Pd(OAc)2/p-benzoquinone-catalyzed anaerobic electrooxidative homocoupling of arylboronic acids, arylboronates and aryltrifluoroborates in DMF and/or water

Article abstract of DOI:10.1002/ejoc.200800631

A new anaerobic electrooxidative homocoupling of arylboronic acids, arylboronates and aryltrifluoroborates was developed in the presence of catalytic amounts of Pd(OAc)₂ and p-benzoquinone, which serves as a redox mediator for the oxidation of transient Pd⁰ to the active Pd^II species. The homocoupling was performed in DMF, DMF/water or in pure water. This electrochemical process avoids the use of a stoichiometric amount of chemical oxidants and subsequent formation of co- and by-products. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800631

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200800631
Pd(OAc)2/p-Benzoquinone-Catalyzed Anaerobic Electrooxidative
Homocoupling of Arylboronic Acids, Arylboronates and Aryltrifluoroborates in
DMF and/or Water
Christian Amatore,*[a] Chama Cammoun,[a] and Anny Jutand*[a]
Keywords: Palladium / Homocoupling / Boron / Borates / Electrooxidation
A new anaerobic electrooxidative homocoupling of aryl-
boronic acids, arylboronates and aryltrifluoroborates was de-
veloped in the presence of catalytic amounts of Pd(OAc)2 and
p-benzoquinone, which serves as a redox mediator for the
oxidation of transient Pd0 to the active PdII species. The
homocoupling was performed in DMF, DMF/water or in pure
water. This electrochemical process avoids the use of a stoi-
chiometric amount of chemical oxidants and subsequent for-
mation of co- and by-products.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
boronic acids ArB(OH)2 performed in the presence of O2
with PPh3 as the ligand has been established.[7] The reaction
proceeds by the formation of the peroxo O2PdL2 complex,
which undergoes transmetallation by ArB(OH)2 leading to
subsequent formation of perborate (HO)2B–OOH as a co-
product. Its hydrolysis releases H2O2, whose reaction with
ArB(OH)2 gives ArOH as a by-product.[7,6a–6c,6h] Pro-
tonolysis of palladium peroxo can also generate H2O2.[8a]
Molecular sieves have been used to decompose H2O2
formed in the catalytic process.[6j]
The use of O2 as an oxidant may also be problematic,
as the rate of homocoupling may be limited by the slow
dissolution[8b] and low solubility of O2 in most solvents,
which limits scale up of the reaction (e.g., [O2] = 5.4 m at
20 °C,[9] 4.5 m at 25 °C[8b] in DMF; [O2] = 1.3 m at 25 °C
in water[8b]).
Introduction
The problem in most PdII-catalyzed reactions arises from
the formation of a Pd0 moiety that must be oxidized back
to the active PdII species involved in the first step of the
catalytic cycle. This problem may be solved by using a
chemical oxidant in stoichiometric amount. This strategy
was successfully developed in the Wacker process,[1] acet-
oxylation of dienes,[2] Heck-type reactions from arenes,[3]
oxidation of alcohols,[4] arylation of arenes by arylboronic
acids[5] and homocoupling of arylboronic acids and aryl-
boronates (Scheme 1).[6]
Scheme 1. Pd-catalyzed homocoupling of arylboronic acids or aryl-
boronates.
We have developed an alternative anaerobic procedure
for PdII-catalyzed reactions, such as Heck-type reactions
performed from arenes[10a] or oxidation of alcohols to alde-
hydes or ketones.[10b] Pd(OAc)2 is used as a catalyst in asso-
ciation with a catalytic amount of p-benzoquinone (BQ),
which serves both as a ligand (cheaper than phosphane) for
Pd0 and as an oxidant, as pioneered by Bäckvall.[2,11] The
latter reaction generates p-hydroquinone (HQ), which is ox-
idized back to p-benzoquinone at an anode (Scheme 2). p-
Benzoquinone serves as a redox catalyst for the oxidation
of Pd0 to PdII.[10,11]
The homocoupling of arylboronic acids or arylboronates
requires either expensive ligands such as phosphanes
(PPh3,[6a,6b,6d,6f,6i] dppp[6f]), a base,[6c–6e,6g] or a cocatalyst:
CuII.[6c,6f] Some reactions are performed in pure water (but
a phase-transfer catalyst is required)[6g] or in organic sol-
vent/water mixtures (where H2O is minor or in equal volu-
me).[6e,6i] O2 (or air) is often used as the oxidant.[6a–6d,6f–6j]
The mechanism of the Pd-catalyzed homocoupling of aryl-
[a] Ecole Normale Supérieure, Département de Chimie, CNRS
UMR 8640
24 Rue Lhomond, 75231 Paris Cedex 5, France
Fax: +33-1-44-32-24-02
E-mail: Anny.Jutand@ens.fr
christian.amatore@ens.fr
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Scheme 2. p-Benzoquinone-mediated oxidation of Pd0 to PdII.
