Mechanical metal activation for Ni-catalyzed, Mn-mediated cross-electrophile coupling between aryl and alkyl bromides

Article abstract of DOI:10.1039/d1nj01732b

Liquid-assisted grinding has been successfully applied to eliminate the requirements of chemical activators and anhydrous solvents in nickel-catalyzed, manganese-mediated cross-electrophile coupling between aryl and alkyl bromides. In addition to the traditional reaction parameters, mechanical ones,e.g.the rotational speed of mill, the filling degree of jar and ball size, have been found to affect the catalytic efficiency remarkably, implying the involvement of the regeneration of nickel(0) species in the rate-determining steps. A combined evaluation of the reaction and mechanical parameters led to an optimal condition under which a variety ofn-alky aromatics with various functional groups could be readily obtained in good yields with a 1 mol% catalyst loading. The practical application of liquid-assisted grinding-enabled aryl/alkyl cross-electrophile coupling has been demonstrated in the gram-scale synthesis of 6-methoxytetralone.

Full text of DOI:10.1039/d1nj01732b

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Mechanical metal activation for Ni-catalyzed, Mn-
mediated cross-electrophile coupling between
aryl and alkyl bromides†
Cite this: New J. Chem., 2021,
45, 11269
Sisi Wu, Weijia Shi * and Gang Zou
*
Liquid-assisted grinding has been successfully applied to eliminate the requirements of chemical
activators and anhydrous solvents in nickel-catalyzed, manganese-mediated cross-electrophile coupling
between aryl and alkyl bromides. In addition to the traditional reaction parameters, mechanical ones,
e.g. the rotational speed of mill, the filling degree of jar and ball size, have been found to affect the
catalytic efficiency remarkably, implying the involvement of the regeneration of nickel(0) species in the
rate-determining steps. A combined evaluation of the reaction and mechanical parameters led to an
optimal condition under which a variety of n-alky aromatics with various functional groups could be
readily obtained in good yields with a 1 mol% catalyst loading. The practical application of liquid-assisted
grinding-enabled aryl/alkyl cross-electrophile coupling has been demonstrated in the gram-scale
synthesis of 6-methoxytetralone.
Received 9th April 2021,
Accepted 21st May 2021
DOI: 10.1039/d1nj01732b
rsc.li/njc
6-methoxytetralone, we found that the addition of a small amount
of water to anhydrous DMA would mainly make the XEC reaction
Introduction
Transition-metal catalyzed cross-coupling of aryl halides represents between 3-bromoanisole and ethyl 4-bromobutyrate slow rather
the most reliable protocol for site-specific introduction of alkyl than quench in the presence of an activator, trimethylsilyl chloride
groups into aromatic rings. Compared with the traditional cross- (TMSCl) (Scheme 1).
coupling with organometallic reagents, the emerging nickel-
These facts imply that the deleterious effects of non-
catalyzed aryl/alkyl cross-coupling of electrophiles, in particular anhydrous solvents can be attributed to their high water
halides (XEC),1 eliminates the requirement of preformation of content as well as the requirement of a large volume in the
organometallics, promising more cost-effective processes and/or traditional solution synthesis. We anticipate that, if solvent-free
higher functional group compatibility. Therefore, many efforts have or liquid-assisted-grinding (LAG) mechanochemical synthesis
been devoted to improving nickel catalyzed aryl/alkyl XEC since the via ball milling is applied,8 it is possible to conduct nickel-
pioneering work by the Weix group in 2010.2 Although the past catalyzed aryl/alkyl XEC without anhydrous solvents since only
few years have witnessed a number of advances in XEC with minimal amounts of solvents are required under LAG conditions.
respect to substrate scope,3 catalysts,4 reductants,5 and mechanistic In addition, ball milling features mechanical activation of the
elucidation,6 the requirement of anhydrous yet strong hydroscopic metal surface for synthesis,9 thus eliminating the need for
solvents, e.g. N,N0-dimethylpropyleneurea (DMPU), N,N0-dimethyl- chemical activators. In fact, Browne et al. have elegantly
acetamide (DMA), etc., is still a challenge in practical applications. demonstrated the applications of mechanical activation of zinc
In sharp contrast to solvents, nickel catalyzed aryl/alkyl XEC
processes appear to be compatible with a small amount of water
in substrates, reagents and even vehicles. For example, hydrous
catalyst precursors, e.g. NiI2ꢀxH2O, can be used in some cases.2,7
Even, vehicles can be used without the rigorous drying procedure
if anhydrous chemicals are used.7 During the development of
School of Chemistry & Molecular Engineering, East China University of Science &
Technology, 130 Meilong Rd, Shanghai, China.
