Conjugate Addition of Diboron Catalyzed by O -Monoacyltartaric Acids

Article abstract of DOI:10.1055/s-0034-1378835

O-3,5-Di(adamantan-1-yl)benzoyltartaric acid catalyzes the conjugate addition of bis(neopentyl glycolato)diboron to α,β-unsaturated ketones with good enantioselectivity. The addition of magnesium sulfate as a dehydrating agent and benzoic acid as a co-cat

Full text of DOI:10.1055/s-0034-1378835

SYNTHESIS
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1
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1
0
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© Georg Thieme Verlag Stuttgart · New York
2015, 47, 2265–2269
2265
special topic
M. Sugiura et al.
Special Topic
Synthesis
Conjugate Addition of Diboron Catalyzed by O-Monoacyltartaric
Acids
Masaharu Sugiura*
Wakoto Ishikawa
Yukinobu Kuboyama
Makoto Nakajima
O
CO2H
Ad
CO2H
O
O
O
O
O
OH
B
B
catalyst
(Ad = 1-adamantyl)
Ad
O
O
Graduate School of Pharmaceutical Sciences, Kumamoto University,
5-1 Oe-honmachi, Chuo-ku, Kumamoto 862-0973, Japan
msugiura@kumamoto-u.ac.jp
O
B
+
MgSO4, PhCO2H
MeCN, 50 °C
R1
R2
O
R1
R2
Received: 15.03.2015
Accepted after revision: 12.05.2015
Published online: 19.06.2015
perature to provide good enantioselectivity, although the
yield was moderate (Table 1, entry 1). The presence of the
catalyst was indispensable because diboron 3a did not react
at all in its absence (entry 2). Unexpectedly, the addition of
methanol, which was effective for the reaction of styrylbo-
ronic acid,10 completely suppressed the reaction (entry 3).
We thus speculated that water might inhibit the reaction as
well. Indeed, addition of magnesium sulfate as a dehydrat-
ing agent effectively improved the chemical yield (entry 4).
Although Na2SO4, CaSO4, CaH2, and molecular sieves were
also tested, MgSO4 afforded the best performance. Heating
at 50 °C with MgSO4 largely increased the yield with a slight
loss of enantioselectivity (entry 5).
Under the conditions of entry 5, other diborons, tetra-
hydoxydiboron (3b), bis(pinacolato)diboron (3c), bis(hex-
ylene glycolato)diboron (3d), and bis(catecholato)diboron
(3e) were examined (entries 6–9). However, none was su-
perior to neopentyl glycolate 3a. Diboron 3b showed low
solubility under the reaction conditions. The low reactivity
of diborons 3c and 3d bearing congested tertiary alcohols
suggests that ligand exchange between the catalyst and a
diol ligand attached to the boron atom is essential for the
current catalysis.11 The catechol ligand on 3e might be too
labile under the reaction conditions.
With diboron 3a, the reaction parameters were further
optimized. Changing the solvent from toluene to dichloro-
methane, ethyl acetate, or diethyl ether decreased both the
yield and selectivity. Acetonitrile also lowered the yield, but
improved the selectivity (entry 10). In combination with
acetonitrile as solvent, benzoic acid was added as a promot-
er of the ligand exchange process, which increased the yield
(entry 11). Furthermore, use of a new catalyst, O-3,5-di(ad-
amantan-1-yl)benzoyltartaric acid (1b) with MgSO4 and
benzoic acid in acetonitrile largely improved the yield and
selectivity (entry 12).
DOI: 10.1055/s-0034-1378835; Art ID: ss-2015-c0172-st
Abstract O-3,5-Di(adamantan-1-yl)benzoyltartaric acid catalyzes the
conjugate addition of bis(neopentyl glycolato)diboron to α,β-unsatu-
rated ketones with good enantioselectivity. The addition of magnesium
sulfate as a dehydrating agent and benzoic acid as a co-catalyst was
found to be effective.
Key words organocatalysts, conjugate addition, diboron
The enantioselective conjugate addition of diboron re-
agents to α,β-unsaturated carbonyl compounds is a useful
method for the construction of a chiral carbon center at-
tached to a boryl group.1 The boryl group can be converted
to a hydroxyl group without loss of optical purity,2 or used
for carbon–carbon bond formation.3 While several asym-
metric transition metal (copper,4 rhodium,5 nickel, and pal-
ladium6) catalysts have been developed, only a few organo-
catalysts [chiral N-heterocyclic carbenes (NHC),7 chiral
phosphines,8 and a prolinol derivative with achiral NHC9]
have been described.
