Palladium-catalyzed intramolecular oxidative C-H sulfuration of aryl thiocarbamates

Article abstract of DOI:10.1002/adsc.201400306

A palladium-catalyzed intramolecular C-H bond sulfuration reaction of aryl thiocarbamates has been developed. This strategy provides a new route to benzo[d][1,3]oxathiol-2-ones with tolerance of a wide range of substituents. Mechanistic studies suggested

Full text of DOI:10.1002/adsc.201400306

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DOI: 10.1002/adsc.201400306
À
Palladium-Catalyzed Intramolecular Oxidative C H Sulfuration
of Aryl Thiocarbamates
Yingwei Zhao,a Yinjun Xie,a Chungu Xia,a,* and Hanmin Huanga,*
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou 730000, Peopleꢀs Republic of China
Fax : (+86)-931-496-8129; e-mail: cgxia@licp.cas.cn or hmhuang@licp.cas.cn
Received: March 26, 2014; Revised: April 20, 2014; Published online: July 17, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400306.
À
Abstract: A palladium-catalyzed intramolecular C
H bond sulfuration reaction of aryl thiocarbamates
has been developed. This strategy provides a new
functional group exchange, leading to inefficient syn-
thetic procedures with poor atom and step economy
and waste generation.[6] A streamlined synthetic ap-
proach to this structural motif would rely on a catalyt-
ic system capable of the direct intramolecular sulfura-
route to benzo[d]ACTHNUTRGNEUNG[1,3]oxathiol-2-ones with tolerance
of a wide range of substituents. Mechanistic studies
suggested that the C H activation–sulfuration to
À
À
tion of aromatic C H bonds. Inspired by the recent
À
afford
2-imino-1,3-benzoxathiole
intermediate
progress on the transition metal-catalyzed C S bond
might involve an electrophilic palladation process.
formation reactions,[7,8] herein, we disclose a palladium
catalyst capable of catalyzing the intramolecular sul-
furation with aryl thiocarbamates leading to benzo[d]-
À
Keywords: benzo[d]
vation; palladium; sulfuration; thiocarbamates
[1,3]oxathiol-2-ones; C H acti-
À
À
ACHTUNGTREN[NUNG 1,3]oxathiol-2-ones via cleavage of C H and C N
bonds.[9] To the best of our knowledge, the reaction
described herein is the first example for the synthesis
À
of such heterocycles via C H bond activation, which
provides a general and flexible approach to a wide
The benzo[d]
N
ACHTUNGERTN[NUNG 1,3]oxathiol-2-ones.
tant structural motif, which exists in a variety of bio-
logically active compounds, pharmaceuticals such as l,3-benzoxathiole salt intermediate (A) has been pro-
antipsychotic[1] and antibacterial drugs,[2] neuroprotec- posed as a useful precursor for the synthesis of
tive agents,[3] and antioxidants[4] (Figure 1). In addi- benzo[d]
[1,3]oxathiol-2-ones via acid-mediated C N
À
tion, it has also been used as an unique building block bond cleavage (Scheme 1). However, for such inter-
for organic synthesis.[5] Current strategies for the con- mediates, there is hardly any catalytic generation pro-
struction of such compounds have largely relied on cess that is accessible. The conventional strategy
toward such a key intermediate generally relied on
some non-catalytic procedures such as Michael addi-
tion of thioureas to benzoquinones[11] or Cu-mediated
thiocyanation of resorcinols,[1,10] which are significant-
ly limited not only by the generally harsh reaction
conditions, but also by the inherent substitution pat-
tern of the substrates imposed by the electronic and
steric properties required by these reactions. Recent
progress on transition metal-catalyzed intramolecular
[8]
À
C H sulfuration together with our continuing stud-
[12]
À
À
ies on the area of C H bond and C N bond activa-
tion[13] have prompted us to envisage that aryl thiocar-
bamates might be potentially ideal substrate candi-
À
dates for C H sulfuration, since they have been
widely utilized as useful starting materials for the syn-
thesis of benzenethiols via the Newman–Kwart rear-
rangement (NKR) reactions.[14]
Figure 1. Important compounds containing the benzo[d]-
ACHTUNGTRENNUNG[1,3]oxathiol-2-one motif.
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Scheme 1. Synthetic routes to benzo[d]ACTHNUTRGNEUG[N 1,3]oxathiol-2-one via 2-imino-1,3-benzoxathiole intermediate.
We anticipated that a thioenolate would be gener- mate (1a) was recovered when the reaction was con-
ated as a directing group to catch a metal species and ducted in the absence of acetic acid.
