Asymmetric cross-coupling of potassium 2-butenyltrifluoroborates with aryl and 1-alkenyl bromides catalyzed by a Pd(OAc)2/Josiphos complex

Article abstract of DOI:10.1246/cl.2006.1368

The asymmetric cross-coupling reaction of [CH₃CH= CHCH ₂BF₃]K with aryl or 1-alkenyl bromides selectively occurred at the γ-carbon of 2-butenylborane moiety with regioselectivities in a range of 84-99%. The enantioselectiv

Full text of DOI:10.1246/cl.2006.1368

1368
Chemistry Letters Vol.35, No.12 (2006)
Asymmetric Cross-coupling of Potassium 2-Butenyltrifluoroborates
with Aryl and 1-Alkenyl Bromides
Catalyzed by a Pd(OAc)2/Josiphos Complex
Yasunori Yamamoto,ꢀ Shingo Takada, and Norio Miyauraꢀ
Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University,
Sapporo 060-8628
(Received September 21, 2006; CL-061100; E-mail: yasu@org-mc.eng.hokudai.ac.jp, miyaura@org-mc.eng.hokudai.ac.jp)
The asymmetric cross-coupling reaction of [CH3CH=
CHCH2BF3]K with aryl or 1-alkenyl bromides selectively oc-
curred at the ꢀ-carbon of 2-butenylborane moiety with regiose-
lectivities in a range of 84–99%. The enantioselectivities were in
a range of 77–90%ee when a palladium/CyPF-t-Bu complex
was used at 80 ꢁC in the presence of K2CO3 (3 equiv.) in
H2O–MeOH (9/1).
a palladium catalyst.7 However, asymmetric version of these
couplings remained unexplored. We report here the first attempts
at asymmetric coupling of an allylic boron reagent via the ꢀ-
selective cross-coupling of potassium (E)-2-butenyltrifluorobo-
rate (1) with aryl or 1-alkenyl bromides catalyzed by a palladi-
um-Josiphos (6f, CyPF-t-Bu) complex (Scheme 1). The regiose-
lectivities of the coupling position and enantioselectivities were
highly sensitive to phosphine ligands employed for palladium
acetate in the reaction between methyl 4-bromobenzoate and 1
(Entries 1–8 in Table 1). Since the reaction was very slow in
the absence of a base as was previously reported in related pal-
ladium-catalyzed coupling reactions of potassium organotri-
fluoroborates,8 K2CO3 (3 equiv.) was used as a suspension in
THF or a solution in aqueous THF or MeOH. Among ligands
screened for optimization of the catalyst, R,S-CyPF-t-Bu (6f)
was found to be the best ligand to achieve 63%ee with 92% ꢀ-
selectivity in refluxing THF (Entry 8), whereas other derivatives
in Josiphos series (6a–6e) were not effective (4–44%ee, Entries
3–7). On the other hand, an analogous ferrocenyl ligand of Man-
dyphos (5a and 5b) resulted in the formation of a ꢀ-coupling
product (31 and 46%ee). Allyltrifluoroborates (1), synthesized
by treatment of allylboronic acids or esters with KHF2, are high-
ly insoluble in common organic solvents. Thus, addition of water
to THF or MeOH significantly improved the yields and enantio-
selectivities owing to the high solubility of 1 in aqueous solvents
(Entries 9–12). The reaction also took place in water (Entry 13,
86%, 81%ee), though the use of an organic solvent was essential
Transition metal-catalyzed allylic substitutions with nucleo-
philes, especially their asymmetric reactions using chiral cata-
lysts, provide an important class of compounds due to the fre-
