Direct difunctionalization of activated alkynes via domino oxidative benzylation/1,4-aryl migration/decarboxylation reactions under metal-free conditions

Article abstract of DOI:10.1039/c5cc10084d

The oxidative difunctionalization of aryl alkynoates via benzyl radical initiated intramolecular 1,4-aryl migration and decarboxylation tandem process was developed in the presence of tert-butyl hydroperoxide as an oxidant, which provides a direct and selective route to a variety of trisubstituted alkenes from simple toluene derivatives and aryl alkynoates.

Full text of DOI:10.1039/c5cc10084d

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Direct Difunctionalization of Activated Alkynes via Domino
Oxidative Benzylation/1,4-Aryl Migration/Decarboxylation
Reactions under Metal-free Conditions
Received 00th January 2015
Accepted 00th January 2015
Tao Miao,a Dong Xia,a Yang Li,a Pinhua Li,a and Lei Wang*a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
An oxidative difunctionalization of aryl alkynoate via benzyl oxidative toluene derivatives as benzylation reagents have been
radical initiated intramolecular 1,4-aryl migration and demonstrated recently.6 For example, Zhang reported a facile
decarboxylation tandem process was developed in presence of protocol for the benzylation of carboxylic acids with toluene
tert-butyl hydroperoxide as an oxidant, which provides a direct via Pdꢀcatalyzed C(sp3)−H functionalization.6a Subsequently,
and selective route to a variety of trisubstituted alkenes from Chatani developed a Niꢀcatalyzed benzylation of orthoꢀposition
simple toluene derivatives and aryl alkynoates.
C−H bonds in aromatic amides with toluene derivatives.6b
Although these methods are popular and attractive, they still
Direct functionalization of C(sp3)
−
H bonds for the construction face the utilization of the transitionꢀmetal catalysts. Therefore,
of C−C and C−X bonds has evolved to be one of the most development of straightforward method for the preparation of
efficient and straightforward synthetic approaches to complex benzylation compounds from commercial available toluene
molecules.1 Although C(sp3)−H functionalization is highly derivatives without transitionꢀmetal is highly desirable.
attractive due to the atomꢀeconomics and environmental
Aryl alkynoates are one kind of the important and versatile
sustainability, it is still a challenging task to synthetic chemists, building blocks in organic synthesis. Recently, the direct
attributing to their relatively inert properties and lack of a difunctionalization of alkynes in aryl alkynoates with free
coordination site for the metal catalyst.2 During the past a few radicals has attracted considerable attention, because it can
years, much effort have been made to overcome these proceed the cyclization with C(sp2) of aromatic moiety and
drawbacks. One of the most common strategy used for the promises a new pathway for the synthesis of functionalized
C(sp3)−H functionalization is transitionꢀmetalꢀcatalyzed C−H heterocycles.7 A range of significantly works using this
bond activation with the assistance of a directing group,3 and pathway such as trifluoromethylation, arylsulfonylation,
the other one is the oxidative crossꢀdehydrogenativeꢀcoupling aryldifluoroacetylation,
acylation,
phosphonation
and
(CDC) of C−H adjacent to heteroatoms, double bonds, phenyl organoselenylation/cyclization with orthoꢀC(sp2) of aryl
groups or cycloalkanes.4 The oxidative CDC reaction is highly alkynoate have been reported, and generated the corresponding
appreciable because it does not require substrate coumarins, which are found in natural products, biological
prefunctionalization, and minimize the number of synthetic active molecules and pharmaceuticals.8 It should be noted that
steps. Among the C(sp3)−H functionalizations via oxidative Li developed an intramolecular electrophilic ipso
CDC, toluene derivatives are found to be stable, low toxic and iodocyclization of para-unactivated aryl alkynoate for synthesis
commercially available precursors, and have been used as ideal of spiro[4,5]trienyl acetates,9 and Liang reported
Cuꢀ
ꢀ
a
ArCO− and ArCOO− surrogates to synthesize functionalized catalyzed difunctionalization of aryl alkynoate to
arylketones and arylesters.5 However, to the best of our spiro[4,5]trienones via a tandem oxidation, cyclization and
knowledge, application of toluene derivatives as benzylation dearomatization.10 Inspired by these remarkable works and
reagents is far more elusive, because they can be oxidized to the based on our ongoing interest in the development of alkyne
corresponding alcohols, aldehydes or acides. Few examples on difunctionalization, we herein disclose a TBHPꢀpromoted
oxidative benzylation of alkynes with toluene derivatives to
trisubstituted alkenes (Scheme 1). The reaction undergoes a
sequential benzylation, intramolecular 1,4ꢀaryl migration and
decarboxylation reaction, providing a novel example of alkyne
a.
Department of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000, P
R China; E-mail: leiwang@chnu.edu.cn
Tel.: +86-561-380-2069; fax: +86-561-309-0518
b.
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
difunctionalization for the construction of two carbonꢀcarbon
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P R China
bonds and release of CO2 as sole byꢀproduct in oneꢀpot under
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
10.1039/x0xx00000x
metalꢀfree conditions.