E-mail: 10111048@mail.ecust.edu.cn, zougang@ecust.edu.cn
† Electronic supplementary information (ESI) available: Experimental procedure,
Scheme 1 Effects of a small amount of water on the Ni-catalyzed,
Mn-mediated XEC between 3-bromoanisole and ethyl 4-bromobutyrate.
characterization data for products, and copies of NMR and HRMS spectra. See
DOI: 10.1039/d1nj01732b
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powders in Reformatsky reaction and Negishi coupling.10 the reaction completed in 1.5 h with 1 mL mgꢁ1 of DMA loading
Herein we report a liquid-assisted grinding-enabled cross- (Z = 1).
electrophile coupling between aryl and alkyl bromides to access
Although there is always some metallic manganese observed
alkyl aromatics without chemical activators under non- after the reaction, when 1.2–1.5 equiv. of Mn powders were
anhydrous conditions.
used 3aa was obtained in lower yields (71–80%). The other
common metals, i.e. Zn, Al and Fe, were also examined as
reductants to regenerate catalytically active nickel species since
the reducing agent has proven to be vital for nickel-catalyzed
cross-electrophile coupling. Zinc powders, commonly used as
the reducing agent in organic reactions, led to only a 38% yield,
while Al and Fe provided no reaction (Table 1, entries 12–14).
Screening of the other common polar solvents, e.g. DMF,
DMPU, DMSO, THF, dioxane and acetonitrile, showed that only
expensive DMPU performed similarly to DMA, while less-polar
dioxane and THF failed completely (Table 1, entries 15–20).
A slight excess (1.2–1.5 equiv.) of the alkyl counterpart appeared
to be necessary to obtain good yields. In fact, a larger excess
(2.0 equiv.) of ethyl 4-bromobutyrate (2a) made it difficult to
obtain analytically pure 3aa (1H NMR) although the yield of the
impure product was 490% (Table 1, entries 21–24). When the
catalyst (bpy)NiCl2 loading was reduced to 1 mol%, the model
reaction became slow and required 9 h to complete, but still
gave 3aa in a comparable yield (Table 1, entry 25).
Results and discussion
Ball-milling is a well-known top-down protocol to increase the
specific surface area and freshness of the surface for particles
of hard materials. For metal powders, the morphological
changes of particles, which strongly depend on the malleability
of the corresponding metals, can directly reflect the effects of
mechanochemical metal activation via ball-milling. In fact, the
size of iron and manganese particles reduced after ball-milling
at 525 rpm for 30 minutes, while the particles of zinc and
aluminum are obviously merged by the force of ball-milling
after the removal of oxide layers (Fig. 1) (see the ESI† for the
original photographs). These results promise the potential to
eliminate the need for a chemical activator in the activation of
metal powders in the nickel-catalyzed, metal-mediated aryl/
alkyl XEC process.
The coupling between 3-bromoanisole (1a) and 1.5 equiv. of
ethyl 4-bromobutyrate (2a) was taken as the model reaction to
explore the mechanical metal activation strategy in the
Ni-catalyzed XEC process. Under solvent-free ball-milling
(525 rpm) in a 50 mL stainless-steel jar and 8 balls (+ =
7 mm), only a trace amount of the desired product (3aa) was
detected in 4 h using a catalyst system, 3 mol% (bpy)NiCl2,
10 mol% KI and Mn (2 equiv.) under nitrogen (Table 1, entry 1).
When a small amount of analytical grade (99%) non-anhydrous
DMA, that is, 0.5 mL mgꢁ1 (Z = 0.5), was used as the assisting
liquid,11 3aa was isolated in 75% yield, implying a key role
played by strong polar solvents. A decrease in KI loading
(5 mol%) led to a lower yield (56%), while a higher loading
(50 mol% KI) failed to increase the yield (Table 1, entries 3 and
4). In contrast, the yields increased to 82–84% when increasing
the loading of DMA (Z = 1, 2) (Table 1, entries 5 and 6). In fact,
In addition to the reaction parameters, mechanical ones
often play an important role in mechanochemical synthesis.
