Selective Hydrogenolysis of Arenols with Hydrosilanes by Nickel Catalysis

Article abstract of DOI:10.1246/cl.150807

Nickel-catalyzed hydrogenolysis of arenols has been developed through the use of hydrosilane as a reductant. Sterically demanding N-heterocyclic carbene (NHC) ligands are crucial for the reaction. The present protocol allows selective cleavage of ArO bonds of arenols over aryl and benzyl ethers.

Full text of DOI:10.1246/cl.150807

Received: August 27, 2015 | Accepted: October 22, 2015 | Web Released: October 29, 2015
CL-150807
Selective Hydrogenolysis of Arenols with Hydrosilanes by Nickel Catalysis
Akito Ohgi and Yoshiaki Nakao*
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510
(E-mail: nakao.yoshiaki.8n@kyoto-u.ac.jp)
Nickel-catalyzed hydrogenolysis of arenols has been
developed through the use of hydrosilane as a reductant.
Sterically demanding N-heterocyclic carbene (NHC) ligands
are crucial for the reaction. The present protocol allows selective
cleavage of Ar-O bonds of arenols over aryl and benzyl ethers.
Table 1. Hydrogenolysis of 4-tert-butylphenol
[Ni(cod)2] (5.0 mol%)
Me NIPr (5.0 mol%)
HSi(OMe)2Me (0.66 mmol)
2
OH
H
toluene, 120 °C, 24 h
tBu
tBu
1a
standard conditions
2a
(0.30 mmol)
Hydrogenolysis of Ar-O bonds in arenols and aryl ethers
has gained much interest because aromatic chemicals for fuels
and value-added chemicals can be obtained from renewable
biomass such as lignin by such transformations.1 However, these
transformations are challenging because the bond dissociation
energy is higher than that of other C-O bonds.2 Selective
hydrogenolysis of hydroxy groups over alkoxy groups in arenes
is particularly difficult, although such a reaction could enable the
regioselective functionalization of arenes derived from lignin-
based chemical feedstocks. Selective hydrogenolysis of phenolic
hydroxy groups over alkoxy groups has generally been achieved
through 2-steps procedures; conversion of the hydroxy groups to
activated forms such as sulfonates, esters, and ethers, followed
by metal-catalyzed reductive cleavage of the resulting activated
Ar-O bonds.3,4 Direct reductive cleavage of phenols has been
R
N
R
N
N
N
SIPr
R = H: IPr
N
N
R = Cl: ClIPr
R = Me: MeIPr
MeO
OMe
R = NMe2: Me NIPr
2
MeIPrOMe
Yield of
2a/%a
Entry Variation from the standard conditions
1
2
3
4
5
6
7
8
9
none
88
<1
<1
4
No Me NIPr
2
PCy3 (10 mol %) instead of Me NIPr
2
IPr instead of Me NIPr
2
5
SIPr instead of Me NIPr
3
reported recently by Ir catalysis with H2 and using an excess
2
amount of LiAlH4/bases.6 Although these methods reduce
Ar-OH bonds exclusively over aromatic unsaturated bonds,
aryl ethers are not compatible under relatively harsh reaction
conditions. Herein, we report that a nickel catalyst bearing an N-
heterocyclic carbene (NHC) ligand enables the hydrogenolysis
of phenolic hydroxy groups with hydrosilanes as the hydrogen
source. Notably, aryl and benzyl ethers are compatible under the
newly developed reaction conditions.
ClIPr instead of Me NIPr
<1
78
85
2
MeIPr instead of Me NIPr
2
MeIPrOMe instead of Me NIPr
2
Me NIPr¢HOTf + tBuONa (0.03 mmol) instead of Me N IPr <1
2
2
10 HSiEt3 instead of HSi(OMe)2Me
11 HSi(OTMS)2Me instead of HSi(OMe)2Me
12 HSi(OEt)3 instead of HSi(OMe)2Me
13 PMHS instead of HSi(OMe)2Me
<1
<1
6
39
aThe yield was determined by GC analysis using undecane as an
internal standard.
We first examined the hydrogenolysis of the Ar-OH bond
in 4-tert-butylphenol (1a) with HSi(OMe)2Me (Table 1). After
screening various parameters, the reaction of 1a (0.30 mmol)
with HSi(OMe)2Me (0.66 mmol) in the presence of [Ni(cod)2]
(5.0 mol %) and Me NIPr7 (5.0 mol %) in toluene at 120 °C for
11), whereas HSi(OEt)3 and PMHS gave 2a in low yields
(Entries 12 and 13).
2
24 h gave tert-butylbenzene (2a) in 88% yield, as estimated by
GC analysis (Entry 1). Different ligands were then examined for
the same reaction (Entries 2-8). 2a was not observed in the
absence of a ligand or with PCy3 as a supporting ligand for
nickel (Entries 2 and 3), whereas PCy3 was reported to be
effective for the nickel-catalyzed hydrogenolysis of Ar-OMe
bonds.4a,4b,4e Among NHC ligands, IPr and SIPr resulted in poor
yields (Entries 4 and 5), although SIPr was known as the ligand
of choice for the nickel-catalyzed hydrogenolysis of Ar-OAr
bonds with H2.4c,4f Less electron-donating ClIPr did not afford
the desired product (Entry 6), while more electron-donating
MeIPr and MeIPrOMe were effective (Entries 7 and 8). The reaction
The reaction on a 1.0 mmol scale under the standard
conditions for 24 h gave 2a in 86% by GC analysis (Table 2,
Entry 1). A range of substituted phenols participated in the hy-
drogenolysis reaction to give the corresponding arene products
2b-2d in modest to excellent yields (Entries 2-6). Notably, the
benzylic C-O bond of silylether 1d (Entry 4) and ortho- and
para-Ar-OMe bonds in 1e and 1f (Entries 5 and 6) were
tolerated under the reaction conditions, allowing exclusive
removal of their hydroxy groups. Both 1- and 2-naphthols were
converted to naphthalene in good yields (Entries 7 and 8).
Substrates containing a carbonyl functionality were not con-
verted under these reaction conditions. In addition, the reaction
of 1b in the presence of tert-butyl benzoate or N-methylacet-
anilide was retarded, whereas the dehydrogenative silylation of
1b was observed. The carbonyl functionalities likely inhibit the
using the NHC ligand generated in situ from Me NIPr¢HOTf
2
t
and BuONa did not give 2a (Entry 9). Different hydrosilanes
were also screened (Entries 10-13). Other hydrosilanes such as
HSiEt3 and HSi(OTMS)2Me did not afford 2a (Entries 10 and
Chem. Lett. 2016, 45, 45–47 | doi:10.1246/cl.150807
© 2016 The Chemical Society of Japan | 45

