A general and efficient suzuki-miyaura cross-coupling protocol using weak base and no water: The essential mole of acetate

Article abstract of DOI:10.1002/ejoc.200900538

A weak base, CsOAc, promotes Suzuki-Miyaura cross-coupling and related Pd-catalyzed reactions under anhydrous conditions as effectively as stronger bases. Aryl triflates exhibit unusual reaction rates, which are comparable to that: of bromoarenes. A negle

Full text of DOI:10.1002/ejoc.200900538

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200900538
A General and Efficient Suzuki–Miyaura Cross-Coupling Protocol Using
Weak Base and No Water: The Essential Role of Acetate
Bing Wang,*[a] Hui-Xia Sun,[a,b] and Zhi-Hua Sun*[c]
Keywords: Suzuki–Miyaura coupling / Cross-coupling / Boranes / Lewis bases / Palladium
A weak base, CsOAc, promotes Suzuki–Miyaura cross-cou- model was proposed to give alternative insight into the key
pling and related Pd-catalyzed reactions under anhydrous
conditions as effectively as stronger bases. Aryl triflates exhi-
bit unusual reaction rates, which are comparable to that of
bromoarenes. A neglected six-membered transition-state
process of transmetalation.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
tive substrates.More importantly, it could provide more in-
sight into the transmetalation process, which to some extent
is still a “mechanistic black box”.[9]
Introduction
Suzuki–Miyaura cross-coupling is dubbed as the “Grig-
nard reaction of the 21st century”, thanks to its versatility
in constructing various types of C–C bonds.[1] Important
advances in effecting the coupling of unreactive substrates[2]
as well as novel nucleophiles[3] have greatly expanded the
scope of this reaction. In particular, the B-alkyl coupling,[4]
an important variation, has attracted much recent attention
and considerable breakthroughs have been made by Fu and
co-workers.[5] Much emphasis was placed on facilitating the
process of oxidative addition and reductive elimination,
however, studies on the base were less prevalent.[6]
Suzuki and Miyaura rationalized the role of bases,
usually hydroxide or alkoxide, in the key process of trans-
metalation within the catalytic cycle.[7] Soderquist[8a] and
Woerpel[8b] demonstrated that this step proceeded with re-
tention of configuration for alkylboranes. So far, a four-
membered µ2-hydroxo-bridged transition-state model[8a] has
been put forward and has since become the textbook
mechanism for transmetalation. As a base is an indispens-
able component in normal Suzuki coupling, with the excep-
tion of that of diazonium salts, the improvement of our
understanding of its role may have a twofold benefit. First,
the scope of the reaction could be broadened by optimizing
this critical parameter, for conventional aqueous strong
bases are incompatible with many base- and moisture-sensi-
Results and Discussion
We have recently developed a protocol for the direct B-
alkyl Suzuki coupling of bromoarenes in the presence of
unmasked acidic or basic functional groups.[10] During the
course of that study, we came across a notable observation;
with K2CO3 as a base, only a trace (Ͻ3%) reaction was
detected for 4-bromobenzoic acid 1a, while the coupling of
its sodium salt 1b realized an appreciable (21%) conversion
in 3 h (Table 1, Entries 1 and 2). Presumably, solubility of
the base may not be the major cause for this result, because
relative to the free acid 1a, its sodium salt is far less soluble
in THF, but exhibits a much higher reaction rate. Instead,
it pointed towards the fact that B-alkyl Suzuki coupling
could be promoted by the carboxylate anion. Stimulated by
this finding, and coupled with our interest in Lewis base
catalysis,[11] we screened a series of carboxylate salts as the
base. Gratifyingly, CsOAc stood out as the most effective
base, affording a 92% yield and full conversion (Entry 5).
