Preparing (Multi)Fluoroarenes as Building Blocks for Synthesis: Nickel-Catalyzed Borylation of Polyfluoroarenes via C-F Bond Cleavage

Article abstract of DOI:10.1021/jacs.6b02337

The [Ni(IMes)₂]-catalyzed transformation of fluoroarenes into arylboronic acid pinacol esters via C-F bond activation and transmetalation with bis(pinacolato)diboron (B₂pin₂) is reported. Various partially fluorinated arenes with different degrees of fluorination were converted into their corresponding boronate esters.

Full text of DOI:10.1021/jacs.6b02337

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Communication
Preparing (Multi)Fluoroarenes as Building Blocks for Syn-thesis:
Nickel-Catalyzed Borylation of Polyfluoroarenes via C-F Bond Cleavage
Jing Zhou, Maximilian Kuntze-Fechner, Rüdiger Bertermann, Ursula Paul, Johannes
H. J. Berthel, Alexandra Friedrich, Zhenting Du, Todd B Marder, and Udo Radius
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b02337 • Publication Date (Web): 12 Apr 2016
Downloaded from http://pubs.acs.org on April 12, 2016
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Preparing (Multi)Fluoroarenes as Building Blocks for Synthesis:
Nickel-Catalyzed Borylation of Polyfluoroarenes via C-F Bond
Cleavage
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Jing Zhou,a,b Maximilian W. Kuntze-Fechner,b Rüdiger Bertermann,b Ursula S. D. Paul,b Johannes H. J.
Berthel,b Alexandra Friedrich,b Zhenting Du,a Todd B. Marder,*b Udo Radius*b
aCollege of Science, Northwest A&F University, Yangling, Shaanxi 712100, P. R. China; bInstitut für Anorganische Chemie,
Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.
Supporting Information Placeholder
THF at 110 °C, whereas Niwa et al. reported borylation using
ABSTRACT: The [Ni(IMes)2]-catalyzed transformation of
bis(pinacolato)diboron (B2pin2) as the boron source, CsF as an
additive and a copper source as a mediator in toluene at 80 °C for
24 h. However, borylation with the [Ni(COD)2]/PCy3 system was
only reported for monofluoroarenes and the reactions strongly
depend on the conditions employed. Martin et al. demonstrated
that the use of other nickel sources or co-ligands (e.g. using an
NHC precursor or PPhCy2 instead of PCy3) leads to a significant
drop in yield and a change of the base (NaOtBu, HCO2Na, CsF) or
the use of B2pin2 as a boron source essentially kills the catalytic
process. The results presented by Niwa et al. on the
[Ni(COD)2]/Cu halide/PCy3-mediated borylation of mono-
fluoroarenes similarly reveal a strong dependence on the copper
source (CuI works whereas other copper sources do not) and the
base employed (CsF works whereas other additives investigated
do not).
fluoroarenes into arylboronic acid pinacol esters via C-F bond
activation and transmetallation with bis(pinacolato)diboron
(B2pin2) is reported. Various partially fluorinated arenes with
different degrees of fluorination were converted into their corre-
sponding boronate esters.
Arylboronic acid esters are versatile reagents in organic synthe-
sis, especially in the widely employed Suzuki-Miyaura cross-
coupling reaction.1 Transition-metal-catalyzed direct C-H boryla-
tion of arenes2 and borylation of aryl halides3 has emerged as one
of the most important mild and attractive routes for the synthesis
of aryl boronates in recent years. Fluoroaromatics play an im-
portant role in pharmaceuticals, agrochemicals and materials,4 and
there is currently much research aimed at the development of new
ways to introduce fluorine or fluorinated building blocks into
organic molecules.4,5 A smart way to achieve this goal would
employ fluoroaromatic boronic acids or boronate esters. The
conversion of fluoroaromatics into arylboronic esters via C-F
bond activation, however, is relatively unexplored and was re-
stricted to noble metal catalysts until recently. Smith and cowork-
ers observed small amounts of C-F bond borylation products (ca.
