Palladium-catalyzed oxidative 6-exo-trig cyclization of 1,6-enynes: Facile synthesis of bicyclo[4.1.0]heptan-5-ones

Article abstract of DOI:10.1039/c5cc04300j

We here describe a new palladium-catalyzed oxidative 6-exo-trig cyclization of 1,6-enynes at room temperature using tBuONO as an oxidant for the synthesis of 3-bicyclo[4.1.0]heptan-5-ones. This cascade strategy involves the hydration, cyclization and cycl

Full text of DOI:10.1039/c5cc04300j

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DOI: 10.1039/C5CC04300J
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Palladium‐Catalyzed Oxidative 6‐exo‐trig Cyclization of 1,6‐Enynes:
Facile Synthesis of Bicyclo[4.1.0]heptan‐5‐ones
Bang Liu, a Ren‐Jie Song,*a Xuan‐Hui Ouyang, a Yang Li, a Ming Hua and Jin‐Heng Li*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We here describe a new palladium‐catalyzed oxidative 6‐exo‐trig SN2 C‐O formation.3d‐f The group of Liu3g have described
cyclization of 1,6‐enynes at room temperature using tBuONO as another new palladium‐catalyzed oxidative 5‐exo‐trig
oxidant for the synthesis of 3‐bicyclo[4.1.0]heptan‐5‐ones. This cyclization/dichlorination of 1,6‐enynes with LiCl in the
cascade strategy achieves hydration, cyclization and presence of H2O2 oxidant leading to dichloro‐contained five‐
cyclopropanation sequence, and represents a new transition‐ membered heterocyclic compounds. Recently, a mild and
metal‐catalyzed oxidative cyclization of 1,6‐enynes through the 6‐ efficient Pd‐catalyzed oxidative diarylating carbocyclization of
exo‐trig fashion.
enynes using arylboronic acids as additional reactants and
benzoquinone as oxidant was illustrated by the group of
Bäckvall3h to stereoselectively assemble tetrahydrofurans and
tetrahydropyranes. There is a lack of a new strategy for the
palladium‐catalyzed oxidative cyclization of 1,n‐enynes. Herein,
we report a novel palladium‐catalyzed oxidative 6‐exo‐trig
Transition‐metal‐catalyzed cyclization of 1,n‐enynes has
become a powerful and atom‐economical method for building
diverse carbo‐ and hetero‐cyclic frameworks.1‐6 Generally, the
majority of these processes utilize three strategies, including
cycloisomerization, skeletal rearrangement and metathesis.1
However, these strategies deliver the carbo‐ and hetero‐cyclic
products with limited complex ring systems because within
them the intermolecular incorporation of new functional
groups into the ring systems from additional functional
reagents is difficult. An alternative strategy includes transition‐
metal‐catalyzed oxidative cyclization of 1,n‐enynes,2‐5 which
offers several advantages over the three classical strategies
because various new functional groups are easily introduced
into the ring systems and the functionalization of both
positions of the π bond in the alkene and alkyne moieties
occur. However, successful palladium‐catalyzed oxidative
cyclization of 1,n‐enynes are quite rare and are limited to the
5‐exo‐trig fashion (Scheme 1a) 2,3 despite the enormous recent
advances enjoyed by Au catalysts.4 The group of Tse3a and the
Group of Sanford3b,3c have independently report a new
palladium‐catalyzed oxidative cyclization of 1,6‐enynes by
using PhI(OAc)2 oxidant in HOAc medium for the synthesis of
bicyclo[3.1.0]hexanes through the 5‐exo‐trig cyclization and
cyclization of 1,6‐enynes (1
) with the aid of tBuONO7,8 at room
temperature for the synthesis of bicyclo[4.1.0]heptan‐5‐one
derivatives (Scheme 1b), which are important core structures
in many natural products and pharmaceuticals.9
Scheme 1 Pd‐Catalyzed Oxidative Cyclization of 1,6‐Enynes.
Our studies began with the cyclization of 4‐methyl‐N‐(2‐
methylallyl)‐N‐(3‐phenylprop‐2‐yn‐1‐yl)benzenesulfonamide
a.State Key Laboratory of Chemo/Biosensing and Chemometrics, College of
Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
E‐mail: srj0731@hnu.edu.cn and E‐mail: jhli@hnu.edu.cn
Prof. Dr. J.‐H. Li
State Key Laboratory of Applied Organic Chemistry Lanzhou University, Lanzhou
730000, China
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
(
1a) for reaction condition optimization (Table 1). After a series
of trials, we found that treatment of 1,6‐enyne 1a with 5 mol%
Pd(OAc)2 and 1.5 equiv tBuONO afforded the desired 3‐
azabicyclo[4.1.0]heptan‐5‐one 2a10 in 75% yield (entry 1).
