Rhodium-Catalyzed cross-coupling of arylboronic acids using vinyl acetate as the electrophilic partner

Article abstract of DOI:10.1055/s-0029-1218283

The first Suzuki-type cross-coupling between arylboronic acids and vinyl acetate is disclosed. The acetoxy moiety as the leaving group is realized in the coupling process. Rhodium complexes were found to be highly effective for promoting this transformati

Full text of DOI:10.1055/s-0029-1218283

LETTER
3151
Rhodium-Catalyzed Cross-Coupling of Arylboronic Acids Using Vinyl
Acetate as the Electrophilic Partner
R
hodium-Catalyze
a
d
Cross-C
o
n
upling of Ar
g
ylboronic
A
cids Wai Lee, Fuk Yee Kwong*
Open Laboratory of Chirotechnology of the Institute of Molecular Technology for Drug Discovery and Synthesis
and Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Kowloon, Hong Kong, P. R. of
China
Fax +85223649932; E-mail: bcfyk@inet.polyu.edu.hk
Received 27 July 2009
A variety of metal complexes were subjected to the reac-
Abstract: The first Suzuki-type cross-coupling between arylboron-
tion in order to test the feasibility for catalyzing this cross-
coupling (Table 1). We chose electronically neutral 4-
tert-butylphenylboronic acid as the model substrate. Con-
ic acids and vinyl acetate is disclosed. The acetoxy moiety as the
leaving group is realized in the coupling process. Rhodium com-
plexes were found to be highly effective for promoting this transfor-
mation. In contrast, the present well-known nickel and palladium ventional Pd(Ph3P)4 complex did not catalyze this process
complexes were inferior for this catalysis when vinyl acetate was
(Table 1, entry 1). Palladium(II) acetate in combination
with indolyl phosphine ligands,12 CM-phos and L1, which
used as the electrophilic partner.
Key words: cross-coupling, catalysis, rhodium, vinyl acetate
we previously found that they were highly active in aryl
mesylate8a–c and chloride13 couplings, respectively, did
not promote this catalysis (Table 1, entries 2 and 3). Com-
mercially available nickel complex was essentially inef-
fective (Table 1, entry 4). To our delight, [Rh(cod)Cl]2
complex which has seldom been used in cross-coupling
reactions, did promote this C–C bond-construction pro-
cess (Table 1, entry 5). Based on this promising result, we
attempted to further improve the catalytic system by uti-
lizing a variety of readily available phosphine-supporting
ligands (Table 1, entries 6–10). Bidentate phosphine
ligand with appropriate carbon-chain length (i.e., n = 3,
dppb) provided the highest desired product yield (Table 1,
entries 10). Apart from neutral complex, we also exam-
ined the cationic rhodium complexes. Interestingly, a
sharp contrast between Rh(cod)OTf and Rh(cod)BF4 was
observed, thus the former afforded much better product
yield than the latter one (Table 1, entries 11 and 12).
To further optimize the reaction conditions,14,15 common-
ly used inorganic bases and solvents in cross-coupling
reactions were screened (Table 2). As base, K3PO4 was
the best choice among various bases surveyed (Table 2,
entries 1–6). As solvent, t-BuOH and dioxane gave mod-
erate product yields, while toluene was superior in this
catalytic transformation (Table 2, entry 5 vs. entries 7–9).
Transition-metal-catalyzed bond-construction protocols
between nucleophilic/organometallic fragments and elec-
trophilic organohalides are highly versatile for specific
carbon–carbon bond formation.1 Aryl or alkenyl iodides
and bromides are usually employed as the electrophiles.
Recently, enormous effort have been undertaken by re-
searchers, especially in a plethora of ligand improve-
ments,2 to enhance the capability for applying relatively
inert electrophile as coupling partners, such as aryl chlo-
rides,3 ethers,4 diazoniums,5 chlorosulfonyls,6 tosylates,7
as well as the most recently aryl mesylates8 and pivalates.9
Though notable advancements have been achieved, the
use of acetoxy moiety as the leaving group remains spo-
radically studied.10 In fact, the acetate electrophiles are
easily accessible from commercial suppliers or from the
esterification of alcohols in the presence of acetic anhy-
dride. Moreover, the side product (acetate salts) generated
in the cross-coupling reactions is generally regarded as
nontoxic. Inspired by the aforementioned beneficial fea-
tures of the acetate leaving group, and our continue inter-
est to explore readily available and economical
electrophiles, we turn to investigate the feasibility of vinyl
acetate couplings.