Other carboxylic salts gave inferior conversions and yields,
for example, the sodium and silver salts afforded only trace
amounts of the coupling product. Here, the normal qualita-
tive correlation of reaction rate with the strength of base in
Suzuki coupling was not observed; the reaction with KOAc
worked, while that with much more basic K2CO3 failed. To
the best of our knowledge, this is the first example in which
a base as weak as acetate effectively promotes B-alkyl Su-
zuki coupling. THF was the solvent of choice, although di-
oxane and polar aprotic solvents such as DMF and NMP
are also suitable. Subsequently, we tested the new protocol
on the reaction using triflate substrate 1c. A near quantita-
[a] Department of Chemistry, Fudan University,
220 Handane Road, Shanghai 200433, China
Fax: +86-21-54237570
E-mail: wangbing@fudan.edu.cn
[b] Institutes of Biomedical Sciences, Fudan University,
138 Yixueyuan Road, Shanghai 200032, China
[c] Shanghai Saijia Chemicals Co., Ltd., Suite 402, No. 54,
Lane 33, Shilong Road, Shanghai 200232, China
E-mail: sungaris@gmail.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200900538.
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Suzuki–Miyaura Cross-Coupling Protocol with Weak Base and No Water
tive yield was obtained, and the reaction rate was surpris- Table 2. B-Alkyl Suzuki coupling of aryl triflates promoted by
CsOAc.
ingly high: the ethylation was complete within 0.5 h under
reflux, or 2 h at room temperature.
Table 1. B-Alkyl Suzuki coupling promoted by carboxylates.[a]
Entry
1, M, X
Base (equiv.)
Conv. [%] 2 [%][b]
1
2
3
4
5
6
1a, H, Br
K2CO3 (3.0)
K2CO3 (3.0)
NaOAc (3.0)
KOAc (3.0)
CsOAc (3.0)
AgOAc (2.0)
NaOBz (3.0)
CsOAc (2.0)
trace
21
trace
58
100
10
14
2a, n.d.
2a, 19[c]
2a, n.d.
2a, 51
2a, 92
2a, 4
1b, Na, Br
1a
1a
1a
1a
7
1a
2a, 12
2c, 99
8[d]
1c, Et, OTf
100
[a] Reaction conditions: 1.0 mmol substrate, 3.0 mmol Et3B (1.0 
in THF, 3.0 mL), 2.0–3.0 mmol base, 5 mL THF, reflux, 3 h, except
otherwise noted. [b] Isolated yields. [c] Isolated as the free acid 2a.
[d] 2.0 equiv. Et3B, reflux, 0.5 h.
[a] Isolated yields. [b] Room temperature, 6 h, 10% diethylation
product.
The scope of the substrate for the present protocol was
then investigated under optimized conditions (Table 2). For
aryl triflate substrates with electron-withdrawing or elec-
tron-donating groups in various substitution patterns, good moiety and a free hydroxyl (Entry 8), proceeded smoothly
to excellent yields were obtained. In general, the yields are in good to excellent yields without the need of excess fluo-
comparable to those obtained by using strong bases. Car- rides or lithium halides as stabilizing additives.[16] Finally,
bonyl functional groups such as aldehydes, ketones, and es- to our satisfaction, the present protocol can also be success-
ters were compatible, even if positioned ortho to the reac- fully applied to reactions with potassium trifluorobor-
tion site. For an α-amino acid derivative prone to base-in- ates[3,17] as the coupling partner (Entries 9 and 10). With
duced racemization (Entry 11), the optical integrity was 5 vol.-% water, the coupling of 1c with phenyl- and styryl
fully preserved, as judged by the chiral HPLC analysis of trifluoroborate gave near quantitative yields. However, it
the product (see Supporting Information). Reactions run should be mentioned that no reaction occurred in the ab-
with tri-n-butylborane gave equally good results (Entries sence of water. Again, the mildness of our protocol could
14–16). It is noteworthy that a monosubstituted (terminal) also mitigate the problems associated with the strongly ba-
epoxide remained intact (Entry15), and the nucleophilic sic aqueous conditions usually required for coupling of or-
pyridine heterocycle posed no problem. With regard to ganotrifluoroborates.