We have reported earlier that O-monoacyltartaric acids
effectively catalyze the conjugate addition of boronic acids
to α,β-unsaturated ketones (enones).10 We therefore envis-
aged that other types of boron compounds could be activat-
ed by those chiral hydroxy acids as well and report here the
results on the activation of diboron compounds.
Using catalyst 1a from our previous work, which was ef-
fective for the conjugate addition of styrylboronic acid to
enones,10 the reaction of chalcone (2a) with various dibo-
rons 3 in toluene at room temperature was initially investi-
gated (Table 1). The crude borated product was directly oxi-
dized with sodium perborate2 in order to evaluate the reac-
tion regardless of the boryl group. To our delight,
bis(neopentyl glycolato)diboron (3a) reacted at room tem-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 2265–2269

2266
M. Sugiura et al.
Special Topic
Synthesis
Under the optimized conditions (Table 1, entry 12), the
reactions of other enones were investigated (Table 2). Al-
though the reaction was found to be quite sensitive to the
enone structure, good enantioselectivities were obtained in
most cases. Higher reactivity was observed with an elec-
tron-rich aromatic substituent at the carbonyl β-position
(entry 3).12
Without the oxidative workup, the β-boryl ketone 5a
could be isolated by chromatography on silica gel (Scheme
1). The enantiomeric excess of 5a indicated that oxidation
of the (neopentyl glycolato)B–C bond proceeded without
loss of optical purity. The enantioselective synthesis of neo-
pentyl glycolates of β-boryl ketones has not been reported
before.
Table 1 Screening of Diborons 3 and Optimization of the Reaction Conditionsa
t-Bu
O
CO2H
OH
CO2H
O
1a
(10 mol%)
O
RO
RO
OR
OR
O
OH
t-Bu
+
B
B
solvent, additive
temp, 24 h
then NaBO3•4H2O
Ph
O
Ph
Ph
Ph
2a
4a
diboron 3
O
B
O
O
O
O
O
O
O
HO
OH
OH
O
O
O
O
B
B
B
B
B
B
B
B
B
HO
O
O
O
3a
3b
3c
3d
3e
Entry
3
Additive
Temp (°C)
Yield (%)b
ee (%)c
1
2d
3
3a
–
r.t.
r.t.
r.t.
r.t.
50
50
50
50
50
50
50
50
31
0
55
–
3a
3a
3a
3a
3b
3c
3d
3e
3a
3a
3a
–
MeOHe
0
–
f
4
MgSO4
MgSO4
MgSO4
MgSO4
MgSO4
MgSO4
MgSO4
44
72
0
55
50
–
f
f
f
f
f
f
5
6
7
7
36
29
–
8
29
0
9
10g
11g
12g,i
40
53
68
57
58
78
MgSO4,f PhCO2Hh
MgSO4,f PhCO2Hh
a Unless otherwise noted, the reaction was carried out using chalcone 2a (0.3 mmol), diboron 3 (0.36 mmol), and catalyst 1a (10 mol%) in toluene (1 mL) for 24
h.
b Isolated yields of 4a.
c Determined by HPLC analysis.
d Without catalyst.
e Amount of additive: 0.72 mmol.
f Amount of additive: 90 mg.
g In MeCN instead of toluene.
h Amount of additive: 0.36 mmol.
i With catalyst 1b instead of 1a.
O
CO2H
Ad
CO2H
O
OH
1b
Ad
(Ad = 1-adamantyl)
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 2265–2269

2267
M. Sugiura et al.
Special Topic
Synthesis
Catalyst 1a was prepared according to our previous report.10a Bis(neo-
pentyl glycolate)diboron (3a), tetrahydroxydiboron (3b), bis(pinaco-
late)diboron (3c), bis(hexylene glycolate)diboron (3d), and bis(cate-
cholate)diboron (3e) were purchased from Boron Molecular or Tokyo
Chemical Industry and used without purification. Toluene (>99%) was
purchased from Nacalai Tesque, Inc. and stored over 4 Å MS pellets.
Anhydrous MeCN was purchased from Wako Pure Chemical Indus-
tries and used without purification.
