Gold-catalyzed cascade oxidative cyclization and arylation of allenoates

Article abstract of DOI:10.1002/ejoc.201300896

An Au^I/Au^III catalytic system was found to be effective for the cascade oxidative arylation and cyclization of allenoates with arylboronic acids to give the corresponding cyclic adducts in moderate yields. This reaction system constitutes a new method for the synthesis of β-aryl-γ-butenolides under mild conditions. Based on the previous mechanistic studies, a proposed Au^I/Au^III redox catalytic cycle has been outlined. An Au^I/Au^III catalytic system was found to be effective for the cascade oxidative arylation and cyclization of allenoates with various arylboronic acids to give the corresponding cyclic adducts in moderate yields. This reaction system enabled the synthesis β-aryl-γ-butenolides under mild conditions. Based on previous mechanistic studies, an Au^I/Au^III redox catalytic cycle has been outlined. Copyright

Full text of DOI:10.1002/ejoc.201300896

FULL PAPER
DOI: 10.1002/ejoc.201300896
Gold-Catalyzed Cascade Oxidative Cyclization and Arylation of Allenoates
Rui Zhang,[a,b] Qin Xu,*[b] Kai Chen,[b] Peng Gu,[b] and Min Shi*[a,b]
Keywords: Synthetic methods / Homogeneous catalysis / Oxidation / Arylation / Cyclization / Gold / Allenes
An AuI/AuIII catalytic system was found to be effective for
the cascade oxidative arylation and cyclization of allenoates
new method for the synthesis of β-aryl-γ-butenolides under
mild conditions. Based on the previous mechanistic studies,
with arylboronic acids to give the corresponding cyclic ad- a proposed AuI/AuIII redox catalytic cycle has been outlined.
ducts in moderate yields. This reaction system constitutes a
workers first prepared the corresponding vinyl AuI com-
Introduction
plexes to examine their reactivity in a stoichiometric man-
Gold was long deemed to be an inert metal until the first ner; they found that the corresponding vinyl AuIII complex
gold-catalyzed reaction was reported in 1972.[1] Since then, could be formed upon treatment of vinyl AuI complex with
homogeneous gold-catalyzed reactions have attracted sig- N-fluoro-N-(phenylsulfonyl)benzenesulfonamide
nificant attention.[2–4] Gold catalysts are typically been used (NFSI).[7a] In the mean time, Hammond and Xu first
as soft π-acids for activation of C–C multiple bonds to initi- proved by XPS measurements the Selectfluor-mediated oxi-
ate various novel transformations in organic synthesis.[5,6] dation of AuI to AuIII in the functionalization of alkynes
However, lately, Hashmi, Zhang, Toste, Gouverneur and upon hydration.[9a] Moreover, Gouverneur and co-workers
other groups have reported several interesting gold-cata- also clearly showed that Selectfluor could oxidize AuI to
lyzed homo- and cross-coupling reactions through a pro- AuIII, and the generated AuIII species could undergo intra-
posed AuI/AuIII redox catalytic cycle by using an external molecular Friedel–Crafts arylation smoothly.[9b] Sub-
F+ oxidant.[7–10] For example, in 2009, Hashmi and co- sequently, Zhang and Toste’s research groups independently
Figure 1. Examples of biologically active β-substituted γ-butenolides.
[a] State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences,
described the aminoarylation reaction of olefins with an
AuI/AuIII redox catalytic cycle. However, Toste and co-
workers proposed a bimolecular reductive elimination
mechanism proceeding via a five-membered cyclic transi-
tion state to explain the reaction outcome.[8a–8c] These find-
ings have remarkably extended the synthetic usefulness of
gold catalysis in organic synthesis.
345 Lingling Road, Shanghai 200032, China
[b] Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, School of Chemistry & Molecular Engineering, East
China University of Science and Technology,
130 Mei Long Road, Shanghai 200237, China
E-mail: Mshi@mail.sioc.ac.cn
www.sioc.ac.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300896.
