One-pot synthesis of conjugated (E)-enynones via two types of cross-coupling reaction

Article abstract of DOI:10.1055/s-0030-1258563

(Trimethylsilyl)ethynyl bromide can be easily transformed into conjugated (E)-enynones, whose skeleton consists of consecutive carbonyl, ethynyl, and (E)-ethenyl units, via a one-pot multicomponent Suzuki-type reaction-Sonogashira reaction sequence. Thus,

Full text of DOI:10.1055/s-0030-1258563

LETTER
2461
One-Pot Synthesis of Conjugated (E)-Enynones via Two Types of
Cross-Coupling Reaction
S
ynthesis of Conjugate
a
d
(
E
)-Enyn
s
ones ayuki Hoshi,* Hirokazu Yamazaki, Mitsuhiro Okimoto
Department of Biotechnology and Environmental Chemistry, Kitami Institute of Technology, 165 Koen-cho, Kitami,
Hokkaido 090-8507, Japan
Fax +81(157)247719; E-mail: hoshi-m@chem.kitami-it.ac.jp
Received 14 July 2010
natural products17 and [2+2]-photocycloadducts.18 How-
ever, the method for preparing such conjugated enynones
is rare,19 and there are no reports, to our knowledge, on
synthesizing conjugated enynones with definite geome-
Abstract: (Trimethylsilyl)ethynyl bromide can be easily trans-
formed into conjugated (E)-enynones, whose skeleton consists of
consecutive carbonyl, ethynyl, and (E)-ethenyl units, via a one-pot
multicomponent Suzuki-type reaction–Sonogashira reaction se-
quence. Thus, a three-component coupling of (trimethylsilyl)ethy- try. As part of our ongoing research aimed at assembling
p-extended conjugation,20 we herein report one-pot syn-
nyl bromide, (E)-alk-1-enyldisiamylborane and acid chloride is
achieved in a two-step, one-pot procedure, in which (E)-alk-1-enyl
thesis of conjugated (E)-enynones, (E)-1-arylalk-4-en-2-
group is installed as nucleophile in the sp-carbon atom attached to
yn-1-ones 3, under mild reaction conditions. Based on the
bromine atom and acyl group is installed as electrophile in the other
copper-catalyzed cross-coupling reaction of (E)-alk-1-
sp-carbon atom.
enyldisiamylboranes 1 with (trimethylsilyl)ethynyl bro-
Key words: (trimethylsilyl)ethynyl bromide, conjugated enynone,
mide leading to the formation of terminal conjugated (E)-
cross-coupling, alkenylborane, acid chloride
enynes 2 (Suzuki-type reaction),21 we envisioned that the
desired compounds 3 could be obtained by reaction of
compounds 2 with aroyl chlorides (Sonogashira reaction)
Conjugated ynones have found a wide range of applica-
in a two-step, one-pot manner as illustrated in Scheme 1.
tions as intermediates in the synthesis of natural products1
Therefore, the main challenge in this one-pot reaction was
and heterocycles,2 as well as substrates in organic synthe-
to investigate the reaction conditions for the Sonogashira
sis through organometallic compounds.3 A great number
reaction.
of methods have been reported for their preparation, and
Optimization of the reaction conditions was explored us-
ing (E)-dec-3-en-1-yne (2a) and benzoyl chloride as mod-
el substrates. Thus, the cross-coupling reaction of (E)-oct-
the methodology can be divided into three categories: (1)
the palladium and/or copper-catalyzed coupling reaction
of terminal alkynes with acid chlorides (Sonogashira reac-
tion),4 (2) the cross-coupling reaction between alkynyl or-
ganometallic reagents and acid chlorides,5–15 and (3) the
palladium and/or copper-catalyzed carbonylative cou-
pling reaction of terminal alkynes with aryl hahides (car-
bonylative Sonogashira reaction).16 Each of them has a
characteristic feature. The cross-coupling reaction using
alkynyl organometallic reagents can proceed well without
employing any base which may give rise to undesired side
reaction with acid chloride. Carbonylative Sonogashira
coupling can be performed even in protic solvent or ionic
liquid. Thus it is possible to choose among many methods
according to circumstances.
1-enyldisiamylborane (1a) (1 mmol) with (trimethylsi-
lyl)ethynyl bromide (0.67 mmol) was carried out in the
presence of Cu(acac)2 (0.05 mmol) and 1 M NaOMe (0.75
mmol) at –15 °C to room temperature overnight to gener-
ate compound 2a (ca. 0.5 mmol).22 After removal of meth-
anol under reduced pressure, the reaction with benzoyl
chloride (1 mmol)23 was conducted in the presence of pal-
ladium source, ligand, and Et3N (1 mmol)24 in anhydrous
THF at room temperature for two hours. Addition of CuI
was unnecessary because Cu(acac)2 had already been
used for the cross-coupling reaction of 1a with (trimethyl-
silyl)ethynyl bromide. The results are summarized in
Table 1. Use of PdCl2(PPh3)2 (2 mol% relative to com-
pound 2a) as a palladium source gave the desired product,
(E)-1-phenylundec-4-en-2-yn-1-one (3aa), albeit in mod-
On the other hand, conjugated enynones, in which the sp-
carbon atom of conjugated ynone connects with an alke-
nyl carbon atom, also play key roles in the synthesis of
Cu(acac)2, 1 M NaOMe
2B
Pd/Base
R2COCl
O
CBr
Me3SiC
R1
R2
R1
R1
1
2
3
Scheme 1 The proposed two-step, three-component coupling process
SYNLETT 2010, No. 16, pp 2461–2464
x
x
.x
x
.2
0
1
0
Advanced online publication: 03.09.2010
DOI: 10.1055/s-0030-1258563; Art ID: U06510ST
© Georg Thieme Verlag Stuttgart · New York

