Synthesis of 3-alkyl spiro[4,5]trienones by copper-catalyzed oxidative ipso-annulation of activated alkynes with unactivated alkanes

Article abstract of DOI:10.1039/c5cc08952b

A Cu-catalyzed oxidative ipso-annulation of activated alkynes with unactivated alkanes for the synthesis of 3-alkyl spiro[4,5]trienones is described. This method allows the formation of two carbon-carbon bonds and one carbon-oxygen bond in a single reaction through a sequence of C-H oxidative coupling, ipso-carbocyclization and dearomatization.

Full text of DOI:10.1039/c5cc08952b

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ARTICLE TYPE
Synthesis of 3-Alkyl Spiro[4,5]trienones by Copper-Catalyzed
Oxidative ipso-Annulation of Activated Alkynes with Unactivated
Alkanes
XuanꢀHui Ouyang,a RenꢀJie Song,*a Bang Liu,a and JinꢀHeng Li*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A Cuꢀcatalyzed oxidative ipsoꢀannulation of activated alkynes with unactivated alkanes for the synthesis of 3ꢀalkyl spiro[4,5]trienones is
describled. This method allows the formation of two carbonꢀcarbon bonds and one carbonꢀoxygen bond in a single reaction through a
sequence of CꢀH oxidative coupling, ispoꢀcarbocyclization and dearomatization.
10
Spirocyclohexadienones represent an essential part of the
skeleton of numerous natural compounds and pharmaceuticals1,
(Eq 3, Scheme 1).9 Although only few approaches have been
developed, this strategy would offer a new door for the
as well as valuable intermediates in organic synthesis1.2 35 incorporation of diverse attack functional reagents that extends
Generally, the spirocyclohexadienone structures are constructed
beyond
the
existing
transformations
to
access
15 via the oxidative spiro cyclization of phenol derivatives3,
transitionꢀmetalꢀcatalyzed intramolecular nucleophilic ipsoꢀ
carbocyclization of 5ꢀ(4ꢀhydroxyaryl)ꢀ1ꢀalkenes or 5ꢀ(4ꢀ
hydroxyaryl)ꢀ1ꢀalkynes,4 and the electrophilic ipsoꢀcyclization of
5ꢀ(4ꢀmethoxyaryl)ꢀ1ꢀalkynes.5 However, these transformations
20 are restrict to a limited substrate scope, especially the attack
functional reagents. Recently, attractive approaches for the
synthesis of spirocyclohexadienones via the oxidative radical
coupling strategy were developed (Scheme 1a). We have reported
radical oxidative ipsoꢀcarbocyclization of alkynes with aldehydes
25 or ethers for the synthesis of 3ꢀsubstituted spiro[4,5]trienones
(Eqs 1 and 2, Scheme 1).8 Very recently, Liu, Liang and coꢀ
workers reported copperꢀcatalyzed difunctionalization of
activated alkynes leading to 3ꢀtrifluoromethly spiro[4,5]trienones
via radical oxidation/tandem cyclization/dearomatization
spirocyclohexadienones.
