Transition metal-free activation of allylic acetates toward regioselective S-allylation of thiols

Article abstract of DOI:10.1016/j.tetlet.2010.02.015

Allylic acetates have been used as allylating agents under transition metal-free condition toward an economical and sustainable regioselective S-allylation of aromatic and aliphatic thiols in the presence of potassium carbonate in DMF.

Full text of DOI:10.1016/j.tetlet.2010.02.015

Tetrahedron Letters 51 (2010) 1902–1905
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Tetrahedron Letters
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Transition metal-free activation of allylic acetates toward regioselective
S-allylation of thiols
*
Amit Saha, Brindaban C. Ranu
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Allylic acetates have been used as allylating agents under transition metal-free condition toward an eco-
nomical and sustainable regioselective S-allylation of aromatic and aliphatic thiols in the presence of
potassium carbonate in DMF.
Received 15 January 2010
Revised 2 February 2010
Accepted 5 February 2010
Available online 8 February 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
S-Allylation
Thiols
Allyl acetate
Transition metal-free
Regioselectivity
R1
K2CO3
DMF, 90 oC
7 - 9 h
The nucleophilic substitution of allylic compounds is a useful
process in organic synthesis and is frequently used as an important
tool in the synthesis of complex organic molecules.1 Usually these
reactions are performed using allyl alcohol derivatives2a–j or allyl
halides2k–o catalyzed by a variety of transition metal (Pd, Ni, and
Ru) complexes.2 Allyl acetates are preferred as allylating agents be-
cause of their easy availability and configurational stability and on
the other hand, the limitation of more highly activated allyl halides
and tosylates.2j However, use of less reactive allyl acetates requires
the involvement of a transition metal catalyst to activate the allylic
system toward nucleophilic addition. Although metal-catalyzed
allylation reaction with carbon, oxygen, and nitrogen nucleophiles
is well studied,3 allylation of sulfur nucleophiles is more challeng-
ing probably because of deactivation of metal catalyst by sulfur
compounds due to their strong co-ordinating properties.4a,b,d
Moreover, metal-catalyzed allylations often led to a mixture of
regioisomers.4 Although a few transition metal-free Lewis/protic
acid-catalyzed procedures have been developed using allyl alco-
hols, these reactions also ended up with the formation of regioi-
somers.5 Thus, a transition metal-free regioselective allylation of
thiols is appreciated. We report here a simple and efficient protocol
for highly regioselective allylation of thiols with allyl acetates in
the presence of a mild base K2CO3 in DMF (Scheme 1). In our at-
tempts for allylic alkylation of thiols by allyl acetates we discov-
ered that the reaction does not require any transition metal
R1
SR2
R2SH
OAc
Scheme 1. Allylation of thiols with allylic acetates.
Table 1
Standardization of the reaction condition for S-allylation
S
Ph
SH
Ph
Base (1.5 equiv)
Solvent, 9 h
OAc
Cl
Cl
Entry
Base
Solvent
Temp. (°C)
Yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
Na2CO3
K3PO4
NaOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
No base
K2CO3
K2CO3
K2CO3
K2CO3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
THF
90
90
90
90
70
rt
90
90
90
Reflux
90
Reflux
90
70
53
15
88 (E:Z = 100:0)
69
0
76a
82b
0
Trace
74
66
0
H2O
CH3CN
Toluene
a
1 equiv of base was used.
Reaction was carried out for 5 h.
* Corresponding author. Tel.: +91 33 24734971; fax: +91 33 24732805.
E-mail address: ocbcr@iacs.res.in (B.C. Ranu).
b
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.015

A. Saha, B. C. Ranu / Tetrahedron Letters 51 (2010) 1902–1905
1903
catalyst. We rationalized that thiols being stronger nucleophiles
compared to amines and alcohols6 are capable of adding directly
to the allylic system without any pre-activation by metal.
aromatic, heteroaromatic, and aliphatic thiols participated in this
reaction uniformly. In addition to allyl acetate, substituted alkyl,
aryl, and heteroaryl allylic acetates such as crotyl, cinnamyl, and
thienyl also underwent reactions without any difficulty. The reac-
tions proceed with high regioselectivity giving only linear allylic
sulfides without any exception. Several substituents such as Cl,
Br, OH, NH2, and OMe on the aromatic rings of thiols and allyl ace-
tates are compatible with this procedure. The trans-allyl acetates
(Table 2, entries 5, 6, 10, 12–14) retain their stereochemistry to
produce the (E)-products; however, the cis allyl acetates (Table 2,
entries 7, 8, and 15) lead to the mixture of (Z)- and (E)-isomers
with (Z)-allyl sulfides predominating. It was observed that when
this mixture of (E)- and (Z)-isomers was subjected to heating at
100 °C in DMF for 36 h in the presence of K2CO3, the (Z)-isomer
was totally converted to (E)-isomer (Eq. (1)). Thus, it is likely that
cis allyl acetate initially produces (Z)-isomer which is partially
transformed into thermodynamically more stable (E)-isomer under
the reaction conditions.
