Rh(ii)/phosphine-cocatalyzed synthesis of dithioketal derivatives from diazo compounds through simultaneous construction of two different C-S bonds

Article abstract of DOI:10.1039/c8cc01656a

Rhodium(ii)/phosphine-cocatalyzed bis-sulfuration of α-diazocarbonyl compounds using thiosulfonates as the sulfenylating agent, which provided two sulfur-containing moieties, was developed via simultaneous inter- and intra-molecular C-S bond formation. This novel protocol provides a rapid synthetic route to dithioketal derivatives in moderate to good yields in an atom-economic process. The transformation is proposed to proceed through phosphine ylide formation followed by S(O₂)-S bond cleavage and rearrangement.

Full text of DOI:10.1039/c8cc01656a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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Rh(II)/Phosphine-cocatalyzed Synthesis of Dithioketal Derivatives
from Diazo Compounds through Simultaneous Construction of
Two Different C-S Bonds
Received 00th January 20xx,
Accepted 00th January 20xx
Changqing Rao, Shaoyu Mai and Qiuling Song*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Previous work:
Rhodium(II)/phosphine-cocatalyzed
bis-sulfuration
of
α-
a) C-S bond formation through nucleophilic addition reactions
diazocarbonyl compounds using thiosulfonates as the
sulfenylating agent which provided two sulfur-containing moieties
was developed via simultaneous inter- and intra-molecular C–S
bonds formation. This novel protocol provides a rapid synthetic
route to dithioketal derivatives in moderate to good yields in an
atom-economic process. The transformation is proposed to
proceed through phosphine ylide formation followed by S(O2)–S
bond cleavage and rearrangement.
X
XH
R1 R2
SR
+
SR
R1 R2
b) C-S bond formation through electrophilic sulfenylation reactions
SR
+
EWG
SR
EWG
EWG
EWG
c) C-S bond formation through cross coupling reaction
S
R1SH or R1S
+
R-X
R1
R
The synthesis of organic sulfur compounds has attracted
significant attentions owing to the wide occurrence of sulfur
skeletons in natural and various biological systems.1
Meanwhile, sulfur-containing molecules could also serve as
important intermediates and ligands in organic synthesis.2 In
the past few decades, great efforts have been devoted to
develop new methods for the construction of C–S bonds.
Representative strategies include a) C-S bond-formation
through nucleophilic addition reaction (Scheme 1a);3 b) C-S
bond-formation through electrophilic sulfenylation reactions
(Scheme 1b);4 c) C-S bond formation through cross coupling
reaction (Scheme 1c).5 Despite these advances, simultaneous
construction of two C-S bonds has rarely been explored,
especially the two newly-built C-S bonds are on the same
carbon atom.6 In 1996, a synthesis of α-alkylsulfanyl-α-
phenylsulfonyl carboxylic esters via benzyltriethylammonium
chloride-catalyzed sulfanylation was achieved in a solid/liquid
phase transfer system.7 Given that dithioketal derivatives have
drawn much attention8 because of their unique motifs, and
this framework is embodied in many drug candidates,
including those used in cancer chemotherapy or as a nontoxic
oral hypocholesterolemic agent, therefore, it becomes highly
desirable to find a facile and efficient strategy to construct this
scaffold.9
X=I, Br, B(OH)2, ect
d) two C-S bonds formation on one carbon atom
SO2R1
OR
N2
R1O2S SR2
R1SO2SR2
Rh(II)/dppp
unknown
OR
R1SO2SR2
K2CO3, TEBA
known
OR
O
O
O
This work
Scheme 1. Patterns for the construction of C-S bonds
wide range of synthetically useful reactions including C-H12 and
X-H insertion reaction (X= N, O, S, P, Si),13 cyclopropanation
reactions14 and ylide formation.15 Recently, the metal-
catalyzed insertion of carbenoids into X–Y (X, Y= C, N, O, Si, S,
etc.) bonds has become
a powerful method for the
construction of versatile chemical frameworks.16 Inspired by
these works, we envision that diazo compounds could be right
starting points to build two C-S bonds simultaneously with a
specific disulfur compounds, namely, sulfenylating agent. At
this point, S-methyl benzenesulfonothioate catches our eyes,
since it contains two different types of sulfur atom and usually
it serves as thiolating reagent with benzenesulfonyl moiety as
a leaving group which was usually discarded as a waste. In the
past decades, diazo compounds have become a readily
available and widely employed precursor to access carbenes.
When it is subjected to S-methyl benzenesulfonothioate, two
different types of C-S bonds might be resulted simultaneously
on the same carbon atom.
