Palladium- and base-free synthesis of conjugated ynones by cross-coupling reactions of alkynylboronates with acid chlorides mediated by CuCl

Article abstract of DOI:10.1016/j.tetlet.2009.11.001

Alkynylboronates can be employed as a practical and versatile precursor for a variety of π-conjugated organic compounds. In the presence of Cu(I) salt, cross-coupling reactions of acid chlorides with alkynylboronates giving rise to the corresponding conju

Full text of DOI:10.1016/j.tetlet.2009.11.001

Tetrahedron Letters 51 (2010) 306–308
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Palladium- and base-free synthesis of conjugated ynones by cross-coupling
reactions of alkynylboronates with acid chlorides mediated by CuCl
*
Yasushi Nishihara , Daisuke Saito, Eiji Inoue, Yoshiaki Okada, Mikihiro Miyazaki, Yoshiaki Inoue,
Kentaro Takagi
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Alkynylboronates can be employed as a practical and versatile precursor for a variety of
p-conjugated
Received 1 October 2009
Revised 29 October 2009
Accepted 2 November 2009
Available online 5 November 2009
organic compounds. In the presence of Cu(I) salt, cross-coupling reactions of acid chlorides with alkynyl-
boronates giving rise to the corresponding conjugated ynones takes place readily in aprotic polar solvents
such as DMI under neutral conditions.
Ó 2009 Elsevier Ltd. All rights reserved.
Conjugated ynones1 have been prominently employed as equiv-
alents of valuable intermediates in organic synthesis as well as var-
ious functional groups in natural product,2 important biologically
active heterocycles,3 and acetylenic scaffoldings.4 Among most fre-
quently employed methods to access ynones, the palladium and/or
copper-catalyzed processes of acid chlorides and terminal alkynes
under basic conditions has been widely employed due to the ver-
satile nature of this protocol, increased functional group tolerance,
and improved yields.3f,5,6 But the reactions were limited by unde-
sired side reactions between the acid chlorides and the added ter-
tiary amines, reducing the yields.
pounds are listed in Table 1. The cross-coupling reaction was
screened with a stoichiometric amount of a copper compound (or
salt) in DMI, which is the best solvent for the homocoupling of alky-
nylsilanes28 and alkynylboronates.25a Counter ions of a halogenated
copper salt dramatically affected the yield of 1,3-diphenyl-2-pro-
pyn-1-one (3a). CuCl was suitable for the present cross-coupling
reaction and 3a was formed in 64% yield (entry 1). However,
100 mol % of CuCl resulted in the lower yield (31%) of the cross-cou-
pled product. Although CuBr can effect the cross-coupling reaction
to give 3a in 50% yield, CuI was found to be inactive at all (entries 2
vs 3). Whereas copper(I) thiophene-2-carboxylate (CuTC)29 fur-
nished 3a in 38% yield (entry 4), other copper(I) compounds such
as Cu2O, CuCN, and CuOAc were not so effective for the formation
Alternatively, the cross-couplings of acid chlorides with the cor-
responding organometallic reagents such as organosilver,7 -sili-
con,8 -cadmium,9 -copper,10 -tin11 -lithium,12 -magnesium,12a
-
zinc,13 -aluminum,14 -thallium,15 -gallium16 -antimony,17 and -in-
dium,18 have attracted much attention. However, there are limita-
tions with each of them.19
Table 1
Copper-mediated cross-coupling reaction of phenylethynylboronate 1a with benzoyl
chloride (2a)a
Ontheother hand, alkynylboronates20 havebeen knownto be the
useful synthetic intermediates. Utility of these reagents in transition
metal-catalyzed reactionshave becomethe subjectof interestdue to
their enhanced reactivity, and successful transformations involve
cycloaddition reactions,21 the synthesisof tetrasubstituted olefins,22
Suzuki–Miyaura cross-coupling,23 and addition to alkynes.24 On the
basis of promising potentialities of alkynylboronates, our efforts
have been made in developing new synthetic methodologies.25
Herein, we report the synthesis of conjugated ynones from CuCl-
mediated cross-coupling reactions of acid chlorides with
alkynylboronates.26,27
Ph
O
Cu compound
O
O
Cl
Ph
Ph
+
Ph
B
DMI
O
80 °C, 24 h
1a
2a
3a
Entry
Copper compound
Yieldb (%)
1
2
3
CuCl
CuBr
CuI
64
50
0
4
5
6
7
8
9
10
CuTC
Cu2O
38
18
<1
0
0
<1
0
CuCN
CuOAc
Cu(OTf)2
Cu(OAc)2
CuCl2
Initially, we examined the reaction between phenylethynylboro-
nate 1a with benzoyl chloride (2a) as standard substrates to optimize
the reaction conditions. The results employing various copper com-
a
Reaction was performed in DMI (3 mL) using 1a (0.2 mmol) and 2a (0.22 mmol)
at 80 °C for 24 h in the presence of a copper compound (0.4 mmol).
* Corresponding author. Tel./fax: +81 86 251 7855.
E-mail address: ynishiha@cc.okayama-u.ac.jp (Y. Nishihara).
b
Yields were determined by GC.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.001

