Synthesis of pinacol arylboronates via cross-coupling reaction of bis(pinacolato)diboron with chloroarenes catalyzed by palladium(0)-tricyclohexylphosphine complexes

Article abstract of DOI:10.1016/S0040-4020(01)00998-X

The cross-coupling reaction of bis(pinacolato)diboron with chloroarenes to yield pinacol arylboronates was carried out in 1,4-dioxane at 80°C in the presence of KOAc (1.5 equiv.) and Pd(dba)₂/2.4PCy₃ (3-6 mol%). The catalyst was also effective to carry out analogous coupling with aryl bromides or triflates under milder conditions than those of the previous procedures catalyzed by PdCl₂(dppf) in DMSO.

Full text of DOI:10.1016/S0040-4020(01)00998-X

TETRAHEDRON
Pergamon
Tetrahedron 57 '2001) 9813±9816
Synthesis of pinacol arylboronates via cross-coupling reaction of
bisꢀpinacolato)diboron with chloroarenes catalyzed by
palladiumꢀ0)±tricyclohexylphosphine complexes
p
Tatsuo Ishiyama, Kousaku Ishida and Norio Miyaura
p
Division of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
Received 13 September 2001; accepted 15October 2001
AbstractÐThe cross-coupling reaction of bis'pinacolato)diboron with chloroarenes to yield pinacol arylboronates was carried out in 1,4-
dioxane at 808C in the presence of KOAc '1.5equiv.) and Pd'dba) /2.4PCy '3±6 mol%). The catalyst was also effective to carry out
2
3
2
analogous coupling with aryl bromides or tri¯ates under milder conditions than those of the previous procedures catalyzed by PdCl 'dppf) in
DMSO. q 2001 Elsevier Science Ltd. All rights reserved.
1.Introduction
oxidative addition to palladium'0) complexes is very
slow, a catalyst in situ generated from Pd'dba) and PCy
2
3
The cross-coupling reaction between tetra'alkoxo)diborons
and aryl halides or tri¯ates in the presence of a base and
PdCl 'dppf) provides a convenient and direct method for the
ef®ciently catalyzed the reaction at 808C. The catalyst can
be also effective to carry out the coupling reaction of less
reactive aryl bromides or tri¯ates bearing electron-donating
substituents under mild conditions.
2
1
synthesis of arylboronates from aryl electrophiles. The
coupling reaction is feasible with various functional groups,
e.g. CO Me, COMe, NO , and CN, which need protection±
2
2
deprotection in the two-step procedure for preparing aryl-
boronic acids or esters from magnesium or lithium
reagents. The protocol offers a direct and ef®cient route
2
to variously functionalized boronic esters, the utility of
which has been amply demonstrated in the synthesis of
natural products, biologically active compounds, and
3
4
5
functional materials. On the other hand, the direct prepara-
tion of arylboronic esters from aryl halides or tri¯ates now
1
b,6
allows a one-pot, two-step procedure
and a solid-phase
7
method for the synthesis of unsymmetrical biaryls. The
cross-coupling reaction of pinacolborane with haloarenes
in the presence of a palladium catalyst and triethylamine
reported by Masuda and Murata is another convenient and
economical variant for the preparation of such pinacol aryl-
8
boronates.
In this paper, we report the palladium-catalyzed coupling
reaction of chloroarenes '1) with bis'pinacolato)diboron '2)
for the synthesis of pinacol arylboronates '3) 'Scheme 1).
Chloroarene is a desirable substrate for the cross-coupling
reactions in combinatorial synthesis since a large number of
derivatives are commercially available. Although their
Keywords: boron and compounds; coupling reactions; aryl halides;
palladium and compounds.
p
Corresponding authors. Tel./fax: 181-11-706-6562;
e-mail: ishiyama@org-mc.eng.hokudai.ac.jp
Scheme 1. Synthesis of pinacol arylboronates via cross-coupling reaction
of diboron with chloroarenes.
