Iridium-catalyzed vinylic C-H borylation of cyclic vinyl ethers by bis(pinacolato)diboron

Article abstract of DOI:10.1246/cl.2008.664

Vinylic C-H borylation of cyclic vinyl ethers by bis(pinacolato)diboron was effectively catalyzed by iridium complexes comprised of 1/2[Ir(OMe)(cod)] ₂ and 4,4′-di-tert-butyl-2.2′-bipyridine in hexane or octane to give the corresponding vinylboron compounds in good yields. The reaction of 1,4-dioxene occurred even at room temperature, whereas the reactions of dihydropyran and dihydrofuran derivatives required a temperature above 80 °C. Although dihydropyran and dihydrofuran themselves produced regioisomeric mixtures of α- and β-borylated products, similar substrates possessing substituents at the γ-position selectively underwent borylation at the α-position. Copyright

Full text of DOI:10.1246/cl.2008.664

664
Chemistry Letters Vol.37, No.6 (2008)
Iridium-catalyzed Vinylic C–H Borylation of Cyclic Vinyl Ethers
by Bis(pinacolato)diboron
Takao Kikuchi, Jun Takagi, Tatsuo Ishiyama,ꢀ and Norio Miyauraꢀ
Division of Chemical Process Engineering, Graduate School of Engineering,
Hokkaido University, Sapporo 060-8628
(Received March 4, 2008; CL-080241; E-mail: ishiyama@eng.hokudai.ac.jp)
Vinylic C–H borylation of cyclic vinyl ethers by bis(pinaco-
afford the corresponding vinylic boron compounds in good
yields (Scheme 1).
lato)diboron was effectively catalyzed by iridium complexes
comprised of 1/2[Ir(OMe)(cod)]2 and 4,40-di-tert-butyl-2,20-
bipyridine in hexane or octane to give the corresponding vinyl-
boron compounds in good yields. The reaction of 1,4-dioxene
occurred even at room temperature, whereas the reactions of di-
hydropyran and dihydrofuran derivatives required a temperature
above 80 ꢁC. Although dihydropyran and dihydrofuran them-
selves produced regioisomeric mixtures of ꢀ- and ꢁ-borylated
products, similar substrates possessing substituents at the ꢂ-
position selectively underwent borylation at the ꢀ-position.
1,4-Dioxene (2.0 mmol), which does not cause regioselec-
tivity problems, was initially chosen as a substrate, and its reac-
tions with pin2B2 (1.0 mmol) were investigated at 25 ꢁC for 8 h
in hexane to optimize IrI precursors (0.03 mmol/Ir) and ligands
(0.03 mmol) (Table 1). Among the catalysts examined, the com-
bination of 1/2[Ir(OMe)(cod)]2 and dtbpy, which exhibited
a high level of activity toward aromatic C–H borylation,6
again catalyzed the vinylic C–H borylation to form 2-boryl-
1,4-dioxene in 81% yield based on boron atoms in the diboron
(Entry 1). The choice of catalyst precursor was crucial for the re-
action. Although the combinations of the IrI precursor possess-
ing basic hydroxy or phenoxy ligand with dtbpy showed similar
catalytic activities (Entries 2 and 3), the less-basic IrI acetate
complex, the chloride complex, or the cationic complex in com-
bination with dtbpy formed no borylated products (Entries 4–6).
The high catalyst efficiency of the alkoxide or hydroxide
complex can be attributed to the more facile formation of a
tris(boryl)IrIII intermediate,6,7 which is reactive toward boryla-
Vinylic boron derivatives are an important class of com-
pounds as versatile intermediates in synthetic organic chemistry,
the utility of which has been amply demonstrated in the synthesis
of natural products, biologically active compounds, and func-
tional organic materials by application of numerous carbon–car-
bon bond-forming reactions.1 Although they have been generally
obtained by hydroboration2 of alkynes or transmetalation3 be-
tween trialkylborates with vinylic lithium or magnesium re-
agents, transition-metal-catalyzed borylation has emerged as
an interesting strategy for their synthesis.4 Among these meth-
ods, vinylic C–H borylation of alkenes by hydroboranes or di-
borons has attracted considerable attention from the viewpoints
