Efficient synthesis of aryl boronates via zinc-catalyzed cross-coupling of alkoxy diboron reagents with aryl halides at room temperature

Article abstract of DOI:10.1021/ol502120q

A zinc(II)/NHC system catalyzes the borylation of aryl halides with diboron (4) reagents in the presence of KOMe at rt. This transformation can be applied to a broad range of substrates with high functional group compatibility. Radical scavenger experiments do not support a radical-mediated process.

Full text of DOI:10.1021/ol502120q

Letter
pubs.acs.org/OrgLett
Efficient Synthesis of Aryl Boronates via Zinc-Catalyzed Cross-
Coupling of Alkoxy Diboron Reagents with Aryl Halides at Room
Temperature
Shubhankar Kumar Bose and Todd B. Marder*
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: A zinc(II)/NHC system catalyzes the boryla-
tion of aryl halides with diboron (4) reagents in the presence
of KOMe at rt. This transformation can be applied to a broad
range of substrates with high functional group compatibility.
Radical scavenger experiments do not support a radical-
mediated process.
The conditions screened for optimizing the catalytic
rylboronic acids and arylboronates have attracted signifi-
Acant attention over the past few decades, due to their
extensive applications in organic synthesis, particularly in
transition-metal-catalyzed C−C, C−O, and C−N bond-forming
reactions, as exemplified by the Suzuki−Miyaura cross-coupling
reaction.1 In comparison with other types of organometallic
carbon nucleophiles, arylboronates are most frequently used
owing to their ease of handling, functional group compatibility,
and low toxicity.1a Recent strategies for preparing arylboronates
include transition-metal-catalyzed cross-coupling reactions of
alkoxy diboron or alkoxy borane reagents with aryl halides2 or
direct C−H borylation of aromatic substrates.3 Significant
examples include Pd-,4 Ni-,5 or Cu-catalyzed6 borylation of aryl
halides, and the selective iridium-catalyzed C−H borylation of
aromatic and heteroaromatic substrates.3,7
In 2009, our group reported that CuI, in the presence of
inexpensive phosphine ligands, catalyzes the borylation of aryl
halides with diboron reagents to generate arylboronic esters in
good to excellent yields.6b Recently, Ito et al. reported the
transition-metal-free borylation of aryl halides using a more
expensive silylborane reagent and metal alkoxide base,8 and
Zhang et al. reported the Cs2CO3 mediated borylation of aryl
iodides, but not aryl bromides, with diborane reagents in the
presence of methanol.9 The Zn-catalyzed borylation of aryl
halides at elevated temperatures (aryl iodides at 75 °C and aryl
bromides at 120 °C) purportedly via a borylzincate generated in
situ from highly pyrophoric diethylzinc, diboron, and metal
alkoxide was very recently reported by Uchiyama et al.10 We
recently developed an efficient Zn-catalyzed borylation of
unactivated alkyl halides to furnish alkylboronates at rt.11 In
comparison to noble metals, zinc catalysis for such processes is
underdeveloped.12 Zinc is an attractive alternative to the
commonly used metals (Pd or Ni), because it is inexpensive
(0.07 € per mol), relatively abundant (0.0076% in the earth
crust), environmentally benign, and less toxic. Herein we
describe a novel Zn-catalyst system for the borylation of aryl
halides with alkoxy diboron reagents under mild conditions.
borylation method using 4-iodotoluene (1a) as the substrate
are shown in Table 1. With ZnBr2 as the zinc source and B2pin2 as
the diboron reagent, the corresponding boronate 4-
MeC6H4Bpin 1b was formed in good yield (85%, entry 2) at rt
in 6 h using IMes (L1) as the ligand (IMes = 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene) in MTBE (methyl-tert-
butylether), while ZnCl2 and ZnI2 (entries 1 and 3) provided
the product in slightly lower yields. In the absence of a Zn source,
only a trace amount of desired product was obtained; thus, any
uncatalyzed reaction is minimal (entry 4). A moderate yield
(58%) was obtained upon using a reduced catalyst loading of 5
mol % (entry 5), whereas a higher concentration gave only a
slightly higher yield (89%, entry 6). Next, the effects of base were
examined (entries 7−12). The use of LiOtBu was ineffective
(entry 7). In contrast to the Zn catalyzed borylation of alkyl
halides, KOtBu was inferior to KOMe (entry 8).11 The
replacement of KOMe by Cs2CO3 or NaOtBu gave only trace
product, whereas NaOMe was only somewhat less effective
(entries 9−11). When base was omitted from the reaction
mixture, no arylboronic ester was formed (entry 12).
