A Metal-Free Synthesis of 2-Alkyl(or Aryl) Thiomethyl-2-cyclohexenones from Cyclic Morita–Baylis–Hillman Bromides

Article abstract of DOI:10.1002/hlca.201600136

Under mild conditions, an efficient and rapid S-allylation of thiols with cyclic Morita–Baylis–Hillman (MBH) bromides without the need of a transition-metal catalyst or an expensive additive is described herein. Treatment of the MBH bromides with various

Full text of DOI:10.1002/hlca.201600136

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FULL PAPER
A Metal-Free Synthesis of 2-Alkyl(or Aryl) Thiomethyl-2-cyclohexenones from Cyclic
Morita–Baylis–Hillman Bromides
by Narjes Baioui, Ahlem Abidi, and Farhat Rezgui*
ꢀ
ꢀ
Laboratoire de Chimie Organique Structurale LR99ES14, Faculte des Sciences de Tunis, Universite de Tunis El Manar,
Campus Universitaire, 2092 Tunis, Tunisie (phone: +216 95 737399; e-mail: rez_far@yahoo.fr)
Under mild conditions, an efficient and rapid S-allylation of thiols with cyclic Morita–Baylis–Hillman (MBH) bromides
without the need of a transition-metal catalyst or an expensive additive is described herein. Treatment of the MBH bromides
with various thiols or ethane-1,2-dithiol in the presence of Et3N regioselectively affords the corresponding 2-alkyl(or aryl)
thiomethyl-2-cyclohexenones or the perhydro benzo[1,4]dithiepinone, respectively, in moderate to good yields (40 – 73%).
The reaction is rapid and carried out in THF at room temperature.
Keywords: Chemoselectivity, Nucleophilic substitution, Sulfur, Sulfur heterocycles, Allylic compounds, S-Allylation.
MBH alcohols as well as cyclic MBH acetates with active
methylene compounds [22]. In 2016, we have described
Introduction
Over the last decades, nucleophilic substitution reactions
of allyl compounds have been developed very well. They
are currently considered as a very important process and
a powerful tool for the construction of new C–C and C–X
an efficient protocol for the synthesis of new series of
c-keto allyl phosphonates in good to excellent yields from
the MBH acetates as starting materials in the presence of
DMAP or imidazole, as promoters of the allylic nucleo-
(X = O, N, S, etc.) bonds that were widely used for the philic substitution [23].
synthesis of a variety of interesting derivatives [1 – 4].
In organic synthetic reactions, the scope and applica-
tions of organosulfur chemistry have increased enor-
To the best of our knowledge, the S-allylation of thi-
ols with cyclic MBH bromides 1a and 1b has not been
extensively studied under catalyst-free conditions. Hence,
mously since S-containing groups serve as important we report in this paper an efficient method for the synthe-
auxiliaries function in synthetic sequences [5]. Among sis of various allyl sulfides 3a – 3l in mild conditions from
them, allyl sulfides have acquired great importance as a the MBH bromides 1a and 1b.
class of useful scaffold in organic synthesis [6 – 8].
Indeed, they are used currently as valuable synthons for
Results and Discussion
various organic transformations [9 – 11], including imida-
tion and subsequent sigmatropic rearrangements [12] and
thio-Claisen rearrangements [13].
They also serve as important synthetic intermediates
The starting materials 1a and 1b were prepared in a two-
step sequence according to previous reports. Indeed,
allylic alcohols 2a and 2b were first prepared through the
in agricultural and pharmaceutical chemistry [14]. Over MBH reaction involving the cyclohex-2-enone [24][25],
the last decades, the Morita–Baylis–Hillman (MBH)
adducts [15 – 18] have been used for the synthesis of
these compounds.
Scheme 1. Synthesis of allyl bromides 1a and 1b from allyl alcohols
In the course of our study on the development of the
chemistry of MBH, we have studied the behavior of cyclic
2a and 2b.
MBH adducts with a large variety of nucleophiles [19].
We have previously reported the S-allylation of alkyl
alcohols and thiols with 2-(hydroxymethyl)cyclohex-2-en-
1-one in the presence of TsOH [20], as well as the behav-
ior of cyclic MBH acetates with thiols in the presence of
NaH in THF [21].
Moreover, we have reported a first DMAP-mediated
i) 2a: HCHO, DMAP, THF/H2O (60%). ii) 2b: MeCHO, imidazole
THF/H2O (40%).
