Planar-chiral ferrocene-based triazolylidene copper complexes: Synthesis, characterization, and catalysis in asymmetric borylation of α,β-unsaturated ester

Article abstract of DOI:10.1039/d0dt03213a

1,2,3-Triazol-5-ylidenes have recently attracted considerable attention as versatile ligands because of their strong electron-donating properties and structural diversities. While some efforts have been devoted to the development of chiral triazolylidene-

Full text of DOI:10.1039/d0dt03213a

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Planar-chiral ferrocene-based triazolylidene
copper complexes: synthesis, characterization,
and catalysis in asymmetric borylation of
α,β-unsaturated ester†
Cite this: DOI: 10.1039/d0dt03213a
a
b
b
b
Ryosuke Haraguchi, * Tatsuro Yamazaki, Koki Torita, Tatsuki Ito and
b
Shin-ichi Fukuzawa
*
1
,2,3-Triazol-5-ylidenes have recently attracted considerable attention as versatile ligands because of
their strong electron-donating properties and structural diversities. While some efforts have been devoted
to the development of chiral triazolylidene-metal complexes, there is no example achieving asymmetric
induction by base-metal complexes with triazolylidene ligands. Herein, we synthesized planar-chiral
ferrocene-based triazolylidene copper complexes, which enabled the asymmetric borylation of methyl
cinnamate with bis(pinacolato)diboron with good enantioselectivity.
Received 15th September 2020,
Accepted 16th November 2020
DOI: 10.1039/d0dt03213a
rsc.li/dalton
8
a
Since the pioneering work by Albrecht, various triazole-based
MIC-metal complexes have been developed. However, despite
Introduction
8
Over the last three decades, N-heterocyclic carbenes (NHCs) the progress, there are limited reports on the synthesis of
9
have significantly contributed to the progress in many research chiral 1,2,3-triazol-5-ylidene-metal complexes, and only few
1
fields, including organometallic chemistry and homogeneous examples of asymmetric catalysis using chiral 1,2,3-triazol-5-
2
catalysis. Moreover, many NHC-metal complexes have been ylidene-metal complexes have been reported. Sankararaman
3
utilized as biologically active molecules in medicinal chem- reported the asymmetric hydrogenation of alkenes using a
4
istry and as promising photosensitizers in material sciences.
[2.2]paracyclophane-based planar-chiral palladium complex as
9
b
While imidazol-2-ylidenes hold an important position in the a catalyst (Scheme 1a). Elsevier developed axially chiral tria-
history of NHC chemistry, 1,2,3-triazol-5-ylidenes have zolylidene rhodium complexes with a binaphthyl backbone for
5
9c
emerged as versatile ligands for transition-metal complexes.
the asymmetric hydrosilylation of ketones (Scheme 1b).
These carbenes are referred to as mesoionic carbenes (MICs), Chen showed that chiral dinuclear gold complexes exhibited
because no canonical resonance form of MICs without intro- high enantioselectivity for the [2 + 2] cycloaddition of allena-
9
e
ducing formal charges can be drawn. The anionic carbon at mides and styrenes (Scheme 1d). Our group recently devel-
the C4-position of a triazole ring endows the C5-carbene
carbon with stronger electron-donating property than those of
normal NHCs such as imidazol-2-ylidenes. In addition, the
precursors of 1,2,3-triazol-5-ylidenes, that is 1,2,3-triazolium
salts, are readily prepared in a structurally diverse manner via
oped a chiral triazolylidene-Pd-PEPPSI (PEPPSI = pyridine,
enhanced, precatalyst, preparation, stabilization, initiation)
1
0
complex with ferrocene-based planar chirality for the asym-
9
d
metric Suzuki–Miyaura cross-coupling reaction. However, all
of these studies are based on the use of noble metals as
catalysts; thus, the development of asymmetric catalysis by
base-metal complexes with chiral mesoionic triazolylidenes is
highly desired. Herein, we synthesized triazolylidene
copper complexes with planar-chiral ferrocenyl groups in
different substitution patterns (Fig. 1) and compared their
catalytic activity and enantioselectivity for the asymmetric bor-
ylation of methyl cinnamate with bis(pinacolato)diboron. All
copper complexes 1a–c showed moderate to good enantio-
selectivity. This study provides the first example to achieve
asymmetric induction in catalysis by triazolylidene base-metal
complexes.