This journal is © The Royal Society of Chemistry 2015
Chem. Commun., 2015, 00, 1ꢀ4 | 1
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atmosphere (Table 1, entry 1). To our delight, the yield of 3a
was enhanced to 70% when the reaction was carried out in 4.0
mL toluene (Table 1, entry 2). Meanwhile, this reaction also
proceeded in comparable yield in the presence of CHP as
oxidant (Table 1, entry 3). The other oxidants including DCP,
DTBP, TBPB and BP gave the inferior results and led to the
formation of 3a in 42–55% yields (Table 1, entries 4–7).
However,
mꢀCPBA, PhI(OAc)2, H2O2 and O2 as oxidants
exhibited no any efficiency to the model reaction (Table 1,
entries 8–11), although O2 has been widely used as a green
oxidant in oxidative crossꢀcoupling reactions. Under the present
reaction conditions, inorganic oxidants, such as K2S2O8 and
Ag2O were also examined, and were ineffective to this
transformation (Table 1, entries 12 and 13). Further screening
revealed that the temperature affected the reaction: 68% yield
of 3a at 150 oC and 42% yield of 3a at 110 oC (Table 1, entries
14 and 15). Changing the amount of TBHP from 2.0 equiv to
3.0 and 1.0 equiv brought a decreasing yield of 56% and 43%
of 3a, respectively (Table 1, entries 16 and 17). When the
model reaction was performed in the absence of TBHP, no 3a
was detected (Table 1, entry 18). In order to reduce the amount
of toluene, mixed solvents, such as benzene and toluene (V:V =
3:1), DMSO and toluene (V:V = 3:1), (DMF) and toluene (V:V
= 3:1), DCE and toluene (V:V = 3:1) were used for the model
reaction. The results indicated that the above mixed solvents
exhibited negative effect significantly (Table 1, entries 19–22).
Scheme 1 Direct difunctionalization of arylalkynoates with toluenes
Table 1 Optimization of the reaction conditions.a
Entry
1
Oxidant
TBHP c
TBHP
CHP
Temp (oC)
130
130
130
130
130
130
130
130
130
130
130
130
130
150
110
130
130
130
130
130
130
130
Yield (%) b
49 d
70
68
42
40
51
55
0
2
3
4
DCP
5
DTBP
TBPB
BP
6
7
8
mꢀCPBA
PhI(OAc)2
H2O2
O
O
9
0
H
R1
R1
10
11
12
13
14
15
16
17
18
19
20
21
22
0
TBHP (2.0 equiv)
130 oC, 16 h
CO2
O2
0
R2
+
R2
K2S2O8
Ag2O
0
3
1a
2
0
Me
TBHP
TBHP
TBHP
TBHP
－
68
42
56 e
43 f
0
0 g
18 h
0 i
0 j
Me
3b,
71%
3a,
3c,
65%
70%
Me
Me
Me
TBHP
TBHP
TBHP
TBHP
Me
3e,
56%
3d, 60%
3f,
62%
a
Reaction conditions: 1a (0.25 mmol), toluene 2a (4.0 mL), oxidant (2.0
equiv), under air for 16 h. b Isolated yield based on 1a. TBHP = tertꢀbutyl
hydroperoxide, 70% solution in water. toluene (2.0 mL). TBHP (3.0
equiv). TBHP (1.0 equiv). benzene and toluene (V:V = 3:1, 4.0 mL) as
c
Cl
F
d
e
f
g
h
i
solvent. DMSO and toluene (V:V = 3:1, 4.0 mL) as solvent. DMF and
toluene (V:V = 3:1, 4.0 mL) as solvent. j mixture of DCE and toluene (V:V =
3:1, 4.0 mL) as solvent. DTBP = diꢀtertꢀbutyl peroxide; DCP = dicumyl
peroxide; BP = benzoyl peroxide; mꢀCPBA = mꢀchloroperbenzoic acid; CHP
= cumene hydroperoxide; TBPB = tertꢀbutyl peroxybenzoate; H2O2 = 30%
aqueous solution.
3g,
54%
3h,
61%
Scheme 2 Reaction of 1a with toluene derivatives [Reaction conditions: 1a
(0.25 mmol), toluene derivatives (2, 4.0 mL), TBHP (2.0 equiv), 130 C, 16
h; isolated yields of products based on 1a].
o
With the optimized reaction conditions in hand, a series of
aryl alkynoate and toluene derivatives as the substrates were
investigated and the results are listed in Scheme 2. First, the
reactions of a number of toluene derivatives with phenyl 3ꢀ
phenylpropiolate (1a) was examined. It was found that the
reactions underwent smoothly with
derivatives as coupling partners as well as solvents, providing
the corresponding products (3a ) in 54 71% yields. The
At the beginning of our investigation, phenyl 3ꢀ
phenylpropiolate (1a) and toluene (2a) were chosen as model
substrates to optimize the conditions for this cascade reaction,
and the results were shown in Table 1. It was pleased to find
that the model reaction provided product 3a in 49% yield using
TBHP (tertꢀbutyl hydroperoxide, 70% solution in water) as an
oxidant and toluene as solvent at 130 °C for 16 h in air
a range of toluene
−
h
−
2 | Chem. Commun., 2015, 00, 1ꢀ4
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functional groups such as methyl, chloro and fluoro groups are propyl, tertꢀbutyl, methoxy, cyano and halogen groups on ester
tolerated under the current system. The substrates containing moiety, and gave the corresponding products in moderate to
more than one methyl group, such as xylene ( ꢀ, ꢀ, ꢀ) and good yields. In particular, the transformation is compatible with
mesitylene were examined, and only the corresponding monoꢀ aryl iodine, aryl bromide and aryl chloride functional groups,
2
o
m
p
functionalized products (3b
−
e) were obtained. It is found that
which are normally reactive in the transitionꢀmetalꢀcatalyzed
coupling reactions for further organic transformations.
