Stoichiometric and Catalytic Aryl?Cl Activation and Borylation using NHC-stabilized Nickel(0) Complexes

Article abstract of DOI:10.1002/chem.201900937

NHC-nickel (NHC=N-heterocyclic carbene) complexes are efficient catalysts for the C?Cl bond borylation of aryl chlorides using NaOAc as a base and B₂pin₂ (pin=pinacolato) as the boron source. The catalysts [Ni₂(ICy)₄(μ-(η²:η²)-COD)] (1, ICy=1,3-dicyclohexylimidazolin-2-ylidene; COD=1,5-cyclooctadiene), [Ni(ICy)₂(η²-C₂H₄)] (2), and [Ni(ICy)₂(η²-COE)] (3, COE=cyclooctene) compare well with other nickel catalysts reported previously for aryl-chloride borylation with the advantage that no further ligands had to be added to the reaction. Borylation also proceeded with B₂neop₂ (neop=neopentylglycolato) as the boron source. Stoichiometric oxidative addition of different aryl chlorides to complex 1 was highly selective affording trans-[Ni(ICy)₂(Cl)(Ar)] (Ar=4-(F₃C)C₆H₄, 11; 4-(MeO)C₆H₄, 12; C₆H₅, 13; 3,5-F₂C₆H₃, 14).

Full text of DOI:10.1002/chem.201900937

A Journal of
Accepted Article
Title: Stoichiometric and Catalytic Aryl-Cl Activation and Borylation
using NHC-stabilized Nickel(0) Complexes
Authors: Laura Kuehn, Dominik G. Jammal, Katharina Lubitz, Todd B.
Marder, and Udo Radius
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10.1002/chem.201900937
Chemistry - A European Journal
Stoichiometric and Catalytic Aryl-Cl Activation and
Borylation using NHC-stabilized Nickel(0) Complexes
Laura Kuehn, Dominik G. Jammal, Katharina Lubitz, Todd B. Marder* and Udo Radius*
Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with
Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.
Corresponding author E-Mail addresses:
todd.marder@uni-wuerzburg.de; u.radius@uni-wuerzburg.de
* Corresponding authors.
Dedicated to Prof. Dr. Jörg Sundermeyer on the occasion of his 60th birthday
Abstract
NHC-nickel complexes are efficient catalysts for the C-Cl bond borylation of aryl chlorides
using NaOAc as a base and B2pin2 as the boron source. The catalysts [Ni2(ICy)4(μ-(η2:η2)-
COD)] 1 (ICy = 1,3-dicyclohexylimidazolin-2-ylidene; COD = cyclooctadiene;), [Ni(ICy)2(η2-
C2H4)] 2 and [Ni(ICy)2(η2-COE)] 3 (COE = cyclooctene) compare well with other nickel catalysts
reported previously for aryl chloride borylation with the advantage that no further ligands have
to be added to the reaction. Borylation also proceeds with B2neop2 as the boron source.
Stoichiometric oxidative addition of different aryl chlorides to complex 1 is highly selective
affording trans-[Ni(ICy)2(Cl)(Ar)] (Ar = 4-(F3C)C6H4,11; 4-(MeO)C6H4,12; C6H5,13; 3,5-F2C6H3,
14).
Introduction
Arylboronic acids and esters represent important precursors for modern organic synthesis.[1]
Transition metal-catalyzed borylations of aryl C-H [2] and C-X bonds[3,4] have become especially
attractive and mild routes to aryl boronates. In 1995, Miyaura and co-workers demonstrated
the first catalytic synthesis of an arylboronic ester, i.e. the Pd(dppf)-catalyzed borylation of aryl
iodides and bromides[3a] using B2pin2 as the boron source.[1b] As early borylation reactions were
often based on precious metal catalysts, first row “Earth abundant” metal catalysts have
become of increasing interest, due to their low cost and toxicity.[5] Zhu and Ma reported the
copper-catalyzed borylation of aryl iodides with pinacolborane (HBpin) in the presence of CuI
1
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and NaH at room temperature, but this reaction does not work with aryl bromides.[4a] Later, our
group reported the first copper-catalyzed borylation of aryl bromides, including electron-rich
and hindered examples, under mild conditions (room temperature) using a phosphine Cu(I)
catalyst and KOtBu.[4b] Our group also reported two efficient systems active at room
temperature, using ZnBr2 and IMes (1,3-dimesitylimidazolin-2-ylidene), or a combination of
ZnCl2 and dtbpy (4,4’-di-tert-butyl-2,2’-dipyridin) for the catalytic borylation of aryl iodides and
bromides.[4c-d] One example was demonstrated for an electron-poor aryl chloride at elevated
temperatures of 50 °C, resulting in only a poor yield of 18%.[4c] Nakamura, Ilies et al. reported
on iron-catalyzed borylation which effectively transforms biaryl chlorides into the corresponding
boronate esters using [Fe(acac)3] and KOtBu in toluene at 130 °C.[4e] However, the borylation
of aryl chlorides such as chlorobenzene or 4-chloroanisol gave only 27% and 46% yields,
respectively, of borylated products. Using biaryl iodides or bromides as substrates, the reaction
suffers from large quantities of hydrodehalogenation byproducts. A cobalt-catalyzed borylation
of aryl halides and pseudohalides, also including aryl chlorides, was reported utilizing
expensive oxazolinylferrocenyl-phosphine ligands and MeLi, a reactive organometallic
reagent.[4f] Another milder method for the borylation of aryl chlorides, employing a Co(II)-NHC
catalyst and KOMe as the base at 50 °C, was recently introduced by Geetharani and co-
workers.[4g] Since Tour et al. demonstrated the first nickel-catalyzed borylation of aryl halides,[6]
various Ni-catalyzed borylation reactions of aryl halides based on [Ni(PR3)2Cl2] complexes