To probe the effectiveness of the optimized catalytic sys-
tem, an array of arylboronic acids were tested (Table 3).
Substituted arylboronic acids were found to be capable to
furnish the corresponding products (Table 3, entries 1–4).
2-Naphthylboronic acid gave 82% yield of the desired
product (Table 3, entry 5). Attempted study using hin-
dered 1-naphthylboronic acid was found to be unsuccess-
ful. Ferrocenylboronic acid decomposed to ferrocene
during the course of the reaction, as judged by GC-MS
analysis.
Successful cross-coupling reactions using the acetoxy
leaving group have seldom been reported. Only allylic
and benzylic acetate substrates were recently disclosed.11
We herein report, to the best of our knowledge, the first
Suzuki-type coupling using vinyl acetate as the electro-
philic partner. Interestingly, our results showed that rho-
dium catalyst systems favored to catalyze this bond-
forming reaction while commonly used palladium and
nickel complexes in cross-coupling reactions were gener-
ally ineffective.
In addition to vinyl acetate, we also attempted to study the
substituted vinyl acetate coupling partner (Scheme 1).
Moderate yield was obtained upon using the previous
vinylation reaction conditions as described in Table 3.
SYNLETT 2009, No. 19, pp 3151–3154
x
x
.x
x
.2
0
0
9
Advanced online publication: 23.10.2009
DOI: 10.1055/s-0029-1218283; Art ID: W11809ST
© Georg Thieme Verlag Stuttgart · New York

3152
H. W. Lee, F. Y. Kwong
LETTER
Table 1 Initial Screening of Metal–Ligand Combination in Vinylation of Arylboronic Acida
t-Bu
t-Bu
metal/ligand (5 mol%)
AcO
+
K3PO4, toluene
110 °C, 24 h
B(OH)2
ligand
Ph2P
Me
PR2
N
N
PPh2
PPh2
PPh2
n
Fe
dppe n = 1
dppp n = 2
dppb n = 3
Cy2P
dppf
L1
CM-phos
Entry
1
Metal
Ligand
Yield (%)b
Pd(Ph3P)4
–
<1
<1
<1
<1
22
28
29
33
58
75
<1
53
2
Pd(OAc)2
CM-phos
L1
3
Pd(OAc)2
4
NiCl2(Ph3P)2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
Rh(cod)BF4
Rh(cod)OTf
2 Ph3P
–
5
6
2 PPh3
dppe
dppf
dppp
dppb
dppb
dppb
7
8
9
10
11
12
a Reaction conditions: metal (5 mol%), ligand (10 mol%), ArB(OH)2 (0.3 mmol), vinyl acetate (6.0 mmol), K3PO4 (0.9 mmol), and toluene (1.0
mL) were placed in a Schlenk tube under nitrogen, and the reaction was subjected at 110 °C for 24 h with continuous stirring.
b GC yields were reported.
Table 2 Screening of Reaction Parameters in Vinylation of Arylboronic Acida
[Rh(cod)Cl]2 (3 mol%)
t-Bu
t-Bu
dppb (6 mol%)
+
AcO
base, solvent
110 °C, 24 h
B(OH)2
Entry
Base
Solvent
toluene
toluene
toluene
toluene
toluene
toluene
t-BuOH
dioxane
DMF
Yield (%)b
1
2
3
4
5
6
7
8
9
Na2CO3
K2CO3
Cs2CO3
Na3PO4
K3PO4
NaOt-Bu
K3PO4
K3PO4
K3PO4
9
22
45
14
64
10
32
38
9
a Reaction conditions: [Rh(cod)Cl]2 (3 mol%), dppb (6 mol%), ArB(OH)2 (0.3 mmol), vinyl acetate (6.0 mmol), base (0.9 mmol), and solvent
(1.0 mL) under N2 at 110 °C for 24 h.
b GC yields were reported.