Table 1 Screening of Optimal Conditionsa
Ph
Ph
catalyst
O
+
radical initiator
N
N
O
O
2a
3aa
1a
40
Entry
1
2
[Cu]
Cu(OAc)2 (10)
CuCl2 (10)
[O] (equiv)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
—
TBHP (3)
DTBP (3)
TBPB (3)
DCP (3)
K2S2O8 (3)
TBHP (2)
TBHP (5)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
T [oC]
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
130
80
Yield (%)b
57 (82)
30 (80)
22 (76)
28 (59)
20 (62)
trace
3
CuOAc (10)
4
Cu2O (10)
5
CuI (10)
a) Previous work:
H
6
Cu(acac)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (5)
Cu(OAc)2 (15)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
R2
7
0
O
8c
9
8 (81)
11 (73)
28 (68)
28 (70)
0
41 (77)
53 (81)
45 (69)
53 (79)
55 (81)
23 (87)
O
CuCl, TBHP
nBuOAc, 120 o
(1)
O
C
Y
O
R1
10
11
12
13
14
15
16
17
18
R2
O
R2
R4CHO
R4
(2)
(3)
TBHP
nBuOAc, 120 o
O
O
C
Y
O
Y
O
R1
R1
NaSO2CF3
CuCl, TBHP
NaOAc, MnO2
Y = NR, O
R2
CF3
CH3CN/H2O, 60 o
C
Y
O
R1
b) This work:
R2
a Reaction conditions: 1a (0.2 mmol), [Cu], oxidant and cyclohexane 2a
(1 mL) for 36 h under argon atmosphere. The number in parentheses is
the yield based on recovered starting materials. DTBP = diꢀtertꢀbutyl
R2
H
Cu(OAc)2, TBHP
(4)
O
+
110 o
C
N
O
N
R
R1
O
peroxide, TBHP = tertꢀbutyl hydroperoxide (5M in decane), TBPB = tertꢀ
R1
R
c
45 butyl peroxybenzoate, DCP = dicumylperoxide.b Isolated yield. TBHP
30
Scheme 1 Oxidative Radical Difunctionalization of Alkynes Leading to
(70% in water) was added.
Spiro[4,5]trienones.
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Direct C(sp3)ꢀH functionalization for the construction of CꢀC
bonds has emerged as an efficient and straightforward method in
modern organic synthesis.6 Despite recent efforts, the
establishment of general and mild strategies for the engagement
of unreactived C(sp3)ꢀH bonds in CꢀC bond forming reactions
45
With the optimal conditions in hand, the substrate scope of this
Cuꢀcatalyzed tandem reaction of alkanes 2 with respect to Nꢀ
methylꢀN,3ꢀdiphenylpropiolamide 1a was first investigated
(Table 2). In the presence of Cu(OAc)2 and TBHP, a variety of
cycloalkanes, such as methylcyclohexane 1b, cyclopentane 1c,
5
still remains a formidable challenge.7 To the best of our 50 cycloheptane 1d, cyclooctane 1e could successfully reacted with
knowledge, approach for the difunctionalization of activated
alkynes with unactivated simple alkanes to generate
spiro[4,5]trienones has never been reported. Herein, we report a
NꢀmethylꢀN,3ꢀdiphenylpropiolamide 1a, allowing easy access to
the corresponding products 3abꢀae in moderate yields. For
example,
diphenylpropiolamide 1a and cyclooctane 2e successfully
alkynes with unactivated alkanes to 3ꢀalkyl spiro[4,5]trienones 55 generated product 3ae in 67% yield. Gratifyingly, a straightꢀchain
the
reaction
between
NꢀmethylꢀN,3ꢀ
10 copperꢀcatalyzed oxidative difunctionalization of activated
via a CꢀH oxidative radical coupling, ispoꢀcarbocyclization and
dearomatization tandem process (Eq 4, Scheme 1).
Our investigation began with oxidative difunctionalization of
15 activated alkynes with unactivated alkanes in the presence of
alkane 2f was found to be viable for the reaction, providing the
product 3af in 36% yield. Notably, ethylbenzene 2g was tested
under the optimal conditions, affording the corresponding product
1ꢀmethylꢀ4ꢀphenylꢀ3ꢀ(1ꢀphenylethyl)ꢀ1ꢀazaspiro[4.5]decaꢀ3,6,9ꢀ
catalytic amount of copper salt and oxidant. (Table 1). In the 60 trieneꢀ2,8ꢀdione
3ag
in
46%
yield.