However, it was difficult to control the stereoselectivity for the
reactions of unsymmetrical substituted aryl alkynoates, and
mixed
substituents attached to alkynyl were evaluated, and they
bearing similar aromatic rings reacted with toluene,
fluorotoluene and ꢀxylene, providing the desired products 3u
in 50–63% yields.
For understanding the mechanism of tandem reaction,
E/Z isomers (3i–t) were obtained. Furthermore, the
pꢀ
Me
i-Pr
Et
p
–
z
some control experiments were conducted, shown in Scheme
SIꢀ1. When TEMPO (2,2,6,6ꢀtetramethylꢀ1ꢀpiperidinyloxy), a
radical scavenger, was added in 2.0 equiv to the standard
reaction, 3a was obtained in 32% yield. Meanwhile, benzyl
3j, 61%
3k, 56%
3i, 70%
-
t Bu
Me
F
Me
.
(PhCH2 ) and vinyl radicals formed in situ were trapped by
TEMPO to form radical adducts (
4 and 5), determined by
HPLCꢀHRMS analysis. Increasing amount of TEMPO to 4.0
equiv, the reaction was completely restrained. These results
revealed that homolysis of tertꢀbutyl hydroperoxide produced
tertꢀbutoxyl and hydroxyl radicals, both of them could abstract
hydrogen from C(sp3)−H of toluene to afford benzyl radical.
Furthermore, an isotopic effect (kH/kD = 3.0) was observed
when equivalent toluene (2a) and D8ꢀtoluene (D8ꢀ2a) were
employed to react with 1a under the standard reaction
conditions, which indicated that cleavage of C(sp3)−H in
toluene should be involved in the rateꢀdetermining step (For
detail see SI).
3l, 54%
3m, 52%
3n, 71%
3q, 69%
3t, 51%
Cl
Br
I
3o, 60%
3p, 51%
MeO
NC
Cl
Cl
On the basis of these experimental results and previous
related report,7,9ꢀ11 a plausible mechanism is depicted in
3r, 43%
3s, 47%
Cl
Cl
Me
Cl
Scheme 4. First,
a
homolytic cleavage of tertꢀbutyl
hydroperoxide produced tertꢀbutoxy radical and hydroxyl
radical, which abstracted hydrogen from C(sp3)−H of toluene to
afford benzyl radical. Then, a selective addition of benzyl
F
radical to αꢀC of ester in 1a gave a vinyl radical
a phenyl group, followed by an intramolecular radical ipso
cyclization of to form II. Subsequently, 1,4ꢀaryl migration on
I stabilized by
3v, 50%
3u, 63%
3w, 55%
Me
Br
ꢀ
Cl
Br
Br
I
ester moiety generated carboxyl radical III, which underwent
decarboxylative process and release of CO2 to afford vinyl
Me
F
radical IV
, trapped by TEMPO. Finally, IV abstracted
hydrogen from 2a to generate product 3a. In addition, the
reaction of aryl alkynoate containing two orthoꢀsubstitutents
(Cl2) on ester moiety with toluene could generate the
corresponding product 3r in 43% yield, which further clarify
the regioselectivity of 1,4ꢀaryl migration step.
3x, 54%
3z, 58%
3y, 51%
Br
Br
Br
Scheme 3 Reaction of 1 with toluene derivatives (2) [Reaction conditions: 1
(0.25 mmol), toluene derivative (2, 4.0 mL), TBHP (2.0 equiv), 130 oC, 16 h;
isolated yields of products based on 1a].
there is no obvious ortho-position effect in the reaction of oꢀ
xylene with 1a, generated 3d in 60% yield. It is worth noting
that the relatively bulky ethylbenzene could be included in the
reaction and 62% yield of 3f was obtained.
After studying the scope of toluene derivatives, we turned
our attention toward substituted aryl alkynoates. As shown in
Scheme 3, aryl alkynoates have a broad scope and high
compatibility with functional groups, such as methyl, ethyl, iso
ꢀ
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toluene derivatives for the synthesis of trisubstituted alkenes
under metal free conditions. Mechanistic studies revealed that it
involves new C(sp3)–C(sp2) and C(sp2)–C(sp2) bond formation
and release of CO2 in oneꢀpot via a radicalꢀinitiated domino
C(sp3)–H bond functionalization, intramolecular 1,4ꢀaryl
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4 | Chem. Commun., 2015, 00, 1ꢀ4
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