were reported.[7] A mixed-ligand system developed by Percec et al. consisting of [Ni(dppp)Cl2]
(dppp = 1,3-bis(diphenylphosphino)propane) and dppf (1,1'-bis(diphenylphosphino)ferrocene)
as the co-ligand turned out to be very efficient for the borylation of aryl chlorides.[7c-e]
Furthermore, this system, as well as [Ni(dppp)Cl2] with PPh3 (triphenylphosphine) as the
additional ligand, turned out to be effective catalysts for the borylation of aryl iodides, bromides
and chlorides bearing electron-rich and electron-deficient ortho-substituents in toluene at
100 °C.[7d] These borylation reactions involved the use of in situ prepared HBneop as the boron
source formed by adding BH3∙DMS (DMS = dimethylsulfide) to a toluene solution of
neopentylglycol, as well as NEt3 as an additive, which has to be freshly distilled from CaH2 for
a satisfying catalytic conversion.[7c-e] In the same year, Murata et al. reported the borylation of
aryl chlorides using pinacolborane (HBpin) as the boron source and catalytic amounts of
[Ni(dppp)Cl2] with Bu4NBr and NEt3 as additives in dioxane at 100 °C.[7g] Yamakawa and co-
workers also effectively borylated a variety of aryl bromides and chlorides in THF at 100 °C
using the nickel phosphine complex [Ni(PMe3)2Cl2]; however, the reaction requires the addition
of CsF and trimethyl(2,2,2-trifluoroethoxy)silane (TMSOCH2CF3).[7i] Molander and co-workers
achieved room temperature borylation for some electron-poor aryl chlorides by the
combination of [Ni(dppp)Cl2] and PPh3 in ethanol using tetrahydroxydiboron (BBA) as the boron
source and DIPEA (N,N-Diisopropylethylamine) as an additive. However, using 4-
2
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chloroanisole, temperatures of 80 °C were required to give a 66% yield of the borylated
product. The borylation of 4-bromotoluene using the same catalyst system only proceeds at
elevated temperatures of 80 °C yielding 84% of the borylated arene.[7j] Our new catalytic
system has the advantage of a readily prepared catalyst requiring no additional ligands. NaOAc
as our additive is mild, inexpensive, nontoxic, and easier to use compared to DIPEA and Et3N
employed by the groups of Percec, Murata and Molander.[7c-e,g,j]
Our group has demonstrated the reactivity of the NHC-stabilized nickel(0) complex [Ni2(IiPr)4(μ-
(η2:η2)-COD)] (IiPr = 1,3-di-iso-propylimidazolin-2-ylidene) towards organic halides[8] and other
substrates[9] in stoichiometric element-element bond activation reactions. The use of this
complex as a catalyst made C-F bonds in per- and polyfluorinated arenes accessible for
Suzuki-Miyaura cross-coupling reactions.[8b] Oxidative addition of aryl chlorides to [Ni2(IiPr)4(μ-
(η2:η2)-COD)] affords complexes of the type trans-[Ni(IiPr)2(Cl)(Ar)].[8g] The application of
[Ni2(IiPr)4(μ-(η2:η2)-COD)] as a catalyst for Suzuki-Miyaura cross-coupling reactions of aryl
chlorides and bromides revealed high catalytic activity in these transformations without
additional ligands.[8g,i,k] Furthermore, in 2016, we reported efficient C-F borylation of a variety
of fluoroarenes catalyzed by the related complex [Ni(IMes)2] in the presence of NMe4F or CsF
as an additive and B2pin2 as the boron source.[10a] We have recently shown that this reaction
can take place at ambient temperature using visible light in the presence of a novel rhodium
biphenyl photocatalyst.[10b] Herein we report the first NHC-nickel catalyzed borylation of aryl
chloride C-Cl bonds and the stoichiometric oxidative addition of aryl-Cl bonds to [Ni2(ICy)4(μ-
(η2:η2)-COD)] 1.
Results and Discussion
We started our catalytic investigation using 4-chlorobenzotrifluoride 4 (1.0 equiv.) as the
substrate, B2pin2 (1.5 equiv.) as the boron source, NMe4F (1.5 equiv.) as the base and
[Ni2(ICy)4(μ-(η2:η2)-COD)] 1 (2.5 mol%)* as the catalyst. (* for synthesis and characterization
of complex 1 see below.) We chose methylcyclohexane as the solvent, as our earlier studies
demonstrated that these conditions are excellent for C-F bond borylation.[10a] From this initial
reaction, the C-Cl bond borylation product 4a was obtained in 67% yield (Table 1, entry 1).
With this promising first result, different bases, solvents, catalysts and boron sources were
screened using standard conditions (100 °C, 24 h). First of all, our studies reveal the
importance of the base, as when no base was used the borylated product was observed in
only 31% yield (Table 1, entry 8), whereas the use of NaOAc or K3PO4 increased the yields of
4a to 99% and 95%, respectively (Table 1, entries 2, 5). Decreasing the amount of NaOAc and
B2pin2 from 1.5 to 1.0 equiv. each only lowered the yield of 4a to 74% (Table 1, entry 3). CsF
3
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and LiOtBu worked well (Table 1, entries 6-7), whereas KF gave 4a in only 17% yield (Table
1, entry 3). Using LitOBu, CsF or KF, trace amounts of the biaryl homocoupling byproduct were
observed by GC-MS. Running the reaction at 80°C gave only traces of borylation product 4a
(Table 1, entry 9).
Table 1. Screening of different bases and solvents for the borylation of 4-chlorobenzotrifluoride 4.
