Diastereoselective Palladium-Catalyzed Conjugate Addition of Arylboronic Acids to α-Substituted Cyclic Enones

Article abstract of DOI:10.1055/s-0036-1590742

A palladium-catalyzed conjugate addition of arylboronic acids to α-substituted cyclic enones was developed to give α,β-disubstituted ketones with high diastereoselectivity. Mechanistic investigation showed that the high diastereoselectivity was realized through epimerization.

Full text of DOI:10.1055/s-0036-1590742

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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–D
letter
A
en
A. Gao et al.
Letter
Synlett
Diastereoselective Palladium-Catalyzed Conjugate Addition of
Arylboronic Acids to α-Substituted Cyclic Enones
Ang Gaoa
Xiu-Yan Liua
Chang-Hua Ding*a
Xue-Long Hou*a,b
O
O
Pd(TFA)2 (5.0 mol%)
L2 (6.0 mol%)
R
R
ArB(OH)2
+
N
N
DMAc, 80 °C
24 h, Ar
n
n
L2
Ar
1
2
3
a State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Ling Ling Road, Shanghai, 200032, P. R. of China
dingch@sioc.ac.cn
R = alkyl, aryl
n = 1, 2
15 examples
10:1–20:1 dr
49–98% yield
xlhou@sioc.ac.cn
b Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Ling Ling Road, Shanghai, 200032, P. R. of China
This paper is dedicated to Professor Victor Snieckus on the oc-
casion of his 80th birthday.
Received: 22.05.2017
O
Accepted after revision: 05.07.2017
Published online: 23.08.2017
(CH2)6COOH
O
O
(CH2)6COOH
DOI: 10.1055/s-0036-1590742; Art ID: st-2017-r0385-l
(CH2)4CH3
HO
H
Abstract A palladium-catalyzed conjugate addition of arylboronic ac-
ids to α-substituted cyclic enones was developed to give α,β-disubsti-
tuted ketones with high diastereoselectivity. Mechanistic investigation
showed that the high diastereoselectivity was realized through epi-
merization.
(CH2)4CH3
HO
HO
PGA1
AH 13205
Equilenin
Figure 1 Natural products containing a α,β-disubstituted cyclic ketone
moiety
Key words palladium, conjugate addition, boronic acid, cyclic enone,
diastereoselectivity
many natural products and medicinal molecules such as
AH13205,9a PGA1,9b and Equilenin9c (Figure 1).
Transition-metal-catalyzed conjugate addition reaction
of α,β-unsaturated ketones has become a powerful tool for
the construction of carbon–carbon bonds.1 A variety of or-
ganometallic nucleophiles, such as organozinc, organoalu-
minum, organomagnesium, organoboron, and organosili-
cons, as well as conjugate addition acceptors comprised of
both cyclic and acyclic ones are employed in the transfor-
mation under transition-metal catalysis.2–4 These reactions
often proceed in high efficiency with excellent stereocon-
trol. Despite these significant advances, there are only iso-
lated examples using α-substituted conjugate addition ac-
ceptors as substrate, and the diastereoselectivity was low-
er.5 Especially, for α-substituted cyclic enones, the only
reports involve highly reactive alkylaluminum,6 alkylmag-
nesium,7 and lithium organocuprate6f reagents under cop-
per catalysis, which could proceed through sequential
asymmetric conjugate addition–enolate trapping with elec-
trophiles in excellent dr and ee.7 During our studies on Pd-
catalyzed reaction using boronic acids,8 we developed a
method that features palladium-catalyzed conjugate addi-
tion of arylboronic acids to α-substituent cyclic enones for
efficient construction of α-alkyl-β-aryl-cyclic ketones with
high diastereoselectivity. The resulting motifs are found in
To initiate our studies, 2-methylcyclopentenone (1a)
and phenylboronic acid (2a) were added to a premixed
solution of palladium acetate and 1,10-phenanthroline (L1)
in DMF at 80 °C for 24 hours, affording the desired trans-
product 3a in 60% yield and with >20:1 dr (Table 1, entry
1).10 The solvent effect on the reaction was investigated.
The reaction was completely suspended when nonpolar sol-
vents such as toluene, 1,2-dimethoxyethane (DME) or 1,2-
dichloroethane (DCE) (entries 2–4) were used. Ketone 3a
was afforded in higher yield in dimethylacetamide (DMAc)
with excellent diastereoselectivity (entry 5 vs. entry 1).
With DMAc as solvent, the effect of palladium source on the
reaction was examined. It was found that PdCl2 and allyl-
palladium chloride dimer did not catalyze the reaction (en-
tries 6 and 7) and [Pd2dba3]·CHCl3 gave an inferior outcome
compared with Pd(OAc)2 (entry 8 vs. entry 5). Pleasingly,
more Lewis acidic palladium trifluoroacetate [Pd(TFA)2]
was found to improve the efficiency of the reaction dramat-
ically, with the yield of 3a being 85% with 20:1 dr, presum-
ably because the presence of a trace amount of trifluoro-
acetic acid favors the protonolysis of the resulting C–Pd
bond (entry 9 vs. entry 5). We then explored the influence
of the ligands on the reaction and found that 2,2′-bipyridine
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–D

