Less hindered ligands give improved catalysts for the nickel catalysed Grignard cross-coupling of aromatic ethers

Article abstract of DOI:10.1039/c7cy01205e

The challenging reaction of unactivated ortho-substituted aromatic ethers with Grignard reagents has been found to be most effectively catalysed using nickel complexes of less sterically hindered ligands. Air stable, cheap, commercially available [NiCl₂(PⁿBu₃)₂] stands out as an improved catalyst for this type of transformation. The improved results with this catalyst even extend to some couplings of a more activated substrate when examined at higher temperatures and at catalyst loadings down to 0.1 mol%. Unusual induction periods in these latter reactions have been related to the by-product magnesium salts acting as co-catalysts.

Full text of DOI:10.1039/c7cy01205e

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Less hindered ligands give improved catalysts for
the nickel catalysed Grignard cross-coupling of
aromatic ethers†
Cite this: DOI: 10.1039/c7cy01205e
Gavin J. Harkness
and Matthew L. Clarke
*
The challenging reaction of unactivated ortho-substituted aromatic ethers with Grignard reagents has been
found to be most effectively catalysed using nickel complexes of less sterically hindered ligands. Air stable,
n
Received 15th June 2017,
Accepted 22nd November 2017
cheap, commercially available [NiCl IJP Bu ) ] stands out as an improved catalyst for this type of transfor-
2
3 2
mation. The improved results with this catalyst even extend to some couplings of a more activated sub-
strate when examined at higher temperatures and at catalyst loadings down to 0.1 mol%. Unusual induc-
tion periods in these latter reactions have been related to the by-product magnesium salts acting as co-
catalysts.
DOI: 10.1039/c7cy01205e
rsc.li/catalysis
ethers. Here we show that, in contrast to most cross-cou-
Introduction
n
plings, less bulky catalysts such as [NiCl IJP Bu ) ] give im-
2
3 2
A key area of modern research is the cross-coupling of organ-
ometallics with aromatic electrophiles via carbon–oxygen
bond cleavage. In particular, intensive research has focused
on expanding this chemistry away from expensive
trifluoromethanesulfonates to cheaper leaving groups that
also deliver less harmful waste streams. The use of simple ar-
omatic ethers is especially desired, but this is a challenging
reaction: naphthyl ethers frequently deliver good results
proved performance in the Grignard cross-coupling of aro-
matic ethers (Scheme 1b).
Results and discussion
1
The model substrate used to search for ligand steric effects
was 2-phenyl anisole. This could be prepared from the imidaz-
ole sulfonate of guaiacol by a Grignard cross-coupling catalysed
by palladium complexes (but unfortunately not the nickel cata-
lysts used for ether cleavage) as described in the ESI .† Our ini-
tial tests of a range of Ni pre-catalysts derived from bidentate,
hemilabile and monodentate ligands did not yield useful
amounts of product in this model coupling; catalysts derived
(Scheme 1a), but removing the second aromatic ring causes a
large fall in reactivity and successful examples are rare and re-
2
,3
quire very specific nucleophile/electrophile combinations.
With the exception of aromatic ethers with ortho-directing
4
,5
oxazoline
groups, examples of coupling ortho-substituted
unactivated anisoles are even more scarce.
2 3
from [NiIJcod) ] and Cy P or bis-dicyclohexylphosphino-ethane
Among the most common catalyst systems used in the
were ineffective. One of the state-of-the-art catalysts for ether
cross-coupling of aromatic ethers are NiIJcod)
2
/PCy
One of the early key papers reported
that using bulky tricyclohexylphosphine as ligand was sub-
3
and
2l
cleavage in the literature uses the NHC ligand IPr, but this
2
e,h,n,t,w,x
2 3 2
[NiCl IJPCy ) ].
was also ineffective (Scheme 2 and see ESI†). Given the hin-
dered nature of the C–O bond in this substrate, we considered
that less sterically hindered phosphines were worthy of investi-
gation. A range of nickel complexes of electron donating phos-
2
x
stantially superior to triethylphosphine, which perhaps led
to smaller cone angle phosphines being neglected.
During
a
project seeking to convert lignin-derived
8,9
phines with relatively small cone angles were therefore inves-
5
–7
2
-methoxyphenol into useful products,
we needed to con-
tigated as catalysts for this especially difficult Grignard cross-
coupling reaction (Scheme 2). All of the catalysts derived from
structively deoxygenate several ortho-substituted anisoles,
leading us to study the Ni-catalysed Grignard coupling of aryl
ligands with smaller cone angles than Cy P outperformed
3
benchmark catalyst, 1, the only exception being the catalyst
2 3 2
with the least sterically hindered phosphine [NiCl IJPMe ) ], 6.
