NHPI- and TBAI-Co-Catalyzed Synthesis of Allylic Esters from Toluene Derivatives and Alkenes

Article abstract of DOI:10.1055/s-0036-1591748

An N -hydroxyphthalimide (NHPI) and tetrabutylammonium iodide (TBAI) co-catalyzed oxidative coupling reaction of toluene derivatives and alkenes has been disclosed. This method can serve as a new strategy to access allylic ester using toluene derivatives as oxyacylating reagent. This metal-free protocol also features the readily available starting materials, broad substrate scope, and mild reaction conditions.

Full text of DOI:10.1055/s-0036-1591748

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© Georg Thieme Verlag Stuttgart · New York
2018, 29, A–E
letter
A
en
C. Li et al.
Letter
Synlett
NHPI- and TBAI-Co-Catalyzed Synthesis of Allylic Esters from
Toluene Derivatives and Alkenes
Chengliang Lia
Hongmei Deng*b
Tao Jinc
Chunju Lic
Xueshun Jia*a,c
Jian Li*a,c
O
NHPI (10 mol%)
TBAI (10 mol%)
O2, balloon
R2
O
R2
R3
R1
+
R1
R3
MeCN, 75 °C, 6–12 h
Toluene derivatives as oxyacylating reagent
Molecular oxygen as green oxidant
Metal-free oxidative coupling
a School of Environmental and Chemical Engineering, Shanghai
University, Shanghai 200444, P. R. of China
b Laboratory for Microstructures, Instrumental Analysis and
Research Center of Shanghai University, Shanghai 200444,
P. R. of China
hmdeng@staff.shu.edu.cn
c Department of Chemistry, Shanghai University, 99 Shangda
Road, Shanghai 200444, P. R. of China
xsjia@mail.shu.edu.cn
lijian@shu.edu.cn
Received: 29.10.2017
Accepted after revision: 06.12.2017
Published online: 15.01.2018
this area.8 After that, studies on the asymmetric version of
this reaction catalyzed by chiral copper salts were also well
documented.9 Furthermore, White and co-workers have
widely investigated the palladium-catalyzed synthesis of
allylic esters from carboxylic acids and monosubstituted
olefins via selective C–H bond oxidation (Scheme 1, b),10
which is believed to be a significant breakthrough. Follow-
ing these works, Wan and co-workers have also developed a
one-pot synthesis of allylic ester using aldehyde as the oxy-
acylating reagent (Scheme 1, c).11 This strategy combined
the aldehydes C–H oxidation and the Kharasch–Sosnovsky
reaction, thus offering a new route to allylic ester directly
from simple olefins and aldehydes. Of late, the Wan group
has also disclosed another metal-free synthesis of allylic es-
ter from carboxylic acid and alkene (Scheme 1, d).12 Al-
though much progress has been achieved, there is still a
continuing demand to develop novel and environmentally
benign methods for the synthesis of allylic esters in an effi-
cient manner.
In spite of their role as commonly used solvents, toluene
and its derivatives have found widely application in organic
synthesis as simplest and readily available starting materi-
als in nature.13,14 From a mechanistic point of view, toluene
derivatives can serve as benzyl precursor15 and acyl
source,16 which makes them widely used in a variety of car-
bon–carbon and carbon–heteroatom bond-forming reac-
tions. In the past years, we became interested in developing
novel transformations using toluene derivatives as versatile
building blocks.17 For instance, we have described a palladium-
catalyzed direct synthesis of Δ2-isoxazolines from toluene
derivatives and olefins.17a In addition, we have also devel-
oped a TBAI-catalyzed direct α-oxyacylation of carbonyl
compounds using simple toluene derivatives as efficient
DOI: 10.1055/s-0036-1591748; Art ID: st-2017-w0801-l
Abstract An N-hydroxyphthalimide (NHPI) and tetrabutylammonium
iodide (TBAI) co-catalyzed oxidative coupling reaction of toluene deriv-
atives and alkenes has been disclosed. This method can serve as a new
strategy to access allylic ester using toluene derivatives as oxyacylating
reagent. This metal-free protocol also features the readily available
starting materials, broad substrate scope, and mild reaction conditions.
Key words oxyacylation, toluene derivatives, oxidative coupling,
allylic ester, metal-free
As a direct C–H functionalization method, cross-dehy-
drogenative coupling (CDC) reaction avoids the prefunc-
tionalization of starting materials and has emerged as a
fundamental and highly valuable tool in organic synthesis.1
It is therefore not surprising that many efforts have been
devoted to the development of new bond-forming strate-
gies with different hybrid C–H substrates using CDC reac-
tion in the past decades.2,3 On the other hand, allylic esters
are considered to be significant structural motifs that are
frequently found in many natural products and bioactive
molecules.4 Traditionally, these building blocks can be syn-
thesized through acylation of the corresponding allylic
alcohols with carboxylic acids or their derivatives in organ-
ic synthesis.5 However, these methods also suffered from
the limited substrate scope and harsh reaction conditions.
To overcome these drawbacks, the direct α-oxyacylation6 of
sp3 C–H bonds of alkenes through oxidative cross-coupling
has drawn much attention from synthetic community.7
Among them, the pioneering Kharasch–Sosnovsky reaction
(Scheme 1, a) that involves a Cu-catalyzed esterification of
an allylic C–H bond is considered as a classical method in
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

