Iron-Catalyzed Intermolecular Hydrothiolation of Internal Alkynes with Thiosalicylic Acids, and Sequential Intramolecular Cyclization Reaction

Article abstract of DOI:10.1021/acs.orglett.7b01953

We demonstrate the iron-catalyzed intermolecular coupling of internal alkynes and thiosalicylic acid derivatives. The reaction was effectively catalyzed by the Fe(acac)₂/1,10-phenanthroline catalyst in toluene/HFIP (hexafluoroisopropyl alcohol)

Full text of DOI:10.1021/acs.orglett.7b01953

Letter
pubs.acs.org/OrgLett
Iron-Catalyzed Intermolecular Hydrothiolation of Internal Alkynes
with Thiosalicylic Acids, and Sequential Intramolecular Cyclization
Reaction
Taro Sonehara, Shogo Murakami, Sae Yamazaki, and Motoi Kawatsura*
Department of Chemistry, College of Humanities & Sciences, Nihon University, Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
S
* Supporting Information
ABSTRACT: We demonstrate the iron-catalyzed intermolecular
coupling of internal alkynes and thiosalicylic acid derivatives. The
reaction was effectively catalyzed by the Fe(acac)2/1,10-phenan-
throline catalyst in toluene/HFIP (hexafluoroisopropyl alcohol) as
the reaction solvent and afforded several types of 1,3-oxathiine
derivatives in moderate to high yields through the intermolecular
hydrothiolation and sequential intramolecular cyclization.
he transition-metal catalyzed hydrofunctionalization of
alkynes is one of the most versatile and atom economical
reaction was realized using a palladium catalyst for the terminal
alkynes.10 Based on this observation, to increase the yield of
3aa, we further investigated the Fe(acac)2/1,10-phen catalyzed
reaction of 1a with 2a in other solvents, such as dioxane,
T
reactions in the field of synthetic organic chemistry. The
hydrothiolation of alkynes is also an important reaction because
the reaction directly provides organosulfur compounds.1
Therefore, there are some reports about the metal-catalyzed
or metal-free hydrothiolation of alkynes,2−4 but most of the
reactions were accomplished for the terminal alkynes, and there
are still limited examples of the reaction of internal alkynes.3
Additionally, iron is one of the most abundant and environ-
mentally friendly metals on the earth, and iron was also
employed as a catalyst for the various types of hydro-
functionalizations of alkynes.5 However, to the best of our
knowledge, there is only one example of the iron-catalyzed
intermolecular hydrothiolation of alkynes.3c On the other hand,
we previously developed several types of iron-catalyzed
reactions6 or metal-catalyzed transformations of the trifluor-
omethyl group possessing internal alkynes.7 Based on this
background, we investigated the iron-catalyzed intermolecular
hydrothiolation of the trifluoromethyl group possessing internal
alkynes and revealed that the reaction with thiosalicylic acids
provided the 1,3-oxathiine derivatives by the intermolecular
hydrothiolation and sequential intramolecular cyclization.
We first examined the reaction of the aryl and trifluoromethyl
group substituted internal alkyne 1a8 with thiosalicylic acid
(2a) in toluene at 120 °C and confirmed that no reactions
occurred (Table 1, entry 1). To investigate this intended
hydrothiolation reaction, we next examined the reactions in the
presence of iron catalysts, such as FeCl3, FeCl2, Fe(acac)3,
Fe(acac)2, Fe(OAc)2, or Fe(OTf)2, with 1,10-phen (1,10-
phenanthroline) as a ligand and revealed that some of the iron
catalysts provided the hydrothiolated product 3aa9 (entries 2−
7). For example, we observed the high conversion of 1a (>98%
conv) in the reaction using Fe(acac)2 and the formation of 3aa
and 4aa10 in 72% and 9% NMR yields, respectively (entry 5).
To the best of our knowledge, there is only one report about
the synthesis of 1,3-oxathiine derivatives, such as 4aa, and the
i
methoxycyclopentane (CPME), PrOH, or hexafluoroisopropyl
alcohol (HFIP) (entries 8−11), and confirmed that, although
the conversion of 1a was slow (80% conv), the reaction in
HFIP gave the 1,3-oxathiine derivative 4aa in 69% NMR yield
without 3aa (entry 11). Although the exact role of the HFIP is
unclear, these results indicated that toluene is a suitable solvent
for the Fe(acac)2/1,10-phen catalyzed intermolecular hydro-
thiolation of 1a with 2a, and the intramolecular cyclization of
3aa to 4aa occurred in the HFIP solvent. Based on this
observation, we optimized the solvent for the selective
formation of 4aa with a high yield and revealed that the
reaction in the mixture of toluene and HFIP (1/1) afforded the
intended products 4aa in 83% NMR yield as the sole product
(entry 12). Furthermore, the highest yield (98% NMR yield,
90% isolated yield) was obtained when the reaction was
performed in toluene/HFIP (3/2) (entry 13).