School of Chemistry, University of St Andrews, EaStCHEM, St Andrews, Fife, UK.
E-mail: mc28@st-andrews.ac.uk; Fax: +44(0) 1334 463808;
Tel: +44(0) 1334 463850
Commercially available, cheap, air stable, and previously
n
untested [NiCl IJP Bu ) ] stands out.
2
3 2
The reactions give good results using reasonably practical
conditions. For example, while excess phosphine ligand, as
well as large excesses of Grignard are often required to gain
†
Electronic supplementary information (ESI) available: Full details of experi-
mental procedures, analytical data and NMR spectra are available in the ESI. See
DOI: 10.1039/c7cy01205e
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Scheme 1 Nickel-catalysed cross-coupling using aryl ethers.
Scheme 2 Effect of ligand cone angle on the challenging nickel-catalysed coupling of 2-methoxybiphenyl.
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2
x
n
satisfactory yields in this type of reaction, [NiCl IJP Bu ) ]
couples extremely well (Table 1, entry 7). However, the in-
2
3 2
works effectively with no additives and actually works better
using 1.5 equivalents of Grignard rather than a large excess
creased sterics imposed by o-TolMgBr gave lower yields
n
(Table 1, entries 5–6). This trend for [NiCl
2
3 2
IJP Bu ) ] to
(
Table 1, entry 3 and ESI†). While this project was not target
outperform bulkier ligands extends to other substrates of
varying electronic and steric properties. Other biphenyl
methyl ether substrates (Table 1, entries 10 and 14) gave high
yields. Coupling of 2-methylanisole required 10 mol% cata-
lyst, (Table 1, entry 12) but also gave best results with catalyst
5. In the coupling of ortho-substituted 1-phenyl-2-
focussed, we felt it important to establish if this catalyst gave
improved results for some other combinations of ether and
Grignard reagent. In fact, other Grignard reagents of varying
steric properties were tested with this substrate and the same
trend was apparent, with catalyst 5 outperforming 1. PhMgBr
Table 1 Nickel-catalysed Grignard cross-coupling of challenging aryl methyl ethers
a
2
b
c
c
Entry
ArOMe
R MgBr
Catalyst (mol%)
Temp (°C)
Time (h)
Conversion (%)
Product (%)
1
p-Tol
p-Tol
p-Tol
p-Tol
1 (5)
1 (5)/L (10)
5 (5)
100
100
100
100
19
19
16
45
36
37
85
98
15
30
79
84 [84]
e
2
e
3
d
4
5 (5)/L (10)
5
6
As above
o-Tol (2.0)
o-Tol (2.0)
1 (5)
5 (5)
100
100
16
16
23
25
8
21 [21]
7
As above
As above
Ph (1.7)
5 (5)
100
16
97
96 [86]
e,f
8
Me (2.4)
Me (2.4)
1 (5)
5 (5)
100
100
16
16
9
48
3
e,f
g
9
46 [27]
10
p-Tol
5 (5)
100
40
79
55 [40]
e
e
1
1
1
2
p-Tol
p-Tol
1 (10)
5 (10)
100
100
64
64
18
76
1
h
56 [40]
1
1
3
4
p-Tol
p-Tol
1 (5)
5 (5)
100
100
16
16
59
84
34
70 [65]
15
As above
o-Tol (2.0)
5 (5)
100
16
51
49 [43]
d
16
17
18
19
p-Tol
p-Tol
p-Tol
p-Tol
1 (1)
1 (1)
5 (1)
5 (1)
80
100
80
16
16
16
16
82
70
54
91
70
60
41
85 [83]
d
100
2
2
0
1
o-Tol (2.0)
o-Tol (2.0)
1 (1)
5 (1)
100
100
16
16
59
76
40
61 [59]
d
d
2
2
2
3
o-Tol
o-Tol
1 (1)
5 (1)
80
80
16
16
>99
>99
95
97 [82]
a
2
Reactions were carried out on the scale of aryl ether (0.50 mmol), Grignard reagent (0.75 mmol in Et O, 0.5 M), 2-MeTHF (2.25 mL) in sealed
1
b
c
vessels unless otherwise noted.
1
Grignard molarity in brackets.
