Synthesis of α-ester-β-keto peroxides via iron-catalyzed carbonylation-peroxidation of α,β-unsaturated esters

Article abstract of DOI:10.1016/j.tet.2012.09.110

A general and practical method for the synthesis of α-ester-β- keto peroxides has been achieved by iron-catalyzed three-component reactions of alkenes, aldehydes, and TBHP. A wide variety of functionalized organic peroxides were synthesized efficiently and selectively.

Full text of DOI:10.1016/j.tet.2012.09.110

Tetrahedron 68 (2012) 10333e10337
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of
a
-estere
b
-keto peroxides via iron-catalyzed
carbonylationeperoxidation of a,b-unsaturated esters
,b,
Kaisheng Liu a, Yuanming Li a, Xiaojian Zheng a, Weiping Liu a, Zhiping Li a
*
a Department of Chemistry, Renmin University of China, Beijing 100872, China
b State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 August 2012
Received in revised form 19 September 2012
Accepted 29 September 2012
Available online 4 October 2012
A general and practical method for the synthesis of a-estereb-keto peroxides has been achieved by iron-
catalyzed three-component reactions of alkenes, aldehydes, and TBHP. A wide variety of functionalized
organic peroxides were synthesized efficiently and selectively.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Iron catalysis
Organic peroxides
Peroxidation
Carbonylation
Alkene di-functionalization
1. Introduction
efficiently and selectively transformed into multisubstituted a-
carbonyl epoxides in the presence of a base catalyst.7 These pre-
liminary results prompt us to further investigate iron-catalyzed
Organic peroxides have received considerable attention in
pharmacy, biochemistry and food chemistry because of their close
relationship with drug design, cell damage, and food safety.1 Or-
ganic peroxides also play as key reactive intermediates in synthetic
chemistry.2 Hydroperoxides and cycloperoxides are typically ob-
tained by the radical processes in the presence of O2 or H2O2
(Scheme 1, Method A).3 Kharasch and co-workers were the pio-
neers to introduce peroxy into organic molecules by the use of
hydroperoxides in the presence of various transition-metal cata-
lysts.4 Since these pioneering works, this strategy has been de-
veloped as an efficient and powerful method for peroxidation
(Scheme 1, Method B).5 In addition, the addition of peroxy radicals
to alkenes (Scheme 1, Method C) and the base-catalyzed nucleo-
philic reactions of hydroperoxides with various electrophiles
(Scheme 1, Method D) were also developed to generate the mixed
peroxides.6 However, the established methods have more-limited
scope. Therefore, new peroxidation methods are highly desirable
and valuable. Recently, we reported an iron-catalyzed three-com-
ponent carbonylationeperoxidation reaction of alkene, aldehyde
and hydroperoxide.7 The methodology allows a carbonyl group8
and a peroxy group to add across C]C bonds of styrene de-
rivatives selectively.9 The generated mixed peroxides could be
three-component reactions of
and tert-butyl hydroperoxide (TBHP). The study highlights that the
densely functionalized peroxides, -estere -keto peroxides, which
are difficult to be obtained through the conventional methods,
were efficiently and selectively built up from three-component
assembling (Scheme 1, Method E).
a,b-unsaturated esters, aldehydes,
a
b
H
FG
R
H
FG = aryl, allyl, carbonyl,
heteroatom, etc.
metal-catalyzed oxidation
Method A
Method B
O2 or H2O2 or ROOH
autoxidation
O
t-BuOOH
H (or R)
R5
Method E
O
O
R2
R3
this work
R
R1
R4
addition
substitution
Method D
ROOH or ROOM
Method C
X
R
Scheme 1. General methods for synthesis of organic peroxides.
* Corresponding author. E-mail address: zhipingli@ruc.edu.cn (Z. Li).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.09.110