Entry
Substrate
Base
Solvent
Yield
[%]a
67
1
2
4
4
4
4
4
4
4
4
4
4
4
4
4
4
NMe4F
NaOAc
NaOAc
KF
methylcyclohexane
methylcyclohexane
methylcyclohexane
methylcyclohexane
methylcyclohexane
methylcyclohexane
methylcyclohexane
methylcyclohexane
methylcyclohexane
n-hexane
>99
74b
17
3
4
5
K3PO4
LiOtBu
CsF
95
6
65
7
59
8
-
34
4c
9
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
10
11
12
13
14
37
toluene
>99
23
acetonitrile
MTBE
33
THF
29
a Reaction conditions: [Ni2(ICy)4(μ-(η2:η2)COD)] 1 (2.5 mol%), 4-chlorobenzotrifluoride 4 (368 mmol,
1.0 equiv.), B2pin2 (1.5 equiv.), base (1.5 equiv.), solvent (5 mL), 100 °C, 24 h. The yields were
determined by GC-MS of a diluted and filtered aliquot of the reaction mixture using biphenyl as an
internal standard. b B2pin2 (1.0 equiv.), base (1.0 equiv.). c 80 °C
With the optimized base in hand, we then investigated the influence of the solvent on our
reaction. Yields of 4a decreased to 37% (n-hexane), 23% (acetonitrile) 33% (MTBE) and 29%
(THF) (Table 1, entries 10,12-14), whereas toluene as the solvent gave a 99% yield of the
borylated product 4a (Table 1, entry 11). Thus, the best solvents are hydrocarbons with a
4
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reasonably high boiling point. The competitive C-H borylation of toluene, reported by Chatani
et al.,[11] was not observed in the presence of an aryl chloride.
Figure 1. Molecular structure of [Ni(ICy)2(η2-COE)] 3 with thermal ellipsoids drawn at the 50% probability
level; hydrogen atoms are omitted for clarity. Selected bond distances [Å] and angles [°]: Ni-C15
1.891(6), Ni-C26 1.892(5), Ni-C1 1.984(6), Ni-C8 1.953(6), Ni-(η2–COE)Centroid 1.225(8), C1-C8
1.414(9), C15-Ni-(η2-COE)Centroid 119.1(2), C26-Ni-(η2-COE)Centroid 123.4(1).
The reaction of [Ni(η4-COD)2] with two equivalents of the free carbene ICy in THF afforded our
dinuclear, COD-bridged complex [Ni2(ICy)4(μ-(η2:η2)-COD)] 1. As a significant amount of the
mononuclear compound [Ni(ICy)2(η4-COD)] 1a arises as a side product in this preparation (our
group also observed this while preparing [Ni2(IiPr)4(μ-(η2:η2)-COD)][9a]), we further reacted 1
with ethylene, cleanly forming [Ni(ICy)2(η2-C2H4)] 2. To verify that our borylation reaction is
actually mediated by the {Ni(ICy)2}-complex fragment and not by any intermediate caused by
the side product 1a, we reproduced our screening reactions using [Ni(ICy)2(η2-C2H4)] 2 as the
catalyst. The yields of the borylated compounds generated by catalyst 2 are in very good
agreement with those observed using catalyst 1 (Table 2, entry 2).
We then investigated the catalytic activity of other NHC-nickel complexes for C-Cl bond
borylation. We prepared the complex [Ni(ICy)2(η2-COE)] 3 (Figure 1); (COE = cyclooctene),
which also formed the borylation product 4a in excellent yields of 99% (Table 2, entry 3). Using
[Ni(η4-COD)2] and one equivalent of the chelating phosphine ligand dppf (dppf = 1,1'-
bis(diphenylphosphino)ferrocene) as a catalyst, borylation was only achieved in moderate
yields of 50% (Table 2, entry 5). In previous work we have shown that [Ni2(IiPr)4(μ-(η2:η2)-
COD)] catalyzes the Suzuki-Miyaura coupling reaction of chlorobenzene with phenylboronic
acid.[8g] Using this catalyst in our standard reaction afforded the borylated compound 4a in a
very good yield of 91% (Table 2, entry 4). Moreover, the carbonyl complex [Ni(IMe)2(CO)2]
(IMe = 1,3-dimethylimidazolin-2-ylidene), bearing the smaller methyl-substituted carbene
5
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ligand, revealed poor catalytic activity under our standard conditions (Table 2, entry 6). The
application of [Ni(IMes)2], with the sterically more demanding NHC IMes, which turned out to
be an efficient catalyst for the C-F borylation of fluoroarenes in our previous work,[10] revealed
only moderate catalytic activity (44% yield) for the borylation of C-Cl bonds under the current
conditions (Table 2, entry 7). Using the Ni(II)-complex [Ni(ICy)2Br2] in our standard reaction
only traces of borylated product 4a were detected.
Table 2. Screening of different nickel-catalyst for the borylation of 4-chlorobenzotrifluoride 4.
Entry
Catalyst (2.5 mol%)
Substrate (1.0 equiv.)
Yield [%]a
1
2
3
4
5
6
7
8
9
[Ni2(ICy)4(μ-(η2:η2)-COD)] 1
[Ni(ICy)2(η2-C2H4)] 2
[Ni(ICy)2(η2-COE)] 3
[Ni2(IiPr)4(μ-(η2:η2)-COD)]
[Ni(η4-COD)2] + dppf
[Ni[(IMe)2(CO)2]
4
4
4
4
4
4
4
4
4
>99
>99b
>99b
91
50b
24b
[Ni(IMes)2]
44b
[Ni(ICy)2Br2]
traces
88
trans-[Ni(ICy)2(Cl)(4-(F3C)C6H4)]
a Reaction conditions: Ni-catalyst (2.5 mol%), 4-chlorobenzotrifluoride 4 (368 mmol, 1.0 equiv.), B2pin2
(1.5 equiv.), NaOAc (1.5 equiv.), methylcyclohexane (5 mL), 100 °C, 24 h. The yields were determined
by GC-MS of a diluted and filtered aliquot of the reaction mixture using biphenyl as an internal standard.
b Ni-catalyst (5.0 mol%)
Using our optimized conditions, we then explored the scope of the nickel-catalyzed C-Cl
borylation using different chloroarenes (4-10). As shown in Table 3, the borylation of
chloroarenes containing either electron withdrawing or donating groups was achieved in good
to excellent yields. For example, using simple chlorobenzene 7, the borylated compound 7a
was generated in 97% yield (Table 3, entry 4). The borylation of 4-chlorotoluene 5 gave 5a in
80% yield (Table 3, entry 2), while 4-chloroanisole 6 generated the borylated product 6a in
nearly quantitative yield (Table 3, entry 3). Borylations of 3,5-difluorochlorobenzene 8 and 4-
fluorochlorobenzene 9 gave good yields of 76% and 90%, respectively, without any
6
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competitive C-F borylation product (Table 3, entries 5-6). Furthermore, we employed the
heteroarene 3-chloropyridine 10 which provided the borylated product 10a in quantitative
yields (Table 3, entry 7).
