Use of triarylstibines in C-arylation reactions

Article abstract of DOI:10.1016/S0040-4039(03)00511-2

A new catalytic system based on triarylstibines for C-arylation of unsaturated compounds is proposed. It was found for a model reaction, the C-arylation of methyl acrylate, that in the presence of equimolar amounts of peroxide and catalytic amounts of a palladium compound (4 mol%), triarylstibines can act as mild efficient arylating agents.

Full text of DOI:10.1016/S0040-4039(03)00511-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3155–3157
Use of triarylstibines in C-arylation reactions
Dmitry V. Moiseev, Vera A. Morugova, Alexey V. Gushchin* and Victor A. Dodonov
Organic Chemistry Department, Nizhnii Novgorod State University, Nizhnii Novgorod, 603950, Russian Federation
Received 22 January 2003; revised 14 February 2003; accepted 20 February 2003
Abstract—A new catalytic system based on triarylstibines for C-arylation of unsaturated compounds is proposed. It was found
for a model reaction, the C-arylation of methyl acrylate, that in the presence of equimolar amounts of peroxide and catalytic
amounts of a palladium compound (4 mol%), triarylstibines can act as mild efficient arylating agents. © 2003 Elsevier Science
Ltd. All rights reserved.
Synthetic applications of organoantimony compounds
are rapidly increasing. Organoantimony(V) derivatives
are efficient reagents in palladium-catalyzed C-C bond
formation.1a–f The application of organoantimony(III)
derivatives is limited since they can give palladium-cata-
lyzed cross-coupling only in the presence of additional
2a–c
oxidants: oxygen, AgOAc, (NH4)2Ce(NO3)6
or in
the presence of stoichiometric amounts of palladium
salts.3a–d
Scheme 1.
In the present work a new catalytic system based on
triarylstibines for C-arylation of unsaturated com-
pounds is proposed. We have found for a model reac-
tion, the C-arylation of methyl acrylate, that in the
presence of equimolar amounts of peroxide and cata-
lytic amounts of a palladium compound (4 mol%),
triarylstibines can act as mild efficient arylating agents
(Scheme 1).
Table 1. C-phenylation reaction of 1 with Ph3Sb/Peroxide/
Li2PdCl4 systemsa
Entry
Peroxide
Yields of 7b (%)
1
2
–
2
171
174
189
164
121
127
26
For the reaction of 1 with 2a (3:1) we tested a number
of peroxides H2O2 3, t-BuOOH 4, (PhCO2)2 5, (cyclo-
C6H11OCO2)2 6 in order to choose the best. All reac-
tions proceeded under standard conditions (AcOH,
50°C, 12 h).4 The results are summarized in Table 1. In
all cases a product yield of 100% corresponds to the
transfer of only one aryl group from the initial
organoantimony compound.
H2O2
3
t-BuOOH
t-BuOOH
(PhCO2)2
(PhCO2)2
(cyclo-C6H11OCO2)2
PhI(OAc)2
–
4c
5
6d
7
8
9e
186
a The reactions were carried out with 0.5 mmol Ph3Sb, 1.5 mmol
methyl acrylate, 0.5 mmol peroxide, 0.02 mmol Li2PdCl4 in AcOH
(4 ml) for 12 h at 50°C under air.
b The yields of methyl cinnamate in all cases were determined by
GLC.
c The ratio 2a:t-BuOOH=1:2.
Keywords: antimony(III); arylation; palladium.
* Corresponding author. Tel.: +7-831-233-7865; fax: +7-831-265-8592;
e-mail: gushchin@mail.nnov.ru
d Acetonitrile was used as solvent.
e Ph3Sb(OAc)2 was utilized instead of 2a.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00511-2