Y. Nishihara et al. / Tetrahedron Letters 51 (2010) 306–308
307
of 3a (entries 5–7). Although we found that Cu(OAc)2 was the best
additive for the homocoupling reaction of alkynylboronates,25a
Cu(OAc)2 as well as other copper(II) compounds were found to be
ineffective for the present reaction (entries 8–10).
the efficiency. Cross-coupling reaction using 1d, which possesses a
sulfur atom gave the corresponding conjugate ynone in 48% yield
(entry 14). Similarly to the case of aliphatic alkynylboronate 1e,
the cross-coupling readily proceeded to provide the corresponding
ynones 3o–3q in good yields (entries 15–17). It is noteworthy that
in all cases no formation of 1,3-diynes was observed in the reaction
mixture, derived from homocoupling reaction of alkynylboronates.
However, aliphatic and alkenyl acid chlorides completely retarded
the reaction with 1a and gave no desired product.
We next surveyed solvents such as DMSO, DMF, Bu2O, DMPU,
TMU, and toluene using a stoichiometric amount of CuCl. Yields
of the cross-coupled product 3a greatly depend on the nature of
the solvent. Among the solvents surveyed in comparison with
CuCl-mediated cross-coupling reaction, DMI proved to be by far
the most effective and afforded 3a. In a sharp contrast, no trace
of the desired product 3a was obtained in other organic solvents.
It is noteworthy that the present reaction system does not require
the palladium catalyst or a base which is normally required for the
activation of organoboron compounds.30
As a result of various combinations of the reaction time and
temperature examined, we found that the reaction completed after
30 min at 120 °C in DMI. With the optimized conditions for the fac-
ile cross-coupling of 1a with 2a in hand, we further explored the
tolerance of this process toward a range of alkynylboronates 1
and the results are summarized in Table 2.31 The reactions of acid
chlorides bearing electron-withdrawing (entries 2–4) and elec-
tron-donating (entry 5) groups with 1a proceeded in moderate to
good yields. Since the present cross-coupling reaction proceeds un-
der neutral condition, acid chloride 2f bearing the ester functional-
ity is compatible to give the corresponding product 3f in 59% (entry
6). Acid chlorides having heteroaromatics such as 2-furyl and 2-
thienyl groups afforded the corresponding cross-coupling products
3g and 3h in 58% and 63% isolated yields, respectively (entries 7
and 8). Acid chloride containing chloride in the ortho position 2i
underwent the reaction to give 3i in good yield (entry 9).
By TLC (hexane/EtOAc = 19:1), the obtained conjugated ynone
3a (Rf = 0.33) was slower moving than the starting acid chloride
2a (Rf = 0.58). All of the cross-coupled products 3 were well charac-
terized by spectroscopic measurements (IR, 1H, and 13C{1H} NMR,
and GC–MS) as well as elemental analyses. In IR spectra, conju-
gated ynꢀo1nes exhibited characteristic strong absorption at 2100–
2210 cm
for the carbon–carbon triple bond and 1600–
1650 cmꢀ1 for the carbonyl group.
Although it has been known that a catalytic amount of CuCl pro-
motes the reaction of alkynylsilanes with acid chlorides,8d a stoi-
chiometric amount of CuCl is required in this reaction. In order
to scavenge the contaminating boron-containing species, we con-
ducted the cross-coupling reaction in the presence of Lewis bases
such as furan, NEt3, and THF. However, we found that our attempt
to obtain the corresponding conjugated ynones was unsuccessful.
As one example of the advantage of the Pd-free reaction condi-
tions, the present reaction of 4-chloromethylbenzoyl chloride with
1a was carried out. In the presence of CuCl under the optimal con-
ditions, the chemoselective cross-coupling occurred to generate
the corresponding conjugated ynone 3r as a sole product in 44%
isolated yield (Eq. 1). The presence of the Pd catalyst resulted in
no formation of the desired product 3r presumably due to the
dechlorination at the benzyl position.
On the other hand, the reactions of acid chlorides 2a–2c with
alkynylboronates 1b–1e bearing various functional groups similarly
underwent the cross-coupling reactions (entries 10–17). To this end
we have prepared a series of alkynylboronates 1 from isopro-
poxy(pinacol)borate with alkynyl lithium.32 These were prepared
in situ from the corresponding terminal alkynes with n-BuLi at
ꢀ78 °C, followed by the subsequent treatment with HCl in diethyl
ether. For a vast number of derivatives of alkynylboronate, the pres-
ence of various substituents, for example, 4-methoxy (entries 10–
12) and 4-chloro (entry 13) on the aromatic ring did not diminish
Cl
Cl
CuCl
1a
Ph
+
Cl
DMI
O
O
120 °C, 30 min
3r: 44 %
ð1Þ
In these cross-coupling reactions it is thought that an alkynyl
Table 2
group of alkynylboronates 1 transmetalates from boron to copper
without a nucleophilic activator such as a base. To gain an insight
into the intermediate alkynylcopper species, the reaction of 1a
with CuCl in 1:2 molar ratio in DMI was carried out. The reaction
proceeded at 120 °C for 1 h under an Ar atmosphere to give
[Cu2Cl(C„CPh)]n (4)33 in 62% yield (Eq. 2). Alkynylcopper 4 was
isolated as a bright yellow solid and elemental analyses (C, H,
and Cl) of the product agree with the calculated values from our
previously reported alkynylcopper 4 derived from alkynylsilane
with CuCl in DMF.33 Once isolated 4 could react with 2a cleanly
to form the corresponding ynone 3a in 43% yield Eq. 3:
Cross-coupling reactions of alkynylboronates 1 with acid chlorides 2a
R'
O
CuCl
O
O
Cl
R'
R
B
+
R
DMI
O
120 °C, 30 min
1
2
3
Entry
R
R0
Products
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
C6H5– (1a)
C6H5– (2a)
4-Cl–C6H4– (2b)
4-CF3–C6H4– (2c)
4-NO2–C6H4– (2d)
4-EtO–C6H4– (2e)
4-MeOCO–C6H4– (2f)
3a
3b
3c
3d
3e
3f
67
73
70
52
44
59
58
63
77
73
67
75
60
48
71
75
72
O
+
[Cu2Cl(C≡ CPh)]n
4: 62%
Ph
B
2
CuCl
DMI
120 °C, 1h
under Ar
ð2Þ
ð3Þ
O
2-Furyl (2g)
2-Thienyl (2h)
2-Cl–C6H4 (2i)
3g
3h
3i
1
Ph
O
4-MeO–C6H4– (1b)
2a
2b
2c
2a
2a
2a
2b
2c
3j
3k
3l
3m
3n
3o
3p
3q
Cl
Ph
Ph
+
4
DMI
120 °C, 1 h
under Ar
O
4-Cl–C6H4– (1c)
2-Thienyl (1d)
n-C6H13– (1e)
2a
3a: 43%
In summary, we have successfully demonstrated copper(I)-
mediated cross-coupling reaction of acid chlorides with alkynyl-
boronates leading to various conjugated ynones in aprotic polar
solvents such as DMI under Pd-free and neutral reaction condi-
a
Conditions: 1 (1.0 mmol); 2 (1.18 mmol); CuCl (2.0 mmol); DMI (2.5 mL).
Isolated yields based on 1.
b