It is well known that the rotational speed of mill, the filling
degree of jar and even ball size could affect the energy input,
impact force and mixing efficiency in ball milling. Indeed, the
filling degree of jar and the rotational speed of mill have proven
to affect the model reaction rate remarkably. When the number
of balls was increased from 8 to 16, the reaction proceeded
faster and could complete within 2 h with a 1 mol% catalyst
loading, affording 3aa in 86% yield, while the use of larger balls
(+ = 10 mm) led to a slight decrease in the yield of 3aa due to
an increase in homo-coupling of 1a. The reaction took a longer
time (4 h) to complete with a lower rotational speed (300 rpm)
(Table 1, entries 27 and 28). Namely, the higher rotational
speed of mill and the filling degree of jar provided a higher
catalytic efficiency. Obviously, these results are consistent with
the mechanical activation of manganese powders, implying
that the activation of manganese and the corresponding
reduction of Ni(II) to Ni(0) by active Mn(0) should be involved
in the rate-determining steps in the catalytic cycle. When
0.1 equiv. of Tempo was added to the reaction mixture, the
coupling could be suppressed remarkably (36%, 2 h). Further, 0.5
equiv. of Tempo blocked the reaction completely. Unfortunately,
similar to a report by Weix et al.,6a attempts to isolate the O-alkyl
Tempo derivative failed. However, coupling of the radical probe,
(bromomethyl)cyclopropane, gave a mixture of 1-(but-3-enyl)-3-
methoxybenzene in 55% yield (GC) and its internal isomers (7%)
as major products along with 10% (GC) normal coupling
product 1-(cyclopropylmethyl)-3-methoxybenzene in 72% total yield
(Scheme 2) (see the ESI† for GC-MS and 1H NMR spectra).
These results indicate that coupling under LAG conditions
should also mainly follow a radical-involved mechanism
established by Weix et al. (Scheme 3). However, given the
Fig. 1 Photographs of metal powders before and after ball-milling: (a) Mn,
(c) Al, (e) Fe and (g) Zn before vs. (b) Mn, (d) Al, (f) Fe and (h) Zn after ball-
milling at 525 rpm for 30 minutes.
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Table 1 Optimization of Ni-catalyzed XEC between 3-bromoanisole 1a and ethyl 4-bromobutyrate 2a under LAG conditionsa
Entry
Ball no.
Ni cat. (mol%)
LAGs (Z)
Red. (equiv.)
2a (equiv.)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (3)
(bpy)NiCl2 (1)
(bpy)NiCl2 (3)
(bpy)NiCl2 (1)
(bpy)NiCl2 (1)
(bpy)NiCl2 (1)
(bpy)NiCl2 (1)
(bpy)NiCl2 (1)
(bpy)NiCl2 (1)
(bpy)NiCl2 (1)
NiCl2 (1)
—
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (1.5)
Mn (1.2)
Mn (2.5)
Al (2)
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.0
0.8
1.2
2.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
4
4
4
4
4
4
1
Trace
75
DMA (0.5)
DMA (0.5)
DMA (0.5)
DMA (1)
DMA (2)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMPU (1)
DMF (1)
DMSO (1)
MeCN (1)
Dioxane (1)
THF (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)/6 mL H2O
DMA (1)/50 mL H2O
DMA (1)/100 mL H2O
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
74c
56d
82(21e)
84
62
84
80
71
83
Trace
38
Trace
83
64
67
17
Trace
Trace
75
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
9
0.5
2
4
1.5
3
6
6
8
4
4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
Zn (2)
Fe (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
Mn (2)
53
77
84(92)f
85
8
16
16
16
16
16
16
16
16
16
16
16
16
16
84
86g
83h
71i
86
84
Trace
54e
Trace
82
81
Trace
10
NiCl2/bpy (1)
NiCl2/Phen (1)
NiCl2/Py (1)
4
4
4
NiCl2/8-HQ (1)
a
b
c
d
Reaction run on a 1.0 mmol scale (1a), 8 stainless-steel balls (+ = 7 mm), 525 rpm. Isolated yields. Used KI (50 mol%). Used KI (5 mol%).
e
f
g
h
i
In air. Impure. No reaction in solution. At 300 rpm. 16 stainless-steel balls (+ = 10 mm).