Table 2 Reaction of Various Enones 2 with Diboron 3a Catalyzed by
Catalyst 1ba
O
CO2H
Ad
CO2H
O
OH
1b
(10 mol%)
O
O
OH
Ad
Melting points were measured with Yanaco MP-J3 and are uncorrect-
ed. 1H and 13C{1H} NMR spectra were recorded in CDCl3 or DMSO-d6
+
R1
R2
3a
R1
R2
MgSO4, PhCO2H
MeCN, 50 °C, 24 h
then NaBO3⋅4H2O
2
4
on
a JEOL JNM-ECX 400 spectrometer. TMS (δ = 0) and CDCl3
(δ = 77.0) served as internal standards for 1H and 13C NMR, respec-
tively. 11B{1H} NMR spectra were recorded in CDCl3 or CD3CN on a
Bruker Avance 600 spectrometer. BF3·OEt2 (δ = 0 ppm) in a glass capil-
lary was used as an external standard for 11B NMR. IR spectra were
recorded on PerkinElmer Frontier spectrometer. Mass spectra were
recorded on a JEOL JMS-700MStation mass spectrometer. Optical ro-
tations were measured on a JASCO P-1010 polarimeter. High-perfor-
mance liquid chromatography (HPLC) was performed on JASCO PU-
2080Plus and UV-2075Plus instruments. TLC analysis was carried out
using Merck silica gel plates. Visualization was accomplished with UV
light, phosphomolybdic acid and/or anisaldehyde. Column chroma-
tography was performed using Kanto Chemical Silica Gel 60N (spher-
ical, neutral, 63–210 μm). The reactions under anhydrous conditions
were carried out using oven- and heating gun-dried glassware under
argon atmosphere.
Entry
R1
Ph
R2
Ph
4
Yield (%)b ee (%)c
1
2
3
4
5
6
7
4a
68
33
78
42
8
78
70
76
49
71
75
79
Ph
Ph
Ph
Ph
4-BrC6H4
4-MeOC6H4
1-naphthyl
i-Pr
4b
4c
4d
4e
4f
4-BrC6H4
4-MeOC6H4
Ph
36
28
Ph
4g
a The reaction was carried out with enone 2 (0.3 mmol), diboron 3a (0.36
mmol), catalyst 1b (10 mol%), MgSO4 (90 mg), and benzoic acid (0.36
mmol) in MeCN (1 mL) at 50 °C for 24 h.
b Isolated yields of ketone 4.
c Determined by HPLC analysis.
Dibenzyl (R,R)-O-3,5-Di(adamantan-1-yl)benzoyltartrate
To a solution of dibenzyl (R,R)-tartrate (356.8 mg, 1.08 mmol) and
3,5-di(adamantan-1-yl)benzoic acid13 (281.2 mg, 0.72 mmol) in an-
hydrous CH2Cl2 (7.2 mL) were added 1-ethyl-3-(3′-dimethylamino-
propyl)carbodiimide hydrochloride (EDC·HCl; 151.8 mg, 0.792 mmol)
and DMAP (8.8 mg, 0.072 mmol) at 0 °C. The mixture was stirred at
r.t. for 17.5 h and evaporated. The residue was purified by silica gel
column chromatography [SiO2 16 g, hexane–CH2Cl2 (1:1), then CH2Cl2
only] to give the title compound as a viscous oil; yield: 213.7 mg
(42%); [α]D19 +20.0 (c 1.360, CHCl3).
O
CO2H
OH
Ad
CO2H
O
O
O
O
1b
(10 mol%)
O
B
Ad
+
3a
Ph
Ph
Ph
Ph
MgSO4, PhCO2H
MeCN, 50 °C, 24 h
2a
5a
IR (ATR): 2903, 2849, 1728, 1599, 1455, 1194, 1123 cm–1
.
64% yield, 79% ee
1H NMR (400 MHz, CDCl3): δ = 7.77 (d, J = 1.9 Hz, 2 H, ArH), 7.61 (t,
J = 1.9 Hz, 1 H, ArH), 7.37–7.29 (m, 5 H, C6H5), 7.22–7.17 (m, 2 H,
C6H5), 7.14–7.08 (m, 3 H, C6H5), 5.69 (d, J = 2.3 Hz, 1 H, ArCO2CH), 5.27
(s, 2 H, PhCH2), 5.25 (d, J = 11.9 Hz, 1 H, PhCH2), 5.13 (d, J = 11.9 Hz, 1
H, PhCH2), 4.92 (dd, J = 7.8, 2.3 Hz, 1 H, HOCH), 3.31 (d, J = 7.8 Hz, 1 H,
OH), 2.17–2.08 (m, 6 H, Ad), 1.98–1.86 (m, 12 H, Ad), 1.85–1.72 (m, 12
H, Ad).
