First iodine-catalyzed deallylation of reactive allyl methylene esters

Article abstract of DOI:10.1080/00397911.2012.682245

C-Allyl cleavage has been developed using the inexpensive and mild reagent iodine in dimethylsulfoxide. A variety of compounds with active methylene groups were C-deallylated using this reagent. This method is efficient and operationally simple in comparison to the methods using transition-metal complexes. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2012.682245

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First Iodine-Catalyzed Deallylation of
Reactive Allyl Methylene Esters
Beena R. Nawghare a & Pradeep D. Lokhande a
a Centre for Advanced Studies, Department of Chemistry, University
of Pune, Pune, India
Accepted author version posted online: 10 Jan 2013.Version of
record first published: 05 Apr 2013.
To cite this article: Beena R. Nawghare & Pradeep D. Lokhande (2013): First Iodine-Catalyzed
Deallylation of Reactive Allyl Methylene Esters, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 43:14, 1955-1963
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Synthetic Communications1, 43: 1955–1963, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.682245
FIRST IODINE-CATALYZED DEALLYLATION OF
REACTIVE ALLYL METHYLENE ESTERS
Beena R. Nawghare and Pradeep D. Lokhande
Centre for Advanced Studies, Department of Chemistry, University of Pune,
Pune, India
GRAPHICAL ABSTRACT
Abstract C-Allyl cleavage has been developed using the inexpensive and mild reagent
iodine in dimethylsulfoxide. A variety of compounds with active methylene groups were
C-deallylated using this reagent. This method is efficient and operationally simple in
comparison to the methods using transition-metal complexes.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords C-Allyl cleavage; dimethylsulfoxide; iodine
INTRODUCTION
Carbon–carbon bonds are the foundation of organic chemistry. Tools for
making such bonds are indispensable for the construction of complex molecules.
In contrast, C-C bond cleavage reactions receive less attention. Oxidative cleavage,[1]
sigmatropic rearrangement[2] and alkene metathesis[3] are key examples on a short
list of synthetic methods that involve C-C bond cleavage. The development of
efficient methods for the selective cleavage of C-C bonds is a challenging subject
in modern organic synthesis.
The dialkylation becomes a significant side reaction in monoalkylation of
active methylene esters with reactive alkyl halides or a,x-dihaloalkanes. It is surpris-
ing that no protective group for the acidic hydrogen of malonic esters or acetoacetic
ester has been developed. Because of the development of transition-metal-complex–
catalyzed deallylation protocols, allyl groups can be introduced as protecting groups
for the protection of active methylene groups in the compound. The use of allyl
Received March 15, 2012.
Address correspondence to Pradeep D. Lokhande, Centre for Advanced Studies, Department of
Chemistry, University of Pune, Pune 411007, India. E-mail: pdlokhande@chem.unipune.ernet.in
1955

1956
B. R. NAWGHARE AND P. D. LOKHANDE
protecting groups for the protection of phenols, alcohols, amines, and carboxylic
acids has become more common because of effective procedures available for their
removal.[4,5]
Transition-metal-complex–catalyzed C-C bond formation and C-C bond cleav-
age under mild conditions are now well-established methodologies in organic
synthesis and are widely used to construct complex molecules. Transition-metal cat-
alysts involving palladium,[6,7] ruthenium,[8] and rhodium[9] have been studied for
deallylation of allyl-alkyl ethers, allyl-carboxylates, and allyl amines. However there
are very few reports available for C-allyl bond cleavages. C-Deallylation of malo-
nates was reported for a titanium-promoted process[10] and was observed during
palladium-catalyzed reactions.[11] The deallylation of allylmalonates has been
reported using Ni,[12] Fe,[13] and Ru complexes.[14] Alternative methods for C-allyl
cleavages are in demand as many transition-metal-free methods for C-allyl cleavages
have been reported for O-allyl and N-allyl cleavages.
RESULTS AND DISCUSSION
A new process for deallylation of aryl allyl ethers[15a,b] and allyl carboxylates[16]
in the presence of a catalytic amount of iodine in dimethylsulfoxide (DMSO) has
been recently reported at ambient temperature. Iodine is easily available and handy,
workup procedure is simple, and no precautions need to be taken to exclude moist-
ure or oxygen from the reaction system. A wide range of substrates undergo deallyla-
tion in the presence of many other functional groups. Earlier use of iodine
(1.5 mmol) in deprotection of the prenyl group in prenyl carboxylate has been
reported.[17] In this protocol, an allyl group was found to be incompatible. The
DMSO-iodine reagent has been used in the deallylation and oxidative cyclization
of 20-allyloxy-chalcones to flavones.[18]
The use of iodine in C-C bond cleavage was reported for retro-Michael
addition reaction on the same 1,5-dicarbonyl compounds. Similarly in the course
of our investigation into the one-step synthesis of c-butenolides, a sequence of reac-
tions intended to convert a-allyl-methyl-phenyl-acetate (1) into a-methyl-c-phenyl
butenolides (2) was performed. A significant quantity of side product (3) was
observed after the workup of these reactions. The identification of cyclization was
a-methyl-c-phenyl butenolide (2) and oxidation was methyl-phenyl-acetate (3)
(15–20% yield).
Further investigation of reactivity of iodine toward oxidation reactions found
that a-allyl-methyl-phenyl-acetate (1) when heated with iodine (1.5 mmol) at 145 ꢀC
oxidized to benzoic acid (4). When a-allyl-methyl-phenyl-acetate (1) was subjected to
oxidation at 60 ꢀC with iodine (1 mmol) in DMSO in the presence of a drop of con-
centrated sulfuric acid, phenyl acetic acid was obtained. The C-allyl cleavage could
be the first reaction. On the basis of this study of iodine-catalyzed oxidation, it is
assumed that successful C-allyl cleavages can be attained using iodine as catalyst
in allyl methylene substrates. After many experiments, the first iodine-promoted
deallylation of various C-allyl groups was successfully completed. Here the develop-
ment of this new iodine-catalyzed reaction is reported.
As seen from inspection of Table 1, a wide variety of groups are tolerated on the
aryl residue, ranging from the electron-withdrawing nitro group to the synthetically

