Palladium/copper-cocatalyzed decarbonylative alkynylation of acyl fluorides with alkynylsilanes: Synthesis of unsymmetrical diarylethynes

Article abstract of DOI:10.1039/d0cc03309j

Palladium/copper-cocatalyzed decarbonylative alkynylation of acyl fluorides with alkynylsilanes is described. This reaction not only effectively inhibits the formation of undesired homocoupled products by avoiding the addition of a base, but also exhibits a wide substrate scope to provide a general access to diverse unsymmetrical diarylethynes.

Full text of DOI:10.1039/d0cc03309j

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DOI: 10.1039/D0CC03309J
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Palladium/copper-cocatalyzed decarbonylative alkynylation of
acyl fluorides with alkynylsilanes: Synthesis of unsymmetrical dia-
rylethynes
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
*b
Qiang Chen, Liyan Fu and Yasushi Nishihara
DOI: 10.1039/x0xx00000x
Palladium/copper-cocatalyzed decarbonylative alkynylation of acyl been employed. However, the development of other coupling
fluorides with alkynylsilanes is described. This reaction not only partners derived from naturally abundant feedstocks is still
effectively inhibits the formation of undesired homocoupled highly demanded. Recently, since acyl fluorides have attracted
products by avoiding the addition of the base, but also exhibits a much attention due to stability, ready availability from the
wide substrate scope to provide a general access to diverse corresponding carboxylic acids as well as their specific
1
5
unsymmetrical diarylethynes.
reactivity,
transformations as an ‘RCO’ or ‘R’ source
decarbonylative or decarbonylative reaction pathway,
respectively. In particular, since the decarbonylative
they have widely been applied in various
1
6
17-21
via non-
Since Sonogashira-Hagihara (S-H) reaction1 has been known as
an effective method to prepare a variety of conjugated
arylethynes and enynes, a large number of applications utilizing
this reaction have been reported. One of the purposes to
optimize this protocol was to minimize the inevitable formation
of homocoupled products that made the work-up process more
tedious and the yields of the desired products lower. To avoid
the formation of homocoupled products, copper-free S-H
reactions have been explored extensively. Furthermore, the
17
trifluoromethylation of acid fluorides has been reported, acyl
fluorides showed unique reactivities which can participated in a
large number of decarbonylative reactions, such as reduction,
2
18
19
20
Suzuki-Miyaura-type arylation, and direct C–H arylation. We
also reported various decarbonylative transformations of acyl
3
21
fluorides, showing different reactivities from those of acyl
4
2
chlorides. However, the construction of C(sp )–C(sp) bonds by
decarbonylative alkynylation of acyl fluorides remains largely
unexplored. We herein report the synthesis of unsymmetrical
“sila”-S-H reactions employed trimethylsilylethynes as the
coupling partner instead of conventional terminal alkynes,
which succeeded in preventing the generation of undesired
diarylethynes
by
Pd/Cu-cocatalyzed
decarbonylative
5
-10
5
homocoupled products.
CsF,
An array of reagents such as K
2
CO
3
,
alkynylation of acyl fluorides with alkynylsilanes under the
neutral conditions.
Initially, as a model reaction we conducted the reaction of
benzoyl fluoride (1a) with (phenylethynyl)trimethylsilane (2a)
under the PdCl
the results are summarized in Table 1. Among a series of
bidentate phosphorus ligands screened, 1,3-
bis(diphenylphosphino)propane (DPPP) was the most efficient,
giving diphenylethyne (3aa) in 58% GC yield (entries 1-3). In a
sharp contrast, monodentate phosphine ligands gave poor
yields of 3aa (Table S1). Subsequently, the influence of pallidum
catalysts was examined and a comparative yield of 3aa was
6
7
tetrabutylammonium
chloride
(TBAC),
and
8
tetrabutylammonium fluoride (TBAF) have been employed to
activate trimethylsilylethynes in “sila”-S-H reactions. Besides,
9
10
Ito and Hosomi, and our group disclosed that copper salts
have specific reactivity for transmetalation, forming
alkynylcopper species. More importantly, these reactions
proceeded under the neutral conditions, which allows the base-
sensitive functional groups to be tolerated.
