Efficient synthesis of isothiochromene derivatives by Pd-catalyzed hydrothiolation reaction

Article abstract of DOI:10.1002/ejoc.201402611

An efficient method for the preparation of isothiochromene derivatives through the palladium-catalyzed intramolecular hydrothiolation reaction of halides and thiourea is reported. Two carbon-sulfur bonds are formed in this one-pot palladium-catalyzed cascade reaction. This new protocol is free from foul-smelling thiols and operates under mild conditions to give isothiochromene derivatives in excellent yields and with excellent 6-endo-dig selectivity. Copyright

Full text of DOI:10.1002/ejoc.201402611

FULL PAPER
DOI: 10.1002/ejoc.201402611
Efficient Synthesis of Isothiochromene Derivatives by Pd-Catalyzed
Hydrothiolation Reaction
Jie Feng,[a] Meifang Lv,[a] Guoping Lu,[a] and Chun Cai*[a]
Keywords: Heterogeneous catalysis / Synthetic methods / Palladium / Sulfur heterocycles / Cyclization / Isothiochromene
derivatives
An efficient method for the preparation of isothiochromene
derivatives through the palladium-catalyzed intramolecular
hydrothiolation reaction of halides and thiourea is reported.
dium-catalyzed cascade reaction. This new protocol is free
from foul-smelling thiols and operates under mild conditions
to give isothiochromene derivatives in excellent yields and
Two carbon–sulfur bonds are formed in this one-pot palla- with excellent 6-endo-dig selectivity.
not been explored extensively. Initially, compounds of this
Introduction
ring system were prepared by the insertion of an alkyne into
the metal–carbon bond of five-membered sulfur-containing
cyclopalladated complexes.[7] Later, 4-iodo-3-substituted
1H-isothiochromenes were synthesized through an electro-
philic cyclization reaction in moderate yields that used
iodine as the electrophilic source.[8] However, these methods
suffer from one or more drawbacks, such as a multi-step
procedure, low yields, long reaction times, and the use of
expensive and hazardous reagents. The synthesis of various
tellurium- or selenium-containing heterocycles, which bear
similar ring structures, by a base-catalyzed intramolecular
cyclization of the tellurol or selenol moieties has been re-
ported,[9] but there was no report on thiol intramolecular
cyclization reactions to synthesize 1H-isothiochromenes.
As for intramolecular cyclization reactions, there exist
two potential products: 6-endo-dig or 5-exo-dig products.
Many reports discuss the regioselectivity in intramolecular
hydroamination and hydrogenation reactions, but, in the
case of intramolecular hydrothiolation reactions, few refer-
ences involve regioselectivity.[9b] Thus, a general protocol
for the selective synthesis of 3-substituted 1H-isothio-
chromenes remains a challenge.
Sulfur-containing heterocyclic compounds commonly ex-
hibit biological activity, and hence are frequently found in
naturally occurring compounds.[1] Among them, isothio-
chromane and isothiochromene ring systems display a wide
range of interesting biological properties as structural
analogs of many bicyclic systems, such as chroman, iso-
chroman, tetrahydroisoquinoline etc., which are widespread
in natural products (Scheme 1).[2] Moreover, these com-
pounds have found important applications in total synthesis
for the preparation of a wide range of functionalized mol-
ecules.[3]
Scheme 1. Examples of sulfur-containing biologically active com-
pounds.
Recently, transition-metal-catalyzed cyclization reac-
tions, especially the sequential reactions that involve a
hydro-element addition, which relies on the catalytic acti-
vation of the C–C triple bond, followed by attack of an
internal nucleophile, has attracted significant interest be-
cause of its synthetic utility and become a rapid and power-
ful approach to prepare cyclic derivatives in organic synthe-
sis.[10] In the past few years, gold complexes, palladium
complexes and so on have emerged as powerful catalysts for
this purpose.[11] These cyclic processes have been success-
fully applied for selective product formation. Among them,
transition-metal-catalyzed hydrothiolation reactions are
well studied.[12] With the aim to selectively synthesize 6-
The approaches to synthesize isothiochromene deriva-
tives commonly include cyclization reactions,[4] free-radical
reactions,[5] and reactions with cyclic sulfur ylides.[6] Despite
a considerable number of reports on the synthesis of iso-
thiochromene derivatives, a survey of the literature revealed
that the synthesis of 3-substituted 1H-isothiochromenes has
[a] College of Chemical Engineering, Nanjing University of
Science & Technology
Address: 200 Xiaolingwei, Nanjing 210094, P. R. China
E-mail: c.cai@mail.njust.edu.cn
http://sce.njust.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402611.