6
the Huisgen reaction of azides and alkynes followed by a
subsequent alkylation. Oxidative [3 + 2] cycloadditions of tria-
7
zenes and alkynes can also provide 1,2,3-triazolium salts.
a
Department of Applied Chemistry, Faculty of Engineering, Chiba Institute of
Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan.
E-mail: haraguchi.ryosuke@p.chibakoudai.jp
b
Department of Applied Chemistry, Institute of Science and Engineering, Chuo
University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
†
Electronic supplementary information (ESI) available. CCDC 2008348 and
008346. For ESI and crystallographic data in CIF or other electronic format see
2
DOI: 10.1039/d0dt03213a
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Scheme 1 Examples of asymmetric catalysis by chiral triazolylidene-metal complexes.
1
1
5
b in 67% yield. On the other hand, 5a and 5c were prepared
by a two-step protocol consisting of copper-catalyzed [3 + 2]
cycloaddition and the subsequent methylation, given that
planar-chiral azide 4 is unstable under the reaction conditions
for the methylation. While this methylation step in the pre-
viously reported method was achieved by the use of dimethyl-
zinc, we found that trimethylaluminium was also successfully
employed to afford the corresponding methylated products.
This finding improved the synthetic route to chiral triazoles
Fig. 1 Planar-chiral ferrocene-based triazolylidene complexes investi- with respect to cost because of trimethylaluminium is much
gated in this study.
inexpensive than dimethylzinc.
The two-step protocol gave 4a and 4c in 46% and 33%
yields, respectively (see Scheme S1 in the ESI† for synthesis of
4
a). The reaction of 4a–c with trimethyloxonium tetrafluoro-
Results and discussion
borate afforded the corresponding triazolium salts, which
underwent metalation at the 5-position of the triazolium rings
in the presence of silver oxide and tetramethylammonium
chloride to give triazolylidene silver complexes in situ. Finally,
the subsequent transmetalation using copper(I) chloride furn-
ished the triazolylidene copper complexes 1a–c (Scheme 3) as
orange solids in good yields.
All complexes were stable toward air and moisture for three
months. The formation of Cu–Ctrz bonds (trz = triazolylidene)
was confirmed by the appearance of a new peak for carbene
Optically active triazoles 5a–c were synthesized according to
the slightly modified procedure from that previously reported
9
d
by our group. Both planar-chiral alkyne 3 and azide 4 were
prepared from commercially available Ugi’s amine in 77%
yield and 60% yield, respectively (see Scheme S1 in the ESI†).
As shown in Scheme 2a, treatment of optically active alkyne 3
with mesityl azide in a mixture of H O/THF (1/1) in the pres-
ence of a catalytic amount of copper(I) chloride and 2-ethynyl-
pyridine at 50 °C for 14 h afforded the corresponding triazole
2
Scheme 2 Preparation of planar-chiral triazoles 5.
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Fig. 3 ORTEP representation of compound 1c. Thermal ellipsoids are at
Scheme 3 Preparation of chiral triazolylidene copper complexes.
5
0% probability level with all hydrogen atoms omitted for clarity.
Selected bond lengths (Å) and angle (°): C(1)–Cu(1) 1.875(7), Cu(1)–Cl(1)
.107(2), C(1)–Cu(1)–Cl(1) 179.2(2).