2
catalysis to optimize reaction conditions and
In regard to the coupling partners in S-H reactions, general
aryl or vinyl (pseudo)halides as well as sodium sulfonates,
arylhydrazines,12 arylsulfonyl hydrazides, and amides have
1
1
13
14
obtained when Pd(OAc)
employed. The combination of Pd(PPh
2
(54%) and Pd
2
(dba)
3
·C
6
H
6
(56%) were
(PPh with
)
3 4
or PdCl
2
3 2
)
a. Graduate School of Natural Science and Technology, Okayama University
1
3
0 mol % of PPh , which showed a high activity in the former
3
-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
b.
reports, also had been tested, but only poor yield of 3aa was
observed (Table S2). Although the yield decreased as the DPPP
loading decreased (entry 4), a comparable yield of 54% could be
1,10
Research Institute for Interdisciplinary Science, Okayama University
-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
E-mail: ynishiha@okayama-u.ac.jp
Electronic Supplementary Information (ESI) available: experimental procedures,
3
†
1
13
1
19
1
spectroscopic data and copies of H, C{ H} and F{ H} NMR spectra. See DOI: obtained even with 15 mol % of DPPP (entry 5). To our delight,
1
0.1039/x0xx00000x
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COMMUNICATION
Journal Name
Table 1 Optimization of the reaction conditionsa
Scheme 1 The influence of substituents on alkynes. a Reaction conditioVniesw: 1A(r0ti.c2lemOmnolinl)e,
DOI: 10.1039/D0CC03309J
2
(0.3 mmol), PdCl (0.02 mmol), DPPP (0.03 mmol), CuI (0.01 mmol), in DMF/toluene
2
PdCl2 (10 mol %)
ligand
(2:3, 0.4 M) at 150 ºC for 24 h under Ar. GC yields using 1-tetradecene as an internal
standard.
O
additive
+
Me3Si
solvent
o
F
150 C, 24 h
1
a
2a
3aa
With the optimized reaction condition in hand, we performed
the reactions of various acyl fluorides 1 with 2a (Table 2). Aa a
result, the desired unsymmetrical diarylethynes 3ba-3ha
bearing electron-donating or –withdrawing groups in the para-
position were obtained in moderate to good yields.
Accordingly, acyl fluorides 1 bearing fluoride and chloride
groups could also be compatible for this transformation; 3ia and
3ja could be obtained in 86% and 74% yields, respectively.
Furthermore, under the identical conditions, the steric effect
yield (%)b
Entry
ligand
DPPE
DPPP
DPPB
DPPP
DPPP
DPPP
DPPP
DPPP
DPPP
DPPP
additive
solvent(s)
1
2
-
DMF
24
-
DMF
58
3
4c
-
DMF
27
-
DMF
9
5d
-
DMF
54
6d
-
DMF/toluene (2/3)
DMF/toluene (2/3)
DMF/toluene (2/3)
DMF/toluene (2/3)
DMF/toluene (2/3)
74
7d
CuI
CuI
CuI
CuI
99
8d,e
9d,e,f
3
was investigated with 1 bearing Me, Ph, and CF substituents in
the ortho-positions, which gave the products in 53-72% yields.
However, 2,4,6-trimethylbenzoyl fluoride having a higher steric
89
98 (84)
14
0d,e,f,g
1
a
Conditions: 1a (0.2 mmol), 2a (0.4 mmol), PdCl (0.02 mmol), ligand (0.04 mmol), CuI bulk, only 27% of 3na was obtained.
