A heterogeneous nickel catalyst for the hydrogenolysis of aryl ethers without arene hydrogenation

Article abstract of DOI:10.1021/ja3085912

A heterogeneous nickel catalyst for the selective hydrogenolysis of aryl ethers to arenes and alcohols generated without an added dative ligand is described. The catalyst is formed in situ from the well-defined soluble nickel precursor Ni(COD)₂ or Ni(CH₂TMS)₂(TMEDA) in the presence of a base additive, such as ^tBuONa. The catalyst selectively cleaves C_Ar-O bonds in aryl ether models of lignin without hydrogenation of aromatic rings, and it operates at loadings down to 0.25 mol % at 1 bar of H₂ pressure. The selectivity of this catalyst for electronically varied aryl ethers differs from that of the homogeneous catalyst reported previously, implying that the two catalysts are distinct from each other.

Full text of DOI:10.1021/ja3085912

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Communication
A Heterogeneous Nickel Catalyst for the Hydrogenolysis
of Aryl Ethers without Arene Hydrogenation
Alexey G. Sergeev, Jonathan D. Webb, and John F. Hartwig
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja3085912 • Publication Date (Web): 19 Nov 2012
Downloaded from http://pubs.acs.org on November 30, 2012
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Journal of the American Chemical Society
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A Heterogeneous Nickel Catalyst for the Hydrogenolysis of
Aryl Ethers without Arene Hydrogenation
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Alexey G. Sergeev, Jonathan D. Webb and John F. Hartwig*
Department of Chemistry, University of California, Berkeley, Berkeley, California 94720-1460, United States
ꢀ
SupportingꢀInformationꢀPlaceholderꢀ
ABSTRACT: A heterogeneous nickel catalyst for the se-
lective hydrogenolysis of aryl ethers to arenes and alco-
hols generated without added dative ligand is described.
The catalyst is formed in situ from the well-defined sol-
alyst loading (typically 20 mol % of Ni) and the SIPr
carbene ligand.
Here we report a highly active heterogeneous and
11
ligandless nickel catalyst for selective hydrogenolysis
of diaryl, benzyl aryl, and benzyl alkyl ethers at much
lower loadings (down to 0.25%) than the SIPr-Ni cata-
lyst without carbenes or phosphines as auxiliary ligands.
In contrast to most known heterogeneous systems, the
catalyst operates at low hydrogen pressure (1 bar) and
does not catalyze the hydrogenation of arenes. The regi-
oselectivity of this system for cleavage of two different
types of C-O bonds is orthogonal to that of the previous-
uble
nickel
precursors
Ni(COD)
2
or
Ni(CH
2
TMS) (TMEDA) in the presence of a base addi-
2
t
tive, such as BuONa. The catalyst selectively cleaves
Ar-O bonds in aryl ether models of lignin without hy-
drogenation of aromatic rings, and it operates at load-
ings down to 0.25 mol% at 1 bar of H pressure. The
C
2
selectivity of this catalyst for electronically varied aryl
ethers differs from that of the homogeneous catalyst
reported previously, implying that the two catalysts are
distinct from each other.
1
0
ly reported homogeneous nickel-carbene catalyst.
The ligandless nickel catalyst reported here was dis-
covered when studying the effects of ligands on the
1
0
nickel-catalyzed hydrogenolysis of aryl ethers. Initial
studies showed that Ni(COD) without added ligand was
less reactive than the combination of Ni(COD) and
SIPr·HCl for the hydrogenolysis of diphenyl ether (eq
).
2
Catalytic hydrogenolysis of the CAr-O bond in aro-
matic ethers is a critical process for the conversion of
the lignin component of plant biomass into aromatic
hydrocarbons, feedstocks for the production of biofuels,
2
1
1
-4
and chemicals. Typically, the hydrogenolysis of CAr-O
bonds is conducted over heterogeneous catalysts that
require high temperatures (>250 ºC) and pressures of
hydrogen (>30 bar). These conditions lead to concomi-
1
,5-7
tant reduction of aromatic rings.
The poor chemose-
lectivity of heterogeneous catalysts for hydrogenolysis
over hydrogenation wastes hydrogen and results in low
yields of arene products. Therefore, the identification of
catalysts that cleave CAr-O bonds selectively would be a
significant advance toward addressing the challenge of
However, we have now found that this ligandless sys-
tem is more active for hydrogenolysis of the types of
1
,12,13
electron-rich diaryl ethers found in lignin
than is
the SIPr-Ni catalyst. Figure 1A provides data on the rel-
ative rates for reaction of electronically varied diaryl
ethers in the presence of the two types of catalysts. Only
75% conversion of the di-o-anisyl ether to arene and
aryl alcohol occurred in the presence of the Ni-SIPr sys-
tem under the conditions that led to full conversion of
the same ether in the presence of the ligandless system.