Scheme 2 Evidence for a radical-involved pathway.
extremely fast ring-opening rearrangement of a cyclopropylcar-
binyl radical, the formation of a small amount of normal
product implied that some minor non-radical pathways should
not be excluded under LAG conditions.
Control experiments on the influence of water content in the
system showed a much better water-compatibility under LAG
Scheme 3 The catalytic cycle proposed by Weix et al.
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conditions than that in the traditional solution. The reaction iodide reacted similarly to bromide (1b). The electronic
could still proceed well when 1–8% volume (6–50 mL) of water factors from aryl bromides affected their cross-coupling more
with respect to DMA (0.6 mL) was added to the system (Table 1, significantly than steric ones, albeit sterically demanding 2,4,6-
entries 30–32), while the 0.5% water content in DMA made the trimethyl bromobenzene completely failed. For example, aryl
reaction rather slow (24 h, 57% yield) in solution even in the bromides bearing an electron-donating group, e.g. Me (1c and
i
presence of 25 mol% TMSCl as an activator (see Scheme 1). 1d), Pr (1e and 1f) and OMe (1g and 1h), at the ortho- or para-
In fact, no reaction occurred in non-anhydrous DMA without position of the phenyl ring, except for NH2 which gave a low
the activator TMSCl in solution (Table 1, entry 27). The reaction yield (33%) due to N-alkylation, reacted with 2a smoothly,
could proceed slowly to give 3aa in 54% yield even in air affording n-alkyl aromatics 3ca–3ha in good yields (81–86%),
(Table 1, entry 33). A direct combination of NiCl2 with 2,20- while those bearing an electron-withdrawing group, e.g. Ac(1j–
bipyridine (bpy) or 1,10-phenanthroline (phen) appeared to 1l), CN (1m), CF3 (1n), F (1o) and Cl (1p), gave lower yields
work, while pyridine itself failed (Table 1, entries 35–38). (47–78%) of the desired products 3ja–3pa. 3-Bromopyridine (1s)
Therefore, after combined evaluation of the reaction and also gave a lower yield than its phenyl analog (1a), while no
mechanical parameters, the optimal conditions were obtained reaction was observed for 1-bromo-4-nitrobenzene. In particular,
as follows: 1.5 molar ratio of alkyl/aryl bromides, 2 equiv. of m-bromoacetophenone (1k) reacted to give the desired product
manganese powders, 1 mol% (bpy)NiCl2 catalyst and 10 mol% 3ka in 70% yield, remarkably higher than its ortho- and para-
KI as the additive, with 16 stainless-steel balls (+ = 7 mm) at isomers (1j, 47% and 1l, 59%), clearly reflecting the electronic
525 rpm under N2.
effect of aryl bromides, that is, electron-rich aryl bromides
The scope and limitations of the LAG-enabled, Ni-catalyzed, perform better than electron-deficient ones. It is worth noting
Mn-mediated cross-electrophile coupling between aryl and that borate (1q, 78%) and tosylate (1r, 71%) groups which could
alkyl bromides/(pseudo)halides were briefly explored under be used in subsequent conventional cross-coupling reactions
the optimal conditions (Table 2).
were tolerable. On the alkyl counterpart, primary bromides could
Phenyl bromide (1b) reacted smoothly with 2a to give 3ba in be efficiently coupled, giving good yields of the corresponding
80% yield. No reaction was observed for phenyl chloride (PhCl), n-alkyl products (81% for 3ab, 69% for 3ac and 70% for 3ad),
phenyl tosylate (PhOTs) and triflate (PhOTf), while phenyl while no reaction was observed for n-hexyl chloride. Surprisingly,
Table 2 Scope of the Ni-catalyzed, Mn-mediated XEC between aryl and alkyl bromides/(pseudo)halides under LAG conditionsa
a
Reaction conditions: 1 (1.0 mmol), 2 (1.5 mmol), 1 mol% (bpy)NiCl2, 10 mol% KI, Mn (2 equiv.), DMA (Z = 1), 16 stainless-steel balls (+ = 7 mm)
b
under a N2 atmosphere at 525 rpm in a planetary ball mill. The N-alkylation product ethyl 4-(4-((4-ethoxy-4-oxobutyl)amino)phenyl)butanoate was
also isolated in 31% yield. No increase in yield (22–25%) with 3–5 mol% catalyst loadings.