B
C. Li et al.
Letter
Synlett
Table 1 Optimization of the Reaction Conditionsa
(a) Classical Kharasch-Sosnovsky reaction
O
O2CPh
[Cu] (cat.)
tBu
+
(1)
Ph
O
conditions
oxidant, Δ
O
+
O
(b) Pd-catalyzed example with carboxylic acid
1a
2a
3a
O
S
Ph
O2CR'
O2CR
Entry
Catalyst
Oxidantb
Solvent
Yield (%)c
[Pd](cat.)
(2)
R'-CO2H
+
R
R
1
2
NHPI
O2
toluene
MeCN
DCE
46
52
37
24
32
0
oxidant, Δ
NHPI
O2
(c) One-pot synthesis starting from aldehyde
3
NHPI
O2
O
(1) TBAI/TBHP
+
(3)
4
NHPI
O2
EtOAc
PhCl
R
H
(2) [Cu] (cat.), Δ
5
NHPI
O2
6
NHPI
O2
DMSO
DMF
(d) Metal-free example on synthesis of allylic ester
TBAI (cat.)
TBHP, Δ
O2CR'
7
NHPI
O2
0
R'-CO2H
(4)
+
8
NHPI
O2
DMAc
dioxane
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
0
9
NHPI
O2
30
61
54
49
0
Scheme 1 Representative examples on the synthesis of allylic esters
10
11d
12e
13
14
15
16
NHPI/TBAI
O2
from alkene and other starting materials
NHPI/TBAB
NHPI/TEBAC
TBAI
O2
O2
oxyacylating reagent.17b Most recently, we have just dis-
closed the coupling reaction of toluene derivatives with iso-
cyanides.17c As a continuation of our previous research,18
herein we wish to disclose a NHPI/TBAI-catalyzed oxidative
coupling reaction of alkene and toluene derivatives. To the
best of our knowledge, no such example has been reported
yet.
O2
NHPI/TBAI
NHPI
TBHP
TBHP
TBHP
19
0
TBAI
26
aUntil otherwise noted, all reactions were carried out with cyclohexene 1a
(1 mmol), toluene 2a (3 mmol), catalyst (10 mol%), oxygen balloon, sol-
vent (8 mL), 75 °C for 6–12 h.
We began our reaction optimization using cyclohexene
1a and toluene2a as model substrates. In the presence of
catalytic amount of N-hydroxyphthalimide (NHPI), heating
the mixture in toluene afforded product 3a in 46% yield (Ta-
ble 1, entry 1). The screening of solvents was then conduct-
ed. The employment of MeCN slightly increased the yield of
3a to 52% (Table 1, entry 2), whereas using other solvent in-
cluding DCE and PhCl did not bring any improvement with
respect to the yield of 3a (Table 1, entries 3–5). Also, no de-
sired product was detected when reactions were conducted
in other solvents such as DMSO and DMF (Table 1, entries
6–8). To our delight, the yield of 3a was increased to 61%
when a new NHPI/TBAI catalyst system was applied (Table
1, entry 10). In sharp contrast, the direct use of tetrabu-
tylammonium iodide (TBAI) as single catalyst led to nega-
tive result (Table 1, entry 13). Several experiments using
TBHP as oxidant were also carried out subsequently, which
brought no improvement on the reaction performance (Ta-
ble 1, entries 14–16).
b TBHP (6.0 equiv) was added.
cIsolated yields.
dTBAB = tetrabutylammonium bromide.
eTEBAC = benzyltriethylammonium chloride.
were proven to be effective partners to yield the corre-
sponding products 3b–m. In sharp contrast, the presence of
a strong electron-withdrawing group seemed to be unfa-
vorable to the formation of product 3. Only trace amount of
product 3n was isolated when the nitro group was present.
Furthermore, no desired product was detected when sub-
strate containing a pyridyl group was used.
After a broad arylmethane substrate scope has been es-
tablished, the possibility of substituted alkenes having an
α-H were next examined. As shown in Scheme 3, a substi-
tuted cyclohexene 1b bearing a phenyl group was firstly
used to react with toluene 1a to produce 4a. Next, the allyl-
ic hexane 1c and allylic benzene 1d were proven to be com-
patible reaction partners to afford 4b and4c. It was also
worthy to note that another interesting reaction took place
when no external alkene was added. In such case, products
4e–g were obtained in which toluene and its derivatives be-
haved as benzyl and oxo-acylating reagent in one reaction.
In contrast, no desired product was detected when a het-
eroatom, such as oxygen or nitrogen, was present at the al-
lylic position of alkene.
With the optimized conditions in hand, we then focused
our attention to the scope and limitation of this oxidative
reaction. As shown in Scheme 2, a series of toluene deriva-
tives 2 having electron-donating and electron-deficient
groups at the aromatic ring was used to react with cyclo-
hexene 1a. Substrates 2 containing substituents such as
tert-butyl, methoxyl, methyl, chloro, bromo, fluoro groups
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