Table 3. Scope of the nickel-catalyzed C-Cl borylation.
Entry
1
Substrate
Product
Boron Source
Yield
[%]a
B2pin2
>99 (85)b
4a
5a
6a
7a
4
5
2
3
B2pin2
B2pin2
B2pin2
80 (61)b
>99 (88)b
97 (79)b
6
7
4
5
8a
9a
B2pin2
B2pin2
76 (63)b
90 (77)b
8
6
7
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9
10
6
7
8
9
10a
6b
B2pin2
B2cat2
99
traces
76 (59)b
6c
B2neop2
6
aReaction conditions: [Ni2(ICy)4(μ-(η2:η2)-COD)] 1 (2.5 mol%), aryl chloride (1.0 equiv.), boron source
(1.5 equiv.), NaOAc (1.5 equiv.), methylcyclohexane (5 mL), 100 °C, 24 h. The yields were determined
by GC-MS of a diluted and filtered aliquot of the reaction mixture using biphenyl as an internal standard.
bIsolated yield after chromatographic workup.
We tested the influence of the nature of the boron source applying B2neop2 instead of B2pin2.
The reaction using 4-chloroanisole 6 as the model substrate yielded the borylated species 6c
in 76% (Table 3, entry 9). Notably, only traces of borylation product 6b were detected utilizing
B2cat2 as the boron source (Table 3, entry 8).
Scheme 1. Synthesis of the oxidative addition products of the type trans-[Ni(ICy)2(Cl)(Ar)]. Ar = 4-
(F3C)C6H4 11, 4-(MeO)C6H4 12, C6H5 13 and 3,5-F2C6H3 14.
In previous work, we have shown that NHC-stabilized-nickel complexes readily insert into C-X
bonds of aryl,[8g,8k,10] benzyl[8i] and aroyl (C(O)Ar)[9l] halides. Using [Ni2(ICy)4(μ-(η2:η2)-COD)] 1,
the aryl chloride C-Cl bond activation occurs cleanly (Scheme 1). The stoichiometric reaction
of 4-chlorobenzotrifluoride 4 with 1 afforded trans-[Ni(ICy)2(Cl)(4-(F3C)C6H4)] 11 in 70%
isolated yield, the exact mechanism of the oxidative addition of Ar-X to [Ni(NHC)2] remains
8
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currently under investigation. In analogous reactions utilizing 4-chloroanisole 6 and 4-
chlorobenzene 7, we isolated the oxidative addition products trans-[Ni(ICy)2(Cl)(4-(MeO)C6H4)]
12 and trans-[Ni(ICy)2(Cl)(C6H5)] 13, respectively. Applying 3,5-difluorochlorobenzene 8
resulted in the formation of the C-Cl activation product trans-[Ni(ICy)2(Cl)(3,5-F2C6H3)] 14
19
exclusively, revealing a singlet at -115.9 ppm in the aryl fluoride region of the F{1H} NMR
spectrum, and no signal in the range characteristic of Ni-F bonds (ca. -350 ppm). Complexes
13
11-14 were characterized by 1H, C{1H} and 19F{1H} (for the fluorine-containing compounds)
NMR spectroscopy, IR spectroscopy, HRMS, elemental analysis and, for 14, by X-ray
diffraction (Figure 2). The structure obtained from X-ray analysis confirms a distorted square
planar coordinated nickel(II) complex.
Figure 2. Molecular structure of trans-[Ni(ICy)2(Cl)(3,5-F2C6H3)] 14 with thermal ellipsoids drawn at the
50% probability level; hydrogen atoms are omitted for clarity. Selected bond distances [Å] and angles
[°]: Ni-C1 1.889(3), Ni-C16 1.901(3), Ni-C31 1.888(2), Ni-Cl 2.226(7), C1-Ni-C16 178.4(1), C1-Ni-C31
89.88(1), C16-Ni-C31 88.83(1), C1-Ni-Cl 89.99(7), C16-Ni-Cl 91.35(7), C31-Ni-Cl 177.06(7).
The reaction using B2pin2, NaOAc, 4-chlorobenzotrifluoride 4 under standard conditions in the
presence of catalytic amounts of the C-Cl bond activation product [Ni(ICy)2(Cl)(4-(F3C)C6H4)]
11 generated the desired borylated product 4a (table 2, entry 9). Concerning the mechanism,
the reaction of [Ni2(ICy)4(μ-(η2:η2)-COD)] 1 with an aryl chloride leads to rapid oxidative addition
of the C-Cl bond with formation of trans-[Ni(ICy)2(Cl)(Ar)]. Interestingly, the reaction of 1 with
stoichiometric amounts of B2pin2 at room temperature indicated the formation of a nickel-boryl
complex (see SI). However, this process does not compete effectively with oxidative addition
of a C-Cl bond to 1, as latter shows a much higher reaction rate. It is clear that the oxidative
addition product trans-[Ni(ICy)2(Cl)(Ar)] is the resting state of the catalytic cycle.
9
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Scheme 2. Proposed mechanism for the NHC-Nickel catalyzed borylation of aryl chlorides.
