Two types of rhodium-catalyzed CS/CS metathesis reactions: formation of CS/CS bonds and CC/SS bonds

Article abstract of DOI:10.1016/j.tetlet.2008.01.040

A rhodium complex catalyzes two types of single bond metathesis reactions of two CS bonds depending on the added ligand: CS/CS to CS/CS metathesis and CS/CS to CC/SS metathesis. In the presence of a catalytic amount of RhH(PPh₃)₄ and 1,1′-bis(diphenylphosphino)ferrocene (dppf), two 1-alkylthioalkynes exchange alkylthio groups to give equilibrium mixtures of four 1-alkylthioalkynes. When tris(p-methoxyphenyl)phosphine or diphenylmethylphosphine is used, 1,3-butadiynes are obtained.

Full text of DOI:10.1016/j.tetlet.2008.01.040

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Tetrahedron Letters 49 (2008) 1593–1597
Two types of rhodium-catalyzed CS/CS metathesis reactions:
formation of CS/CS bonds and CC/SS bonds
Mieko Arisawa a, Yoko Tagami a, Masahiko Yamaguchi a,b,
*
a Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba, Sendai 980-8578, Japan
b WPI Advanced Institute for Material Research, Tohoku University, Aoba, Sendai, Japan
Received 3 December 2007; revised 26 December 2007; accepted 10 January 2008
Available online 15 January 2008
Abstract
A rhodium complex catalyzes two types of single bond metathesis reactions of two CS bonds depending on the added ligand: CS/CS
to CS/CS metathesis and CS/CS to CC/SS metathesis. In the presence of a catalytic amount of RhH(PPh3)4 and 1,10-bis(diphenylphos-
phino)ferrocene (dppf), two 1-alkylthioalkynes exchange alkylthio groups to give equilibrium mixtures of four 1-alkylthioalkynes. When
tris(p-methoxyphenyl)phosphine or diphenylmethylphosphine is used, 1,3-butadiynes are obtained.
Ó 2008 Elsevier Ltd. All rights reserved.
We propose the use of transition metal catalyzed reac-
tions, in particular single bond metathesis reactions, for
the efficient synthesis of organosulfur compounds.1 Several
such reactions have been developed: SS/SS metathesis for
disulfide exchange;2 CH/SS metathesis of 1-alkynes to give
1-alkylthioalkynes;3 CS/SS metathesis for the alkylthio
exchange of 1-alkylthioalkynes;3 SS/HH metathesis to
reduce disulfides to thiols;4 PP/SS metathesis to give
thiophosphinates;5 PS/SS metathesis for the alkylthio
exchange of thiophosphinates.5 In order for such reactions
to proceed effectively, the catalyst must be able to cleave
two single bonds and to exchange the combination of the
groups.
The above reactions using organic disulfides for one of
the substrates give products of a single metathesis mode.
In contrast, the single bond metathesis of CS/CS bond
can undergo two modes of the reaction, CS/CS to CS/
CS metathesis and CS/CS to CC/SS metathesis. Control
of the reaction mode using a single metal complex is an
interesting subject, which leads to the understanding on
the properties of the catalysts, and provides novel syn-
thetic method for the CC bond formation. Described
herein is that a rhodium complex in the presence of differ-
ent phosphine ligands catalyzes either the alkylthio
exchange reaction of 1-alkylthioalkynes or the coupling
reaction to give 1,3-butadiyne, which are CS/CS to CS/
CS metathesis and CS/CS to CC/SS metathesis, respec-
tively (Scheme 1). Although symmetrical 1,3-butadiynes
are readily prepared from 1-alkynes by copper catalyzed
oxidation reaction,6 a notable aspect of the present reac-
tion is to provide a method for catalytic CC bond forma-
tion using organosulfur compounds liberating disulfides.
The change of single bond formation mode by ligand in
the transition metal reaction has a few precedents in the
reductive elimination of palladium(IV) complex.7 For
example, depending on the ligand and substituent,
PdBrMe2(CH2Ar)(L2) complexes give ethane or aryl-
ethane. Its application to catalytic reaction as indicated
in this study may be important.
CS/CS to
+
+
C
C
S
C
C
S
S
S
CS/CS metathesis
+
C
S
C S
CS/CS to
CC/SS metathesis
*
Corresponding author. Tel.: +81 22 795 6812; fax: +81 22 795 6811.
Scheme 1.
E-mail address: yama@mail.pharm.tohoku.ac.jp (M. Yamaguchi).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.01.040