of economy, efficiency, and environmental benignity.5 Recently,
we demonstrated that iridium complexes generated from
1/2[IrCl(cod)]2 or 1/2[Ir(OMe)(cod)]2 and 2,20-bipyridine
(bpy) or 4,40-di-tert-butyl-2,20-bipyridine (dtbpy) efficiently cat-
alyzed aromatic C–H borylation6 by bis(pinacolato)diboron
(pin2B2, pin = Me4C2O2) or pinacolborane (pinBH). The com-
bination of 1/2[Ir(OMe)(cod)]2 and dtbpy showed a high level
of catalytic activity and allowed room-temperature borylation
of arenes and heteroarenes with a stoichiometric amount of sub-
strate to produce the corresponding aromatic boron compounds
in high yields. This high level of activity prompted us to apply
the catalyst to the borylation of alkenes that have sp2 C–H bonds
similar to aromatic substrates. We report herein the vinylic C–H
borylation of cyclic vinyl ethers by pin2B2 catalyzed by iridium
complexes generated from 1/2[Ir(OMe)(cod)]2 and dtbpy to
Table 1. Reaction conditions for 1,4-dioxenea
IrI precursor-
ligand (3 mol %)
O
O
+
2
2 pinB
Y
pin2B2
hexane/25 °C/8 h
O
O
(1.0 equiv)
(2.0 equiv)
Y
ligand =
N
N
Entry IrI precursor
Ligand
Yield/%b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1/2[Ir(OMe)(cod)]2
dtbpy
dtbpy
dtbpy
dtbpy
dtbpy
dtbpy
none
bpy
81
71
72
0
0
0
0
50
0
75
55
0
71
0
1/2[Ir(OH)(cod)]2
1/2[Ir(OPh)(cod)]2
1/2[Ir(OAc)(cod)]2
1/2[IrCl(cod)]2
[Ir(cod)2]BF4
1/2[Ir(OMe)(cod)]2
1/2[Ir(OMe)(cod)]2
1/2[Ir(OMe)(cod)]2
1/2[Ir(OMe)(cod)]2
1/2[Ir(OMe)(cod)]2
1/2[Ir(OMe)(cod)]2
1/2[Ir(OMe)(cod)]2 4,40-di-MeO-bpy
1/2[Ir(OMe)(cod)]2
4,40-di-Cl-bpy
1/2[Ir(OMe)(cod)]2 4,40-di-O2N-bpy
3,30-di-Me-bpy
4,40-di-Me-bpy
5,50-di-Me-bpy
6,60-di-Me-bpy
1/2[Ir(OMe)(cod)]2-
dtbpy (3 mol %)
X
X
+
pin2B2
2
n
2 pinB
n
O
O
n = 1, 2
X = O, CH2, CHR, CRR'
0
t-Bu
t-Bu
aA mixture of a pin2B2 (1.0 mmol), 1,4-dioxene (2.0 mmol),
an Ir precursor (0.03 mmol/Ir), a ligand (0.03 mmol), and
hexane (6 mL) was stirred at 25 ꢁC for 8 h. bGC yields based
on boron atom in pin2B2.
O
O
O
O
pin2B2
=
B B
dtbpy =
N
N
Scheme 1.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.6 (2008)
665
Table 2. C–H borylation of cyclic vinyl ethersa
1,4-dioxene, the reactions were carried out by using 3.0 equiv of
the substrates at 80 ꢁC in octane in a sealed tube. At that time, we
assumed that the borylation of these substrates selectively pro-
ceeds at the ꢀ-position, because an electronegative oxygen atom
would activate the ꢀ-C–H bond. However, dihydropyran and
dihydrofuran themselves produced regioisomeric mixtures of
ꢀ- and ꢁ-borylated products in ratios of 75:25 and 49:51, respec-
tively (Entries 1 and 5). Our previous studies on aromatic C–H
borylation demonstrated that the reaction does not occur at
sterically hindered C–H bonds ortho to a substituent.6 Thus,
we examined substrates possessing substituents at the ꢂ-position
to increase the ꢀ-selectivity of the borylation. Actually, intro-
duction of substituents at the ꢂ-position sufficiently improved
the ꢀ-selectivity. The reactions of ꢂ-substituted and ꢂ,ꢂ-disub-
stituted dihydropyran derivatives yielded ꢀ-borylated products
as single isomers (Entries 2–4), while those of the corresponding
dihydrofuran derivatives still yielded small amounts of ꢁ-bory-
lated products (Entries 6 and 7). It notable that substrates having
both vinylic and aromatic C–H bonds selectively underwent
borylation at the vinylic C–H bond to produce only vinylboron
compounds (Entry 4).
Entry
1
Product
Yield/%b
+
64 (75:25)
pinB
pinB
pinB
pinB
pinB
pinB
pinB
pinB
O
O
O
O
O
O
O
Me
2
3
4
5
6
7
61
Me
Me
65
81
+
+
+
75 (49:51)
71 (86:14)
73 (95:5)
pinB
pinB
O
O
O
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas ‘‘Advanced Molecular Transforma-
tions of Carbon Resources’’ from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Bu
Bu
References
Me
1
2
3
4
5
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Published on the web (Advance View) June 5, 2008; doi:10.1246/cl.2008.664