The borylation of trans-[Ni(ICy)2(Cl)(Ar)] (i.e. transmetalation of boron to Ni) seems to be the
rate-limiting process in this catalytic cycle,[10a] as we observe 11 as the resting state in the
reaction mixture under catalytic conditions with 50 mol% catalyst loading (see SI). It was not
possible to isolate or observe trans-[Ni(ICy)2(Bpin)(Ar)]. The boryl transfer reagent is either
B2pin2, which would react with a Ni-OAc species, or its possible adduct Na[B2pin2(OAc)].[12]
However, the treatment of 11 with NaOAc did not reveal the formation of a Ni-acetate complex,
in contrast to what the group of Miyaura observed for the analogous trans-[Pd(PPh3)2(Br)(Ph)]
complex.[3a] Furthermore, we could not observe any reaction or interaction between B2pin2 and
stoichiometric amounts of NaOAc either at room temperature or at 100 °C, although KOAc is
capable of activating B2pin2 for the metal-free borylation of aryldiazonium salts.[12] Our
proposed mechanism is shown in Scheme 2. In the first step, the reaction of the {Ni(ICy)2}-
moiety I with the aryl chloride leads to oxidative addition of the C-Cl bond forming trans-
[Ni(ICy)2(Cl)(Ar)] II. The acetate anion coordinates to B2pin2 forming the very weakly bound
adduct Na[B2pin2(OAc)] with an activated B-B bond.[12] Complex II then reacts with
Na[B2pin2(OAc)] by boryl transfer to give trans-[Ni(ICy)2(Bpin)(Ar)] III, pinB-OAc and NaCl. A
final reductive elimination step, after ligand elimination, from a three coordinate species IV
gives the desired borylated product Ar-Bpin and regenerates {Ni(ICy)2} I.
10
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Conclusion
We have developed an efficient procedure for the C-Cl borylation of aryl chlorides using
[Ni2(ICy)4(μ-(η2:η2)-COD)] 1 as the catalyst precursor, NaOAc as the base and B2pin2 as the
boron source. [Ni2(IiPr)4(μ-(η2:η2)-COD)] is also an efficient catalyst for our borylation reaction.
The catalysts compare well with other nickel catalysts reported so far[7] with the advantage that
no further ligand or additive is required other than the very mild base NaOAc. This is the first
example of an NHC-nickel-mediated borylation of aryl chlorides. Furthermore, we have
demonstrated that stoichiometric reactions of 1 with different aryl chlorides generate C-Cl
oxidative addition products of the type trans-[Ni(ICy)2(Cl)(Ar)] 11-14.
Experimental Section
General
All reactions and subsequent manipulations were performed under an argon atmosphere using
standard Schlenk techniques as reported previously[13] or in a glovebox (Innovative Technology
Inc. and Braun Uni Lab). All reactions were carried out in oven-dried glassware. Toluene,
hexane, MTBE and THF were purified by distillation from an appropriate drying agent (sodium
with benzophenone as indicator). C6D6 and CDCl3 were purchased from Sigma-Aldrich.
Commercially available methylcyclohexane was degassed and dried over molecular sieves.
[Ni(η4-COD)2],[14] [Ni(ICy)2Br2],[15] [Ni(IMes)2],[16] [Ni2(IiPr)4(μ-(η2:η2)-COD)],[9a] [Ni(IMe)2
(CO)2],[17] ICy[18] and B2cat2[19] were prepared according to published procedures. The diboron
reagents B2pin2, and B2neop2 were a generous gift from AllyChem Co. Ltd. Anhydrous NMe4F
is commercially available; for this work, however, it was synthesized according to a literature
procedure.[20] All other reagents were purchased from Aldrich or ABCR. NMR spectra were
recorded at 298 K using Bruker Avance 400 (1H, 400 MHz; 13C, 100 MHz, 11B, 128 MHz; 19F,
1
376 MHz) spectrometers. H NMR chemical shifts are reported relative to TMS and were
referenced via residual proton resonances of the corresponding deuterated solvent (CDHCl2:
5.32 ppm; CHCl3: 7.26 ppm; C6D5H: 7.16 ppm) whereas 13C{1H} NMR spectra are reported
relative to TMS using the natural-abundance carbon resonances (CD2Cl2: 53.84 ppm; CDCl3:
77.16 ppm; C6D6: 128.0 ppm).19F{1H} NMR spectra are reported relative to external CFCl3.
Coupling constants are given in Hertz. Elemental analyses were performed in the
microanalytical laboratory of the Institute of Inorganic Chemistry, Universität Würzburg, using
an Elementar vario micro cube. GC-MS analyses were performed using a Thermo Fisher
Scientific Trace 1310 gas chromatograph (column: TG-SQC 5% phenyl methyl siloxane, 15 m,
∅ 0.25 mm, film 0.25 μm; injector: 250 °C; oven: 40 °C (2 min), 40 °C to 280 °C; carrier gas:
He (1.2 mL min–1). High-resolution mass spectra were obtained using a Thermo Scientific
11
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10.1002/chem.201900937
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Exactive Plus spectrometer equipped with an Orbitrap Mass Analyzer. Ionizations were
accomplished in Liquid Injection Field Desorption Ionization mode using a LIFDI 700 from
Linden CMS with 10 kV at the emitter and an accelerating voltage of 5 V.
Synthesis
[Ni2(ICy)4(μ-(η2:η2)-COD)] 1
In a Schlenk-tube, [Ni(η4-COD)2] (406 mg, 1.48 mmol, 1.0 equiv.) was dissolved in THF
(80 mL) and cooled to -78 °C. A solution of 1,3-dicyclohexylimidazolin-2-ylidene (ICy) (686 mg,
2.95 mmol, 2.0 equiv.) in THF (20 mL) was added slowly. The yellow solution was allowed to
warm to room temperature and stirred overnight. All volatiles were removed under reduced
pressure and the residue was suspended in n-hexane (20 mL). The product was collected by
filtration, washed with n-hexane (10 mL) and dried in vacuo. Yield: 697 mg (1.33 mmol, 90%)
of a bright yellow solid. 1H NMR (400 MHz, 25 °C, C6D6): δ = 1.04-2.18 (m, 84H, Cy-CH2, COD-
CH2), 2.54 (m, 4H, COD-CH2), 2.87 (m, 4H, COD-CH), 5.10 (m, 4H, NCH), 6.61 (s, 4H,
13
NCHCHN) ppm. C{1H} NMR (100 MHz, 25 °C, C6D6): δ = 25.8-35.1 (Cy-CH2), 38.6 (COD-
CH2), 54.7 (COD-CH), 57.6 (NCH), 114.8 (NCCN), 205.3 (NCN) ppm. Elemental Analysis for
C68H108N8Ni2 [1155.05 g/mol]: calculated (found): C 70.71 (70.30), H 9.43 (9.73), N 9.70 (9.44).