To standardize the reaction conditions a representative reaction
of 4-chloro thiophenol and cinnamyl acetate was carried out with a
variety of bases in different solvents at varied temperature and
time. The best result was obtained using 1.5 equiv of K2CO3 in
DMF at 90 °C for 9 h (Table 1). As illustrated in Table 1, the reaction
did not proceed at all in the absence of base. Probably, the base
K2CO3 accelerates the thiol toward nucleophilic attack generating
the more nucleophilic thiolate anion and also participates in the
neutralization of acetic acid formed during the reaction.
The experimental procedure is very simple.7 A mixture of allyl
acetate and thiol was heated in DMF at 90 °C in the presence of
K2CO3 for a period of time as required for the completion of the
reaction (TLC). A wide range of diversely substituted thiols under-
went allylations by a variety of allyl acetates to provide the corre-
sponding allyl sulfides. The results are summarized in Table 2. The
Table 2
Allylation of thiols with various allylic acetates
Entry
1
Thiol
PhSH
Allylic acetate
Product
Time (h)
8
Yielda (%)
86
Ref.
4d
OAc
PhS
S
OAc
OAc
2
3
4
5
6
4-OH–C6H4SH
2-NH2–C6H4SH
4-Cl–C6H4SH
4-Cl–C6H4SH
4-Cl–C6H4SH
8
8
8
7
9
78
75
90
77
88
4a
8
HO
S
NH2
OAc
OAc
S
9
Cl
Cl
Cl
S
S
10
Ph
Ph
Ph
OAc
OAc
Ph
S
7
8
4-Cl–C6H4SH
4-Cl–C6H4SH
9
9
85
82
Cl
Cl
E : Z = 32 : 68
S
OAc
OAc
5
5
E : Z = 17 : 83
S
9
4-Cl–C6H4SH
4-Br–C6H4SH
9
8
85
78
11
4a
Cl
Br
OAc
S
S
10
S
OAc
11
12
8
9
89
73
N
SH
N
S
OAc
OAc
PhH2CS
PhCH2SH
MeO
OMe
n-BuS
13
14
n-BuSH
7
9
75
83
MeO
Ph
OMe
SH
S
Ph
Ph
OAc
OAc
10
10
10
Ph
SH
S
15
9
84
10
E : Z = 27 : 73
a
Isolated yields of pure products.

1904
A. Saha, B. C. Ranu / Tetrahedron Letters 51 (2010) 1902–1905
4-hydroxythiophenol (Table 2, entry 2) no N- or O-allylation was
SC H -(4-Cl)
6
4
observed even in the presence of excess allyl acetate.
SC H -(4-Cl)
6
4
K CO
2
OAc
3
In summary, we have developed a transition metal-free simple
protocol for S-allylation of thiols with allyl acetates. The reactions
are highly regio- and chemoselective. The complete retention of
stereochemistry was maintained in the reactions of trans-allyl ace-
tates. This procedure has general applicability to the reactions of a
wide variety of aromatic, aliphatic, and heteroaromatic thiols with
aliphatic, aromatic, and heteroaromatic allylic acetates. To the best
of our knowledge, we are not aware of any transition metal-free S-
allylation of thiols by allylic acetates with such a wide scope. On
extensive literature search on S-allylations by allylic acetates we
have found only two reports of transition metal-free allylation of
thiols using diallyl carbonate12a and Baylis–Hillman acetates.12b
However, no generalizations have been made.12 Nevertheless, be-
cause of its simplicity and low cost in operation and use of environ-
mentally benign reagents, this procedure will provide an easy
access to synthetically useful allylic sulfides1c,13 by S-allylation of
thiols.