2462
M. Hoshi et al.
LETTER
Table 1 Effect of Pd Sources and Ligands on the Cross-Coupling with Benzoyl Chloridea
O
2B
Pd, Et3N
PhCOCl
Cu(acac)2, 1 M NaOMe
Me3SiC CBr
Ph
C6H13-n
_15 °C to r.t.
C6H13-n
C6H13-n
r.t.
1a
2a
3aa
Entry
Pd source
PdCl2(PPh3)2
Pd(PPh3)4
PdCl2
(mmol)
Ligand
(mmol)
Yield of 3aa (%)b
1
2
0.01
0.01
0.01
0.005
0.01
0.01
0.005
0.01
0.01
0.01
0.005
–
–
47
70
0
–
–
3
–
–
4
Pd2(dba)3⋅CHCl3
Pd(OAc)2
PdCl2
–
–
0
5
–
–
0
6
Ph3P
0.02
0.02
0.02
0.02
0.02
0.01
80
86
86
82
78
84
7
Pd2(dba)3⋅CHCl3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Ph3P
8
Ph3P
9
(2-furyl)3P
(4-MeOPh)3P
Ph3P
10
11
a Reaction conditions: (1) 1a (1 mmol), Me3SiC≡CBr (0.67 mmol), Cu(acac)2 (0.05 mmol), 1 M NaOMe (0.75 mmol), –15 °C to r.t., overnight;
(2) PhCOCl (1 mmol), Pd source, ligand, Et3N (1 mmol), r.t., 2 h.
b Isolated yields based on the amount of 2a (0.5 mmol) formed.
erate yield (Table 1, entry 1). When Pd(PPh3)4 (2 mol%) partner (Table 2, entries 5–10). Using condensed aroyl
was used in place of PdCl2(PPh3)2, the yield of product chloride such as 2-naphthoyl chloride, the cross-coupling
3aa improved considerably (Table 1, entry 2). Ligand- reaction could also proceed smoothly (Table 2, entries 11
free palladium sources, such as PdCl2, Pd(OAc)2, and and 12). In addition, heteroaroyl chlorides such as 2-fu-
Pd2(dba)3·CHCl3, did not lead to any desired product at all royl and 2-thiophenecarbonyl chlorides were good sub-
(Table 1, entries 3–5). But, in contrast, the palladium strates for this coupling reaction (Table 2, entries 13–16).
source in combination with Ph3P (4 mol%) led to high
In summary, we have developed a two-step, one-pot,
yields of product 3aa (Table 1, entries 6–8), indicating
three-component synthesis of (E)-1-arylalk-4-en-2-yn-1-
that phosphine ligand is crucial to the success of this cou-
ones 3 using a Suzuki-type reaction–Sonogashira reaction
sequence. This protocol is the first method for preparing
compounds 3 in good to high yields. The extended scope
of this transformation will be reported in due course.
pling reaction. For our further studies, Pd(OAc)2 is prefer-
able to PdCl2 and Pd2(dba)3·CHCl3 in terms of the yield
and cost. Among phosphine ligands screened, Ph3P
proved to be the best ligand (Table 1, entry 8).25 Reducing
the loading of Pd(OAc)2/Ph3P showed a decreasing ten-
References and Notes
dency in the yield of product 3aa (Table 1, entry 11).
(1) For example, see: (a) Thompson, C. F.; Jamison, T. F.;
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2002, 67, 9200.
Having established our optimized conditions for the se-
quential coupling reaction, we examined the scope of the
one-pot synthesis of (E)-1-arylalk-4-en-2-yn-1-ones 3.
The results are shown in Table 2. Different types of com-
pounds 2 underwent smooth cross-coupling with a variety
of aroyl chlorides to afford products 3 in good to high
yields. This coupling reaction was successfully applied to
compound 2 with a structurally and electronically diverse
substituent R1 (Table 2, entries 1–4). The cross-coupling
reaction with various aroyl chlorides was carried out using
both (E)-dec-3-en-1-yne (2a) and (E)-4-phenylbut-3-en-
1-yne (2b). It is noteworthy that the use of compound 2b
provides p-conjugated molecules (Table 2, entries 2, 6, 8,
10, 12, 14, and 16). Electron-donating as well as electron-
withdrawing aroyl chlorides could be used as coupling
(3) For example, see: (a) Dodero, V. I.; Koll, L. C.; Faraoni, M.
B.; Mitchell, T. N.; Podestá, J. C. J. Org. Chem. 2003, 68,
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126, 13942.
Synlett 2010, No. 16, 2461–2464 © Thieme Stuttgart · New York