chemoselectivity, in contrast to the observations by Fu, and
To verify the rate enhancement of aryl triflates, we car-
Brown and co-workers, certain aryl triflates reacted prefer- ried out an intermolecular competition study using 1z and
entially with respect to bromoarenes at both room tempera- 1aa as substrates, to eliminate possible electronic interac-
ture and at reflux (Entries 5, 12), and this may complement tions in the intramolecular examples (Scheme 1). By as-
the usual order of reactivity manifested in most catalytic suming the para-ethyl and -methyl ester possess virtually
systems.[1a,12]
the same electronic and steric effects, these two substrates
The potential of CsOAc in other mechanistically related represent the true intrinsic reactivities of aryl triflates and
reactions was then probed (Table 3). Aryl bromides under- bromides, respectively, in the present protocol. By measur-
went coupling in THF under reflux in excellent yields (En- ing the ratio of Et/Me esters in the crude coupling products
tries 1, 5, 6). Notably, it was demonstrated that CsOAc also by GC-MS, a rough estimate of kOTf/kBr could thus be ob-
promoted aryl–aryl coupling by using aryl boronic acids tained. It turned out that at room temperature, the reactiv-
(pKa ≈ 9); generally, bases stronger than acetates are re- ity of triflate 1z for B-alkyl coupling was indeed higher than
quired (Entries 2–6).[13] The anhydrous conditions allowed that of bromide 1aa, while in the analogous B-aryl cou-
the use of moisture-sensitive 2-formylphenylboronic acid, pling, the aryl bromide was slightly more reactive. On the
which is known to undergo protodeboronation under aque- other hand, elevated temperatures (70 °C) increased the re-
ous conditions.[14] It is also worth noting that highly base- action rate of the aryl bromide reaction by twice as much
labile phenolic acetyl protection was well tolerated (Entry relative to that of the triflate reaction, and 1aa is the pre-
3).[15] Furthermore, Stille coupling between 1c and a couple ferred substrate in both types of coupling. Thus, in retro-
of stannanes, including one with a trisubstituted alkenyl spect, the triflate-selective coupling of 1h and 1o do involve
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B. Wang, H.-X. Sun, Z.-H. Sun
SHORT COMMUNICATION
Table 3. Applications of CsOAc in cross-coupling reactions.[a]
determining step. On the other hand, the displacement of
Br– by AcO– from the covalent intermediate L2ArPdBr is
conceivably slower, and this may instead become part of the
rate-determining step for the coupling of aryl bromides. The
temperature dependence of the relative reactivity in the
above kinetic study also supports this hypothesis.
Unlike Cs2CO3, the mixing of CsOAc with Et3B showed
no evidence of quaternization of boron, as indicated by 11
B
NMR spectroscopy, hence borane must enter the catalytic
cycle after the formation of L2ArPdOAc. Thus, we conjec-
ture that, in this protocol where monodentate strong bases
such as HO– or RO– are precluded and the only nucleo-
phile/base available is the acetoxy anion, it is the carbonyl
oxygen of the acetatopalladium(II) intermediate, rather
than the oxygen bonded to Pd, that coordinates with the
electrophilic boron atom, thus activating the B–C bond.[20]
Therefore, the transmetalation is likely to proceed via a six-
membered SE2 (cyclic) transition state (TS2, Figure 1),
which is distinct from the conventional mechanism (TS1, Y
= H, alkyl).[21]
[a] All reactions performed on a 0.50–1.0-mmol scale. [b] Isolated
yields. [c] 3.0 equiv. PhB(OH)2 and 4.0 equiv. CsOAc were used. [d]
PMP = p-methoxyphenyl. [e] 5 vol.-% water was added.
Figure 1. Comparison of transition-state models for Suzuki cou-
pling and related reactions.