10334
K. Liu et al. / Tetrahedron 68 (2012) 10333e10337
2. Results and discussion
Table 2
The scope of aldehydesa
2.1. Optimization of the reaction conditions
O
OO-t-Bu
O
The reaction of benzyl methacrylate (1a) with benzaldehyde
(2a) and TBHP (3) was chosen to establish the suitable reaction
conditions (Table 1). The yields of the desired carbon-
ylationeperoxidation product 4aa were substantially improved
through increasing the amount of 2a and 3 (entries 1e4). It is worth
noting that 4aa was obtained in the absence of catalyst, albeit in
31% yield (41% for 6 h) (entry 5). Furthermore, the catalyst
screening with various iron sources (entries 6e13) as well as other
metal salts (entries 14e17) indicated FeCl2 as the best catalyst for
the transformation.10 To our satisfied, an excellent result was ob-
tained using 4.0 equiv of TBHP (3) in CH3CN at 85 ꢀC (91%, entry 18).
OBn
FeCl2 (2.5 mol %)
Me
OBn
+
+t-BuOOH
RCHO
R
MeCN (3 mL)
85 oC, 1 h
Me
1a
2
3
4
O
Entry
1
2
4
Yield(%)b
O
O
2b
2c
4ab
4ac
89(75)
92(78)
MeO
Me
2
3
O
2d
4ad
80(72)
Table 1
Br
Optimization of the reaction conditionsa
O
O
OO-t-Bu
O
4
5
2e
2f
4ae
4af
70(58)
55(46)
OBn
catalyst
AcHN
NC
Me
OBn
+
+t-BuOOH
PhCHO
2a
Ph
Me
O
1a
3
4aa
O
Entry
2a (equiv)
3 (equiv)
Catalyst (2.5 mol %)
4aa yield (%)b
O
1
2
3
4
5
6
7
8
1
1
3
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
1
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
4
FeCl2
FeCl2
FeCl2
FeCl2
d
10
28
74
82
6
2g
4ag
73(63)
HN
31(41)c
73
FeCl3
O
7
8
2h
2i
4ah
4ai
66(56)
55(45)
Fe(OAc)2
Fe2(CO)9
FeBr2
FeCl3$6H2O
FeSO4$7H2O
Fe(acac)2
Fe(acac)3
CuCl
CuBr2
CoCl2
Mn(OAc)2
FeCl2
68
70
56
50
41
30
40
72
49
52
74
91
S
9
O
O
10
11
12
13
14
15
16
17
18
O
9
2j
4aj
27(13)
O
10
11
12
2k
2l
4ak
4al
73(72)
58(46)
84(80)
a
Conditions: 1a (0.5 mmol), catalyst (2.5 mol %), MeCN (3 mL), 85 ꢀC, 1 h, under
nitrogen atmosphere.
O
b
Reported yields were based on 1a and determined by 1H NMR using an internal
standard.
c
6 h.
O
O
2m
4am
2.2. The scope of aldehydes
Me
13
2n
2o
4an
4ao
60(45)c
57(51)c
With the optimized conditions in hand, the scope of aldehydes
(2) was investigated by the use of 1a as a model substrate (Table 2).
Both aromatic and aliphatic aldehydes were successfully applied to
the present transformation. Electron-donating substituted benzal-
dehydes (2b and 2c) afforded the desired peroxides in excellent
yields (entries 1 and 2), while benzaldehydes with electron-
withdrawing group (2def) retarded the efficiency of the trans-
formation (entries 3e5). Heteroaromatic aldehydes (2gei) also led
to the corresponding peroxides (entries 6e8). 1-Naphthaldehyde
(2j) showed a lower efficiency due to the steric effect (entry 9).
Aliphatic aldehydes could also be used to give the desired peroxides
under the standard conditions (entries 10e12). Importantly, the
reactions of formamides (2n and 2o) with 1a led to the expected
peroxides, albeit with the use of large excess amount of formam-
ides in the absence of MeCN (entries 13e14). Interestingly, the
decarbonylation product (5) instead of the desired carbon-
ylationeperoxidation product was obtained quantitatively when
pivaldehyde 2p was applied under the standard conditions (Eq. 1).
N
Me
Et
O
14
N
Et
a
Conditions: 1a (0.5 mmol), 85 ꢀC, 1 h, under nitrogen atmosphere.
Reported yields were based on 1a and determined by 1H NMR using an internal
b
standard; isolated yields were given in parentheses.
c
2k (10 mmol); without MeCN.
t-BuOO
Bu-t
O
FeCl2 (2.5 mol %)
MeCN, 85 oC, 1 h
1a
3
+
+
Bu-t
(1)
Me
BnO2C
2p
5
99%

