The gold-catalyzed formal hydration, decarboxylation, and [4+2] cycloaddition of alkyne derivatives featuring L2/Z-type diphosphinoborane ligands

Article abstract of DOI:10.1246/cl.180610

The catalytic formal hydration of alkynes and decarbox-ylation of alkynoic acid were developed using a Au catalyst featuring a Z-ligand. Furthermore, the intramolecular [4+2] cycloaddition of the alkynoic acid-alkene derivative for the formation of the oxabicyclo[4.4.0] skeleton also proceeded.

Full text of DOI:10.1246/cl.180610

Received: July 11, 2018 | Accepted: August 3, 2018 | Web Released: August 11, 2018
CL-180610
The Gold-catalyzed Formal Hydration, Decarboxylation, and [4+2] Cycloaddition
of Alkyne Derivatives Featuring L2/Z-type Diphosphinoborane Ligands
Chiaki Matsumoto, Masayuki Yamada, Xun Dong, Chisato Mukai, and Fuyuhiko Inagaki*
Division of Pharmaceutical Sciences, Graduate School of Medical Sciences, Kanazawa University,
Kakuma-machi, Kanazawa 920-1192, Japan
E-mail: finagaki@p.kanazawa-u.ac.jp
The catalytic formal hydration of alkynes and decarbox-
ylation of alkynoic acid were developed using a Au catalyst
featuring a Z-ligand. Furthermore, the intramolecular [4+2]
cycloaddition of the alkynoic acid-alkene derivative for the
formation of the oxabicyclo[4.4.0] skeleton also proceeded.
and oxygen atom of the alcohol was detected by the 11B NMR
shift, which suggested activating of the reactivity of the O atom
by chelation assistance.8b Based on this, we speculated that the
chelation assistance of the Z-ligand might activate the reactiv-
ities of the other oxygen atoms, such as the intermolecular
alcohol or intramolecular carboxylic acid, for the catalytic
reactions. We now report the catalytic formal hydration,
decarboxylation, and formal [4+2] cycloaddition by using
alkynes and oxygen atoms featuring the Z-type ligand.
The metal-catalyzed formal hydration of alkynes is one of
the most important reactions for making the ketone unit. As
a representative metal species, Hg(II) is well known for the
hydration reaction. However, there is a significant disadvantage
of being toxic. For avoiding the use of a mercury salt, the
hydrations using Au,9 Pt,10 Ru11 or Ag12 have been explored.
In most cases of hydration reactions using Au(I) catalysts, a
high reaction temperature is required. For example, hydration
with gold catalysts bearing NHC ligands is conducted at 50-
120 °C.9d,9l,9m Only one example of the NHC-Au catalyzed
hydration using 3-hexyne at room temperature was recently
reported by Zuccaccia et al. Also, Hu and Wu’s group reported
the hydration of alkynes at room temperature with a Au
catalyst bearing isocyanide ligands. Although the reactions were
effective for several arylalkynes and alkylalkynes, the addition
of a large amount of -OH sources as the solvents, such as MeOH
and water, must be required. Therefore, developing more
effective gold catalysts for the alkyne hydration at room
temperature is still a challenging task. At the beginning, the
addition of 4 mol % of Au(DPB)SbF6 (L2/Z-type ligands) to
the solution of octyn-3-ol (4a) and MeOH (5 equiv) in DCE at
room temperature for 24 h followed by hydrolysis gave the
desired ketone 5a in 87% yield (Table 1, Entry 1). When the
Keywords: Z-Ligand
| Gold | Catalytic reaction
For organometallic compounds, there are three types of
ligands, the L-, X- and Z-types categorized by Green.1 Although
the X-ligand forming a covalent bond with a metal (M-X) and L-
ligand providing two electrons to a metal (M←:L) are generally
used, the σ-acceptor type Z-ligand (M:¼Z) metal complexes
and their catalytic reactions are still limited.2-7 We recently
focused on the syntheses of the gold(I) complexes Au(DPBR)X 1
featuring a Z-type ligand (boron atom) and its applications to
catalytic reactions. The Z-Au catalysts were effective for the
construction of 7-membered ring systems not only from enynes
but also from yne-diols 2 (Figure 1).8 Based on the previous
study, the interaction between the boron atom of the Z-ligand
Table 1. Au(I)-catalyzed formal hydration of alkyne 4a
1) cat. Au+ (4 mol%)
MeOH (5 eq.)
DCE, rt
OH
HO
4
2) SiO2
4
4a
5a O
entry
1
yield
87%
Au cat.
form
time
+
B
Au(DPB)SbF6
24 h
P Au P
+
+
Au(PPh3)2SbF6
Au(PPh3)SbF6
2
3
24 h
trace
41%
P Au P
1.5 h
P
Au
Figure 1. Catalytic reactions of alkynes and oxygen atom
featuring Au(DPBR)X 1.
Chem. Lett. 2018, 47, 1321–1323 | doi:10.1246/cl.180610
© 2018 The Chemical Society of Japan | 1321

