Synthesis of benzylboronates via palladium-catalyzed borylation of benzyl halides with pinacolborane

Article abstract of DOI:10.1081/SCC-120003413

Various benzyl halides were borylated with pinacolborane in the presence of ⁱPr₂NEt and a catalytic amount of PdCl₂(PPh₃)₂ to afford the corresponding benzylboronates in good yields.

Full text of DOI:10.1081/SCC-120003413

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SYNTHESIS OF BENZYLBORONATES VIA PALLADIUM-
CATALYZED BORYLATION OF BENZYL HALIDES WITH
PINACOLBORANE
a
a
a
b
Miki Murata , Takashi Oyama , Shinji Watanabe & Yuzuru Masuda
a
Department of Materials Science, Kitami Institute of Technology, Kitami, 090-8507, Japan
Department of Materials Science, Kitami Institute of Technology, Kitami, 090-8507, Japan
b
Version of record first published: 16 Aug 2006.
To cite this article: Miki Murata, Takashi Oyama, Shinji Watanabe & Yuzuru Masuda (2002): SYNTHESIS OF BENZYLBORONATES
VIA PALLADIUM-CATALYZED BORYLATION OF BENZYL HALIDES WITH PINACOLBORANE, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 32:16, 2513-2517
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 16, pp. 2513–2517, 2002
SYNTHESIS OF BENZYLBORONATES VIA
PALLADIUM-CATALYZED BORYLATION
OF BENZYL HALIDES WITH
PINACOLBORANE
Miki Murata,* Takashi Oyama, Shinji Watanabe,
and Yuzuru Masuda*
Department of Materials Science, Kitami Institute of
Technology, Kitami, 090-8507, Japan
ABSTRACT
Various benzyl halides were borylated with pinacolborane in
i
the presence of Pr NEt and a catalytic amount of
2
PdCl (PPh ) to afford the corresponding benzylboronates
2
3 2
in good yields.
The palladium-catalyzed cross-coupling reactions of bismetallic
reagents, such as diborons, have been shown to provide versatile methods
[
1]
for direct carbon-metal bond formation from organic electrophiles. On the
other hand, we recently developed palladium-catalyzed metalation reactions
of aryl and alkenyl halides utilizing metal hydrides, such as dialkoxy-
[2]
[3]
boranes and trialkoxysilanes, as metalating reagents. Also, we reported
that a novel platinum(0)-catalyzed coupling reaction of allyl halides with
[
4]
dialkoxyboranes provided the corresponding allylboronates. A suitable
*
Corresponding authors. E-mail: muratamk@mail.kitami-it.ac.jp
513
2
DOI: 10.1081/SCC-120003413
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2
514
MURATA ET AL.
catalyst system varied in the substrates as mentioned above. During the
course of our investigations, we examined a synthesis of benzylboranes 3
focusing on what kind of transition metals catalyzed a borylation of benzyl
halides 1. We here report the borylation of 1 with pinacolborane 2 as a
boron source (Eq. 1).
(1)
[
2]
Analogous with the borylation of aryl and alkenyl electrophiles,
the reaction was also achieved with the aid of a suitable palladium catalyst
and a tertiary amine. The reaction of benzyl chloride 1a (Z ¼ H, X ¼ Cl)
with 1.5 equiv of pinacolborane 2 in the presence of PdCl (PPh )
2
3 2
i
and Pr NEt gave the corresponding benzylboronates 3a (96% GLC
2
i
yield based on 1a) (Table 1, Entry 1). Pr NEt was recognized to be the
2
most effective base for the formation of 3a, but only a moderate yield
was obtained for 1a in the presence of Et N (48% yield), which contributed
3
[2,4]
to our previous borylation.
The reaction was efficiently catalyzed by the
palladium complexes having a monodentate phosphine ligand, such as
Pd(PPh3)4 (77% yield) and PdCl (PPh ) . Platinum complexes, which
2
3 2
[
4]
were effective catalysts for the borylation of allyl halides, were ineffective.
The phosphine ligand of the palladium catalyst affected the borylation
markedly; i.e., the PdCl (dppf)-catalyzed reaction of 1a resulted in only
2
an 8% yield, whereas PdCl (dppf) could be used satisfactorily for the
2
[2]
borylation of aryl halides. In testing the three solvents, CH ClCH Cl,
2
2
dioxane (94% yield of 3a), and toluene (77%), it was found that the
solvent did not have a significant impact on the yield of the borylation.

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SYNTHESIS OF BENZYLBORONATES
Table 1. Synthesis of Benzylboronate 3 (Eq. 1)
2515
a
Yield (%)
Entry
Halides
Product
Time (h)
3
4
b
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
3a
3a
3c
3d
3e
3f
6
6
6
8
8
6
4
4
96
79
77
83
(70)
(73)
(72)
(74)
–
–
b
18
15
b
–
b
–
b
–
1g
1h
3g
3h
b
–
a
GLC yields were based on 1. Yields in parentheses were isolated yields.
a and 4e–h were not precisely determined because of difficulty on GLC analysis.
b
4
Under reaction conditions similar to those described above, various
benzyl halides 1 were borylated efficiently with 2 to afford the desired 3 in
good yields. Comparing the leaving groups of benzyl halides, the reaction
of 1b resulted in fewer yields. (Table 1, Entry 2). Although the desired
products 3 were contaminated with small amounts of the reduced by-
products ArCH 4, both of the yield and the selectivity are almost indepen-
3
dent of electronic characteristics of the substituent on aromatic ring
(Entries 3–5). The present reaction was tolerated by various functional
groups, e.g., CO Me and CN (Entries 4 and 5). Since aryl bromides were
2
ꢀ
[2]
negligibly reactive at 80 C, 4-bromobenzyl bromide 1g (Z ¼ 4-Br, X ¼ Br)
was preferentially borylated at the benzylic position under the present
condition (Entry 7). Interestingly, the carbon–carbon double bond was
i
inert to hydroboration in the presence of PdCl (PPh ) and Pr NEt,
2
3 2
2
although pinacolborane 2 has been widely recognized as a potent hydro-
[
boration reagent (Entry 8).
5]
In conclusion, the present borylation of benzyl halides 1 with pinacol-
borane 2 provides a synthetically useful access to benzylboronates 3.
Mechanistic studies and investigations of the coupling with other halides
are currently underway.
EXPERIMENTAL
All the experiments were carried out under an argon atmosphere.
CH ClCH Cl was distilled from CaH before use. Pinacolborane was
2
2
2