LETTER
Synthesis of Conjugated (E)-Enynones
2463
Table 2 Synthesis of (E)-1-Arylalk-4-en-2-yn-1-onesa
O
Pd(OAc)2, Ph3P
ArCOCl, Et3N
2B
Cu(acac)2, 1 M NaOMe
Me3SiC CBr
_15 °C to r.t.
R1
R1
Ar
r.t.
3
1
R1
2
Entry
1
R1
Ar
Product
3aa
3ba
3ca
Yield (%)b
n-C6H13
Ph
Ph
86
90
92
88
90
85
79
77
78
82
87
77
76
86
79
79
2
Ph
3
cyclohex-1-enyl
Cl(CH2)3
n-C6H13
Ph
Ph
4
Ph
3da
3ab
3bb
3ac
5
2-Tol
2-Tol
4-Tol
4-Tol
6
7
n-C6H13
Ph
8
3bc
3ad
3bd
3ae
9
n-C6H13
Ph
4-ClC6H5
4-ClC6H5
2-naphthyl
2-naphthyl
2-furyl
10
11
12
13
14
15
16
n-C6H13
Ph
3be
3af
n-C6H13
Ph
2-furyl
3bf
n-C6H13
Ph
2-thienyl
2-thienyl
3ag
3bg
a Reaction conditions: (1) 1 (1 mmol), Me3SiC≡CBr (0.67 mmol), Cu(acac)2 (0.05 mmol), 1 M NaOMe (0.75 mmol), –15 °C to r.t., overnight;
(2) ArCOCl (1 mmol), Pd(OAc)2 (0.01 mmol), Ph3P (0.02 mmol), Et3N (1 mmol), r.t., 2 h.
b Isolated yields based on the amount of 2 (0.5 mmol) formed.26
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Synlett 2010, No. 16, 2461–2464 © Thieme Stuttgart · New York