C-ALLYL CLEAVAGE BY I2-DMSO
1957
Table 1. C-Allyl cleavage in a-allyl methyl-phenyl-acetate using I2-DMSO
Entry
1
R1
R2
R3
3
Yield (%) 3(a-o)a,b
1
2
1a
1b
1c
1d
1e
1f
H
4-Cl
4-CH3O
4-NO2
2-NO2
3-NO2
H
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
3a
3b
3c
3d
3e
3f
78
71
74
61
63
71
59
70
67
60
58
54
52
62
59
H
3
H
4
H
5
H
H
6
7
1g
1h
1i
-CH2C6H5
-CH2CH2COOCH3
-CH2CH(CH3)COOCH3
3g
3h
3i
8
H
9
H
=
-CH2CH CH2
10
11
12
13
14
15
1j
H
3j
=
-CH2CH CH2 3k
=
-CH2CH CH2
1k
1l
H
H
CH3
-C6H5
H
H
3l
3m
3n
30
1m
1n
H
Br
CH3
CH3
CH3
1o 4-CH2CH(CH3)2
CH3
aIsolated yields of the products.
bProducts are characterized by spectral analysis.
useful halides. Even the oxidation-prone methoxy group was tolerated in the reac-
tion. The hydrolysis-prone ester group and carbonyl group in the side chain were
well tolerated. The cleavage of C-allyl group was observed in preference to aryl,
alkyl, and benzyl groups located at the methylene carbon atom.
Further attention was focused on the C-allyl cleavage reaction of various a-
allyl-phenyl-acetates under the present reaction conditions. These reactions involve
the selective removal of the allyl group observed in the presence of ester, benzyl,
nitro, chloro, and methoxy groups. The entries 11 and 12 demonstrate the possibility
of removal of the O-allyl group in preference to the C-allyl group.
Because we observed that the C-allyl group is cleaved at 60 ꢀC using 1 mmol of
iodine, we were interested in investigating whether the allyl group can be cleaved at
room temperature so that it can be cleaved selectively in the presence of other
groups. Therefore we performed the deprotection at room temperature and increas-
ing the molar ratio of iodine to 1:1.5 mmol. The reaction was completed in 12 h. To
our surprise, the C-allyl group cleaved cleanly and the methyl-phenyl-acetate was
isolated in 90% yield. This reaction also demonstrates that ester groups are not
cleaved and no oxidation product was observed. Further attention focused on the
selective C-allyl cleavage of the allyl carboxylic ester (Table 1, entries 11 and 12).
As indicated, a considerable difference in reactivity was again observed for both
O-allyl and C-allyl group deprotection. The O-allyl cleavage in 1k and 1l occurs
selectively after 3 h at room temperature while C-allyl cleavage completed in 12 h
at room temperature.
Catalytic activity of iodine was studied by considering a-allyl methyl-phenyl-
acetate (1a) as a model substrate. It was observed that increase in the quantity of
iodine gives greater yield (Table 2).
When a-diallyl ester (1j) reacts with iodine in the presence of DMSO, iodine
=
activates the C C bond and forms a three-membered iodonium intermediate. The