Pd-free Tsuji–Trost-type reaction of cyclic and acyclic
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DOI: 10.1002/hlca.201600136

Helv. Chim. Acta 2016, 99, 704 – 708
705
further dimerization was performed using a combination
of Mn metal and CuCl2. Moreover, the corresponding Sn
and Zn reagents have also been prepared by the same
research group and in situ involved in a Barbier reaction
using various aldehydes [27]. In our part, we make our
contribution by studying the behavior of compounds 1a
and 1b toward a wide range of thiols as well as ethane-
1,2-dithiol as a 1,4-bidentate nucleophile.
Scheme 2. Conversion of allyl bromides 1a and 1b into allyl sulfides
3a – 3l.
In our first attempt, a mixture of MBH bromide 1a
(4 mmol) and thiophenol (6 mmol) was carried out in
THF without any additive. After stirring 1 h at room tem-
perature, we observed a total conversion of the allylic
followed by their direct conversion into the corresponding
allylic bromides 1a and 1b using aqueous 48% HBr bromide 1a into the corresponding sulfide 3a with a mod-
(Scheme 1) [26].
est yield (45%), presumably because the presence of an
It is noted that these allyl bromides 1a and 1b are of acidic medium (HBr) may inhibit the progress of the
limited stability, particularly when neat. Therefore, they
reaction. Therefore, in order to improve this modest
were prepared from the MBH alcohols 2a and 2b and yield, we thought that a more efficient route would be
then directly used without further purification. It is note-
worthy that the chemistry of these derivatives has not
been previously well developed. Indeed, only the ally bro-
mide 1a was first synthesized by Handy et al. [26] and its
implemented using Et3N as a base to trap the excess of
HBr formed in the reaction medium. Thus, we investi-
gated the behavior of the bromide 1a toward thiophenol
in the presence of Et3N (1.5 equiv.) as additive in THF as
Table. S-Allylation of thiols with the allyl bromides 1a and 1b
Reaction time [h] Product 3
0.5 3a [21]
Entry
1
R1
Thiol
PhSH
Yield [%]
72
H (1a)
2
H (1a)
4-Cl–C6H4SH
0.5
3b
68
3
4
5
6
H (1a)
H (1a)
H (1a)
H (1a)
PrSH
0.5
0.5
0.5
0.5
3c
61
63
70
73
iPrSH
3d
BuSH
3e [21]
3f [21]
Me(CH2)5SH
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Helv. Chim. Acta 2016, 99, 704 – 708
Table. (cont.)
Entry
7
R1
Thiol
Reaction time [h]
0.5
Product 3
3g [21]
Yield [%]
70
H (1a)
MeOOCCH2SH
8
9
H (1a)
EtOOC(CH2)2SH
0.5
3h
65
52
54
40
55
Me (1b)
Me (1b)
Me (1b)
Me (1b)
PhSH
4
3i [21]
3j [21]
3k [21]
3l [21]
10
11
12
PrSH
4
BuSH
4
Me(CH2)5SH
4
solvent. Under these conditions and after 30 min, we have 40 – 55% moderate yields (Scheme 2, Table, Entries
successfully observed a metal-free total conversion of the
bromide 1a into the corresponding S-allylation product 3a
in 72% yield (Scheme 2).
9 – 12).
We believe that the reaction mechanism starts with a
conjugate addition of a thiol onto the Michael acceptor
1a, followed by the elimination of HBr, affording the
intermediate I. Subsequent second b0-conjugate addition
of a thiol onto the intermediate I, then elimination of a
In order to demonstrate the scope and the limitation
of this rapid and efficient synthetic methodology, we
investigated the behavior of a variety of thiols under the
above conditions (Et3N in THF at room temperature) molecule of thiol, provides the allyl sulfide 3a, which is
toward the MBH bromides 1a and 1b. Our results, listed
therefore the result of two consecutive SN20 reactions
in the Table, showed the total conversion of the MBH through
one-pot addition–elimination sequence1)
bromides 1a and 1b into the corresponding allyl sulfides (Scheme 3).
a
3a – l in modest to good yields ranging from 40% to 73%
(Scheme 2, Table, Entries 1 – 12).
However, when 1 equiv. of ethane-1,2-dithiol, as a 1,4-
bidentate nucleophile, was left to react with the allyl bro-
Under the above optimized reaction conditions, we mide 1a, under the previous conditions, we have obtained,
have observed that the six-membered allylic bromide 1a via a one-pot two-step sequence, the perhydrobenzo[1,4]-
rapidly reacted with primary thiols (Scheme 2, Table, dithiepinone 4 in 50% overall yield (Scheme 4).