In contrast, conditions that led to full conversion of the
trifluoromethyl-substituted ether in the presence of the
Ni-SIPr system occurred to only 41% conversion in the
presence of the ligandless system and formed products
from the reduction of both C-O and C-F bonds.
3
converting lignocellulose biomass into simple arenes.
To address the problem of selective reduction of CAr-O
bonds in aryl ethers, several groups have developed cat-
alytic systems that are based on soluble nickel complex-
8
-10
es and hydride donors, such as silanes
and alumino-
hydrides. Reactions with hydrogen would be more
practical, and we recently reported nickel-N-
1
0
a
heterocyclic carbene catalyst for the selective reduction
of the CAr-O bond in aryl ethers with hydrogen at 1 bar
1
0
pressure. The catalyst converted aryl ethers into arenes
and alcohols in high yields without arene hydrogenation.
Despite these advantages, the process required high cat-
To test whether this trend in reactivity also applies to
the cleavage of two different CAr-O bonds within the
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Journal of the American Chemical Society
Page 2 of 5
same ether, we conducted the hydrogenolysis of un-
symmetrical diaryl ethers. As shown in Figure 1B, the
ligandless nickel catalyst cleaved the CAr-O bond adja-
cent to the most electron-rich arene ring of 4-hydroxy
diphenyl ether to form 2 equiv of phenol as the sole
product. In contrast, the Ni-SIPr catalyst preferentially
cleaved the CAr-O bond adjacent to the more electron-
deficient ring to form predominantly hydroquinone
base occurred to completion and formed benzene and
phenol in nearly quantitative yields (Table 1, Entry 4).
1
2
3
4
5
6
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9
1
1
1
1
1
1
1
1
1
1
2
2
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2
2
2
2
2
2
2
3
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14
Tableꢀ 1.ꢀ Effectꢀ ofꢀ baseꢀ andꢀ nickelꢀ precursorꢀ onꢀ
hydrogenolysisꢀ ofꢀ diphenylꢀ etherꢀ catalyzedꢀ byꢀ theꢀ
aꢀ
ligandlessꢀnickel. ꢀ
(
67%) and benzene (48%). These results on both the
intermolecular and intramolecular competition experi-
ments clearly indicate that the catalytic species generat-
ed without added ligand is distinct from the species gen-
erated with added SIPr ligand.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
tBuONa,ꢀ Conv.,ꢀ
Entryꢀ
GCꢀYields
Figureꢀ1.ꢀDifferencesꢀinꢀreactivityꢀandꢀselectivityꢀbetweenꢀ
theꢀligandlessꢀandꢀcarbene-ligatedꢀnickelꢀcatalysts.ꢀ
equivꢀ
%ꢀ
A,ꢀ%ꢀ B,ꢀ%ꢀ C,ꢀ%ꢀ D,ꢀ%ꢀ E,ꢀ%ꢀ
b
c
1
2ꢀ
3
4ꢀ
5ꢀ
ꢀ
0ꢀ
0.2ꢀ
1ꢀ
2.5ꢀ
5ꢀ
72 ꢀ
58ꢀ
18ꢀ
84ꢀ
99ꢀ
99ꢀ
2ꢀ
8ꢀ
77ꢀ
97ꢀ
96ꢀ
12ꢀ
0ꢀ
0ꢀ
0ꢀ
0ꢀ
53ꢀ
8ꢀ
5ꢀ
<5ꢀ
<5ꢀ
6 ꢀ
0ꢀ
0ꢀ
0ꢀ
0ꢀ
A. Orthogonal reactivity
ꢀ
d
19 ꢀ
O
HO
+
84dꢀ
ꢀ
20 mol% "Ni"
tBuONa, m-xylene,
120 °C, 16 h
+
H2
100ꢀ
100ꢀ
(
1 bar)
R1
R2
R1
R2
(
a)ꢀ Reactionꢀ conditions:ꢀ diarylꢀ etherꢀ (1ꢀ equiv.),ꢀ hydrogenꢀ (1ꢀ bar
t
Relative reactivities of diaryl ethers (reaction time 16 h)
OMe
gaugeꢀpressureꢀatꢀrt),ꢀNi(COD)2ꢀ(0.2ꢀequiv.),ꢀ BuONaꢀ(0-5ꢀequiv.),ꢀm-
xylene,ꢀ 120ꢀ C,ꢀ 96ꢀ h;ꢀ (b)ꢀ Dimethylcyclohexanesꢀ wereꢀ observedꢀ as
o
OMe
O
O
O
productsꢀofꢀm-xyleneꢀhydrogenation;ꢀ(c)ꢀUncorrectedꢀGCꢀyield;ꢀ(d)ꢀNo
furtherꢀconversionꢀwasꢀobservedꢀafterꢀ48ꢀh.ꢀ
t
CF3
The fate of the nickel in the absence of BuONa was