Diazo compounds are versatile and powerful synthetic
building blocks that have been extensively explored in modern
synthetic organic chemistry and their rich chemistry has
attracted great interest.10 One of main reasons is that diazo
compounds can generate carbenes or metal carbenoids in
situ,11 which are reactive intermediates that could undergo a
In order to examine our hypothesis, diazoacetate 1a and S-
methyl benzenesulfonothioate (2a) were chosen as model
substrates. To our delight, we observed that when a 2:1
mixture
of
diazoacetate
1a
and
S-methyl
benzenesulfonothioate (2a) was catalyzed by 5 mol % of CuCl2,
α-methylsulfanyl-α-phenylsulfonyl carboxylic ester (3a) was
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obtained in 16% yield with 10 mol% tricyclohexyl phosphine as
cocatalyst in DCE at 70 oC for 12 h (Table 1, entry 1).
Subsequent metal screening suggested that Rh2(OAc)4 was the
optimal one among CuCl2, Cu(acac)2, Cu(OAc)2 , Pd(OAc)2, and
Pd2(dba)3 (see the Supporting Information). Temperature
examination revealed that higher temperature has deteriorate
effect on the reaction, and 25 oC gave the best yield of the
desired product 3a (Table 1, entries 4-5). Further fine tuning
on the loading of catalysts as well as the combination of
triphenylphosphine (PPh3), 1,4-bis(diphenylphosphino)butane
(dppb) and 1,3-bis(diphenylphosphino)propane (dppp) were
tested, and 1,3-bis(diphenylphosphino)propane (dppp)
appeared to be the best choice for the reaction (Table 1,
entries 7, 8–11).
Next, we proceeded to explore the generality of the
reaction under the optimized reaction conditions. As shown in
Scheme 2, a series of diazo compounds could be employed as
substrates in the reaction with 2a, affording the corresponding
dithioketal derivatives 3a-3l efficiently. It is noteworthy that
both electron-withdrawing and electron-donating groups
(fluoro, chloro, bromo, NO2, tBu, and methoxyl) on the
substrate 1a and 2a indicated that 1.5:1 of 1a:2a with 1 mol%
of Rh2(OAc)4 rendered the optimal result (Table 1, entries 6-8).
In view that the choice of cocatalyst is crucial for the success of
this transformation, several commonly used electron-rich
organophosphine compounds, such as tricyclohexyl phosphine,
Table 1. Optimization of the reaction conditions
aromatic ring of
1 were well tolerated under the reaction
conditions, and the position of substituents has little influence
on the efficiency of the transformation (Scheme 2, 3b-3c, 3f-
3k). Most remarkably, heteroaryl acetate derived diazo
compounds, such as benzothiophene was also compatible
under the standard conditions, leading to desired product 3l in
64% yield.
entry 1a
:
2a
Catalyst
[x mol%]
Cocatalyst T[oC]/[h] Yielda[%]
[10 mol%]
Furthermore, the scope of thiosulfonate derivatives were
investigated (Scheme 3). Not surprisingly,
a variety of
1
2
2:1
2:1
CuCl2/5
Cy3P
Cy3P
Cy3P
Cy3P
Cy3P
Cy3P
Cy3P
Cy3P
PPh3
dppb
dppp
70/12
70/12
70/12
70/12
25/12
25/3
16
75
79
80
94
89
89
71
25
82
94
thiosulfonates underwent the insertion smoothly and afforded
the target products in good yields.
Rh2(OAc)4/5
Rh2(OAc)4/2.5
Rh2(OAc)4/1
Rh2(OAc)4/1
Rh2(OAc)4/1
Rh2(OAc)4/1
Rh2(OAc)4/1
Rh2(OAc)4/1
Rh2(OAc)4/1
Rh2(OAc)4/1
3
2:1
Scheme 3. Scope of thiosulfonatesa
4
2:1
5
2:1
6
2:1
7
1.5:1
1:1
25/3
8
25/3
9
1.5:1
1.5:1
1.5:1
25/3
10
11
25/3
25/3
a
Reaction conditions: 2a (0.2 mmol), in DCE (2.0 mL), N2.
Isolated yields.
Scheme 2. Scope of diazo compoundsa
Rh2(OAc)4 (1 mol%)
dppp (10 mol%)
N2
O
S
O
S
CO2R1
PhSO2SMe
2a
+
CO2R1
DCE (2 mL), rt, N2
R
R
1
3
O
S
O
O
a
O
S
S
O
S
S
S
S
Unless otherwise noted, all reactions were run with 1a (0.3
2 (0.2 mmol), Rh2(OAc)4 (1 mol%), dppp (10 mol%),
S
O
O
O
CO2Me
CO2Me
CO2Bn
CO2Me
mmol),
DCE (2 mL) in a schlenk tube under N2 atmosphere at 25 oC for
3 h. b scale up to 5 mmol.