Entries 1 – 8) affording, within 30 min, in 61 – 73%
yields, the corresponding allyl sulfides 3a – 3h. This allyla-
tion reaction also worked with the six-membered bromide
1) Regarding the reaction mechanism, one of the referee sug-
1b but the latter is less reactive than its homologous com-
pound 1a, presumably as it is slightly hindered at the
a-allyl bromide C-atom bearing a Me group. The corre-
sponding sulfides 3i – 3l were obtained within 4 h, in
gested that the tertiary amine Et3N could have two roles: it
could also act as a catalyst for the reaction in a similar man-
ner than in the MBH reactions, giving initially the 1,4-addi-
tion product, and neutralizing HBr.
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Helv. Chim. Acta 2016, 99, 704 – 708
707
Scheme 3. Proposed mechanism for the conversion of 1a into 3a.
Scheme 4. Proposed mechanism for the conversion of 1a into 1,4-dithiepine derivative 4.
1
We believe that, in this study, the reaction mechanism 66v/S spectrometer; v in cmꢀ1. H- and 13C-NMR spectra:
~
starts with a b-conjugate addition of the thiol onto the Bruker AC-300 spectrometer, 300 and 75 MHz, resp., in
Michael acceptor 1a, followed by the elimination of HBr,
CDCl3; d in ppm rel. to Me4Si as internal standard, J in
yielding the intermediate I0. The latter subsequently Hz. HR-ESI-MS: Autospec Ultima micromass mass spec-
reacts in an intramolecular b0-conjugate addition onto the trometer at 70 eV; in m/z.
cyclic enone subunit, to finally afford the bicyclic
1,4-dithiepine derivative 4 (Scheme 4).
Typical Procedure for the Preparation of Sulfur
Compounds (3a – l)
Conclusions
To a soln. of allyl bromides (4 mmol) 1a or 1b in THF
were added 6 mmol of Et3N and a soln. of thiol (6 mmol)
In summary, we have developed a rapid and efficient pro-
tocol for the synthesis of allyl sulfides 3a – 3l from the in THF (2 ml). The mixture was stirred at r.t., then
reaction of a variety of thiols onto the corresponding cyc-
lic MBH bromides 1a and 1b in moderate to good yields.
Mild and metal-free catalyzed reaction conditions are the
quenched with H2O, and extracted with CH2Cl2
(3 9 10 ml). The combined org. layers were dried
(MgSO4), filtered, and concentrated in vacuo. The result-
attractive features of this synthetic methodology. We ing residue was purified by FC on silica gel (PE/Et2O),
believe these functionalized allyl sulfides would be of
much importance in organic chemistry.
affording compounds 3a – 3l or 4.
2-[(Phenylsulfanyl)methyl]cyclohex-2-en-1-one (3a) [21].
Yield: 72%. Yellow oil. IR (CHCl3): 1680, 1590, 1480,
1
We gratefully acknowledge financial support of this work
by the Ministry of Higher Education of Tunisia.
1440. H-NMR (300 MHz, CDCl3): 7.31 – 7.17 (m, 5 H);
6.76 (t, J = 4.0, 1 H); 3.70 – 3.69 (d, J = 3, 2 H);
2.46 – 2.41 (m, 2 H); 2.32 – 2.26 (m, 2 H); 1.99 – 1.92 (m,
2 H). 13C-NMR (75 MHz, CDCl3): 198.0; 147.0; 136.1;
135.2; 130.2; 128.8; 126.4; 38.3; 32.6; 26.0; 22.8. EI-MS: 53
(46), 79 (55.4), 81 (98), 110 (54.5), 185 (36.6), 218
(C13H14OS, M+, 100).
Supplementary Material
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/hlca.201600136.
2-{[(4-Chlorophenyl)sulfanyl]methyl}cyclohex-2-en-1-one
(3b). Yield: 68%. Yellow oil. 1H-NMR (300 MHz,
CDCl3): 7.25 – 7.22 (m, 4 H); 6.77 (t, J = 3.5, 1 H); 3.68
(d, J = 3, 2 H); 2.48 – 2.43 (m, 2 H); 2.35 – 2.30 (m, 2 H);
2.02 – 1.93 (m, 2 H). 13C-NMR (75 MHz, CDCl3): 197.7;
147.4; 134.9; 134.6; 132.4; 131.5; 128.9; 38.2; 32.8; 26.0;
22.8. EI-MS: 53 (73), 81 (100), 108 (44), 144 (14), 252
(C13H13ClOS, M+, 94).
