The iron(III) chloride-mediated 1,4-addition of mercaptans to α,β-unsaturated ketones and esters under solvent free conditions

Article abstract of DOI:10.1016/j.tetlet.2006.07.151

The 1,4-addition of various thiols to α,β-unsaturated ketones was completed rapidly in the presence of a catalytic amount (2-3 mol %) of anhydrous iron(III) chloride under solvent free conditions and an air atmosphere. Anhydrous iron(III) chloride is more active than that of other ferric salts. With more reactive and/or less steric reagents (1a-c and/or 2a-2c), expeditious conditions (short reaction times at room temperature) could be employed. With less reactive and/or steric reagents (1d-g and/or 2d-e), a slight increase in reaction time was required, but high yields were obtained. The FeCl₃ catalyst causes preferential interactions with α,β-unsaturated ketones present in the reaction.

Full text of DOI:10.1016/j.tetlet.2006.07.151

Tetrahedron Letters 47 (2006) 7375–7380
The iron(III) chloride-mediated 1,4-addition of mercaptans to
a,b-unsaturated ketones and esters under solvent free conditions
Cheng-Ming Chu, Wan-Ju Huang, Chaowei Lu, Pohsi Wu,
Ju-Tsung Liu and Ching-Fa Yao*
Department of Chemistry, National Taiwan Normal University, 88, Sec. 4, Tingchow Road, Taipei 116, Taiwan, ROC
Received 28 May 2006; revised 27 July 2006; accepted 31 July 2006
Available online 28 August 2006
Abstract—The 1,4-addition of various thiols to a,b-unsaturated ketones was completed rapidly in the presence of a catalytic amount
(2–3 mol %) of anhydrous iron(III) chloride under solvent free conditions and an air atmosphere. Anhydrous iron(III) chloride is
more active than that of other ferric salts. With more reactive and/or less steric reagents (1a–c and/or 2a–2c), expeditious conditions
(short reaction times at room temperature) could be employed. With less reactive and/or steric reagents (1d–g and/or 2d–e), a slight
increase in reaction time was required, but high yields were obtained. The FeCl3 catalyst causes preferential interactions with a,b-
unsaturated ketones present in the reaction.
ꢀ 2006 Published by Elsevier Ltd.
1. Introduction
2-one, etc.), which cannot react in water even under
forcible conditions.8
The base catalyzed 1,4-addition of thiols to a,b-unsatu-
rated carbonyl compounds to form carbon–sulfur bonds
constitutes a key reaction in a variety of biosynthetic
processes as well as in organic synthesis.1,2 Tradition-
ally, the 1,4-addition of mercaptans is catalyzed by
strong bases such as alkali metal alkoxides, hydroxides,3
and amines.4,5 Alternatively, these reactions can also
proceed using different Lewis acids such as Hf(OTf)3,6a
InBr3,6b Bi(NO3)3,6c Bi(OTf)3,6d InCl3,6e Cu(BF4)2.6f
and Zn(ClO4)2Æ6H2O.6g However the use of either
strongly acidic or basic conditions frequently leads to
the formation of undesirable side products owing to
competing reactions, such as polymerization, self-con-
densation and rearrangements.7 Chakraborti et al. re-
cently reported on the conjugate addition of thiols to
a,b-unsaturated carbonyl compounds in water at room
temperature without the use of any metal catalyst lead-
ing to the easy and highly efficient synthesis of a-sulfido
carbonyl compounds. However, there has been no
straightforward synthesis of these uncomplicated enones
and methyl acrylate to confirm the preparation of typi-
cal enones (e.g., chalcone and trans-4-phenyl-3-butene-
In addition to the above description, these reactions can
also be conducted with zeolites and other heterogeneous
systems.9 Since organosulfur compounds have become
increasingly useful and important in the synthesis of bio-
logically active compounds such as the calcium antagonist
diltiazem,10 the development of simple, convenient and
environmentally benign approaches are highly desirable.
Iron has enormous practical advantages as a catalyst
due to its low cost, ample supply, environmental friend-
liness and lack of toxicity. Iron catalysis therefore is cur-
rently being intensively studied to achieve a controlled
organic synthesis, especially in the vinylogous Michael
reaction and cross-coupling reactions.11 Considerable
progress in this field has recently been made by Naka-
mura12 and others.13 Iron catalysis not only improves
many known catalytic processes but also brings about
new possibilities in synthesis. Thus, our goal was to eval-
uate the catalytic efficiency of various commercially
available ferric mediated catalysts as a new and efficient
catalyst for thia-Michael addition reactions.
2. Results and discussion
Keywords: 1,4-Addition; Anhydrous iron(III) chloride; a,b-Unsatu-
rated ketones; Solvent free.
*
In a typical reaction, trans-4-phenyl-3-butene-2-one 1d
was used as a representative of a,b-unsaturated ketone
Corresponding author. Tel./fax: +886
cheyaocf@scc.ntnu.edu.tw
2
29309092; e-mail:
0040-4039/$ - see front matter ꢀ 2006 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2006.07.151