In the course of the synthesis of 1, varying amounts of the mononuclear compound
[Ni(Cy2Im)2(η4-COD)] 1a were detected as a side product. HRMS-LIFDI (m/z): calculated
(found) for C68H108N8Ni2 1 [(Cy2Im)2Ni]+ 522.3227 (522.3226). IR (ATR [cm-1]): 휈̃ = 506 (vw),
543 (w), 558 (w), 668 (s), 687 (w), 760 (vw), 793 (w), 818 (w), 892 (w), 928 (vw), 974 (w), 988
(m), 1022 (vw), 1082 (w), 1134 (w), 1215 (vs), 1269 (m), 1370 (vw), 1387 (w), 1406 (w), 1447
(vw), 2852 (w), 2923 (m).
[Ni(ICy)2(η2-C2H4)] 2
In a Schlenk-tube, ethylene was passed through a solution of complex 1 (300 mg, 260 µmol)
in toluene (30 mL) at room temperature. All volatiles were removed under reduced pressure
and the residue was suspended in cooled n-hexane (20 mL). The product was collected by
filtration, washed with n-hexane (5 mL) and dried in vacuo. Yield: 191 mg (346 µmol, 67%) of
a yellow solid. 1H NMR (400 MHz, 25 °C, C6D6): δ = 1.00-2.13 (m, 40H, Cy-CH2), 1.92 (s, 4H,
η2-C2H4),
5.09
(m,
4H,
NCH),
6.59
(s,
4H,
NCHCHN)
ppm.
13C{1H} NMR (100 MHz, 25 °C, C6D6): δ = 25.9 (Cy-CH2), 34.6 (Cy-CH2), 26.6 (η2-C2H4),
58.2 (NCH), 115.1 (NCCN), 204.1 (NCN) ppm. Elemental Analysis for C32H52N4Ni
[551.49 g/mol]: calculated (found): C 69.69 (69.55), H 9.50 (9.30), N 10.16 (9.88). HRMS-LIFDI
(m/z): calculated (found) for C30H48N4Ni 1 [M-C2H4]+ 522.3227 (522.3220).
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[Ni(ICy)2(η2-COE)] 3
In a Schlenk-tube, [Ni(ICy)2Br2] (160 mg, 23.0 µmol, 1.0 equiv.) was dissolved in THF (7 mL).
With rapid stirring, cyclooctene (COE) (37.5 µL, 28.1 µmol, 1.2 equiv.) and KC8 (101 mg,
74.0 µmol, 3.2 equiv.) were added at -78 °C. The reaction mixture was allowed to warm to
room temperature and stirred overnight. The dark suspension was filtered through a pad of
Celite and washed with THF (12 mL). All volatiles were removed under reduced pressure, the
crude product was redissolved in toluene (10 mL) and filtered again through a pad of Celite.
The solvent was removed under reduced pressure, the residue washed with n-hexane (2 mL)
and dried in vacuo. Yield: 74.0 mg (12.0 µmol, 52%) of a yellow solid. For X-Ray diffraction:
A saturated solution of [Ni(ICy)2(η2-COE)] 3 in n-hexane was cooled to -30 °C to obtain single-
crystals suitable for X-ray diffraction. 1H NMR (400 MHz, 25 °C, C6D6): δ = 1.45-2.17 (m, 52H,
Cy/COE-CH2), 2.44 (m, 2H, COE-CH), 5.04 (m, 4H, NCH), 6.61 (s, 4H, NCHCHN) ppm.
13C{1H} NMR (100 MHz, 25 °C, C6D6): δ = 26.0-35.0 (Cy/COE-CH2), 48.9 (COE-CH), 57.8
(NCH), 115.1 (NCCN), 204.9 (NCN) ppm. Elemental Analysis for C38H62N4Ni [633.54 g/mol]:
calculated (found): C 72.03 (71.62), H 9.86 (9.78), N 8.84 (8.78). HRMS-LIFDI (m/z):
calculated (found) for C30H48N4Ni [M-COE]+ 522.3227 (522.3216). IR (ATR [cm-1]): 휈̃ = 444 (w),
505 (w), 538 (w), 556 (m), 670 (vs), 689 (m), 730 (vw), 761 (vw), 815 (m), 863 (vw), 892 (m),
974 (w), 989 (m), 1021 (w), 1084 (w), 1136 (vw), 1215 (vs), 1266 (m), 1347 (vw), 1367 (vw),
1385 (w), 1407 (w), 1449 (w), 2825 (vw), 2853 (m), 2922 (m).
General Procedures for the Synthesis of Organoboronic Esters
Method A
A mixture of [Ni2(ICy)4(μ-(η2:η2)-COD)] 1 (2.5 mol%), NaOAc (1.5 equiv.) and B2pin2
(1.5 equiv.) was dissolved in methylcyclohexane (5 mL) in a Schlenk-tube equipped with a
magnetic stirring bar. The aryl chloride (1.0 equiv.) was then added to the solution. The
reaction mixture was heated at 100 °C for 24 h. Afterwards, the products and yields were
determined by GC-MS with biphenyl as an internal standard.