Unfortunately,
presence
of
Cu(OAc)2
and
TBHP,
NꢀmethylꢀN,3ꢀ
diphenylmethane 2h was not a suitable substrate more because of
the steric effect. Furthermore, toluene derivatives showed good
compatibility (Products 3alꢀak).
diphenylpropiolamide 1a successfully underwent the ipsoꢀ
annualtion reaction with cyclohexane 2a, furnishing the desired
20 3ꢀcyclohexylꢀ1ꢀmethylꢀ4ꢀphenylꢀ1ꢀazaspiro[4.5]decaꢀ3,6,9ꢀ
trieneꢀ2,8ꢀdione 3aa in 57% yield (entry 1). Encouraged by these
results, a series of other Cu salts, namely CuCl2, CuOAc, Cu2O,
CuI and Cu(acac)2 were tested (entries 3ꢀ7): all the copper salts
showed catalytic activity, but they were less effective than
25 Cu(OAc)2 (entry 1 vs entries 2ꢀ6). It should be noted that no
detectable amounts of product 3aa was observed in the absence
of oxidant (entry 7). A number of other oxidants, such as TBHP
(70% in water) DTBP, TBPB, DCP and K2S2O8, were
subsequently examined, and they were less effective than DTBP
30 (entry1 vs entries 8ꢀ12). Further, K2S2O8 had no effect the
reaction (entry 12). Screening on the amount of TBHP and
Cu(OAc)2 reaveled that combined 10 mol% of Cu(OAc)2 with 2
equivalents of TBHP gave the best results (entry 1vs entries 13ꢀ
16). Finally, the effect of reaction temperature were investigated:
35 higher reaction temperature (at 130 oC) did not improve the yield
compared with the results at 110 oC (entry 17), but lower
temperature (at 80 oC) dramatically reduced the yield (entry 18).
Table 3 Scope of NꢀArylpropiolamides (1)a
65
Ph
Ph
R = Bn, 3ba, 41% (67%)
O
R1 = Me, 3ea, 48% (73%)
R1 = Ph, 3fa, 31% (60%)
O
R = Allyl, 3ca, 39% (60%)
R = H, 3da, trace
N
N
O
O
R1
R
Ph
Ph
Ph
MeO
O
O
O
N
O
N
N
O
O
MeO
3ga, 40% (70%)
3ha, 48% (68%)
3ia, 43% (65%)
NC
Table 2 Scope of Alkanes (2)a
OMe
O
O
O
N
O
N
O
N
O
3ja, 37% (68%)
3la, 45% (69%)
3ka, 51% (77%)
N
40
S
O
O
O
N
O
N
O
N
O
3ma, 65% (82%)
3na, 38% (66%)
3oa, trace
a
Reaction conditions: 1 (0.2 mmol), Cu(OAc)2 (10 mol%), TBHP (3
equiv, 5 M in decane) and cyclohexane 2a (1 mL) at 110 oC under argon
atmosphere for 36 h. The number in parentheses is the yield based on
70 recovered starting materials.
We next examined the scope of various propiolamides 1 for
this oxidative C(sp3)ꢀH functionalization/ispoꢀcarbocyclization
tandem reaction under the standard reaction conditions. As shown
in Table 3, we are pleased to find amides with NꢀBn or Nꢀallyl
75 were viable substrates for the reaction (Products 3baꢀca), but a Nꢀ
H group resulted in no reaction. Subsequently, the substitutes
effect of the Nꢀaryl moiety was investigated. Nꢀ
Arylpropiolamideswith Me, Ph or OMe groups were wellꢀ
a
Reaction conditions: 1a (0.2 mmol), Cu(OAc)2 (10 mol%), TBHP (3
equiv, 5 M in decane) and 2 (1 mL) at 110 oC under argon atmosphere for
36 h, isolated yield. The number in parentheses is the yield based on
recovered starting materials.