c
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Paper
Experimental
General information
Reactions were carried out under a nitrogen atmosphere unless
otherwise stated. All mechanochemical reactions were conducted
in a planetary ball mill (QM-3SP2) equipped with four (50 mL)
milling jars made of stainless steel and 8 or 16 stainless-steel balls
(+ = 7 or 10 mm) at 300 rpm or 525 rpm. Chemicals obtained
from commercial sources were used as received. (bpy)NiCl2 was
prepared according to a previously reported procedure.12 The
reaction progress was monitored by TLC. Column chromatography
was performed on 300–400 mesh silica gel. 1H and 13C NMR
spectra were recorded in CDCl3 at ambient temperature.
New compounds 3ba, 3ka and 3ca were further characterized
by HRMS.
Scheme 4 Synthesis of 6-methoxytetralone on a gram scale.
lower yields (46–49%) were obtained for n-hexyl iodide in experiments.
Under the optimal conditions, n-hexyl tosylate reacted with 1a
to give 3ab in a very low yield (8%), which could increase to 47%
with 1 equiv. of KI, consistent with the conversion of alkyl
tosylate into iodide. Unfortunately, the coupling of secondary
acyclic (2e) and cyclic bromides (2f and 2g) with 1a under
Typical procedure
A mixture of aryl bromide (1.0 mmol), alkyl bromide (1.5 mmol),
(bpy)NiCl2 (1 mol%), Mn powder (2 equiv.), DMA (Z = 1) and KI
(10 mol%) were added in a 50 mL stainless steel milling jar. The
milling jar was flushed with N2 through the lid with a gas inlet,
screwed tightly and then placed on the planetary ball mill at
525 rpm using 16 stainless-steel balls (+ = 7 mm) for 2–6 h.
After the elapsed time, the reaction mixture was transferred
from the milling jar into a flask with the aid of CH2Cl2 (3 ꢂ 5 mL)
and then concentrated by a rotavapor to afford the crude
product, which was purified by flash column chromatography
on silica gel using a gradient of ethyl acetate in petroleum ether
(see the ESI† for details).
the LAG conditions was not satisfactory and
a higher
catalyst loading (3–5 mol%) appeared to fail in increasing the
yields. Instead, self-coupling of 1a took place to give 3,30-
dimethoxybiphenyl as the major isolated product.
To demonstrate the application of the LAG-enabled, Ni-
catalyzed, Mn-mediated aryl/n-alkyl XEC, a gram-scale synthesis
of 6-methoxytetralone, a key starting material for a synthetic
progestogen, Norgestrel, was carried out (Scheme 4). Under the
optimal conditions, the reaction of 3-bromoanisole (1a)
with ethyl 4-bromobutyrate (2a, 1.5 equiv.) could be readily
conducted on a gram scale at the same size (50 mL) and
filling degree (16 ball, + = 7 mm) of the jar for a 1 mmol
substrate loading to afford 3aa in 81% yield (5.39 g) in 3 h.
Hydrolysis of the ester 3aa, followed by subsequent treatment
with SOCl2 and AlCl3, provided 6-methoxytetralone in
75% yield.
Author contributions
Supervision, G. Z.; conceptualization, W. S. & G. Z.; investigation,
S. W. & W. S.; writing – original draft preparation, W. S.; writing –
review & editing, G. Z. & W. S.
Conflicts of interest
Conclusions
There are no conflicts to declare.
In summary, liquid-assisted grinding (DMA, Z = 1) has been
used to enable an efficient nickel-catalyzed, manganese-
mediated cross-electrophile coupling between aryl and n-alkyl
bromides without the requirement of chemical activators and
anhydrous solvents. The catalytic efficiency has proven to
correlate well with the mechanical activation of the metal
surface and mixing of heterogeneous materials, implying that
the reduction of Ni(II) to Ni(0) by active Mn(0) should be
involved in the rate-determining steps. A variety of n-alky
aromatics with various functional groups could be obtained
in good yields with a low catalyst loading (1 mol%). Elimination
of the need for chemical activators and compatibility
with non-anhydrous solvents make the nickel-catalyzed
Acknowledgements
We are grateful for the financial support provided by the
National Natural Science Foundation of China (21472041).
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