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Efficient Synthesis of Isothiochromene Derivatives
endo-dig or 5-exo-dig isothiochromene derivatives, we re- (Table 1, Entry 5). Like intramolecular hydroamination and
port herein the use of palladium complexes to selectively hydrogenation reactions, both the 6-endo-dig and 5-exo-dig
synthesize isothiochromene derivatives through intramolec- products were observed in the hydrothiolation reaction. The
ular hydrothiolation reaction of 2-(phenylethynyl)benz- 6-endo-dig mode cyclization was preferred in the palladium-
ylthiol.
catalyzed case. Ligand and temperature were found to influ-
We tried to use other sulfur sources to replace thiol be- ence the selectivity between 6-endo and 5-exo products.
cause of its disagreeable smell. In continuation of our work With an appropriate ligand [Pd(dppf)Cl2 showed the best
on “thiolation” reactions, we focused our attention on the result here], 6-endo-dig products could be obtained selec-
synthesis of 3-substituted 1H-isothiochromene derivatives tively at room temperature. An increase in the reaction tem-
by using thiourea as the “sulfur” source. Thus, we designed perature sped up the reaction but decreased the selectivity
a cascade reaction through the in situ generation of S-alkyl- (Table 1, Entry 8). It is also worth mentioning that pincer-
isothiouronium salts in place of thiols,[13] which are formed palladium complexes survived the reaction with high yields
by halides and thiourea. The thiolation protocol and the and selectivities (Table 1, Entry 12). Other transition-metal
intramolecular hydrothiolation reaction can proceed in se- catalysts were also tested but gave unsatisfactory results
quence in one pot without the need for multi-step reactions. (Table 1, Entries 9–11).
Reaction conditions were then screened to establish the
optimum conditions. Different “sulfur” sources were ex-
Results and Discussion
plored. Sulfur and sodium thiosulfate were found to be inef-
fective for this reaction (Table 2, Entries 3 and 4). The use
of Na2S led to a series of byproducts (Table 2, Entry 2),
whereas excellent yields were observed in the presence of
thiourea (almost quantitative). Screening bases revealed
that stronger bases performed better under the reaction
conditions. The reaction was initially conducted in water;
however, the disulfide product was obtained. (Table 2, En-
try 8). Other solvents, such as toluene and acetonitrile, were
tested but none were suitable for this reaction (Table 2, En-
tries 9 and 10).
We began our studies by exploring catalysts for the intra-
molecular hydrothiolation reaction. Some commercially
available palladium catalysts and some other transition-
metal catalysts were initially screened in the intramolecular
hydrothiolation reaction of 2-(phenylethynyl)benzyl brom-
ide with thiourea (Table 1). No products were obtained in
the absence of base (Table 1, Entry 1). A small amount of
2-(phenylethynyl)benzyl bromide was converted only in the
presence of base (Table 1, Entry 2). The addition of palla-
dium catalyst accelerated the reaction (Table 1, Entries 3–8
and 12). Among them, Pd(dppf)Cl2 [dppf = 1,1Ј-bis(di-
phenylphosphino)ferrocene] showed the best results
Table 2. Optimum conditions of intramolecular hydrothiolation.[a]
Table 1. Catalyst selection for intramolecular hydrothiolation reac-
tions.[a]
Entry
Sulfur source
Base
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
9
thiourea
Na2S
K2CO3
K2CO3
K2CO3
K2CO3
NaOH
NaOAc
Et3N
K2CO3
K2CO3
K2CO3
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
H2O
96
52
0
23
97
32
0
0
11
46
S
Entry
Base
Catalyst
Yield [%][b]
6-endo/5-exo[c]
Na2S2O3
thiourea
thiourea
thiourea
thiourea
thiourea
thiourea
1
2
3
4
5
–
Pd(dppf)Cl2
–
Pd(OAc)2
PdCl2
Pd(dppf)Cl2
Pd(PPh3)2Cl2
0
–
–
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
11
64
56
98
83
75
96
0
95:5
91:9
100:0
96:4
90:10
64:36
–
78:22
–
99:1
toluene
CH3CN
6
7
10
[d]
K2CO3 Pd(amphos)Cl2
[a] Reaction conditions: halides (0.5 mmol), sulfur source
(1 mmol), base (0.5 mmol), Pd(dppf)Cl2 (0.005 mmol), solvent
(2 mL), room temp., 2 h. [b] Isolated yield of 6-endo-dig product.
