Zinc-Catalyzed Dual C-X and C-H Borylation of Aryl Halides

Article abstract of DOI:10.1002/anie.201505603

A zinc-catalyzed combined C-X and C-H borylation of aryl halides using B₂pin₂ (pin=OCMe₂CMe₂O) to produce the corresponding 1,2-diborylarenes under mild conditions was developed. Catalytic C-H bond activation occurs ortho to the halide groups if such a site is available or meta to the halide if the ortho position is already substituted. This method thus represents a novel use of a groupXII catalyst for C-H borylation. This transformation does not proceed via a free aryne intermediate, but a radical process seems to be involved. Two B or not two B: A novel catalytic system based on a Zn^II-dtbpy precursor was developed for the preparation of 1,2-diborylarenes. This method represents a new type of catalytic process for diborylation of aryl halides via both C-X and C-H activation.

Full text of DOI:10.1002/anie.201505603

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201505603
German Edition: DOI: 10.1002/ange.201505603
À
C H Activation
Zinc-Catalyzed Dual C–X and C–H Borylation of Aryl Halides
Shubhankar Kumar Bose, Andrea Deißenberger, Antonius Eichhorn, Patrick G. Steel,
Zhenyang Lin, and Todd B. Marder*
À
À
Abstract: A zinc-catalyzed combined C X and C H boryla-
tion of aryl halides using B2pin2 (pin = OCMe2CMe2O) to
produce the corresponding 1,2-diborylarenes under mild
monoborylated products (Table 1). Classical methods for the
preparation of 1,2-disubstituted arenes necessitate the avail-
ability of the appropriate dihaloarenes.[12,14] Recently, Yosh-
ida reported that the Pt- or Cu-catalyzed diboration of the
À
conditions was developed. Catalytic C H bond activation
occurs ortho to the halide groups if such a site is available or
meta to the halide if the ortho position is already substituted.
This method thus represents a novel use of a group XII catalyst
Table 1: Optimization of the reaction conditions for the zinc-catalyzed
diborylation of iodobenzene (1a).[a]
À
for C H borylation. This transformation does not proceed via
a free aryne intermediate, but a radical process seems to be
involved.
A
rylboronic acids and arylboronates are widely used as
versatile building blocks in organic synthesis and they are
important synthetic intermediates for transition-metal-cata-
[1]
À
lyzed C C and carbon–heteroatom bond-forming reactions.
Traditional methods for their synthesis are based on the
reaction of aryl Grignard or lithium reagents with boron
electrophiles.[2] More recently, transition-metal-catalyzed bor-
ylation of aryl halides[3] and the direct C H borylation of
Entry Catalyst
Ligand Base
Solvent [2 mL]
Product Yield
[%][b]
À
arenes[4–6] have become increasingly important from the
viewpoint of efficiency and functional-group compatibility.