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C. Li et al.
Letter
Synlett
R3
NHPI (10 mol%)
TBAI (10 mol%)
O2, balloon
O
R2
O
4
O
NHPI (10 mol%)
TBAI (10 mol%)
O2, balloon
R1
R1
+
R2
R3
CH3
MeCN, 75 °C, 6–12 h
O
+
1a
2
3
MeCN, 75 °C, 6–12 h
1
2a
O
O
O
O
O
O
O
O
O
O
O
O
O
3b: 67%
3c: 73%
3d: 53%
O
4a: 52%
4b: 64%
4c: 55%
O
O
O
O
O
O
O
O
3e: 68%
3f: 61%
3g: 56%
O
O
Cl
Br
O
CH3
O
O
O
O
4d: 37%
4e: 44%
4f: 57%
Cl
O
O
Cl
O
Cl
3h: 60%
3i: 52%
3j: 47%
O
O
O
O
O
F
O
HN
O
O
O
O
CH3
O
O
4g: 58%
ND
ND
O
O
O
3k: 65%
3l: 56%
3m: 39%
Scheme 3 Reagents and conditions: alkene 1 (1 mmol), toluene 2a (3
mmol), TBAI (10 mol%), NHPI (10 mol%), O2 balloon, 75 °C in MeCN (8
mL). Isolated yields after silica gel chromatography.
O
NO2
N
O
O
O
O
O
O
TEMPO (4 equiv)
O
NHPI (10 mol%)
TBAI (10 mol%)
O
trace
ND
3n: 65%
O2, balloon
+
(1)
O
Scheme 2 Scope of the reaction with respect to the arylmethane sub-
strate. Reagents and conditions: cyclohexene 1a (1 mmol), arylmethane
2 (3 mmol), TBAI (10 mol%), NHPI (10 mol%), O2 balloon, 75 °C in MeCN
(8 mL). Isolated yields after silica gel chromatography.
MeCN, 75 °C, 6 h
1a
1a
2a
3a, 0%
NHPI (10 mol%)
TBAI (10 mol%)
O2, balloon
CHO
+
3a, 75%
(2)
(3)
Several mechanistic experiments were next carried out
to have more insight into the present oxidative coupling re-
action. As shown in Scheme 4, the coupling reaction was
completely inhibited when excessive radical scavenger
2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) was added
to the mixture of 1a and 2a under optimized conditions
(Scheme 4, eq. 1), therefore suggesting a free radical pro-
cess. Furthermore, the reaction of benzaldehyde 5 and cy-
clohexene 1a worked well to yield 3a in 75% yield (Scheme
4, eq. 2). This result also indicated that aldehyde was possi-
ble reaction intermediate. Finally, benzoic acid 6 was also
used to experience the optimized conditions. In such case,
product 3a was also isolated in good performance (Scheme
4, eq. 3).
MeCN, 75 °C, 6 h
5
NHPI (10 mol%)
TBAI (10 mol%)
O2, balloon
COOH
1a
+
3a, 83%
MeCN, 75 °C, 6 h
6
Scheme 4 Preliminary mechanistic studies
radical.19 Then the benzyl radical A is generated in the pres-
ence of PINO radical. Next, The in situ formed benzyl radical
A is trapped by O2 to afford the peroxide radical B, which
then eliminates a hydroxyl radical to produce benzaldehyde
intermediate. In the presence of O2, benzaldehyde is further
oxidized to benzoic acid,20 which then reacts with the
hydroxyl radical to yield acyloxy radical C. Finally, the cou-
pling of C and D essentially yields the final product.21
On the basis of the aforementioned mechanistic obser-
vation, a plausible mechanism is depicted in Scheme 5 to
explain this oxidative coupling reaction. The beginning of
this reaction involves the homolysis of NHPI to the PINO
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