Method B
This method was performed on a preparative scale. A mixture of [Ni2(ICy)4(μ-(η2:η2)-COD)] 1
(2.5 mol%), NaOAc (1.5 equiv.) and B2pin2 (1.5 equiv.) was dissolved in methylcyclohexane
(50 mL) in a Schlenk-tube equipped with a magnetic stirring bar. The aryl chloride (1.0 equiv.)
was then added to the solution. The reaction mixture was heated at 100 °C for 24 h and was
then filtered and the remaining solid was washed with diethyl ether (10 mL Et2O). The filtrate
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was concentrated in vacuo, purified by silica-gel column chromatography with n-hexane and
then an n-hexane and ethyl acetate mixture (n-hexane/EtOAc =100/1) as eluent. The solvent
of the product-containing fraction of the eluent was evaporated in vacuo.
trans-[Ni(ICy)2(Cl)(4-(F3C)C6H4)] 11
In a Schlenk-tube, [Ni2(ICy)4(μ-(η2:η2)-COD)] 1 (100 mg, 86.6 µmol, 1.0 equiv.) was dissolved
in toluene (7 mL) and 4-chlorobenzotrifluoride 4 (25.6 µL, 191 µmol, 2.2 equiv.) was added at
room temperature and the reaction was stirred overnight. All volatiles were removed under
reduced pressure and the residue was suspended in n-hexane (3 mL). The product was
collected by filtration, washed with n-hexane (2 mL) and dried in vacuo. Yield: 94.3 mg
1
(134 µmol, 71%) of a colorless solid. H NMR (400 MHz, 25 °C, C6D6): 1.06-2.85 (m, 40H,
3
3
CH2), 6.18 (tt, 4H, JHax-Heq = 12.2 Hz, JHax-Hax = 3.8 Hz, N-CH), 6.29 (s, 4H, NCHCHN), 7.08
(d, 2H, 3JH-H = 7.8 Hz, aryl-CHo), 7.71 (d, 2H, 3JH-H = 7.8 Hz, aryl-CHm) ppm. 13C{1H} NMR (125
MHz, 25 °C, CD2Cl2): 25.9-34.8 (CH2), 60.1 (NCH), 117.0 (NCCN), 121.2 (3JC-F = 4 Hz, aryl-
Cm), 122.6 (2JC-F = 31 Hz, aryl-Cp), 124.5 (aryl-Cipso), 125.9 (1JC-F = 271 Hz, CF3), 137.1 (aryl-
Co), 167.9 (aryl-Cipso), 181.9 (NCN) ppm. 19F{1H} NMR (376 MHz, 25 °C, C6D6): -61.2 (s) ppm.
Elemental analysis for C37H52ClF3N4Ni [704.00 g/mol]: calculated (found): C 63.13 (63.19), H
7.45 (7.60), N 7.96 (7.81). HRMS-LIFDI (m/z): calculated (found) for C37H52F3N4Ni [M-Cl]+
667.3492 (667.3484). IR (ATR [cm-1]): 휈̃ = 565 (vw), 598 (vw), 696 (vs), 734 (vw), 756 (vw),
813 (s), 897 (w), 998 (w), 1009 (m), 1030 (vw), 1069 (s), 1106 (s), 1153 (m), 1191 (w), 1234
(m), 1274 (w), 1290 (s), 1318 (s), 1348 (vw), 1381 (w), 1403 (w), 1423 (w), 1445 (w), 1463
(vw), 1582 (w), 2845 (w), 2917 (w), 2936 (w).
trans-[Ni(ICy)2(Cl)(4-MeOC6H4)] 12
In a Schlenk-tube, [Ni2(ICy)4(μ-(η2:η2)-COD)] 1 (90.0 mg, 77.9 µmol, 1.0 equiv.) was dissolved
in toluene (7 mL) and 4-chloroanisole 6 (20.8 µL, 171 µmol, 2.2 equiv.) was added at room
temperature and the reaction was stirred overnight. All volatiles were removed under reduced
pressure and the residue was suspended in n-hexane (3 mL). The product was collected by
filtration, washed with n-hexane (2 mL) and dried in vacuo. Yield: 61.9 mg (92.9 µmol, 54%) of
1
a pale yellow solid. H NMR (400 MHz, 25 °C, C6D6): 1.08-1.83 (m, 32H, CH2), 1.98 (d, 4H,
3
3JHax-Heq = 12.1 Hz, NCHCHaxHeq) 2.90 (d, 4H, JHax-Heq = 12.1 Hz, NCHCHaxHeq), 3.30 (s, 3H,
3
3
CH3), 6.31 (tt, 4H, JHax-Heq = 12.1 Hz, JHax-Hax = 3.7 Hz, N-CH), 6.36 (s, 4H, NCHCHN), 6.65
3
3
(d, 2H, JH-H = 8.6 Hz, aryl-CHo), 7.41 (d, 2H, JH-H = 8.6 Hz, aryl-CHm) ppm. 13C{1H} NMR
(100 MHz, 25 °C, C6D6): 26.0-34.7 (CH2), 54.5 (CH3), 59.8 (N-CH), 116.3 (NCCN), 112.5 (aryl-
CHm), 137.9 (ary-CHo), 143.5 (C-CH3), 155.8 (aryl-Cipso) ppm. Elemental analysis for
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C37H55ClN4NiO [666.02 g/mol]: calculated (found): C 66.73 (66.42), H 8.32 (8.32), N 8.41
(8.41). HRMS-LIFDI (m/z): calculated (found) for C37H55N4NiO [M-Cl]+ 629.3724 (629.3717).