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Oxidative Cyclization and Arylation of Allenoates
In 2008, Hammond and co-workers first synthesized and
isolated the stable organogold(I) compound and disclosed
evidence for the reaction mechanism in the gold-catalyzed
cyclization of allenoate.[11] Because β-substituted γ-butenol-
ides are very important building blocks in organic synthesis
and are widely found in a variety of natural products and
medicinally important compounds (Figure 1),[12] the devel-
opment of a simple synthetic method that allows access to
this important structural motif is highly desirable. Cur-
rently, these compounds are almost always obtained
through a two-step process: cyclization of allenoate and
subsequent β-substitution.[11,13] On the basis of the above
findings, we attempted to further explore a straightforward
new synthetic approach for the construction of the β-aryl-
γ-butenolide structure motif by applying an AuI/AuIII cata-
lytic process to allenoates. Herein, we wish to report a novel
cascade oxidative arylation and cyclization of allenoates
with arylboronic acids, affording the corresponding β-aryl-
γ-butenolides in moderate yields under mild conditions.
(10 mol-%) in CH3CN/H2O were aimed at establishing the
optimal conditions; the results of these experiments are
summarized in Table 1. We found that the desired β-aryl-γ-
butenolide 3a was isolated in 60% yield along with oxidat-
ive homo-coupling product 4a derived from allenoate 1a in
35% yield as a pair of diastereoisomers (1:1). An examina-
tion of solvent effects revealed that CH3CN was the best
solvent with respect to the yield of 3a (Table 1, entries 1–
5). Elevating the reaction temperature to 60 °C dramatically
decreased the yield of 3a (Table 1, entry 6). When the reac-
tion temperature was reduced to room temperature, the de-
sired product 3a was obtained in trace amounts (Table 1,
entry 22). The yield of 3a was increased to 65% when 1a in
CH3CN (1.0 mL) was slowly added into the reaction mix-
ture over two hours by using a syringe pump (Table 1, en-
try 7). Reducing the amount of phenylboronic acid em-
ployed also decreased the yield of 3a (Table 1, entries 8 and
9). An examination of various gold catalysts established
that [PPh3AuCl] was the best choice for this transformation
(Table 1, entries 1 and 10–16). Notably, AuCl3 could also
promote the reaction, but it was less efficient, delivering 3a
in 15% yield along with 4a in 70% yield (Table 1, entry 16).
It was found that Selectfluor was the best oxidant, affording
Results and Discussion
Initial studies using allenoate 1a as substrate with phen-
ylboronic acid (2a; 10 equiv.) in the presence of [PPh3AuCl]
Table 1. Optimization of the reaction conditions for oxidative cyclization and arylation.[a]
Entry
Cat.
x
Oxidant
Solvent
Yield (%)[b]
3a
4a
1
PPh3AuCl
PPh3AuCl
PPh3AuCl
PPh3AuCl
PPh3AuCl
PPh3AuCl
PPh3AuCl
PPh3AuCl
10.0
10.0
10.0
10.0
10.0
10.0
10.0
3.0
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
NFSI
PhI(OAc)2
tBuOOH
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
CH3CN/H2O
acetone/H2O
toluene/H2O
DCE/H2O
60
49
18
33
55
37
65
35
51
60
33
trace
59
52
26
15
46
35
40
53
57
20
45
26
42
28
16
37
60
15
21
16
70
44
2[c]
3[c]
4[c]
5[c]
THF/H2O
6[d]
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
THF/H2O
7[e]
8[e]
9[e]
PPh3AuCl
5.0
10[e]
11[e]
12[e]
13[e]
14[e]
15[e]
16
(p-CF3C6H4)3PAuCl
(p-MeC6H4)3PAuCl
tBu3PAuCl
PPh3AuNTf2
(dppm)Au2Br2
(1,1Ј-biphenyl-2-yl)tBu2AuCl 10.0
AuCl3
PPh3AuCl
PPh3AuCl
PPh3AuCl
PPh3AuCl
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
2.0
2.0
10.0
2.0
17[e]
18[e]
19[e]
20[f,c]
21[g,c]
22[h]
23
trace
trace
30
29
trace
41
trace
trace
10
12
trace
20
PPh3AuCl
PPh3AuCl
PPh3AuCl/AgNTf2
PPh3AuCl/Pd2(dba)3
THF/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
24
2.0
complex
[a] Reaction conditions: allenoate 1a (0.2 mmol), catalyst (10 mol-%), PhB(OH)2 2a (x equiv.), oxidant (2.5 equiv.), solvent (2.0 mL), H2O
(36 μL), stirred at 40 °C, 2 h. [b] Isolated yield. [c] The reaction mixture was stirred for 24 h. [d] The reaction mixture was stirred at 60 °C.