DMF
Base
SC H -(4-Cl)
4-Cl-C H SH
+
6
4
o
110
C
Me
14%
6
4
12 h
(1)
(3)
(2)
SC H -(4-Cl)
OAc
6
4
K CO , DMF
2
3
4-Cl-C H SH
+
6
4
o
110 C, 12 h
6
(5)
6
(4)
5%
Scheme 2. Reaction of 4-chlorothiophenol with secondary/branched allylic
acetates.
Base
SAr
SAr
ð1Þ
SAr
The reaction is believed to proceed through SN2 path with an
-addition of the thiolate anion leading to linear allylic sulfide as
a
Acknowledgments
a sole product. It has been observed that this procedure is not very
effective for secondary/branched allylic acetates probably due to
steric factors. When an aromatic-branched allyl acetate 1 was
heated with 4-chloro thiophenol at 110 °C for 12 h under usual
reaction conditions a vinylic sulfide 3 was isolated in low yield
Financial support from DST, New Delhi, Govt of India under J. C.
Bose National Fellowship to B.C.R. (Grant No. SR/S2/JCB-11/2008) is
gratefully acknowledged. A.S. thanks CSIR, New Delhi, for his
fellowship.
(14%). Possibly, the compound 3 resulted from the a-attack of thiol
followed by isomerization to the conjugated system. For general-
ization, when an aliphatic-branched allyl acetate 4 was subjected
to similar treatment, the corresponding allylic sulfide was obtained
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.02.015.
(5%) by the
a-addition (Scheme 2). The absence of a phenyl ring
adjacent to the double bond does not facilitate isomerization in
this case. Thus, exclusive
the SN2 path.
a-addition to all allyl acetates supports
References and notes
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of the peak intensity of the product with that of the starting mate-
rial in the 1H NMR spectrum of crude reaction mixture after an
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intermediate time period of 1.25 h) versus
r (substituent constant)
for the reaction of 4-chlorothiophenol with different substituted
cinnamyl acetates under identical reaction conditions (Figure 1)
shows a linear correlation with a small positive slope (
which suggests the SN2 reaction path.
q = 0.225)
In general, the reactions are very clean and high yielding. In
addition to regioselectivity this procedure also shows chemoselec-
tivity. In S-allylations of 2-aminothiophenol (Table 2, entry 3) and
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2006, 348, 1841–1845; (c) Tamura, S.; Shiomi, A.; Kaneko, M.; Ye, Y.; Yoshida,
M.; Yoshikawa, M.; Kimura, T.; Kobayashi, M.; Murakami, M. Bioorg. Med. Chem.
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7. General experimental procedure for S-allylation of thiols with allyl acetates.
Representative procedure for the reaction of cinnamyl acetate and 4-
chlorothiophenol (Table 2, entry 6): A mixture of cinnamyl acetate (1 mmol,
176 mg), 4-chlorothiophenol (1 mmol, 145 mg) and K2CO3 (1.5 mmol, 207 mg)
was heated in DMF (2 mL) at 90 °C for 9 h under argon. After completion of the
Figure 1. Hammett Plot.

A. Saha, B. C. Ranu / Tetrahedron Letters 51 (2010) 1902–1905
1905
reaction (TLC), the reaction mixture was diluted with ether. The organic layer
was dried over anhydrous Na2SO4 and was concentrated under reduced
pressure. The product was obtained as a pure white solid (mp 84–87 °C) after
column chromatography using hexane (229 mg, 88%). The compound was
characterized by its spectroscopic data and elemental analysis, IR (KBr): 2926,
yellow liquid; IR (neat): 3028, 2908, 2833, 1606, 1510, 1286, 1247, 1174,
1031, 964, 823 cmꢀ1 1H NMR (300 MHz, CDCl3) d 3.23 (2H, d, J = 7.3 Hz), 3.73
;
(2H, s), 3.83 (3H, s), 6.01–6.11 (1H, m), 6.38 (1H, d, J = 15.6 Hz), 6.89 (2H, d,
J = 8.6 Hz), 7.26–7.39 (7H, m); 13C NMR (75 MHz, CDCl3) d 33.9, 35.2, 55.3,
114.1 (2C), 123.6, 127.0, 127.6 (2C), 128.6 (2C), 129.1 (2C), 129.6, 132.0, 138.5,
159.3; HRMS Calcd for C17H18OSK ([M+K]+) 309.0715; Found: 308.9709.