5%) in the iridium-catalyzed borylation of pentafluorobenzene
and in the rhodium-catalyzed borylation of 1,3,5-
trifluorobenzene.6 C-F bond borylation promoted by
[Rh(SiPh3)(PMe3)3] was reported by Marder, Perutz and cowork-
ers7 in stoichiometric reactions, and Braun et al. developed the
Rh(I)-catalyzed borylation of pentafluoropyridine, hexafluoropro-
pene and other fluorinated aromatics.8 Furthermore, Zhang et al.
reported that commercially available [Rh(cod)2]BF4 catalyzes
ortho-selective C-F bond borylation of polyfluoroarenes with
B2pin2.9 All of the above protocols employ noble metal catalysts.
Although much work has been done in the field of nickel-
mediated C-F bond activation,5,10 applications of nickel phosphine
complexes in catalytic C-F bond borylation appeared just recently.
Thus, Martin et al.11 and Niwa and Hosoya et al.12 reported in
2015 the borylation of monofluoroarenes using in situ generated
nickel phosphine complexes from the reaction of [Ni(COD)2]
(COD = 1,5-cyclooctadiene) and PCy3 (> 4 eq.) (Scheme 1).
Martin et al. chose bis(neopentylglycolato)diboron (B2neop2) as
the boron source and NaOPh as a base to promote the reaction in
We are currently developing convenient methodologies to gen-
erate and use suitable, partially fluorinated organic precursors.
One strategy we follow employs (i) C-F borylation of polyfluoro-
aromatics and (ii) use of the resulting fluoroaryl boronic ester in
subsequent Suzuki-Miyaura coupling reactions. The reports by
Martin et al.11 and Niwa and Hosoya et al.12 prompted us to dis-
close our results on the NHC nickel-catalyzed borylation of
polyfluorinated aromatics (Scheme 1).
Scheme 1. Nickel-Catalyzed Borylation of Fluoroaromat-
ics.
B(OR)2
B(OR)2
F
[Ni(COD)2]/
PCy3
Ni(IMes)2
R = H, F
R = H, aryl, ..
Fn
Rn
Rn
defluoroborylation
defluoroborylation
Martin et al., JACS 2015
+
generation of fluorinated
building blocks
this work
Niwa et al., JACS 2015
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We have demonstrated earlier that [Ni2(iPr2Im)4(COD)] (iPr2Im
is crucial and beneficial (Table S3). Interestingly, the addition of
= 1,3-diisopropyl-imidazolin-2-ylidene),10a reacts with fluorinated
arenes with high chemo- and regioselectivity and catalyzes C-F
bond transformations.10 Thus, we envisioned this complex as a
precatalyst for C-F bond borylation, but our initial experiments
revealed that the nickel compound decomposes in the presence of
B2pin2 or B2cat2. As an alternative, the complex [Ni(IMes)2] with
0.5 equivalents of water-free NMe4F instead of CsF provided a
superior yield, especially, if methylcyclopentane was used as the
solvent (Table S4, Table 1, entries 5-7). However, considering the
fact that water-free NMe4F is relatively expensive,15 we also
provide the conditions and results for reactions using much
cheaper and commercially available fluoride sources such as CsF,
KF, or NBu4F (Tables S1-S4). CsF and KF were moderately
effective and provide the product in 50 % yield, while the use of
water-containing TBAF shuts the reaction down completely. With
water-free NMe4F in hand, the conditions as given in Table 1,
entry 7 are the best for the formation of 2a (1.1 eq. C6F3H3, 1 eq.
B2pin2, 10 mol% [Ni(IMes)2], 0.5 eq. NMe4F, methylcyclopen-
tane, 80 °C, 15 h).