Cl
tBu
3a, 84%
3b, 82%
3c, 90%
3d, 85%
The easy availability of dithioketal derivative 3t, which was
obtained in 82% yield when scaled up to 5 mmol, opens new
synthetic opportunities for a suitable functionalization on the
S-methyl group due to its well-known reactivity upon different
O
S
O
S
O
S
S
O
S
S
S
O
S
O
O
O
CO2Et
CO2Et
CO2Et
CO2Et
MeO
Br
Br
OMe
3e, 94%
3f, 77%
conditions. For example, methyl-sulfonyl derivative
4 was
3g, 93%
3h, 95%
readily obtained in almost quantitative yield under oxidizing
condition with m-CPBA as an oxidant; meanwhile, a reductive
condition with LAH as reducing reagent directly
O
O
S
O
O
S
O
S
O
S
S
S
O
S
S
O
CO2Et
MeO
CO2Me
CO2Et
CO2Et
S
NO2
F
3j, 88%
3k, 72%
3l, 64%
3i, 44%
a Unless otherwise noted, all reactions were run with
1
(0.3
mmol), 2a (0.2 mmol), Rh2(OAc)4 (1 mol%), dppp (10 mol%),
DCE (2 mL) in a Schlenk tube under N2 atmosphere at 25 oC for
3 h.
Scheme 4. Further synthetic elaborations of 3t under different
redox conditions
2 | J. Name., 2012, 00, 1-3
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converted 3t into 2-tosyl-phenylacetate
5
,
with the dppp was mixed with diazo compound 1a and Rh2(OAc)4
(1:1:0.05) (Fig. 1c). This new resonance was assigned to a
substitution of MeS group by a hydride from LAH (Scheme 4).
In order to gain insights into the efficient S(O2)-S bond zwitterion intermediate. Moreover, when TsSMe was added to
insertion into the diazo compounds, a series of control the above solution of diazo compound 1a, dppp and Rh2(OAc)4,
reactions were carried out. First, in the absence of Rh (II) and the mixture was stirred for 3 hours, we can see two new
catalyst, desired product 3e was not detected (Scheme 5a). peaks at -17.5 ppm and 32.6 ppm shown up, undoubtedly, the
When the reaction was performed without co-catalyst dppp, peak at -17.5 ppm is the recovery of catalyst dppp, and the
no 3e was ever obtained (Scheme 5b) and one at 32.6 ppm represents the oxidized phosphine compound
the material was fully recovered. These two experiments that derived by dppp (Fig. 1d, 31P NMR), which has been
indicated that both Rh (II) catalyst and dppp play key roles for proven by the reaction between TsSMe and dppp only, in this
the success of this transformation. To further confirm that an reaction, 1,3-bis(diphenylphosphino)propane was fully
intramolecular-concerted process was involved in the oxidized into oxyphosphine (Fig. 1e, 31P NMR). It is of note that
transformation via the cleavage of S(O2)-S bond and
simultaneous insertion of carbene moiety, a crossover reaction
was run with two different thiosulfonates and only the
corresponding target products 3o and 3e were obtained (60%
and 85% yields respectively) and no crossover intermolecular
products 3o’ and 3e’ were ever detected (Scheme 5c).
Therefore this result ruled out the SN1 pathway, strongly
supporting a concerted cleavage-attack route. When the
reaction was performed with Wittig reagent
6 and
thiosulfonate 2a in the presence of KtOBu, no 3e was ever
obtained (Scheme 5d) and the material was fully recovered.
This experiment suggested that phosphine ylide itself could
not promote this transformation, therefore it further
underscored the importance of both Rh(II) salt and cocatalyst
1,3-bis(diphenylphosphino)propane (dppp) for the reaction.
Figure 1. 31P NMR and in CDCl3. (31P NMR-a ) dppp ; (31P NMR-
b) dppp:Rh2(OAc)4 =(1:1); (31P NMR-c) dppp: Diazo:Rh2(OAc)4
=0.2:1:0.05; (31P NMR-d) dppp:Diazo:TsSMe:Rh2(OAc)4
=0.2:1:1:0.05; (31P NMR-e) dppp: TsSMe= 1:1
O
O
N2
O
MeS
S
On the basis of previous reports as well as our own
experimental observations, a plausible mechanism for the
formation of 3e is depicted in Scheme 6. The formation of the
observed product is first initiated by the Rh(II)-catalyzed
SMe
dppp (10 mol%)
(a)
S
CO2Et
+
O
CO2Et
3e N.D
DCE (2 mL), 25 oC, 3 h
1e, 0.3 mmol
2a, 0.2 mmol
O
O
N2
decomposition of diazo-compound
corresponding Rh(II)-carbenoid intermediate
N2. Carbenoid subsequently reacts with dppp to form a
rhodium-containing phosphorus ylide , which might undergo
two reaction pathways (path and II). However, the
phosphorus ylide , which is generated from intermediate by
the release of Rh salt, can be excluded as the possible reaction
1
to afford the
O
SMe
MeS
Rh2(OAc)4 (1 mol%)
S
(b)
S
CO2Et
A
via the loss of
+
O
DCE (2 mL), 25 oC, 3 h
CO2Et
A
1e, 0.3 mmol
2a, 0.2 mmol
3e trace
B
O
O
O
O
O
S
SPh
I
SPh
SPh
S
S
O
CO2Et
C
B
CO2Et
N2
Rh2(OAc)4 (1 mol%)
dppp (10 mol%)
3o', N.D.