2
(
0.02 mmol), solvent (1 mL), 150 ºC, 24 h. bGC yields using 1-tetradecene as an internal
standard and an isolated yield is shown in parentheses. DPPP (0.02 mol). dDPPP (0.03
c
Table 2 Scope of acyl fluoridesa
mmol). 2a (0.3 mmol). CuI (0.01 mmol). g Benzoyl chloride instead of benzoyl fluoride.
e
f
PdCl2 (10 mol %)
DPPP (15 mol %)
CuI (5 mol %)
O
after several optimization of the solvents (Table S4), a mixed
solvent system of DMF and toluene in a 2:3 ratio increased the
yield of 3aa to 74% (entry 6). Furthermore, 10 mol % of CuI as
the additional catalyst dramatically improved the yield of 3aa
F
+
Me3Si
R
DMF/toluene (2/3)
R
150 ºC, 24 h
1
2a
3
(
entry 7). A series of additives, such as PhCO
PhCO K proved to be less efficient than CuI (Table S5). With a
decrease of the amount of 2a to 1.5 equiv, 3aa was obtained in
9% yield (entry 8). Although the copper sources were
2 2
Li, PhCO Na, and
2
Me
tBu
Ph
3aa: 84%
3ba: 67%
3ca: 71%
3da: 80%
8
screened, CuI was found to be the best (Table S6). Since the
decreased loading of CuI to 5 mol % also resulted in 98% yield
of 3aa, the conditions in entry 9 was adopted to be optimum. It
is worth noting that when benzoyl chloride was employed
instead of benzoyl fluoride, only 14% of 3aa was detected,
which demonstrated the unique nature of acyl fluorides in the
O
O
O
NC
3ea: 63%
3fa: 83%
3ga: 76%
3ha: 62%
Ph
OMe
OPh
Me
Ph
present decarbonylative alkynylation (entry 10).
It is
noteworthy to mention that no formation of alkynylketones
without decarbonylation was observed in all cases.
F
Cl
3ia: 86%
3ja: 74%
3ka: 72%
3la: 53%
Next, we investigated the effect of substituents on alkynes.
As shown in Scheme 1, when the reaction was conducted with
a terminal alkyne (R = H), only 10% yield of 3aa was obtained.
The other silyl groups were also evaluated. As a result, as steric
bulk of silyl groups increases, the reactivity slightly decreased to
CF3
Me
Me
Me
3ma: 69%
3na: 27%
3oa: 74%
3pa: 57%
give 3aa in 82% (SiEt
contrast, when the bulkier SiMe
3
) and 81% (SiMe
tBu group was employed, no
2
Ph) yields. In a sharp
2
desired product was formed, indicating that the steric
hindrance of the substituents might retard the coordination of
O
O
S
N
3qa: 59%
3ra: 62%
3sa: 71%
3ta: 81%
copper to
a triple bond in 2 and the subsequent
transmetalation, forming alkynylcopper (I) species.
NC
iBu
S
N
Acetylsalicylic
acid
O
O
PdCl2 (10 mol %)
Me
O
DPPP (15 mol %)
CuI (5 mol %)
O
Me
Febuxostat
F
+
R
3ua: 55%
3va: 52%
3wa: 27%
DMF/toluene (2/3)
a
150 ºC, 24 h
Reaction conditions: 1 (0.2 mmol), 2a (0.3 mmol, 1.5 equiv), PdCl
2
(0.02 mmol), DPPP
3
aa
1a
2 (1.5 equiv)
(
0.03 mmol), CuI (0.01 mmol), in DMF/toluene (2:3, 0.4 M) at 150 ºC for 24 h under Ar.
R = H; 10%
R = SiMe2Ph; 81%
R = SiMe2tBu; 0%
R = SiEt3; 82%
Isolated yields.
2
| J. Name., 2012, 00, 1-3
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COMMUNICATION
PdCl2 (10 mol %)
DPPP (15 mol %)
CuI (5 mol %)
Besides, the cross-coupled products with polycyclic (3oa-3pa)
and heterocyclic (3qa-3ta) scaffolds were afforded in moderate
to high yields. In regard to the demonstration of the present
reaction, acyl fluorides derived from biologically active
molecules (3ua-3va) were also suitable substrates. We also
O
View Article Online
DOI: 10.1039/D0CC03309J
F
+
Me Si
3
SiMe3
DMF/toluene (2/3)
150 ºC, 24 h
52%
1o
4
5
(
2 equiv)
(0.2 mmol)
a
Scheme 2 Reaction of Benzoyl Fluoride with Bis(trimethylsilyl)acetylene.