À
selectivity to deliver the catalyst to the proximal C H
bond. After cleavage of the aryl C H bond under important for the C N bond cleavage, but also plays
These results revealed that acetic acid is not only
À
À
À
suitable conditions, the cyclometalated intermediate an important role for promoting the C H bond cleav-
B might be formed, which is converted into the key age. Other commonly used carboxylic acids such as
intermediate A by reductive elimination.
propionic acid and trifluoroacetic acid, were also used
To test our above mentioned proposal, the model instead of acetic acid, but they proved to be ineffi-
cyclization of O-phenyl N,N-dimethylthiocarbamate cient (Table 1, entries 7 and 8). The catalytic amount
(1a) was used to identify the optimal reaction condi- of TsOH·H2O has
tions. Initially, the catalytic system containing entry 9), which can be explained in terms of increas-
Pd(OAc)2, benzoquinone (BQ), acetic acid and a cata- ing electrophilicity of the Pd(II) center by replace-
a beneficial effect (Table 1,
ACHTUNGTRENNUNG
lytic amount of para-methylbenzenesulfonic acid
(TsOH·H2O) was employed in this model reaction. To
our delight, we found that under these conditions the
substrate was converted to the desired benzo[d]-
Table 1. Effect of reaction parameters.[a]
ACHTUNGTRENNUNG[1,3]oxathiol-2-one (2a) smoothly when the reaction
was performed at 1008C for 12 h (Table 1, entry 1).
Ligand screening demonstrated that the best result
was obtained in the absence of ligands, which is in
sharp contrast to the previous reports that an excess
of phosphine ligands is generally required for sustain-
ing the catalytic cycle by suppressing the catalyst poi-
soning.[15] A control experiment showed that no reac-
tion occurred in the absence of the Pd catalyst (see
the Supporting Information). Other terminal oxidants
Entry Oxidant
Acid
T [8C] Yield [%][b]
1
BQ
CH3CO2H
100
100
100
100
100
100
70
0
0
0
0
trace
18
0
49
18
75
77
2
Cu
O2
Na2S2O8
BQ
ACHTNUGNERTN(UNG OAc)2·H2O CH3CO2H
3[c]
4
CH3CO2H
CH3CO2H
–
5
6
BQ
CH3CO2H[d]
such as CuACHTUNGTRENNUNG(OAc)2·H2O, Na2S2O8 and molecular
7
BQ
CH3CH2CO2H 100
oxygen were less efficient (Table 1, entries 2–4). The
8
BQ
BQ
BQ
BQ
CF3CO2H
CH3CO2H
CH3CO2H
CH3CO2H
CH3CO2H
100
100
60
À
acetic acid was believed to the promote the C N
9[e]
10
11
12
bond cleavage of 2-iminium-1,3-benzoxathiole salt in-
termediate (A) as well as act as an oxygen source to
deliver the final product with the elimination of the
dimethylamino moiety. A few control experiments
were carried out to gain a better understand of this
process. Control reactions in the absence of acetic
acid or in the presence of 1.2 equivalents of acetic
acid failed to give the desired product (Table 1, en-
tries 5 and 6). Furthermore, over 90% of thiocarba-
120
150
BQ
[a]
All the reactions were performed on a 1-mmol scale in
sealed tubes. 10 mol% of TsOH·H2O additive, 1.5 mL of
carboxylic acid and 1 mL of toluene were used.
Isolated yield.
1 atm of oxygen.
1.2 mmol of acetic acid were used.
No TsOH·H2O was added.
[b]
[c]
[d]
[e]
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Palladium-Catalyzed Intramolecular Oxidative C H Sulfuration
ment of its coordinating anion, resulting in faster met- ortho-sulfuration product 2g in 83% yield, which was
[16]
À
alation of the aromatic C H bond. Higher temper- confirmed by a single crystal X-ray diffraction analy-
ature results in a slightly higher yield (Table 1, en- sis[17] (see the Supporting Information), while the C
tries 11 and 12) and lowering of the reaction tempera- H activation did not happen on the 2-phenyl group.
À
ture to 608C led to a sluggish reaction (Table 1, The meta-methoxy-substituted precursor cyclized to
entry 10).
give a mixture of 4-substituted product 2i and 6-sub-
With the optimized reaction conditions in hand, we stituted product 2i’ in 92% total yield with 3.6:1 regio-
next explored the scope and generality of this process selectivity. Similarly, 2-naphthyl thiocarbamate also
(Scheme 2). A variety of substituted aryl thiocarba- reacted to afford a mixture of 2k and 2k’. Both exam-
mates 1, which could be readily prepared from the ples indicate that the cyclization tends to take place
condensation of corresponding phenols with dime- at the side of less steric hindrance. Compared to the
thylthiocarbamoyl chloride, was subjected to the opti- electron-rich thiocarbamates, the electron-deficient
mized reaction conditions. All of these aryl thiocarba- thiocarbamates exhibited lower reactivity. For the thi-
mates with electron-donating substituents such as ocarbamates containing electron-withdrawing groups
alkyl and methoxy groups on the phenyl ring gave on the para position of the phenyl ring, the reactivity
high yields, irrespective of whether the substituent could be efficiently enhanced by increasing the reac-
was on the ortho or para position (2b–2f). The reac- tion temperature (2l, 2m, 2n, and 2o). However, in
tion of 2-phenyl-substituted substrate 1g led to the the case of ortho-substituted substrates 2q, 2r and 2s,
higher temperature led to lower yield. It was notewor-
thy that halide substituents (Br and Cl) were tolerat-
ed, as this is advantageous for further transformations
with transition metal catalysis. In addition, a series of
other functional groups, such as ester (2n), nitro (2m),
aldehyde (2q), ketone (2o and 2s), acetamide (2p),
were also tolerated. The observed electron effect is
consistent with a mechanism of electrophilic pallada-
À
tion which have been observed in many oxidative C
H functionalization reactions.[8]
To get some insight into the mechanism of the pres-
ent reaction, some control experiments were carried
out under the standard reaction conditions
(Scheme 3). A stoichiometric amount of PdACHTUNGTRENNUNG(OAc)2
was allowed to react with 1a under the standard con-
ditions in the absence of BQ [Eq. (1)]. A yield of
66% of 2a could be obtained, which indicated that
a Pd(II)-Pd(0) cycle might be involved in this reac-
tion.