t
1
00%
62%
41%
different from the fate in the presence of BuONa. Under
conversion with the ligandless Nia
t
the reaction conditions in the absence of BuONa, the
100%b
100%c
7
5%
nickel precursor rapidly formed a mirror on the walls of
conversion with the carbene Ni
the vessel, leaving a colorless solution. In contrast, un-
t
der the reaction conditions with 0.2 equiv of BuONa,
B. Orthogonal selectivity
the nickel remained in solution. The solution was either
black or dark brown, depending on the amount of base
0% Ni(COD)2a
tBuONa
HO
2
2
Ligandless Ni
Carbene Ni
H2
(
see the supporting information for photographs).
Conversion: 100%
97%
OH
O
1 bar
m-xylene,
120 °C,
16h
HO
+
2
4
0% Ni(COD)2,
0% SIPr·HCl
HO
67%
HO
48%
tBuONa
Conversion: 67%
aCatalyst loadings were not minimized.
b75% at 100 oC; c100% at 100 o
A change in the nickel precursor led to reactions that
C
5%
occur with lower loadings than those conducted with
Ni(COD) . Reactions initiated by a series of well-
The effect of the reaction components on the hydro-
genolysis of diaryl ethers helped reveal the origin of the
unusual regios- and chemoselectivity of the ligandless
2
defined nickel precursors for the model hydrogenolysis
of diphenyl ether (Table S2) indicated that
t
catalyst system. In particular, the BuONa base had a
Ni(CH
lyst than did Ni(COD)
of diphenyl ether initiated by Ni(CH
The reactions conducted with 20 mol% of
Ni(CH TMS) (TMEDA) in place of Ni(COD) formed
benzene and phenol quantitatively in 16 h instead of 48
h. The same reaction with just mol% of
Ni(CH TMS) (TMEDA) occurred without any decrease
2
TMS)
2
(TMEDA) generated a more active cata-
. Eq 2 summarizes the reactions
TMS) (TMEDA).
dramatic influence on catalyst activity, stability, and
chemoselectivity for hydrogenolysis of the CAr-O bond
over hydrogenation of the aromatic ring (Table 1).
2
2
2
t
In the absence of BuONa (Table 1, Entry 1), the cata-
2
2
2
lyst was not regioselective. Under these conditions, the
products included benzene (58%), phenol (2%), cyclo-
hexane (12%), cyclohexanol (53%) and phenyl cyclo-
hexyl ether (6%) from competitive CAr-O bond hydro-
2
2
2
in yield of the hydrogenolysis products after 96 h. The
reaction at 140 °C led to 90% conversion to phenol and
benzene after 24 h.
genolysis and arene hydrogenation. In the presence of
t
0
.2 equiv of BuONa, with respect to diphenyl ether, the
catalytic activity was low (Table 1, Entry 2). However,
the reaction conducted with one equivalent of base oc-
curred to a higher conversion (84%) and formed ben-
zene (84%), phenol (77%) and only traces of cyclohex-
anol (Table 1, Entry 3). The reaction with 2.5 equiv of
Tables 2 and 3 summarize the selective hydrogenolysis
2 2
of various diaryl ethers with Ni(CH TMS) (TMEDA) as
t
precatalyst and BuONa (2.5 equiv) as base. These reac-
tions occurred at just 1 bar of hydrogen pressure and ran
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Journal of the American Chemical Society
Tableꢀ 3.ꢀ Hydrogenolysisꢀ ofꢀ unsymmetricalꢀ diarylꢀ ethersꢀ
to completion with 0.25-10 mol% Ni. No competing
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
hydrogenation of arenes was observed.
catalyzedꢀ byꢀ theꢀ ligandlessꢀ nickelꢀ catalystꢀ atꢀ hydrogenꢀ
pressureꢀofꢀ1ꢀbar.