Screening of the amount of tBuONO revealed that the identical
yield to that of 1.5 equiv tBuONO was achieved at a loading of
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2 equiv tBuONO (entry 2), but the yield decreased from 75% to reaction temperatures (entry 16) and solvents (entries 17‐19)
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DOI: 10.1039/C5CC04300J
58% by decreasing tBuONO to 1 equiv (entry 3). However, in examined, it turned out that the use of CH2ClCH2Cl medium at
the absence of tBuONO, the reaction did not deliver the room temperature was optimal for the reaction (entry 1). The
detectable amounts of 2a (entry 4). In light of these results, a results indicated that O2 favoured the reaction (entries 1, 20
number of oxidants, namely nAmONO, AgNO2, Fe(NO3)3 and and 21).The reaction under air or O2 offered 2a in good yield
PhI(OAc)2, were subsequently examined, and they were less (entries 1 and 20), but under argon the reactivity of 1,6‐enyne
effective than tBuONO (entry 1 versus entries 5‐8). Two nitriles, 1a was suppressed in a lower yield (entry 21).
nAmONO, and Fe(NO3)3, effected the reaction but decreased
After establishing the optimal reaction conditions, we
the yields of 2a to 68% and 30%, respectively (entries 5 and 7). applied the palladium‐catalyzed oxidative 6‐exo‐trig cyclization
However, both AgNO2 (entry 6) and PhI(OAc)2 (entry 8) were protocol to a variety of 1,6‐enynes 1b‐z. As shown in Table 2,
inert for the cyclization reaction. Interestingly, the amount of the substitution effect on the nitrogen atom had a crucial
water had a fundamental influence on the reaction, and 4 effect on the reactivity of substrates (2b‐d). While substrates
equiv H2O was found to be the preferred choice (entry 1 1b and 1c with a Et group or an Ac group have no reactivity for
versus entries 9 and 10). The results demonstrated that the Pd the reaction (2b and 2c), substrates 1d with a PhSO2 group was
catalyst played an important role in the cyclization reaction, successfully converted into the corresponding 3‐
and its amount affected the reaction in terms of yields (entries azabicyclo[4.1.0]heptan‐5‐one 2d in 70% yield. In light of the
11‐13). It should be noted that 2a cannot be detected without results, a wide range of N‐Ts‐substituted 1,6‐enyens 1e‐z were
Pd catalysts (entry 11), and 5 mol% Pd(OAc)2 gave the best employed for the cyclization reaction under the optimal
results (entry 1 versus entries 12 and 13). Pd(TFA)2 had the conditions (2e‐z). To our delight, both electron‐donating (2e
same catalytic activity as Pd(OAc)2 (entry 14), but PdCl2 2f and 2i) and electron‐withdrawing (2g 2h 2j and 2k)
,
,
,
showed less activity (entry 15). Among the effect of the
aromatic substituents were well‐tolerated at the terminal
alkyne. For example, 1,6‐enynes 1e and 1i with a p‐MeC6H4
group or a o‐MeC6H4 group were successfully converted into
2a and 2i in high yields. Moreover, halide substituents (Cl and
Br) were compatible with the optimal conditions (2g and 2j),
thereby providing chances for further manipulation at the
halogenated positions. 1,6‐Enynes 1h and 1k having a stronger
electron‐withdrawing aromatic substituent (p‐CNC6H4 or p‐
CO2MeC6H4) were also viable substrates for assembling 3‐
azabicyclo[4.1.0]heptan‐5‐ones 2h and 2k in moderate yield,
albeit with lower reactivity. Using heteroaryl alkyne 1l
successfully reacted with H2O, Pd(OAc)2 and tBuONO, giving 2l
in 73% yield. It should be noted that terminal alkyne 1m and
aliphatic alkynes 1n‐o were consistent with the optimal
conditions, and allowed the formation of 2m‐2o in moderate
to good yields. However, electron‐deficient alkyne 1p or
electron‐deficient alkene 1r were not suitable for the oxidative
Table 1. Screening of the Reaction Conditions.a
[M] [mol%] Additive [equiv]
Entry
1
2
Solvent
CH2ClCH2Cl
CH2ClCH2Cl
Yield [%]b
Pd(OAc)2 (5)
Pd(OAc)2 (5)
tBuONO (1.5)
tBuONO (2)
75
76
58
0
Pd(OAc)2 (5)
Pd(OAc)2 (5)
tBuONO (1)
CH2ClCH2Cl
CH2ClCH2Cl
3
4

Pd(OAc)2 (5) nAmONO (1.5) CH2ClCH2Cl
Pd(OAc)2 (5) AgNO2 (1.5) CH2ClCH2Cl
Pd(OAc)2 (5) Fe(NO3)3 (1.5) CH2ClCH2Cl
Pd(OAc)2 (5) PhI(OAc)2 (1.5) CH2ClCH2Cl
68
trace
30
trace
67
71
0
5
6
7
8
cyclization reaction (1p and 1r). Gratifyingly, 1,6‐enyne 1q,
9c
bearing a Ph group at the 2 position of the allyl moiety, was
found to smoothly deliver 2q in 63% yield. An internal alkene
1s was also viable substrate, stereoselectively giving 2s
according to 2D NMR analysis (see Supporting Information).