1-(3-Butylsulfanyl-propenyl)-4-methoxy-benzene (Table 2, entry 13): colorless
liquid; IR (neat): 2956, 2929, 1606, 1510, 1462, 1286, 1249, 1176, 1033, 964,
2854, 1475, 1446, 1388, 966, 808 cmꢀ1 1H NMR (300 MHz, CDCl3) d 3.60 (2H,
;
d, J = 7.0 Hz), 6.09–6.19 (1H, m), 6.34 (1H, d, J = 15.7 Hz), 7.15–7.24 (9H, m); 13C
NMR (75 MHz, CDCl3) d 37.4, 124.7, 126.4 (2C), 127.8, 128.6 (2C), 129.0 (2C),
131.8 (2C), 132.6, 133.1, 134.3, 136.6; Anal. Calcd for C15H13ClS: C, 69.08; H,
5.02. Found: C, 69.11; H, 5.06. This procedure was followed for all the reactions
listed in Table 2. Many of these products are known compounds and were
easily identified by comparison of their spectroscopic data with those reported
(see Refs. in Table 2). The unknown compounds (Table 2, entries 7, 8, 10, 12–
15) were properly characterized by their spectroscopic data (IR, 1H NMR, 13C
NMR, and HRMS) which are presented below in order of their entries. The
purity of all compounds was also checked by 1H NMR and 13C NMR.
831 cmꢀ1 1H NMR (300 MHz, CDCl3) d 0.90 (3H, t, J = 7.2 Hz), 1.36–1.44 (2H,
;
m), 1.52–1.60 (2H, m), 2.49 (2H, t, J = 7.5 Hz), 3.28 (2H, dd, J1 = 7.3 Hz,
J2 = 1.1 Hz), 3.79 (3H, s), 5.99–6.09 (1H, m), 6.37 (1H, d, J = 15.6 Hz), 6.85 (2H,
d, J = 8.7 Hz), 7.30 (2H, d, J = 8.7 Hz); 13C NMR (75 MHz, CDCl3) d 13.7, 22.0,
30.5, 31.5, 34.4, 55.3, 114.0 (2C), 124.0, 127.4 (2C), 129.6, 131.4, 159.1; HRMS
Calcd for C14H20OSK ([M+K]+) 275.0872; Found: 275.0317.
E-(3-Dodecylsulfanyl-propenyl)-benzene (Table 2, entry 14): colorless liquid; IR
(neat): 2926, 2852, 1460, 1217, 962, 758 cmꢀ1 1H NMR (300 MHz, CDCl3) d
;
1-Chloro-4-(3-phenyl-allylsulfanyl)-benzene (as a mixture of E and Z isomers)
0.89 (3H, t, J = 6.9 Hz), 1.26–1.39 (18H, m), 1.56–1.61 (2H, m), 2.49 (2H, t,
J = 7.5 Hz), 3.30 (2H, dd, J1 = 7.3 Hz, J2 = 1.1 Hz), 6.16–6.24 (1H, m), 6.43 (1H, d,
J = 15.7 Hz), 7.23–7.39 (5H, m); 13C NMR (75 MHz, CDCl3) d 14.25, 22.82, 29.03,
29.30, 29.40, 29.55 (2C), 29.66, 29.76, 29.80, 30.93, 32.05, 34.45, 126.40 (3C),
127.60, 128.68 (2C), 132.01, 136.95; HRMS Calcd for C21H34SK ([M+K]+)
357.2018; Found: 357.2549.
(Table 2, entry 7): colorless viscous liquid; IR (neat): 3024, 2904, 1475, 1448,
1388, 1095, 968, 810 cmꢀ1 1H NMR (300 MHz, CDCl3) d 3.70 (2H, d, J = 7.0 Hz),
;
3.80 (2H, d, J = 7.7 Hz), 5.76–5.82 (1H, m), 6.16–6.29 (1H, m), 6.44 (1H, d,
J = 15.7 Hz), 6.60 (1H, d, J = 11.3 Hz), 7.16–7.40 (18H, m); 13C NMR (75 MHz,
CDCl3) d 32.24, 37.41, 124.72, 126.41 (2C), 126.78, 127.35, 127.76, 128.42 (2C),
128.63 (2C), 128.77 (2C), 128.95 (2C), 129.03 (2C), 130.89 (2C), 131.76 (2C),
132.16, 132.35, 132.55, 133.12, 134.38, 134.49, 136.34, 136.61; Anal. Calcd. for
C15H13ClS: C, 69.08; H, 5.02. Found: C, 69.12; H, 5.05.