Eur. J. Org. Chem. 2008, 4567–4570
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4567

C. Amatore, C. Cammoun, A. Jutand
SHORT COMMUNICATION
The same strategy is now used for the homocoupling of Table 1. Pd(OAc)2/p-benzoquinone-catalyzed oxidative homocoup-
ling of arylboronic acids and arylboronates in DMF and/or water.
arylboronic acids or arylboronates, performed under anaer-
obic conditions, in DMF, pure H2O or H2O/DMF (4:1).
The homocoupling was also extended to the unreported
homocoupling of aryltrifluoroborates (Scheme 3).
Scheme 3. Electrooxidative homocoupling of nucleophilic boron
derivatives.
Results and Discussion
For all Ar–B reagents, the electrolyses were carried out
at the same potential of +0.75 V vs. SCE (the oxidation
potential of p-hydroquinone). The electrolyses were inter-
rupted when the current dropped to background levels.
The homocoupling of arylboronic acids was first per-
formed in DMF at 80 °C affording the biaryls in good
yields under anaerobic conditions (Table 1). When per-
formed in the absence of p-benzoquinone, the current
dropped to background levels after 1 h (Table 1, entry 1)
due to the formation of nonelectroactive palladium black.
Consequently, the remaining PhB(OH)2 could not be fur-
ther converted into biphenyl and the reaction could not go
[a] 1 mmol in 10 mL of solvent, either DMF (containing
nBu4NBF4, 0.3 ), H2O (containing Na2SO4, 0.3 ) or H2O/DMF
to completion. In contrast, the reaction went to completion
and was quantitative when performed in the presence of
10 mol-% of p-benzoquinone (Table 1, entry 2). This em-
phasizes the crucial role of p-benzoquinone, which may
serve as a ligand for the transient Pd0[11] species and oxidize
it in a faster reaction than its decomposition to Pd black.
Some reactions afforded quantitative yields after passage of
1 Faraday per mol of Ar–B(OH)2 through the cell (Table 1,
entries 2,11,13).
(4:1) (containing Na2SO4, 0.3 ). [b] Yields related to the initial
ArB(OR)2 were determined after work up by 1H NMR spec-
troscopy by using Cl2CHCHCl2 as an internal standard. [c] Iso-
lated yield in parentheses. [d] Without p-benzoquinone. [e] 5 mmol
in 50 mL of H2O/DMF (4:1) (containing Na2SO4, 0.3 ).
Interestingly, the homocoupling of arylboronic acids was
also performed in pure water or in H2O/DMF (4:1) with
Na2SO4 as supporting electrolyte. Some DMF was added to
avoid precipitation of the hydrophobic biaryl at the carbon
anode. The reaction was scaled up to 5 mmol (Table 1, en-
try 5) with a yield even better than on a 1-mmol scale
(Table 1, entry 4). The reaction was relatively fast, as
3.7 mmol were converted within 1 h (Table 1, entry 5). All
homocouplings were regiospecific and compatible with
Scheme 4.
functional groups.[12] A mechanism for the oxidative homo- transmetallation on Pd(OAc)2 by ArB(OH)2 is not so sensi-
coupling of arylboronic acids is proposed in Scheme 4. tive to the nucleophilicity of arylboronic acids (Scheme 4).
