Bifunctional phosphine ligand-enabled gold-catalyzed direct cycloisomerization of alkynyl ketones to 2,5-disubstituted furans

Article abstract of DOI:10.1039/d0cc01238f

An efficient synthesis of 2,5-disubstituted furans directly from alkynyl ketones has been developed via tandem gold(i)-catalyzed isomerization of alkynyl ketones to allenyl ketones and cycloisomerization. The key to the success of this chemistry is the use of a biphenyl-2-ylphosphine ligand featuring a critical remote tertiary amino group.

Full text of DOI:10.1039/d0cc01238f

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Bifunctional Phosphine Ligand-Enabled Gold-Catalyzed Direct
Cycloisomerization of Alkynyl Ketones to 2,5-Disubstituted Furans
Xiaojun Hu,a Bingwei Zhou,a Hongwei Jin,a Yunkui Liu,*a,b and Liming Zhang*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
An efficient synthesis of 2,5-disubstituted furans directly from allene subunit without the use of strong base, thus providing
alkynyl ketones has been developed via tandem gold(I)-catalyzed many opportunities for further utilization of the in situ
isomerization of alkynyl ketones to allenyl ketones and generated allene species. For example, by using a biphenyl-2-yl
cycloisomerization. The key to the success of this chemistry is the phosphine ligand featuring a remote aniline or tertiary amine
use of a biphenyl-2-ylphosphine ligand featuring a critical remote moiety (L3-L7), a range of gold-catalyzed transformations
tertiary amino group.
involving in situ generated allene intermediates were
realized,5e-5i including asymmetric versions (Scheme 1B).5h Very
recently, we have even realized in situ generation of highly
electron-rich furans directly from alkynyl amides by using
L3Au+ as the catalyst (Scheme 1C).5j Despite these progresses,
the development of new and versatile gold-catalyzed
transformations enabled by the aforementioned bifunctional
phosphine ligands is still highly appealing and would render
remarkable synthetic values.
Furan is an important class of heteroarenes broadly found in
natural and biologically important molecule.6 In addition,
furans are frequently used as building blocks in organic
synthesis,7 medicinal chemistry,8 and materials science.9
Over the past twenty years, the chemical community has
witnessed tremendous development in homogeneous gold
catalysis.1-3 The evolution of homogeneous gold catalysis
largely relies on the design and application of a broad range of
ligands to achieve highly selective reactions (chemoselective,
regioselective, and/or enantioselective), among which
phosphine-2 and NHC-carbene-ligands3 are most common. In
the past several years, our group has successfully explored a
series of biphenyl-2-ylphosphine ligands4 featuring a critical
remote basic moiety such as amide, aniline or tertiary amine
moiety (Scheme 1A).5 These ligands permit cooperative gold
catalysis (Scheme 1A) and have enabled exceptional and/or
unprecedented reactivities including 1) significantly
accelerating the addition of HNu across alkynes and
dramatically lowering the catalyst loading, as exemplified in
the nucleophilic addition of carboxylic acid or in situ generated
hydrazoic acid to alkynes by using WangPhos (L1) featuring a
remote amide moiety;5a,5b 2) achieving novel accelerative
asymmetric gold catalysis in the cyclization of 4-allen-1-ols by
using a chiral version of WangPhos (L2);5c 3) in situ generating
Generally, there are two strategies to synthesize multi-
10-12
substituted furans:
one involves functionalization of an
existing furan-containing precursor by introduction of new
substituents;10,11 the other strategy involves the construction
of furan scaffolds through cyclization of acyclic substrates.10,12
For the latter strategy, significant attention has particularly
been paid to the development of catalytic approaches, aiming
at cycloisomerization of unsaturated acyclic precursors into
furans under rather mild and/or neutral conditions.12a-h It was
reported that silver,12a-c rhodium,12a palladium,12d,e and
gold(III)12f,g can assist cycloisomerization of allenyl ketones into
furans. However, these methods all suffer from the tedious
preparation of the starting allenyl substrates, and gold(III)-
catalyzed cycloisomerization of allenyl ketones to furans may
also accompany with the formation of undesired dimerization
products12f or lead to problems with acid-sensitive substrates
by using trifluoroacetic acid.12g Several more convenient
procedures involving direct conversion of alkynyl ketones to
substituted furans have been reported.12h-k However, these
procedures suffered from harsh reaction conditions (100 °C)
and the need to use the excessive amount of bases,12h low
yields,12i-k or low selectivity and difficult separation due to
-allenylgold species from TBS-terminated 3-phenylprop-1-
ynes for nucleophilic propargylation of aldehydes by using
L3Au+.5d Particularly, such bifunctional ligands also enabled
gold-catalyzed in situ isomerization of propargylic moiety into
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production of several furan derivatives.12j,k We herein report a
bifunctional phosphine ligand-enabled gold-catalyzed
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DOI: 10.1039/D0CC01238F
Scheme 1 Development of biphenyl-2-ylphosphines featuring a remote basic group for gold catalysis: (A) Design and selected
examples, (B) gold-catalyzed isomerization of propargylic moiety into allene subunit and further transformations, (C) gold-
catalyzed in situ generation of highly electron-rich furans directly from alkynyl amides and further trapping in intermolecular or
intramolecular D-A reactions, and (D) gold-catalyzed synthesis of 2,5-disubstituted furans directly from alknyl ketones without
additional base (this work).
