indole to 2a, further probing of the
influence of the structure 2 and 3 on the outcome of the reaction
was undertaken (Table 2).[12] The substituent on the ester had a
rather dramatic effect on enantiocontrol, the trend being smaller
substituents led to higher enantiomeric ratios.[12] Methyl esters
(2) resulted in the best stereocontrol observed in the reaction.
Alkylidene malonates where R1 = cyclohexyl afforded 4a in
excellent yield with high levels of enantiocontrol (98%, 75% ee,
entry 1), Various substitution patterns about the indole scaffold
enabled the preparation of 4b-4g. Alkyl and aryl substituents in
the 5-position of indole were well tolerated, producing 4b-4f in
excellent yields and high enantiomeric excess (entries 2-6).
10
11
12
13
H
4j
H
4k
4l
2-naphthyl
H
n-propyl
H
4m
[a] See supporting information for details on the experimental procedure, TFIP
1,1,1-trifluoroispropanol; [b] Isolated yields after flash column
=
chromatography on silica gel; [c] Enantiomeric ratio determined by HPLC; [d]
Yield determined by 1H NMR spectroscopy, 30 mol % 1; [e] 40 mol % 1
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10.1002/chem.201801304
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substituents led to higher enantiomeric ratios.12 Methyl esters (2)
resulted in the best stereocontrol observed in the reaction.
Alkylidene malonates where R1 = cyclohexyl afforded 4a in
excellent yield with high levels of enantiocontrol (98%, 75% ee,
entry 1). Various substitution patterns about the indole scaffold
enabled the preparation of 4b-4g. More specifically, alkyl and
aryl substituents in the 5-position of indole were well tolerated,
producing 4b-4f in excellent yields and high enantiomeric
excess (entries 2-6). Substituents at the 7-position of indole
were also well accepted. For example, 7-methylindole afforded
4g in 96% yield and 75% ee (entry 7). For a comparison to
alkylidene malonate 2a, several arylidene malonates were
tested as substrates in the reaction. Interestingly, the arylidene
malonates did not afford enantiomeric excesses as high as 2a.
For instance, arylidene malonates containing either electron
donating or withdrawing substituents yielded 4h-4l in low to
excellent yield (16-99%) but with modest levels of enantiomeric
excess (30-56% ee, entries 8-12). The importance of the
alkylidene malonate structure on enantioselectivity was further
highlighted when malonate 2 (R1 = n-propyl) gave rise to 4m in
92% yield and 30% ee (entry 13).
role of the silanediol in the reaction. Experimental evidence
collected during the optimization of the reaction (Table 1)
Scheme 2. Unique combination of silanediol and Cu(OTf)2
O
O
MeO
OMe
NH
20 mol % HBD
20 mol % Cu(OTf)2
+
2a
3a
*
TFIP (50 mol %)
toluene, – 28°C
4a
HBD: Hydrogen Bond Donor:
iPr
tBu
S
iPr
N
OH
Si
N
N
H
Me
H
OH
O
N
Ph
1: 99% yield
5: no reaction
72% ee
OMe
Si
Ph
Ph
OH
OH
OH
OH
The reaction is amenable to scale-up (Scheme 1). When
2a was subjected to reaction with indole under the influence of
silanediol and copper cocatalysis, 4a was produced in 85% yield
and 74% ee. The process is synthetically useful as the
enantioenriched product is easily crystallized from isopropanol
and hexanes giving rise to 4a in an excellent enantiomeric
excess of 97%.
OMe
6: 25% yield
7: 28% yield
racemic
racemic
8: 32% yield
racemic
suggested that neither the silanediol or Cu(OTf)2 alone were
efficient catalysts: there is something unique about the
combination of the two. The observation had been made (Table
1) that the triflate counterion was required for a successful
reaction. Furthermore, it has been previously demonstrated that
silanediols can recognize triflate anions6a, thus there is the
possibility that the silanediol is activating Cu(OTf)2 through
anion-binding to the triflate to create copper-silanediol catalyst
complex I (see Scheme 4). Alternatively, a reaction pathway in
which the silanediol may be operating as a ligand on copper was
also considered.
Scheme 1. Scale up and recrystallization
O
O
O
O
MeO
OMe
20 mol % 1
MeO
OMe
NH
20 mol % Cu(OTf)2
+
*
TFIP (50 mol %)
toluene, – 28°C
NH
4a
2a
3a
85% yield, 74% ee
66% yield, 97% ee
crystallization
Efforts to probe the role of the silanediol and plausible
catalytic species began with UV-Vis studies of the Cu(OTf)2 in
the presence of silanediol 1 and a competing ligand, achiral
bisoxazoline 9 (Scheme 3). In the first set of experiments, it was
noted that the combination of Cu(OTf)2 and 1 did not produce a
band in the spectrum that is indicative of a d-d transition,
whereas the combination of Cu(OTf)2 and 9 did produce a band
with a lmax ~ 754nm (Scheme 3a). In a second experiment, the
titration of 1-5 equiv of 1 into a solution of Cu(OTf)2 and 9 did not
significantly alter the UV-Vis spectrum (Scheme 3b). These data
suggest that the silanediol may not be operating as a ligand, and
will likely not displace a bisoxazoline ligand on copper.