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020'01)00998-X

9814
T. Ishiyama et al. / Tetrahedron 57 ꢀ2001) 9813±9816
2
.Results and discussion
combination of Pd'dba) and PCy '2.4 equiv.) was ®nally
2 3
recognized to be the most ef®cient catalyst to achieve a
quantitative conversion and an almost quantitative yield at
2
.1. Reaction conditions
808C 'entry 10). On the other hand, the palladium±P't-Bu)3
complex resulted in low yields at temperatures in the range
of 20±808C 'entry 11).
Various phosphine ligands are effective in stabilizing the
palladium'0) species; however, the stoichiometry of phos-
9
9,10
phine to palladium and the bulkiness or donating ability
of phosphine ligands change the reactivity of catalysts
KOAc was found to be the best base for the coupling of
diboron '2) with bromo- or iodoarenes because a stronger
1
1
toward oxidative addition and transmetalation. Palladium
complexes having fewer than four phosphines, a weakly
coordinating ligand such as AsPh , or a bulky phosphine
base, such as K PO and K CO , prompted the further
3
3
4
2
1
2
coupling of arylboronates '3) with haloarenes, resulting in
a competitive formation of homo-coupling biaryls '36±60%
yields). The use of KOAc was also signi®cant in the present
3
1
3
such as P'o-tolyl)3, provide a highly reactive catalyst
because of the ready formation of the coordinatively
unsaturated species. Another role of the ligand is electron
donation to the palladium'0) metal center, which was amply
demonstrated in the cross-coupling reaction of chloro-
arenes. Bulky and highly donating ligands, such as
coupling reaction catalyzed by a palladium±PCy complex.
3
The reaction was almost quantitative with KOAc '94%),
whereas use of K CO , K PO , and KOPh resulted in 85,
2
3
3
4
75, and 51% yields, respectively.
1
4
15
P't-Bu)3,
o-'di-t-butylphosphino)biphenyl '5),
and
1
6
N-heterocyclic carbenes, were recognized to be highly
effective for chloroarenes even at room temperature. The
high reactivity of the complexes involving these ligands is
attributable to the strong electron-donating ability of the
ligand to the metal center and the ready dissociation of
the ligand to generate a coordinatively unsaturated species.
It was also noted that less bulky phosphines, such as
2.2. Scope and limitation
The borylation of the representative chloroarenes is
summarized in Table 2. The reaction was very fast, complet-
ing within 6 h for activated chloroarenes possessing an
electron-withdrawing group such as NO , CN, CHO, and
2
CO Me 'entries 1±6). However, the presence of ortho-
2
substituents 'entries 2 and 4) or electron-donating sub-
stituents 'entries 8±13) slowed down the reaction signi®-
cantly, thus requiring prolongation of the time to 48 h or a
higher catalyst loading '6 mol%) to complete the reaction.
Analogous to the previous borylation of aryl bromides,
iodides, and tri¯ates, the reaction tolerated various func-
tional groups. The reaction is applicable for the borylation
of heteroaromatic chlorides such as 3-chloropyridine 'entry
16), 2-chlorobenzo[b]furan 'entry 17), 4-chloroindole 'entry
18), and presumably other derivatives reported previously;
however, 2-chloropyridine failed to yield the corresponding
1
7
'
phosphine 'PCy3),
dicyclohexylphosphino)arenes 'e.g. 7) and tricyclohexyl-
1
4,18
are practical ligands giving a more
stable complex at high temperature.