8[e]
9
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Pd(dppf)Cl2
AgCF3SO3
CuI
10
11
12
42
0
99
AuCl3
pincer-Pd[f]
With the optimized conditions in hand, a series of alk-
ynylbenzyl bromides were chosen to establish the scope and
generality of the method (Table 3). Initially, the alk-
ynylbenzyl bromides were synthesized from the related
alcohols. o-Iodobenzyl alcohol and alkyne underwent Son-
ogashira coupling smoothly in the presence of Pd(PPh3)2Cl2
with Et3N as base and solvent simultaneously. The obtained
alkynylbenzyl alcohols were brominated with PBr3 to give
[a] Reaction conditions: halides (0.5 mmol), thiourea (1 mmol),
K2CO3 (0.5 mmol), catalyst (0.005 mmol), DMSO (2 mL; con-
tained a trace amount of water), room temp., 2 h. [b] Isolated yield.
[c] Determined by 1H NMR spectroscopy. [d] amphos = di-tert-
butyl[4-(dimethylamino)phenyl]phosphine. [e] The reaction was
conducted at 110 °C for 15 min. [f] Pd[(1E,1ЈE)-1,1Ј-(1,3-phenyl-
ene)bis(N-phenylmethanimine)]Br was used (see the Supporting In-
formation, chapter 3.1).
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J. Feng, M. Lv, G. Lu, C. Cai
FULL PAPER
the desired alkynylbenzyl bromides in good to excellent drawing and electron-donating substituents survived the re-
yields (Scheme 2).[14] The effect of the R1 group was also action conditions well to give the 6-endo-dig products selec-
investigated. Excellent yields were obtained when R1 was tively in most cases. Substituents that bore electron-with-
an aryl group. It is noteworthy that both electron-with- drawing groups showed higher yields (Table 3, Entry 5). As
Table 3. Substrate scope of the intramolecular hydrothiolation.[a]
[a] Reaction conditions: halides (0.5 mmol), thiourea (1 mmol), K2CO3 (0.5 mmol), Pd(dppf)Cl2 (0.005 mmol), DMSO (2 mL; contained
a trace amount of water), room temp., 2 h. [b] Isolated yield. [c] Determined by 1H NMR spectroscopy. [d] Yield of debromination
product.
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Efficient Synthesis of Isothiochromene Derivatives
expected, there is a slight decrease of the yield when hin-
dered R1 groups were used (Table 3, Entries 6 and 7). Inter-
estingly, a mixture of 5-exo-dig and 6-endo-dig products was
observed when R1 was an alkylalkyne. Yields of 29 and
60%, respectively, were obtained with 2-decynylbenzyl
bromide and thiourea (Table 3, Entry 11).
Scheme 2. Synthesis of alkynylbenzyl bromides.
Alkynylbenzyl bromides with different substituents on
the benzyl ring were tested. When R3 was a halogen or an
electronic-donating group, the corresponding alkynylbenzyl
bromides converted into isothiochromanes smoothly in
good to excellent yields (Table 3, Entries 12 and 13). How-
ever, when R3 was a nitro group, the debromination product
was formed unexpectedly in 45% yield. Likewise, we also
explored substituted α-substituted alkynylbenzyl bromides.
Unlike the hydroamination reaction reported by Santos,[15]
α-substituted groups did not appear to influence selectivity
between 6-endo-dig and 5-exo-dig products in this palla-
dium-catalyzed protocol (Table 3, Entries 8 and 9).
Scheme 4. Potential application of the protocol for the synthesis of
a bioactive lasofoxifene analogue.
To expand our methodology, we carried out the hydro-
genation reaction of the generated double bond to give rise
to isothiochromane derivatives. The reduction was easily ac-
complished by using a Pd/C-catalyzed hydrogenation pro-
cess with excellent yields (Scheme 3).
Scheme 3. Reduction of isothiochromenes.
Scheme 5. Proposed mechanism for the Pd-catalyzed hydrothiol-
ation reaction.