Conversions andd yields were determined by
H
NeMR using
-methylnaphthalene as an internal standard [yield of isolated product in square brackets]. Reaction performed in a Schlenk flask. 2.1 equiv.
f
g
h
Grignard added. PhMe as reaction solvent. 27% yield isolated with 1-methylnaphthalene impurity; total product obtained = 24%. 40% yield
isolated with 1-methylnaphthalene impurity; total product obtained = 38%.
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n
methoxynaphthalene at 100 °C, [NiCl IJP Bu ) ] is a better cat-
For the reactions described here, we suggest the use of
2
3 2
n
alyst than catalyst 1 for coupling either p-TolMgBr or
o-TolMgBr (Table 1, entries 19 and 21). At lower temperature,
for this more activated aromatic, catalyst 1 was actually the
faster catalyst. The simpler substrate methoxynaphthalene,
even using a hindered Grignard reagent, ortho-tolyl magne-
sium bromide, can be coupled well with either catalyst (en-
tries 22 and 23).
A final ortho-substituted substrate of particular interest
was 2-(2-methoxyphenyl)-pyridine, since there is potential for
an ortho-directing effect to have an impact on the nickel ca-
talysis. At room temperature, a significant amount of the de-
sired product is formed in the presence of the nickel catalysts
cheap, air stable, commercially available [NiCl IJP Bu ) ] as
2
3 2
the preferred catalyst.
There is possibly an argument that high catalyst loadings
are perfectly acceptable with some cheap iron-based cross-
1
0
coupling catalysts (used for more activated electrophiles),
but the ligands often have a significant cost, so large scale
cross-couplings are likely to need substrate : catalyst ratios
less than 100 : 1. In the case of nickel, the requirement to re-
1
1
move all the nickel due to its toxicity creates further costs.
Couplings at lower catalyst loadings than the loadings of 1 to
5 mol% used in the literature were studied next using
2-methoxynaphthalene as substrate.
(Scheme 3). However this is let down by low chemoselectivity
In the reaction of p-TolMgBr with 2-methoxynaphthalene
with unknown side products, and the demethylated phenol
being present. The ortho-directing group therefore has a pro-
found and positive effect on the reactivity of the substrate
with Ni catalysts, although this can be difficult to control.
Even lower chemoselectivity was observed at higher tempera-
tures. In fact, while there is no sign of a background
at 80 °C using [NiCl
2
IJPCy
3
)
2
], there is no advantage gained
n
when using [NiCl IJP Bu ) ] (Table 2, entries 1 and 3). How-
2
3 2
ever, at 100 °C, we were able to deliver high yields of product
using 0.25 mol% of [NiCl
n
2 3 2
IJP Bu ) ]; better than can be
achieved using [NiCl IJPCy ) ] as catalyst (Table 2, entries 5
2
3 2
and 6). From a mechanistic viewpoint, there is clearly a li-
gand steric effect on the productivity of these reactions at
higher temperatures, with smaller cone angle, electron-
donating phosphine-based catalysts giving better results. This
(uncatalysed) reaction at room temperature, more
chemoselective coupling can be observed without any catalyst
at 80 °C (71% isolated yield). This appears to be the first
4
Meyers ether cleavage that has been reported using a pyridyl
3 n
could be a direct effect in that a less crowded NiIJPR ) species
directing group and highlights that this chemistry is poten-
tially general whenever there is a nearby mesomerically
electron-withdrawing group that also can coordinate to
magnesium.
of some form undergoes C–O bond cleavage more effectively,
or an indirect effect relating to catalyst stability. Initial data
from sampling experiments throughout this work suggest
both catalysts 1 and 5 remain active for extended periods of
time, suggesting the former explanation of a ligand effect on
the catalytic cycle. Time sampling experiments for reactions
carried out at 0.1 mol% of catalyst showed quite unusual be-
haviour (Fig. 1).
We also examined some further examples of Ni/NHC cata-
lyst systems with varying steric properties and found a simi-
lar trend regarding ligand sterics; the NHC ligands, IDM and
1
3
ICy with the smallest% buried volume gave improved re-
sults relative to generally more favoured NHC ligands such as
IPr, IPr* and SiPr (see ESI†). Ni/ICy has been used in ether
cleavages using alkynyl and alkyl Grignard reagents
Most catalysts would give highest rates at the highest sub-
strate concentration (i.e. most catalysts being first order in
substrate), but also when catalyst is freshest: it is normal for
some degradation to further limit rates after days have
passed for the majority of cross-coupling catalysts. The re-
sults obtained were therefore quite unexpected. There is al-
most no conversion in the first 16 hours for either catalyst.