[e] Allenoate 1a (in 1.0 mL CH3CN) was added to the reaction mixture over 2 h by using a syringe pump. [f] THF (1.0 mL), H2O (36 μL)
used. [g] THF (0.5 mL), H2O (36 μL). [h] The reaction mixture was stirred at room temperature.
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R. Zhang, Q. Xu, K. Chen, P. Gu, M. Shi
FULL PAPER
the desired product 3a in 65% yield, compared with the (Table 1, entries 17–19). It was clear that 10 equiv. 2a is re-
other oxidants such as NFSI, PhI(OAc)2 or tBuOOH quired in this oxidative cyclization and arylation process
because when 2.0 equiv. 2a was used 3a was afforded in
lower yields (Table 1, entries 20–21). Furthermore, the yield
of 3a decreased to 41% when [PPh3AuCl]/[AgNTf2] was
used as the catalyst (Table 1, entry 23). In the presence of
[PPh3AuCl]/[Pd2(dba)3], complex product mixtures were
obtained under otherwise identical conditions (Table 1, en-
try 24). More detailed reaction conditions are summarized
in the Supporting Information as Table SI-1. The structure
of butenolide 3a was unambiguously determined by X-ray
diffraction analysis; its crystal structure is shown in Fig-
ure 2 and the CIF data are presented in the Supporting In-
formation.
Having established the optimal reaction conditions, the
substrate scope of this oxidative arylation and cyclization
reaction with various allenoates and diverse arylboronic ac-
ids was evaluated; the results are summarized in Table 2.
This oxidative transformation was compatible with a wide
range of arylboronic acids including para-, meta-, and or-
tho-substituted derivatives. Arylboronic acids bearing a
methyl group at the ortho-position gave the desired product
in lower yields, perhaps due to steric effects (Table 2, en-
tries 1–9 and 16–20). It was clear that, regardless of whether
electron-poor or -rich arylboronic acids were used in this
reaction, all reactions proceeded smoothly, affording the de-
Figure 2. ORTEP drawing of butenolide 3a.
Table 2. Cascade oxidative cyclization and arylation of allenoates.[a]
Entry
1
2
Ar
Yield (%)[b]
3
R1
R2
R3
4
1
2
3
4
5
6
7
8
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
H
H
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
Et
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1c
1d
1e
2-MeC6H4
3-MeC6H4
4-MeC6H4
3,5-Me2C6H3
4,4Ј-biphenyl
4-tBuC6H3
3-FC6H4
3,5-F2C6H3
3,5-Cl2C6H3
4-ClC6H4
4-FC6H4
2-FC6H4
4-CHOC6H4
4-OHC6H4
Ph
4-MeC6H4
3-FC6H4
3,5-Cl2C6H3
3-MeC6H4
2-MeC6H4
Ph
2b
2c
2d
2e
2f
2g
2h
2i
32 (3b)
46 (3c)
63 (3d)
55 (3e)
30 (3f)
60 (3g)
65 (3h)
30 (3i)
50 (3j)
29 (3k)
42 (3l)
42 (3m)
complex
complex
64 (3n)
65 (3o)
60 (3p)
38 (3q)
51 (3r)
30 (3s)
50 (3t)
complex
21 (3n)
26 (4a)
40 (4a)[c]
26 (4a)[c]
21 (4a)[c]
35 (4a)[c]
27 (4a)[c]
21 (4a)[c]
50 (4a)[c]
16 (4a)[c]
55 (4a)[c]
32 (4a)
50 (4a)[c]
–
–
–
–
–
–
–
–
9
2j
2k
2l
10
11
12
13
14
15
16
17
18
19
20
21
22
23[d]
2m
2n
2o
2a
2d
2h
2j
2c
2b
2a
2a
2a
24 (4b)
Ph
Ph
Bn
–
[a] Reaction conditions: to a mixture of arylboronic acid 2 (2.0 mmol), [PPh3AuCl] (10 mol-%), Selectfluor (0.5 mmol) and CH3CN/H2O
(1.0 mL/10.0 equiv.) was added allenoate 1 (0.2 mmol) in CH3CN (1.0 mL) at 40 °C over 2 h. [b] Isolated yield. [c] The dimer was mixed
with ArB(OH)2. [d] The reaction mixture was stirred at 60 °C for 48 h.