quent occurrence of allylic fragments in natural products or in
synthetic intermediates.1,2 A number of efficient chiral catalysts
have been developed for those purposes and have considerably
expanded the scope of enantioselective C–C and C–heteroatom
bond formation.3–5 The cross-coupling reaction between allylic
metal reagents and electrophiles is another practical protocol
for allylation of organic halides or triflates. Thus, there are excel-
lent precedents achieved by palladium-catalyzed coupling reac-
tions of boron, silicon, and tin compounds. Among studies using
allylic metal reagents as the coupling partners, perfect control of
the coupling position with phosphine ligands was first achieved
by Hiyama, Hatanaka, and co-workers by using allyltrifluoro
silanes.6
We recently reported that allylation of aryl and 1-alkenyl
halides with [RCH=CHCH2BF3]K (R = Me, Ph) selectively
occurs at the ꢀ-carbon when a bulky and donating 1,10-bis(di-
t-butylphosphino)ferrocene (D-t-BPF) was used as a ligand of
Table 1. Effects of ligands and solventsa
Entry Ligand Solvent
Yield/%b 3/4 %ee of 3c
R
RX (2)
1
2
3
4
5
6
7
8
9
5a
5b
6a
6b
6c
6d
6e
6f
THF
THF
THF
THF
THF
THF
THF
THF
98
84
63
64
57
73
54
96
66
93
62
93
86
93/7 31
96/4 46
+
BF3K
R
Pd(OAc)2
chiral ligand
K2CO3
21/79
14/86
4
5
1
3
4
83/17 44 (R)
99/1 10 (R)
83/17 26 (R)
92/8 63 (R)
92/8 70 (R)
95/5 82 (R)
91/9 80 (R)
93/7 82 (R)
93/7 81 (R)
R= aryl or 1-alkenyl (see, Table 2)
Me
NMe2
Ph
R12P
PR1
2
Fe
PR2
Fe
Ph
2
6f
THF–H2O (9/1)
THF–H2O (1/4)
MeOH–H2O (1/1)
MeOH–H2O (1/9)
H2O
10 6f
11 6f
12 6f
13 6f
PR1
NMe2
2
Mandyphos series (S,S,R)
5a: R1 = Cy
Josiphos series (R,S)
6a: R1 = Cy, R2 = 3,5-(CH3)2-4-(CH3O)C6H2
5b: R1 = 3,5-(CH3)2-4-(CH3O)C6H2 6b: R1 = Cy, R2 = 3,5-(CF3)2C6H3
6c: R1 = Cy, R2 = Ph
aAll reactions were carried out at 80 ꢁC for 22 h in the presence of
Pd(OAc)2 (3 mol %), chiral ligand (3.6 mol %), methyl 4-bromo-
benzoate (1 mmol), 1 (2.5 mmol), and K2CO3 (3 mmol). bIsolated
yield by chromatography. cEnantiomer excess was determined by
a chiral stationary column.
6d: R1 = Cy, R2 = Cy
6e: R1 = t-Bu, R2 = 4-CF3C6H4
6f: R1 = t-Bu, R2 = Cy (R,S-CyPF-t-Bu)
Scheme 1. Asymmetric coupling giving 3.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.12 (2006)
1369
Table 2. Asymmetric cross-coupling giving 3a
References and Notes
1
a) Handbook of Organopalladium Chemistry for Organic
Synthesis, ed. by E.-I. Negishi, John Wiley & Sons, New York,
2002. b) T. Graening, H.-G. Schmalz, Angew. Chem., Int. Ed.
2003, 42, 2580.
Entry
2
Yield/%b
93
3/4
%ee of 3c
82
Br
Br
1
2
2a
2b
2c
2d
2e
2f
93/7
MeO2C
2
3
H. Miyabe, Y. Takemoto, Synlett 2005, 1641.
a) H. Yorimitsu, K. Oshima, Angew. Chem., Int. Ed. 2005, 44,
4435. b) A. W. van Zijl, L. A. Arnold, A. J. Minnaard, B. L.
Feringa, Adv. Synth. Catal. 2004, 346, 413. c) K. Tissot-Croset,
85
89/11
91/9
79
CO2Me
Br
3
97d
70
80
´
A. Alexakis, Tetrahedron Lett. 2004, 45, 7375. d) F. Lopez,
Ac
Br
Br
Br
A. W. van Zijl, A. J. Minnaard, B. L. Feringa, Chem. Commun.
2006, 409.
4
90/10
84/16
89/11
84/16
84/16
88/12
>99/1
>99/1
90
F3C
4
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704.