K. Liu et al. / Tetrahedron 68 (2012) 10333e10337
Table 3 (continued )
10335
This result indicated that decarbonylation competes with carbon-
Entry
1
4
Yield(%)b
ylation kinetically depended on the nature of substituent of
aldehydes.11
O
O
OMe
OAc
9
1j
4ja
85(79)
2.3. The scope of terminal alkenes
Subsequently, the scope of
a-substituted acrylates was in-
vestigated under the standard conditions (Table 3). A verity of
functional groups was well compatible with this transformation
under optimized conditions and afforded the corresponding per-
oxides in good to excellent yields (entries 1e9). The scope of this
carbonylationeperoxidation could be extended to the BayliseHill-
man products bearing O-protected groups, however, the trans-
formation showed no diastereoselectivity (entries 10e12). 3-
Methylenedihydrofuran-2,5-dione 1n also reacted smoothly with
2a (entry 13). It should be noted that a dimeric product 6 was ob-
tained in 20% yield together with the desired peroxide 4oa when
methyl acrylate was tested under the standard reaction conditions
OMe
OAc
10
1k
4ka
4la
99(94)c
Ph
O
OMe
11
12
1l
94(84)d
OBoc
Ph
O
OMe
OTBDMS
Table 3
1m
1n
4ma
4na
96(89)e
72(53)f
The reactions of terminal alkenesa
O
Ph
OO-t-Bu
R2
O
R1
R2
FeCl2 (2.5 mol %)
+
+ t-BuOOH
PhCHO
O
Ph
MeCN (3 mL)
85 oC, 1 h
R1
1
2a
3
4
O
13
O
O
Entry
1
4
Yield(%)b
a
Conditions: 1 (0.5 mmol), 85 ꢀC, 1 h.
Yields were based on 1 and determined by 1H NMR using an internal standard;
b
O
O
isolated yields were given in parentheses.
O
c
1
2
3
1b
1c
1d
4ba
91(73)
80(61)
87(70)
Two diastereomers (3:2).
Two diastereomers (2:1).
Two diastereomers (1:1).
3 h.
d
e
Me
f
O
O
O
O
t-BuOO
Ph
4oa
4ca
4da
, 71%
Me
MeOOC
FeCl2 (2.5 mol %)
MeCN, 85 oC, 1 h t-BuOO
+ 2a + 3
+
(2)
O
MeO2C
O
Ph
1o
O
MeOOC
6, 20%
MeOOC
Me
OH
O
(Eq. 2). This result indicated that
the high selectivity.
a-substituent is important to gain
4
1e
4ea
82(61)
O
Me
2.4. The scope of internal alkenes
O
OEt
5
6
1f
4fa
88(82)
89(65)
It is encouraging that the implementation of this synthetic
strategy to tri-/tetra-substituted acrylates could also give the de-
sired carbonylationeperoxidation products (Table 4), which vali-
dates the reaction as a general and practical method for synthesis of
multifunctional organic peroxides. In most cases, the correspond-
ing peroxides were efficiently obtained using internal alkenes
Ph
O
OMe
1g
4ga
NHAc
bearing a b-alkyl (entries 1e3, 7e11), b-phenyl (entries 4e6, 13), b-
ester (entry 12) in excellent yields. Noticeably, aromatic aldehydes
showed lower reactivity than aliphatic aldehydes in some cases, for
examples, entry 1 versus entries 2e3 and entry 4 versus entries
5e6. Methylenyl Meldrum’s acid derivatives (1xez) were also ap-
plicable for the present transformation (entries 14e17). Impor-
tantly, b,b-disubstituted substrate 1z reacted with 2a to generate
two vicinal quaternary carbon centers, albeit with a much lower
efficiency (entry 17).
O
OMe
7
8
1h
1i
4ha
4ia
80(70)
86(80)
COOMe
OMe
O
OMe

