Borylation of Aryldiazonium Ions with N-Heterocyclic Carbene-Palladium Catalysts Formed without Added Base

Article abstract of DOI:10.1021/ol035857q

(Matrix presented) A highly efficient catalytic borylation process with aryldiazonium ions was developed using a carbene-palladium catalyst formed in situ to give arylpinacolatoborane products. An X-ray structure for the N-heterocyclic carbene-palladium complex, used as the catalyst formed from bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolium chloride, was obtained without added base.

Full text of DOI:10.1021/ol035857q

ORGANIC
LETTERS
2
003
Vol. 5, No. 24
635-4638
Borylation of Aryldiazonium Ions with
N-Heterocyclic Carbene−Palladium
Catalysts Formed without Added Base
4
†
†
Yudao Ma, Chun Song, Wei Jiang, Guoping Xue, John F. Cannon,
†
Xiaomei Wang, and Merritt B. Andrus*
Brigham Young UniVersity, Department of Chemistry and Biochemistry, C100 BNSN,
ProVo, Utah 84602, and Chemistry College of Shandong UniVersity,
Shanda Road #100, Jinan, Shandong, 250100, P. R. China
mbandrus@chemdept.byu.edu
Received September 24, 2003
ABSTRACT
A highly efficient catalytic borylation process with aryldiazonium ions was developed using a carbene−palladium catalyst formed in situ to
give arylpinacolatoborane products. An X-ray structure for the N-heterocyclic carbene−palladium complex, used as the catalyst formed from
bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolium chloride, was obtained without added base.
Catalyzed reactions with N-heterocyclic carbene (NHC)-
palladium complexes have become the subject of intense
of bis-cyclophane NHC ligands was recently shown to
catalyze arylborane additions to enones with high selectivity.7
We have reported base-free conditions with NHC catalysts
for efficient Heck and Suzuki couplings using reactive
1
interest due to their enhanced reactivity and stability.
2
Successful transformations now include Heck, Suzuki-
Miyaura, Hiyama, and Kumada couplings together with
hydrogenation reactions. In addition, NHC ligands hold great
3
4
5
8
aryldiazonium ions, as part of an effort to develop milder,
2
low-temperature conditions for these transformations. 4,5-
Dehydroimidazolium carbene-palladium complexes are now
reported for borylation of aryldiazonium ions at room
temperature without added base. X-ray structures are reported
promise for catalyzing asymmetric transformations, but to
6
date, this effort remains largely undeveloped. A new class
†
2
for the palladium(II) iodide-H IPr (N,N-bis-2,6-diisopropyl-
Chemistry College of Shandong University.
(
1) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290-1309.
b) Herrmann, W. A.; Kocher, C. Angew. Chem., Int. Ed. Engl. 1997, 36,
162-2187.
2) (a) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G.
4,5-dihydroimidazolium carbene) complex and, for the first
time, the palladium(II) chloride complex formed in the
absence of base. This unambiguous structure determination
suggests an alternative mode for base-free carbene formation.
Imidazolium carbene ligands provide enhanced stability
and reactivity compared to phosphines through strong σ-bond
donation to the metal, together with attenuated back-bonding
(
2
(
R. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 2371-2373. (b) Yang, C.;
Lee, H. M.; Nolan, S. P. Org. Lett. 2001, 3, 1511-1514.
(3) (a) Zhang, C.; Huang, J.; Trudell, M. L.; Nolan, S. P. J. Org. Chem.
1
2
1
999, 64, 3804-3806. (b) F u¨ rstner, A.; Leitner, A. Synlett 2001, 290-
92. (c) Grasa, G. A.; Hillier, A. C.; Nolan, S. P. Org. Lett. 2001, 3, 1077-
080.
1
a
(
4) Lee, H. M.; Nolan, S. P. Org. Lett. 2000, 2, 2053-2056.
via N-lone pair donation. This combination of electronic
effects renders the metal more electron rich, allowing for a
(5) (a) Huang, J.; Nolan, S. P. J. Am. Chem. Soc. 1999, 121, 9889-
9
890. (b) B o¨ lm, V. P. W.; Weskamp, T.; Gst o¨ ttmayr, C. W. K.; Herrman,
W. A. Angew. Chem., Int. Ed. 2000, 39, 1602-1604.
6) For recent efforts, see: (a) Perry, M. C.; Cui, X.; Powell, M. T.;
Hou, D. R.; Reibenspies, J. H.; Burgess, K. J. Am. Chem. Soc. 2003, 125,
13-124. (b) Glorius, F.; Altenhoff, G.; Goddard, R.; Lehmann, C. Chem.
Commun. 2002, 2704-2705.
(
(7) Ma,Y.; Song, C.; Ma, C.; Sun, Z.; Chai, Q.; Andrus, M. B. Angew.
Chem., Int. Ed. 2003, 42, in press.
(8) (a) Andrus, M. B.; Song, C. Org. Lett. 2001, 3, 3761-3764. (b)
Andrus, M. B.; Song, C.; Zhang, J. Org. Lett. 2002, 4, 2079-2082.
1
1
0.1021/ol035857q CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/23/2003