The heterocoupling of PhB(OH)2 (1 mmol) with 4- This is in agreement with the fact that the rate of the homo-
CNC6H4B(OH)2 (2 mmol) was tested under the experimen- coupling of arylboronic acids reported in Table 1 is not sig-
tal conditions of Table 1 in DMF. A nonoptimized reaction nificantly affected by the electron-donating or -withdrawing
delivered
PhC6H4CN
a
mixture of isolated PhPh (0.22 mmol), properties of the substituent on the Ar group.
(0.39 mmol) and 4-CNC6H4–C6H4CN The homocoupling of arylboronates was performed in
(0.51 mmol). 4-CNC6H4B(OH)2 was expected to be less re- DMF or H2O/DMF (4:1) (Table 1, entries 16–19) although
active than PhB(OH)2 in the transmetallation of Pd(OAc)2 they are less water soluble than arylboronic acids. The
and was consequently added in a larger amount to compen- homocoupling of aryboronate 1 did not afford expected bi-
sate its lower reactivity. Nevertheless, 4-CNC6H4–C6H4CN- aryl 2 (Scheme 5). First of all, a faster C–H activation by
4 was formed in the largest amount. This suggests that the Pd(OAc)2 than transmetallation was postulated, as ob-
4568
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 4567–4570

Pd(OAc)2/p-Benzoquinone-Catalyzed Electrooxidative Homocoupling
Scheme 5. Tentative Pd(OAc)2/p-benzoquinone-catalyzed electrooxidative reactions from 1.
served in Heck-type reactions performed with arenes substi- in DMF (Table 2). The electrooxidative homocoupling of
–
tuted by the ortho-directing acetamido group.[10a] Such a ionic PhBF3 K+, which could serve as supporting electro-
reaction could have served as a quencher for the PdII cata- lyte, was tested in DMF in the absence of the usual support-
lyst, but when 1 was treated with CH2=CHCO2nBu under ing electrolyte nBu4NBF4. The reaction was slower and
the experimental conditions developed by us for the vinyla- 32% of Ph–Ph was formed (Table 2). Because aryltrifluoro-
tion of arenes,[10a] expected alkene 3 was not produced borates are water soluble, the homocoupling was performed
(Scheme 5). Instead, the reaction gave compound 4 formed in pure water. However, the reactions were more efficient
in an unprecedented electrooxidative Heck reaction,[13] when conducted in H2O/DMF (4:1) (Table 2). The homo-
which involves monotransmetallation of Pd(OAc)2 by the coupling was regiospecific and good compatibility was ob-
arylboronate. The reason why 2 was not formed must arise served in the para-bromo derivative, which indicates that
from the unfavourable second transmetallation from the Pd0 species formed in the catalytic cycle was oxidized
Pd(OAc)2 (Scheme 4).
to PdII before undergoing oxidative addition at the C–Br
To the best of our knowledge, the PdII-catalyzed homo- bond.
coupling of aryltrifluoroborates in the presence of a stoi-
chiometric oxidant (as O2 or air) has never been re-
ported.[14] It was first checked that the aryltrifluoroborates
investigated herein were not oxidized in DMF in the range
0 to +1.6 V vs. SCE (gold disk electrode, d = 1 mm, scan
rate = 0.5 Vs–1). In the presence of Pd(OAc)2 (10 mol-%)
and p-benzoquinone (10 mol-%), the oxidative homocoup-
ling proceeded with good yields under anaerobic conditions
Conclusions
We developed a simple anaerobic, quite fast electrooxid-
ative homocoupling of aryl boronic acids, arylboronates
and aryltrifluoroborates catalyzed by ligandless Pd(OAc)2
and p-benzoquinone. This procedure avoids the use of stoi-
chiometric chemical oxidants (as O2), the formation of per-
oxo moieties and subsequent by-products. The homocoup-
ling can be performed in pure water. The reaction involves
cheap and water-soluble supporting electrolyte (Na2SO4) as
well as cheap and water-soluble p-benzoquinone as a redox
mediator. Work is in progress on the heterocoupling of aryl-
boron derivatives and on the homocoupling of aryl and vi-
nylstannanes.
Table 2. Pd(OAc)2/p-benzoquinone-catalyzed oxidative homocoup-
ling of aryltrifluoroborates (K+ salts) in DMF and/or water.