5
86
80 (R)e
85
i-Bu
6
95
Ph
Br
Br
7
2g
2h
2i
86
86
t-Bu
8
99
79
PhO
5
6
Br
9
90
83 (R)e
82
10
11
2j
47
Br
Br
2k
76
77
PhCH2O
aAll reactions were carried out at 80 ꢁC for 22 h in H2O/MeOH
(9/1) in the presence of Pd(OAc)2 (3 mol %), CyPF-t-Bu (5a,
3.6 mol %), bromoarene or bromoalkene (1 mmol), 1a (2.5 mmol),
and K2CO3 (3 mmol). bIsolated yield by chromatography.
cEnantiomer excess was determined by a chiral stationary column.
d1a: 2.0 mmol, time: 4.5 h. eConfiguration determined by Ref. 5b.
7
8
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to dissolve the bromide substrates. The absolute configuration
of methyl 4-(1-methylallyl)benzeate thus obtained by a series
of Josiphos ligands (Entries 3–13) was established to be R
ˆ
A. Batey, T. D. Quach, Tetrahedron Lett. 2001, 42, 9099.
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General procedure for asymmetric ꢀ-selective coupling
reaction (Table 2): A flask charged with potassium 2-butenyl-
trifluoroborate (2.5 mmol), Pd(OAc)2 (3 mol %), CyPF-t-Bu
(3.6 mol %) and K2CO3 (3.0 mmol) was flushed with nitrogen.
H2O/MeOH (9/1, 5 mL) and bromoarene (1.0 mmol) were
then added. The resulting mixture was stirred at 80 ꢁC for
22 h. Isolated yields determined by chromatography on silica
gel are shown in Table 2. Enantiomer excess was determined
22
(½ꢁꢂD ꢃ12:7 (c 0.58, CHCl3)) by the specific rotation reported
for (S)-isomer (½ꢁꢂD þ12 (c 0.9, CHCl3)3d) and retention times
of two enantiomers in HPLC analysis.
Under these conditions9 optimized for methyl 4-bromoben-
zoate, the representative aryl bromides possessing a donation or
withdrawing substituent resulted in 79–90%ee with ꢀ-selectivi-
ties in a range of 84–91% (Table 2, Entries 3–9).10 Among the
bromoarenes employed, 4-bromoanisole exceptionally predomi-
nated the formation of a ꢁ-coupling product (4) (75%, 3/
4 = 45/55) for an unknown reason. Aryl bromides possessing
a meta-substituent (Entry 2) gave comparable results to those
having a para-substituent, but ortho derivatives such as methyl
2-bromobenzoate (81%, 3/4 = 28/72) and 2-bromoanisole
(79%, 3/4 = 28/72) resulted in very low enantioselectivities
(less than 10%). Two alkenyl bromides achieved perfect ꢀ-
selectivities without any difficulty with enantioselectivities
comparable to those of aryl bromides (Entries 10 and 11).
In conclusion, CyPF-t-Bu in Josiphos series was found to be
an excellent ligand for asymmetric coupling of 1 with aryl or
1-alkenyl bromides with high ꢀ-selectivities. Although there is
a precedent for X-ray structure of Pd(Ph)(Br)(CyPF-t-Bu), kinet-
ic study suggested the formation of a cationic palladium(II)
intermediate such as [Pd(Ph)(CyPF-t-Bu)]þ before transmetala-
tion with 1. Mechanisms of transmetalation and enantioselection
will be reported elsewhere.
9
by a chiral stationary column.
23
10 Specific rotations ½ꢁꢂD were 3b: ꢃ4:33 (c 0.44, CHCl3), 3c:
ꢃ11:8 (c 0.59, benzene), 3d: ꢃ6:65 (c 0.81, benzene), 3e:
ꢃ6:05 (c 1.03, CHCl3), 3f: ꢃ15:27 (c 0.67, benzene), 3g:
ꢃ9:49 (c 0.57, benzene), 3h: ꢃ9:46 (c 0.56, benzene), 3i:
ꢃ10:18 (c 0.46, benzene), 3j: ꢃ3:64 (c 0.68, benzene), 3k:
ꢃ1:58 (c 0.41, benzene).
Published on the web (Advance View) November 4, 2006; doi:10.1246/cl.2006.1368