2o, 0.1 mmol
+
3o, 60%
(c)
CO2Et
O
O
S
O
O
+
DCE (2 mL), 25 oC, 3 h
O
SMe
MeS
intermediate by the control experiment d in Scheme 5 (path I).
MeS
S
S
1e, 0.3 mmol
O
CO2Et
CO2Et
MeS
2
SO2Ph
COOEt
2a, 0.1 mmol
3e', N.D.
PR3
COOEt
PR3
COOEt
3e, 85%
3e
Ph
Ph
O
O
Ph
O
S
O
C
SO2Ph
Rh
SMe
KtOBu (2 equiv)
MeS
P
S
(d)
MeS
Br-
CO2Et
path a
path I
R3P
+
O
DCE (2 mL), 25 oC, 3 h
Ph
O
CO2Et
PR3
S
O
2
MeS
SO2Ph
COOEt
Rh
EtO
E
Rh
O
MeS
b
COOEt
a
6, 0.3 mmol
2a, 0.2 mmol
3e N.D. with sm fully recovered
R3P
Rh
path II
EtO
B
MeS
A
O
O
3e
Scheme 5. Control experiments
R3P
S
O
EtO
D
N2
Rh2OAc4
path b
COOEt
To further elucidate the mechanism of such a dual-catalyst
system, we carried out several reactions with 31P NMR
spectroscopic analyses on the experimental processes (Fig. 1,
31P NMR). The 31P NMR spectroscopy studies provide further
supports for the assumed key catalytic active species in the
reaction. The cocatalyst dppp alone showed a 31P peak at -17.5
ppm. (Fig. 1a); the shift of this signal has not changed when
dppp was mixed with Rh2(OAc)4 (1:1) (Fig. 1b), indicating that
dppp does not serve as a ligand with Rh(II) salt, it should be a
cocatalyst. There is a new peak at 26.1 ppm appeared, when
F
1
Scheme 6. Proposed mechanism for the formation of 3e
In terms of Path II, as shown in the reaction details
D, two
possible pathways might be occurred leading to the desired
+
product. Given that the strong leaving ability of the PR3 motif
on intermediate
nucleophilic substitution reaction between the sulfur atom of
the thiosulfonate and intermediate , generating final
B, the sulfur ylide E is obtained via direct
2
B
product 3e through a Stevens-type rearrangement (path a). On
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Tetrahedron Lett. 1998, 39, 7113; c) W. Ando, Acc. Chem.Res.
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B. Wladislaw, L. Marzorati, N. F. Claro Jr, C. D. Vitta. Synthesis,
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the other hand, intermediate
B
attacks on the PhSO2-motif of
thiosulfonate
2 rendering intermediate F, which produces 3e
7
8
via the SMe 1, 2-migration and the departure of 1,3-
bis(diphenylphosphino)propane (path b).
In summary, we have developed an unprecedented
rhodium(II)-catalyzed direct sulfenylation reaction between
thiosulfonates with diazo compounds which simultaneously
build dual C-S bonds by sulfur ylide formation followed by
Thirupathi, H. Gao, C-H. Tunga, Z. Xu, Org. Chem. Front., 2018
Accepted Manuscript.
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9
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S(O2)–S bond cleavage and rearrangement.
A series of
dithioketal derivatives were obtained in moderate to high
yields by the metal-catalyzed insertion of carbenoids into
S(O2)–S. Most importantly, the substrate scope was extended
by successful execution of α-diazocarbonyl compounds and
thiosulfonates as suitable substrates for this transformation.
The present protocol provides a versatile method to establish
two important functional groups which are key structural
elements and are frequently encountered in many natural
products and biomolecules.
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ACKNOWLEDGMENT
Financial support from the National Natural Science
Foundation (21772046), the Natural Science Foundation of
Fujian Province (2016J01064), the Recruitment Program of
Global Experts (1000 Talents Plan), Program of Innovative
Research Team of Huaqiao University (Z14X0047), the
Graduate Innovation Fund of Huaqiao University (to C. Rao).
We also thank Instrumental Analysis Center of Huaqiao
University for analysis support.
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