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C.-M. Chu et al. / Tetrahedron Letters 47 (2006) 7375–7380
1. Initially, endeavours were mainly focused on the effi-
ciency of the uncatalyzed reaction. Several blank reac-
tions were conducted at this stage between 1d and
1.1 equiv of thiophenol 2a under solvent free conditions
under different atmospheres. Only a slight conversion of
1d to 3da was observed, suggesting that the rate of the
uncatalyzed reaction is not favourable for 1,4-addition.
No significant change was observed in the reaction mix-
ture, when the reactions were conducted under different
atmospheres such as Ar, air and pure oxygen.
Fe(III)Cl3 (Table 2). The catalyst was compatible with
various solvents, affording high yields of 3da.
The results indicate that anhydrous Fe(III)Cl3 retained
its catalytic activity in solvents with poor and weak
coordinating properties, irrespective of the high polarity
(e = 37) of a hydroxylic solvent such as DMF (entry 8).
In the presence of 2 mol % anhydrous iron(III) chloride,
the reaction in an aprotic solvent such as in acetone and
DMF gave only 84% and 28% yields of 3da, respectively,
along with some unreacted 1a after 96 h (entries 3 and
8). The inferior results are probably due to its specific
affinity towards the cation, which interferes with the
formation of a coordinate bond between the carbonyl
oxygen atom and the Fe3+ ion. Although a similar
interference of coordination between the carbonyl sub-
strate and the Fe3+ ion would be expected to take place
in a protic solvent such as MeOH, the hydrogen bond-
ing effect of MeOH probably compensates for this. It
is noteworthy that the treatment of 1d with 2a in
2 mol % of anhydrous Fe(III)Cl3, in a solvent free sys-
tem, afforded the corresponding Michael adduct 3da in
quantitative yield within 10 min at ambient temperature
(entry 2 of Table 1).
When 1d was treated with thiophenol 2a under neat con-
ditions in the absence or presence of FeCl3 at room tem-
perature, the reactions were monitored by 1H NMR and
the yields were confirmed by GC and the optimum
results are shown in Table 1.
The best result was observed when the reaction was car-
ried out in the presence of 2 mol % of anhydrous
Fe(III)Cl3 to quantitatively afford the desired conjugate
addition product 3aa after 10 min (entries 4–5 of Table
1). The poor catalytic activity of Fe(II) can probably
be explained by its lower oxidation state so that the oxo-
philicity of Fe(II) ion significantly decreased compared
to that of Fe(III) ion. The parallel of the catalytic activ-
ity of the metal chloride was also observed in metal
nitronate-catalyzed and sulfonate-catalyzed reactions.
However, the inferior results obtained for Fe(III)-
(NO3)3Æ9H2O and Fe(III)(SO4)3(NH4)2SO4Æ24H2O (en-
tries 7 and 8) are probably due to the poor electron-
withdrawing nature, which makes the central metal
ion more oxophilic. Thus, Fe(III) salts are susceptible
to hydrolytic decomposition in the presence of
trace amounts of moisture and lose their catalytic
properties.
In this reaction, the efficiency of the metallic catalyst was
strongly influenced by the nature of the metal ion. The
reactivity of various thiols and enones was examined
in anhydrous FeCl3 and the results are presented in
Eq. 1 and Table 3.
Among the above reagents, anhydrous FeCl3 was found
to be superior in terms of conversion. The yields are
always very high (>90%) and the product is formed
within a few minutes. For example, the treatment of
2-cyclohexen-1-one 1a with 2a in a solvent free system
in the presence of 2 mol % of anhydrous FeCl3 gave
the corresponding 1,4-adduct in 94% yield (entry 1 of
A solvent effect was observed in the reaction of 1d with
2a in the presence of a catalytic amount of anhydrous
Table 1. Reaction of 2a with 1d in the presence of various ferrous catalyst under air atmosphere
O
O
Cata. (2 mol%)
neat, rt, air
+
PhSH
Ph
Ph
SPh
3da
1d
2a
Entry
Catalysta
Time
3dab (%)
Recovered 1d (%)b
1
2
3
4
5
6
7
8
Non-additivec
Non-additive
Non-additived
3 h
3 h
3 h
tr
2
4
99
98
96
tr
—
—
tr
c
Fe(III)Cl3
10 min
10 min
10 min
8 h
72 h
72 h
72 h
72 h
98
99
99
99
92
61
68
75
Fe(III)Cl3
Fe(III)Cl3
d
Fe(III)(NO3)3Æ9H2O
Fe(III)(SO4)3(NH4)2SO4Æ24H2O
Fe(II)Cl2
Fe(II)SO4Æ7H2O
Fe(II)SO4(NH4)2SO4Æ6H2O
8
9
10
11
39
32
25
a All reactions were performed by using 2 mmol of 1d and 2.2 mmol of 2a in the presence of different ferrous catalyst under air atmosphere.
b The crude mixture was worked up in ice cold brine solution and then extracted with diethyl ether, the yield was measured by GC by using
naphthalene as the internal standard.
c The reaction was carried out under argon atmosphere.
d The reaction was carried out under oxygen atmosphere.