In addition, S-phenyl dimethylcarbamothioate (3a)
could not transfer to the same product 2a under the
standard conditions [Eq. (2)]. This result indicates
Scheme 2. Substrate generality. All the reactions were per-
formed on a 1-mmol scale in sealed tubes without the extru-
sion of air. 10 mol% of TsOH·H2O additive, 1.5 mL of car-
boxylate acid and 1 mL of toluene were used. Values in pa-
renthesis indicate the yield of the reaction carried out at
1508C.
Scheme 3. Control experiments.
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that the Pd(0) promoted Newman–Kwart rearrange-
ment process, which was reported by Lloyd-Jones,[14b]
is not involved in our catalytic system. Moreover, the
free radical mechanism can be ruled out because this
reaction is not affected by a radical scavenger [62%
yield was still obtained in the presence of TEMPO,
Eq. (3)].
On the basis of these results, a tentative reaction
pathway involving a Pd(II)-Pd(0) redox catalytic cycle
is proposed as shown in Scheme 4. The thioenolate
was formed in the presence of acid, which reacted
with PdACHTUNGTRENNUNG(OAc)2 to furnish the palladium-S-enolate
species C. Electrophilic palladation took place to give
the cyclopalladium species B via cleavage of the aryl
À
C H bond. Subsequent reductive elimination of B re-
leased the key iminium salt A and Pd(0) which can be
reoxidized by BQ in the presence of acetic acid to fur-
nish the catalytic cycle. An intramolecular O–N ex-
change reaction occurred in the intermediate A to de-
liver the desired product 2a and DMA via C N bond
cleavage.
Figure 2. The reaction profile monitored by in situ IR.
À
To probe the feasibility of the pathway and detect which corresponded to the mass of [AÀOAc]+. An-
the formation of 2-iminium-1,3-benzoxathiole salt A, other peak at m/z=262.0508 was also detected, corre-
we attempted to use in situ IR to monitor the stan- sponding to the mass of [D+Na]+. Moreover,
dard reaction. A reaction profile was obtained in the a strong peak at m/z=285.9525, assigned to the mass
cyclization of O-phenyl N,N-dimethylthiocarbamate of cyclometalated Pd(II) complex [BÀOAc]+, was
(1a) under the standard conditions (Figure 2). Within also detected in the reaction of 1a promoted by stoi-
15 min, O-phenyl N,N-dimethylthiocarbamate (1a) chiometric PdACTHNURGTNEUNG(OAc)2 at room temperature, which re-
À
was detected to be consumed to more than 80% and vealed that the C H cleavage can proceed even at
with the concomitant disappearance of 1a, the room temperature (see the Supporting Information).
benzo[d]ACHTUNGTRENNUNG[1,3]oxathiol-2-one (2a) started to be pro- These above results support that the intermediates A,
duced slowly and an induction period was observed, B and D are most likely involved in the course of this
thus implying that an intermediate existed in the cyclization reaction.
course of this cyclization reaction, but outside of this
The kinetic isotope effect (kH/kD =2.7) was ob-
Pd-mediated catalytic cycle. Indeed, the HR-MS served by comparison with the consumption rates of
À
(ESI) analysis of the crude reaction mixture of the 1a and 1a-d5, suggesting that C H bond cleavage is
standard reaction showed a peak at m/z=180.0474, the rate-determining step for the Pd-mediated catalyt-
Scheme 4. Plausible reaction mechanism.
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Palladium-Catalyzed Intramolecular Oxidative C H Sulfuration
À
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analysis of the crude solution of the standard reaction
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Experimental Section
General Procedure
To a 25-mL dried Young-type tube were added aryl thiocar-
bamate 1 (1 mmol), PdACHTNUGTRNEUNG(OAc)2 (0.05 mmol, 11.2 mg), benzo-
quinone (1.1 mmol, 119 mg), para-methylbenzenesulfonic
acid (0.1 mmol, 19 mg), acetic acid (1.5 mL), and toluene
(1 mL). Then the Young-type tube was sealed and the re-
sulting mixture was stirred at 1208C [caution!] for 12 h.
After cooling to room temperature, the reaction mixture
was concentrated under reduced pressure and the residue
was purified by chromatography on silica gel with ethyl ace-
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We gratefully acknowledge the National Natural Science
Foundation of China (21203222, 21133011 and 21173241).
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