2
1
3
carbon in 1a–c at 165–167 ppm in the C NMR spectrum,
which is consistent with the reported values in the range of
1
2
1
60–170 ppm for triazolylidene copper complexes. High-
Table 1 Comparison of 1b and 1c with other [Cu(NHC)Cl] complexes
resolution of electrospray mass spectra of 1a–c provided the
additional evidence for the formation of triazolylidene copper Complex
a
Cu–Cl (Å)
%Vbur
complexes, with the fragment peaks at m/z 656.0257 (calcu-
1
b
2.117(3)
2.137(2)
2.132(6)
2.111(2)
2.115(1)
48.8
47.9
43.8
36.2
51.3
+
lated for [1a + Na] 656.0256), m/z 917.2748 (calculated for 1c
+
13d
6
7
8
[
[
2*1b − CuCl ] 917.2718), and m/z 917.2690 (calculated for
2
+
1
3e
2*1c − CuCl
2
] 917.2718) being observed.
13f
While the molecular structures of triazolylidene copper
a
Parameters applied for SambVca calculations: sphere radius 3.50 Å;
complexes 1b and 1c were unambiguously determined by X-ray
diffraction analysis (Fig. 2 and 3), several attempts to grow
single crystals of 1a suitable for X-ray diffraction were unsuc-
cessful. Single crystals of 1b and 1c were grown by slow
diffusion of diethyl ether into a saturated chloroform solution
of the corresponding complexes.
bond length 2.00 Å; mesh spacing 0.10 Å; Bondi radius 1.17; H atoms
are excluded.
The copper complexes exhibited the expected linear geome-
try with Ctrz–Cu–Cl angles of 177.3(3)° and 179.2(2)° in 1b and
.
1
c, respectively. The bond length of Cu–Cl in 1b was slightly
longer than that in 1c, which was probably because of a stron-
ger trans-effect in 1b. This indicates that the position of the
ferrocenyl group on the triazolium ring influences the elec-
tron-donating properties of triazolylidene ligands.
4
7.9% in 1c are close to that for the bulkiest NHC copper
1
3f
complex 8
NHC copper complexes 6
The catalytic activity of triazolylidene copper complexes 1a–
c for the asymmetric borylation of methyl cinnamate with bis
and significantly higher than those for bulky
1
3d
13e
and 7.
The steric properties of 1b and 1c were evaluated by means
of the percent buried volume analysis (%Vbur), as shown in
1
3a
Table 1.
The obtained %Vbur values of 48.8% in 1b and
1
4
(
pinacolato)diboron were examined. The results are shown
in Table 2. Treatment of methyl cinnamate (9) with bis(pinaco-
lato)diboron (1.1 eq.) in the presence of copper complex 1c
(
5.0 mol%) and sodium tert-butoxide (1.1 eq.) in THF at 0 °C
for 2 h gave the borylated compound, which underwent oxi-
dation with NaBO ·4H O to afford methyl 3-hydroxy-3-phenyl-
3
2
propanoate (10) in 53% yield with 47% ee (entry 1). This is the
first time that asymmetric induction was observed using a
chiral triazolylidene copper complex as a catalyst. The use of
1
,4-dimesityl-3-methyl-1,2,3-triazol-5-ylidene copper chloride
1
2a
complex (1d)
as a catalyst decreased the yield of 10 by 30%
compared to 1c, which suggests that the planar-chiral ferroce-
nyl moiety is crucial for both good catalytic efficiency and
enantioselectivity (entry 2). When bases other than sodium
tert-butoxide were used in the reaction, the enantioselectivity
was improved without loss of product yield (entries 3 and 4).
The use of a catalytic amount of sodium tert-butoxide slightly
Fig. 2 ORTEP representation of compound 1b. Thermal ellipsoids are
at 50% probability level with all hydrogen atoms omitted for clarity.
Selected bond lengths (Å) and angle (°): C(1)–Cu(1) 1.888(8), Cu(1)–Cl(1)
2.111(2), C(1)–Cu(1)–Cl(1) 177.3(3).