The high reaction rate suggests that TS2 is energetically
favorable because of less strain in its geometry.[22] TS2 also
finds analogy in the protolysis of alkylboranes in which
weaker carboxylic acids are much more effective than min-
eral acids, for the former are unique in being able to adopt
six-membered transition structures by coordination through
the carbonyl group (TS3).[22,23] Moreover, important recent
studies on topics as diverse as Pd-catalyzed direct aryl-
ation,[24] aerobic oxidation,[25] Ru-catalyzed C–H acti-
vation,[26] and Suzuki-type coupling[27] all suggest the inter-
mediacy of carboxylato–metal species and the nucleophilic
role of the carbonyl oxygen atom. Thus, the efficiency of
the acetate-promoted Suzuki coupling fits logically in this
context. Transition states analogous to TS2 could also be
invoked for the coupling by using boronic acids (X = OH),
organotrifluoroborates (X = F), and stannanes, as well as
for Suzuki reactions employing cesium carbonate[10] or
K3PO4 as the base, whose bidentate character resembles
that of AcO–.
electronic interactions between the two substituents. Never-
theless, the enhanced reactivity of triflates is still note-
worthy and useful.
Scheme 1. Kinetic study on electrophile reactivity.
The paradoxical high efficiency of the weak base CsOAc,
the generality of the Pd-catalyzed couplings it promoted,
and the unusual enhanced reactivity of aryl triflates suggest
that a neglected mode of action could be present, probably
within the transmetalation process.[18] As the TfO– anion
Conclusions
In summary, the weak base CsOAc promotes Suzuki
is non-coordinating, the cationic PdII species formed upon coupling and related Pd-catalyzed reactions as effectively as
oxidative addition would be rapidly trapped by CsOAc to stronger bases. Its mildness offers an attractive option when
irreversibly form an acetato complex L2ArPdOAc,[19] and base-sensitivity needs to be addressed. In addition, the reac-
the subsequent transmetalation is most probably the rate- tivity of aryl triflates is comparable to that of bromoarenes
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Suzuki–Miyaura Cross-Coupling Protocol with Weak Base and No Water
and markedly higher than usual, and, in some cases, good
to excellent triflate-selectivity (Ͼ6:1) could be achieved.
[1]
Selected reviews: a) N. Miyaura, A. Suzuki, Chem. Rev. 1995,
95, 2457–2483; b) N. Miyaura in Metal-Catalyzed Cross-Cou-
pling Reactions, 2nd ed. (Eds: A. de Meijere, F. Diederich),
Wiley-VCH, Weinheim, 2004, pp. 41–123; c) A. Suzuki in Bo-
ronic Acids (Ed.: D. G. Hall), Wiley-VCH, Weinheim, 2005, pp.
123–170; d) A. Suzuki in Modern Arene Chemistry (Ed.: D.
Astruc), Wiley-VCH, Weinheim, 2002, pp. 53–98; e) A. Suzuki,
H. C. Brown, Suzuki Coupling, Organic Syntheses via Boranes,
Aldrich Chemical Company, Milwaukee WI, 2003, vol. 3; f) A.
Suzuki, J. Organomet. Chem. 2002, 653, 147–168; g) N. Mi-
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Lahiri, D. Kashinath, Tetrahedron 2002, 58, 9633–9695.
For a review of the coupling of aryl chlorides, see: F. C. Littke,
G. C. Fu, Angew. Chem. 2002, 114, 4350–4386; Angew. Chem.
Int. Ed. 2002, 41, 4176–4211 and references cited therein.
For excellent reviews, see: a) G. A. Molander, N. Ellis, Acc.
Chem. Res. 2007, 40, 275–286; b) S. Darses, J.-P. Genet, Chem.
Rev. 2008, 108, 288–325.
Mechanistically, a neglected six-membered transition-state
model is proposed to give alternative insight into the key
process of transmetalation. It differs from the textbook
mechanism not only in the composition and geometry of
the TS, but in that it suggests a new mode of action for
bidentate bases in Suzuki coupling reactions. Awareness of
the essential and versatile roles of carboxylate anions as η1
ligands capable of activating reaction components through
the nucleophilic carbonyl oxygen atom could have a signifi-
[2]
cant bearing on the design of novel metal-catalyzed reac-
tions such as cross-coupling and C–H functionalization.
