Metal-free carbon-carbon cross-couplings between the ion pairs in sulfonium tetraphenylborates

Article abstract of DOI:10.1016/j.tetlet.2014.05.102

A series of sulfonium tetraphenylborates can be readily prepared by the metathesis of sulfonium halides with sodium tetraphenylborates. After heating at 120-150 °C, the sulfonium tetraphenylborates can smoothly undergo the cross-couplings between the tetraphenylborate anions and the sulfonium cations in the absence of a metal catalyst. For carbonylmethyl-, benzyl-, and allylsulfoniums, the corresponding carbonylmethyl-phenyl, benzyl-phenyl, and allyl-phenyl cross-coupling products can be obtained in 22-76% yields. An interionic electron-transfer mechanism for this cross-coupling reaction is proposed.

Full text of DOI:10.1016/j.tetlet.2014.05.102

Accepted Manuscript
Metal-free carbon-carbon cross-couplings between the ion pairs in sulfonium
tetraphenylborates
Mei-Li Xu, Wenhua Huang
PII:
S0040-4039(14)00933-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.05.102
TETL 44693
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
23 April 2014
18 May 2014
26 May 2014
Please cite this article as: Xu, M-L., Huang, W., Metal-free carbon-carbon cross-couplings between the ion pairs in
sulfonium tetraphenylborates, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.05.102
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Graphical Abstract
To create your abstract, type over the instructions in the template box below.
Fonts or abstract dimensions should not be changed or altered.
Metal-free carbon-carbon cross-couplings
between the ion pairs in sulfonium
tetraphenylborates
Leave this area blank for abstract info.
Mei-Li Xu and Wenhua Huang*

1
Tetrahedron Letters
journal homepage: www.els evier.com
Metal-free carbon-carbon cross-couplings between the ion pairs in sulfonium
tetraphenylborates
Mei-Li Xu and Wenhua Huang∗
Department of Chemistry, Tianjin University, 92 Weijin Road, Tianjin 300072, P. R. China
ARTICLE INFO
ABSTRACT
Article history:
Received
A series of sulfonium tetraphenylborates can be readily prepared by the metathesis of sulfonium
halides with sodium tetraphenylborates. After heating at 120-150 °C, the sulfonium
tetraphenylborates can smoothly undergo the cross-couplings between the tetraphenylborate
anions and the sulfonium cations in the absence of a metal catalyst. For carbonylmethyl-,
benzyl-, and allylsulfoniums, the corresponding carbonylmethyl-phenyl, benzyl-phenyl, and
allyl-phenyl cross-coupling products can be obtained in 22-76% yields. An interionic electron-
transfer mechanism for this cross-coupling reaction is proposed.
Received in revised form
Accepted
Available online
Keywords:
Metal-free
2009 Elsevier Ltd. All rights reserved.
Sulfonium tetraphenylborate
Phenylation
Cross-coupling
Carbon-carbon bond formation