C.-M. Chu et al. / Tetrahedron Letters 47 (2006) 7375–7380
7377
the conjugate addition. The resonance effect of the
methyl group in 1d (R = Me) increased the electron den-
sity at the carbonyl oxygen and enhanced the coordinat-
ing ability of the compound with Fe3+. On the contrary,
the phenyl group in 1e and 1f (R = Ph), decreased the
electron density at the carbonyl oxygen by inductive
and resonance effects. Therefore, the reaction of 1e
and 1f with 2a required a longer time and a higher tem-
perature compared to the corresponding reaction of 1d
(entries 12–18, Table 3). It is noteworthy that no
byproducts resulting from 1,2-addition or bis-addition
were observed by 1H NMR and GC. Moreover, the
reactions were clean, high yielding and sometimes quan-
titative. Compared to conventional methods, enhanced
reaction rates, improved yields and excellent 1,4-selec-
tivity are features observed in catalysis by anhydrous
Fe(III)Cl3.
O
O
R3
R2
Fe(III)Cl3 (2 - 3 mol%)
neat, air
R3
R4SH
R
R
+
ð1Þ
R1
R1
SR4
R2
1
2
3
-
1a: R + R1 = -(CH2)3-, R2 = R3 = H
1b: R + R1 = -(CH2)2 , R2 = R3 = H
1c: R = Me, R1 = R2 = R3 = H
2a: R4 = Ph
2b: R4 = c-C6H11
2c: R4 = C6H5CH2
2d: R4 = C3H7
1d: R = Me, R2 = Ph, R1 = R3 = H
1e: R = Ph, R2 = Ph, R1 = R3 = H
1f: R = Ph, R1 = R2 = Me, R3 = H
1g: R = Me, R2 + R3 = -(CH2)4- , R1 = H
2e: R4 = CH2=CHCH2
One plausible mechanism for this reaction is proposed
and is based on the oxidizing characteristics of Fe(III),
which is shown as Scheme 1. The use of a strong oxidiz-
ing catalyst such as Fe(III)Cl3 could easily form a strong
coordinate bond with the carbonyl oxygen of the a,b-
unsaturated ketone, which in turn, increases the electro-
philicity of the b-carbon in assisting the conjugate addi-
tion reaction to proceed under mild conditions with
short reaction times.
Table 3). The reaction proceeded rapidly and was com-
plete at room temperature within 5 min without the
necessity of any acid or base catalyst. The product also
could be easily isolated by simple extraction with diethyl
ether. Encouraged by the result using 1a and 2a, we
turned our attention to various thiols and enones. Inter-
estingly, numerous cyclic and acyclic enones including
benzilideneacetone 1d underwent a 1,4-addition with a
range of thiols under mild reaction conditions to afford
the corresponding Michael adducts (Table 3).
The excellent results obtained for FeCl3 in promoting
the addition of various thiols to different enones encour-
aged us to design and optimize synthetic strategies for
a,b-unsaturated ester catalyzed processes. The 1,4-addi-
tion of thiophenol 2a to the resulting 1,4-adduct ester
took place smoothly as shown by the data in Table 4.
It is noteworthy that the highly steric cyclohexanethiol
2b and the unstable allyl mercaptan 2e also afforded
the 1,4-adduct in 96% and 91% yield, respectively, after
isolation (entries 13 and 16). Sterically hindered enones
1e–g also gave the desired 1,4-adducts in excellent yields
(entries 17–19, Table 3). Furthermore, the bonding res-
onance effect of the b-phenyl substituent in 1d and 1e
makes these enones less electrophilic compared to
1a–c. The combined steric and resonance effects of the
b-phenyl substituent make conjugate addition reactions
of 1d and 1e, with particular thiols, less active compared
to those of 1a–c (entries 1–17 of Table 3). The similarity
of the results for 1d, 1e and 1f suggest that the a,b-unsat-
urated ketone exhibited electronic effects in controlling
It is noteworthy that increased catalytic amounts
(15 mol %) of anhydrous Fe(III)Cl3 and an extended
reaction time are necessary for a complete reaction.
The reaction is dependent on sterically hindered and elec-
tronic effects of the esters for achieving good yields
(entries 1, 2 and 4 of Table 4) except for the trans-ethyl
cinnamate (entry 3), in which case the reaction was very
difficult. This torpor is presumably due to the presence of
a
Table 2. Reaction of 1a with 2a in various solvents under the catalytic influence of FeCl3
O
O
FeCl3 (2 mol%)
+
PhSH
rt, air
Ph
Ph
SPh
3da
1d
2a
Entry
Solventa
Time (h)
3dab (%)
Recovered 1db (%)
1
2
3
4
5
6
7
8
CH2Cl2
MeCN
Me2CO
Et2O
CH3(CH2)4CH3
MeOH
EtOAc
4
2.5
96
2.5
2
28
2
96
100
100
84
100
100
100
100
28
—
—
16
—
—
—
—
44
DMF
a All reactions were performed by using 2 mmol of 1d and 2.2 mmol of 2a in 2 mL of argon degassed solvent under air atmosphere.
b The crude mixture was worked up in ice cold brine solution and then extracted with diethyl ether, the yield was measured by GC by using
naphthalene as the internal standard.