A screening of representative phosphine ligands reported in
the cross-coupling reactions of chloroarenes is summarized
in Table 1. The catalysts were in situ generated by stirring a
mixture of Pd'dba) '3 mol%) and a phosphine ligand '3.3±
2
7.2 mol%) at room temperature to be used directly for the
coupling reaction of diboron '2) '1.1 equiv.) and 4-chloro-
benzaldehyde '1.0 equiv.) in the presence of KOAc
1.5equiv.) in dioxane. Use of triarylphosphines commonly
'
resulted in low conversions and low yields due to their low
electron-donating ability 'entries 2±5), whereas more
electron-rich tris'p-methoxyphenyl)phosphine exhibited
better catalyst activity than triphenylphosphine 'entries 2
and 4). Among the four phosphine ligands developed by
Buchwald and Guram '4±7), 4, 5, and 7 achieved high
yields in a range of 68±78% 'entries 6±9). However, a
Table 2. Synthesis of pinacol arylboronates
a
b
Entry
Chloroarene
Mol%
Time 'h)
Yield '%)
1
2
3
4
5
6
7
8
9
1
4-O
2-O
4-NCC
2-NCC
2
NC
NC
6
H
H
4
Cl
Cl
3
3
3
3
3
3
6
6
3
6
6
3
6
3
4
48
4
48
6
6
48
48
48
48
48
48
48
48
88
72
98
72
94
87
91
92
86
78
92
70
73
77
2
6
4
6
H
H
4
Cl
Cl
6
4
4-OHCC
6
H
4
Cl
Cl
4-MeO CC
2
6
H
4
Table 1. Effect of ligands in the reaction of bis'pinacolato)diboron '2,
C
6
H
5
Cl
1
8
.1 mmol) with 4-chlorobenzaldehyde '1.0 mmol) in dioxane '6 ml) at
08C for 16 h in the presence of KOAc '1.5mmol), Pd'dba)
4-MeC
2-MeC
4-MeOC
3-MeOC
2-MeOC
6
H
H
4
Cl
Cl
2
'3 mol%),
and a phosphine ligand '3.3±7.2 mol%)
6
4
0
6
6
6
H
H
H
4
4
4
Cl
Cl
Cl
a
b
Entry
Ligand
Ligand 'equiv.)
Yield '%)
11
2
1
1
2
3
4
None
PPh
P'o-MeC
1
13
14
4-Me
2-Chloronaphthalene
2 6 4
NC H Cl
3
2.4
2.4
2.4
1.1
1.1
2.4
1.1
1.1
2.4
2.4
19
36
44
18
78
68
10
71
94
58
H
4
)
3
152-Chloropyridine
16 3-Chloropyridine
3
48
0
6
P'p-MeOC
6
H
4
)
3
3
3
48
48
82
73
c
5
6
7
8
9
1
1
dppf
4
5
6
7
1
7
0
1
PCy
P't-Bu)
3
1
8
3
48
72
3
a
b
c
Equivalents to palladium metal.
GC yields based on 4-chlorobenzaldehyde.
a
Mol% of catalyst.
GC yields based on chloroarenes.
0
b
1,1 -Bis'diphenylphosphino)ferrocene.

T. Ishiyama et al. / Tetrahedron 57 ꢀ2001) 9813±9816
9815
inlet, and a condenser was charged with Pd'dba) '0.017 g,
2
0
0
.03 mmol, 3 mol%) and tricyclohexylphosphine 'PCy3,
.020 g, 0.072 mmol), and ¯ushed with nitrogen. Dioxane
'
6 ml) was added and the resulting mixture was then stirred
for 30 min at room temperature. Bis'pinacolato)diboron '2,
.279 g, 1.1 mmol), KOAc '0.147 g, 1.5mmol), and a
0
chloroarene '1.0 mmol) were added successively. After
being stirred at 808C for the period shown in Table 2, the
reaction mixture was treated with water '5ml) at room
temperature. The product was extracted with benzene,
washed with brine, and dried over MgSO . Kugelrohr
4
distillation gave an analytically pure sample.
The following arylboronates were prepared by the general
procedure, unless otherwise noted.
3.2.1. 2-ꢀ4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-
benzonitrile ꢀentry 4). H NMR '400 MHz, CDCl , TMS)
1
3
d 1.39 's, 12H), 7.52 'dt, Jˆ1.5, 7.4 Hz, 1H), 7.57 'dt,
Scheme 2. Cross-coupling reaction of dibron with electron-rich aryl elec-
trophiles.
Jˆ1.5, 7.4 Hz, 1H), 7.70 'dd, Jˆ1.5, 7.3 Hz, 1H), 7.88 'd,
1
3
Jˆ7.3 Hz, 1H); C NMR '100 MHz, CDCl , TMS) d
3
2
4.77, 84.77, 117.24, 118.96, 131.06, 131.54, 133.40,
1
1
boronate 'entry 15). Since the boronic acids and esters
adjacent to a heteroatom are highly susceptible to hydrolytic
135.82; B NMR '128 MHz, CDCl , BF ´OEt ) d 30.74;
3 3 2
exact mass calcd for C H BNO 229.1274, found
2
1
3
16
1
9
protodeboronation, the coupling reaction can be often
followed by hydrolytic boron±carbon bond cleavage.