a
Hydrogenolysis of the electron-neutral diphenyl, di-m-
tolyl, and di-p-tolyl ethers formed arenes and phenols in
excellent yield with 2-10 mol% of Ni (Table 2, Entries
1
2 2
-3). Ni(CH TMS) (TMEDA) also catalyzed the selec-
tive hydrogenolysis of di-o-anisyl ether, an electron-rich
diaryl ether which is representative of the 4-O-5 struc-
tural motif in lignin. Di-o-anisyl ether formed anisole
(
99%) and guaiacol (99%) in excellent yields in the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
presence of only 0.25 mol% of the Ni catalyst (Table 2,
Entry 4). By contrast 10 mol % of the Ni catalyst was
required for cleavage of the electron deficient di-m-
anisyl ether to form anisole (68%) and m-
methoxyphenol (66%) (Table 2, Entry 5).
ArH,ꢀ%ꢀ
ArOH,ꢀ%ꢀ
Entryꢀ Rꢀ
Ni,ꢀ%ꢀ Time,ꢀhꢀ
1ꢀ
2ꢀ
3ꢀ
4ꢀ
1
2
3
4ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
o-Meꢀ
m-Meꢀ
p-Meꢀ
p- Buꢀ
ꢀ
10ꢀ
5ꢀ
5ꢀ
5ꢀ
ꢀ
96ꢀ
48ꢀ
48ꢀ
48ꢀ
ꢀ
22ꢀ
18ꢀ
13ꢀ
3ꢀ
60ꢀ
82ꢀ
85ꢀ
90ꢀ
ꢀ
17ꢀ
17ꢀ
11ꢀ
3ꢀ
63ꢀ
75ꢀ
81ꢀ
90ꢀ
ꢀ
Tableꢀ2.ꢀHydrogenolysisꢀofꢀsymmetricalꢀdiarylꢀethersꢀcat-
t
alyzedꢀbyꢀtheꢀligandlessꢀnickelꢀcatalystꢀatꢀhydrogenꢀpres-
a
ꢀ
ꢀ
sureꢀofꢀ1ꢀbar. ꢀ
5
6
7
ꢀ
ꢀ
ꢀ
ꢀ
o-OMeꢀ 0.5ꢀ
m-OMeꢀ 10ꢀ
48ꢀ
48ꢀ
48ꢀ
ꢀ
45ꢀ
21ꢀ
15ꢀ
ꢀ
54ꢀ
44ꢀ
22ꢀ
18ꢀ
ꢀ
53ꢀ
45ꢀ
2ꢀ
ꢀ
46ꢀ
8ꢀ
ꢀ
p-OMeꢀ
ꢀ
10ꢀ
ꢀ
8
9
ꢀ
ꢀ
o-OHꢀ
m-OHꢀ
5ꢀ
10ꢀ
0.5ꢀ
48ꢀ
48ꢀ
41ꢀ
80ꢀ
75ꢀ
97ꢀ
9ꢀ
8ꢀ
-ꢀ
80ꢀ
75ꢀ
97ꢀ
-ꢀ
-ꢀ
ꢀ
10ꢀ p-OHꢀ
-ꢀ
(
a)ꢀSeeꢀTableꢀ2ꢀforꢀdetailsꢀofꢀtheꢀexperimentalꢀsetup.ꢀ
Entryꢀ
Rꢀ
Ni,ꢀ%ꢀ
Time,ꢀhꢀ
ArH,ꢀ%ꢀ
ArOH,ꢀ%ꢀ
Diaryl ethers that contain a free phenolic group also
1
2
3
4ꢀ
5
ꢀ
ꢀ
ꢀ
Hꢀ
m-Meꢀ
p-Meꢀ
o-OMeꢀ
m-OMeꢀ
2ꢀ
5ꢀ
10ꢀ
0.25ꢀ
10ꢀ
96ꢀ
96ꢀ
96ꢀ
48ꢀ
48ꢀ
99ꢀ
97ꢀ
94ꢀ
99ꢀ
68ꢀ
99ꢀ
93ꢀ
91ꢀ
99ꢀ
70bꢀ
underwent hydrogenolysis. Two equivalents of phenol
were obtained in good to high yield (75-97%) and high
selectivity from the cleavage of o-, m- and p-hydroxy
diphenyl ethers (Table 3, Entries 8-10) with 0.5-10
mol% Ni.