2-[(Propylsulfanyl)methyl]cyclohex-2-en-1-one (3c). Yield:
61%. Yellow oil. 1H-NMR (300 MHz, CDCl3): 6.92 (t,
J = 3.0, 1 H); 3.30 (d, J = 3, 2 H); 2.48 – 2.41 (m, 6 H);
2.05 – 1.97 (m, 2 H); 1.63 – 1.56 (m, 2 H); 1.00 – 0.95 (m,
Experimental Part
General
Anal. thin-layer chromatography (TLC): precoated SiO2
60 F254 plates; visualization by UV light (254 nm). Flash
chromatography (FC): SiO2 60 and a gradient solvent sys-
tem (petroleum ether (PE)/Et2O as eluent). M.p.: Elec-
trothermal 9100 apparatus; uncorrected. IR Spectra: IFS
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Helv. Chim. Acta 2016, 99, 704 – 708
3 H). 13C-NMR (75 MHz, CDCl3): 198.2; 146.5; 136.4; (75 MHz, CDCl3): 197.7; 145.1; 141.8; 38.6; 35.8; 33.6;
38.2; 34.2; 29.9; 26.0; 23.0; 22.7; 13.4. EI-MS: 82 (90), 110 26.1; 22.9; 22.8; 21.4; 13.5. HR-ESI-MS: 221.0971
(100), 141 (89), 184 (C10H16OS, M+, 55).
2-{[(1-Methylethyl)sulfanyl]methyl}cyclohex-2-en-1-one
(3d). Yield: 63%. Yellow oil. 1H-NMR (300 MHz,
([M + Na]+, C11H18NaOS+; calc. 221.0976).
2-[1-(Butylsulfanyl)ethyl]cyclohex-2-en-1-one (3k) [21].
1
Yield: 40%. Yellow oil. IR: 1672.02. H-NMR (300 MHz,
CDCl3): 6.95 (t, J = 4.5, 1 H); 3.33 (d, J = 1.1, 2 H); CDCl3): 7.03 (t, J = 4.2, 1 H); 4.04 (q, J = 7.0, 1 H);
2.92 – 2.81 (m, 1 H); 2.48 – 2.40 (m, 4 H); 2.05 – 199 (m, 2.49 – 2.42 (m, 6 H); 2.04 – 1.97 (m, 2 H); 1.56 – 1.51 (m,
2 H); 1.27 – 1.24 (m, 6 H). 13C-NMR (75 MHz, CDCl3): 2 H); 1.43 – 1.39 (m, 2 H); 1.34 (d, J = 7.0, 3 H); 0.89 (t,
197.8; 146.8; 136.3; 38.3; 35.0; 28.6; 26.0; 23.3; 22.9. EI-MS:
53 (46), 79 (69), 82 (75), 110 (100), 141 (24), 184
(C10H16OS, M+, 52).
J = 7.5, 3 H). 13C-NMR (75 MHz, CDCl3): 197.8; 145.2;
141.8; 38.9; 35.8; 31.6; 31.2; 26.1; 22.8; 22.1; 21.7; 13.7.
HR-ESI-MS: 235.1127 ([M + Na]+, C12H20NaOS+; calc.
2-[(Butylsulfanyl)methyl]cyclohex-2-en-1-one (3e) [21]. 235.1133).
Yield: 70%. Yellow oil. IR (CHCl3): 1662. 1H-NMR
2-[1-(Hexylsulfanyl)ethyl]cyclohex-2-en-1-one (3l) [21].
(300 MHz, CDCl3): 6.91 (t, J = 4.0, 1 H); 3.30 – 3.29 (d, Yield: 55%. Yellow oil. IR: 1674. 1H-NMR (300 MHz,
J = 3, 2 H); 2.49 – 2.38 (m, 6 H); 2.05 – 1.97 (m, 2 H); CDCl3): 7.03 (t, J = 4.2, 1 H); 4.05 (q, J = 7.1, 1 H);
1.61 – 1.51 (m, 2 H); 1.45 – 1.33 (m, 2 H); 0.91 (t, J = 4.0,
2.48 – 2.41 (m, 6 H); 2.04 – 1.95 (m, 2 H); 1.59 – 1.49 (m,
3 H). 13C-NMR (75 MHz, CDCl3): 198.1; 146.5; 136.5; 2 H); 1.37 – 1.24 (m, 6 H); 1.33 (d, J = 7.0, 3 H); 0.88 (t,
38.4; 32.0; 31.5; 30.1; 26.0; 23.0; 22.0; 13.6. EI-MS: 53 (63), J = 7.5, 3 H). 13C-NMR (75 MHz, CDCl3): 197.5; 144.9;
82 (97), 110 (100), 141 (60), 198 (C11H18OS, M+, 33).
142.0; 38.6; 36.0; 31.7; 31.5; 29.6; 28.7; 26.1; 22.9; 22.5;
2-[(Hexylsulfanyl)methyl]cyclohex-2-en-1-one (3f) [21]. 21.5; 14.0. HR-ESI-MS: 263.1440
([M + Na]+,
C14H24NaOS+; calc. 263.1445).