4
5
6
7
8
9
the sterically more demanding NHC IMes (IMes
= 1,3-
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dimesitylimidazolin-2-ylidene), which is sufficiently stable in the
presence of diboron esters,13 was used in the current study.
Table 1. Optimization of the Borylation of 1,2,3-C6F3H3
1a/B2pin2
1.1:1
1.1:1
1.1:1
3:1
1.1:1
1.1:1
1.1:1
Base
Solvent
THF
Yield(%)a,b
1
2
3
4
5
6
CsF (1 eq.)
CsF (1 eq.)
CsF (1 eq.)
CsF (1 eq.)
NMe4F (1 eq.)
NMe4F (0.5 eq.)
NMe4F (0.5 eq.)
39
50
50
75
62
76
79
Using these optimized conditions, we then explored the scope of
the nickel-catalyzed C-F bond borylation using different
fluoroarenes. As shown in Table 2, the borylation of fluoroarenes
containing different degrees of fluorination was achieved in mod-
erate to good yields. For example, using 1,3,5-C6F3H3 (1b),
mono-borylated 2b was obtained in 93 % yield (Table 2, entry 2;).
Borylations of 1,2,3-trifluorobenzene (1a) and 1,2,3,5-C6F4H2
(1d) were also accomplished with good selectivity and yield
(Table 2, entries 1, 4). For other substrates such as 1,2,4-C6F3H3
(1c), 1,2,4,5-C6F4H2 (1e) and C6F5H (1f), the temperature had to
be raised to 100°C for successful borylation (Table 2, entries 3, 5,
6). For 1,2,4-C6F3H3 (1c), borylation of the 2-position (2c) pre-
vails and only minor amounts of the isomers from borylation of
the 1-(2c’’) or of the 4-positions (2c’) (ratio approximately 10:1:1,
judged from 19F NMR spectroscopy) were formed. For 1,2,4,5-
C6F4H2 (1e), diborylation to give 2e’ was observed when using
the standard reaction conditions. However, the formation of
mono-borylation product (2e) and diborylation product (2e’) as
the main product can be controlled by the ratio of fluoroaromatic
and diboron reagent used. One equivalent of 1,2,4,5-C6F4H2, 2
equivalents B2pin2 and CsF leads to diborylation product 2e’ in 73
% yield. If the amount of 1,2,4,5-C6F4H2 is increased to 4 equiva-
lents, the monoborylation product 2e was obtained in good yield
with only small amounts of 2e’. Compounds 2e and 2e’ can be
separated easily by column chromatography (see SI). The boryla-
tion of C6F5H occurs not at the most nucleophilic 3-position, as
probably expected, but at the position ortho to the hydrogen sub-
stituent. However, selective ortho-C-F bond activation of C6F5H
at nickel was observed before.16 We were unsuccessful in improv-
ing the reaction with [Ni(IMes)2] from being essentially stoichio-
metric in nickel (12 % yield at 100 °C). However, we found that
the use of [Ni(IPr)2], bearing the sterically more demanding NHC
C9H12
C6H12
C9H12
C9H12
C9H12
C6H12
7
a
Reaction conditions: 29.0 mg (0.22 mmol) 1,2,3-C6F3H3 1a; [Ni(IMes)2]
(10 mol%), solvent (1 mL), 80 °C, 15 h. The yields were determined by in
situ 19F NMR spectroscopy.b Trace amounts of isomeric C-F and C-H
borylation products were detected by GC-MS. C9H12 = mesitylene; C6H12
= methylcyclopentane. For more details see the SI.