Interesting observations were made while probing the
possible role of the silanediol. First, it was found that the
silanediol is a uniquely effective cocatalyst: all of our attempts to
catalyze the reaction with a combination of Cu(OTf)2 and other
popular dual hydrogen bond donors (HBDs) were entirely
fruitless (Scheme 2). For instance, the combination of Cu(OTf)2
and popular chiral thiourea catalyst 5 yielded no reaction. The
addition of enantiopure BINOL (7) and VANOL (8) resulted in
low yields of racemic product. Evidence of the importance of the
silanediol functional group was found when bis-dimethoxy
silanediol 6 gave rise to racemic 4a in 25% yield.
To better understand the silanediol mode of action, an
experiment was set up to in which 20 mol % of achiral bis-
oxazoline 9 was added to an otherwise standard reaction
(Scheme 3c). Based upon the UV-Vis data, it was reasoned that
the bisoxazoline would be a superior ligand on copper than the
silanediol and if 4a was prepared in enatiomerically enriched
Given the unique ability of silanediol 1 to cooperate with
catalytic copper(II) triflate to affect the enantioselective addition
reaction of 3 and 2, we became curious to learn more about the
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form from this reaction then that may suggest the silanediol is
operating as an anion binding catalyst, not as a ligand. Indeed,
4a was isolated in 90% yield with 27% ee, possibly providing
evidence for the reaction pathway depicted in Scheme 4. More
specifically, the catalytic cyclic may begin with the silanediol
interacting with the Cu(OTf)2 to generate enhanced Lewis acid
species I.13 The complexation of I with the alkylidene malonate 2
may give rise to II. The subsequent enantioselective addition of
indole 3a to II yields intermediate III which, after proton
transfers, affords desired product 4 and frees complex I for
continued participation in the catalytic cycle.
Scheme 4. Possible catalytic pathway
1 + Cu(OTf)2
I
Si
S
O
H
O
H
Cu
2a
4a
OTf
O
H
O
N
F3C
O
Cy
O
Enhanced
Lewis Acid
Cy
*
Scheme 3. Probing of plausible role of silanediol
OMe
MeO
OMe
MeO
*
*
O
O
O
Cu
Si
O
Cu
L
Si
O
H
O
O
H
H
L
H
H
IV
N
II
O
O
*
Tf
Tf
Cy
*
Si
OMe
MeO
3a
O
O
H
O
O
III
H
Cu
L
O
Tf
Experimental Section
Dimethyl cyclohexylidene malonate (113 mg, 0.5 mmol, 1.0 eq),
Cu(OTf)2 (36 mg, 0.1 mmol, 0.2 eq), trifluoroisopropanol (22.6µL,
0.25 mmol, 0.5 eq) and toluene (5mL) were added into a 20 mL
screw top reaction vial with a teflon coated septum. The flask
was purged with dry N2 and the reaction mixture stirred for 15
minutes or until a homogenous slurry was obtained. The
reaction vial was then cooled to –78°C in a dry ice/acetone bath.
2.4 mL of 0.05 M silanediol stock solution (82 mg, 0.24 mmol,
0.2 eq) in toluene and 2.6 mL of indole solution (88 mg, 0.75
mmol, 1.5 eq) in toluene were added into the reaction vial
dropwise. The reaction vial was transferred into a lab freezer (–
28°C) and stirred overnight. The reaction was quenched with 2
mL of DI water, stirred for 10 minutes then extracted with EtOAc
3x10mL and dried over Na2SO4. Solvent was removed from the
combined organic layer under vacuum to obtain crude product.
The crude product was purified by silica gel column
chromatography (eluent = 4:1, Hexanes:EtOAc). The resulting
material was purified further by silica gel column
To conclude, silanediols work with Cu(OTf)2 to generate an
enhanced catalyst system with applications in enantioselective
synthesis. Herein, silanediol and copper catalysis has been
demonstrated in the reaction of indoles and alkylidene
malonates to afford desirable adducts with high levels of
enantiocontrol. Our preliminary studies suggest the silanediol
may be operating as an anion-binding catalyst to activate copper
triflate, but further investigations are needed to better
understand the operation of the catalyst system. Ongoing
studies in our laboratory are dedicated toward uncovering the
full potential of silanediol and transition metal cocatalysis in the
context of asymmetric synthesis.