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C.-M. Chu et al. / Tetrahedron Letters 47 (2006) 7375–7380
Table 3. Fe(III)Cl3-Catalyzed conjugate addition of thiols to a,b-unsaturated ketonesa
Entry
1
2
Temp (ꢁC)/time
Product 3
Yieldb (%)
1
2
3
4
5
2a
2b
2c
2d
2e
rt/5 min
rt/5 min
rt/5 min
rt/5 min
rt/5 min
3aa
3ab
3ac
3ad
3ae
94
93
95
94
93
O
O
O
O
SR4
6
2a
rt/5 min
3ba
90
SPh
7
8
9
10
11
2a
2b
2c
2d
2e
rt/5 min
rt/5 min
rt/5 min
rt/5 min
rt/5 min
3ca
3cb
3cc
3cd
3ce
92
92
93
90
91
O
O
O
SR4
12
13
14
15
16
2a
2b
2c
2d
2e
rt/10 min
rt/10 min
rt/10 min
rt/10 min
rt/10 min
3da
3db
3dc
3dd
3de
94
96
93
92
91
O
Ph
Ph
SR4
O
O
O
17c
2a
50/10 min
3ea
96
Ph
Ph
Ph
Ph
Ph
O
SPh
18d
19
2a
2a
50/20 min
50/12 min
Ph
3fa
94
SPh
O
O
3ga
92 (98:1)e
PhS
a All reactions were performed by using 1 equiv (2 mmol) of enones 1 and 1.1 equiv (2.2 mmol) of mercaptan 2 in the presence of 2 mol % of
Fe(III)Cl3 under solvent free condition and air atmosphere.
b Isolated yields.
c Reaction was carried out in 1 mL of EtOAc.
d 3 mol % of FeCl3 was used.
e The distereomeric ratio was determined by GC.
O
O
the 2-phenyl group of 4d, which sterically hinders the
approaching thiol 2a. Furthermore, the inferior results
are probably due to the specific affinity and the electronic
effect of the methoxyl group in the ester, which interferes
with the formation of a coordinate bond between the car-
bonyl oxygen atom and the Fe3+ ion. However, a double
electronic effect of a a-carbonyl ester substituent pro-
ceeded efficiently, giving excellent yields within 30 min
at room temperature (Table 4, entry 4).
Fe(III)
1
O
Fe(III)
Fe(II)
H
S
O
RSH
R
Fe(II)
In summary, we describe a simple, convenient and effi-
cient protocol for the 1,4-conjugate addition of thiols
to a,b-unsaturated ketones and esters using anhydrous
iron chloride as catalyst under mild and neutral condi-
tions. Anhydrous iron chloride plays the role of a pro-
moter. The enones exhibit an enhanced reactivity in
anhydrous iron chloride thereby reducing the reaction
times and significantly improving the yields. The simple
experimental procedure, which is environmentally
friendly, is expected to contribute to the development
RS
RS
O
H
2
O
3
Scheme 1. Plausible cyclization mechanism of Fe(III)Cl3 as a catalyst
for Michael addition of thiols to a,b-unsaturated ketones.