more favorable oxidative addition step. Typical NHC com-
plexes, formed by treatment of an imidazolium salt 1 with
base, give the free carbene 2 (Scheme 1). Free carbenes of
Table 1. Effect of Ligand and Conditions for Aryl Borylation
Scheme 1
T, time, yield,a bi-Ar,
entry ligand
Ar
°C
h
%
%
1
2
3
4
5
6
7
4
4
5
5
6
6
7
mesityl
2,6-di-i-PrPh
23
65
23
65
2
57
50
79
68
31
10
45
1
2
1
2
1
2
28
17
31
4,5-dehydro-bis-mesityl 23
65
4,5-dehydro-
23
bis(2,6-di-i-PrPh)
8
9
0
1
2
(none)
65
23 24
65 12
23
23
23
1
41
21
37b
68b
77c
53d
38
23
trace
24
1
1
1
5
5
5
2,6-di-i-PrPh
2
2
2
28a
this type, depending on the size of the nitrogen substituent,
have been shown to be stable in solution and in crystalline
form. Treatment with a metal gives the carbene complex,
many of which are air stable and can be chromatographed.
Alternatively, the NHC-Pd complex can be formed in situ
without added base from the imidazolium precursor. The
imidazolium NHC precursors employed in this study include
the nonaromatic N,N-bis-mesityl-4,5-dihydroimidazolium
13
a
Yields are for isolated, chromatographed materials. b Reaction was
9
c
-
performed with added K2CO3 (1 equiv). Imidazolium anion was BF4 in
place of Cl . Reaction was performed in methanol as solvent.
-
d
out the ligand (entry 8) produced even more biaryl formation
at 65 °C. At room temperature without added ligand, a low
yield of product, 21%, was obtained after a 24 h reaction
period. When base was used in the absence of ligand at
elevated temperature, again the yield was low (entry 10).
With K
product was again seen (entry 11). Catalyst, Pd(OAc)
, at 0.5 mol % gave a 69% yield. Use of 0.1 mol % lowered
chloride 4 (H
dihydrochloride 5 (H
2
IMes), N,N-bis-2,6-diisopropylphenyl-4,5-
IPr) and the corresponding aromatic
2
analogues 6 (IMes‚HCl) and 7 (IPr‚HCl).
Borylation of aryldiazonium tetrafluoroborates with bis-
2
CO
3
and ligand 5, a significant amount of biaryl
2
and
(pinacolato)diborane, both used as 1 equiv, was optimized
5
using the various NHC ligand complexes formed in situ
without added base (Table 1). Ligand 4 gave only moderate
yields of product (entries 1 and 2). Imidazolium 5 used with
palladium acetate in THF proved optimal giving borylated
product after only 2 h at room temperature in 79% isolated
yield (entry 3). Importantly, the product in this case was
obtained without the formation of biaryl side product, a
drawback commonly encountered with this process. Higher
temperature led to formation of significant amounts of biaryl
coupling. The aromatic imidazolium ligands 6 and 7 were
inferior giving lower yields together with more biaryl
the yield to 58%. Catalyst used at 0.01 mol % gave 58%
product after 6 h at room temperature. Palladium/5 1:2, at 1
mol %, also gave a 79% yield after 2 h, while a 1:3 ratio of
palladium/ligand gave a slightly lower 77% yield. Changing
-
the counterion to BF
4
in place of chloride had little effect
in this case, lowering the yield to 77% (entry 12). Use of
dioxane, DMF, or methanol as solvent in place of THF gave
large amounts of biaryl formation (entry 13).
Borylation couplings involving tetraalkoxydiboron deriva-
tives including bis(pinacolato)diborane with aryl halides have
been reported previously using Pd
with added base at elevated temperatures. Use of excess
3-4 equiv) aryldiazonium ions has been reported recently
to couple with bis(pinacolato)diborane using PdCl (dppf)
2
2 3 2 3 2
(dba) or PdCl (PPh )
14
formation at elevated temperature (entries 6 and 8). Leaving
1
0
(
(
9) Arduengo, A. J. Acc. Chem. Res. 1999, 32, 913-920.
(10) (a) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60,
2
7
508-7510. (b) Takagi, J.; Takahashi, K.; Ishiyama, T.; Miyaura, N. J.
Am. Chem. Soc. 2002, 124, 8001-8006.
11) Willis, D. M.; Strongin, R. M. Tetrahedron Lett. 2000, 41, 8683-
686.
11
at 3 mol % in methanol at reflux. IMes‚HCl 6 has also
been employed recently with palladium acetate and added
potassium acetate as base to couple aryl chlorides in THF at
(
8
12) Furstner, A.; Seidel, G. Org. Lett. 2002, 4, 541-543.
12
2
reflux. The more active aryldiazonium ions used as 1 equiv
are now shown to couple with high yield at room temperature
without added base.
2
2
:1 NHC-Pd complex formed with base, see: (b) Gstottmayr, C. W. K.;
Bohm, V. P. W.; Herdtweck, E.; Grosche, M.; Herrmann, W. R. Angew.
Chem., Int. Ed. 2002, 41, 1363-1365.
(14) Dible, B. R.; Sigman, M. S. J. Am. Chem. Soc. 2003, 125, 872-
873.
4636
Org. Lett., Vol. 5, No. 24, 2003