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2
516
MURATA ET AL.
Table 2. NMR, IR, and MS Data of Benzylboronate 3
1
13
H NMR
(ppm)
C NMR
(ppm)
IR
(cm
HRMS
þ
À1
3
3
)
(M )
a
1.23 (s, 12 H), 2.29
24.69,
124.79, 128.22,
128.96, 138.61
83.38,
2979, 1144
calcd 218.1478,
found 218.1457
(
(
s, 2 H), 7.1–7.3
m, 5 H)
3
3
3
3
3
3
c
1.23 (s, 12 H), 2.22
s, 2 H), 3.77 (s, 3
H), 6.79 (d, J ¼ 8.8
Hz, 2 H), 7.09 (d,
J ¼ 8.8 Hz, 2 H)
1.22 (s, 12 H), 2.35
24.73,
55.22,
2978, 1246,
1144
calcd 248.1584,
found 248.1590
(
83.36, 113.76,
129.81, 130.49,
157.12
d
e
f
24.69,
51.89,
2991, 1717,
1327, 1140,
1110
calcd 276.1533,
found 276.1514
(
s, 2 H), 3.89 (s, 3
83.638, 126.90,
128.94, 129.64,
144.73, 167.32
H), 7.24 (d, J ¼ 8.4
Hz, 2 H), 7.91 (d,
J ¼ 8.4 Hz, 2 H)
1.23 (s, 12 H), 2.35
24.70, 108.69,
119.35, 129.70,
132.06, 145.01
2983, 2226,
1143
calcd 243.1430,
found 243.1434
(
s, 2 H), 7.23 (d,
J ¼ 8.3 Hz, 2 H),
7
Hz, 2 H)
.52 (d, J ¼ 8.3
1.23 (s, 12 H), 2.25
21.38,
24.69,
2979, 1144
calcd 232.1634,
found 232.1642
(
(
s, 2 H), 6.9–7.0
m, 3 H), 7.12 (t,
83.35, 125.57,
125.94, 128.11,
129.84, 137.69,
138.45
24.72,
J ¼ 7.5 Hz, 1 H)
g
h
1.22 (s, 12 H), 2.23
(
83.57,
2981, 1210,
1141, 677
calcd 296.0583,
found 296.0564
s, 2 H), 7.10 (d,
J ¼ 8.3 Hz, 2 H),
118.56, 130.74,
131.25, 137.71
7
Hz, 2 H)
.34 (d, J ¼ 8.3
1.23 (s, 12 H), 2.28
22.87,
81.59,
2979, 1477,
1457, 1331,
983, 968
calcd 244.1634,
found 244.1629
(
s, 2 H), 5.15 (d,
J ¼ 10.8 Hz, 1 H),
.67 (d, J ¼ 17.5
Hz, H), 6.68
110.54, 124.32,
127.25, 134.44,
134.98, 136.92
5
1
(dd, J ¼ 17.5, 10.8
Hz, 1 H), 7.14 (d,
J ¼ 7.9 Hz, 2 H),
7
Hz, 2 H)
.29 (d, J ¼ 7.9

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SYNTHESIS OF BENZYLBORONATES
2517
[
5a]
synthesized as reported previously.
products except for 1d and 1e, which were prepared from the corresponding
4
All benzyl halides were commercial
[
6]
by chlorination at benzylic position.
Typical Procedure
To a solution of PdCl (PPh ) (22 mg, 0.03 mmol) in CH ClCH Cl
2
3 2
2
2
i
(4 mL), benzyl halides 1 (1.0 mmol), Pr NEt (0.52 mL, 3.0 mmol), and
2
pinacolborane 2 (0.22 mL, 1.5 mmol) were added. After being stirred for
4–8 h at 80 C, the mixture was extracted using benzene. The extract was
washed with brine, dried over MgSO , and concentrated. The residue was
ꢀ
4
purified by Kugelrohr distillation to give the corresponding benzylboronates
3
as oil. The products’ 3 spectra data are listed in Table 2.
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7508; (b) Ishiyama, T.; Itoh, Y.; Kitano, T.; Miyaura, N. Tetrahedron
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Tetrahedron Lett. 1996, 37, 6889; (d) Ahiko, T.-a.; Ishiyama, T.;
Miyaura, N. Chem. Lett. 1997, 811.
2
3
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4
5
877.
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3
Pereira, S.; Srebnik M. J. Am. Chem. Soc. 1996, 118, 909; (d) Pereira, S.;
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Fujikawa, R.; Yamada, A.; Miyaura, N. Chem. Lett. 1999, 1069.
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6
Received in Japan June 25, 2001

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