Table 1 Optimization of the reaction conditionsa
Entry
Catalyst (x mol %)
Halide abstractor (y mol %)
Solvent
Temp (ºC)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Ph3PAuCl (5)
L1AuCl (5)
L3AuCl (5)
L4AuCl (5)
L5AuCl (5)
NaBARF (10)
NaBARF (10)
NaBARF (10)
NaBARF (10)
NaBARF (10)
NaBARF (10)
NaBARF (10)
NaBARF (2)
AgNTf2 (10)
AgOTf (10)
NaBARF (10)
NaBARF (10)
NaBARF (10)
NaBARF (10)
NaBARF (10)
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
CH2Cl2
toluene
DCE
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
12
12
12
12
12
12
2
0
0
96
86
88
0
96
89
87
(Ad-JohnPhos)AuCl (5)
L3AuCl (2)
L3AuCl (2)
L3AuCl (2)
L3AuCl (2)
L3AuCl (2)
L3AuCl (2)
L3AuCl (2)
2
12
12
2
2
2
83
92,c 94d
90
89
Ph3PAuCl (2)
Ad-JohnPhos)AuCl (2)
2
2
0,e 0f
tracee
DCE
a Reaction conditions: 1a (0.2 mmol), gold catalyst (x mol % based on 1a), halide abstractor (y mol % based on 1a), solvent (2 mL,
the initial substrate concentration is 0.1 M), at T C under N2 atmosphere unless otherwise noted. b Yield was determined by
o
HPLC using biphenyl as the internal standard. c,d The initial substrate concentration is 0.05 M and 0.2 M, respectively. e In the
presence of 10 mol % of Et3N. f In the presence of 10 mol % of DIPEA (N, N-Diisopropylethylamine).
desired product 2a in DCE at 60
synthesis of 2,5-disubstituted furans directly from alkynyl The use of
ketones. This reaction avoids the use of additional bases and
tolerates a range of functional group under the nearly neutral WangPhos (L1) coordinated-gold(I) catalyst still did not afford
conditions (Scheme 1D). 2a (entry 2, Table 1), presumably due to the weak basicity of
°C for 12 h (entry 1, Table 1).
Initially, 1-phenylhept-2-yn-1-one 1a was selected as the the remote amide moiety. Gratifyingly, when switching to
model substrate to investigate the optimal reaction conditions ligand L3 featuring a more basic tertiary amine moiety, the
(Table 1).
A commonly used Ph3PAuCl (5 mol %) in reaction indeed occurred and 2a was obtained in 96% yield
combination with NaBARF (10 mol %) failed to deliver the (entry 3, Table 1). L4 and L5 possessing similar tertiary amine
2 | J. Name., 2012, 00, 1-3
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d
moieties gave inferior results to that of L3 (entries 4, 5 vs 3, prolonged to 4 h. The reaction temperature wasViienwcrAertiaclseeOdnlitnoe
DOI: 10.1039/D0CC01238F
Table 1), suggesting that the location of the basic moiety at the 80 °C and the reaction time was prolonged to 12 h.
biphenyl scaffold of the bifunctional ligands is also an under the standard reaction conditions, and the corresponding
important factor for the reaction. In sharp contrast, with the furan derivatives were obtained in excellent yields (2o and 2p
,
related biphenyl-2-ylphosphine ligand Ad-JohnPhos lacking the Table 2). In addition, when R1 is a benzyl or styryl group, the
tertiary amino moiety in L3, the reaction led to none of the reaction also proceeded smoothly, leading to the target furan
desired product 2a, confirming that the remote tertiary amine in 94% and 97% yield, respectively (2q, 2r, Table 2). Next, the
group is crucial for the effectiveness of the gold(I) catalyst scope of R2 was surveyed. It was found that R2 can be alkyl
(entry 6 vs 3, Table 1). The catalyst loading could be lowered to groups substituted by various functional groups including
2 mol % without affecting the reaction outcome and moreover, phenyl, chloro, acid-sentitive THP ether, and acid/base-
the reaction completed in 2 h (entry 7). The less use of sensitive TMS ether (2s
NaBARF (2 mol %) resulted in a reduced yield of 2a (89%, entry groups also worked well and afforded excellent yields of
8, Table 1). Other reactions including halide abstractor, solvent, products 2x 2z
Table 2). Interestingly, R2 can be
and substrate concentration (entries 9-13, Table 1) were also heterofunctional group such as phenoxy, N-methyl-N-
varied but led to slightly inferior results. A non-bifunctionalized tosylamino, and even chloro (2za 2zc, Table 2). Previously,
-
2w, Table 2). Substrates with R2 as aryl
(
-
,
a
-
gold complex in combination with a base system (e.g. gold(III)-catalyzed construction of halo-substituted furans
Ph3PAuCl/NaBARF/Et3N, Ph3PAuCl/NaBARF/DIEPA or (Ad- required haloallenones as substrates which are difficult to
JohnPhos)AuCl/NaBARF/Et3N) failed to trigger the reaction, access.12l
suggesting that the bifunctionalized gold complex L3Au+
To further demonstrate the synthetic potential of the
played key role in the reaction (entries 14, 15 vs 7, Table 1).