tolerated,
affording
the
corresponding
3ꢀalkyl
80 spiro[4,5]trienonesin moderate yields (Products 3eaꢀha). For
example, amides 1e and 1g with a Me group at the ortho or meta
2
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DOI: 10.1039/C5CC08952B
position successfully underwent the tandem transformation,
45 Conclusions
furnishing the desired products 3ea and 3ga in 48% and 40%
yield, respectively. Using naphthaleneꢀderived substrate 1i, 43%
yield of 4'ꢀcyclohexylꢀ1'ꢀmethylꢀ3'ꢀphenylꢀ4Hꢀspiro[naphthaleneꢀ
1,2'ꢀpyrrole]ꢀ4,5'(1'H)ꢀdione 3ia was isolated. Extensive
screening revealed that electronꢀdeficient or electronꢀrich
substitutes gropus, including CN, Me, and OMe, on the aromatic
ring at the terminal alkyne were tolerated well (Products 3jaꢀla).
Notably, the amides heteroaryl alkyne 1m and 1n reacted with
10 cyclohexane 2a, giving the desired products 3ma and 3na in 65%
and 38% yield, respectively. Unfortunately, NꢀmethylꢀNꢀ
phenylbutꢀ2ꢀynamide 1o was not suitable substrate for the
reaction.
In summary, we have illustrated the first copperꢀcatalyzed CꢀH
oxidative coupling and ipsoꢀcyclization of Nꢀarylpropiolamides
with unactivated alkanes for the synthesis of 3ꢀalkyl
spiro[4,5]trienones using TBHP oxidant. This method achieves
50 alkyne difunctionalization through a sequence of CꢀH oxidative
coupling, ispoꢀcarbocyclization and dearomatization, and
represents a new shortcut to oneꢀstep formation of two CꢀC bonds
and one CꢀO double bond. Applications of this method in organic
synthesis are currently underway in our laboratory.
5
55
This work was supported by the NSFC (Nos. 21402046,
21172060 and 21472039) and Hunan Provincial Natural Science
Foundation of China (No. 13JJ2018). Dr. R.ꢀJ. S. also thanks the
Fundamental Research Funds for the Central Universities.
To understand mechanism of the current reaction, a radical
15 inhibitor, TEMPO (2,2,6,6ꢀtetramethylpiperidinyloxyl), was
added to the oxidative C(sp3)ꢀH functionalization/ispoꢀ
carbocyclization reaction (Scheme 2): Adding 2 equiv of TEMPO
completely inhibited the conversion of Nꢀarylpropiolamides 1a;
However, substrate 2a was treated with TEMPO to form 1ꢀ
20 (cyclohexyloxy)ꢀ2,2,6,6ꢀtetramethylpiperidine (4) in 89% yield
(Eq 5 in Scheme 2). It is indicated that the radical process might
be involved in this reaction. In addition, treatment of substrate 1a
Notes and references
60 aState 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 jhli@hnu.edu.cn
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
with cyclohexane 2a, H218O, Cu(OAc)
and TBHP afforded 79.2%
65 DOI: 10.1039/b000000x/
2
18Oꢀcontaining product, which were determined by GCꢀMS
25 analysis, suggesting that the newlyꢀformed oxygen atom is from
water (Eq 6 in Scheme 2).
1
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Scheme 2 Control Experiment.
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30
Based on the above results and previous reports, we proposed a
possible mechanism for this system (Scheme 3).7ꢀ9 Initially,
TBHP readily split into tertꢀbutoxy radicals in the presence of
Cu(OAc)2 under heating conditions. Abstraction of a CꢀH bond in
cyclohexane 2a was subsequently by the tertꢀbutoxy radical to
85
35 yield alkyl radical A. Addition of the radical A to the C≡C bond
of Nꢀarylpropiolamide 1a results in the formation of radical
intermediate B, which upon intramolecular ipsoꢀcyclization gives
radical intermediate C. oxidation of intermediate C by the CuIII
species forms cation intermediate D. Finally, nucleophilic
40 addition and oxidation of intermediate D with H2O and TBHP
takes place to afford 3ꢀalkyl spiro[4,5]trienones 3aa.
90
95
4
100
105
110
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