3156
D. V. Moisee6 et al. / Tetrahedron Letters 44 (2003) 3155–3157
All the peroxides exhibited high activity. Peroxides 3
and 4 turned out to be the most effective, giving the
following yields of the reaction product—methyl cinna-
mate 7 171% and 174%, respectively. Peroxide 5 was
less efficient (164%). In the case of 6 the yield was
minimal (127%). A two-fold excess of hydroperoxide 4
ensured an increase in the product yield but only to a
small extent (Table 1, entry 4). The yield of 7 did not
exceed the stoichiometric yield based on palladium in
the absence of the oxidant (Table 1, entry 1). This fact
shows that a catalytic reaction does not occur.
they display similar efficiencies (Table 1, entries 3 and
9). Nevertheless, the former differs advantageously
from the latter in that the synthesis, isolation and
purification necessary for triphenylantimony diacetate
are not needed.
To compare with the peroxides, we used another
1c
known oxidant PhI(OAc)2 which gives Ph3Sb(OAc)2
on interacting with 2a as well.
Ph3Sb+PhI(OAc)2“Ph3Sb(OAc)2+PhI
However, as seen from our results, the yield of the
target product proved to be low (Table 1, entry 8).
Moreover, as it turned out, the addition of an equimo-
lar amount of PhI inexplicably inhibits the phenylation
reaction under these conditions.
The principal role of the peroxides in this system is the
oxidation of Sb(III) to Sb(V) in the form of triarylanti-
mony diacetates (Scheme 2). The mechanism of this
interaction has been well studied.5a,b Triarylantimony
diacetates are known to be effective arylating agents.1e,f
In this C-arylation reaction, other triaryl derivatives of
antimony(III) were examined. All reactions were car-
ried out under the standard conditions (AcOH, 50°C,
12 h) using tert-butyl hydroperoxide 4. The results are
given in Table 2.
Scheme 2.
With peroxide 5, the availability of an acid is not
obligatory since Ar3Sb transforms to triarylantimony
dibenzoate without participation of an acid. Therefore,
reactions with 5 can be carried out in neutral solvents,
e.g. acetonitrile, but the rate becomes lower (Table 1,
entry 6).
As seen, for 2b and 2c the yields of the C-arylated
products remained high (180 and 192%, respectively).
In the case of the ortho-substituted derivative of anti-
mony 2d the product yield decreased sharply to 125%.
With the most sterically hindered 2e, no reaction was
observed at all. Thus, the C-arylation reaction is sensi-
tive to steric hindrance in the aromatic ring, especially,
in the ortho-position.
Ar3Sb+(PhCO2)2“Ar3Sb(O2CPh)2
While comparing the new systems Ph3Sb/peroxide with
the known system based on Ph3Sb(OAc)2, it is seen that
Table 2. C-arylation reactions of 1 with Ar3Sb/t-BuOOH/Li2PdCl4 systemsa

D. V. Moisee6 et al. / Tetrahedron Letters 44 (2003) 3155–3157
3157
pares favorably with the earlier proposed system based
on triarylantimony dicarboxylates.
References
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As should be expected, better donor substituents
reduced the rate of the arylation reaction. So, in the
case of 2f, the yield of the product was smaller, com-
pared with 2b (Table 2, entries 1 and 5).
The catalytic mechanism is represented in Scheme 3.
Triphenylantimony diacetate formed during the prelim-
inary stage (Scheme 2) interacts with the active form of
the catalyst—Pd(0). As a result, intermediate 8 is
formed and it phenylates 1 immediately with formation
of the reaction product 7, the catalyst being trans-
formed again to the active form and the cycle being
repeated.
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4. Typical procedure for C-phenylation reaction: Ph3Sb (0.177
g, 0.5 mmol), Li2PdCl4 (5.2 mg, 0.02 mmol) and methyl
acrylate (0.135 ml, 1.5 mmol) in acetic acid (4 ml) were
placed in a 50 ml tube. The tube was sealed and the
reaction mixture was kept for 12 h at 50°C. The solvent
was evaporated under reduced pressure. The solid residue
was filtered from inorganic products through a short
column on silica gel eluting with hexane–diethyl ether (v/v
4:1) mixture. The filtrate was analyzed by GLC. Methyl
cinnamate (0.151 g) was found.
The antimony(III) derivative 9 does not take part later
on in the reaction because it is not able to oxidize Pd(0)
to Pd(II). The yield of 7 can reach only 100% of initial
2a. As was shown earlier,1f,2a in the presence of oxygen
antimony compounds Ph2SbX can transfer the second
phenyl group onto palladium. Then the yield can attain
200% of 2a corresponding to the transfer of two of the
three possible phenyl groups of the starting
organometallic compound.
Hence, we offer a new catalytic system for C-arylation
of unsaturated compounds based on triarylstilbines in
the presence of equimolar amounts of a peroxide and a
catalytic amount of Li2PdCl4 (4 mol%). The role of the
peroxides is in the in situ preparation of triarylanti-
mony diacetates without isolation and purification of
the latter. H2O2 and t-BuOOH can be considered as the
best peroxides. This system enables the reaction to be
conducted at low temperatures with transfer of two of
the three possible aryl groups. The new system com-
5. (a) Dodonov, V. A.; Zinov’eva, T. I. Metallorg. Khim.
1992, 5, 1265–1271 [Organomet. Chem. USSR, 1992, 5
(Engl. Transl.)]; (b) Hiatt, R.; McColeman, C.; Howe, G.
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