[3]
Experimental Section
[4]
a) N. Miyaura, T. Ishiyama, H. Sasaki, M. Ishikawa, M. Satoh,
A. Suzuki, J. Am. Chem. Soc. 1989, 111, 314–321. For a review,
see: b) S. R. Chemler, D. Trauner, S. J. Danishefsky, Angew.
Chem. 2001, 113, 4676–4701; Angew. Chem. Int. Ed. 2001, 40,
4544–4568.
General Information: All reactions were performed in oven-dried
glassware under an atmosphere of argon. All 1H NMR and 13C
NMR spectra were recorded at ambient temperature in CDCl3 un-
less otherwise noted. Chemical shifts are reported in parts per mil-
lion as follows: chemical shift, multiplicity (s = singlet, d = doublet,
t = triplet, q = quartet, m = multiplet, br. = broad), coupling con-
stant, and integration. Optical rotations were measured at ambient
temperature, and concentrations are reported in g per 100 mL. GC-
MS spectra were recorded on an Agilent 6890N apparatus coupled
with an Agilent 5975 mass spectrometer, by using an HP-5MS col-
umn (30 mϫ250 µmϫ0.25 µm). Melting points were uncorrected.
THF was distilled from sodium benzophenone ketyl, and all other
solvents and chemical reagents were used as received.
[5]
For leading references, see: a) B. Saito, G. C. Fu, J. Am. Chem.
Soc. 2007, 129, 9602–9603; b) F. Gonzalez-Bobes, G. C. Fu, J.
Am. Chem. Soc. 2006, 128, 5360–5361; c) J. Zhou, G. C. Fu, J.
Am. Chem. Soc. 2004, 126, 1340–1341; d) M. R. Netherton,
G. C. Fu, Angew. Chem. 2002, 114, 4066–4068; Angew. Chem.
Int. Ed. 2002, 41, 3910–3912; e) J. H. Kirchhoff, M. R. Nether-
ton, I. D. Hills, G. C. Fu, J. Am. Chem. Soc. 2002, 124, 13662–
13663; f) J. H. Kirchhoff, C. Dai, G. C. Fu, Angew. Chem. 2002,
114, 2025–2027; Angew. Chem. Int. Ed. 2002, 41, 1945–1947;
g) M. R. Netherton, C. Dai, K. Neuschütz, G. C. Fu, J. Am.
Chem. Soc. 2001, 123, 10099–10100.
For a recent DFT study of the role of the base in Suzuki cou-
pling, see: A. A. C. Braga, N. H. Morgon, G. Ujaque, F. Ma-
seras, J. Am. Chem. Soc. 2005, 127, 9298–9307. However, this
paper does not seem to cover the mechanism of B-alkyl cou-
pling.
N. Miyaura, J. Organomet. Chem. 2002, 653, 54–57 and refer-
ences cited therein.
a) K. Matos, J. A. Soderquist, J. Org. Chem. 1998, 63, 461–470;
b) B. H. Ridgway, K. A. Woerpel, J. Org. Chem. 1998, 63, 458–
460.
This situation parallels that of Stille coupling, for a review, see:
P. Espinet, A. M. Echavarren, Angew. Chem. 2004, 116, 4808–
4839; Angew. Chem. Int. Ed. 2004, 43, 4704–4734.
a) B. Wang, H.-X. Sun, Z.-H. Sun, G.-Q. Lin, Adv. Synth. Ca-
tal. 2009, 351, 415–422; b) H.-X. Sun, Z.-H. Sun, B. Wang,
Tetrahedron Lett. 2009, 50, 1596–1599.
a) B. Wang, H.-X. Sun, Z.-H. Sun, J. Org. Chem. 2009, 74,
1781–1784; b) B. Wang, H.-X. Sun, B. Chen, Z.-H. Sun, Green
Chem. 2009, DOI: 10.1039/B905443J.
a) G. Espino, A. Kurbangalieva, J. M. Brown, Chem. Commun.