1958
B. R. NAWGHARE AND P. D. LOKHANDE
Table 2. Catalytic activity of iodine in C-deallylation
No.
Starting (1)
Product (3)
I2 (mmol)
Temp. ꢀC
Time (h)
Yield (%)a,b
1
2
3
4
5
6
1a
1a
1a
1a
1a
1a
3a
3a
3a
3a
3a
3a
0.1
0.2
0.5
0.8
1.0
1.2
60
60
60
60
60
60
0.5
0.5
0.5
0.5
0.5
0.5
20
38
52
55
58
59
aIsolated yields of the products.
bProducts are characterized by spectral analysis.
carbonyl oxygen of ester attacks the electron-deficient carbon of iodonium ion,
which results in the cleavage of C-C bond, giving allyl vinyl ether. Further
attack of iodine shows O-deallylation of allyl vinyl ether resulting in the formation
of mono allylic ester (3j). The second C-allyl group is removed by repeating the
steps (a) to (d). However, no mechanistic study has been done; this mechanism is
proposed on the basis of earlier observations. Recently O-deallylation in allyl ethers
was reported using iodine (10 mol%) in polyethyleneglycol-400 as a green reaction
solvent.[19]
The treatment of a-allyl-diethylmalonate (5a) with 1 mol of iodine in DMSO in
the presence of a catalytic amount of sulfuric acid at 60 ꢀC for 30 min gave a deal-
lylated product diethylmalonate (6a) in an isolated yield of 58% (Table 3). It is also
interesting to find that one allyl group of diethyl malonate (5b) selectively cleaved
when 1-equivalent of iodine was used. Both allyl groups cleaved with 2 equivalent
of iodine (6b).
Similar results were observed when allyl derivatives of ethyl-aceto-acetate
(7a–c) and acetyl acetone (7d–f) were treated with iodine. For this, various allylic
compounds were prepared using conventional techniques (K2CO3, allyl bromide,
DMSO) in good yields. These C-allyl compounds were then subjected to cleavage
conditions, which involved heating in DMSO with 1 mmol of iodine and monitoring
the reaction by thin-layer chromatography (TLC). The results are summarized in
Table 4. The reactions were completed in 30 min at 60 ꢀC in the presence of a drop
of concentrated sulfuric acid.
The C-allyl cleavage of a series of the allyl substrates (7a–f) having an allyl
functionality at reactive methylene group proceeded with chemoselectivity at the
Table 3. C-Allyl cleavage in diethylmalonate using I2-DMSO
Entry
5
R4
6
Yield (%))a, b
1
2
3
4
5
5a
5b
5c
5d
5e
H
-CH2CH CH2
6a
6b
6c
6d
6e
58
57
54
51
56
=
-CH2C6H5
-CH2CH2COOCH3
-CH2CH(CH3)COOCH3
aIsolated yields of the products.
bProducts are characterized by spectral analysis.

C-ALLYL CLEAVAGE BY I2-DMSO
1959
Table 4. C-Allyl cleavage in reactive methylene compounds using I2-DMSO
Entry
7
R5
R6
R7
8
Yield (%)a,b
1
2
3
4
5
6
7a
7b
7c
7d
7e
7f
-COCH3
-COCH3
-COCH3
-COCH3
-COCH3
-COCH3
-OC2H5
-OC2H5
-OC2H5
-CH3
H
8a
8b
8c
8d
8e
8f
75
64
65
59
50
57
=
-CH2CH CH2
-CH2C6H5
H
-CH3
-CH3
-CH2C6H5
=
-CH2CH CH2
aIsolated yields of the products.
bProducts are characterized by spectral analysis.
Table 5. Cyclization of allylic compound to a-pyrone using I2-DMSO
Entry
9
R8
R9
R10
10
Yield (%)a,b
1
2
3
9a
9b
9c
H
H
H
-OCH3
-OCH3
-OCH3
10a
10b
10c
90
83
87
-OCH3
-OCH3
Cl
aIsolated yields of the products.
bProducts are characterized by spectral analysis.
allyl end. Treatment of substances 7a–f afforded products 8a–f in 50–75% yields. For
bis-allyl substrate (7b,7f), 2 equivalents of iodine is necessary for double deallylation,
however, mono-deallylation proceeded smoothly. The reaction of 7c,7e occurred
smoothly at the allyl end eventhough benzyl group was located at the same carbon
atom (Table 4).
a-Pyrones are found in a large number of natural products[20] that display a
wide variety of biological activities.[21] The synthesis of 2-pyrones is either by
traditional approaches[22] or by new approaches[23] and transition-metal-catalyzed
procedures.[24]
Pursuing the competitive deallylation,we further investigated the effect of
the side chain substituent in the cyclization of 5,6,7-member ring, which success-
fully involved allyl functionality (Table 5). We often observed the formation of cyclic
product a-pyrone along with the desired deallylated product. When the percentage
Scheme 1. Cyclization and oxidation of a-allyl methyl-phenyl-acetate.