We started our investigation using 1,2,3-trifluorobenzene 1a (1
eq.) as the model substrate, B2pin2 (1 eq.) as the boron source,
CsF (1 eq.) as an additive, [Ni(IMes)2] (10 mol %) as the catalyst
and THF as the solvent of choice. A C-F borylation product was
obtained from this reaction in 39 % yield (judged by 19F NMR
spectroscopy; Table 1, entry 1), which was identified as com-
pound 2a (Scheme 2), i.e. the product from a defluoroborylation
at the 1-position. In addition to 2a, the regioisomer 2a’ (boryla-
tion at the 2-position), traces of the hydrodefluorina-
tion/borylation products 2g and 2h and a C-H borylation product
were detected by GC-MS analysis. Interestingly, trace amounts of
the diborylation compound 3a, presumably formed by subsequent
C-H- and C-F-borylation, were detected as well. Note that the
preference of NHC nickel insertion into the C-F bond at the 1-
position of 1,2,3-C6F3H3 was observed before.10d With this prom-
ising first result, different solvents, bases, and different stoichio-
metric ratios of fluorobenzenes and B2pin2 were screened using
standardized conditions (80°C, 15 h) to assess the scope and
limitations of this reaction (Table 1, Tables S1 – S4 in the SI).
First of all, our studies clearly reveal that the nature of the solvent
used is very important. For hexane, mesitylene or methylcyclo-
pentane instead of THF the yields of 2a increase to 45 % (hexane)
and 50 % (mesitylene or methylcyclopentane), respectively (Table
S1; Table 1, entries 2, 3), whereas acetonitrile shuts the reaction
down completely (Table S1). Methyl t-butyl ether works similarly
well compared to THF; in this case, however, the C-F/C-H di-
borylation product 3a and the alkylation product 4a (Scheme 2)
were detected in trace amounts (Table S1). The latter compound
implies C-O bond cleavage of MTBE under catalytic conditions.14
In general, yields and turnover rates are low, but some improve-
ment with respect to the consumption of B2pin2 was achieved if
the amount of fluoroarene was increased to 3 equivalents (75 %
2a; Table S2, Table 1, entry 4). An increase of the amount of
B2pin2 (and CsF) to 2 equivalents leads to the formation of a
mixture of monoborylated 2a and bisborylated 3a. A screening of
different additives further reveals that the choice of the base also
dramatically affects the reaction and that the presence of fluoride
1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene IPr, as
a
precatalyst is beneficial to this reaction, and formation of the
borylation product was observed in more than 80 % yield. For
1,2-C6F2H4 (1g) and 1,3- C6F2H4 (1h), C-F borylation products
were formed in somewhat lower yields, with trace amounts of C-
H bond borylation observed by GC-MS (Table 2, entries 7, 8).
Borylation of monofluorobenzene leads to PhBpin 2i in only 20 %
yield (Table 2, entry 9). Moreover, we observe no reaction or only
traces of products for the borylation of perfluorinated substrates
such as hexafluorobenzene or octafluorotoluene. We also provide
yields of the isolated product using CsF as a base in Table 2. All
products have been characterized by multinuclear NMR spectros-
copy and high resolution mass spectroscopy and, for 3a, 2e’ and
2f’, by X-ray diffraction (see SI).