ꢀ
(a)ꢀConditions:ꢀdiarylꢀetherꢀ(1ꢀequiv.),ꢀhydrogenꢀ(1ꢀbarꢀgaugeꢀpressure
t
atꢀrt),ꢀNi(CH
2
TMS)
2
(TMEDA)ꢀ(0.25-10ꢀmol%),ꢀ BuONaꢀ(2.5ꢀequiv.),ꢀm-
xylene,ꢀ120ꢀ C.ꢀ(b)Totalꢀyieldꢀofꢀ3-methoxyphenolꢀ(66%)ꢀandꢀphenol
4%).ꢀ
o
Tableꢀ4.ꢀHydrogenolysisꢀofꢀbenzylꢀethersꢀcatalyzedꢀbyꢀtheꢀ
ꢀa
(
ligandlessꢀnickelꢀcatalystꢀatꢀhydrogenꢀpressureꢀofꢀ1ꢀbar. ꢀ
The hydrogenolysis of unsymmetrical diaryl ethers
Table 3) provided further insight into the electronic
effects on the reaction. Alkyl substituted diaryl ethers
Table 3, Entries 1-4) were cleaved in good yield (80-
9%) to the corresponding arenes and phenols with 5-10
(
(
9
mol% Ni. The regioselectivity of these reactions favored
arene and phenol products from cleavage of the CAr-O
bond adjacent to the unsubstituted ring. However, the
hydrogenolysis of substrates bearing oxygenated sub-
stituents favored arene and phenol products from the
cleavage of the CAr-O bond adjacent to the more elec-
tron rich arene ring (Table 3, Entries 5-10).
2
Entryꢀ
1b
Benzylꢀetherꢀ
ꢀ
Ni,ꢀ%ꢀ ArCH
3
,ꢀ%ꢀ R OH,ꢀ%ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
OPh
2ꢀ
98ꢀ
81ꢀ
95ꢀ
87ꢀ
ꢀ
2ꢀ
2
ꢀ
ꢀ
ꢀ
ꢀ
The unsymmetrical 4-O-5 lignin structural model o-
anisyl phenyl ether was cleaved to arenes (99%) and
phenols (98%) in excellent yield (Table 3, Entry 5) in
the presence of 0.5 mol% of the nickel catalyst. Recall
that di-o-anisyl ether (Table 2, Entry 4) reacted quantita-
tively in the presence of 0.25 mol% of the Ni catalyst.
The loadings of the ligandless catalyst required for full
cleavage of the 4-O-5 lignin model substrates were 40
and 80 times lower than that required by the Ni carbene
3ꢀ
0.25ꢀ
93ꢀ
98ꢀ
Ndꢀ
ꢀ
ꢀ
4
ꢀ
20ꢀ
99cꢀ
ꢀ
(
a)ꢀSeeꢀTableꢀ2ꢀforꢀdetailsꢀofꢀtheꢀexperimentalꢀsetup.ꢀ(b)ꢀ96ꢀhꢀ(c) In
theꢀpresenceꢀofꢀAlMe ꢀ(1ꢀequiv.).ꢀ
3
The ligandless nickel system also catalyzes the hydro-
genolysis of benzyl aryl and benzyl methyl ether, which
are two additional types of linkages in lignin (Table 4).
Benzyl aryl ethers were converted to methyl-substituted
arenes and phenols in excellent yields in the presence of
1
0
catalyst, reflecting a directing effect of o-methoxy
substituents. The m- and p- constitutional isomers (Table
3
, entries 6 and 7) of o-anisyl phenyl ether were less
reactive under the hydrogenolysis reaction conditions.
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0
3
.25-2 mol% of the nickel catalyst (Table 4, Entries 1-
). Benzyl methyl ether was unreactive under standard
catalyst and the development of supported catalytic sys-
tems that display the same selectivity for hydrogenolysis
over hydrogenation.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
reaction conditions, but full conversion could be
achieved in the presence of 1 equiv of AlMe (Table 5,
3
Entry 5).
ASSOCIATED CONTENT
Having observed the cleavage of the C–O bonds in
various ethers, we evaluated the relative reactivity of
these ethers toward the ligandless system (For the full
primary data, see Figures S1-S5). We observed that di-
aryl ethers and benzyl aryl ethers reacted at comparable
rates, and both underwent hydrogenolysis faster than
benzyl alkyl ethers. Even though unactivated diaryl and
benzyl aryl ethers reacted at comparable rates (Figure
S2), the electron-rich di-o-anisyl ether was selectively
reduced in the presence of the electron-neutral p-tert-
butylbenzyl p-tert-butylphenyl ether (eq 3). This reduc-
tion of an unactivated diaryl ether in the presence of a
benzyl aryl ether is unusual.