1960
B. R. NAWGHARE AND P. D. LOKHANDE
Scheme 2. Plausible mechanism for C-allyl cleavage.
mole of iodine increases at the same temperature, a-pyrone was the sole product
with a yield of 90% (Scheme 5). Iodine acted as Lewis acid, which promotes the
cyclization.
Scheme 3. C-Allyl cleavage in diethylmalonate.
Scheme 4. C-Allyl cleavage in reactive methylene compounds.

C-ALLYL CLEAVAGE BY I2-DMSO
1961
Scheme 5. Cyclization of allylic compound to a-pyrone.
Scheme 6. Plausible mechanism for synthesis of a-pyrone.
CONCLUSION
An efficient C-deallylation procedure for a variety of allylic compounds using
iodine in DMSO reagent has been successfully developed. The reaction proceeds
using a weak oxidizing agent in a short time. The ease of handling the reagent
encourages using it for allyl deprotection in reactive methylene compounds.
EXPERIMENTAL
TLC was performed on E-Merck precoated 60 F254 plates, and the spots were
rendered visible by exposure to ultraviolet light and iodine. Melting points were
determined with an Electrothermal model 9100 apparatus and are uncorrected.
Infrared (IR) spectra were recorded on a Shimadzu 8000 spectrophotometer. The
1H and 13C NMR spectra were recorded on Varian spectrometer. Chemical shifts
(d) are reported in parts per million (ppm) and with reference to tetramethylsilane
(TMS) as internal standard. Mass spectra (GCMS) were recorded on a Shimadzu
Q 5050 spectrometer.
General Procedure for C-Deallylation
Iodine (1 mmol) was added to a solution of allyloxy compound (1)(1 mmol) in
DMSO (3 ml). The reddish reaction mixture was heated in an oil bath at 60 ꢀC for

1962
B. R. NAWGHARE AND P. D. LOKHANDE
30 min. After cooling, the reaction mixture was diluted with ice-cold water, and
iodine was removed by addition of saturated solution of sodium thiosulfate and
washing with water. The product 3 was extracted with ethyl acetate and dried over
Na2SO4, and the solvent was evaporated. The product was purified by column
chromatography (hexane = ethyl acetate, 9:1).
General Procedure for Cyclization of Allylic Compound to a-Pyrone
Iodine (2 mmol) was added to a solution of allyloxy compound (9) (1 mmol) in
DMSO (3 ml). The reddish reaction mixture was heated in an oil bath at 130 ꢀC for
2 h. After cooling, the reaction mixture was diluted with ice-cold water and iodine
was removed by addition of saturated solution of sodium thiosulfate and washing
with water. The product 10 was extracted with ethyl acetate and dried over Na2SO4,
and the solvent was evaporated. The product was purified by column chromato-
graphy (hexane = ethyl acetate, 9:1).
Methyl 2-Phenylpent-4-enoate (1a)
IR: 1630, 1742 cmꢁ1. 1H NMR (300 MHz, CDCl3): d 3.02 (m, 1H), 3.67 (s, 3H),
4.51 (m, 2H), 5.22 (dd, J ¼ 10.2, 20.4 Hz, 2H), 5.93 (m, 1H), 7.28 (m, 5H). 13C NMR
(75 MHz, CDCl3): d 174.3, 62.6, 62.2, 39.9. MS (m=z): 190 (Mþion). Anal. calcd. for
C12H14O2: C, 75.76; H, 7.42%. Found: C, 75.98; H, 7.21%.
Methyl 2-(4-Nitro phenyl)acetate (2d)
1
IR: 1734 cmꢁ1. H NMR (300 MHz, CDCl3): d 2.67 (s, 3H), 3.68 (s, 2H),
7.28 (m, 4H). 13C NMR ( 75 MHz, CDCl3): d 174.8, 63.5, 42.3. MS (m=z): 195
(Mþion). Anal. calcd. for C9H9NO4: C, 55.39; H, 4.65%. Found: C, 55.58; H, 4.4%.
Please see the Supplemental Material, available online, for further details.
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