After around a day, the reaction begins to accelerate with, as
we now expected, the smaller cone angle tri-n-butylphosphine
catalysts proceeding faster. A time profile where the degree of
conversion increases in this way can either imply an induc-
tion period where a pre-catalyst is converted into the active
species, or autocatalysis from a product of the reaction. Since
the reduction of NiL Cl to Ni(0) using a Grignard is a fast re-
2
q,r
recently,
The findings with the carbenes are consistent
with a direct and strong ligand steric effect on these
reactions.
2
2
action, this is not likely to be the origin of the induction pe-
riod. When considering the mechanism for this type of reac-
tion, the formation of some higher order Ni nanoparticle or
multinuclear species is not outside the realms of possibility
and could exhibit a time profile like this. However, an added
possibility in these reactions is that various magnesium salts
that could be formed during these reactions might promote
the reaction. Using this experimental set-up, without in situ
monitoring, it is not possible to study the kinetics of these re-
actions and fully study autocatalytic behaviour and other
Scheme 3 A potentially chelating substituent enables Ni-catalysed
ether cleavage at room temperature, and also an uncatalysed reaction
at higher temperature (% product vs. internal standard is shown, with
conversions for Ni-catalysed reactions both 85%; 71% isolated yield
from uncatalysed rct., see ESI†).
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Table 2 Effect of catalyst and catalyst loading in a cross coupling of an activated substrate
a
b
b
Entry
Catalyst (mol%)
Temp (°C)
Conversion (%)
Product (%)
1
2
3
4
5
1 (0.5)
1 (0.5)
5 (0.5)
5 (0.5)
5 (0.25)
5 (0.1)
80
100
80
100
100
100
39
75
17
93
83
62
38
59
16
89
71
56
c
6
a
2
Reactions were carried out on the scale of aryl ether (0.50 mmol), Grignard reagent (0.75 mmol in Et O, 0.5 M, 1.5 equiv.), 2-MeTHF (2.25
1
b
mL) in sealed vessels for 16 hours unless otherwise noted. Conversions and yields were determined by H NMR using 1-methylnaphthalene as
an internal standard [yield of isolated product in square brackets]. 69 hours reaction time.
c
Fig. 1 Time profile for the reaction of 2-methoxynaphthalene with p-tolyl magnesium bromide using 0.1 mol% of the Ni catalysts shown at 100
°C. “30% MeOH” = 0.3 equiv. of MeOH added to solution containing catalyst and substrate, prior to Grignard addition, “30% BrMgOMe” = 0.3
equiv. of MeOH added to Grignard prior to addition of solution containing catalyst and substrate, (see ESI† for further details).
promotion effects. However, the preliminary data does seem
to indicate a possible role of Mg salts other than the often-
postulated role for the Grignard as promoter. First of all an
excess of anhydrous magnesium iodide was added to the re-
action in order to see if there was any indication that just a
simple Mg salt could remove the induction period: which it
clearly does (Fig. 1, brown line). A more realistic concentra-
tion and identity of Mg salt were studied next. Some of the
likely Mg salts formed during the reactions were added by
quenching a slightly larger excess of Grignard with methanol
to make ‘MgIJOMe)Br’. This was done in two different ways,
just in case either methanol had a negative effect on the cata-
lyst. While this study does not delve into the details of the
promotion effect (or even if there is significance in the im-
proved performance when methanol is added to the catalyst
first), it is clear the magnesium salts reduce the induction pe-
riod. A full kinetic study using in situ techniques on a range
of Ni catalysed Ni–O bond cleavages using various promoters
would be welcome.
The role of the magnesium salts produced during the cou-
pling reactions could therefore be important in the mecha-
nism for the couplings of naphthalenes at least (a prelimi-
nary attempt to use these additives to improve yields in one
of the more problematic couplings in Table 1 did not signifi-
cantly improve the result). The data are consistent with the
induction period being a result of autocatalysis from the Mg
salts produced during the reaction, presumably assisting in
the C–O bond cleavage. There have been previous proposals
for a nickelate complex undergoing oxidative addition after
2
a
1
2a
transmetalation from the Grignard,
or an alternative in
which the Grignard or an organoaluminium reagent acts as
an activator for the otherwise conventional oxidative addition
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2
y,3a
process.