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Table 2 Asymmetric borylation of methyl cinnamate with bis(pinaco-
lato)diboron
Experimental section
a
General
All manipulations of oxygen- and moisture-sensitive materials
were conducted under argon or nitrogen atmosphere in a flame
dried Schlenk flask. Nuclear magnetic resonance spectra were
taken on a JEOL ECA spectrometer using tetramethylsilane for
b
c
1
Entry
[Cu]
Base
Solvent
Yield (%)
ee (%)
H NMR as an internal standard (δ = 0 ppm) when CDCl was
3
13
used as a solvent, and using CDCl
3
for C NMR as an internal
1
2
3
4
1c
1d
1c
1c
1c
1c
1c
1c
1b
1a
NaOtBu
NaOtBu
KOtBu
LiOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
LiOtBu
LiOtBu
THF
THF
THF
THF
THF
53
23
53
52
48
51
38
36
50
45
47
—
52
52
48
44
53
25
52
60
1
standard (δ = 77.16 ppm) when CDCl
3
was used as a solvent. H
13
NMR and C NMR data are reported as follows: chemical shift,
multiplicity (s = singlet, d = doublet, t = triplet, q = quartet,
quint = quintet, sext = sextet, sept = septet, br = broad, m = mul-
tiplet), coupling constants (Hz), and integration. High-resolu-
d
5
6
7
8
9
CH
Et
2 2
Cl
O
2
Toluene
THF
THF
tion mass spectra (HRMS) were measured by
a JEOL
1
0
JMS-T100LC AccuTOF. Infrared (IR) spectra were measured by
an FT/IR-4100ST spectrometer. High performance liquid chrom-
atography (HPLC) was performed on a JASCO MD-2010 Plus
a
Standard conditions: 9 (0.20 mmol), (Bpin)
2
(0.22 mmol), base
0.22 mmol), catalyst (0.010 mmol) in THF (2.0 mL) at 0 °C for 2 h.
(
b
c
Isolated yields. Determined by HPLC analysis of the isolated 10 system with UV and CD detectors and chiral column of Daicel,
d
using a chiral stationary phase column (Chiralcel OJ-H). 5 mol% of
Chiralpak OJ-H. X-ray crystallographic analysis was performed
on VariMax/Saturn CCD diffractometer.
NaOtBu was used.
General procedure for preparation of chiral triazolylidene
copper complexes 1a
To a 50 mL Schlenk flask were added sequentially triazolium salt,
silver(I) oxide (1.0 eq.), NMe Cl (1.0 eq.), and dry CH Cl under a
.
4
2
2
nitrogen atmosphere. The mixture was stirred in the dark at room
temperature for 23 h. To the mixture was added CuCl (1.0 eq.),
and the resulting mixture was stirred at 40 °C for 14 h followed by
filtration through a pad of Celite. The filtrate was concentrated in
vacuo to afford the crude product. Purification by silica gel
column chromatography gave 1 as an orange solid.
decreased the yield of 10 without loss of enantioselectivity
(entries 1 and 5). Moreover, the effect of solvents on catalytic
activity and enantioselectivity was investigated. Among the sol-
vents investigated, THF gave the best result with respect to
both yield and enantioselectivity (entries 1, 6–8). It is note-
worthy that switching the positions of the chiral ferrocenyl
substituent and the mesityl group had negligible influence on
1a
Yield: 57% (0.12 g); orange solid; R 0.23 (hexane/ethyl acetate =
f
2
5
1
5
(
5
/1); [α] +90.7 (c 0.1, CHCl
3
); H NMR (500 MHz, CDCl
dd, J = 2.6, 1.6 Hz, 1H), 4.44 (s, 5H), 4.39–4.33 (m, 2H), 4.35 (s,
H), 4.33–4.31 (m, 1H), 4.27–4.25 (m, 1H), 4.24–4.21 (m, 1H),
3
) δ: 5.16
both yield and enantioselectivity (entries
1
and 9).
D
Furthermore, the enantioselectivity was slightly improved
when copper complex 1a bearing two planar-chiral ferrocenyl
1
5,16
3.94 (s, 3H), 3.60 (qq, J = 6.8, 6.8 Hz, 1H), 3.06 (qq, J = 6.8, 6.8
Hz, 1H), 1.40–1.34 (m, 6H), 0.93 (d, J = 6.8 Hz, 3H), 0.64 (d, J =
units was used (entry 10).