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Oxidative Cyclization and Arylation of Allenoates
sire products in moderate yields (Table 2, entries 1–12). Un- zation oxidative cross-coupling was also successful with
fortunately, when arylboronic acids having OH or CHO benzyl allenoate 1e. When the reaction was stirred at 60 °C
functional groups were employed in this reaction, complex for 48 h, the desired product 3n was afforded in 21% yield,
product mixtures were formed (Table 2, entries 13 and 14). presumably because release of a benzyl group is more diffi-
The generality of the reaction with respect to allenoates cult than a tert-butyl group under identical conditions.
was also examined by using 1b–e as substrates under the
To establish a plausible reaction mechanism, several con-
standard conditions.[14] Allenoate 1b also afforded the de- trol reactions were performed (Scheme 1). The related inter-
sired oxidative cyclization and arylation products in moder- mediate gold(I) complex 5 was synthesized in 68% yield
ate yields (30–65%) with various arylboronic acids, but according to a reported procedure[15] and its structure was
without formation of the corresponding homocoupling confirmed by X-ray diffraction analysis [see Figure 3 and
product derived from 1b (Table 2, entries 15–20). By using Scheme 1, Equation (1)]. Treatment of intermediate 5 af-
allenoate 1c as the substrate, in which R1 = H, R2 = Me, forded the desired oxidative cyclization and arylation cross-
R3 = tBu, the reaction also proceeded efficiently, giving 3t coupling product 3a in 70% yield, together with bibutenol-
in 50% yield, albeit along with 24% yield of the homocou- ide 4a (15% yield), which is consistent with above reaction
pling product bibutenolide 4b (Table 2, entry 21). Ethyl all- outcome, suggesting that gold(I) complex 5 was indeed
enoate 1d was not tolerated in this catalytic system, provid- formed in the above reaction and it is the real intermediate
ing complex product mixtures (Table 2, entry 22). The cycli- in this cross-coupling reaction [Scheme 1, Equation (2)].
Scheme 1. Control experiments.
Eur. J. Org. Chem. 2013, 7366–7371
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R. Zhang, Q. Xu, K. Chen, P. Gu, M. Shi
FULL PAPER
Treatment of 1a and 1c without any arylboronic acid under
the optimized conditions delivered the corresponding bibut-
enolide 4a and 4b in 65 and 62% yield, respectively
[Scheme 1, Equation (3) and Equation (4)]. No reaction oc-
curred in the absence of Selectfluor [Scheme 1, Equation
(5)] and the 31P NMR spectroscopy indicated that no
change took place when a mixture of [PPh3AuCl] and Se-
lectfluor was stirred at 40 °C for 20 min, which is consistent
with the observations reported by Hammond’s group.[9a]
Moreover, BnOH was detected by GC–MS analysis of the
crude reaction mixture when allenoate 1e was used as the
substrate under the standard conditions, indicating that the
gold-catalyzed cyclization of 1 combined with the hydroly-
sis of the ester moiety (see the Supporting Information).
Scheme 2. Proposed reaction mechanism.