Supportingꢀ Information.ꢀ Experimentalꢀ procedures,ꢀ
Tablesꢀ S1-S4,ꢀ Figuresꢀ S1-S10,ꢀ photographsꢀ ofꢀ reactionꢀ
mixtures,ꢀ reactionꢀ progressꢀ graphs.ꢀ Thisꢀ materialꢀ isꢀ
availableꢀ freeꢀ ofꢀ chargeꢀ viaꢀ theꢀ Internetꢀ atꢀ
http://pubs.acs.org.ꢀ
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AUTHOR INFORMATION
Corresponding Author
jhartwig@berkeley.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
tBu
OMe
OMe
We thank Energy and Biosciences Institute for support of
this work as well as the Molecular Foundry and Virginia
Altoe for TEM analysis. Work at the Molecular Foundry
was supported by the Office of Science, Office of Basic
Energy Sciences, of the U.S. Department of Energy under
Contract No. DE-AC02-05CH11231.
O
O
5% Ni(TMEDA)(CH
2
TMS)
2
+
tBu
tBuONa (2.5 equiv.),
m-xylene, 120 °C,
+ H2
1 equiv
1 equiv 1 bar
OMe
1
6 h
tBu
OMe
HO
O
(3)
+
+
tBu
REFERENCES
99%
99%
100% unreacted
(
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(
8
2
and
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tivity of homogeneous catalysts.
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(
1
8) Alvarez-Bercedo, P.; Martin, R. J. Am. Chem. Soc. 2010, 132,
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ligandless nickel catalyst for the hydrogenolysis of di-
phenyl ether was completely suppressed by an excess of
added mercury (Table S4). This result contrasts the lack
of an effect of mercury on the hydrogenolysis of aryl
ethers catalyzed by the SIPr-Ni system. These data
suggest that heterogeneous nickel clusters or particles
catalyze the hydrogenolysis reactions conducted without
added dative ligand. Indeed, TEM analysis (Figure S6-
S10) of an aliquot of a reaction mixture revealed the
presence of 2.5±0.4 nm nickel particles. Indeed X-Ray
EDS analysis of these nanoparticles indicated that they
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(
10) Sergeev, A. G.; Hartwig, J. F. Science 2011, 332, 439.
1
0
(11) For the reductive cleavage of C-S bonds using ligandless nickel
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(
12) Nimz, H. Angew. Chem. Int. Ed. Engl. 1974, 13, 313.
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Bioprod. Bioref. 2008, 2, 58.
(
14) The identity of the base also affected the catalytic activity.
Reactions conducted with various bases (2.5 equiv) in the model
hydrogenolysis of diphenyl ether (Table S1) for 16 h showed that the
t
highest conversions were obtained in the presence of BuONa (62%)
1
7-19
t
and PentONa (70%). The reactions conducted in the presence of
contain both nickel and sodium. Caubère and Fort
have shown that BuONa can stabilize nickel nanoparti-
cles and we propose that the BuONa stabilizes the
t
t
t
BuOK or BuOLi and reactions conducted with less bulky alkoxide
i
t
bases containing hydrogens α to oxygen, such as OMe and O Pr, led
to low conversions (<15%). No hydrogenolysis occurred from
t
“
ligandless” system of our studies.
reactions with weaker bases. Given the similar efficiency of BuONa
t
t
In conclusion, we have developed a highly active het-
and PentONa, we chose to conduct further studies with BuONa (2.5
equiv) as base.
erogeneous nickel catalyst for selective hydrogenolysis
of diaryl, benzyl aryl and benzyl alkyl ethers to form
arenes and alcohols as the exclusive products. The cata-
lyst generated from the well-defined soluble nickel pre-
cursor Ni(CH TMS) (TMEDA) operates at low hydro-
2 2
gen pressure (1 bar) and loadings as low as 0.25 mol%
(15) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A 2003, 198, 317.
(
(
16) Crabtree, R. H. Chem. Rev. 2012, 112, 1536.
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(
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(19) Illy, S.; Tillement, O.; Machizaud, F.; Dubois, J. M.; Massicot,
F.; Fort, Y.; Ghanbaja, J. Philos. Mag. A 1999, 79, 1021.
Future work will be aimed at the characterization of this
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