Yield: 73%. Yellow oil. IR: 1673. 1H-NMR (300 MHz,
CDCl3): 6.91 (t, J = 4.2, 1 H); 3.29 (d, J = 3, 2 H);
2.48 – 2.41 (m, 6 H); 2.07 – 1.97 (m, 2 H); 1.61 – 1.52 (m,
2 H); 1.39 – 1.25 (m, 6 H); 0.88 (t, J = 7.5, 3 H). 13C-
NMR (75 MHz, CDCl3): 198.1; 146.6; 136.5; 38.4; 32.3;
31.5; 30.3; 29.7; 28.6; 26.1; 23.0; 22.8; 14.0. HR-ESI-MS:
249.1284 ([M + Na]+, C13H22NaOS+; calc. 249.1289).
Typical Procedure for the Preparation of Sulfur
Compound (4)
To a soln. of allyl bromide (4 mmol) 1a in THF were
added 4 mmol of Et3N and a soln. of ethane-1,2-dithiol
Methyl {[(6-Oxocyclohex-1-en-1-yl)methyl]sulfanyl}acetate (4 mmol) in THF (2 ml). Following the previous proce-
(3g). Yield: 70%. Colorless oil. 1H-NMR (300 MHz,
dure for compounds 3a – 3l, the compound was
CDCl3): 6.96 (t, J = 3, 1 H); 3.74 (s, 3 H); 3.40 (d, J = 0.9, obtained pure.
4
2 H); 3.19 (s, 2 H); 2.49 – 2.42 (m, 4 H); 2.07 – 1.98 (m, 2
Octahydro-6H-1,4-benzodithiepin-6-one (4) [21]. Yield:
H). 13C-NMR (75 MHz, CDCl3): 198.2; 170.9; 147.9; 50%. White solid. M.p. 150 – 152 °C. IR (CHCl3): 1710,
1
135.3; 52.2; 38.2; 32.9; 30.4; 25.9; 22.9. EI-MS: 53 (20), 79 1410. H-NMR (300 MHz, CDCl3): 4.32 – 2.66 (m, 8 H);
(40), 109 (13), 141 (100), 155 (21), 214 (C10H14O3S, M+, 2.53 – 1.64 (m, 6 H). 13C-NMR (75 MHz, CDCl3): 209.1;
16).
207.9; 60.4; 55.4; 50.2; 50.0; 42.0; 40.6; 39.4; 37.5; 37.0;
33.4; 33.3; 33.0; 25.4; 23.0. EI-MS: 97 (22.3), 110 (54.1),
174 (16.9), 202 (C9H14OS2, M+, 100).
Ethyl 3-{[(6-Oxocyclohex-1-en-1-yl)methyl]sulfanyl}propano-
ate (3h). Yield: 65%. Yellow oil. 1H-NMR (300 MHz,
CDCl3): 6.94 (t, J = 3, 1 H); 4.12 (q, J = 9, 2 H); 3.32
(d, J = 3, 2 H); 2.76 – 2.71 (m, 2 H); 2.62 – 2.60 (m, 2 H);
2.46 – 2.44 (m, 4 H); 2.06 – 1.99 (m, 2 H); 1.27 (t, J = 6, 3
H). 13C-NMR (75 MHz, CDCl3): 198.2; 171.9; 147.1;
136.1; 60.6; 38.0; 34.3; 30.2; 26.8; 25.8; 22.7; 13.8. EI-MS:
82 (54), 110 (63), 141 (100), 197 (20), 242 (C12H18O3S,
M+, 19).
2-[1-(Phenylsulfanyl)ethyl]cyclohex-2-en-1-one (3i) [21].
Yield: 52%. Yellow oil. IR: 1670. 1H-NMR (300 MHz,
CDCl3): 7.34 – 7.19 (m, 5 H); 6.85 (t, J = 3, 1 H); 4.46 (q,
J = 7.1, 1 H); 2.42 (t, J = 7.5, 2 H); 2.35 – 2.29 (m, 2 H);
1.97 – 1.9 (m, 2 H); 1.34 (d, J = 7.1, 3 H). 13C-NMR
(75 MHz, CDCl3): 197.3; 145.7; 140.4; 135.0; 135.5; 128.6;
126.9; 39.0; 38.4; 25.9; 22.6; 20.8. HR-ESI-MS: 255.0814
([M + Na]+, C14H16NaOS+; calc. 255.0819).
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Received May 10, 2016
Accepted July 4, 2016
€
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