To highlight the potential of this methodology, isothio-
chromene 3a – a crucial intermediate for a lasofoxifene ana-
logue – was synthesized in a one-pot, odorless procedure
(Scheme 4), rather than by using foul-smelling thiol and
organolithium reagents in a multistep protocol as pre-
viously reported.[2f]
Conclusions
Finally, a proposed mechanism for the protocol is illus-
We have developed a method to synthesize 3-substituted
trated in Scheme 5. The reaction proceeds by in situ genera- 1H-isothiochromene derivatives by means of a palladium-
tion of an S-alkylisothiouronium salt, which is hydrolyzed catalyzed intramolecular hydrothiolation reaction. Excel-
to produce a thiolate moiety and urea. The generated thio- lent regioselectivity and product yields are obtained ef-
late ion and the palladium complex react to form Pd–S in- ficiently in the presence of Pd(dppf)Cl2 under mild condi-
termediate A, which can react with the alkyne to form pal- tions. Two C–S bonds are formed in the one-pot cascade
ladium hydride intermediate B. Reductive elimination yields reaction, and this newly developed procedure is free of foul-
the final products and regenerates the catalyst. The alkyne smelling thiols. In addition, the isothiochromene derivatives
may also be activated by the palladium complex, which is react easily to form the respective isothiochromane deriva-
not shown.
tives and other useful synthetic intermediates.
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J. Feng, M. Lv, G. Lu, C. Cai
FULL PAPER
Experimental Section
Acknowledgments
General Remarks: All the reagents were commercially available and
used without any further purification. Dimethyl sulfoxide (DMSO)
was used without drying. Alkynylbenzyl bromides were synthesized
according to literature procedures.[14] GC–MS analyses were per-
formed with an Agilent 7890A-5975C instrument (Column: DB-
5 MS). 1H NMR spectroscopic data was recorded with a Bruker
DRX 500, and tetramethylsilane was used as a reference. Elemental
analysis was performed with a Yanagimoto MT3CHN instrument.
We gratefully acknowledge the National Natural Science Founda-
tion of Jiangsu Province (BK 20131346) for financial support.
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General Procedure for the Synthesis of Alkynylbenzyl Bromides: To
a mixture of alkyne and o-iodobenzyl alcohol (10 mmol) in piper-
idine (20 mL) were added Pd(PPh3)2Cl2 (70.2 mg, 0.1 mmol). The
mixture was stirred at 70 °C under argon for 12 h. After that, cold
water (30 mL) was added to the mixture, and the resulting aqueous
mixture was extracted with diethyl ether (3ϫ 20 mL). The com-
bined organic extracts were washed with hydrochloric acid (3ϫ
20 mL), satd. aqueous NaHCO3 (20 mL), and then dried (Na2SO4).
The solvent was removed in vacuo. The red residual oil was purified
by silica gel chromatography (hexane/ethyl acetate, 90:10) to give
the pure alkynylbenzyl alcohol. To a mixture of the alkynylbenzyl
alcohol (5 mmol) and pyridine (0.51 g, 6.5 mmol) in chloroform
(10 mL) at 0 °C was slowly added phosphorus tribromide (1.5 g,
5.5 mmol) over 1 h. After the addition, the mixture was stirred at
room temperature for 12 h. Upon completion of the reaction, the
mixture was poured onto ice/water. The resulting aqueous mixture
was extracted with CH2Cl2 (3ϫ 20 mL), and the combined organic
extracts were washed with brine (2ϫ 20 mL) and then dried
(Na2SO4). After removal of the organic solvent in vacuo, the resid-
ual oil was purified by silica gel chromatography (n-hexane) to give
the pure benzyl bromide.
General Procedure for the Synthesis of the Isothiochromene Deriva-
tives:
A mixture of halide (0.5 mmol), thiourea (1.0 mmol),
Pd(dppf)Cl2 (0.005 mmol) and K2CO3 (0.5 mmol) in DMSO
(2.0 mL) was stirred at room temperature for 2 h. Upon completion
of the reaction, the mixture was extracted with diethyl ether (3ϫ
5.0 mL). The volatiles were removed in vacuo to afford the crude
product. Column chromatography on silica gel (hexane/ethyl acet-
ate, 99:1) afforded the pure desired product.
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General Procedure for the Reduction of the Isothiochromenes: A
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(1.0 mL) was introduced in a pressure reactor. The suspension was
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pad of Celite® and washed with ethyl acetate. Evaporation of the
solvents rendered compound 3j without further purification.
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General Procedure for the Synthesis of 4a: A mixture of 3a
(0.5 mmol) and K2CO3 (0.5 mmol) in CH3CN (1.0 mL) was stirred
at 0 °C. Then iodine was added slowly. The mixture was then stirred
at 80 °C for another 24 h. Upon completion of the reaction, the
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cle): General procedures, characterization data, and copies of the
1H NMR and 13C NMR spectra.
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Efficient Synthesis of Isothiochromene Derivatives
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Received: May 21, 2014
Published Online: July 15, 2014
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