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M. Arisawa et al. / Tetrahedron Letters 49 (2008) 1593–1597
It was previously found in our laboratory that a
RhH(PPh3)4 (1 mol%)
rhodium complex derived from RhH(PPh3)4 and 1,10-
bis(diphenylphosphino)ferrocene (dppf) was capable to
cleave CS bond of 1-alkylthioalkynes:3 the reaction of a
1-alkylthioalkyne and a thiol gave a 1-alkyne and an alkyl-
thio exchanged 1-alkylthioalkyne; the reaction of a 1-alkyl-
thioalkyne and a disulfide gave an alkylthio exchanged
1-alkylthioalkyne. It was therefore considered that alkyl-
thio exchange of 1-alkylthioalkynes could take place by
the treatment of two 1-alkylthioalkynes in the presence
of the rhodium complex. When a mixture of 1-butylthio–
2-triisopropylsilylethyne 1, and 1-cyclohexylthio–2-tri-
ethylsilylethyne 2 in acetone was heated at reflux for 2 h
in the presence of RhH(PPh3)4 (1 mol %) and dppf
(2 mol %), a mixture of 1-cyclohexylthio–2-triisopropyl-
silylethyne 3 (37%), 1-butylthio-2-triethylsilylethyne 4
(29%), 1 (31%), and 2 (39%) was formed (Scheme 2). The
ligand dppf is essential in the alkylthio exchange reaction,
and no reaction occurs in its absence. This is an equili-
brium giving comparable amounts of the possible prod-
ucts. The alkylthio exchange reaction of 1-hexylthio-2-
(2,4,6-trimethylphenyl)ethyne 5 and 1 was slower, and a
mixture of 1-alkylthioalkynes containing 1-hexylthio-2-tri-
isopropylsilylethyne 6 (40%), 1-butylthio-2-(2,4,6-trimethyl-
phenyl)ethyne 7 (41%), 1 (43%), and 5 (49%) was formed
after 8 h. It is shown that RhH(PPh3)4 and dppf catalyze
the cleavage of two CS bonds and alkylthio exchange reac-
tion, CS/CS to CS/CS metathesis.
(p-MeOC6H4)3P (3 mol%)
+
SiR3
(R'S)2
R3Si
SR'
R3Si
acetone, refl., 2 h
4: R = Et, R' =
1: R = -Pr, R' =
2: R = Et, R' =
n
-C4H9
-C4H9
-C6H11
8
9
8
74%
73%
71%
i
n
c
Scheme 3.
(Scheme 3). Analogously, 1 was converted to the corres-
ponding diyne 9 in 73% yield. The alkylthio group could
be secondary cyclohexyl as shown by the example of 2 to
give 8. The use of electron-rich monodentate phosphine
was essential, and the use of dppf (4%), dppe (not
detected), and dppb (trace) gave a very small amount of
dimer 8. Monodentate phosphine ligands, Ph3P (51%)
and (p-ClC6H4)3P (43%), were less effective.
The ligand effect in the dimerization of aryl substituted
1-alkylthioalkynes is slightly different from the silylalkynes.
When 1-hexylthio-2-(2,4,6-trimethylphenyl)ethyne 5 was
treated with RhH(PPh3)4 (5 mol %) and MePh2P
(15 mol %) in refluxing acetone for 12 h, dimer 10 was
obtained in 56% yield (Scheme 4). The ligand effect follows:
(p-MeOC6H4)3P, 40% (reaction time, 9 h); {2,4,6-
(MeO)3C6H2}3P, 30% (9 h); (p-ClC6H4)3P, 9% (2 h);
EtPPh2, 57% (12 h); Me2PPh, 8% (12 h); Bu3P, 22% (9 h);
(2-furyl)3P, 7% (9 h); without phosphine, 16% (12 h). The
1-alkylthioalkynes required bulky aromatic groups for
effective dimerization, and the reaction of 1-hexylthio-2-
phenylethyne gave various products without forming 1,4-
diphenyl-1,3-butadiyne. The bulky substituents may be
retarding the oligomerization of alkynes. It was confirmed
that this reaction was not reversible, and the reaction of 10
and dioctyl disulfide in the presence of RhH(PPh3)4
Next examined was the other mode of the metathesis,
CS/CS to CC/SS metathesis. The reaction of 1-butylthio-
2-triethylsilylethyne 4 in refluxing acetone with RhH-
(PPh3)4 (1 mol %) and (p-MeOC6H4)3P (3 mol %) for 2 h
gave 1,4-bis(triethylsilyl)-1,3-butadiyne 8 in 74% yield,
which was accompanied by dibutyl disulfide in 54%
RhH(PPh3)4 (1 mol%)
dppf (2 mol%)
+
i-Pr3Si
S(n-C4H9)
Et3Si
S(c-C6H11
)
acetone, refl., 2 h
1
2
i
-Pr3Si
S(c-C6H11)
+
1 31%
+
Et3Si
S(n-C4H9)
2 39%
+
3 37%
4 29%
Me
Me
RhH(PPh3)4 (1 mol%)
dppf (2 mol%)
S(n-C6H13)
i-Pr3Si
S(
n-C4H9)
Me
+
acetone, refl., 8 h
1
5
Me
+
1 43%
+
5 49%
Me
+
S(n-C4H9)
i
-Pr3Si
S(n-C6H13
)
Me
6 40%
7 41%
PPh2
PPh2
Fe
dppf
Scheme 2.