With the optimal reaction conditions in hand, we examined
the reactions of the trifluoromethyl group substituted internal
alkynes 1b−z′′ with 2a, and the results are summarized in
Table 2. The reaction of the phenyl and trifluoromethyl group
substituted internal alkyne 1b with 2a gave the desired
trifluoromethyl group possessing 1,3-oxathiine derivatives 4ba
in 87% yield (Table 2, entry 1). The reactions of other alkynes,
such as 1c and 1d, which possess alkyl substituents on the
benzene ring, also afforded the intended products 4ca and 4da
in 89% and 98% yields, respectively (entries 2 and 3). On the
other hand, we confirmed that the reaction of the o-tolyl group
substituted alkyne 1e resulted in a low yield (26%), and we
observed the formation of hydrothiolated product 3ea in the
1H NMR of crude materials (entry 4). The reactions of 1f−h,
Received: June 26, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01953
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Iron-Catalyzed Reaction of 1a with 2a
Table 2. Iron-Catalyzed Reaction of Trifluoromethyl Group
Substituted Internal Alkynes 1b−z with 2a in Toluene/
a
HFIP
b
entry
1
yield (%) of 4
1
2
3
1b (R = Ph)
87 (4ba)
89 (4ca)
98 (4da)
26 (4ea)
80 (4fa)
83 (4ga)
82 (4ha)
87 (4ia)
50 (4ja)
58 (4ka)
64 (4la)
58 (4ma)
37 (4na)
39 (4oa)
42 (4pa)
<2 (4qa)
97 (4ra)
63 (4sa)
46 (4ta)
43 (4ua)
90 (4va)
74 (4wa)
72 (4xa)
<2 (4ya)
73 (4za)
77 (4z′a)
79 (4z″a)
b
1c (R = 3-MeC6H4)
1d (R = 4-tBuC6H4)
1e (R = 2-MeC6H4)
1f (R = 4-MeOC6H4)
1g (R = 3-MeOC6H4)
1h (R = 2-MeOC6H4)
1i (R = 4-FC6H4)
yield (%)
entry
[Fe]
solvent
toluene
3aa
4aa
c
4
1
−
<2
<2
16
39
72
46
<2
71
19
64
<2
<2
<2
<2
<2
<2
<2
5
5
6
7
8
9
2
FeCl3
toluene
3
FeCl2
toluene
4
Fe(acac)3
Fe(acac)2
Fe(OAc)2
Fe(OTf)2
Fe(acac)2
Fe(acac)2
Fe(acac)2
Fe(acac)2
Fe(acac)2
Fe(acac)2
Fe(acac)2
toluene
5
toluene
9
1j (R = 3-FC6H4)
c
6
toluene
9
10
1k (R = 2-FC6H4)
7
toluene
<2
<2
<2
<2
69
83
98 (90)
11
12
1l (R = 4-BrC6H4)
8
dioxane
1m (R = 3-BrC6H4)
1n (R = 2-BrC6H4)
1o (R = 4-CF3C6H4)
1p (R = 3-CF3C6H4)
1q (R = 2-CF3C6H4)
1r (R = 4-PhC6H4)
1s (R = 4-CF3OC6H4)
1t (R = 4-EtO2CC6H4)
1u (R = 1-naphthyl)
1v (R = 3,3-Me2C6H3)
1w (R = 3-Cl-4-MeC6H3)
1x (R = 2-thienyl)
c
9
CPME
iPrOH
13
c
10
11
12
13
14
HFIP
15
16
17
18
19
20
21
22
23
24
25
26
27
toluene/HFIP (1/1)
toluene/HFIP (3/2)
toluene/HFIP (3/2)
c
d
c
14
(85)
a
Reaction conditions: 1a (0.1 mmol), 2a (1.1 equiv), Fe(acac)2 (10
mol %), and 1,10-phen (20 mol %) in solvent (0.2 mL) at 120 °C for
b
16 h. Yields are determined by 1H NMR of crude materials using an
c
internal standard (phenanthrene). Isolated yield is shown in
d
parentheses. 1 mmol scale.
1y (R = 4-NO2C6H4)
1z (R = PhCH2CH2CH2)
1z′ (R = PhCH2CH2)
1z″ (R = C6H13)
which possess a methoxy group on the aromatic ring, gave the
desired products 4fa−ha in over 80% yields (entries 5−7). We
next investigated the reactions of alkynes 1i−n, possessing a
fluoro- or bromobenzene as a substituent, and obtained the
intended products 4ia−ma in the range of 50−87% yields
(entries 8−12), but the yield of 4na was low (entry 13).