1b
1c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
ZnCl2
ZnBr2
ZnI2
ZnEt2
Zn(OAc)2 L1
L1
L1
L1
L1
MeOK MTBE
MeOK MTBE
56
45
43
84
10
0
30
56
50
trace
trace
35
trace
5
37[c]
20
27
9
0
0
15
24
35[c–e]
0
0
14
0
0
0
0
0
17
0
0
0
0
À
The most efficient catalysts for C H borylation of arenes are
based on precious metals such as Rh or Ir.[4–7] Similarly, aryl
halide borylation typically utilizes Pd or Ni catalysts,[3a,b]
although the inexpensive, abundant, and environmentally
more acceptable Cu,[8a–e] Fe,[8f] or even Zn[9] now represent
attractive alternatives to the commonly used expensive and
toxic noble metals. However, the development of zinc
catalysis is still in its infancy.[9–11]
MeOK MTBE
MeOK MTBE
MeOK MTBE
MeOK MTBE
MeOK MTBE
MeOK MTBE
MeOK MTBE
MeOK MTBE
MeOK MTBE
MeOK MTBE
MeOK MTBE
MeOK MTBE
MeOK MTBE
Cs2CO3 MTBE
tBuOK MTBE
MeONa MTBE
–
L1
L2
L3
L3
L4
L5
L6
L7
L8
–
L1
L1
L1
L1
L1
L1
ZnCl2
ZnCl2
ZnBr2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
We recently reported an efficient catalytic system using
ZnBr2 and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene
(IMes) for the cross-coupling of aryl and heteroaryl halides
with diboron reagents.[11] We have now found that the
combination of ZnCl2 and 4,4’-di-tert-butyl-2,2’-bipyridine
0
7
À
(dtbpy; L1) borylates not only the C X bond of haloben-
trace
25
0
trace
3
26
À
zenes, but also a C H bond ortho to the halide group, thereby
yielding valuable 1,2-bis(Bpin)benzenes[12,13] along with the
–
MTBE
20[f] CuI
MeOK MTBE
MeOK MTBE
MeOK MTBE
MeOK MTBE
21[g] NiBr2
[*] Dr. S. K. Bose, A. Deißenberger, A. Eichhorn, Prof. Dr. T. B. Marder
Institut für Anorganische Chemie
22[h] Pd(OAc)2 L1
23[i] ZnCl2
L1
trace
0
Julius-Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
E-mail: todd.marder@uni-wuerzburg.de
[a] Reaction conditions: C6H5I, 1a (0.5 mmol, 1 equiv), catalyst
(10 mol%), ligand (20 mol%), base (1.5 equiv), B2pin2 (1.5 equiv),
MTBE (2 mL), at room temperature unless otherwise stated. [b] The
yields were determined by GC–MS analysis vs. a calibrated internal
standard and are averages of two runs. [c] Complete conversion was
observed, as determined by GC–MS analysis (see Table S8). [d] The
reaction was performed using 2 equiv of B2pin2 and 2 equiv of MeOK.
[e] The reaction was performed at 508C. [f] 2 mol% of CuI catalyst was
used. [g] 2 mol% of anhydrous NiBr2 used. A similar result was obtained
with NiCl2. [h] 2 mol% of Pd(OAc)2 catalyst was used. [i] 18 mL (1 mmol)
of water was added.
Prof. Dr. P. G. Steel
Department of Chemistry, Durham University
South Road, Durham DH1 3LE (UK)
Prof. Dr. Z. Lin
Department of Chemistry
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505603.
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ꢀ
Table 2: Screening of aryl halides for the zinc-catalyzed diborylation
reaction.
highly reactive C C bond of arynes at elevated temperatures
(Pt catalyst at 808C, Cu catalyst at 1008C) yielded 1,2-
diborylarenes.[13] This route entails preparing the appropriate
ortho-(trimethylsilyl)aryl triflate derivative from a suitable
precursor.[15] Our novel Zn-catalyzed diborylation of aryl
À
À
halides via both C X and C H activation under mild
conditions represents a new type of catalytic process.
Using 1a as the substrate, we screened a range of
conditions, zinc sources, ligands, bases, and solvents to
assess the scope and limitations of this diborylation reaction
(Table 1; see Tables S1–S7 in the Supporting Information for
details). Diborylated product 1c was obtained in the highest
yield using 1.5 equivalents of B2pin2 and MeOK in the
presence of ZnCl2 as the catalyst and L1 (dtbpy) as the
ligand at ambient temperature in methyl tert-butyl ether
(MTBE), with complete conversion after 8 h (entry 1), while
ZnBr2 and ZnI2 provided the product in lower yields
[9b]
(entries 2 and 3). ZnEt2 yielded predominantly the mono-
borylation product 1b, and Zn(OAc)2 showed very low
catalytic activity (entries 4 and 5). In the absence of a Zn
À
source, the borylation reaction does not occur at either the C
[3d]
À
X or the C H bond (entry 6) in this solvent.