Our working hypothesis is that for these acti-
with a crimp cap and flushed with argon for 30 minutes.
vated substrates, oxidative addition, or conceivably trans-
metalation, is promoted by the Mg salts, formally
2-Methoxybiphenyl
(92.1
mg,
0.50
mmol)
and
1-methylnaphthalene (60 μL, 0.42 mmol, internal standard)
were added to a flame dried Schlenk flask under an inert at-
mosphere. 2-MeTHF (2.25 mL) was then added to the
[MgBrIJOMe)], that are formed during the reaction.
Schlenk flask to make a solution. A t
0
sample (approximately
1
Conclusions
10 μL) was taken and analysed by H NMR (to calibrate the
ratio of internal standard to starting material). The solution
containing the electrophile and internal standard was added
to the nickel catalyst via syringe. PhMgBr (439 μ, 0.75 mmol,
Of course, streamlined ligands have been used in other exam-
2
r,14
ples of cross-coupling catalysis,
but they are more often
overlooked. Meanwhile, examples of a unambiguous ligand
steric effect favouring smaller ligands are unusual. There is a
tendency for methodology chemists to initially minimise re-
action temperatures, aiming at convenience for lab-scale re-
search, which might lead to streamlined ligands giving poor
performance under those conditions. However, both less acti-
vated reactions and reactions needing to be run using low
catalyst loadings will often be operated at higher tempera-
tures. We suggest there is a case for re-examining stream-
lined ligands in other reactions at higher temperatures. The
1
.7 M in Et O) was added dropwise over 15 minutes. The re-
2
action mixture was stirred vigorously in an oil bath at 100 °C
for 16 hours. Upon cooling to room temperature, approxi-
mately 20 μL of the crude reaction mixture was added to a
3
vial and quenched with CDCl . The resulting mixture was
then filtered through a small cotton wool plug into an NMR
1
tube and a H NMR was run to assess the ratio between SM
and desired product. Saturated aqueous ammonium chloride
(
5 mL) was added to the reaction mixture and the aqueous
phase was extracted three times with ethyl acetate (3 × 5 mL),
dried over sodium sulfate and concentrated under reduced
pressure. Purification via column chromatography on silica
gel (eluent petroleum ether) gave 1,1′:2′,1″-terphenyl (98.6 mg,
3
ligand n-Bu P is perhaps especially worthwhile since it is al-
ready produced at large scale, with a good range of metal
salts commercially available or made in a trivial manner. The
exact nature of the active catalyst in these reactions is
unclear, but the fact that elevated temperatures seem to be
needed to reach the working temperature of the catalyst,
along with the very low activity of bidentate systems, may
suggest the active species is formed after a quite challenging
ligand dissociation. Once this has occurred, it releases a
highly active and unhindered catalyst. The less-hindered cata-
8
6%) as a colourless oil with data in accordance with the lit-
1
5
erature. δH (500 MHz, CDCl ); 7.49–7.42 (4H, m, C H),
3
Ar
C
7.26–7.20 (6H, m, CArH), 7.19–7.14 (4H, m, CArH); δ (125
MHz, CDCl
3
) 141.6 (2C, CAr), 140.7 (2C, CAr), 130.7 (2C, CArH),
1
30.0 (4C, C H), 128.0 (4C, C H), 127.6 (2C, C H), 126.6
Ar
Ar
Ar
+
+
+
(
2C, CArH); m/z HRMS (EI ) C18
H14 ([M] ) requires 230.1096;
found 230.1092 (−1.7 ppm).
3
lyst has been validated to be more effective than Cy P/nickel
in over 10 challenging cross-couplings, but also to have an ef-
fect on activated substrates when carried out at higher tem-
perature. This aspect also includes what seem to be the low-
est catalysts loadings used in this type of Ni catalysis. A full
kinetic and mechanistic study covering a full range of ligands
and substrates at high and low temperatures would be a use-
ful topic for further study. Target-focussed applications of
Ni complexes of streamlined ligands at higher temperatures
should also be considered following on from this work.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
1
2
The authors thanks the EPSRC for funding in the initial
stages of this work from EP/J018139/1 and from the DTG.
Notes and references
Experimental
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able in ESI.†
All manipulations were carried out under an inert atmo-
sphere of nitrogen or argon using standard Schlenk tech-
niques. Solvents were dried and degassed before use.
Research data that underpins this article (NMR f.i.d.'s)
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Example procedure of Ni-catalysed Grignard cross-
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