D
C. Li et al.
Letter
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O
O2
H
O
OH
OH
5
6
O
O
OH
H
H
B
O
O
H2O
O2, heat
H
N
OH
N
O
NHPI
PINO
O
O
O
O2
O
C
CH3
PINO
O
PINO
TBAI
A
O
D
Scheme 5 Proposed mechanism
In conclusion, a NHPI/TBAI co-catalyzed oxidative cou-
pling of alkene and toluene derivatives has been dis-
closed.22 This protocol allows for the synthesis of allylic
ester in an efficient manner. The present strategy still have
several distinguished features: (i) metal-free oxidation con-
ditions; (ii) the employment of toluene and its derivatives
as readily available oxo-acylating precursors; (iii) the use of
molecular oxygen as green oxidant, which meets the re-
quirement of green chemistry; and (iv) milder reaction con-
ditions. In this light, this method has potential to be further
applied.
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(22) Experimental Procedure and Characterization Data
A 50 mL three-necked round-bottom flask equipped with a
magnetic stir bar was charged with toluene derivatives 2 (3.0
mmol), olefin 1 (1.0 mmol), NHPI (16.3 mg, 10 mol%), and Bu4NI
(36.9 mg, 10 mol%) in MeCN (8 mL) at room temperature. O2
was bubbled into the mixture at 75 °C for 6–12 h. After the reac-
tion was completed, it was monitored by TLC. The resulting
solution was poured into NaCl (15 mL), extracted with DCM
(twice). The combined organic layers were dried over anhy-
drous Na2SO4 and solvents were removed in vacuo. The residue
was purified by PLC Silica gel plate (eluent: petroleum
ether/ethyl acetate = 30:1) to give the desired product.
Compound 3a: 123 mg, 61% yield; colorless oil. 1H NMR (400
MHz, CDCl3): δ = 8.07–8.05 (m, 2 H), 7.56–7.52 (m, 1 H), 7.44–
7.41 (m, 2 H), 6.02–5.98 (m, 1 H), 5.86–5.82 (m, 1 H), 5.52–5.51
(m, 1 H), 2.17–2.10 (m, 1 H), 2.08–2.02 (m, 2 H), 1.94–1.92 (m, 2
H),1.74–1.61 (m, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
166.2, 132.8, 132.7, 130.8, 129.6, 128.3, 125.7, 68.6, 28.4, 24.9,
18.9 ppm.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E