H
Table 2. Hydrogenolysis of arenols with hydrosilanes cata-
lyzed by nickel
OH
+ H Si
R
R
[Ni(cod)2] (5.0 mol%)
2
1
Me2
NIPr (5.0 mol%)
OH
H
L
Ni
HSi(OMe)2Me (2.2 mmol)
H2
OSi
cat. LNi(0)
R
R
LNi(0)
H
toluene, 120 °C, 24 h
R
1
2
5
(1.0 mmol)
R
3
O
Yield of 2
Si
Si
Entry
1
2
/%a
L
Ni
OSi
OH
H
H Si
R
1
2
3
⎯ (86)
94b (99)b
47b (52)b
4
tBu
Ph
tBu
1a
1b
2a
2b
Scheme 1. Plausible mechanism for nickel-catalyzed hydro-
genolysis of arenols with hydrosilanes.
OH
H
[Ni(cod)2]
H2 (2.0 atm)
(5.0 mol%)
2a
Ph
Me NIPr
2
120 °C, 24 h
3% (GC)
(5.0 mol%)
1a
+
OH
H
Ar–OSi
HSi(OMe)2Me
(1.2 mmol)
–H2
toluene-d8
rt, 1 h
HSi(OMe)2Me
(1.0 mmol)
Ph
Ph
2a
78% (GC)
120 °C, 24 h
1c
1d
2b
2c
Scheme 2. Hydrogenolysis of aryl silyl ether.
OH
H
4c
59 (⎯)
⎯ (62)
⎯ (69)
TBSO
TBSO
catalysis to give aryl silyl ether 3 with H2 evolution, which was
reported very recently8 and confirmed by us using 1H NMR and/
or GC-MS. Then, oxidative addition of the Ar-O bond of the
aryl silyl ethers thus formed to nickel(0) gives nickel(II)
intermediate 4. Transmetalation between 4 and hydrosilanes
giving 5 is followed by reductive elimination to give arene
products 2 and regenerate nickel(0). On the other hand, we
cannot exclude another catalytic cycle involving a silyl-Ni(I)
species.4e
Since H2 is generated in the initial dehydrogenative
silylation step, one can argue that the subsequent hydrogenolysis
proceeds with H2 rather than with hydrosilanes.4c To verify this
possibility, the reaction of 1a with an equimolar amount of
HSi(OMe)2Me was performed to confirm quantitative formation
of the corresponding aryl silyl ether and liberation of H2 from an
open reaction vessel by 1H NMR (Scheme 2). The resulting
mixture was transferred to a closed vessel, pressurized with H2,
and heated at 120 °C for 24 h to show a trace amount of 2a,
whereas treatment of the residue with the hydrosilane reagent
gave 2a in 78% yield. These experiments support the proposed
mechanism in which the hydrosilane serves as a reductant in the
hydrogenolysis step.
OH
H
5
MeO
MeO
1e
1f
2d
2d
OH
H
6
OMe
OH
OMe
H
7
8
89b (98)b
80b (90)b
1g
1h
2e
2e
OH
H
The silyl ether intermediate in the present hydrogenolysis
was also confirmed by the reactions of trimethylsilyl ethers 6a
and 6b derived from 1a and 1b, respectively (eq 1). The reaction
conditions optimized for the hydrogenolysis of arenols also
effect the reduction of 6a and 6b. The corresponding methyl
ethers 7a and 7b reacted sluggishly, showing that aryl silyl
ethers are more reactive than alkyl aryl ethers under the present
conditions for achieving the chemoselectivity observed with 1e
and 1f (Entries 5 and 6 of Table 1), although the rationale for the
reactivity difference is yet to be elucidated.
aIsolated yields. Values in parenthesis are yields determined by GC
analysis using undecane as an internal standard. bAverage of two
independent reaction runs. c MeIPr was used instead of Me N IPr.
2
subsequent hydrogenolysis step (vide infra) in a way that is
elusive at present.
A plausible reaction mechanism is depicted in Scheme 1.
Although details remain yet to be clarified, we propose that the
dehydrogenative silylation of arenols first proceeds by nickel
46 | Chem. Lett. 2016, 45, 45–47 | doi:10.1246/cl.150807
© 2016 The Chemical Society of Japan