Different NHC ligands that were examined (L2−L5) provided
lower yields than L1 (entries 13−16). Ligand is essential for this
catalytic borylation; no significant yield was obtained when the
ligand was omitted (entry 17). Finally, we investigated the effect
of the solvents (Table 1, entries 18−22). The use of THF
afforded results comparable to those for MTBE (79% yield, entry
18), and both benzene and toluene provided the product in
slightly lower yields (entries 19 and 20, respectively). The use of
the more polar coordinating solvents N,N-dimethylformamide
(DMF) or acetonitrile (CH3CN) resulted in a moderate or
limited reaction, respectively (entries 21 and 22).
With the optimized reaction conditions identified, we
examined the scope of the present borylation reaction with
different aryl halides, as summarized in Table 2. Aryl halides with
Received: July 18, 2014
© XXXX American Chemical Society
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Table 1. Optimization of the Reaction Conditions for the
Zinc-Catalyzed Borylation of 4-MeC6H4I, 1a
Table 2. Screening of Aryl Halides for the Zinc Catalyzed
a
a
Borylation Reaction
a
Reaction conditions: 4-MeC6H4I, 1a (0.5 mmol, 1 equiv), Zn source
(10 mol %), ligand (20 mol %), base (1.5 equiv), B2pin2 (1.5 equiv),
b
MTBE (2 mL), at rt. The yields were determined by GC-MS analysis
c
vs a calibrated internal standard and are averages of two runs. Isolated
d
yield after chromatographic workup. Complete conversion was
e
observed, determined by GC-MS analysis (see Table S5). 10 mol
f
% of L1 used. 30 mol % of L1 used.
different electronic properties as well as different halogen
substituents were investigated. Neutral (2a, 3a) or electron-
withdrawing substituents, such as ester (4a), cyano (5a), and
trifluoromethyl (6a) groups, are well tolerated, affording the
desired arylboronates in moderate to good yields. Aryl bromides
can also be utilized (7a, 12a, 16a, 19a−23a). Furthermore, the
effects of the halides were investigated with para-CF3-containing
substrates (6a−8a); 4-bromobenzotrifluoride is readily con-
verted into 7b (yield = 65%), but 4-chlorobenzotrifluoride was
only borylated in 18% yield (NMR), even at higher temperature
(50 °C), and the addition of (Bu4N)I did not facilitate the
transformation.
4-Bromophenyl boronate (9b) was the sole borylated product
from the reaction of 4-bromophenyl iodide (9a) with 1 equiv of
B2pin2/KOMe. 1,4-Diiodobenzene (10a) also underwent
predominantly monoborylation when only 1 equiv of the
borylating reagent was used (Table 2, entry 10), but with 3
equiv of B2pin2/KOMe, the 1,4-diborylated product 11b was
formed in good yield (entry 11). Reactions with electron-rich
substrates containing o- or p-Me or -OMe groups (1a, 13a−16a)
also gave the desired boronates (1b, 13b−16b, respectively) in
moderate to good yields, and 4-bromo-N,N-dimethylaniline
(12a), containing a strong π-donor substituent, gave a 62% yield
a
Reaction conditions: aryl halide (typically 0.5 mmol, 1 equiv), B2pin2
(1.5 equiv), ZnBr2 (10 mol %), L1 (20 mol %), KOMe (1.5 equiv),
MTBE, rt for 6−12 h (see Supporting Information for details).
b
Isolated yield after chromatographic workup unless otherwise stated.
c
d
Approximate ArH yields were estimated by GC-MS analysis. Yield
e
1
was determined by H NMR analysis. ArH peak was not separated
f
g
from the solvent peak. The reaction was performed at 50 °C. The
reaction was performed using 1 equiv of B2pin2 and 1 equiv of KOMe.
h
1,4-(Bpin)2C6H4 (7%) and C6H5Bpin (4%) were also detected by
i
GC-MS. The reaction was performed using 3 equiv of B2pin2 and 3
equiv of KOMe. C6H5Bpin (25%) and 4-IC6H4Bpin (3%) were also
detected by GC-MS. The diboron reagent changes from B2pin2 to
B2neop2.
j
k
of the corresponding arylboronic ester, along with a small
amount of dimethylaniline via hydrodehalogenation.6b
Sterically congested, ortho-substituted phenyl derivatives were
borylated in moderate to good yields (entries 2, 14−16).