C.-M. Chu et al. / Tetrahedron Letters 47 (2006) 7375–7380
Table 4. FeCl3-Catalyzed conjugate addition of thiophenol to a,b-unsaturated estersa
7379
O
O
R3
Cata. (mol%)
neat, rt, air
R3
R1
RO
PhSH
+
RO
R2
SPh
R2
R1
4(a-d)
R2
2
5(a-d)
Entry
R
R1
R3
Catalyst (mol %)
Time
5b (%)
1
2
3
4
Me
Me
Et
H
H
Ph
Ph
H
H
H
H
H
Me
H
FeCl3 (15)
FeCl3 (15)
FeCl3 (15)
FeCl3 (5)
2 h
2 h
4.5 h
30 min
5a (88)
5b (76)
5c (15)
5d (95)
Me
CO2Me
a All reactions were performed by using 1 equiv (2 mmol) of esters 4 and 1.1 equiv (2.2 mmol) of thiophenol 2a in the presence of Fe(III)Cl3 under
solvent free condition and air atmosphere.
b Isolated yields.
of a green strategy for the conjugate addition of thiols to
enones. Further exploration of the scope of such addi-
tion reactions with complex structures of biological sig-
nificance is currently underway.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2006.07.151.
3. Experimental
References and notes
3.1. General procedure for the 1,4-addition of thiophenol
2a to trans-4-phenyl-3-butene-2-one 1d in the presence of
anhydrous Fe(III)Cl3 to generate 4-phenyl-4-phenylsulfa-
nyl-butan-2-one (3da)14 (entry 12 of Table 3)
1. (a) Fluharty, A. L. In The Chemistry of the Thiol Group;
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Typical experimental procedures: In a typical experi-
ment, trans-4-phenyl-3-butene-2-one 1d (0.292 g,
2.0 mmol) and thiophenol 2a (0.249 g, 2.2 mmol) were
mixed together and then anhydrous Fe(III)Cl3
(0.0065 g, 0.02 mmol) was added and the solution stirred
at room temperature under an air atmosphere for
10 min. After the completion of the reaction (monitored
by TLC and GC), the crude mixture was worked up in
ice cold brine solution and then extracted with diethyl
ether solution (3 · 10 mL). The combined ether extract
was dried over anhydrous MgSO4, filtered and then con-
centrated in vacuo, and the resulting product was
directly charged on a small silica gel column and eluted
with a mixture of ethyl acetate/n-hexane (1:25) to afford
the pure 1,4-adduct 3da as a colourless oil (0.481 g, 94%
yield) 1H NMR (400 MHz, CDCl3) d 7.31–7.24 (m, 6H),
7.24–7.16 (m, 4H), 4.70 (dd, J = 7.8, 6.8 Hz, 1H), 3.08
(dd, J = 14.0, 7.8 Hz, 1H), 3.02 (dd, J = 14.0, 6.8 Hz,
1H), 2.05 (s, 3H); 13C NMR (100 MHz, CDCl3) d
205.64, 141.24, 134.25, 133.08, 129.02, 128.67, 127.89,
127.80, 127.61, 49.71, 48.26, 30.87. m/z (relative inten-
sity) 256 (M+, 71), 148 (11), 147 (99), 135 (9), 111
(11), 109 (100), 105 (14), 104 (41), 103 (41), 91 (18), 77
(44), 65 (37), 51 (23). HRMS calcd for C16H16OS
256.0922, found 256.0915.
7. Novak, L.; Kolontis, P.; Szantay, C.; Aszodi, D.; Kajtar,
M. Tetrahedron 1982, 38, 153.
8. In our studies, a solution of deionized water was treated
with a mixture of 2-cyclohexen-1-one and thiophenol
precisely under the conditions given in Tale’s paper as
follows. Only 15% of the expected product was observed
by NMR analysis. As expected, similar results were
observed and only 35% and 20% of product, for a 24 h
reaction were found when chalcone and trans-4-phenyl-3-
Acknowledgement
Financial support provided by the National Science
Council of the Republic of China is gratefully
acknowledged.

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C.-M. Chu et al. / Tetrahedron Letters 47 (2006) 7375–7380
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