Conclusions
We have found an interesting AuI/AuIII redox catalytic
system for the cascade oxidative arylation and cyclization of
allenoates in CH3CN/H2O with various arylboronic acids,
giving β-aryl-γ-butenolides derivatives in moderate yields
under mild conditions. This catalytic system is applicable to
both electron-poor and electron-rich arylboronic acids. We
believe that this is a significant step towards the develop-
ment of cascade reactions combining traditional gold catal-
ysis. Further studies to find more efficient catalytic systems
based on gold catalysis or other metal catalytic systems are
underway.
Figure 3. ORTEP drawing of gold(I) complex 5.
Experimental Section
General Procedure: [PPh3AuCl] (10 mol-%, 0.02 mmol) was dis-
solved in CH3CN/H2O (1.0 mL/36 μL) in a flame-dried Schlenk
tube equipped with a septum cap and stirring bar. Arylboronic acid
(2.0 mmol) and Selectfluor (0.5 mmol) were added under argon,
then a solution of allenoate (0.2 mmol) in CH3CN (1.0 mL) was
slowly added over 2 h by using a syringe pump, and the reaction
mixture was stirred at 40 °C for another 10 min. H2O (5 mL) was
added, the mixture was extracted with EtOAc and the combined
organic phases were filtered through a thin layer of Celite. The
filtrate was washed with brine (5 mL), dried with anhydrous
Na2SO4, concentrated under reduced pressure, and purified by
flash chromatography on silica gel (EtOAc/petroleum ether, 1:6) to
give the pure product.
On the basis of above investigation and on the results
of previous mechanistic studies conducted by Hashmi,[7a]
Toste,[8c] Hammond,[11] Zhang,[8a] and Gouverneur,[10b]
a
plausible catalytic cycle has been outlined in Scheme 2. The
reaction of gold(I) complex 10 with 1a gives intermediate
vinyl-gold complex 5 through a cyclization process. Sub-
sequently, the gold(III) complex 7 is formed through oxi-
dation of 5 with Selectfluor, which reacts with arylboronic
acid through a transmetalation process to give intermediate
8. This process is favored by the strong B–F bond and weak
Au–F bond.[16] Reductive elimination of intermediate 8
gives the desired product 3a and regenerates the gold cata-
lyst 10. Furthermore, the gold(III) complex 7 can react with
a second molecule of 1a to produce intermediate 9, which
undergoes reductive elimination to give the homocoupling
product bibutenolide 4a, also along with the regeneration
of the gold catalyst 10.
5-Methyl-4-phenylfuran-2(5H)-one (3a): Yield: 65% (23 mg); white
solid.[17] IR (CH Cl ): ν = 2960, 2925, 2864, 1751, 1621, 1450, 1376,
˜
2
2
1259, 1080, 1015, 934, 860, 796, 690 cm–1. 1H NMR (400 MHz,
CDCl3, TMS): δ = 7.51–7.47 (m, 5 H), 6.28 (d, J = 1.2 Hz, 1 H),
5.57 (qd, J = 1.2, 6.8 Hz, 1 H), 1.54 (d, J = 6.8 Hz, 3 H) ppm. 13C
NMR (100 MHz, CDCl3, TMS): δ = 172.6, 168.9, 131.3, 129.9,
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Oxidative Cyclization and Arylation of Allenoates
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[M+], 131.0 (100), 103.1 (27), 77.0 (6). HRMS (EI): Calcd. for
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cle): Spectroscopic data of compounds and detailed descriptions of
experimental procedures.
Acknowledgments
The authors thank the Shanghai Municipal Committee of Science
and Technology (11JC1402600), National Basic Research Program
of China (973)-2009CB825300, the Fundamental Research Funds
for the Central Universities and the National Natural Science
Foundation of China (NSFC) for financial support (grant numbers
21072206, 21121062, 20472096, 20872162, 20672127, 21121062,
and 20732008).
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Received: June 19, 2013
Published Online: September 24, 2013
Eur. J. Org. Chem. 2013, 7366–7371
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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