B
A. Gao et al.
Letter
Synlett
(L2) delivered the product 3a in 98% yield with >20:1 dr
(entry 10). However, other types of bidentate nitrogen li-
gands, such as pyridinyloxazoline L3, bis-oxazoline L4 or
pyridinyl imidazoline L5, showed no catalytic activity in the
transformation (entries 11–13). We also found that the ste-
ric hindrance greatly affects the reaction. Only a trace
amount of product 3a was observed when 2,9-dimethyl-
1,10-phenanthroline (L6), with two extra methyl at the
ortho-position of nitrogen, was used as ligand (entry 14).
Notably, both the yield and the dr value declined signifi-
cantly when the reaction temperature was reduced (vide
infra). The yield diminished to 59% and the dr value de-
creased to 5:1 when the reaction was conducted at 50 °C
(entry 15). Further lowering the temperature to ambient af-
forded the product 3a in 40% with 1.2:1 dr (entry 16).
Table 2 Substrate Scope for the Palladium-Catalyzed Conjugate Addi-
tions of Arylboronic Acids with α-Substituted Cyclic Enonesa
O
O
Pd(TFA)2 (5.0 mol%)
L2 (6.0 mol%)
R
R
ArB(OH)2
+
DMAc, 80 °C
24 h, Ar
n
n
Ar
1
2
3
Entry
n, R (1)
Ar (2)
Yield (%)b
drc
>20:1
Table 1 Optimization of Reaction Conditions for Pd-Catalyzed Conju-
gate Addition of Phenylboronic Acid (2a) to 2-Methylcyclopentenone
(1a)a
1
2
1, Me (1a)
1, Me (1a)
1, Me (1a)
1, Me (1a)
1, Me (1a)
1, Me (1a)
1, Me (1a)
1, n-Bu (1b)
1, Ph (1c)
2, Me (1d)
2, Me (1d)
2, Me (1d)
2, n-Bu (1e)
2, Ph (1f)
o-MeC6H4 (2b)
m-MeC6H4 (2c)
p-MeC6H4 (2d)
p-BrC6H4 (2e)
2-naphthyl (2f)
p-CF3C6H4 (2g)
p-MeOC6H4 (2h)
Ph (2a)
85 (3b)
82 (3c)
84 (3d)
75 (3e)
89 (3f)
50 (3g)
60 (3h)
56 (3i)
80 (3j)
90 (3k)
50 (3l)
49 (3m)
69 (3n)
60 (3o)
N.R.
>20:1
>20:1
>20:1
>20:1
18:1
15:1
>20:1
>20:1
16:1
10:1
19:1
>20:1
>20:1
–
3
[Pd] (5 mol%)
O
O
4
L (6 mol%)
PhB(OH)2
(2.0 equiv)
5
+
solvent, Ar
80 °C, 24 h
6
Ph
3a
1a
2a
7
8
O
9
Ph (2a)
N
N
N
N
N
N
10
11
12
13
14
15
16
Ph (2a)
L1
L4
L2
L5
L3
p-CF3C6H4 (2g)
p-MeOC6H4 (2h)
Ph (2a)
Ts
N
Ph
Ph
O
N
O
N
N
N
N
N
Ph (2a)
L6
1, Me (1a)
1, Me (1a)
2-furanyl (2i)
(E)-PhCH=CH- (2j)
Entry
[Pd]
Ligand
Solvent
Yield (%)b drc
N.R.
–
a Molar ratio of Pd(TFA)2/L2/1/2=5:6:100:200.
1
2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L2
L2
DMF
60
>20:1
b Isolated yield.
toluene
DME
trace
trace
trace
72
–
–
c Determined by 1H NMR spectroscopy or GC chromatography with crude
product.
3
4
DCE
–
The substrate scope of the reaction was examined under
the optimized reaction conditions; the results are shown in
Table 2.11 Both five- and six-membered cyclic enones with
α-substituents reacted with arylboronic acids smoothly to
afford conjugate addition products in good to excellent
yields with high dr. Cyclic enones with α-phenyl or α-butyl
substituents were also viable substrates, providing the cor-
responding products in 56–80% yields with excellent dr (en-
tries 8, 9, 13, and 14). A wide range of arylboronic acids
were suitable for the reaction. Arylboronic acids with the
substituent at different positions of the phenyl ring could
be used to afford the corresponding products in high yield
with excellent dr (entries 1–3). It was worth noting that a
bromine substituent on the boronic acid 2e was also toler-
ated in this transformation, delivering the product 3e,
which is convenient for further transformation, in 75% yield
and >20:1 dr (entry 4). The reaction of boronic acid 2f, with
a bulky naphthyl group, also proceeded efficiently to afford
the product 3f in 89% yield in >20:1 dr (entry 5). Reaction of
5
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
>20:1
–
6
N.R.
N.R.
70
7
[Pd(C3H5)Cl]2
Pd2dba3·CHCl3
Pd(TFA)2
–
8
16:1
>20:1
>20:1
–
9
85
10
11
12
13
14
15d
16e
Pd(TFA)2
98
Pd(TFA)2
trace
N.R.
trace
trace
59
Pd(TFA)2
–
Pd(TFA)2
–
Pd(TFA)2
–
Pd(TFA)2
5:1
1.2:1
Pd(TFA)2
40
a Molar ratio of [Pd]/L/1a/2a = 5:6:100:200.
b Isolated yield.
c Determined by GC chromatography analysis of the crude product.
d Run at 50 °C.
e Run at room temperature.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–D

