Rhodium-catalyzed borylation of aryl and alkenyl pivalates through the cleavage of carbonoxygen bonds

Article abstract of DOI:10.1246/cl.141084

Rhodium-catalyzed borylation reactions of aryl and alkenyl pivalates, using a diboron reagent, via the cleavage of carbonoxygen bonds have been developed. The inert nature of the pivalate moiety enables relatively complex aryl boronates to be synthesized via the tandem cross-coupling of carbonhalogen and carbonoxygen bonds.

Full text of DOI:10.1246/cl.141084

CL-141084
Received: November 25, 2014 | Accepted: December 10, 2014 | Web Released: December 19, 2014
Rhodium-catalyzed Borylation of Aryl and Alkenyl Pivalates
through the Cleavage of Carbon­Oxygen Bonds
1
1
1,2,3
and Naoto Chatani*1
Hirotaka Kinuta, Junya Hasegawa, Mamoru Tobisu,*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871
1
2
Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871
3
ESICB, Kyoto University, Katsura, Kyoto 615-8510
(
E-mail: tobisu@chem.eng.osaka-u.ac.jp, chatani@chem.eng.osaka-u.ac.jp)
Rhodium-catalyzed borylation reactions of aryl and alkenyl
pivalates, using a diboron reagent, via the cleavage of carbon­
oxygen bonds have been developed. The inert nature of the
pivalate moiety enables relatively complex aryl boronates to be
synthesized via the tandem cross-coupling of carbon­halogen
and carbon­oxygen bonds.
polysubstituted arenes by combination with a directed ortho
carbon­hydrogen bond functionalization process. Here, we
7
report a rhodium-based catalytic system for aryl pivalate
borylation (Scheme 1c). Although activation of the aromatic
carbon­oxygen bond of an aryl ester has been reported to
8
proceed stoichiometrically at a rhodium center, this is the first
report of a rhodium-catalyzed reaction that involves such a bond
9
,10
activation process.
Arylboronic acids and their derivatives are versatile reagents
for the catalytic synthesis of carbon­carbon and carbon­het-
eroatom bonds. Traditionally, they have been prepared by the
Previously, we developed rhodium-catalyzed reactions of
nitriles through cleavage of a carbon­cyano bond, in which the
use of organosilicon or organoboron reagents is crucial. Based
1
11
reactions of organomagnesium or organolithium reagents with
trialkylborates; however, functional group compatibility is
limited because organometallic reagents are strong nucleophi-
on these results, we became interested in whether the postulated
silylrhodium or borylrhodium species can activate other un-
reactive bonds, including carbon­oxygen bonds.
1
c
les. Transition-metal-catalyzed methods have also been devel-
oped, and these enable the synthesis of structurally diverse
functionalized aryl boronates. Catalytic methods that can trans-
form a carbon­halogen bond to a carbon­boron bond are among
First, we investigated the reaction of 2-naphthyl pivalate
(1a) with bis(neopentylglycolato)diboron [B (nep) ] in the
2
2
presence of a rhodium(I) catalyst (Table 1). Although none of
the expected borylated product 2 was formed when the reaction
2
the most powerful methods. The replacement of aryl halides
was performed using [RhCl(cod)] (cod: cyclooctadiene) alone,
2
with phenol derivatives in these catalytic borylation processes
would be beneficial in terms of environmental benignity and
ready availability of the starting materials. Activated sulfonates
such as triflates can be used as alternatives to aryl halides
addition of PPh3 provided 2 in 41% yield (Entry 2). Further
investigation showed that the addition of an electron-rich
phosphine, i.e., P(4-MeOC H ) , slightly improved the yield
6
4 3
of 2 (Entry 3), whereas an electron-deficient ligand did not
(Entry 8). Other ligands such as PCy3 and N-heterocyclic
(Scheme 1a); however, fluorinated reagents are expensive, and
1
2
they generate harmful wastes, therefore greener processes need
carbenes were ineffective. This reaction was also significantly
affected by the nature of the leaving group. Aryl acetate 1b
(R = Me) was decomposed under the catalytic conditions to
3
to be developed. Recent advances in the development of
palladium- or nickel-catalyzed borylations have enabled the use
4
of less reactive sulfonates such as tosylates and mesylates.
Table 1. Optimization of reaction conditionsa
Moreover, Shi and co-workers reported a nickel-catalyzed
borylation of aryl carbamates that involves the cleavage of a
relatively inert C(aryl)­O bond (Scheme 1b).5 This method
enables the use of inexpensive and easily handled phenol
derivatives, and can be used for the regioselective synthesis of
2
equiv
B2(nep)2
,6
[RhCl(cod)]2 5 mol%
O
R
ligand
O
B
30 mol%
OH
Ar
+
Ar
toluene
O
Ar
O
130 °C, 15 h
2
3
t
R = Bu (1a), Me (1b), NEt (1c)
Ar = 2-naphthyl
2
a) aryl sulfonates: many example
R1
NMR yields/%
O
B(OR)2
cat. Pd or Ni
B (OR)4 or HB(OR)2
Entry
Substrate
Ligand
S
2
3
1
O
O
2
1
2
3
4
5
1a
1a
1a
1b
1c
1a
1a
1a
none
PPh3
0
0
0
0
58
0
0
99
58
50
41
80
31
26
59
R2 = CF , p-tol, Me, etc.
3
41
50
0
10
66
78
31
b) Shi
O
P(4-MeOC6H4)3
P(4-MeOC6H4)3
P(4-MeOC6H4)3
P(4-MeOC6H4)3
P(4-MeOC6H4)3
P(4-CF3C6H4)3
O
R2
B
cat. Ni
O
O
B (nep)2
2
b
6
7
R2 = NMe , Bu, O Bu, etc.
t
t
2
b,c
0
0
c) this work
O
8
O
tBu
B
cat. Rh
aConditions:
O
1
(0.50 mmol), B2(nep)2 (1.0 mmol), [RhCl(cod)]2
O
B2(nep)2
(0.025 mmol), ligand (0.15 mmol), toluene (0.50 mL) for 15 h at
b
1
30 °C. [RhCl(cod)]2 (0.050 mmol), ligand (0.30 mmol) were used.
c
B2(nep)2, the catalyst, and the ligand were added in two batches at
and 3 h.
Scheme 1. Aryl boronate synthesis via carbon­oxygen bond
cleavage.
0
366 | Chem. Lett. 2015, 44, 366–368 | doi:10.1246/cl.141084
© 2015 The Chemical Society of Japan