Experimental Section
General Procedure for Preparative Electrolysis Performed from Po-
tassium Aryltrifluoroborates: The electrosynthesis of 4,4Ј-dimeth-
ylbiphenyl (Table 2) was performed under an atmosphere of argon
at 80 °C in a two-compartment cell. The two compartments were
separated by a sintered glass disk. The cathode was a nickel foam
(ca. 1 cm2 surface area). The anode was a carbon cloth (ca. 4 cm2
surface area). The reference was a saturated calomel electrode sepa-
rated from the solution by a bridge filled with a solution of Na2SO4
(0.3 ) in H2O/DMF (4:1). The anodic and cathodic compartments
were respectively filled with 10 and 2 mL of H2O/DMF (4:1) con-
taining Na2SO4 (0.3 ). Potassium 4-methylphenyltrifluoroborate
(198 mg, 1 mmol) was added into the anodic compartment fol-
lowed by sublimed p-benzoquinone (11 mg, 0.1 mmol) and
Pd(OAc)2 (22 mg, 0.1 mmol). Acetic acid (150 µL, 2.6 mmol) was
introduced into the cathodic compartment (reduction of protons
[a] 1 mmol of ArBF3K in 10 mL of solvent, DMF (containing
nBu4NBF4, 0.3 ), H2O (containing Na2SO4, 0.3 ) or H2O/DMF
(4:1) (containing Na2SO4, 0.3 ). [b] Yields related to the initial
–
1
ArBF3 were determined after work up by H NMR spectroscopy
by using Cl2CHCHCl2 as an internal standard. The amount of re-
–
covered ArBF3 (remaining in the aqueous phase) was not deter-
mined. [c] Without nBu4NBF4.
Eur. J. Org. Chem. 2008, 4567–4570
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4569

C. Amatore, C. Cammoun, A. Jutand
SHORT COMMUNICATION
[5] a) Z. Shi, B. Li, X. Wan, J. Cheng, Z. Fang, B. Cao, Y. Wang,
Angew. Chem. Int. Ed. 2007, 46, 5554; b) S.-D. Yang, C.-L.
Sun, Z. Fang, B.-J. Li, Y.-Z. Li, Z.-J. Shi, Angew. Chem. Int.
Ed. 2008, 47, 1473.
during the electrolysis). The electrolysis was carried out at a con-
trolled potential of +0.75 V by using a Tacussel PJT 35-2 potentios-
tat. The electrolysis was stopped when the current dropped to back-
ground levels, after 23 min. A charge of 110 C was passed through
the cell (theoretical charge: 106 C). After cooling to room tempera-
ture, the anodic compartment was hydrolyzed with water (40 mL).
After extraction with diethyl ether, the organic phase was dried
with MgSO4, and the solvents were evaporated. The yield of 4,4Ј-
dimethylbiphenyl (94%) was determined on the crude product by
1H NMR (250 MHz) spectroscopy by using Cl2CHCHCl2
(0.5 mmol) as an internal standard. 4,4Ј-Dimethylbiphenyl was iso-
lated as a pure compound (white powder) by flash chromatography
(petroleum ether/ethyl acetate, 95:5). M.p. 119 °C. 1H NMR
(250 MHz, CDCl3): δ = 2.31 (s, 6 H, CH3), 7.16 (d, J = 7.6 Hz, 4
H, o-H relative to CH3), 7.41 (d, J = 7.6 Hz, 4 H, m-H relative to
CH3) ppm. 13C NMR (62.89 MHz, CDCl3): δ = 20.03, 125.76,
128.39, 135.63, 137.25 ppm. MS (EI): m/z = 182 [M]+, 167 (100),
152. Data are similar to those of an authentic sample.[15]
[6] a) M. Moreno-Mañas, M. Pérez, R. Pleixats, J. Org. Chem.
1996, 61, 2346; b) M. A. Aramendia, M. Lafont, M. Moreno-
Mañas, M. Pérez, R. Pleixats, J. Org. Chem. 1999, 64, 3592; c)
A. Smith, E. M. Campi, W. R. Jackson, S. Marcuccio, C. G. M.
Naeslund, G. B. Deacon, Synlett 1997, 131; d) M. S. Wong,
X. L. Zhang, Tetrahedron Lett. 2001, 42, 4087; e) G. W. Kab-
alka, L. Wang, Tetrahedron Lett. 2002, 43, 3067; f) D. J. Koza,
E. Carita, Synthesis 2002, 2183; g) J. P. Parrish, Y. C. Jung, R. J.
Floyd, K. W. Jung, Tetrahedron Lett. 2002, 43, 7899; h) H.