10336
K. Liu et al. / Tetrahedron 68 (2012) 10333e10337
Table 4
2.5. A proposed reaction mechanism
Based on our results and literature reports,12 a tentative reaction
The reactions of internal alkenesa
O
OO-t-Bu
R1
R2
OR45
O
R3
mechanism for iron-catalyzed three-component reactions of a,b-
FeCl2 (2.5 mol %)
t-BuOOH
OR4
+R CHO+
R5
unsaturated esters, aldehydes, and hydroperoxide is given in
Scheme 2.7 Alkyloxy and alkylperoxy radicals are generated from
a serious of steps, including steps (a) and (b), by iron catalyst.
Subsequently, hydrogen abstraction by oxyl radicals, such as tert-
butoxyl radical, gives acyl radical (step c). The nucleophilic radical
MeCN (3 mL)
85 oC, 3 h
R3
R1
R2
O
1
2
3
4
Entry
1
1
2
4, yield(%)b
COOMe
addition of acyl radical to C]C bond leads to a regioselective a-
1p
1q
1r
2a
4pa, 74(55)c
ester radical (step d). Finally, the desired peroxide is formed by
Me
Ph
Me
radical coupling (step e).5f,13
2
3
2k
2m
4pk, 96(64)c
4pm, 96(92)c
t-BuOOH
+
+
Fe+2
Fe+3OH
+
+
(a)
t-BuO
COOMe
COOMe
Fe+2
O
t-BuOOH
Fe+3OH
t-BuOO (b)
4
2a
4qa, 36(30)
H2O
O
5
6
2k
2l
4qk, 85(75)
4ql, 87(74)
(c)
+
t-BuO
Ph
O
Ph
Ph
t-BuOH
O
+
COOEt
COOEt
COOMe
Me
COOMe
(d)
7
8
9
2k
2l
4rk, 99(91)
4rl, 94(87)
4sl, 99(87)
Ph
Me
Pr
Me
O
OOBu-t
O
Ph
COOMe
Me
(e)
t-BuOO
+
COOEt
COOEt
COOEt
COOEt
Ph
COOMe
Me
1s
1t
2l
Scheme
2. A
tentative
reaction
mechanism
for
iron-catalyzed
carbonylationeperoxidation.
3. Conclusion
10
11
2l
2l
4tl, 74(64)
Me
Me
A general and practical method for the synthesis of multifunc-
tional peroxides has been developed by iron-catalyzed three-
COOEt
O
component reactions of
a,b-unsaturated esters, aldehydes, and
1u
4ul, 71(63)d
hydroperoxide. The densely functionalized peroxides, 2-peroxy-
1,4-dicarbonyls, were efficiently and selectively synthesized with
a broad scopes of the substrates. The results are the basis of the
studies on these organic peroxides. The works on the reactivity of
the obtained peroxides are undergoing in this lab.
Me
Me
COOMe
12
13
1v
2a
2a
4va, 99(90)d
4wa, 99(90)d
MeOOC
MeOOC
4. Experimental section
COOMe
1w
4.1. General information
Ph
CN
1H NMR spectra were recorded on Bruker 400 MHz spectrom-
eter and the chemical shifts were reported in parts per million (d)
O
O
relative to internal standard TMS (0 ppm) for CDCl3. The peak
patterns are indicated as follows: s, singlet; d, doublet; dd, doublet
of doublet; t, triplet; m, multiplet; q, quartet. The coupling con-
stants, J, are reported in Hertz (Hz). 13C NMR spectra were obtained
at Bruker 100 MHz and referenced to the internal solvent signals
(central peak is 77.0 ppm in CDCl3). CDCl3 was used as the NMR
solvent. Mass spectra were obtained on a VG ZAB-HS mass spec-
trometer. APEX II (Bruker Inc.) was used for HRMS and ESI-MS. IR
spectra were recorded by a Nicolet 5MX-S infrared spectrometer.
Flash column chromatography was performed over silica gel
200e300. All reagents were weighed and handled in air at room
temperature. Unless otherwise noted, all reactions were performed
under a nitrogen atmosphere. All reagents were purchased from
Alfa, Acros, Aldrich, and TCI and used without further purification.
14
15
1x
1y
2a
2l
4xa, 90(90)
4xl, 68(63)
Me
O
MeO
O
O
O
16
2a
4ya, 87(81)
Ph
O
O
Me
Me
O
17
1z
2a
4za, 18(18)
O
O
a
Conditions: 1 (0.5 mmol), 85 ꢀC, 1 h, under nitrogen atmosphere.
4.2. General procedure for the products 4
b
Reported yields were based on 1 and determined by 1H NMR using an internal
standard; isolated yields were given in parentheses.
c
To a mixture of alkene 1 (0.5 mmol), aldehyde 2 (2.5 mmol),
and FeCl2 (1.6 mg, 0.0125 mmol), acetonitrile (3.0 mL) was
Two diastereomers (2:1).
d
Two diastereomers (3:2).

K. Liu et al. / Tetrahedron 68 (2012) 10333e10337
10337
added under nitrogen at room temperature. Then tert-butyl hy-
References and notes
droperoxide 3 (TBHP, 2.0 mmol, 5e6 M in decane) was dropped
into the mixture under nitrogen at room temperature. The
resulting mixture was stirred under 85 ꢀC for 1 h. The tempera-
ture of reaction was cooled to room temperature. The resulting
reaction solution was directly filtered through a pad of silica by
ethyl acetate. The solvent was evaporated in vacuo to give the
crude products. NMR yields are determined by 1H NMR using
dibromomethane as an internal standard. Solvent was evaporated
and the residue was purified by flash column chromatography on
silica gel with ethyl acetate/petroleum ether as eluent to afford
the pure product 4.
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d
7.95 (d, J¼7.2 Hz, 2H), 7.54
(t, J¼7.2 Hz, 1H), 7.43 (t, J¼7.2 Hz, 2H), 7.38e7.26 (m, 5H), 5.20 (s,
2H), 3.73 (d, J¼16.4 Hz, 1H), 3.42 (d, J¼16.4 Hz, 1H), 1.67 (s, 3H), 1.13
(s, 9H); 13C NMR (101 MHz, CDCl3)
d 196.7, 171.3, 137.1, 135.8, 133.1,
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Acknowledgements
Financial support by the National Science Foundation of China
(20832002, 21072223, 21272267), State Key Laboratory of Heavy
Oil Processing (2012-1-06), the Fundamental Research Funds for
the Central Universities, and the Research Funds of Renmin Uni-
versity of China (10XNL017).
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Supplementary data
Experimental details, characterization data, copies of 1H NMR
and 13C NMR spectra for all products. Supplementary data related
to this article can be found at http://dx.doi.org/10.1016/
j.tet.2012.09.110.