Table 2. Substrate scopea
Rh Cl
B (nep)
A
2
2
[RhCl(cod)]2
10 mol%
O
O
(nep)B Cl
P(4-MeOC H ) 60 mol%
6
4 3
C
R
OPiv + B2(nep)2
R
B
Rh B(nep)
tBu
toluene
30 °C, 15 h t
OPiv = OCO Bu)
O
Ar B(nep)
Ar
1
B
2
equiv
(
O
Entry
1
Pivalate
Yield/%b Entry
Pivalate
Yield/%b
t
BuCO B(nep)
2
OPiv
OPiv
7
5
60
(23)
6
7
8
B(nep)
(
24)
Rh Ar
N
(nep)B Rh Ar
8
1
a
D
E
Ph
OPiv
53
OPiv
5
5
(27)
2
Ph
B (nep)
2
2
(
32)
+
+
9
tBu
OPiv
OPiv
O
tBu
4
5
56
O
(
(
20)
O
O
OPiv
[
Rh]
O
1
0
[
Rh] B(nep)
B
5
7
3
(nep)
N
(38)
51
F
G
9
0
O
13)
MeO
4
OPiv
O
OPiv
Scheme 2. A plausible mechanism.
11
7
(
3
22)
16
1
OPiv
6
(68)
MeOC H B(OH)
2
F3C
OPiv
6
4
OPiv
1
2
[PdCl (dppf)]
2
6
8
OPiv
5
Na CO
3
dioxane/H O
(
29)
Br
B (nep)
2
0
97)
11
1
4
2
MeO
15 73%
(
tBu
reflux
7
13
2 equiv
10 mol%
4 3 60 mol%
2
2
B(nep)
[RhCl(cod)]2
aReaction conditions: substrate (0.50 mmol), B2(nep)2 (0.50 mmol),
P(4-MeOC H )
6
[
(
[
RhCl(cod)]2 (0.025 mmol), P(4-MeOC6H4)3 (0.15 mmol), toluene
0.50 mL) for 3 h at 130 °C, and then B2(nep)2 (0.50 mmol),
toluene
30 °C, 15 h
1
MeO
16 71%
RhCl(cod)]2 (0.025 mmol), and P(4-MeOC6H4)3 (0.15 mmol) were
b
added before heating for 12 h at 130 °C. Numbers in parentheses refer
to yields of recovered starting materials.
Scheme 3. Sequential Suzuki­Miyaura coupling/borylation of
pivalate.
give naphthol 3 selectively (Entry 4). Aryl carbamate 1c
oxygen bonds were more reactive than phenyl carbon­oxygen
bonds, was similar to that observed in the nickel-catalyzed
carbon­oxygen bond activation reactions.
(
R = NEt ), which is a good substrate for nickel-catalyzed
2
5
7a,7b,13
borylation reactions, exhibited much lower reactivity than 1a in
this rhodium-catalyzed system (Entry 5). Increasing the catalyst
loading led to the formation of 2 in 66% yield (Entry 6). Further
improvement was achieved by adding B (nep) , the catalyst, and
A possible mechanism for the rhodium-catalyzed borylation
of aryl pivalates is outlined in Scheme 2. First, borylrhodium B
is generated via the oxidative addition of B (nep) to chloro-
2
2
2
2
the ligand in two batches (Entry 7). One unique feature of this
reaction is that the borylation proceeds without adding external
bases, which is in contrast to the typical palladium- and nickel-
rhodium precatalyst A/reductive elimination of chloroborane
11i
C. Subsequent carbon­oxygen bond cleavage is mediated by
borylrhodium B to form arylrhodium D. Oxidative addition of
B2(nep)2 to complex D forms a rhodium(III) species E, which
eventually leads to the formation of a borylated product, with
regeneration of borylrhodium B via reductive elimination.
Although the details of the mechanism of carbon­oxygen bond
cleavage are still unclear, two activation modes are possible.
One involves a concerted pathway via a six-membered transition
state F, induced by the interaction between the boryl ligand and
2
catalyzed borylation.
With the optimized conditions in hand, we investigated the