M. Arisawa et al. / Tetrahedron Letters 49 (2008) 1593–1597
1595
R1
R1
R1
R1
RhH(PPh3)4 (5 mol%)
MePh2P (15 mol%)
R2
R2
R2
S(n-C6H13)
acetone, refl., 12 h
R1
R1
5: R1 = R2 = Me
10 56%
49%
R1 = Me, R2 = H
R1 = R2 =
i-Pr
50%
Scheme 4.
(5 mol %) and MePPh2 (15 mol %) in 1,3-dimethyl-2-imid-
azolidinone (DMI) at 132 °C for 12 h resulted in the
recovery of 10 in 93% yield. The irreversibility was further
confirmed by the lack of crossover in the reaction of 8 and
10 under the conditions.
The dimerization of alkyl substituted 1-alkylthioalkynes
required higher temperatures. When 1-hexylthio-2-(1-
methoxycyclohexyl)ethyne 11 was treated with RhH-
(PPh3)4 (5 mol %) and MePPh2 (15 mol %) in DMI at
132 °C for 12 h, dimer 12 was obtained in 46% yield
(Scheme 5). This reaction competed with catalyst deactiva-
tion. The reaction in refluxing acetone gave only 4% yield
of 12. As for the substrate, the tertiary ether structure
was critical for the effective reaction, and 1-butylthio-3,3-
dimethyl-1-butyne was recovered unchanged under the
conditions.
for 12 h gave homodimers 9 (30%) and 10 (23%), which
was accompanied by the cross-coupling product 13
(17%). The result indicated a comparable efficiency in the
homo- and cross-coupling, and appreciable difference in
the reactivity of 1 and 5 was not observed in the coupling.
The alkylthio exchange products 6 (10%) and 7 (11%) were
formed as well as the starting materials 1 and 5, which indi-
cated that RhH(PPh3)4-MePh2P complex catalyzed both
CS/CS to CS/CS metathesis and CS/CS to CC/SS meta-
thesis. The attempt of the cross-coupling of the less reactive
alkyl substituted 1-alkylthioalkyne 11 and reactive 1 in
refluxing acetone gave only a small amount of 9 without
forming the unsymmetrical butadiyne. Thus, the facility
of the CS bond cleavage appears to be an important factor
in the present reaction.
Mechanistically, it may be acceptable that the initial step
is the oxidative addition of a 1-alkylthioalkyne with rho-
dium(I) complex to form alkynylrhodium(III) complex
(Scheme 7). The direct metathesis reaction of CRh and
CS bonds to form either CC or CS bonds for the next step
To know the difference in the reactivity of the substrate,
cross-coupling experiments were conducted (Scheme 6).
The reaction of 1 and 5 in the presence of RhH(PPh3)4
(5 mol %) and MePh2P (15 mol %) in refluxing acetone
RhH(PPh3)4 (5 mol%)
MePh2P (15 mol%)
DMI, 132 ºC, 12 h
OR2
R1
R2O
OR2
R1
R1
R1
R1
S(n-C6H13)
R1
11: R1 = (CH2)5, R2 = Me
12 46%
35%
R1 = (CH2)4, R2 = Bn
R1 = -Pr, R2 = Me
i
36%
Scheme 5.
i
-Pr3Si
S(
1
n
-C4H9)
i
-Pr3Si
Si(i-Pr )
3
30%
Me
9
RhH(PPh3)4 (15 mol%)
MePh2P (30 mol%)
+
Me
+
+
Me
Me
Me
acetone, refl., 12 h
Me
S(n-C6H13)
Me
Me
Me
Me
10 23%
5
13 17%
Si(i
-Pr3)
Me
+
Me
Me
Me
Me
i-Pr3Si
S(n
-C6H13
)
S(n
-C4H9)
6 10%
7 11%
Scheme 6.