Unfortunately, the yields of the reactions of 1o−q, which
possess a trifluoromethyl group on the benzene ring, were also
low, and the reaction of 1q with 2a did not provide any 1,3-
oxathiine derivatives (entries 14−16). We further examined the
reaction of several aryl and trifluoromethyl group substituted
internal alkynes 1r−w and confirmed that the reactions gave
the desired products 4ra−wa in the range of 43−97% yields
(entries 17−22). Furthermore, we demonstrated the reaction of
1x, which possesses a 2-thienyl group, and succeeded in
obtaining 4xa in 72% yield (entry 23), but we also confirmed
that the reaction of the p-nitrophenyl group substituted alkyne
1y resulted in no reaction (entry 24). It should be noted that
the present catalyst system allowed the reaction of the alkyl
group substituted internal alkyne 1z, 1z′, and 1z″, and we
obtained the corresponding product 4za, 4z′a, and 4z″a in
73%, 77%, and 79% yields, respectively (entries 25−27).
Based on the results for the reaction of the trifluoromethyl
group possessing internal alkynes, we next investigated the
reaction of other types of internal alkynes 5a−o, which possess
several substituents instead of the trifluoromethyl group, with
2a. As shown in Scheme 1, the reaction of ethyl 3-
arylpropiolate 5a−c, which possess an ester group instead of
the trifluoromethyl group, smoothly proceeded, and the
intended products 6aa−ca were obtained in the range 60−
a
Reaction conditions: 1b−z′′ (0.1 mmol), 2a (1.1 equiv), Fe(acac)2
(10 mol %), and 1,10-phen (20 mol %) in toluene/HFIP (3/2) (0.25
b
mL) at 120 °C for 16 h. Yields are isolated products by silica gel
chromatography. Reaction time: 48 h.
c
86%. We also examined the reactions of alkynes 5d and 5e,
which possess a ketone group, and succeeded in obtaining the
desired products 6da and 6ea in 55% and 67% yield,
respectively. Furthermore, we revealed that the reaction of
the internal alkyne 5f, which has an alkyl group instead of an
aryl group, smoothly proceeded and afforded the desired
product 6fa in 91% yield. The reaction of acetylene-
dicarboxylate (5g) with 2a provided the corresponding product
6ga in 78% yield, but the reactions of diphenylacetylene (5h)
or 1-phenyl-1-propyne (5i) resulted in no reaction. We further
examined the reactions of the terminal alkynes 5j−o with 2a,
and the results are also shown in Scheme 1. For the reactions of
phenylacetylene 5j and its analogues 5k−m, we confirmed that
the Markovnikov type product 6la was obtained in 61% yield,
but the reactions of other arylacetylenes 5j, 5k, and 5m resulted
in low yields and low regioselectivities. On the other hand, the
reactions of the propiolate esters 5n and 5o smoothly
proceeded in high yields, and the anti-Markovnikov adducts
6na (98%) and 6oa (96%) were obtained as a single
regioisomer, respectively.
B
DOI: 10.1021/acs.orglett.7b01953
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Iron-Catalyzed Reaction of 5a−o with 2a in
Scheme 2. Iron-Catalyzed Reaction of 1a or 5a with 2b−j in
a
a
Toluene/HFIP
Toluene/HFIP
a
Reaction conditions: 5a−o (0.1 mmol), 2a (1.1 equiv), Fe(acac)2 (10
mol %), and 1,10-phen (20 mol %) in toluene/HFIP (3/2) (0.25 mL)
at 120 °C for 16 h. Yields are isolated products by silica gel
chromatography
We next demonstrated the reactions of 1a or 5a with several
thiosalicylic acid derivatives 2b−j, and the results are
summarized in Scheme 2. For the trifluoromethyl group
substituted alkyne 1a, the reactions with 2g and 2h exhibited
slightly decreased yields, but other reactions with 2b−f and 2i
afforded the intended products 4ab−af and 4ai in good to high
yields (70−93%), respectively. Unfortunately, the reaction with
2-mercaptonicotinic acid (2j) gave the intended product 4aj,
but the yield was low (22%). We also examined the reactions of
ethyl 3-phenylpropionate (5a) with 2b−j and confirmed that
most of the reactions afforded the desired products in moderate
to high yields, although we confirmed that the yields were low
for 2g and 2j.