However, 2-iodo-1,3,5-trimethylbenzene (17a; disubstituted at
both ortho positions) reacted to give the borylation product in
only 16% yield (17b), along with unreacted starting material, as
B
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Letter
indicated by GC-MS analysis. This zinc-catalyzed method is also
applicable to the borylation of a variety of heteroaryl halides, such
as thiophene, benzothiophene, quinoline, and furan systems
(entries 18−23). Furthermore, this protocol shows not only a
wide scope with respect to the aryl halide substrate but also that
bis(neopentylglycolato)diboron13 can be substituted for bis-
(pinacolato)diboron, giving the corresponding aryl boronates
(24b−26b) in moderate yields.
This zinc-catalyzed borylation reaction can also be successfully
applied to the alkenyl bromide 27a (β-bromostyrene; E/Z =
88:12) to give a 67% yield of the corresponding alkenyl boronate
ester 27b (mixture containing E/Z = 90:10), but at 50 °C
(Scheme 1). The borylation reaction does have some substrate
28a at 50 °C afforded arylboronic ester product 28b in 37% yield,
along with the cyclized (borylmethyl)indane product 28c in 21%
yield. The formation of cyclized product 28c suggested the
possibility of a radical mechanism, although insertion of alkene
into a Zn-aryl bond cannot be excluded.16 Therefore, we
examined the reaction of 6a in the presence of a radical scavenger
(Scheme 3). Addition of 9,10-dihydroanthracene or TEMPO
Scheme 3. Mechanistic Investigations with 9,10-
Dihydroanthracene as a Radical Scavenger
Scheme 1. Borylation of β-Bromostyrene, 27a
had no effect, indicating that the borylation does not appear to be
a radical process (similar results were obtained with 1a, 12a, and
13a; increasing the radical inhibitor to 3 equiv did not inhibit the
borylation reaction).
To shed further light on this reaction mechanism, we
endeavored to prepare a Zn−boryl complex. First, we prepared
the Zn−bis(aryloxide) complex (NHC)Zn(O-2,6-iPr2C6H3)2 via
alcoholysis of (NHC)ZnEt2 (NHC = L1) with 2,6-diisopropyl-
phenol.17 Reaction of this Zn−bis(aryloxide) with 2 equiv of
B2pin2 in [D8]THF at rt was examined by in situ 11B NMR
spectroscopy. The original 11B signal of B2pin2 at δ = 30.5 ppm
was partially transformed into two new signals at δ = 38.7 and
21.7 ppm. The former resonance could be assigned to a Zn−
boryl complex,6b,12a,18 and the latter to the aryloxy-Bpin species
(low intensity signals between 5 and 2 ppm were also
observed).19 Interestingly, when 4-iodotoluene (1a) was added
to the above mixture, the signal at δ = 38.7 ppm assigned to the
Zn−boryl complex partially disappeared and a signal corre-
sponding to the 4-tolylBpin appeared at δ = 30.9 ppm
(overlapped with the B2pin2 signal) in the 11B NMR spectrum
(the presence of the arylboronic ester product was confirmed by
GC-MS). Therefore, it is possible that the mechanism of the
present borylation reactions is related in some ways to that of the
previously reported Cu-catalyzed borylation reactions.6b Further
detailed mechanistic investigations are underway to establish
unambiguously the reaction pathway.
In summary, we have demonstrated a novel and efficient Zn-
catalyzed cross-coupling reaction of aryl halides with diboron
reagents, under mild conditions. The low associated cost and low
toxicity of Zn support it as an attractive alternative to the widely
employed Pd- and Ni-based systems. This method tolerates a
variety of functional groups with both iodo- and bromo-
substituted aryls (although alcohol and aldehyde are not
compatible) and can be extended to heteroaryl halides, providing
easy access to the corresponding aryl- and heteroarylboronates in
good yields. Our Zn(II)−NHC catalyst is much more active than
that recently reported by Uchiyama et al., and it avoids the use of
pyrophoric Et2Zn.
restrictions. For example, when hydroxy- or aldehyde-substituted
arenes were used, the aryl boronate was detected in only trace
amounts by GC-MS and was not readily isolated.
