Palladium-catalyzed cross-coupling of aryldiazonium tetrafluoroborate salts with arylboronic esters

Article abstract of DOI:10.1016/S0040-4039(00)01076-5

A novel palladium-catalyzed reaction of aryldiazonium tetrafluoroborate salts with arylboronic esters furnishes cross-coupled products in moderate yields. The reaction proceeds at relatively mild temperatures and in aqueous media. (C) 2000 Published by Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01076-5

Tetrahedron Letters 41 (2000) 6271±6274
Palladium-catalyzed cross-coupling of aryldiazonium
tetra¯uoroborate salts with arylboronic esters
Douglas M. Willis and Robert M. Strongin*
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, USA
Received 9 June 2000; revised 23 June 2000; accepted 26 June 2000
Abstract
A novel palladium-catalyzed reaction of aryldiazonium tetra¯uoroborate salts with arylboronic esters
furnishes cross-coupled products in moderate yields. The reaction proceeds at relatively mild temperatures
and in aqueous media. # 2000 Published by Elsevier Science Ltd.
Palladium-catalyzed coupling reactions have attracted signi®cant attention during the past two
1
decades. Of recent interest is the palladium-catalyzed cross-coupling of aryldiazonium tetra-
2
¯
uoroborate salts with arylboronic acids . In addition, aryldiazonium tetra¯uoroborate salts
3
have also been successfully cross-coupled to organotri¯uoroborates. Suitable conditions for the
successful direct, catalytic cross-coupling of aryldiazonium tetra¯uoroborate salts to arylboronic
esters, however, had not been found.4
Scheme 1.
*
Corresponding author. Tel: +1 225-388-3238; fax: +1 225-388-3458; e-mail: rob.strongin@chem.lsu.edu
0040-4039/00/$ - see front matter # 2000 Published by Elsevier Science Ltd.
PII: S0040-4039(00)01076-5

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272
The relatively mild preparation of arylboronic esters tolerates a broad range of functional
5
6
groups and a€ords access to one-pot conversion to cross-coupled products. The diazonium
electrophile, which can be synthesized from inexpensive, readily available anilines, is the most
7
powerful nucleofuge used in di€erential cross-couplings. The cross-coupling of aryldiazoniums
with arylboronates would thus be of broad synthetic utility. As part of our program towards
ecient syntheses of well-de®ned oligoaromatic materials8 for sensory applications,9 we
decided to explore the palladium-catalyzed cross-coupling of aryldiazonium tetra¯uoroborate
10
salts 1 with arylboronic esters 2 (Scheme 1). Herein we report our preliminary ®ndings concerning
the synthesis of unsymmetric biaryls in moderate yields in aqueous MeOH, at mild temperatures.
We initially screened the reaction of 1a (1.0 mmol) and 2a (1.5 mmol) in a variety of solvents
(
(
2 mL) in the presence of CsF or KF (2 equiv.)11 at room temperature for 24 h, employing PdCl2
5±10 mol%) as the catalyst. We monitored reaction progress by GC/MS. In anhydrous or 50%
Table 1
Cross-coupling of aryldiazonium tetra¯uoroborate salts 1 with arylboronic esters 2

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aqueous MeOH solvent, complete consumption of starting materials and the best relative
conversion (54%) to cross-coupled product 3a was observed in these initial attempts. The polar
aprotic solvents DMF and MeCN led to only trace consumption of starting materials. In two
anhydrous polar aprotic solvents (THF or Me CO), no reaction occurred; however, with 50%
2
12
aqueous solutions of THF or Me CO we observed complete consumption of starting materials.
2
Based on these ®ndings, we decided to study the cross coupling of several aryldiazonium
tetra¯uoroborate salts with arylboronates using 75% MeOH as the solvent, CsF as the base and
the more aqueous soluble Na PdCl as the catalyst (Table 1).
2
4
ꢀ
As shown in Table 1, the cross-coupling reactions, after 20±24 h at 40 C under inert atmosphere
a€orded unsymmetric biaryls 3 in moderate yields.13 Aryldiazonium tetra¯uoroborate salts 1
containing electron donating, electron withdrawing or sterically encumbered ortho-substituted
moieties furnished cross-coupled products. The reaction of 4-iodophenyldiazonium tetra¯uoroborate
salt 1c with 2b furnished 4-iodo-2 -methylbiphenyl 3c as the only cross-coupled product,
highlighting the outstanding regioselectivity a€orded by the diazonium moiety.
0
In conclusion, the current study demonstrates that readily available aryldiazonium
tetra¯uoroborate salts react with functional group tolerant arylboronic esters in aqueous solvents
at mild temperatures to form isolable cross-coupled products. Ongoing optimization studies are
aimed at further elucidating the mechanism, scope and limitations of the reaction.
Acknowledgements
We gratefully acknowledge the Arnold and Mabel Beckman Foundation for support through
the Beckman Young Investigator program. D.M.W. thanks the Graduate Education for
Minorities (GEM) and Huel Perkins Foundations, as well as Procter and Gamble and Louisiana
State University for generous fellowships. We also thank Professors William Crowe and David
Spivak for helpful discussions.
References
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1

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1
1. Fluoride mediated couplings of arylboronic esters and acids: (a) Wright, S. W.; Hageman, D. L.; McClure, L. D.
J. Org. Chem. 1994, 59, 6095. (b) Desurmont, G.; Dalton, S.; Giolando, D. M.; Srebnik, M. J. Org. Chem. 1996,
6
1, 7943.
1
2. A mixture of desired product 3 and homocoupled by-products characteristically precipitates from these reactions
under the described aqueous conditions.
3. General procedure for cross-coupling of aryldiazonium tetra¯uoroborate salts 1 and arylboronic esters 2.
1
2 4
Aryldiazonium tetra¯uoroborate salt (1.0 mmol), arylboronic ester (1.5 mmol), Na PdCl (0.10 mmol) and CsF
(
mL) was added via syringe and the reaction was allowed to proceed at 40 C in a sand bath until completion.
1.5 mmol) were added to a reaction vessel purged thoroughly with N
2
in the dark. Degassed 75% MeOH/H
ꢀ
2
O (2
The solution was cooled to room temperature, concentrated, and the residue extracted with EtOAc (3Â10 mL).
4
The organic layer was dried with MgSO , passed through Celite and the ®ltrate concentrated. Silica gel
chromatography (pentane or 0±10% EtOAc in hexanes) a€orded the respective biaryls, 3a±g, terphenyl 3h and
naphthyl derivative 3i. Physical data for the cross-coupled products were consistent with the literature: the purity
of each product was veri®ed by GC/MS (tetradecane internal standard) and 1H NMR. Alkyl- and
benzyldiazonium tetra¯uoroborate salts were unreactive under these conditions.