Synlett 2009, No. 19, 3151–3154 © Thieme Stuttgart · New York

LETTER
Rhodium-Catalyzed Cross-Coupling of Arylboronic Acids
3153
[Rh(cod)Cl]2
dppb
Table 3 Rhodium-Catalyzed Vinylation of ArB(OH)2 Using Vinyl
Acetatea
[Rh(cod)Cl]2 (3 mol%)
dppb (6 mol%)
AcO
P
ArB(OH)2
AcO
Ar
+
K3PO4, toluene
110 °C, 24 h
R
Rh Cl
P
Entry Arb(OH)2
Product
Yield (%)b
proposed
B(OH)2
B(OH)2
B(OH)2
catalytic cycle
1
68
80
2c
Cl
t-Bu
t-Bu
P
R
Cl
Rh OAc
P
Rh
3
55
60
P
P
BuO
BuO
B(OH)2
+
4
B(OH)2(OAc)
(base salt)
R
base
Me2N
Ph
Me2N
B(OH)2
B(OH)2
Scheme 2 Proposed catalytic cycle for rhodium-catalyzed cross-
coupling between vinyl acetate and arylboronic acids
5
69
82
Ph
ing the oxidative addition of alkenyl acetate to rhodium(I)
center, a similar vinyl(aceto)ruthenium(II) complex was
reported by Komiya and co-workers.16 Following the
transmetalation of arylboronic acid to the rhodium com-
plex and subsequent reductive elimination of the arylvinyl
rhodium intermediate, the desired vinylarene is afforded.
During the galley proof processing, a closely related liter-
ature appeared.17
6
B(OH)2
7
8
9
trace
S
S
trace
B(OH)2
In summary, we have disclosed the first vinylation of aryl-
boronic acid using vinyl acetate as a new coupling partner.
Notably, we demonstrated that the acetoxy moiety was ef-
fective to act as a leaving group in the rhodium-catalyzed
cross-coupling reaction. The application of the acetoxy
leaving group both potentially provides a better environ-
mental concern and atom economy and offers a new and
attractive direction for cross-coupling in complementary
to vinylhalides and sulfonates. Further fine-tunings of the
catalytic system to increase the catalytic efficiency and
exploration on aryl acetate coupling are currently under
way.
B(OH)2
decomp.
Fe
Fe
a Reaction conditions: [Rh(cod)Cl]2 (3 mol%), dppb (6 mol%),
ArB(OH)2 (0.3 mmol), vinyl acetate (6.0 mmol), K3PO4 (0.9 mmol),
and toluene (1.0 mL) were placed in a Schlenk tube under nitrogen,
and the reaction was subjected at 110 °C for 24 h with continuous stir-
ring; reaction times were not optimized in each case.
b Isolated yields.
c [Rh(cod)Cl]2 (5 mol%), dppb (10 mol%) were used.
Although vinyl acetate worked well in this catalytic sys-
tem, aryl acetate did not give any desired coupling product
under the same reaction conditions.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
We propose that the catalytic cycle proceeds through a
similar mechanistic pathway as the well-known Suzuki–
Miyaura coupling (Scheme 2). The Rh–dppb complex is
suggested to undergo oxidative addition of the olefinic C–
O bond in vinyl acetate to afford a vinylrhodium(III) com-
plex. Although there has been no literature report regard-
Acknowledgment
We thank the Research Grants Council of Hong Kong (Project No.
CERG: PolyU 5005/07P) and University Grants Committee Areas
of Excellence Scheme (Project No. AoE/P-10/01) for financial sup-
port.
[Rh(cod)Cl]2 (3 mol%)
dppb (6 mol%)
B(OH)2
+
AcO
K3PO4, toluene
110 °C, 24 h
35%
Scheme 1 Rhodium-catalyzed coupling of substituted vinyl acetate with arylboronic acid
Synlett 2009, No. 19, 3151–3154 © Thieme Stuttgart · New York

3154
H. W. Lee, F. Y. Kwong
LETTER
S. L. J. Am. Chem. Soc. 2008, 130, 13552. (e) Zhang, L.;
Qing, J.; Yang, P.; Wu, J. Org. Lett. 2008, 10, 4971. For
Ni-catalyzed ArOMs couplings, see: (f) Percec, V.; Bae, J.-
Y.; Hill, D. H. J. Org. Chem. 1995, 60, 1060. (g) Percec,
V.; Bae, J.-Y.; Hill, D. H. J. Org. Chem. 1995, 60, 1066.