Yoshida, Y. Yamaryo, J. Ohshita, A. Kunai, Tetrahedron Lett.
2003, 44, 1541; i) S. Punna, D. D. Diaz, M. G. Finn, Synlett
2004, 2351; j) C. Zhou, R. C. Larock, J. Org. Chem. 2006, 71,
3184.
[7] a) C. Adamo, C. Amatore, I. Ciofini, A. Jutand, H. Lakmini,
J. Am. Chem. Soc. 2006, 128, 6829; b) H. Lakmini, I. Ciofini,
A. Jutand, C. Amatore, C. Adamo, J. Phys. Chem. A, 2008,
DOI: 10.1021/jp801948 (special issue).
[8] a) S. S. Stahl, J. L. Thormann, R. C. Nelson, M. A. Kozee, J.
Am. Chem. Soc. 2001, 123, 7188; b) B. A. Steinhoff, S. S. Stahl,
J. Am. Chem. Soc. 2006, 128, 4348.
The electrosyntheses of the biaryls reported in Tables 1 and 2 are
depicted in the Supporting Information
Supporting Information (see footnote on the first page of this arti-
[9] C. Amatore, S. Aziz, A. Jutand, G. Meyer, P. Cocolios, New J.
Chem. 1995, 19, 1047.
1
cle): H and 13C NMR spectra and MS of the biaryl products.
[10] a) C. Amatore, C. Cammoun, A. Jutand, Adv. Synth. Catal.
2007, 349, 292; b) C. Amatore, C. Cammoun, A. Jutand, Syn-
lett 2007, 2173.
[11] a) J.-E. Bäckvall, A. Gogoll, J. Chem. Soc., Chem. Commun.
1987, 1236; b) H. Grennberg, A. Gogoll, J.-E. Bäckvall, Orga-
nometallics 1993, 12, 1790.
Acknowledgments
This work was supported in part by the Centre National de la Re-
cherche Scientifique (UMR CNRS-ENS-UPMC 8640) and the
Ministère de la Recherche (Ecole Normale Supérieure). We thank
Johnson Matthey for generous loan of palladium salt.
[12] The fate of the B(OH)2 moiety released in the reaction was not
investigated. B(OH)3 must be formed after work up.
[13] For air-promoted oxidative Heck reactions from arylboronic
acids, see: a) Y. C. Jung, R. K. Mishra, C. H. Yoon, K. W.
Jung, Org. Lett. 2003, 5, 2231; b) M. M. S. Andappan, P. Nils-
son, M. Larhed, Molecules 2003, 7, 97; c) P.-A. Enquist, P.
Nilsson, P. Sjöberg, M. Larhed, J. Org. Chem. 2006, 71, 8779.
[14] A recent paper reports the homocoupling of potassium aryltri-
fluoroborates under aerobic conditions in the presence of gold
nanoclusters. See: H. Sakurai, H. Tsunoyama, T. Tsukuda, J.
Organomet. Chem. 2007, 692, 368.
[1] P. M. Henry in Handbook of Organopalladium Chemistry for
Organic Synthesis (Ed.: E.-i. Negishi), Wiley-VCH, New York,
2002, vol. 2, p. 2119.
[2] a) J.-E. Bäckvall, Acc. Chem. Res. 1983, 16, 335; b) J.-E.
Bäckvall, Pure Appl. Chem. 1992, 64, 429.
[3] M. D. K. Boele, G. P. F. van Strijdonck, A. H. M. de Vries,
P. C. J. Kamer, J. G. de Vries, P. W. N. M. van Leeuwen, J. Am.
Chem. Soc. 2002, 124, 1586.
[15] A. Jutand, A. Mosleh, J. Org. Chem. 1997, 62, 261.
Received: June 25, 2008
Published Online: August 15, 2008
[4] K. P. Peterson, R. C. Larock, J. Org. Chem. 1998, 63, 3185.
4570
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 4567–4570