13C NMR (100 MHz, CDCl3): δ = 170.7, 166.5, 165.8, 151.2, 135.0,
134.3, 128.49, 128.32, 128.26, 128.03, 127.96, 127.1, 124.0, 73.3, 70.7,
68.1, 67.5, 43.1, 36.6, 36.4, 28.8.
Scheme 1 Reaction without oxidative workup
For the organocatalytic methods that use N-heterocyclic
carbenes8,10 or phosphines,9 activation of the diboron spe-
cies by these nucleophilic catalysts (to generate tetravalent
boron intermediates) has been suggested. Meanwhile, the
current catalyst system likely proceeds through a ligand ex-
change mechanism (generation of chiral trivalent boron
species).11 To discern this possibility, a stoichiometric reac-
tion of diboron 3a with catalyst 1a in CD3CN at 50 °C in the
presence of MgSO4 was performed and monitored by 1H
and 11B NMR spectroscopy.13 However, no significant
change was observed.14,15 This step might be energetically
unfavorable. Further study is necessary to clarify the pre-
cise mechanism.
MS (FAB+, CHCl3 + NBA + NaI): m/z (%) = 725 (28, [M + Na]+), 373 (100,
[Ad2C6H3CO]+), 135 (71, [Ad]+), 91 (93, [Bn]+).
HRMS (FAB+, CHCl3 + NBA + NaI): m/z [M + Na]+ calcd for C45H50O7Na:
725.3454; found: 725.3457.
(R,R)-O-3,5-Di(adamantan-1-yl)benzoyltartaric Acid (1b)
To a solution of dibenzyl (R,R)-O-3,5-di(adamantan-1-yl)benzoyltar-
trate (105.0 mg, 0.15 mmol) in EtOAc (4.5 mL) under argon atmo-
sphere was added 10% Pd/C (10.5 mg). The argon was replaced by H2
gas using a balloon and the reaction mixture was stirred at r.t. for 4 h.
In summary, we have demonstrated that O-monoacyl-
tartaric acids catalyze the conjugate addition of diborons to
α,β-unsaturated ketones with good enantioselectivity. Fur-
ther improvements and applications are now under investi-
gation.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 2265–2269

2268
M. Sugiura et al.
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The mixture was filtered through a Celite pad. The filtrate was con-
centrated under vacuum to give 1b as a colorless solid; yield: 78.6 mg
(quant); mp 186–189 °C; [α]D19 –3.4 (c 1.28, EtOH).
filtrate was washed with sat. aq NaHCO3 (2 × 10 mL) and brine (10
mL). The organic layer was dried (Na2SO4), filtered, and concentrated
under vacuum. The residue was purified by column chromatography
on silica gel (SiO2 2.2 g, hexanes–EtOAc, 20:1 to 5:1) to give 5a as a
viscous oil; yield: 61.4 mg (64%); 79% ee (R); [α]D19 –32.3 (c 1.565, CHCl3).
IR (ATR): 2901, 2847, 1728, 1701, 1600, 1210, 1118, 765 cm–1
.
1H NMR (400 MHz, DMSO-d6): δ = 7.79 (s, 2 H), 7.66 (s, 1 H), 5.41 (s, 1
H), 4.64 (s, 1 H), 2.13–2.02 (m, 6 H), 1.95–1.83 (m, 12 H), 1.82–1.68
(m, 12 H).
HPLC: Chiralcel AD-H, hexane–i-PrOH (24:1), flow rate = 1.0 mL/min,
UV detection at 240 nm, tR = 10.4 min (S), 14.0 min (R).
IR (ATR): 1681, 1599, 1580, 1476, 1253, 750 cm–1
.
13C NMR (100 MHz, DMSO-d6): δ = 172.1, 168.4, 165.7, 151.2, 128.6,
126.8, 123.3, 74.0, 70.1, 42.5, 36.07, 36.02, 28.3.