(h) Ueda, M.; Saitoh, A.; Oh-tani, S.; Miyaura, N.
Tetrahedron 1998, 54, 13079.
References and Notes
(1) (a) Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.,
Vol. 1-2; de Meijere, A.; Diederich, F., Eds.; Wiley-VCH:
Weinheim, 2004. (b) Beller, M.; Bolm, C. Transition Metals
for Organic Synthesis, Building Blocks and Fine Chemicals,
2nd ed., Vol. 1-2; Wiley-VCH: Weinheim, 2004.
(c) Handbook of Organopalladium for Organic Synthesis,
Vol. 1-2; Nigeshi, E., Ed.; Wiley-Interscience: New York,
2002. (d) Tsuji, J. Palladium Reagents and Catalysts, 2nd
ed.; Wiley: Chichester, 2004. (e) Yin, L. X.; Liebscher, J.
Chem. Rev. 2007, 107, 133. (f) Corbet, J.-P.; Mignani, G.
Chem. Rev. 2006, 106, 2651. (g) Roglans, A.; Pla-Quintana,
A.; Moreno-Mañas, M. Chem. Rev. 2006, 106, 4622.
(h) Molander, G. A.; Ellis, N. Acc. Chem. Res. 2007, 40, 275.
(2) For recent reviews on ligand developments, see: (a) Surry,
D. S.; Buchwald, S. L. Angew. Chem. Int. Ed. 2008, 47,
6338. (b) Martin, R.; Buchwald, S. L. Acc. Chem. Res. 2008,
41, 1461. (c) Hartwig, J. F. Acc. Chem. Res. 2008, 41, 1534.
(d) Kwong, F. Y.; Chan, A. S. C. Synlett 2008, 1440.
(3) For a pertinent review on aryl chloride couplings, see: Littke,
A. F.; Fu, G. C. Angew. Chem. Int. Ed. 2002, 41, 4176.
(4) Tobisu, M.; Shimasaki, T.; Chatani, N. Angew. Chem. Int.
Ed. 2008, 47, 4866.
(9) (a) Guan, B.-T.; Wang, Y.; Li, B.-J.; Yu, D.-G.; Shi, Z.-J.
J. Am. Chem. Soc. 2008, 130, 14468. (b) Quasdorf, K. W.;
Tian, X.; Garg, N. K. J. Am. Chem. Soc. 2008, 130, 14422.
(c) Li, B.-J.; Li, Y.-Z.; Lu, X.-Y.; Liu, J.; Guan, B.-T.; Shi,
Z.-J. Angew. Chem. Int. Ed. 2008, 47, 10124. For a related
decarboxylative coupling, see: (d) Gooßen, L. J.; Deng, G.;
Levy, L. M. Science 2006, 313, 662.
(10) To our knowledge, there is no literature report on cross-
coupling of vinyl acetate before. Only oxidative coupling
and 2-phenylpyridine C–H activation/coupling were
reported, see: (a) Amatore, M.; Gosmini, C.; Périchon, J.
Eur. J. Org. Chem. 2005, 989. (b) Matsuura, Y.; Tamura,
M.; Kochi, T.; Sato, M.; Chatani, N.; Kakiuchi, F. J. Am.
Chem. Soc. 2007, 129, 9858.
(11) (a) Valle, L. D.; Stille, J. K.; Hegedus, L. S. J. Org. Chem.
1990, 55, 3019. (b) Uozumi, Y.; Danjo, H.; Hayashi, T.
J. Org. Chem. 1999, 64, 3384. (c) Bouyssi, D.; Gerusz, V.;
Balme, G. Eur. J. Org. Chem. 2002, 2445. (d) Kuwano, R.;
Yokogi, M. Chem. Commun. 2005, 5899.