C
A. Gao et al.
Letter
Synlett
aryl boronic acids with electron-withdrawing or donating
substituents on the phenyl ring afforded the corresponding
products in moderate yields with high dr (entries 6, 7, 11,
and 12). Heteroaromatic boronic acid 2i and alkenyl boron-
ic acid 2j were used to react with 2-methyl-cyclopentenone
(1a); unfortunately, no reaction occurred (entries 15 and
16).
dine. The reaction is applicable to various five- and six-
membered cyclic enones with α-alkyl- and aryl substitu-
ents as well as a wide range of arylboronic acids. Prelimi-
nary mechanistic studies revealed that the high diastereo-
selectivity was realized through an epimerization process.
Funding Information
During our investigations on the mechanism, we ob-
served that the use of deuterated phenylboronic acid 2a led
to the product 3a with 52% α-deuterium incorporation
(Scheme 1, a). The experiment indicated that the α-proton
of the product 3a may derive from boronic acid and trace
moisture present in the reaction system. In addition, simply
extending the reaction time improved both the yield and
the diastereoselectivity significantly (Table 1, entry 16 vs.
Scheme 1, b). These experiments suggested an epimeriza-
tion process may take place during the reaction. Control ex-
periments showed that the diastereoselectivity remained
unchanged at room temperature by treatment of the prod-
uct 3a with 1.2:1 dr under the reaction conditions without
the extra proton source even for 6 days (Scheme 1, c). How-
ever, after 24 hours at 80 °C, the diastereoselectivity in-
creased from 1.2:1 to 10:1 and 3a was recovered in 95%
yield (Scheme 1, c). These results revealed that the high dia-
stereoselectivity of the reaction is achieved through an epi-
merization process12 and both proton source and reaction
temperature play important roles.
The authors acknowledge financial support from the National Natural
Science Foundation of China (NSFC) (21532010, 21372242,
21472214, 21421091), the Strategic Priority Research Program of the
Chinese Academy of Sciences (XDB20030100), the NSFC and the Re-
search Grants Council of Hong Kong Joint Research Scheme
(21361162001), the Technology Commission of Shanghai Municipali-
ty, and the Croucher Foundation of Hong Kong.
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1590742.
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References and Notes
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(52% deuterated)
O
D
B(OD)2
O
Pd(TFA)2 (5.0 mol%)
L2 (6.0 mol%)
+
(a)
80 °C, Ar
DMAc, 24 h
1a
2a-D
D > 90%
(2.0 equiv)
3a-D
93% yield
20:1 dr
O
O
Pd(TFA)2 (5.0 mol%)
L2 (6.0 mol%)
PhB(OH)2
(2.0 equiv)
+
(b)
r.t., Ar
DMAc, 6 d
Ph
3a
1a
2a
86% yield
>20:1 dr
O
O
O
Pd(TFA)2 (5.0 mol%)
L2 (6.0 mol%)
Pd(TFA)2 (5.0 mol%)
L2 (6.0 mol%)
(c)
r.t., Ar
DMAc, 6 d
80 °C, Ar
DMAc, 24 h
Ph
3a
Ph
Ph
3a
3a
1.2:1
1.2:1 dr
95% yield
dr
10:1 dr
Scheme 1 Exploration of the reaction mechanism
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In summary, we have developed a highly diastereoselec-
tive conjugate addition reaction of arylboronic acids to α-
substituted cyclic enones catalyzed by Pd(TFA)2/2,2′-bipyri-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–D