1. Introduction
pairs in sulfonium tetraphenylborates: phenylation of
carbonylmethyl-, benzyl-, and allylsulfoniums.
Organoboron compounds have been widely used in the
transition-metal-catalyzed cross-coupling reactions with organic
halides or pseudohalides to form carbon-carbon bonds.1 In recent
years, this cross-coupling reaction has been extended from the
initial alkenyl-alkenyl, aryl-alkenyl, and aryl-aryl couplings2 to
aryl-allyl3 and aryl-benzyl4 couplings. Moreover, α-arylation of
acetamides5 and acetates6 using arylboronic acid derivatives has
also been reported. Sodium tetraphenylborate as a stable and
commercial available source of the phenyl group has also been
used in the transition-metal-catalyzed cross-couplings, such as
the Suzuki couplings with benzylic halides and 2-halopyridines
catalyzed by a mixed Fe-Zn catalyst,7 a modified Suzuki cross-
coupling with aryl halides,8 and the ligandless Suzuki coupling
with aryl bromides.9 On the other hand, sulfonium salts can
replace the corresponding halide, in particular those unstable
halides such benzylic halides, to participate the palladium- and
nickel-catalyzed cross-coupling reactions with organoboron.10
Scheme 1 Synthesis of sulfonium tetraphenylborates and
the cross-couplings between their ion pairs
The sulfonium tetraphenylborates are readily prepared by
anion-exchange with sodium tetraphenylborate from sulfonium
halides, as shown in Scheme 1, and are bench-stable crystal
solids. We therefore have prepared a variety of sulfonium
tetraphenylborates including benzylsulfoniums, allylsulfoniums,
and carbonylmethylsulfoniums. Initially, phenacylthiolanium
tetraphenylborate 2a was employed as a model compound for
exploring the reaction condition, as shown in Table 1. When 2a
in xylene was heated at 150 °C for 2 h, 1,2-diphenylethanone 3a
was obtained in 52% yield (entry 1). Replacing xylene with
toluene and performing the reaction at 120 °C for 10 h increased
the yield of 3a to 74% (entry 2). Whether increasing or reducing
Although the transition-metal-catalyzed cross-coupling has
become a powerful tool in organic synthesis in the last decades,
the development of efficient processes that do not require a metal
catalyst is still highly desirable due to the difficulty in disposing
and removing the metal catalyst from the reaction residue.11 In
this context, Ryu and co-workers have developed a transition-
metal-free cross-coupling reaction of allylic bromides with aryl-
and vinylboronic acids.12 And Barluenga and co-workers have
reported a metal-free coupling reaction between boronic acids
and tosylhydrazones.13 Herein, we present the preliminary results
o—n —a m—etal-free carbon-carbon coupling reaction between the ion
∗ Corresponding author. Tel.: +86-022-27403475; fax: +086-27403475; e-mail: huangwhxiao@yahoo.com