IR (ATR [cm-1]): 휈̃ = 510 (vw), 565 (vw), 579 (vs), 690 (vs), 758 (vw), 799 (m), 813 (w), 897
(w), 986 (vw), 998 (vw), 1026 (w), 1091 (vw), 1170 (w), 1197 (w), 1223 (m), 1234 (w), 1258
(ww), 1289 (vw), 1381 (w), 1403 (w), 1403 (w), 1423 (w), 1446 (w), 1464 (w), 1476 (w), 1574
(vw), 2847 (w), 2920 (m).
trans-[Ni(ICy)2(Cl)(C6H5)] 13
In a Schlenk-tube, [Ni2(ICy)4(μ-(η2:η2)-COD)] 1 (100 mg, 86.6 µmol, 1.0 equiv.) was dissolved
in toluene (7 mL) and chlorobenzene 7 (19.3 µL, 190 µmol, 2.2 equiv.) was added at room
temperature and the reaction was stirred overnight. All volatiles were removed under reduced
pressure and the residue was suspended in n-hexane (3 mL). The product was collected by
filtration, washed with n-hexane (2 mL) and dried in vacuo. Yield: 76.3 mg (120 µmol, 63%) of
a colorless solid. 1H NMR (400 MHz, 25 °C, CD2Cl2): 1.30-2.52 (m, 40H, CH2), 6.05 (m, 4H, N-
CH), 6.49 (t, 1H, 3JH-H = 7.2 Hz, aryl-CHp), 6.68 (m, 2H, 3JH-H = 7.2 Hz, aryl-CHm), 6.78 (s, 4H,
NCHCHN), 7.37 (d, 2H, 3JH-H = 7.4 Hz, aryl-CHo) ppm. 13C{1H} NMR (100 MHz, 25 °C, CD2Cl2):
25.9-34.9 (CH2), 59.9 (NCH), 116.7 (NCCN), 120.4 (aryl-Cp), 125.7 (aryl-Co), 137.3 (aryl-Cm),
157.5 (aryl-Cipso), 183.6 (NCN) ppm. Elemental analysis for C36H53ClN4Ni [635.99 g/mol]:
calculated (found): C 67.99 (67.60), H 8.40 (8.73), N 8.81 (8.59). HRMS-LIFDI (m/z):
calculated (found) for C36H53N4Ni [M-Cl]+ 599.3618 (599.3611). IR (ATR [cm-1]): 휈̃ = 490 (vw),
567 (vw), 692 (vs), 735 (m), 758 (vw), 819 (vw), 894 (w), 983 (vw), 997 (vw), 1019 (vw), 1055
(vw), 1194 (w), 1234 (w), 1262 (vw), 1290 (vw), 1382 (vw), 1423 (w), 1446 (w), 1563 (vw),
2853 (w), 2924 (w).
trans-[Ni(ICy)2(Cl)(3,5-F2C6H3)] 14
In a Schlenk-tube, [Ni2(ICy)4(μ-(η2:η2)-COD)] 1 (91 mg, 78.8 µmol, 1.0 equiv.) was dissolved in
toluene (7 mL) and 3,5-difluorochlorobenzene 8 (19.3 µL, 173 µmol, 2.2 equiv.) was added at
room temperature and the reaction was stirred overnight. All volatiles were removed under
reduced pressure and the residue was suspended in n-hexane (3 mL). The product was
collected by filtration, washed with n-hexane (2 mL) and dried in vacuo. Yield: 76.0 mg
(113 µmol, 65%) of a colorless solid. For X-Ray diffraction: Single-crystals suitable for X-ray
diffraction were obtained by slow evaporation of the solvent of a saturated solution of
1
[Ni(ICy)2(Cl)(3,5-F2C6H3)] in benzene. H NMR (400 MHz, 25 °C, C6D6): 1.04-1.80 (m, 32H,
3
3
CH2), 1.99 (d, 4H, JHax-Heq = 12.0 Hz, NCHCHaxHeq), 2.81 (d, 4H, JHax-Heq = 12.0 Hz,
3
3
NCHCHaxHeq), 6.22 (m, 1H, aryl-CHp), 6.22 (tt, 4H, JHax-Heq = 12.0 Hz, JHax-Hax = 3.9 Hz, N-
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3
4
CH), 6.29 (s, 4H, NCHCHN), 7.29 (m, 2H, JF-H = 6.5 Hz, JH-H = 2.3 Hz, aryl-CHo) ppm.
2
13C{1H} NMR (100 MHz, 25 °C, C6D6): 26.5-34.6 (CH2), 60.0 (NCH), 95.9 (t, JC-F = 25.4 Hz
2
4
aryl-Cp), 116.6 (NCCN), 118.9 (m, JC-F = 14.4 Hz, JC-F = 3.4 Hz, aryl-Co), 160.6 (dd,
3
1JC-F = 250.4 Hz, JC-F = 11.1 Hz, aryl-Cm), 165.8 (Ni-Cipso), 182.4 (NCN) ppm. 19F{1H} NMR
(376 MHz, 25 °C, C6D6): -115.9 (s) ppm. Elemental analysis for C36H51ClF2N4Ni
[671.97 g/mol]: calculated (found): C 64.35 (64.62), H 7.65 (7.91), N 8.34 (8.48). IR (ATR [cm-
1]): 휈̃ = 512 (vw), 525 (vw), 568 (vw), 694 (vs), 758 (vw), 805 (w), 820 (w), 849 (w), 895 (w),
964 (w), 985 (vw), 996 (vw), 1029 (vw), 1107 (w), 1194 (m), 1235 (w), 1269 (vw), 1292 (vw),
1347 (vw), 1397 (w), 1424 (w), 1441 (w), 1464 (w), 1564 (w), 1587 (m), 2853 (w), 2925 (m).
3044 (vw).
Crystallographic Details
Crystal data collection and processing parameters are given in the Supporting
Information. Crystallographic data for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as supplementary publication no.s
CCDC 1897241 (3) and 1897242 (14). Copies of the data can be obtained free of charge on
application to CCDC.
Acknowledgements
This work was supported by funds from the Julius-Maximilians-Universität Würzburg and the
Deutsche Forschungsgemeinschaft (DFG). We thank AllyChem Co. Ltd. for a generous gift of
diboron(4) reagents.
Keywords
Nickel; N-heterocyclic carbene; NHC; Borylation; Boronate
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