D
A. Gao et al.
Letter
Synlett
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(11) Typical experimental procedure: To a flame-dried 25 mL
Schlenk tube were added Pd(TFA)2 (4.2 mg, 0.0125 mmol),
ligand L2 (2.3 mg, 0.015 mmol), and anhydrous DMAc (1.0 mL).
The resulting mixture was stirred for 60 min. Arylboronic acid
2b (68.0 mg, 0.5 mmol) and ketone 1a (24.0 mg, 0.25 mmol)
were added subsequently. The resulting reaction mixture was
stirred at 80 °C for 24 h. Water (10 mL) was added and the
aqueous phase was extracted with ethyl acetate (2 × 10 mL).
The combined organic phase was washed with water (3 × 10
mL) and brine (10 mL). The organic phase was dried over anhy-
drous Na2SO4 and filtered. After the volatile materials were
removed in vacuo, the ratio of two diastereoisomers was deter-
mined by GC chromatography. The resulting residue was then
subjected to flash chromatography on silica gel with petroleum
ether and ethyl acetate as eluent to give product 3b (39.9 mg,
85%) as a colorless oil. 1H NMR (400 MHz, CDCl3): δ = 7.33–7.01
(m, 5 H), 3.10 (td, J = 12.1, 5.2 Hz, 1 H), 2.61–2.48 (m, 1 H), 2.43–
2.17 (m, 7 H), 1.87–1.71 (m, 1 H), 1.01 (dd, J = 6.8, 1.3 Hz, 3 H).
13C NMR (101 MHz, CDCl3): δ = 219.7, 140.3, 136.4, 130.6, 126.5,
126.4, 124.7, 51.0, 46.0, 37.6, 29.1, 19.8, 12.1. IR (neat): 2962,
1738, 1492, 1459, 1406, 1260, 1147, 1090, 1021, 798, 761, 732.
MS (EI): m/z (%) = 187 (60) [M+], 132 (90), 114 (100), 90 (50).
HRMS (EI): m/z [M]+ calcd for C13H16O: 188.1201; found:
188.1199.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–D