2
Tetrahedron Letters
the reaction time led to the slightly lower yield (entries 3 and
solvents, such as diisopropyl ketone, DMSO, and isobutyl methyl
ketone, but the yield of 3a was not improved (entries 6-8).
Dimethylphenacylsulfonium 4 afforded 3a in a low yield after 9
h, whereas diphenylphenacylsulfonium 5 gave 3a in 68% yield
(entries 9 and 10) in a very short time (1 h).
4). When the reaction was carried out in acetonitrile at 90 °C,
only trace of 3a was detected and 2a was recovered in 90% yield
(entry 5). Because of the poor solubility of 2a in toluene, we tried
to enhance its solubility by combining toluene with some polar
Table 1. Initial observations on the cross-couplings between the ion pairs in sulfonium tetraphenylborates a
O
R1
S
Ph4B-
O
R2
Ph
Ph
Ph
3a
Entry
R1
R2
Sulfonium salt
Solvent
Xylene
Toluene
Toluene
Toluene
CH3CN
T (°C)
150
120
120
120
90
Time (h)
Yield (%)b
1
2
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
2a
2a
2a
2a
2a
2a
2a
2a
4
2
10
8
52
74
3
66
4
12
10
4
68
5
Trace c
6
iPrCOPri: toluene (1:1)
DMSO: toluene (1:1)
iBuCOMe: toluene (1:1)
Toluene
120
120
120
120
120
57
7
4
5
8
4
23
9
Me
Ph
Me
Ph
9
37
10
5
Toluene
1
68
a Reaction conditions: sulfonium salt (0.4 mmol), solvent (8 mL), heated at T °C (bath temperature), the reactions were monitored by TLC.
b Isolated yield.
c 2a was recovered in 90% yield.
Next
we
examined
a
variety
of
thiolanium
transfer of alkyl from tetraalkylborates to the pyridinium cations
to afford the various nucleophilic adducts. It has also been
reported16 that dimethylphenacylsulfonium butyldiphenylborate
tetraphenylborates,14 and the results are summarized in Table 2.
For arylacylthiolaniums, the reactions was tolerant of electron
withdrawing and donating groups at para-position of benzene
ring, affording 3b-d in 53-76% yields (entries 2-4); for 2e with a
methoxy group at ortho-position, however, 3e was obtained in
only 28% yield (entry 5). For 2f with a naphthalene moiety, the
corresponding phenylation product 3f was obtained in 53% yield
(entry 6). The phenylation also underwent at α-position of
carboxymethylthiolaniums 2g and 2h to give 3g and 3h in 73%
and 60% yields, respectively (entries 7 and 8). It is noteworthy
that the double bond in the cinnamyl group of 2h can survive the
can
undergo
photo-induced
electron-transfer
from
butyldiphenylborate to dimethylphenacylsulfonium to generate
butyl and phenacyl radicals as initiators for photopolymerization.
Based on these studies, we propose a reaction mechanism for the
cross-couplings between the ion pairs in sulfonium
tetraphenylborates, as is depicted in Scheme 2. Upon heating, 2
undergoes an interionic electron-transfer to generate
tetraphenylboranyl and S-substituted thiolanyl radicals. The
tetraphenylboranyl radical decomposes to Ph3B and the phenyl
radical which further reacts with the thiolanyl radical to produce
the cross-coupling product 3 and Ph3B-tetrahydrothiophene
Lewis pair. The interionic electron-transfer might undergo
through the contact ion pair, so the polar solvent such as
acetonitrile is unfavorable (Table 1, entries 5-8). Furthermore,
the coupling between the phenyl radical and the R group of the
thiolanyl radical might also occur within the solvent cage. And
the attack of the phenyl radical prefers to the carbon adjacent to
sulfur atom, which might account for the tolerance of the nitro
group (Table 2, entries 3 and 15). The fact that biphenyl was
detected and isolated from the reaction mixture supports the
formation of the phenyl radical, and thus this radical coupling
mechanism.
reaction.
Three
acetamide-containing
thiolanium
tetraphenylborates 2i-k gave the phenylation products 3i-k in
37%-74% yields but 2i and 2j required higher reaction
temperature, and 2j also needed
a
polar solvent
(diisopropylketone) to promote the reaction (entries 9-11).
Allylic thiolanium tetraphenylborates 2l and m afforded phenyl-
allyl cross-coupling products 3l and 3m in 73% and 62% yields,
respectively; and the double bond in the cinnamyl group partially
isomerized from the sole trans to a 20:80 mixture of cis: trans
(entries 12-13). Finally, three benzylthiolaniums could also
undergo cross-couplings with tetraphenylborates but the coupling
yields depended on the group attached to the benzyl moiety,
being higher with an electron-withdrawing group than with an
electron-donating group (entries 14-16).
In summary, we have discovered a metal-free carbon-carbon
cross-coupling reaction between the ion pairs in sulfonium
tetraphenylborates. This reaction provides a new methodology
for three types of carbon-carbon cross-coupling: carbonylmethyl-
phenyl, allyl-phenyl, and benzyl-phenyl. More examples and the
detailed reaction mechanism are currently under investigation.
At this stage, we do not know the exact mechanism of this
cross-coupling reaction. It has been reported15 that pyridinium
tetraalkylborates can undergo the interionic charge-transfer
interaction between tetraalkylborate anions and the pyridinium
cations under thermal or photochemical condition, leading to the

