558
Published on the web April 24, 2010
Rhodium-catalyzed Dehydroborylation of Styrenes
with Naphthalene-1,8-diaminatoborane [(dan)BH]: New Synthesis
of Masked ¢-Borylstyrenes as New Phenylene-Vinylene Cross-coupling Modules
Noriyuki Iwadate1 and Michinori Suginome*1,2
1Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510
2JST, CREST, Katsura, Nishikyo-ku, Kyoto 615-8510
(Received March 19, 2010; CL-100272; E-mail: suginome@sbchem.kyoto-u.ac.jp)
Table 1. Optimization of dehydroborylation of styrene with
Styrene derivatives underwent dehydroborylation with
(dan)BHa
naphthalene-1,8-diaminatoborane [(dan)BH] in the presence of
a cationic rhodium complex, giving ¢-borylstyrene derivatives
in good yields. Thus prepared ¢-borylstyrenes bearing a chlorine
or B(pin) group on their aromatic rings were utilized for the
synthesis of highly conjugated molecules through stepwise
cross-coupling, taking advantage of the dan group as an effective
protective group for a boronyl group.
H
N
B
H
N
H
B(dan)
(dan)BH
catalyst
B(dan)
B(dan)
dioxane
60 °C, 4 h
2a'
via hydroboration
2a''
2a
1
via dehydrogenative
borylation
Organoboronic acids play important roles in synthetic
organic chemistry as versatile synthetic intermediates exhibiting
balanced stability and reactivity.1 Recent interest has focused on
the control of their reactivity by easily attachable and/or
removable groups on the boron atoms. For instance, reactivity
improvement has been accomplished by introduction of tri-
Entry Catalyst
Ligand Yield 2ab/% 2a:2a¤:2a¤¤
1
2
3
4
5
6
7
[IrCl(cod)]2
[Ir(cod)2]BF4
[RuCl2(p-cymene)]2
Ru3(CO)12
[RhCl(cod)]2
[Rh(cod)2]BF4
[Rh(cod)2]BF4
®
®
®
®
®
®
27
26
6
44
30:51:19
28:48:24
6:8:86
82:0:18
®
99c:0.6:0.7
5:86:9
0
methylolethane
(2-hydroxymethyl-2-methyl-1,3-propanediol)
through formation of its borate salts.2 Introduction of an ortho-
directing group derived from 2-(pyrazolyl)aniline onto the boron
atom has enabled Ru-catalyzed ortho-silylation of arylboronic
acids.3 Increasing attention has also been paid to a protective
group for a boronyl group [B(OH)2], which provides reliable
ways to highly functionalized organoboronic acids.4,5 Through
temporary protection of a boronyl group, iterative Suzuki-
Miyaura coupling has been accomplished for the sequence-
selective synthesis of oligoarene derivatives.4-6
99 (91)
5
PPh3
aA mixture of 1a (0.3 mmol), styrene (0.75 mmol), transition-
metal complex (1.5 ¯mol, Ir, Ru, or Rh), and ligand (3.3 ¯mol)
in dioxane was stirred at 60 °C for 4 h under a nitrogen
atmosphere. bGC yield. Isolated yield is shown in the
c
parentheses. >99% E.
We have been involved in the chemistry of naphthalene-1,8-
diaminatoborane [(dan)BH], of which the dan group serves as a
highly efficient protective group for a boronyl group.4,6 The dan-
protected organoboronic acids have been prepared conveniently
by condensation of the corresponding organoboronic acids with
1,8-diaminonaphthalene. On the other hand, we have recently
established a method for direct introduction of a B(dan) group
into organic molecules by using B(dan)-based boron reagents.7
The direct synthesis of dan-protected organoboronic acids seems
highly beneficial in that one can escape handling organoboronic
acids, which are sometimes unstable and difficult to purify. In
this report, selective synthesis of masked (E)-¢-styrylboronic
acids via transition-metal-catalyzed dehydroborylation8-10 with
(dan)BH is described. Although we have recently reported the
synthesis of masked (E)-¢-styrylboronic acids as coupling
modules for the synthesis of oligo(phenylene-vinylene)s via
hydroboration of 1-arylalkynes,7 the present reaction provides a
more selective way to the modules from styrenes, which are
much less expensive and more easily available than arylalkynes.
Dehydroborylation of styrene with (dan)BH was examined
in the presence of transition-metal catalysts (Table 1). Iridium
catalysts, which we found effective for hydroboration of alkynes
with (dan)BH, afforded a mixture of hydroboration and
dehydroborylation products (Entries 1 and 2). Although a
ruthenium chloride complex resulted in low conversion of 1, the
desirable alkenylborane was obtained as a major product in the
presence of a ruthenium carbonyl complex albeit in low yield
(Entries 3 and 4). A neutral rhodium catalyst known as an
effective catalyst for dehydroborylation with pinacolborane
completely failed to catalyze the reaction with (dan)BH
(Entry 5). Cationic rhodium catalyst was found to be the best
catalyst for dehydroborylation with (dan)BH, resulting in high
yield (99%), product selectivity (99%), and stereoselectivity
(>99% E) (Entry 6). Use of PPh3 as a ligand significantly
lowered the catalyst activity (Entry 7). In these reactions, styrene
also served as a receptor of eliminated hydrogen atoms, forming
ethylbenzene as a by-product. The dehydroborylation product
was easily separated from the hydroboration products as well as
from ethylbenzene by silica gel column chromatography.
Under the optimized reaction conditions, dehydroborylation
of several styrene derivatives were carried out (Table 2).
Styrenes bearing p-substituents such as methyl, methoxy,
Chem. Lett. 2010, 39, 558-560
© 2010 The Chemical Society of Japan
Iwadate, Noriyuki