The choice of ligand is crucial for this unusual diboryla-
tion reaction. Several substituted 2,2’-bipyridine and 1,10-
phenanthroline ligands were investigated (L2, L4–L8), which
provided lower yields than L1 and L3 (entries 7–14). The 5,5’-
di-Me-bpy (L3) ligand displayed comparable reactivity to L1
when used in combination with 2 equiv of B2pin2 and MeOK
(entry 9). Ligand is essential for this catalytic process; no
borylated products were observed when the ligand was
omitted (entry 15). Next, the effects of the base were
examined (entries 16–19). The replacement of MeOK by
Cs2CO3 or tBuOK resulted in limited or no reaction, whereas
MeONa was only somewhat less effective than MeOK
(entries 16–18). There was no reaction in the absence of
a base (entry 19).
The possible involvement of copper, nickel, or palladium
contamination in the catalyst was eliminated by the observa-
tion that copper[8a–e] and nickel[3a] salts provided trace or very
small amounts of 1b and no 1c (entries 20 and 21), and
Pd(OAc)2 provided a low yield of monoborylation product 1b
and no diborylation product 1c under the standard reaction
conditions (entry 22). The reaction is moderately sensitive to
air[16] but very sensitive to moisture; after the addition of
2 equiv of water, no significant amounts of the arylboronic
esters were formed (entry 23).
Further screening of metal salts, ligands, and bases
identified the two best catalyst systems. System A comprises
ZnCl2, L1, and 1.5 equiv of B2pin2 and MeOK in MTBE
(entry 1), and system B comprises ZnBr2, L3, and 2 equiv of
B2pin2 and MeOK in MTBE (entry 9).
With optimized reaction conditions in hand, we examined
the substrate scope of the diborylation reaction (Table 2).
[a] Approximate ArH yields were estimated by GC–MS analysis.
[b] Reaction conditions (A): aryl halide (0.5 mmol, 1 equiv), B2pin2 (1.5 equiv), ZnCl2 (10 mol%), Ligand: L1 (20 mol%), MeOK (1.5 equiv), MTBE
(2 mL), at room temperature unless otherwise stated. [c] Yields were determined by GC–MS analysis vs. a calibrated internal standard and are averages
of two experiments. [d] Reaction conditions (B): aryl halide (0.5 mmol, 1 equiv), B2pin2 (2 equiv), ZnBr2 (10 mol%), Ligand: L3 (20 mol%), MeOK
(2 equiv), MTBE (2 mL), at room temperature unless otherwise stated. [e] Yield was determined by 1H NMR analysis using 1,1,2,2-tetrachloroethane as
an internal standard. [f] The reaction was performed at 508C for 12 h. [g] GC–MS analysis of the crude reaction mixture revealed the presence of a small
amount of one more isomer of the diborylation product. [h] The structure of 3c was confirmed by single-crystal X-ray diffraction (see Figure S7).
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Substituted aryl halides underwent diborylation reactions
exclusively at the position ortho to the halide group,
irrespective of the electronic nature of the substituent. Aryl
halides possessing electron-donating substituents, such as p-
methyl, 3,5-di-methyl, p-methoxy, and p-N,N-dimethylamino
groups (2a–5a), are well tolerated, providing 1,2-diborylar-
enes in moderate yields. Aryl bromides (6a and 7a) are less
reactive, with the yields of the bis(boronate) being lower even
at higher temperature. An electron-withdrawing CF3 group
(8a) at the para position yielded diborylation product 8c in
lower yield, along with the monoborylation product 8b. The
sterically congested, ortho-tolyl derivative 10a was dibory-
lated in 34% yield.
so a two-step process is unlikely. Indeed, the isolated
monoborylated product 2b, which would arise from initial
À
À
C X borylation, did not undergo a second (C H) borylation
reaction under the standard conditions. We do note that there
is an induction period for the formation of both products.