substrate scope (Table 2). This borylation reaction proceeded
successfully with polyaromatic substrates, including naphtha-
lene (Entries 1­4) and phenanthrene derivatives (Entry 5). The
carbon­oxygen bond in aryl methyl ether was completely inert
under these conditions (Entry 4), in contrast to the case for
6
14
nickel catalysis. Nitrogen-containing functional groups such as
the carbonyl group of the substrate. Another possibility is
tertiary amines (Entry 3) and a quinoline moiety (Entry 6) were
also tolerated. Alkenyl pivalates also participated in this
reaction; 2,2- (Entry 7) and 1,2-disubstituted (Entry 8) alkenyl
pivalates afforded the corresponding alkenyl boronates. It is
worth noting that when the 4-hydroxycoumarin derivative 11
underwent this borylation, the coumarin skeleton, which bears
an aryl ester moiety, remained intact (Entry 9). Simple phenol
derivatives were found to be significantly less reactive (Entries
oxidative addition to form G, which could be facilitated by the
strong σ-donating ability of the boryl ligand.
1
5­17
The stability of the pivalate moiety under typical palladium
catalytic conditions enables a sequential cross-coupling reaction
(Scheme 3); for example, 14 was converted to boronate 16 via
the tandem Suzuki­Miyaura arylation of the bromide/borylation
of the pivalate.
In conclusion, we have developed a rhodium-catalyzed
borylation reaction of aryl and alkenyl pivalates, using B2(nep)2
10 and 11). The reactivity trend, i.e., naphthyl and vinyl carbon­
Chem. Lett. 2015, 44, 366–368 | doi:10.1246/cl.141084
© 2015 The Chemical Society of Japan | 367

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by a rhodium catalyst. Although carbon­oxygen bond activation
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8
9
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1
0
This work was supported by Grants-in-Aid for Scientific
Research on Innovative Areas “Molecular Activation Directed
toward Straightforward Synthesis” from MEXT, Japan. M.T. was
also supported by the “Elements Strategy Initiative to Form Core
Research Center” from MEXT. H.K. expresses his special thanks
for a JSPS Research Fellowship for Young Scientists. We also
thank the Instrumental Analysis Center, Faculty of Engineering,
Osaka University, for assistance with MS and HRMS.
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17 A mechanism involving reductive elimination of the borylated product
from G to form Rh­OPiv, followed by oxidative addition of B2(nep)2 and
reductive elimination of (nep)B­OPiv may also be possible.
18 We recently found the rhodium-catalyzed borylation of aryl 2-pyridyl
ethers via the cleavage of C(aryl)-O bonds: H. Kinuta, M. Tobisu, N.
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368 | Chem. Lett. 2015, 44, 366–368 | doi:10.1246/cl.141084
© 2015 The Chemical Society of Japan