308
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kynes protected with the boron moiety in the absence of the palla-
dium catalyst, side reactions leading to the conjugated diynes by
the Pd-catalyzed homocoupling reactions of terminal alkynes,
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Acknowledgment
19. For instance, (1) The silver or copper(I) acetylide-acyl chloride routes are quite
variable in yield, and acetal or ester functions in the acetylide are often cleaved
by the acyl chloride. (2) The AlCl3-catalyzed reaction of acyl chlorides with
alkynylsilanes uses strongly acidic conditions. (3) Almost all the methods
reported so far make use of an excess of the acid chloride (1.5–2 mol per mole
of phenyl acetylene) and NEt3 (2–3 equiv).
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Y.N. acknowledges the financial assistance from National Sci-
ence Council, ROC (NSC 098-2811-M-002-027) for stay in Taiwan.
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31. Representative procedure: To the 20 mL of a Schlenk tube were successively
added CuCl (198 mg, 2 mmol), 1a (228 mg, 1 mmol), DMI (2.5 mL), and 2a
(128
lL, 1.1 mmol). After being stirred for 30 min at 120 °C under Ar the
reaction was quenched with 1 M hydrochloric acid (2 mL) and the reaction
mixture was extracted with diethyl ether (10 mL ꢁ 2). The combined ethereal
layer was washed with brine, and dried over MgSO4. Filtration and
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