The palladium‐catalyzed oxidative 6‐exo‐trig cyclization
protocol could be applicable to the synthesis of 3‐
Pd(OAc)2 (5)
Pd(OAc)2 (5)

tBuONO (1.5)
tBuONO (1.5)
tBuONO (1.5)
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2Cl2
MeCN
HOAc
CH2ClCH2Cl
CH2ClCH2Cl
10d
11
12
13
14
15
16e
17
18
19
20f
21g
Pd(OAc)2 (10) tBuONO (1.5)
76
64
73
34
76
64
62
30
74
59
Pd(OAc)2 (2)
Pd(TFA)2 (5)
PdCl2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
tBuONO (1.5)
tBuONO (1.5)
tBuONO (1.5)
tBuONO (1.5)
tBuONO (1.5)
tBuONO (1.5)
tBuONO (1.5)
tBuONO (1.5)
tBuONO (1.5)
oxabicyclo[4.1.0]heptan‐5‐ones
(
2t and 2u
)
and 1H‐
cyclopropa[b]naphthalen‐2(1aH)‐ones (2v‐aa). 1,6‐Enynes 1t
and 1u, having a Me group or a Ph group at the 2 position of
the allyl moiety, worked well with H2O, Pd(OAc)2 and tBuONO,
offering 2t and 2u in moderate yields. For 1‐allyl‐2‐
ethynylbenzenes
1v‐ab
,
the
corresponding
1H‐
cyclopropa[b]naphthalen‐2(1aH)‐ones 2v‐ab were successfully
constructed in moderate to high yields, although 10 mol%
AgSbF6 was required to improve the cyclizaition process. For
example, treatment of 1,6‐enyne 1v with H2O, Pd(OAc)2 and
tBuONO for 24 h afforded 2v in 45% yield, whereas with
additional AgSbF6 increased the yield to 77% yield.
a
Reaction conditions: 1a (0.2 mmol), H2O (4 equiv), [Pd] (5% mol),
additive, and anhydrous solvent (1 mL) at room temperature under
air atmosphere for 16 h. b Yield of isolated product. c H2O (2 equiv).
d
e
o
f
The coversion of 1a is 89%. H2O (8 equiv). At 40 C. Under O2
atmosphere. g Under argon atmosphere.
2 | J. Name., 2012, 00, 1‐3
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Table 2. Substrate Scope.a
The mechanism outlined in Scheme 3 for the oxidative
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cyclization reaction is proposed on thDeObI:a1s0i.s103o9f/Ct5hCeC0a4b3o0v0eJ
results and previous reports.2,3,7,8 Initially, complex of the
active PdII species with the C‐C double bond and the C‐C triple
bond of 1,6‐enyne 1a affords intermediate
A, followed by
hydration of the C‐C triple bond with the active PdII species
and H2O gives intermediate
B
. Cyclization with the C‐C double
easily occurs to produce
bond within intermediate
B
NC
MeO
Cl
Ph
O
intermediate
oxidative cyclization of intermediate
C
. A PdIV intermediate D8l‐n is formed from
C
by NO2 and air, 8 in‐situ
N
R1
generated from tBuONO and H2O.7 Finally, reductive
O
O
O
O
R1 = Et, 2b, trace
elimination of the PdIV intermediate
regenerates the active PdII species.
D delivers product 2a and
N
R1 = Ac, 2c, trace
N
Ts
N
N
Ts
Ts
Ts
R
1 = PhSO2, 2d, 70%
2e, 67%
2h, 41%b
2f, 73%
2g, 77%
MeO2C
Br
S
H
O
O
O
O
O
N
N
N
Ts
N
N
Ts
Ts
Ts
Ts
2k, 62%b
Ph
2m, 85%
2l, 73%
2j, 74%
2i, 70%
H
Ph
O
O
O
O
Ph
O
O
N
N
N
O
N
Ts
N
Ts
Ts
Ts
Ts
2q, 63%
2p, trace
2r, trace
R4
2n, 66%
Ph
2o, 45%
Ph
O
R4 = H, 2v, 45%b
R4 = H, 2v, 77%b,c
O
Scheme 3. Possible Mechanism.