The optimized reaction conditions with palladium acetate
and imidazolium 5 as catalyst in THF at room temperature
were applied to a variety of aryldiazonium tetrafluoroborate
substrates (Table 2). Electron-rich and -deficient aryldiazo-
Scheme 2
Table 2. Palladium-Carbene-Catalyzed Aryl Borylation
2
The structure adopts a flat, four-membered ring µ -bridged
arrangement with square-planar palladium where the halides
are twisted out of plane relative to the heterocycle nitrogens.
This planar arrangement has been observed in an X-ray
structure of the aromatic NHC complex formed with IMes
1
3
6
and added base. A related nickel halide dimer formed
1
4
using 7 has also been reported recently.
Previously, NHC-metal X-ray structures have not been
obtained in the absence of added base from the N,N-bis-
15
arylimidazolium salts 4-7. To establish its formation under
these and comparable coupling conditions, we were pleased
to find that chloride 5, when treated with palladium(II)
acetate 2:1 without added base or halide in THF, also
produced suitable crystals for X-ray analysis (Scheme 3).16
Scheme 3
nium ions all gave excellent yields of borylated products
again used at 1:1 stoichiometry with bis(pinacolato)borane.
All yields reported are for isolated, chromatographically pure
products. Reactions with the phenyl- and tolyldiazonium ions
were complete in 1.5 h with high yields. The anisyl substrates
reacted within 1 h, but with slightly lower yields. Acetyl-
and nitrobenzenediazonium ions also gave high yields.
P-Bromophenyl reacted with the highest yield at 95%. This
product has the added potential of further cross-coupling
reactions through either the pinacolborane or the bromide.
Ortho-substituted substrates, including the very hindered 2,6-
dimethylphenyldiazonium ion, all gave good to excellent
yields. The 1,4-bis-diazonium ion also gave a good yield of
the bis(pinacolato)diborane product under these conditions.
Additional studies were pursued to explore the nature of
the NHC-palladium catalyst controlling the process. Dark
The orange palladium-NHC complex formed was chro-
matographed and obtained in 37% isolated yield based on
the dimer formed. In contrast to the diiodide structure, this
2
palladium-carbene µ -dimer is twisted out of plane adopting
(15) X-ray structures of N,N-dimethylimidazolium‚palladium NHC com-
plexes formed without base have been reported: (a) Herrmann, W. A.;
Elison, M.; Fischer, J.; K o¨ cher, C.; Artus, G. R. J. Angew. Chem., Int. Ed.
Engl. 1995, 34, 2371-2374. For a more recent example with a chelating
bis-imidazolium NHC‚Pd complex, see: (b) Clyne, D. S.; Jin, J.; Genest,
E.; Gallucci, J. C.; RajanBabu, T. V. Org. Lett. 2000. 2, 1125-1128.
2
red crystals were obtained from the reaction of H IPr 5 with
sodium tert-butoxide, NaI, and palladium(II) acetate to give
(16) Hydrogens are omitted for clarity. See the Supporting Information
a dimeric palladium(II) iodide NHC complex (Scheme 2).
for full details.
Org. Lett., Vol. 5, No. 24, 2003
4637