Addition of anthracene, 9,10-dimethylanthracene, 1,3-
diphenylisobenzofuran, or 2,5-dimethylfuran as benzyne
traps did not generate Diels–Alder adducts, thus indicating
that our diborylation reaction does not generate a free
benzyne intermediate resulting from base-promoted dehy-
drohalogenation. However, careful analysis of the crude
reaction mixture with added anthracene revealed the forma-
tion of a small amount of 9-phenyl-9,10-dihydroanthracene,[17]
suggesting the possibility of a radical mechanism (Scheme S1
in the Supporting Information). Furthermore, no vic-dibor-
ylbenzene (1c) was observed by GC–MS analysis when
benzyne, generated in situ from 2-(trimethylsilyl)phenyl tri-
flate and KF/[18]crown-6, was reacted with B2pin2 in the
presence of ZnCl2 and L1 as the ligand.[9b, 13]
In the absence of a combination of directing groups and
specific catalysts,[5] the regioselectivity of the Rh- or Ir-
À
catalyzed C H borylation of arenes is primarily controlled by
steric effects[6] and, consequently, the reactions typically occur
at unhindered sites.[4–7] To study the steric effect of the
substituents in our zinc-catalyzed reaction, we investigated
the diborylation of meta-substituted aryl halides (Scheme 1).
The borylation of sterically congested iodomesitylene
(13a), which is substituted at both ortho positions, afforded
the unexpected 1,3-diborylarene product 13c, as confirmed by
X-ray diffraction (Figure 1). It is significant that in the case of
À
À
13a, the C H activation took place at a remote aryl C H
À
bond rather than a weaker benzylic C H bond (compare 2a,
3a, 7a, and 10a). The formation of 13c further excluded the
possibility of an aryne intermediate.
When the reaction was performed in the presence of
1 equiv of the radical inhibitor 9,10-dihydroanthracene, the
desired products were obtained in lower yields (1b: 26%; 1c:
18%). Increasing the radical inhibitor to 7 equiv shuts down
the reaction almost completely (4% of 1b and no 1c detected
by GC–MS) and a similar result was obtained when using
3 equiv of 2,2,6,6-tetramethylpiperidin-1-yloxyl (TEMPO) as
the inhibitor. It thus seemed that the borylation reaction
Scheme 1. Borylation of 1-iodo-3-methylbenzene (11a) and 1-iodo-3-
methoxybenzene (12a; the structure of 12c was confirmed by single-
crystal X-ray diffraction; see Figure S8).
Significantly, the zinc-catalyzed diboryla-
tion of 11a occurred preferentially at the
more hindered site, ortho to the substituent,
thereby yielding 1,2-bis(Bpin)-3-methyl-
benzene (10c) accompanied by a small
amount of 1,2-bis(Bpin)-4-methylbenzene
(2c). A similar result was observed for the
corresponding 1-iodo-3-methoxybenzene
Figure 1. Borylation of 1-iodo-2,4,6-trimethylbenzene (13a) along with the molecular struc-
ture of 13c (see Figure S9 for details).
(12a), thus providing further evidence
À
that the regioselectivity of the C H bory-
lation is not dominated by steric effects of
the substituents.
might involve one-electron processes. A radical-clock experi-
The mechanism of this Zn-catalyzed diborylation of aryl
halides is not immediately obvious. To probe the possibility of
ment using substrate 14a under the standard conditions
afforded the major acyclic mono and diborylation products
14b and 14c in 42 and 26% yield, respectively, and no 5-exo-
trig cyclization product 14d was detected (Scheme 2; a trace
amount of one more isomeric diborylation product was also
observed by GC–MS analysis). However, an experiment using
1-bromo-2-(but-3-en-1-yl)benzene (15a; Scheme 3) gave
monoborylated product 15b, along with cyclized (borylme-
thyl)indane (15d); no diborylation product 15c was detected
by GC–MS (small amounts of hydrodehalogenation byprod-
ucts 4-phenyl-1-butene and 1-methylindane were also
À
a two-step process, in which the C X borylation product
À
subsequently undergoes ortho C H borylation, we monitored
the reaction of 2a by GC–MS analysis. As shown in Figure S1
in the Supporting Information, the monoborylated product
2b and diborylated product 2c were formed in maximum
yields of 50% and 34%, respectively, within 8 h. The roughly
similar shapes of the graphs for the time dependence of the
formation of the two products indicate that both products are
formed concurrently under the standard reaction conditions,
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Scheme 2. Borylation of 1-(allyloxy)-3-iodobenzene (14a).