present method, a 2 mmol-scale synthesis of 2a was also tried,
With the optimized reaction conditions (Table 1, entry 7) in and the target furan 2a was obtained in 88% yield (eq. 1).
hand, we set out to investigate the scope of substrates and
functional groups (Table 2). First, alkynyl aryl ketones were
investigated (R1 = aryl). Generally, both electron-donating and
electron-withdrawing substituents on the benzene ring of
alkynyl aryl ketones
conditions and disubstituted furans
excellent yields (2a 2p, Table 2). The position of substituents
1
were compatible with the reaction
2
were obtained in good to
-
on the benzene ring (ortho-, meta-, and para-) has no
significant impact on the reaction time and yields.
Heteroarenes such as thiophene and furan are also tolerated
On the basis of previous literature,5e-5j,12f-k a mechanism for
this gold(I)-catalyzed cycloisomerization is proposed in Scheme
2. Firstly, the bifunctional phosphine ligand enables gold(I)-
catalyzed in situ isomerization of alkynyl ketone 1a to the
Table 2 Substrate Scope of Alkynyl Ketones 1.a,b
allenyl ketone intermediate
group deprotonating an Au(I)-activated alkyne complex
followed by protodeauration of the resulting intermediate B 5e-
C via the remote tertiary amine
A
.
5j
Then, activation of the C=C=C moiety in
by the nucleophilic attack by the carbonyl oxygen at the
terminal allene carbon leads to intermediate D 12f,g
Finally,
C
by L3Au+ followed
.
D
undergoes deprotonation followed by protodeauration to
deliver 2a and regeneration of the catalyst L3Au+ (path a).12f,g
Alternatively, activation of the allenyl moiety by delivery of the
aReaction conditions:
1 (0.2 mmol), L3AuCl (2 mol %), NaBARF
o
(10 mol %), dry DCE 2 mL, 60 C for 2 h under N2 atmosphere
unless otherwise noted. bIsolated yields. cThe reaction
temperature was increased to 80 °C and the reaction time was
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Scheme 2 Proposed mechanism for the L3Au+-catalyzed
6
a) Comprehensive Heterocyclic Chemistry III, ed. A.
View Article OnliRne.
Katritzky, Elsevier, 2008; b) Five-membered Heterocycles:
DOI: 10.1039/D0CC01238F
transformation of 1a to 2a
proton from the N atom in intermediate
carbon of the allenyl moiety followed by intramolecular
cyclization delivers intermediate undergoes sequential
.
Furan, ed. J. Alvarez-Builla, J. J. Vaquero and J. Barluenga,
Wiley-VCH, 2011; c) Heterocycles in Life and Society: An
Introduction to Heterocyclic Chemistry, Biochemistry and
Applications, ed. A. F. Pozharskii, A. R. Katritzky and A. T.
Soldatenkov, Wiley, 2nd edn, 2011.
B
to the central
F. F
deprotonation and protodeauration also gives 2a (path b).
7
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In summary,
a bifunctional phosphine ligand-enabled gold(I)-
catalyzed direct cycloisomerization of alkynyl ketones to access 2,5-
disubstituted furans has been successfully achieved. The reaction
has characteristic advantages of broad functional group tolerance,
high yields, and no need for additional bases. The rational design of
a biphenyl-2-ylphosphine ligand (L3) featuring a critical remote
tertiary amine moiety is a key factor for the effectiveness of the
gold(I) catalyst.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgments
We are grateful to the Natural Science Foundation of China
(Grant No. 21772176 and 21372201) and Zhejiang Province
(Grant No. LY20B020013) for financial support. Y. L. thanks the
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Graphical Abstract for TOC:
Bifunctional Phosphine Ligand-Enabled Gold-Catalyzed Direct
Cycloisomerization of Alkynyl Ketones to 2,5-Disubstituted
Furans
Xiaojun Hu,a Bingwei Zhou,a Hongwei Jin,a Yunkui Liu,*a,b and
Liming Zhang*b
A gold(I)-catalyzed direct cycloisomerization of alkynyl ketones to
2,5-disubstituted furans enabled by a bifunctional phosphine
ligand was successfully achieved.
29 examples,
up to 98% yield.
L3
AuCl (2 mol %)
NaBARF (10 mol %)
O
R2
O
R1
R1
DCE, 60-80 o
C
R2
PAd2
N
+
L3
Au
+
L3
Au
L3
O
R1
R2
R1 = aryl, heteroaryl, alkyl, styryl etc.
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