chromatography (eluent
=
100% dichloromethane). After
removal of the solvent under vacuum, dimethyl 2-
(cyclohexyl(1H-indol-3-yl)methyl)malonate was obtained as an
off white solid (159mg, 0.46 mmol, 93% yield). 1H NMR (500
MHz, CDCl3) δ 8.03 (s, 1H), 7.66 (ddt, J = 8.0, 1.5, 0.8 Hz, 1H),
7.32 (dt, J = 8.0, 0.9 Hz, 1H), 7.16 (ddd, J = 8.2, 7.0, 1.2 Hz, 1H),
7.10 (ddd, J = 8.0, 7.0, 1.1 Hz, 1H), 7.03 (d, J = 2.4 Hz, 1H),
4.03 (d, J = 10.8 Hz, 1H), 3.78 (dd, J = 10.8, 4.9 Hz, 1H), 3.73 (s,
3H), 3.35 (s, 3H), 1.78 – 1.55 (m, 6H), 1.31 – 1.08 (m, 2H), 1.02
– 0.81 (m, 3H). 13C NMR (126 MHz, CDCl3) δ 169.62, 168.96,
135.76, 128.42, 122.87, 121.85, 119.72, 119.41, 113.90, 111.02,
55.69, 52.64, 52.26, 42.04, 41.16, 32.33, 28.79, 26.68, 26.48,
26.32. IR (neat) 3413, 2926, 2853, 1755, 1726, 1457, 1431 cm-1.
HPLC: 86.0:14.0 e.r., 72% ee, Chiralpak AS-H column (10%
iPrOH/Hexanes, 1 mL/min, 225 nm); tR (minor) = 8.55 min, tR
(major) = 23.80 min. Rf = 0.25 (4:1, Hexanes:EtOAc). α ꢁꢂ = -
ꢀ
9.1 (c = 4.0, CH2Cl2).
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10.1002/chem.201801304
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COMMUNICATION
G. Schafer, M. D. Visco, J. C. Galluci, A. E. Mattson Eur. J. Org. Chem.
2015, 525-533; c) A. G. Schafer, J. M. Wieting, T. J. Fisher, A. E.
Mattson Angew. Chem. Int. Ed. 2013, 52, 11321-11324.
For reviews that include information on anion-binding catalysis, see: a)
M. D. Visco, J. Attard, Y. Guan, A. E. Mattson Tetrahedron Lett. 2017,
b) N. Busschaert, C. Caltagirone, W. Van Rossom, P. A. Gale Chem.
Rev. 2015, 115, 8038-8155; c) K. Brak, E. N. Jacobsen Angew. Chem.
Int. Ed. 2013, 52, 534-561.
Acknowledgements
The National Science Foundation (1362030), the National
Institutes of Health (1R35GM12480401), and Worcester
Polytechnic Institute are gratefully acknowledged for providing
support for our studies.
[7]
[8]
[9]
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Chem. Int. Ed. 2005, 44, 1924-1942; (b) T. Akiyama, K. Mori Chem.
Rev. 2015, 115, 9277-9306.
Keywords: silanediol • cooperative catalysis • copper •
enantioselective • Friedel-Crafts
[1]
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[10] Cationic copper and chiral anions have been used to render reactions
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Lett. 2000, 2, 4165-4168.
[2]
For examples of the medicinal applications of silanediols, see: (e) S.
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Hansen, K. A. Jorgensen Chem. Commun. 2001, 347-348.
[12] See supporting information for examples of additional substrates that
were tested.
[3]
[4]
For an application of silanediol molecular recognition in sensing, see: S.
Kondo, Y. Bie, M. Yamamura Org. Lett. 2013, 15, 520-523.
For select examples of silanediols in achiral catalysis, see: (g) N T.
Tran, T. Min, A. K. Franz Chem. Eur. J. 2011, 17, 9897; (h) A. G.
Schafer, J. M. Wieting, A. E. Mattson Org. Lett. 2011, 13, 5228-5232; (i)
A. M. Hardman, A. E. Mattson ChemSusChem 2014, 7, 3275-3278.
For an early report of silanediols in anion recognition, see: S. Kondo, T.
Harada, R. Tanaka, M. Unno Org. Lett. 2006, 8, 4621-4624.
[5]
[6]
[13] A recent example of squaramide activation of silyl trilfates has been
reported: S. M. Banik, A. Levina, A. M. Hyde, E. N. Jacobsen Science
2017, 358, 761-764.
For enantioselective reactions catalysed by silanediols, see: a) A. M.
Hardman-Baldwin, M. D. Visco, J. M. Wieting, C. Stern, S, Kondo, A. E.
Mattson Org. Lett 2016, 18, 2883-2885; b) J. M. Wieting, T. J. Fisher, A.
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10.1002/chem.201801304
Chemistry - A European Journal
COMMUNICATION
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COMMUNICATION
Better Together: Silanediol and
copper(II) triflate work together to
create an enhanced catalyst system
for enantioselective heterocycle
functionalization. This unique catalyst
combination activates alkylidene
malonates for reaction with indoles,
giving rise to desirable products in
excellent yield and high levels of
enantiomeric excess.
Yong Guan, Jonathan Attard, Michael D.
Visco, Thomas J. Fisher, Anita E.
Mattson
Page No. – Page No.
Enantioselective Catalyst Systems
from Copper(II) Triflate and BINOL-
Silanediol
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