(0.02 mmol %), DPPP (0.03 mmol), CuI
0.01 mmol), in DMF / toluene (2:3, 0.4 M) at 150 ºC for 24 h under Ar.
Reaction
tried to extend the scope of acyl fluorides to cinnamoyl fluoride conditions: 1o (0.4 mmol), 4 (0.2 mmol), PdCl
2
(
(
1w), but only 27% of conjugated enynes 3wa was obtained.
Unfortunately, benzoyl fluorides 1 bearing an OMe, NO , NMe
or Br group in the 4-position were not compatible in this Because of non-basic conditions, the base-sensitive substrates
reaction (see, ESI).
could be tolerated to afford 3af in 50% yield. Besides, aliphatic
2
2
,
In order to elucidate an electronic effect of the substituents alkynylsilanes were also compatible to give 3og and 3oh, albeit
on the coupling partners 1 and 2, we performed a series of in moderate yields. We successfully attained a large-scale
substrate combinations that afford the identical products. As synthesis by the reaction of 1a with trimethyl(4-
shown in Table 3, acyl fluorides 1 with electron-neutral and - tolylethynyl)silane, giving 1.10 g of 3ba in 57% yield.
withdrawing groups gave higher yield than that with electron-
3
In the “Sila”-S-H reactions, a SiMe group not only can work
donating groups, indicating that electronically poor acyl as a directly reactive group for transmetalation with a copper
fluorides can render oxidation addition to Pd(0) much easier, salt, but can help us to handle volatile compounds. Based on
leading to the higher yields of 3. To our surprise, when an acyl this idea, we utilized
a
commercially available
fluoride bearing an OMe group was employed, no trace of the bis(trimethylsilyl)ethyne (4) as the ethyne gas surrogate
desired products 3ac and 3xc were detected, along with the (Scheme 2). The reaction 4 with 2 equiv of acyl fluoride 1o gave
formation of 3aa and 3ad (up to 15% yield), which might be 5 in 52% yield. This synthetic method demonstrated convenient
generated from the Ph-group transfer between acyl fluorides usage of 4 for the synthesis of symmetrical diarylethynes.
2
2
and the 1,3-bis(diphenylphosphino)propane (DPPP) ligand.
Combining all the fundamental reactions reported in the
Whereas alkynylsilanes 2 with an OMe group could afford 3ac catalytic cycle,2,10,17,23 a plausible reaction mechanism is shown
and 3xc in 83% and 81% yields, respectively. On the contrary, in Scheme 3. Initially, acyl fluorides 1 are subjected to oxidative
alkynylsilanes 2 with electron-deficient groups gave 3xb and addition with Pd(0) via C–F bond cleavage to provide complex
3
ae in lower yield.
We assumed that electron-rich A. Meanwhile, transmetalation between alkynylsilanes 2 and a
alkynylsilanes can accelerate the coordination of copper to a copper salt takes place to form alkynylcopper species B.
2
4
triple bond in 2 to facilitate the transmetalation. These results Sequentially, a second transmetalation between A and B, which
suggest that the choice of the substrates is highly important for might be the rate-determining step, affords an
the success of complementary synthesis of the target acyl(alkynyl)palladium(II) complex C.
The formation of
molecules.
intermediate D by decarbonylation from C, followed by CO
extrusion from D may occur to produce intermediate E, from
which reductive elimination produces the cross-coupled
product 3, regenerating the initial Pd(0) catalyst. Considering
the result shown in Table 1, entry 6, where the product was
obtained in 74% yield even in the absence of CuI, a direct
transmetalation between the intermediate A and alkynylsilanes
Table 3 Scope of alkynylsilanesa
R2
PdCl2 (10 mol %)
O
DPPP (15 mol %)
CuI (5 mol %)
+
Me3Si
R2
F
DMF/toluene (2/3)
150 ºC, 24 h
R1
R1
1
2
3
2
certainly takes place with the assistance of the
OMe
CF3
1
7,18
thermodynamically stable Si–F bond formation.