13
13
6
.8 Hz, 3H) ppm; C NMR (125 MHz, CDCl ) δ: C NMR
3
(
125 MHz, CDCl
3
)δ: 166.7(Ccarbene), 146.2(Ctrz-4), 98.0(CFero), 96.1
Conclusions
(CFero), 91.8(CFero), 71.2(CFero), 71.1(CFero), 71.1(CFero), 67.6(CFero),
Planar-chiral ferrocene-based triazolylidene copper com- 67.1(CFero), 66.4(CFero), 66.3(CFero), 65.3(CFero), 64.8(CFero), 37.0
plexes were successfully prepared and characterized by NMR, (CMe), 26.7(Ci-Pr), 25.9(Ci-Pr), 25.4(Ci-Pr), 24.6(Ci-Pr), 22.1(Ci-Pr), 21.6
HRMS, and X-ray diffraction analysis. The catalytic perform- (Ci-Pr) ppm IR (KBr): 3455.8, 3092.3, 2958.3, 2924.5, 2866.7,
ance of the copper complexes was evaluated for the asym- 1628.6, 1560.1, 1542.8, 1457.9, 1424.2, 1410.7, 1380.8, 1360.5,
metric borylation of methyl cinnamate with bis(pinacolato) 1262.2, 1231.3, 1170.6, 1106.9, 1067.4, 1036.5, 1000.9, 970.0,
diboron. All copper complexes were found to exhibit moder- 946.9, 875.5, 824.4, 749.2, 733.8, 720.3, 688.5, 678.8, 669.2,
ate enantioselectivity for the reaction. To the best of our 644.1, 621.9, 566.0, 540.0 cm− ; ESI-HRMS (m/z): [M + Na] calcd
1
+
33 3 2
knowledge, this is the first example that achieved asymmetric for C29H N ClCuFe Na 656.0256, found 656.0257.
induction employing a triazolylidene base-metal complex as a
catalyst. To provide a more effective chiral environment
within proximity of metal center, the development of triazoly- Yield: 71% (0.047 g); orange solid; R
1b
f
0.69 (dichloromethane);
); H NMR (500 MHz, CDCl ) δ: 7.02
2
5
1
lidene ligands with bulkier planar-chiral ferrocenyl groups is [α] −40.3 (c 0.01, CHCl
3
3
D
currently in progress.
(s, 1H), 6.99 (s, 1H), 4.42 (s, 5H), 4.40–4.36 (m, 2H), 4.36–4.33
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m, 1H), 3.98 (s, 3H), 3.79–3.68 (m, 1H), 2.36 (s, 3H), 2.16 (s,
Paper
(
Conflicts of interest
3
H), 2.00 (s, 3H), 1.41 (d, J = 6.8 Hz, 3H), 0.70 (d, J = 6.8 Hz,
1
3
3
H) ppm; C NMR (125 MHz, CDCl
3
) δ: 166.8(Ccarbene), 146.9 There are no conflicts to declare.
(
1
6
C
trz-4), 140.5(Cbenz), 136.4(Cbenz), 134.1(Cbenz), 133.9(Cbenz),
29.6(Cbenz), 129.6(Cbenz), 98.0(CFero), 71.2(CFero), 70.8(CFero),
7.7(CFero), 67.2(CFero), 66.6(CFero), 37.0(CMe), 26.9(CMe), 25.3
Me), 21.5(CMe), 21.3(Ci-Pr), 17.9(Ci-Pr), 17.5(Ci-Pr) ppm; IR
Acknowledgements
(
(
C
KBr): 3464.5, 3092.3, 3080.7, 2956.3, 2920.7, 2862.8, 1654.6, We thank Dr Yoshiaki Tanabe for X-ray crystallographic ana-
1
1
1
9
6
638.2, 1628.6, 1618.0, 1609.3, 1560.1, 1542.8, 1509.0, 1490.7, lysis. This work was partially supported by Chuo University
458.9, 1449.2, 1439.6, 1407.8, 1337.9, 1360.5, 1322.0, 1291.1, grant for special research.