Although the reaction in dioxane can be much more
insensitive to such protodeboronation than that in DMSO,
the 2-borylpyridine was quantitatively led to free pyridine.
Such side-reaction giving the reduction product of halo-
arenes in small amounts was also observed in the reactions
of 2-chloronitrobenzene and 2-chlorobenzonitrile 'entries 2
and 4).
229.1270.
3.2.2. 3-ꢀ4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-
anisole ꢀentry 11). H NMR '400 MHz, CDCl , TMS) d
1
3
1.35's, 12H), 3.84 's, 3H), 7.01 'dd, Jˆ2.7, 8.1 Hz, 1H),
7.30 't, Jˆ7.8 Hz, 1H), 7.33 'd, Jˆ2.7 Hz, 1H), 7.40 'd,
1
3
Jˆ7.1 Hz, 1H); C NMR '100 MHz, CDCl , TMS) d
3
24.85, 55.24, 83.82, 117.91, 118.64, 127.17, 128.93,
159.02; B NMR '128 MHz, CDCl , BF ´OEt ) d 30.56;
1
1
3
3
2
exact mass calcd for C H BO 234.1427, found 234.1432.
1
3
19
3
The use of such highly electron-donating ligand 'PCy ) was
3
also advantageous to signi®cantly shorten the reaction times
of the borylation of electron-rich aryl bromides or tri¯ates
3.2.3. 3-ꢀ4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-
pyridine ꢀentry 16). H NMR '400 MHz, CDCl , TMS) d
1
3
'
previous conditions using PdCl 'dppf) in DMSO, even for a
Scheme 2). The borylation of iodoarenes is fast under the
1.36 's, 12H), 7.28 'ddd, Jˆ0.9, 4.9, 7.6 Hz, 1H), 8.05'dt,
Jˆ7.6, 1.8 Hz, 1H), 8.67 'dd, Jˆ2.0, 4.9 Hz, 1H), 8.95's,
2
1
3
dimethylamino derivative; however, analogous reactions of
the bromides or tri¯ates are signi®cantly slow. In contrast to
the palladium±dppf complex, Pd'dba) /PCy completed the
1H); C NMR '100 MHz, CDCl , TMS) d 24.84, 84.20,
3
11
123.05, 142.18, 151.98, 155.48; B NMR '128 MHz,
CDCl , BF ´OEt ) d 31.00; exact mass calcd for
3
2
3
3
2
reactions of both 4-methoxy and 4-dimethylamino deriva-
tives within 7 h. Although the ef®ciency of Pd'dba) /PCy
C H BNO 205.1274, found 205.1281.
11 16 2
2
3
has not yet been studied in detail for other halides or
tri¯ates, the catalyst would serve to achieve complete
conversions within shorter reaction times than those of the
3.2.4. 2-ꢀ4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-
benzo[b]furan ꢀentry 17). H NMR '400 MHz, CDCl ,
1
3
TMS) d 1.39 's, 12H), 7.23 't, Jˆ7.4 Hz, 1H), 7.34 'dt,
previously reported procedure using PdCl 'dppf).
2
Jˆ1.2, 7.8 Hz, 1H), 7.40 's, 1H), 7.57 'd, Jˆ8.5Hz, 1H),
1
3
7
.63 'd, Jˆ7.8 Hz, 1H); C NMR '100 MHz, CDCl , TMS)
3
d 24.77, 84.68, 111.97, 119.53, 121.88, 122.71, 125.93,
127.48, 157.51; B NMR '128 MHz, CDCl , BF ´OEt ) d
3 3 2
1
1
3.Experimental
27.51; exact mass calcd for C H BO 244.1270, found
244.1264.
14 17 3
3
.1. Reagents
2
0
21
Pd'dba)2 and bis'pinacolato)diboron were synthesized
by the reported procedures. All phosphine ligands
purchased from Strem were used directly without further
puri®cation. 1,4-Dioxane was distilled from sodium and
KOAc was dried in vacuo.