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M. Arisawa et al. / Tetrahedron Letters 49 (2008) 1593–1597
Rh(V)
1) oxidative addition
S
+
S
S
Rh(III)
Rh(III)
Rh(I)
S
Rh(III)
2) ligand exchange
Scheme 7.
CS/CS to CS/CS metathesis
Bidentate Ligand
S
S
+
P
dppf
Rh
S
S
S
S
P
+
RhH(PPh3)4
P
Rh
P
S
+
S
Monodentate Ligand
-MeOC6H4)3P, Ph2PMe
(p
S S
CS/CS to CC/SS metathesis
Scheme 8.
may not be likely, since an alkynylrhodium(III) would not
possess such reactivity to attack carbon or sulfur atom of
another 1-alkylthioalkyne. Rather, ligand coupling on rho-
dium metal may be probable, and the next possible path-
ways will be either (1) oxidative addition of thioalkyne to
form rhodium(V) complex, or (2) ligand exchange between
the rhodium(III) complex.
or CC/SS bond. It is an interesting method to probe the
reactivity of rhodium complex possessing C–Rh–S struc-
ture, which plays important roles in the single bond
metathesis reactions of organosulfur compounds. It should
also be noted that the present work provides a possibility
to develop catalyzed coupling reaction of organosulfur
compounds.
A possible explanation of the ligand effect can be pro-
vided based on the Rh(V) mechanism (Scheme 8).8 Two
thioalkynes subsequently undergo oxidative addition with
rhodium(I) intermediate to give octahedral rhodium(V)
species possessing two alkynyl and two alkylthio ligands.
The dppf ligand occupies the cis-configuration and the
trans-configuration of alkylthio and phosphine group
assuming the trans-effect of phosphorous and sulfur
groups. Here, two alkynyl groups possessing the trans-
configuration do not undergo reductive elimination to
form butadiyne, and only alkylthio exchange proceeds
(CS/CS to CS/CS metathesis). In case of monophosphine
complex, two phosphines take the trans-configuration,
and reductive elimination to form either butadiyne or
alkylthio exchanged products take place (CS/CS to CS/
CS metathesis and CS/CS to CC/SS metathesis).
Acknowledgments
This work was supported by JSPS (Nos. 16109001 and
17689001). M.A. expresses her thanks to the Grant-
in-Aid for Scientific Research on Priority Areas, ‘Advanced
Molecular Transformations of Carbon Resources’ from
MEXT (Nos. 18037005 and 19020008).
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2008.01.040.
References and notes
It is shown that a rhodium complex in the presence of
different phosphine ligand catalyzes single bond metathesis
reactions of CS and CS bonds to form either CS/CS bond
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