(5) Darses, S.; Jeffery, T.; Genet, J.-P.; Brayer, J.-L.; Demoute,
J.-P. Tetrahedron Lett. 1996, 37, 3857.
(6) (a) Dabbaka, S. R.; Vogel, P. J. Am. Chem. Soc. 2003, 125,
15292. (b) Dabbaka, S. R.; Vogel, P. Org. Lett. 2004, 6, 95.
(7) For ArOTs in Pd-catalyzed C–C bond couplings, see:
(a) Nguyen, H. N.; Huang, X.; Buchwald, S. L. J. Am. Chem.
Soc. 2003, 125, 11818. (b) So, C. M.; Lau, C. P.; Chan, A. S.
C.; Kwong, F. Y. J. Org. Chem. 2008, 73, 7734. (c) Zhang,
L.; Meng, T.; Wu, J. J. Org. Chem. 2007, 72, 9346.
(d) Roy, A. H.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125,
8704. (e) Limmert, M. E.; Roy, A. H.; Hartwig, J. F. J. Org.
Chem. 2005, 70, 9364. (f) Ackermann, L.; Althammer, A.
Org. Lett. 2006, 8, 3457. (g) Hansen, A. L.; Ebran, J.-P.;
Ahlquist, M.; Norrby, P.-O.; Skrydstrup, T. Angew. Chem.
Int. Ed. 2006, 45, 3349. For Ni-catalyzed C–C couplings of
ArOTs, see: (h) Zim, D.; Lando, V. R.; Dupont, J.;
(12) For our recent developments on indolyl phosphine ligands,
see: (a) So, C. M.; Yeung, C. C.; Lau, C. P.; Kwong, F. Y.
J. Org. Chem. 2008, 73, 7803. (b) Lee, H. W.; Lam, F. L.;
So, C. M.; Lau, C. P.; Chan, A. S. C.; Kwong, F. Y. Angew.
Chem. Int. Ed. 2009, 48; DOI: 10.1002/anie.200904033.
(13) So, C. M.; Lau, C. P.; Kwong, F. Y. Org. Lett. 2007, 9, 2795.
(14) We have attempted to use MW reactor instead of
conventional heating to perform this reaction. Under the
same reaction conditions except MW heating at 130 °C for
1 h, only 25% yield of p-tert-butylstyrene was obtained. For
our recent reference on rhodium catalysis under MW
conditions, see: Lee, H. W.; Lee, L. N.; Chan, A. S. C.;
Kwong, F. Y. Eur. J. Org. Chem. 2008, 3403.
(15) We tried to lower the loading of vinyl acetate to 2 equiv
(with respected to arylboronic acid). However, the product
yield dropped significantly.
(16) (a) Komiya, S.; Suzuki, J.-I.; Miki, K.; Kasai, N. Chem. Lett.
1987, 16, 1287. (b) Planas, J. G.; Marumo, T.; Ichikawa, Y.;
Hirano, M.; Komiya, S. J. Chem. Soc., Dalton Trans. 2000,
2613.
Monteiro, A. L. Org. Lett. 2001, 3, 3049. (i) Tang, Z. Y.;
Hu, Q. S. J. Am. Chem. Soc. 2004, 126, 3058. (j) Percec,
V.; Golding, G. M.; Smidrkal, J.; Weichold, O. J. Org.
Chem. 2004, 69, 3447.
(8) For ArOMs in Pd-catalyzed couplings, see: (a) So, C. M.;
Zhou, Z.; Lau, C. P.; Kwong, F. Y. Angew. Chem. Int. Ed.
2008, 47, 6402. (b) So, C. M.; Lau, C. P.; Kwong, F. Y.
Angew. Chem. Int. Ed. 2008, 47, 8059. (c) So, C. M.; Lee,
H. W.; Lau, C. P.; Kwong, F. Y. Org. Lett. 2009, 11, 317.
(d) Fors, B. P.; Watson, D. A.; Biscoe, M. R.; Buchwald,
(17) Yu, J.-Y.; Kuwano, R. Angew. Chem. Int. Ed. 2009, DOI:
101002/anie. 200903146.
Synlett 2009, No. 19, 3151–3154 © Thieme Stuttgart · New York