As illustrated in Table 2, in some cases, the formation of the
hydrodehalogenation byproduct, ArH, was observed. The
hydrodehalogenating byproduct was previously detected during
Pd- and Ni-catalyzed coupling reactions of aryl halides with
dialkoxyhydroboranes,4c−e,h,5a−c and recently in Cu-catalyzed
borylation reactions involving alkoxy diboron reagents.6b We
attempted to determine the hydrogen source in our borylation
reaction by conducting the reaction with 4-iodoanisole 13a using
10 mol % ZnBr2, 20 mol % L1, 1.5 equiv of B2pin2, and KOMe in
[D8]THF for 12 h at rt; however, no deuterium incorporation
into the hydrodehalogenation product was observed by GC-MS.
Thus, solvent is not a hydrogen atom source in this reaction.
Moreover, the addition of D2O (1 equiv) to the reaction mixture
shut down the reaction almost completely (trace of 13b,
determined by GC-MS analysis). However, the borylation of 13a
in the presence of 20 mol % D2O led to selective deuteration at
the para position of the resulting anisole (determined by GC-MS
analysis and 2H NMR spectroscopy),14 along with the borylation
product in only 8% yield (13b). Hydrodehalogenation was also
observed when the borylation of 13a was conducted using
rigorously dried solvent in an argon-filled glovebox, but in small
amounts (observed by GC-MS). Surface OH groups on the glass
vessels were ruled out as the cause of this undesired side reaction
by a test using plasticware.
The mechanism of this Zn-catalyzed borylation of aryl halides
remains under investigation. We first explored the possibility of a
radical-mediated mechanism by performing the reaction on o-(3-
butenyl)bromobenzene (28a) (Scheme 2). The corresponding
aryl radical has been reported to undergo 5-exo-trig cyclization to
form 1-methylindane after H-atom abstraction.15 The reaction of
Scheme 2. Borylation of o-(3-Butenyl)bromobenzene, 28a
ASSOCIATED CONTENT
■
S
* Supporting Information
Details of experimental procedures and 1H, 13C, 11B spectra and
GC-MS of the products. This material is available free of charge
via the Internet at http://pubs.acs.org.
C
dx.doi.org/10.1021/ol502120q | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Y.; Bannwarth, P.; Olivero, S.; Dunach, E.; Chavant, P. Y. Org. Lett. 2014,
16, 2366.
̃
AUTHOR INFORMATION
■
Corresponding Author
(7) (a) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.; Anastasi, N. R.;
Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390. (b) Cho, J.-Y.; Tse, M. K.;
Holmes, D.; Maleczka, R. E.; Smith, M. R., III. Science 2002, 295, 305.
(c) Mkhalid, I. A. I.; Coventry, D. N.; Albesa-Jove, D.; Batsanov, A. S.;
Howard, J. A. K.; Perutz, R. N.; Marder, T. B. Angew. Chem., Int. Ed.
2006, 45, 489. (d) Harrisson, P.; Morris, J.; Steel, P. G.; Marder, T. B.
Synlett 2009, 147. (e) Tajuddin, H.; Shukla, L.; Maxwell, A. C.; Marder,
T. B.; Steel, P. G. Org. Lett. 2010, 12, 5700. (f) Hurst, T. E.; Macklin, T.
*E-mail: todd.marder@uni-wuerzburg.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
K.; Becker, M.; Hartmann, E.; Kugel, W.; Parisienne-La Salle, J.-C.;
̈
S.K.B. thanks the Alexander von Humboldt Foundation (AvH)
for a postdoctoral fellowship. T.B.M. thanks AllylChem Co. Ltd.
for a generous gift of B2pin2 and B2neop2, and the Deutsche
Forschungsgemeinschaft (DFG) for funding.
Batsanov, A. S.; Marder, T. B.; Snieckus, V. Chem.Eur. J. 2010, 16,
8155. (g) Crawford, A. G.; Liu, Z.; Mkhalid, I. A. I.; Thibault, M.-H.;
Schwarz, N.; Alcaraz, G.; Steffen, A.; Collings, J. C.; Batsanov, A. S.;
Howard, J. A. K.; Marder, T. B. Chem.Eur. J. 2012, 18, 5022.
(h) Tajuddin, H.; Harrisson, P.; Bitterlich, B.; Collings, J. C.; Sim, N.;
Batsanov, A. S.; Cheung, M. S.; Kawamorita, S.; Maxwell, A. C.; Shukla,
L.; Morris, J.; Lin, Z.; Marder, T. B.; Steel, P. G. Chem. Sci. 2012, 3, 3505.
(i) Kawamorita, S.; Murakami, R.; Iwai, T.; Sawamura, M. J. Am. Chem.
Soc. 2013, 135, 2947. (j) Konishi, S.; Kawamorita, S.; Iwai, T.; Steel, P.
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