a puckered, nonplanar, four-membered ring core where the
chlorides now are not found in a plane.
To account for the formation of the NHC complex in the
absence of base, an alternative mechanism is proposed
palladium gives the dichloro-Pd NHC which dimerizes to
give the observed structure. Oxidative insertion has also been
proposed as an alternative for base-free conditions for an
intermediate leading to metal-hydride NHC products with
1
8
(Scheme 4). Instead of a free carbene intermediate formed
palladium(0) starting materials.
These conditions for active palladium carbene catalyst
formation and its use in THF at room temperature for highly
efficient borylation coupling should find other applications
to related processes. The characterization of the palladium
chloride complex unambiguously demonstrates that added
base is not needed for carbene formation or catalyst turnover.
An alternative mechanism for palladium-NHC formation
for this case is proposed.
Scheme 4
Acknowledgment. We are grateful for the support
provided by Brigham Young University and the National
Institutes of Health (GM57275). We also thank Bruce
Jackson for MS analyses.
with strong base, palladium acetate adds directly to the
imidazolium 5 (Ar ) 2,6-diisopropylphenyl) C2 position to
give adduct 8. This adduct can undergo ligand exchange to
give the chloropalladium acetate 9.17 Deprotonation by
acetate at the activated C2 position then gives carbene
complex 10. Loss of acetate and transfer of the π-electrons
from carbon to palladium gives 11. Addition of chloride to
Supporting Information Available: Experimental pro-
cedures, characterization, and X-ray data. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL035857Q
(
18) (a) McGuinness, D. S.; Cavell, K. J.; Yates, B. F.; Skelton, B. W.;
(
17) Triazolium ions have been shown to undergo nucleophilic addition
White, A. H. J. Am. Chem. Soc. 2001, 123, 8317-8328. (b) Grundemann,
S.; Albrecht, M.; Kovacevic, A.; Faller, J. W.; Crabtree, R. H. J. Chem.
Soc., Dalton Trans. 2002, 2163-2167. (c) Duin, M. A.; Clement, N. D.;
Cavell, K. J.; Elsevier: C. J. Chem. Commun. 2003, 400-401.
with methoxide at the electrophilic carbon position: Enders, D.; Breuer,
K.; Raabe, G.; Runsink, J.; Teles, J. H.; Melder, J. P.; Ebel, K.; Brode, S.
Angew. Chem., Int. Ed. Engl. 1995, 34, 1021-1024.
4638
Org. Lett., Vol. 5, No. 24, 2003