3ac
3ad
MeO
F3
C
83%
0%
75%
67%
R
O
Ar
3
LnPd0
Me
CF3
OMe
CF3
Ar
F
1
reductive
elimination
oxidative
addtion
3xb
3xc
Ar
F3C
Me
F3C
MeO
PdLn
71%
39%
81%
0%
R
E
O
O
O
B
Ar
PdLn
Me
O
CO
CO extrusion
F
A
Bu
CO
Cu
R
Ar
B
PdLn
3ae: 43%
3af: 50%
3og: 54%
3oh: 29% (52%)b
trans-
metalation
decarbonylation
O
a
R
D
Reaction conditions: 1 (0.2 mmol), 2 (0.03 mmol 1.5 equiv), PdCl
2
(0.02 mmol), DPPP
SiMe3
X
Ar
PdLn
C
CuX
(
X = F or I)
(0.03 mmol), CuI (0.01 mmol), in DMF/toluene (2:3, 0.4 M) at 150 ºC for 24 h under Ar.
R
Isolated yields.
Me Si
R
3
2
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J. Name., 2013, 00, 1-3 | 3
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Journal Name
Scheme 3 Plausible catalytic cycle.
14 (a) W. Srimontree, A. Chatupheeraphat, H.V-Hiew. ALrtiiacloe ,OnMline.
Rueping, Org. Lett., 2017, 19, 3091; (b) L. Liu, D. Zhou, M. Liu,
DOI: 10.1039/D0CC03309J
Y. Zhou, T. Chen, Org. Lett., 2018, 20, 2741.
In conclusion, we have developed Pd/Cu-cocatalyzed
decarbonylative alkynylation of various acyl fluorides with
ether, halo, cyano, ketone, and ester functional groups and
nitrogen,
oxygen, sulfur-containing heterocyclic compounds in moderate
to good yield. To clarify the orders of transmetalation and
decarbonylation in the catalytic cycle, further mechanistic
studies by isolating the intermediate complexes and by detailed
DFT calculations are currently underway in our laboratory.
We gratefully thank the SC-NMR Laboratory (Okayama
University) for the NMR spectral measurements.
1
5 For selected reviews on transition-metal-catalyzed
transformations of acyl fluorides, see: (a) N. Blanchard, V.
Bizet, Angew. Chem. Int. Ed., 2019, 58, 6814; (b) Q. Zhao, M.
Szostak, ChemSusChem, 2019, 12, 2983; (c) Y. Ogiwara, N.
Sakai, Angew. Chem., Int. Ed., 2020, 59, 574.
1
6 (a) Y. Zhang, T. Rovis, J. Am. Chem. Soc., 2004, 126, 15964; (b)
Y. Ogiwara, Y. Maegawa, D. Sakino, N. Sakai, Chem. Lett.,
2
016, 45, 790; (c) Y. Ogiwara, D. Sakino, Y. Sakurai, N. Sakai,
Eur. J. Org. Chem., 2017, 4324; (d) A. Boreux, K. Indukuri, F.
Gagosz, O. Riant, ACS Catal., 2017, 7, 8200; (e) Y. Ogiwara, Y.
Iino, N. Sakai, Chem.-Eur. J., 2019, 25, 6513; (f) F. Pan, P. Guo,
C. Li, P. Su, X. Shu, Org. Lett., 2019, 21, 3701; (g) J. Han, W.
Zhou, P. Zhang, H. Wang, R. Zhang, H. Wu, J. Zhang, ACS Catal.,
2
019, 9, 6890; (h) Y. Ogiwara, S. Hosaka, N. Sakai,
Organometallics, 2020, 39, 856.
Conflicts of interest
There are no conflicts to declare.
1
1
1
7 S. T. Keaveney, F. Schoenebeck, Angew. Chem. Int. Ed., 2018,
57, 4073.
8 Y. Ogiwara, Y. Sakurai, H. Hattori, N. Sakai, Org. Lett., 2018,
2
0, 4204.
9 C. A. Malapit, J. R. Bour, C. E. Brigham, M. S. Sanford, Nature,
018, 563, 100.
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