Au(PPh3)2SbF6 (L2-type ligands), which did not contain the
boron atom at the ligand, was treated, the reaction hardly
proceeded (Entry 2). Carriedo et al. reported the hydration with
the bis(phosphine) gold(I) cation at high temperature (100-
150 °C). These results suggest that the σ-acceptance of Z-ligand
would activate the formal hydration. Although the reaction with
Au(PPh3)SbF6 was much faster than the others, the product 5a
was obtained in moderate yield (41%, Entry 3).
Our next effort focused on the exploration of the scope and
limitations. These results are summarized in Table 2. When the
4-pentynyl pivalate (4b) was treated under the best conditions
in Table 1, the desired product 5b (96%) was observed in high
yield (Entry 1). The alkyne 4c bearing carboxylic acid was also
applicable for the formal hydration, which provided 5c in 92%
yield (Entry 2). The reactions with benzyloxymethyl, phenethyl,
and N-protected aminopropyl alkynes 4d-f provided the desired
ketones 5d-f in moderate yields (Entries 3-5). Formal hydration
with 4-methoxyphenylacetylene (4g) gave the arylmethylketone
5g (87%) in good yield (Entry 6). On the other hand, internal
alkynes 4h,i bearing phenyl or ester moieties did not afford the
desired products 5h,i (Entries 7 and 8).
Carboxylic acids, which are present in many natural
products and commercially available compounds, are interesting
functional groups that lead to various reactions including the
classical decarboxylation of β-dicarboxylic acid by heating.
Recently, several types of catalytic decarboxylations of aryl-
carboxylic acids have been reported by using Cu,13 Ag,14 and
Au15 under heating conditions. However, the decarboxylation of
alkynoic acid is still limited. Kolarovič and co-worker16 reported
the Pd or Cu-catalyzed decarboxylation of alkynoic acid at
60 °C. Also, Pd-catalyzed decarboxylative coupling of alkynoic
acids and aryl halides was reported by several groups.17
However, as far as we know, the reaction requires on elevated
temperature. After several screenings, we found that the treat-
ment of 2 mol % of [Au(DPB)SbF6]2(cod) and alkynoic acid 6e
in DCE under Ar afforded the decarboxylative product 4e (50%)
in moderate yield at room temperature (Scheme 1). Although
decarboxylation of arylalkynoic acid 6j also gave the desired
product 4j, the chemical yield (24%) was rather low. When the
alkylalkynoic acid 6k was treated with the same catalyst in air,
the product was not the terminal alkyne but a ketone 5k (50%
yield). Decarboxylation and hydration with moisture would give
the resulting ketone 5k.
Table 2. Au(DPB)SbF6-catalyzed formal hydration of alkynes 4.
1) Au(DPB)SbF6 (4 mol%)
O
MeOH (5 eq.), DCE, rt
R2
R1
R2
R1
2) SiO2
4
5
entry
result
substrate
O
1
2
3
PivO
PivO
HO2C
BnO
3
5b: 8 h, 96%
4b
O
6
HO2C
6
5c: 3.5 h, 92%
4c
O
3
4
BnO
Our final task in this research was the intramolecular
[4+2] cycloaddition of the alkynoic acid-alkene. In 2012, Yu
and Shin’s group18 reported the Au(JohnPhos)SbF6-catalyzed
intermolecular [4+2] cycloaddition of alkynoic acid and
alkene to form the α,β-unsaturated δ-lactone. However, the
intramolecular [4+2] cycloaddition is unreported. When the
alkynoic acid-alkene derivative 7 was treated with 2 mol % of
5d: 18 h, 55%
4d
O
2
Ph
Ph
2
5e: 24 h, 60%
4e
O
O
O
3
[Au(DPB)SbF6]2(cod)
N
5
(2 mol%)
N
CO2H
O
3
2
2
DCE, Ar, rt, 24 h
Ph
Ph
O
6e
6j
4e
5f: 24 h, 69%
4f
50% yield
O
[Au(DPB)SbF6]2(cod)
(2 mol%)
6
7
8
MeO
Ph
CO2H
Ph
MeO
DCE, Ar, rt, 24 h
4g
4h
4j
5g: 18 h, 87%
24% yield
O
Ph
Ph
Ph
Ph
[Au(DPB)SbF6]2(cod)
(2 mol%)
5h: 24 h, no reaction
CO2H
7
7
O
DCE, air, rt, 24 h
O
6k
5k
MeO2C
CO2Me
CO2Me
MeO2C
50% yield
4i
5i: 24 h, complex mixture
Scheme 1.
1322 | Chem. Lett. 2018, 47, 1321–1323 | doi:10.1246/cl.180610
© 2018 The Chemical Society of Japan

MeO
MeO
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(2 mol%)
CO2H
MeO
MeO
O
O
DCE, rt
8
9
7
8
Au(JohnPhos)SbF6
70% yield
[Au(DPB)SbF6]2(cod) 96% yield
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Scheme 2.
Au(JohnPhos)SbF6, the desired oxabicyclo[4.4.0] derivative 8
was observed in 70% yield (Scheme 2). For the reaction with
[Au(DPB)SbF6]2(cod), the cyclized product 8 was provided in
a better yield (96%).
In summary, we found that the Au catalyst featuring the Z-
ligand worked not only for the formal hydration of alkynes, but
also the decarboxylation of alkynoic acid at room temperature.
In addition, the intramolecular [4+2] cycloaddition of the
alkynoic acid-alkene derivative for the formation of the
oxabicyclo[4.4.0] skeleton was also developed by using the
Z-Au catalyst. Further investigation of the intramolecular [4+2]
cycloaddition is currently in progress.
This study was supported by JSPS KAKENHI Grant
Number JP18H04244 in Precisely Designed Catalysts with
Customized Scaffolding to F. I.
Supporting Information is available on http://dx.doi.org/
10.1246/cl.180610.
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Chem. Lett. 2018, 47, 1321–1323 | doi:10.1246/cl.180610
© 2018 The Chemical Society of Japan | 1323