2007, 1742–1744; b) F. C. Littke, C. Dai, G. C. Fu, J. Am.
Chem. Soc. 2000, 122, 4020–4028; c) T. Oh-e, N. Miyaura, A.
Suzuki, J. Org. Chem. 1993, 58, 2201–2208.
NaHCO3 (pKa1 6.35) or Et3N (pKa 10.8) were only occasion-
ally effective for Suzuki coupling, and both are much more
basic than CsOAc (pKa 4.75). On the other hand, the fre-
quently used K3PO4 (pKa3 12.67) is a much stronger base. For
a survey of the acidity of boronic acids, see: D. G. Hall (Ed.:
D. G. Hall) in Boronic Acids Wiley-VCH, Weinheim, 2005, pp.
10.
a) S. Gronowitz, V. Bobosik, K. Lawitz, Chem. Scripta 1984,
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General Procedure: Triethylborane (1.0  in THF, 2.0 mL,
2.0 mmol) was added to a suspension of aryl triflate (1.0 mmol),
Pd(dppf)Cl2 (15 mg, 0.02 mmol), and CsOAc (384 mg, 2.0 mmol)
in THF (3.0 mL) under an argon atmosphere, and the mixture was
heated at reflux for 0.5–6 h until all the starting material was con-
sumed. After cooling, the reaction mixture was diluted with diethyl
ether, washed with aq. NaHCO3 and brine, dried (Na2SO4), and
concentrated. The residue was purified by silica gel flash column
chromatography.
[6]
[7]
[8]
2-[(3-Butylphenoxy)methyl]oxirane (2r): Triflate 1r (129 mg,
0.43 mmol) coupled with nBu3B to afford 2r (70 mg, 79% yield) as
a colorless oil. 1H NMR (CDCl3, 300 MHz): δ = 7.18 (t, J =
7.8 Hz, 1 H, Ar-H), 6.82–6.70 (m, 3 H, Ar-H), 4.19 (dd, J = 10.5,
3.0 Hz, 1 H, 3-Ha), 3.96 (dd, J = 10.8, 5.7 Hz, 1 H, 3-Hb), 3.36 (m,
1 H, 2-H), 2.90 (dd, J = 5.1, 3.9 Hz, 1 H, 1-Ha), 2.75 (dd, J = 5.1,
2.4 Hz, 1 H, 1-Hb), 2.58 (t, J = 7.8 Hz, 2 H, ArCH2), 1.65–1.53
(m, 2 H, ArCH2CH2), 1.42–1.27 (m, 2 H, CH2CH3), 0.92 (t, J =
7.2 Hz, 3 H, CH3) ppm. 13C NMR (CDCl3, 100 MHz): δ = 158.4,
144.7, 129.2, 121.4, 114.9, 111.4, 68.6, 50.2, 44.8, 35.6, 33.5, 22.3,
[9]
[10]
[11]
[12]
14.0 ppm. HR-ESI-MS: calcd. for C13H18O2Na [M
229.1204; found 229.1199.
+
Na]+
Supporting Information (see footnote on the first page of this arti-
cle): Characterization data and NMR spectra for new compounds
are presented.
[13]
[14]
Acknowledgments
We thank the National Natural Science Foundation of China for
a Major Program Funding (20832005) and a Youth Funding
(20602008). Financial support from Fudan University
(EYH1615003) is gratefully acknowledged.
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B. Wang, H.-X. Sun, Z.-H. Sun
SHORT COMMUNICATION
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[27] Recently, Garg and Shi reported the Ni-catalyzed Suzuki-type
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[21] For a more complex ternary TS model for CuTC-mediated
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Received: May 15, 2009
Published Online: June 22, 2009
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