3
Table 2. Metal-free cross-couplings between the ion pairs in thiolanium tetraphenylborates a
Entry
1
2
3
Yield % Entry
2
3
Yield %
74 c
74
9
2
3
4
5
6
76
53
63
28
53
10
11
12
13
14
37 d
52
73
62 e
41
O2N
O2N
7
8
73
15
65 f
S
Ph4B-
Ph
2o
3o
60 b
16
22 g
a Reaction conditions: 2 (0.4 mmol), toluene (8 mL), heated at 120 °C, the reactions were monitored by TLC.
b Using iPrCOPri as solvent.
Scheme 2 Proposed reaction mechanism
c Using xylene as solvent at 150 °C.
+
2
R
S
Ph4B
d Using iPrCOPri: xylene (1:1) as solvent at 150 °C.
e cis: trans = 20: 80, determined by 1H-NMR.
f
Using iBuCOMe: toluene (1:1) as solvent.
Ph3B
S
g Using iPrCOPri: toluene (1:1) as solvent.
3
Ph3B + Ph
+
R
S

4
Tetrahedron
Acknowledgments
We thank the National Natural Science Foundation of China
for their financial support of our program (Grant No. 21272170).
References and notes
1. Alonso, F.; Beletskaya, I. P.; Yus, M. Tetrahedron 2008, 64,
3047-3101.
2. Suzuki, A. Angew. Chem. Int. Ed. 2011, 50, 2-18.
3. (a) Kurosawa, H.; Kajimaru, H.; Miyoshi, M.-A.; Ohnishi, H.;
Ikeda, I. Journal of Molecular Catalysis 1992, 74, 481-488; (b)
Moreno-Manas, M.; Pajuelo, F.; Pleixats, R. J. Org. Chem. 1995,
60, 2396-2397.
4. (a) Chowdhury, S.; Georghiou, P. E. Tetrahedron Lett. 1999, 40,
7599-7603; (b) Inés, B.; Moreno, I.; SanMartin, R.; Domínguez,
E. J. Org. Chem. 2008, 73, 8448-8451.
5. (a) Duan, Y.-Z.; Deng, M.-Z. Tetrahedron Lett. 2003, 44, 3423-
3426; (b) Molander, G. A.; Traister, K. M.; Barcellos, T. J. Org.
Chem. 2013, 78, 4123-4131.
6. (a) Gooßen, L. J. Chem. Commun. 2001, 669-670; (b) Liu, X.-x.;
Deng, M.-z. Chem. Commun. 2002, 622-623; (c) Peng, Z.-Y.;
Wang, J.-P.; Cheng, J.; Xie, X.-m.; Zhang, Z. Tetrahedron 2010,
66, 8238-8241; (d) Lei, A.; Zhang, X. Tetrahedron Lett. 2002, 43,
2525-2528.
7. Bedford, R. B.; Hall, M. A.; Hodges, G. R.; Huwe, M.; Wilkinson,
M. C. Chem. Commun. 2009, 6430-6432.
8. Bumagin, N. A.; Bykov, V. V. Tetrahedron 1997, 53, 14437-
14450.
9. Lu, G.; Franzén, R.; Zhang, Q.; Xu, Y. Tetrahedron Lett. 2005,
46, 4255-4259.
10. Srogl, J.; Allred, G. D.; Liebeskind, L. S. J. Am. Chem. Soc. 1997,
119, 12376-12377.
11. Arancon, R. A. D.; Lin, C. S. K.; Vargas, C.; Luque, R. Org.
Biomol. Chem. 2014, 12, 10-35.
12. Ueda, M.; Nishimura, K.; Kashima, R.; Ryu, I. Synlett 2012, 23,
1085-1089.
13. Barluenga, J.; Tomas-Gamasa, M.; Aznar, F.; Valdes, C. Nat.
Chem. 2009, 1, 494-499.
14. Typical procedure for the cross-couplings between the ion pairs in
sulfonium tetraphenylborates. To a two-neck flask equipped with
a condenser which is connected to a nitrogen balloon, thiolanium
tetraphenylborate 2a (210.6 mg, 0.4 mmol) and toluene (8 mL)
which was degassed by purging nitrogen, was added by syringe.
The reaction mixture was heated in an oil bath (bath temperature,
120 °C). The reaction was monitored by TLC. After reaction, the
reaction mixture was cooled down to room temperature,
evaporated to remove solvent, and then separated by preparative
TLC (silica gel). 1,2-Diphenylethanone (3a). A white solid,
-1
yield: 74%%. IR (KBr, cm ) 1687 (C=O). 1H-NMR (400 MHz,
CDCl3) δ 4.33 (s, 2 H), 7.27-7.34 (m, 3 H), 7.35-7.40 (m, 2 H),
7.47-7.52 (m, 2 H), 7.56-7.62 (m, 1 H), 8.04-8.09 (m, 2 H). 13C-
NMR (100 MHz, CDCl3) δ 45.5, 126.9, 128.66, 128.70, 128.73,
129.5, 133.2, 134.6, 136.6, 197.6.
15. Zhu, D.; Kochi, J. K. Organometallics 1999, 18, 161-172.
16. Toba, Y.; Yasuike, M.; Usui, Y. Chem. Commun. 1997, 675-676.
.
Supplementary Material
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/.........