266.0, 1230.4, 1185.0, 1131.1, 1106.0, 1071.3, 1035.6, 1001.8,
60.4, 852.4, 823.5, 812.8, 756.9, 737.6, 681.7, 671.1, 654.7,
−
1
22.9, 594.9, 583.4, 569.9, 537.1 cm ; ESI-HRMS (m/z): [2M −
Notes and references
+
CuCl ] calcd for C H N CuFe 917.2718, found 917.2748.
2
50 58
6
2
1
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5
1
[
(
(
D
3 3
α] +83.1 (c 0.008, CHCl ); H NMR (500 MHz, CDCl ) δ: 6.99
s, 1H), 6.98 (s, 1H), 5.22–5.11 (m, 1H), 4.35 (s, 5H), 4.30–4.21
m, 2H), 3.81 (s, 3H), 3.32–3.12 (m, 1H), 2.34 (s, 3H), 2.11 (s,
3
3
H), 2.02 (s, 3H), 1.39 (d, J = 6.8 Hz, 3H), 0.97 (d, J = 6.8 Hz,
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1
1
1
7
5
637.3, 1611.2, 1560.1, 1458.9, 1439.6, 1406.8, 1379.8, 1364.4,
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CuFe 917.2718, found 917.2690.
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N
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Chem., Int. Ed., 2001, 40, 2004; (b) F. Himo, T. Lovell,
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(
0.22 mmol, 18 mg) and catalyst 1a (0.010 mmol, 6.3 mg) in a
glovebox. After the vial was taken out of the glovebox, THF
1 mL) was added to the reaction mixture. The mixture was
(
stirred at room temperature for 20 min, and a THF (0.5 mL)
solution of bis(pinacolato)diboron (0.22 mmol, 56 mg) was
added to the reaction mixture at 0 °C. After the resulting
mixture was stirred at 0 °C for 10 min, a THF solution of
methyl cinnamate (0.20 mmol, 32 mg) was added to the
mixture. The mixture was stirred at 0 °C for 2 h and then was
quenched with NaBO ·4H O and water. After the resulting
3
2
mixture was stirred at room temperature for 4 h, the mixture
was extracted by ethyl acetate. The combined organic layers
were washed with brine, dried over magnesium sulfate, and
concentrated in vacuo. Purification by preparative TLC (PTLC)
(hexane/ethyl acetate = 4/1) gave 10 as a colorless oil. Enantio
excess of 10 was determined by HPLC analysis with a chiral
stationary phase column (Daicel, Chiralpak OD-H, n-hexane/
7 (a) W. Wirschun, M. Winkler, K. Lutz and J. C. Jochims,
J. Chem. Soc., Perkin Trans. 1, 1998, 1755; (b) J. Bouffard,
B. K. Keitz, R. Tonner, G. Guisado-Barrios, G. Frenking,
−
1
i-PrOH = 95/5, flow rate = 0.5 mL min , λ = 225 nm, 30 °C):
tmajor = 31.1 min, tminor = 46.7 min.
This journal is © The Royal Society of Chemistry 2020
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lation, the product was obtained in 27% yield and 46% ee.
The result indicated that the counter anion of copper tria-
zolylidene complexes was both crucial for the reaction
efficiency and the enantioselectivity, while the reason is
not unclear at this time.
2
1
1
8575; (b) U. Siemeling, Eur. J. Inorg. Chem., 2012, 2012, 16 The reaction did not proceed in the absence of copper com-
3
523.
plexes. In contrast, the use of copper chloride (CuCl) as a
catalyst in the borylation reaction provided 10 in 51% yield.
The elemental analysis of 1a revealed that some impurity was
included in 1a, which indicated the possibility that residual
CuCl may promote the background reaction to decrease the
enantioselectivity in the asymmetric borylation.
1
1 (a) H. Hiroki, K. Ogata and S.-i. Fukuzawa, Synlett, 2013,
43; (b) Although we observed a trace amount of the tri-
8
azole product (less than 1% yield) generated from planar
chiral azide 4 and ethynylpyridine in the Scheme 2(b), 5b
was obtained as a major product.
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