2464
M. Hoshi et al.
LETTER
(12) Alkynylindium: (a) Pérez, I.; Sestelo, J. P.; Sarandeses, L. A.
J. Am. Chem. Soc. 2001, 123, 4155. (b) Augé, J.; Lubin-
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819.
amount of benzoyl chloride was employed in this one-pot
reaction. Indeed, using a stoichiometric amount of benzoyl
chloride (0.5 mmol), a decrease in the yield of product 3aa
was observed.
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2953.
(24) Among amine bases including i-Pr2NEt, Et3N was the base
of choice for the cross-coupling reaction with benzoyl
chloride.
(25) To a solution of BH3 (1 mmol) in THF (3 mL) was added 2-
methylbut-2-ene (0.14 g, 2 mmol) dropwise at –15 °C under
argon, and the mixture was stirred for 2 h at 0 °C to form a
solution of disiamylborane in THF. To this solution was
added oct-1-yne (0.11 g, 1 mmol) dropwise at –15 °C, and
the mixture was stirred for 2 h at 0 °C. A solution of (E)-oct-
1-enyldisiamylborane (1a, 1 mmol) in THF, thus prepared,
was cooled to –15 °C, and Cu(acac)2 (0.013 g, 0.05 mmol)
was added to the solution under a flow of argon, followed
by dropwise addition of (trimethylsilyl)ethynyl bromide
(0.119 g, 0.67 mmol) and NaOMe (1 M, 0.75 mL, 0.75
mmol). The resulting mixture was allowed to warm
gradually to r.t. and stirred overnight. Methanol resulting
from 1 M NaOMe was removed under reduced pressure,
accompanied by the solvent. After addition of THF (3 mL)
to the residue under argon, the resulting mixture including
(E)-dec-3-en-1-yne (2a) was cooled to 0 °C, and Pd(OAc)2
(0.002 g, 0.01 mmol) and Ph3P (0.005 g, 0.02 mmol) were
added successively under a flow of argon, followed by
dropwise addition of benzoyl chloride (0.141 g, 1 mmol) and
Et3N (0.101 g, 1 mmol). The resultant mixture was stirred for
2 h at r.t. and then oxidized by the successive addition of 3
M NaOH (1 mL) and 30% H2O2 (0.5 mL) at 0 °C. After
being stirred for 1 h at this temperature, the mixture was
extracted three times with Et2O. The combined extracts were
washed with brine, dried over Na2SO4, and concentrated.
The residue was purified by flash chromatography on silica
gel, with hexane–CH2Cl2 (1:1) as eluent, to give (E)-1-
phenylundec-4-en-2-yn-1-one (3aa, 0.103 g, 86%).
Compound 3aa: 1H NMR (500 MHz, CDCl3): d = 0.89 (t,
J = 7.1 Hz, 3 H), 1.25–1.35 (m, 6 H), 1.42–1.49 (m, 2 H),
2.21–2.26 (m, 2 H), 5.74 (dt, J = 16.1, 1.5 Hz, 1 H), 6.63 (dt,
J = 16.1, 7.1 Hz, 1 H), 7.46–7.50 (m, 2 H), 7.58–7.62 (m, 1
H), 8.13–8.16 (m, 2 H). 13C NMR (125 MHz, CDCl3): d =
14.06 (CH3), 22.56 (CH2), 28.22 (CH2), 28.78 (CH2), 31.59
(CH2), 33.64 (CH2), 86.05 (≡C), 92.85 (≡C), 107.65 (=CH),
128.50 (2 × =CH), 129.50 (2 × =CH), 133.90 (=CH), 136.92
(=C), 153.10 (=CH), 178.11 (C=O). IR (neat): 2954, 2927,
2856, 2183, 1641, 1620, 1596, 1579, 1448, 1313, 1265,
1174, 956, 937, 700 cm–1. HRMS (EI): m/z calcd for
C17H20O: 240.1514; found: 240.1508.
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(22) Compound 2a was formed in about 75% GC yield based on
Me3SiC≡CBr employed, see ref. 21.
(23) Considering that acid chloride would be consumed by
reaction with residual both NaOMe and MeOH, an excess
(26) Compounds 2b–d were formed in 72–74% GC yields based
on Me3SiC≡CBr employed; unpublished results.
Synlett 2010, No. 16, 2461–2464 © Thieme Stuttgart · New York