O
H
R2
R4 = Me, 2w, 80%b,c
R4 = Cl, 2x, 77%b,c
N
O
In summary, we have report a new strategy for the oxidative
6‐exo‐trig cyclization by using Pd(OAc)2 as the catalyst and
tBuONO as the oxidant. This 1,6‐enyne cyclization method
proceeds through hydration, 6‐exo‐trig cyclization and
cyclopropanation cascade, and provides a mild and selective
access to diverse bicyclo[4.1.0]heptan‐5‐one skeletons,
Ts
R = Me, 2t, 65%
R = Ph, 2u, 50%
CO2Me
O
2s, 45%
S
O
O
O
Ph
Ph
Cl
Cl
H
CO2Me
CO2Me
2z, 59b,c
CO2Me
2y, 76%b,c
2ab, 54%b,c
2aa, 59%b,c
a
including
oxabicyclo[4.1.0]heptan‐5‐ones
3‐azabicyclo[4.1.0]heptan‐5‐ones,
and
3‐
1H‐
Reaction conditions: 1 (0.2 mmol), H2O (4 equiv), Pd(OAc)2 (5
mol%), tBuONO (1.5 equiv), and anhydrous CH2ClCH2Cl (1 mL). b
For 24 h. c AgSbF6 (10 mol%) was added.
cyclopropa[b]naphthalen‐2(1aH)‐ones, in moderate to good
yields with excellent functional group tolerance and
stereoselectivity. Further applications of this catalytic oxidative
cyclization method in synthesis and detailed mechanistic
studies are currently underway in our laboratory.
Some control experiments were performed to understand
the mechanism (Scheme 2). We found that the radical
inhibitors (TEMPO and hydroquinone) had no effect on the
reaction of 1,6‐enyne 1a, ruling out a radical process [Eq (1)].
Subsequently, an 18O‐Labeled experiment by using H218O was
investigated [Eq (2)]. The result showed that 18O‐containing
product 2a‐18O was isolated in 71% yield, suggesting that H2O
was the major resource of the oxygen atom in the new formed
carbonyl group.
Notes and references
‡ We thank the Natural Science Foundation of China (Nos.
21172060, 21472039 and 21402046), Specialized Research Fund
for the Doctoral Program of Higher Education (No.
20120161110041) and Hunan Provincial Natural Science
Foundation of China (No. 13JJ2018) for financial support. Dr. R.‐J.
S. also thanks the Fundamental Research Funds for the Central
Universities.
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Diao, A.‐H. Liu, X. Han, B. Li, L.‐N. He and X.‐M. Liu, Curr. Org.
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P. Kanoo, H. J. Jeon, M. L. Foo, A. Wakamiya, Y. Murata and S.
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papers on the use of nitrites or NOx within the Pd‐catalyzed
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1981, 103, 2894; (e) M. A. Andrews and C.W. F. Cheng, J. Am.
Chem. Soc. 1982, 104, 4268; (f) F. Mares, S. E. Diamond and F.
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Chem. Front. 2014, 1, 34; for selected special reviews on ‐
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Hashmi, in Modern Gold Catalyzed Synthesis (Eds.: A. S. K.
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5
For papers on the other oxidative cyclization of 1,n‐enynes
under the Cu‐catalyzed conditions: (a) Z.‐Q. Wang, W.‐W.
Zhang, L.‐B. Gong, R.‐Y. Tang, X.‐H. Yang, Y. Liu and J.‐H. Li,
Angew. Chem. Int. Ed. 2011, 50, 8968; (b) J. Zhang, D. Wu, X.
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Morandi and R. H. Grubbs, Angew. Chem. Int. Ed. 2013, 52
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X.‐R. Song, X.‐Y. Liu and Y.‐M. Liang, Chem. Eur. J. 2013, 19
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E. Guzmán and R. H. Grubbs, Angew. Chem. Int. Ed. 2014, 53,
14420; (e) Y.‐T. He, L.‐H. Li, Z.‐Z. Zhou, H.‐L. Hua, Y.‐F. Qiu, X.‐
Y. Liu and Y.‐M. Liang, Org. Lett. 2014, 16, 3896; under
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For representative papers on the cyclopropanation via the 6‐
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(a) R. Grigg, R. Rasul, J. Redpath and D. Wilson, Tetrahedron
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(
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Oberhuber, M. P. Muñoz, E. Buñuel, C. Nevado, D. J.
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crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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