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Scheme 2. Borylation Products and Side-Products
Table 2. Scope of the Ni-catalyzed C-F borylation
4
5
6
7
8
Substrate
Product and Yield
9
1
2
3
1,2,3-C6F3H3
1,3,5-C6F3H3
1,2,4-C6F3H3
1-Bpin-2,3-C6F2H3(2a) 79 % (38 %)b
1-Bpin-3,5-C6F2H3(2b) 93 % (75 %)b
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1-Bpin-2,4-C6F2H3(2c) 77
2c:2c’:2c’’=10:1:1 c
%
(60 %)c
4
5
1,2,3,5-
C6F4H2
1-Bpin-2,3,5-C6F3H2(2d) 99 % (71 %)b
2d:2d’=10:1
1,2,4,5-
C6F4H2
1-Bpin-2,4,5-C6F3H2(2e) 70 % (65 %)d
1,4-bisBpin-2,5-C6F2H2(2e’) 85 %e,f (73 %)e
1-Bpin-2,3,4,5-C6F4H (2f) 85 %f (80 %)b,f
1-Bpin-2-C6FH4 (2g) 48 % g (30 %) g
1-Bpin-3-C6FH4 (2h) 50 % g (35 %) g
C6H5Bpin (2i) 20 % g, h
6
7
8
9
C6F5H
1,2-C6F2H4
1,3-C6F2H4
C6FH5
a Reaction conditions (unless specified otherwise): B2pin2 (0.2 mmol),
fluoroarene (0.22 mmol), [Ni(IMes)2] (10 mol %), NMe4F (0.1 mmol),
methylcyclopentane (1 mL), 80 °C, 15 h. Yields determined by 19F NMR
b
spectroscopy based on B2pin2. Reaction conditions: B2pin2 (2 mmol),
fluoroarene (2.2 mmol), [Ni(IMes)2] (10 mol %), CsF (2 mmol), methyl-
cyclopentane (10 mL), 80 °C, 15 h. Isolated yields. methylcyclohexane,
100 °C.
c
d
e
2e (4 eq.), methylcyclohexane, 100 °C. B2pin2 (2 eq.),
fluoroarene (1 eq.), CsF (2 eq.), methylcyclohexane (20 mL), 100 °C, 15
h. f [Ni(IPr)2] (10 mol %) instead of [Ni(IMes)2]. Fluoroarene (2 eq., 0.4
g
h
mmol). Yield was determined by GC-MS using C12H26 as the internal
standard. For more details see the SI.
In previous work we have shown that NHC nickel complexes
cis/trans isomerization of the boryl complex) delivers the
borylated fluoroaromatic ArF-Bpin and regenerates [Ni(IMes)2] I.
The borylation of the nickel fluoride complex seems to be the rate
limiting process in the sequence, as we (i) observe larger quanti-
ties of the nickel fluoride as the resting state in the reaction mix-
tures under catalytic conditions (see SI) and (ii) failed to isolate
complexes trans-[Ni(IMes)2(Bpin)(ArF)] so far from the stoichi-
readily insert into C-F bonds of polyfluorinated aromatics.10
Using [Ni(IMes)2], C-F bond activation also occurs cleanly. The
reaction of 1,2,3,5-C6F4H2 (1d) with [Ni(IMes)2] affords the
nickel fluoride complex trans-[Ni(IMes)2(F)(2,3,5-C6F3H2)] 5 in
good isolated yield (see SI). This reaction is quantitative accord-
ing to 1H and 19F NMR spectroscopy. Complex 5 has been charac-
terized by proton and fluorine NMR spectroscopy, mass spec-
trometry, elemental analysis and X-ray diffraction. Most signifi-
cantly, the Ni-F ligand of 5 gives rise to a resonance at –344.5
ppm in the 19F NMR spectrum.
ometric
B2pin2/NMe4F.
reaction
of
trans-[Ni(IMes)2(F)(ArF)]
with
Scheme 3. Proposed Mechanism for the NHC-Nickel-
Catalyzed Borylation of Fluoroarenes.
The reaction of [Ni(IMes)2] with 1,2,3,5-C6F4H2 (1d) in the
presence of B2pin2 and NMe4F leads to the initial formation of the
C-F bond activation product 5 (see SI). The boryl transfer reagent
should be either B2pin2 or its fluoride adduct, i.e.
[NMe4][B2pin2F] 7.17 The reaction of 5 with an excess of B2pin2
and NMe4F using catalytic conditions leads to borylated
fluoroarene, complex 5 and [NMe4][F2Bpin]. We favor at present
a mechanism as outlined in Scheme 3. The reaction of [Ni(IMes)2]
I with the fluoroarene leads in a first step to oxidative addition of
the C-F bond with formation of trans-[Ni(IMes)2(F)(ArF)] II.