To elucidate the reactions pathway, we examined the
cyclization reaction of 3aa to 4aa under different reaction
conditions. As shown in Table 3, we confirmed that the
cyclization occurred when the HFIP was used as the solvent
(Table 3, entries 2 and 4), and the highest yield was obtained
when the 3aa was treated in HFIP without an iron catalyst
(entry 4). These results clearly indicated that the Fe(acac)2/
1,10-phen catalyzes the intermolecular hydrothiolation of
a
Reaction conditions: 1a or 5a (0.1 mmol), 2b−j (1.1 equiv),
Fe(acac)2 (10 mol %), and 1,10-phen (20 mol %) in toluene/HFIP
(3/2) (0.25 mL) at 120 °C for 16 h. Yields are isolated products by
silica gel chromatography
internal alkynes 1 with thiosalicylic acid analogues 2 in toluene,
and the intramolecular cyclization reaction of 3 to 4 proceeded
in HFIP.
In conclusion, we demonstrated the iron-catalyzed inter-
molecular coupling of alkynes with thiosalicylic acid and
revealed that the reaction provides 1,3-oxathiine derivatives
through the intermolecular hydrothiolation and sequential
intramolecular cyclization. We also investigated the reactions of
several types of alkynes and thiosalicylic acid derivatives and
succeeded in obtaining a variety of 1,3-oxathiine derivatives in
moderate to high yields. We further confirmed that the
intermolecular hydrothiolation was effectively catalyzed by
Fe(acac)2/1,10-phen, and HFIP promoted the intramolecular
cyclization reaction.
C
DOI: 10.1021/acs.orglett.7b01953
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
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entry
condition
solvent
3aa
4aa
1
2
3
4
−
−
toluene
HFIP
61
<2
88
<2
27
92
11
95
́
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HFIP
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no catalyst
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Reaction conditions: 3aa (0.1 mmol), Fe(acac)2 (10 mol %), and
1,10-phen (20 mol %) in toluene/HFIP (3/2) (0.25 mL) at 120 °C
for 16 h. Yields are isolated products by silica gel chromatography.
b
1
NMR yields were determined by H NMR of crude materials.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01953.
Experimental procedures and spectral data for the
products (PDF)
X-ray crystallographic data for compound 3aa (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kawatsur@chs.nihon-u.ac.jp.
ORCID
Motoi Kawatsura: 0000-0002-8341-6866
Notes
(5) Greenhalgh, M. D.; Jones, A. S.; Thomas, S. P. ChemCatChem
2015, 7, 190.
The authors declare no competing financial interest.
(6) (a) Kude, K.; Hayase, S.; Kawatsura, M.; Itoh, T. Heteroat. Chem.
2011, 22, 397. (b) Kawatsura, M.; Kajita, K.; Hayase, S.; Itoh, T.
Synlett 2010, 2010, 1243. (c) Kobayashi, J.; Matsui, S.; Ogiso, K.;
Hayase, S.; Kawatsura, M.; Itoh, T. Tetrahedron 2010, 66, 3917.
(d) Fujiwara, M.; Kawatsura, M.; Hayase, S.; Nanjo, M.; Itoh, T. Adv.
Synth. Catal. 2009, 351, 123. (e) Kawatsura, M.; Higuchi, Y.; Hayase,
S.; Nanjo, M.; Itoh, T. Synlett 2008, 2008, 1009. (f) Kawatsura, M.;
Komatsu, Y.; Yamamoto, M.; Hayase, S.; Itoh, T. Tetrahedron 2008,
64, 3488. (g) Kawatsura, M.; Komatsu, Y.; Yamamoto, M.; Hayase, S.;
Itoh, T. Tetrahedron Lett. 2007, 48, 6480. (h) Kawatsura, M.; Fujiwara,
M.; Uehara, H.; Nomura, S.; Hayase, S.; Itoh, T. Chem. Lett. 2008, 37,
794. (i) Itoh, T.; Uehara, H.; Ogiso, K.; Nomura, S.; Hayase, S.;
Kawatsura, M. Chem. Lett. 2007, 36, 50. (j) Uehara, H.; Nomura, S.;
Hayase, S.; Kawatsura, M.; Itoh, T. Electrochemistry 2006, 74, 635.
(7) (a) Ishikawa, T.; Sonehara, T.; Minakawa, M.; Kawatsura, M. Org.
Lett. 2016, 18, 1422. (b) Ishikawa, T.; Sonehara, T.; Murakami, S.;
Minakawa, M.; Kawatsura, M. Synlett 2016, 27, 2029. (c) Minakawa,
M.; Ishikawa, T.; Namioka, J.; Hirooka, S.; Zhou, B.; Kawatsura, M.
RSC Adv. 2014, 4, 41353. (d) Kawatsura, M.; Yamamoto, M.;
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ACKNOWLEDGMENTS
This work was supported by the Individual Research Expense
of College of Humanities and Sciences at Nihon University.
■
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DOI: 10.1021/acs.orglett.7b01953
Org. Lett. XXXX, XXX, XXX−XXX