Scheme 3. Borylation of 1-bromo-2-(but-3-en-1-yl)benzene (15a).
observed). Formation of the cyclized product 15d supports
the possibility of a radical mechanism.
Itami and co-workers have reported a transition-metal-
free tBuOK-promoted coupling of electron-deficient nitrogen
heterocycles with aryl iodides.[18a] Recently, Jutand and Lei
et al. reported that the interaction between 1,10-phenanthro-
line (Phen) and tBuOK generates the 1,10-phenanthroline
radical anion PhenCÀ and the tBuOC radical.[18b] Furthermore,
Phen serves as a relay for the reduction of phenyl bromide to
the phenyl radical via PhenCÀ. Echegoyen et al. reported
[Ru(bpy)3]0, formed by reductive electrocrystallization of
[Ru(bpy)3](PF6)2, and the crystal structure provides evidence
that the extra electrons reside on the bpy ligands.[18c]
Interestingly, the borylation of 2-fluoroiodobenzene afforded
monoborylated product 16b, along with the arylated dtbpy
product 16d; no diborylated product was observed by GC–
MS (Scheme 4). The formation of coupling product 16d
Figure 2. A plausible mechanism for pathway (B).
either a step-wise or concerted (III) fashion), giving dibory-
lated arene and regenerating the [Zn-dtbpy]CÀ radical anion.
The regioselectivity shown in Scheme 1 can be explained by
a concerted mode, in which ArXCÀ undergoes nucleophilic
substitution (releasing XÀ), which in turn promotes the
electrophillic substitution (III).[19c] For the formation of 13c,
the regioselectivity can be explained by a fast, step-wise
addition process.
We have discovered a novel process that yields 1,2-
À
À
diborylarenes via Zn-catalyzed C X and C H activation of
aryl halides under mild conditions. Preliminary investigations
À
À
show that the C X/C H diborylation reaction does not
involve a free aryne, but one-electron processes seem to be
involved.
Scheme 4. Borylation of 2-fluoroiodobenzene (16a).
suggested the possibility of dtbpy reduction followed by
attack at the aryl iodide. In the absence of a Zn source, neither
the borylation nor arylation occurred (see Table S9), thus
suggesting the intermediacy of a Zn-dtbpy complex. Running
the reaction with 1a under standard conditions but in the dark
(Table S7, entry 9) had no effect on the conversion or product
distribution; therefore, the borylations are not triggered by
a photoexcited state.
Acknowledgements
T.B.M. thanks AllylChem Co. Ltd. for a generous gift of
B2pin2, and the DFG for funding. S.K.B. thanks the Alexander
von Humboldt Foundation for a postdoctoral fellowship.
À
Keywords: boronate esters · C H activation · cross-coupling ·
homogeneous catalysis · nitrogen heterocycles
To account for the observed products, we speculate that
there are two independent pathways occurring. One pathway
(A) gives monoborylated products via metathesis between
a (dtbpy)ZnII-boryl complex and ArX.[8b,11] In the second
pathway (B; Figure 2) leading to the diborylated products,
MeOK reacts with B2pin2 to generate the adduct
K+[B2pin2OMe]À,[19a,b] which reduces a (dtbpy)nZnIIXY com-
plex (n = 1 or 2; X,Y= OMe, Bpin or both) to produce a ZnII-
stabilized [dtbpy]CÀ radical. This can pass the electron to ArX
to form ArXCÀ (II), which reacts with K+[B2pin2OMe]À (in
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Received: June 17, 2015
Published online: August 18, 2015
Angew. Chem. Int. Ed. 2015, 54, 11843 –11847
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