3.2.5. 4-ꢀ4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)in-
dole ꢀentry 18). H NMR '400 MHz, CDCl , TMS) d 1.39
1
3
's, 12H), 7.06 'ddd, Jˆ1.0, 2.1, 3.0 Hz, 1H), 7.21 'dd,
Jˆ7.1, 8.1 Hz, 1H), 7.26 't, Jˆ2.8 Hz, 1H), 7.50 'd,
Jˆ8.1 Hz, 1H), 7.64 'dd, Jˆ0.7, 7.1 Hz, 1H), 8.17 'br s,
1
3
1
H); C NMR '100 MHz, CDCl , TMS) d 24.97, 83.38,
3
3
.2. Representative procedure ꢀTable 2)
104.50, 113.96, 121.30, 124.58, 127.84, 132.49, 135.09;
B NMR '128 MHz, CDCl , BF ´OEt ) d 31.05; exact
1
1
3
3
2
A 25ml ¯ask assembled a magnetic stirring bar, a septum
mass calcd for C14H18BNO 243.1430, found 243.1422.
2

9816
T. Ishiyama et al. / Tetrahedron 57 ꢀ2001) 9813±9816
Syntheses of 4-'4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)nitrobenzene 'entry 1), 2-'4,4,5,5-tetramethyl-1,3,2-
7. 'a) Gosselin, F.; Betsbrugge, J. V.; Hatam, M.; Lubell, W. D.
J. Org. Chem. 1999, 64, 2486. 'b) Tempest, P. A.; Armstrong,
R. W. J. Am. Chem. Soc. 1997, 119, 7607. 'c) Piettre, S. R.;
Baltzer, S. Tetrahedron Lett. 1997, 38, 1197. 'd) Brown, S. D.;
Armstrong, R. W. J. Am. Chem. Soc. 1996, 118, 6331.
8. 'a) Murata, M.; Oyama, T.; Watanabe, S.; Masuda, Y. J. Org.
Chem. 2000, 65, 164. 'b) Murata, M.; Watanabe, S.; Masuda,
Y. J. Org. Chem. 1997, 62, 6458.
1
,8
1
dioxaborolan-2-yl)nitrobenzene 'entry 2), 4-'4,4,5,5-tetra-
1
,8
methyl-1,3,2-dioxaborolan-2-yl)benzonitrile 'entry 3), 4-
'
'
4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzaldehyde
entry 5), methyl 4-'4,4,5,5-tetramethyl-1,3,2-dioxaboro-
1
1
lan-2-yl)benzoate 'entry 6), '4,4,5,5-tetramethyl-1,3,2-
1
,8
dioxaborolan-2-yl)benzene 'entry 7),
methyl-1,3,2-dioxaborolan-2-yl)toluene 'entry 8), 2-
4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)toluene 'entry
4-'4,4,5,5-tetra-
8
9. Tolman, C. A. Chem. Rev. 1977, 77, 313 and references
therein.
'
8
9
'
), 4-'4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)anisole
entry 10), 2-'4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
10. Fernandez, A. L.; Reyes, C.; Prock, A.; Giering, W. P.
J. Chem. Soc., Perkin Trans. 2 2000, 1033 and references
therein.
1
,8
1
yl)anisole 'entry 12),
methyl-1,3,2-dioxaborolan-2-yl)aniline 'entry 13), and
-'4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)naphthalene
N,N-dimethyl-4-'4,4,5,5-tetra-
1
,8
11. For reviews: 'a) Suzuki, A. In Metal-Catalyzed Cross-
Coupling Reactions, Diederich, F., Stang, P. J., Eds.; Wiley-
VCH: Weinheim, 1998 p 49. 'b) Miyaura, N. Advances in
Metal±Organic Chemistry; Liebeskind, L. S., Ed.; JAI:
London, 1998; Vol. 6, p. 187. 'c) Miyaura, N.; Suzuki, A.
Chem. Rev. 1995, 95, 2457.
2
'
8
entry 14) were previously reported.
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