Although an initial “kinetic” C-H bond activation step5d,e would
explain the regioselectivity of the borylation reaction (borylation
occurs at a site adjacent to an hydrogen substituent), we can not
observe such an species even at low temperatures (see Figure S 68
– S70). The complex trans-[Ni(IMes)2(F)(ArF)] II reacts then
with [NMe4][B2pin2F] by boryl transfer to give trans-
[Ni(IMes)2(Bpin)(ArF)] III and [NMe4][F2Bpin]. A final reductive
elimination step (e.g. after ligand elimination from a three coordi-
In summary, we have developed an efficient procedure for the C-
F borylation of fluoroaromatic compounds using [Ni(IMes)2]
([Ni(IPr)2] for pentafluorobenzene) as a catalyst, NMe4F or CsF
as a fluoride source, and B2pin2 as the boron source. In combina-
tion with the wide versatility of borylarene transformations, this
nate species as indicated in Scheme
3
or after
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method will enable the variation of C-F-bonds in polyfluoroarenes
in the future, e.g. replacing selected fluorine substituents with
other groups such as other halides, amides, carboxylate or ether
functionalities. On the other hand, typical C-C bond formation
steps will allow the introduction of partially fluorinated arenes
into larger organic molecules. Detailed investigations concerning
the applications and mechanistic details of this reaction are in
progress.
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ASSOCIATED CONTENT
Supporting Information
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2015, 127, 9203.
(10) (a) Schaub, T.; Radius, U., Chem. Eur. J. 2005, 11, 5024; (b) Schaub,
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1
Experimental details, spectroscopic data, copies of H, 13C{1H},
11B, 19F{1H} and 19F NMR spectra and HRMS data. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*u.radius@uni-wuerzburg.de,
*todd.marder@uni-wuerzburg.de
Present Addresses
Institut für Anorganische Chemie, Julius-Maximilians-Universität
Würzburg, Am Hubland, 97074 Würzburg, Germany
(13) Pietsch, S.; Paul, U.; Cade, I. A.; Ingleson, M. J.; Radius, U.; Marder,
T. B., Chem. Eur. J. 2015, 21, 9018.
(14) C-O bond cleavage of ethers by NHC nickel complexes is a known
process and may be used for the catalytic transformation of ethers, see for
example: (a) A. G. Sergeev, J. F. Hartwig, Science 2011, 332, 439; (b)
Tollefson, E. J.; Hanna, L. E.; Jarvo, E. R., Acc. Chem. Res. 2015, 48,
2344; (c) Liu, C.; Peterson, C.; Wilson, A. K., J. Phys. Chem. 2013, 117,
5140.
(15) e.g. Aldrich 459315: 5g ca. 300 €
(16) Johnson, S. A.; Taylor, E. T.; Cruise, S. J., Organometallics 2009, 28,
3842.
(17) The [NMe4][B2pin2F] salt was not detected in the reaction mixtures
we have investigated in C6D6 and methylcyclopentane. However, it is
known that it is only sparingly soluble in these solvents, but soluble in
acetonitrile and in hot THF. We recently reported the preparation, isola-
tion and utilization of [NMe4][B2pin2F] for boryl transfer: Pietsch, S.;
Neeve, E. C.; Apperley, D. C.; Bertermann, R.; Mo, F.; Qiu, D.; Cheung,
M. S.; Dang, L.; Wang, J.; Radius, U.; Lin, Z.; Kleeberg, C.; Marder, T.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by funds from University of Würzburg
and the DFG (Ra720/12-1). We are grateful to the China Scholar-
ship Council for providing a scholarship to Jing Zhou. We thank
Prof. Dr. Maik Finze and Mathias Häring for a donation of anhy-
drous NMe4F and AllyChem Co. Ltd. For a gift of B2pin2. Dedi-
cated to Prof. em. Dr. Hansgeorg Schnöckel on the occasion of his
75th birthday.
B.,
Chem.
Eur.
J.
2015,
21,
7082.
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