Difficult Substrate Conversion Reactions” (No. 15H05799) from
MEXT.
[Ni(cod)2] (5.0 mol%)
Me NIPr (5.0 mol%)
2
OR2
HSi(OMe)2Me (0.36 mmol)
ð1Þ
2a or 2b
Supporting Information is available on http://dx.doi.org/
10.1246/cl.150807.
toluene, 120 °C, 24 h
R1
6 or 7
(0.30 mmol)
R1
R2
GC Yield (%)
References and Notes
tBu SiMe3 (6a)
Ph SiMe3 (6b)
tBu Me
(7a)
Ph Me (7b)
45 (2a)
96 (2b)
1
For a review, see: M. Saidi, F. Samimi, D. Karimipourfard, T.
Nimmanwudipong, B. C. Gates, M. R. Rahimpour, Energy
Environ. Sci. 2014, 7, 103.
4 (2a)
46 (2b)
In conclusion, we have developed the nickel-catalyzed
hydrogenolysis of arenols using hydrosilanes as a reducing
agent. Unreactive Ar-OH bonds can be activated without
conventional pre-activation through ester formation. In this
transformation, hydrosilane plays two key roles; in situ activa-
tion of arenols through the formation of aryl silyl ethers and
reduction of the Ar-O bond of the silyl ethers thus formed, both
catalyzed by nickel/NHC. It is worth noting that aryl alkyl
ethers and benzyl silyl ethers are compatible under the reaction
conditions, whereas nickel catalysis has been demonstrated
to effect the activation and functionalization of their C-O
bonds.4c,4f,4g The present protocol can be complementary to
these known nickel-catalyzed protocols to access variously
substituted arenes from lignin-derived chemical feedstocks.
Further studies on nickel-catalyzed direct transformations of
arenols are currently underway in our laboratory.
2
3
E. Furimsky, Appl. Catal., A 2000, 199, 147.
A. Mori, T. Mizusaki, T. Ikawa, T. Maegawa, Y. Monguchi, H.
Sajiki, Chem.®Eur. J. 2007, 13, 1432, and references therein.
a) P. Álvarez-Bercedo, R. Martin, J. Am. Chem. Soc. 2010,
132, 17352. b) M. Tobisu, K. Yamakawa, T. Shimasaki, N.
Chatani, Chem. Commun. 2011, 47, 2946. c) A. G. Sergeev,
J. F. Hartwig, Science 2011, 332, 439. d) A. G. Sergeev, J. D.
Webb, J. F. Hartwig, J. Am. Chem. Soc. 2012, 134, 20226.
e) J. Cornella, E. Gómez-Bengoa, R. Martin, J. Am. Chem.
Soc. 2013, 135, 1997. f) B. Sawatlon, T. Wititsuwannakul, Y.
Tantirungrotechai, P. Surawatanawong, Dalton Trans. 2014,
43, 18123. g) M. Tobisu, T. Morioka, A. Ohtsuki, N. Chatani,
Chem. Sci. 2015, 6, 3410.
4
5
6
S. Kusumoto, K. Nozaki, Nat. Commun. 2015, 6, 6296.
H. Xu, B. Yu, H. Zhang, Y. Zhao, Z. Yang, J. Xu, B. Han, Z.
Liu, Chem. Commun. 2015, 51, 12212.
7
8
Y. Zhang, V. César, G. Storch, N. Lugan, G. Lavigne, Angew.
Chem., Int. Ed. 2014, 53, 6482.
H. Ohta, N. Miyoshi, Y. Sakata, Y. Okamoto, M. Hayashi, Y.
Watanabe, Tetrahedron Lett. 2015, 56, 2910.
This work has been supported financially by a Grant-in-Aid
for Scientific Research on Innovative Areas “Precise Formation
of a Catalyst Having a Specified Field for Use in Extremely
Chem. Lett. 2016, 45, 45–47 | doi:10.1246/cl.150807
© 2016 The Chemical Society of Japan | 47