1H NMR (400 MHz, CDCl3): δ = 7.98 (d, J = 7.3 Hz, 2 H, C6H5CO), 7.54 (t,
J = 7.3 Hz, 1 H, C6H5CO), 7.44 (t, J = 7.3 Hz, 2 H, C6H5CO), 7.33–7.26 (m,
4 H, C6H5), 7.19–7.12 (m, 1 H, C6H5), 3.62 (d, J = 10.8 Hz, 2 H,
2 × OCHH), 3.59 (dd, J = 18.3, 11.0 Hz, 1 H, PhCOCHH), 3.55 (d, J = 10.8
Hz, 2 H, 2 × OCHH), 3.34 (dd, J = 18.3, 5.0 Hz, 1 H, PhCOCHH), 2.70 (dd,
J = 11.0, 5.0 Hz, 1 H, CHB), 0.89 (s, 6 H, 2 × CH3).
MS (FAB+, DMSO + NBA + NaI): m/z (%) = 567 (9, [M + 2 Na]+), 545 (7,
[M + Na]+), 435 (20, [M + H – 2 CO2]+), 373 (39, [Ad2C6H3CO]+), 135
(100, [Ad]+).
HRMS (FAB+, DMSO + NBA + NaI): m/z [M + Na]+ calcd for C31H38O7Na:
545.2515; found: 545.2516.
13C NMR (100 MHz, CDCl3): δ = 200.4, 143.1, 136.8, 132.8, 128.42,
128.40, 128.10, 128.00, 125.3, 72.0, 42.6, 31.8, 30.5 (br), 21.8.
11B{1H} NMR (192.5 MHz, CDCl3): δ = 29.1.
HRMS (FAB+, CHCl3 + NBA + NaI): m/z [M]+ calcd for C20H23BO3Na:
Conjugate Addition of Diboron 3a to Enone 2 Followed by Oxida-
tion; General Procedure
A 20-mL, screw-top test tube charged with MgSO4 (90 mg) was dried
under vacuum using heating gun. After cooling to r.t. under argon at-
mosphere, an enone 2 (0.3 mmol), catalyst 1b (15.7 mg, 10 mol%), and
MeCN (1 mL) were successively added to the test tube. After stirring
at r.t. for 30 min, bis(neopentyl glycolate)diboron (3a; 81.3 mg, 0.36
mmol) and benzoic acid (44.0 mg, 0.36 mmol) were added to the mix-
ture. The mixture was heated at 50 °C for 24 h, cooled to r.t., and fil-
tered through a Celite pad and washed with EtOAc (30 mL). The or-
ganic layer was concentrated under vacuum. The residue was diluted
with THF–H2O (3:1, 3 mL) and treated with NaBO3·4H2O (332 mg, 3.6
mmol). After stirring at r.t. for 2 h, the mixture was extracted with
EtOAc (3 × 20 mL). The combined organic layers were washed with
brine (20 mL), dried (Na2SO4), filtered, and concentrated under vacu-
um. The residue was purified by column chromatography on silica gel
(hexane–EtOAc) to give the product 4 (Table 2). The enantiomeric ex-
cess of 4 was determined by HPLC analysis using a chiral stationary
phase column.13
345.1642; found: 345.1643.
Acknowledgment
This work was partially supported by JSPS KAKENHI Grant Numbers
23590009 and 26460010.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1378835.
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References
(1) In addition to the conjugate addition, there are several enantio-
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According to the general procedure, the reaction of chalcone (2a; 62.5
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(c 0.765, CHCl3) for 78% ee (R) {Lit.16 [α]D20 +60.8 (c 1.0, CHCl3) for 88%
ee (R)}.
HPLC: Chiralcel OD-H, hexane–i-PrOH (85:15), flow rate
mL/min, UV detection at 254
= 0.7
nm) tR = 13.2 min (S), 14.7 min (R).
The products 4a–g are known. For further details, see the Supporting
Information.
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A 20-mL, screw-top test tube charged with MgSO4 (90 mg) was dried
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mol%), and MeCN (1 mL) were successively added to the test tube. Af-
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mg, 0.36 mmol) and benzoic acid (43.5 mg, 0.36 mmol) were added to
the mixture. The mixture was heated at 50 °C for 24 h, cooled to r.t.,
and filtered through a Celite pad and washed with EtOAc (30 mL). The
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(13) See the Supporting Information.
(14) In the previous study (ref. 10b), smooth complexation between
catalyst 1a and styrylboronic acid was observed.
(15) Participation of MgSO4 in the enantio-determining step is
unlikely, because the identical selectivity was observed even
without MgSO4 (Table 1, entries 1 and 4).
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© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 2265–2269