(3-Dodecylsulfanyl-propenyl)-benzene (as a mixture of E and Z isomers) (Table 2,
entry 15): colorless liquid; IR (neat): 2924, 2852, 1492, 1464, 1446, 1220, 1074,
962, 808 cmꢀ1 1H NMR (300 MHz, CDCl3) d 0.91 (6H, t, J = 6.7 Hz), 1.29–1.63
;
1-Chloro-4-non-2-enylsulfanyl-benzene (as a mixture of E and Z isomers) (Table 2,
entry 8): colorless liquid; IR (neat): 2955, 2926, 2854, 1475, 1388, 1222, 1095,
(40H, m), 2.45–2.53 (4H, m), 3.32 (2H, d, J = 7.3 Hz), 3.42 (2H, dd, J1 = 7.8 Hz,
J2 = 1.1 Hz), 5.72–5.82 (1H, m), 6.16–6.26 (1H, m), 6.45 (1H, d, J = 15.7 Hz), 6.59
(1H, d, J = 11.4 Hz), 7.21–7.38 (10H, m); 13C NMR (75 MHz, CDCl3) d 28.92,
29.02 (2C), 29.33 (2C), 29.40, 29.50 (4C), 29.63 (2C), 29.70 (4C), 29.80 (4C),
30.91, 31.55, 32.04, 32.66, 34.43, 39.55, 126.40 (3C), 127.09, 127.56, 128.49
(2C), 128.52, 128.65 (2C), 128.89 (2C), 131.30, 132.00, 136.76 (2C); HRMS Calcd
for C21H34SK ([M+K]+) 357.2018; Found: 357.0979.
1012, 964, 815 cmꢀ1 1H NMR (300 MHz, CDCl3) d 0.90 (6H, t, J = 6.9 Hz), 1.26–
;
1.32 (16H, m), 1.96–1.98 (4H, m), 3.48 (2H, d, J = 6.0 Hz), 3.54 (2H, d, J = 6.4 Hz),
5.46–5.55 (4H, m), 7.22–7.30 (8H, m); 13C NMR (75 MHz, CDCl3) d 14.21 (2C),
22.73 (2C), 27.34, 28.82, 29.08 (2C), 29.24, 29.52, 31.71, 31.81, 32.35, 36.89,
124.19, 124.64, 126.03 (2C), 128.90 (2C), 129.00 (2C), 131.47 (2C), 131.65 (2C),
132.42, 134.14, 134.97, 135.07; HRMS Calcd for C15H21ClSK ([M+K]+) 307.0690;
Found: 307.0964.
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J. P.; Billing, D. G. Tetrahedron Lett. 2004, 45, 9171–9175.
2-[3-(4-Bromo-phenylsulfanyl)-propenyl]-thiophene (Table 2, entry 10): pale
yellow solid, mp 89–91 °C; IR (KBr): 3024, 2955, 1471, 1384, 1089, 1003,
9. Tang, R.-Y.; Zhong, P.; Lin, Q.-L. Synthesis 2007, 85–97.
10. Atkinson, R. S.; Awad, S. B. Chem. Commun. 1975, 651–652.
11. Saxena, A. K.; Kumar, A.; Mozumdar, S. Appl. Catal., A: General 2007, 317, 210–
215.
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K.; Thijs, L.; Zwanenburg, B. Tetrahedron Lett. 1999, 40, 981–984.
13. Dai, C.; Zhu, S.; Yu, Z.; Cohen, T. J. Am. Chem. Soc. 2001, 123, 30–34.
945, 806 cmꢀ1 1H NMR (300 MHz, CDCl3) d 3.65 (2H, d, J = 7.2 Hz), 6.05–6.12
;
(1H, m), 6.55 (1H, d, J = 15.5 Hz), 6.90–6.96 (2H, m), 7.14–7.16 (1H, m), 7.24
(2H, d, J = 8.4 Hz), 7.41 (2H, d, J = 8.4 Hz); 13C NMR (75 MHz, CDCl3) d 37.0,
120.4, 124.1, 124.3, 125.6, 126.1, 127.3, 131.9 (4C), 134.9, 141.4; HRMS Calcd
for C13H11BrS2K ([M+K]+) 348.9123; Found: 348.8244.
1-(3-Benzylsulfanyl-propenyl)-4-methoxy-benzene (Table 2, entry 12): pale