Copper-Catalyzed Boron-Selective C(sp2)-C(sp3) Oxidative Cross-Coupling of Arylboronic Acids and Alkyltrifluoroborates Involving a Single-Electron Transmetalation Process

Article abstract of DOI:10.1021/acscatal.5b02524

A rapid and highly selective oxidative cross-coupling reaction between readily available and shelf-stable arylboronic acids and primary or secondary potassium alkyltrifluoroborates was devised and developed, which works under mild conditions using copper(II) acetate as the catalyst and silver oxide as the oxidant. Initial experimental results indicate that a single-electron transmetalation process is involved. This approach effectively bypasses the problems associated with the traditional cross-coupling reactions of alkylboronates and thus provides a complementary method in building C(sp²)-C(sp³) bonds.

Full text of DOI:10.1021/acscatal.5b02524

Letter
pubs.acs.org/acscatalysis
2
3
Copper-Catalyzed Boron-Selective C(sp )−C(sp ) Oxidative Cross-
Coupling of Arylboronic Acids and Alkyltrifluoroborates Involving a
Single-Electron Transmetalation Process
Siyi Ding, Liang Xu, and Pengfei Li*
Center for Organic Chemistry, Frontier Institute of Science and Technology (FIST), Xi’an Jiaotong University, 99 Yanxiang Road,
Xi’an, Shaanxi 710054, China
*
S Supporting Information
ABSTRACT: A rapid and highly selective oxidative cross-coupling reaction
between readily available and shelf-stable arylboronic acids and primary or
secondary potassium alkyltrifluoroborates was devised and developed, which
works under mild conditions using copper(II) acetate as the catalyst and
silver oxide as the oxidant. Initial experimental results indicate that a single-
electron transmetalation process is involved. This approach effectively
bypasses the problems associated with the traditional cross-coupling
reactions of alkylboronates and thus provides a complementary method in
2
3
building C(sp )−C(sp ) bonds.
KEYWORDS: cross-coupling, oxidation, single-electron transmetalation, organoboron, copper catalysis
4
alladium-catalyzed carbon−carbon cross-coupling reac-
tions of organometallic reagents and organic electrophiles,
elimination of alkyl palladium intermediates. In this regard,
P
first-row transition metals such as nickel have been effective as
especially the Suzuki−Miyaura reaction, have proven to be
catalysts in coupling of alkyl electrophiles by overcoming some
1
,2
3b,5−8
indispensable tools in organic synthesis. Mechanistically, the
catalytic cycle includes three two-electron processes that are
oxidative addition (OA), transmetalation (TM), and reductive
elimination (RE) (Scheme 1a). Although this scenario has been
of the above limitations.
Highly reactive and air-,
moisture-sensitive alkyl nucleophiles such as Grignard reagents,
alkylzincs, and alkyl boranes were also successfully used in
2
3
3c,9
forming C(sp )−C(sp ) bonds.
However, the more user-
2
2
very successful for C(sp )−C(sp ) bond formation, convenient
and reliable methods involving C(sp )-type reactants still
remain challenging because for alkyl electrophiles and/or
metallic reagents, some or all of these steps are slower, and the
friendly alkylboronic acid derivatives are much slower in
transmetalation, and thus, high temperatures and long reaction
times are generally necessary (Scheme 1a),
3
3a,10
although for
3
some substrates neighboring functional groups may facilitate
11
reactions are very often complicated due to facile β-hydride
the transmetalation. Very recently, Molander and co-workers
developed an elegant nickel/photoredox dual catalytic method
which allowed for efficient use of alkyltrifluoroborates in cross-
2
3
Scheme 1. Approaches toward C(sp )−C(sp ) Bonds
12
coupling under mild conditions. Thus, in the presence of
visible-light photoredox catalyst, a benzylic or later on a
secondary alkyltrifluoroborate can be oxidized to generate an
alkyl radical that undergoes facile single-electron trans-
metalation onto a nickel catalyst (Scheme 1b). Independently,
MacMillan and co-workers successfully coupled α-heteroatom
aliphatic acids, via photoredox decarboxylation to form the
13,14
radicals, with aryl halides in a similar manner (Scheme 1b).
In the past decade, oxidative cross-coupling reactions,
mechanistically via two consecutive transmetalation, have
emerged as an alternative approach to construct C−C
15,16
bonds.
The inherent challenge of these reactions is to
achieve high chemoselectivity between two organometallic
reagents avoiding undesired homocoupling processes. For
Received: November 10, 2015
Revised: January 21, 2016
©
XXXX American Chemical Society
1329
DOI: 10.1021/acscatal.5b02524
ACS Catal. 2016, 6, 1329−1333

ACS Catalysis
Letter
2
3
formation of C(sp )−C(sp ) bonds (Scheme 1c), in particular,
Table 1. Variations from the “Standard” Reaction
Conditions of the Cross-Coupling of 1a and 2a
a
Lei developed a selective palladium-catalyzed cross-coupling
17
between arylzinc and trialkylindium reagents. Cahiez used
diorganozinc to realize this type of coupling in the presence of
1
8
the iron catalyst. Severin attempted similar reaction with
Grignard reagents, but in most cases, a mixture of various
19
coupling products was formed. In contrast, a selective
2
3
b
b
oxidative C(sp )−C(sp ) cross-coupling between two organo-
boron partners has not been reported. This reaction is
interesting and potentially more advantageous because, in
comparison with other organometallic reagents, organoboronic
acid derivatives are more readily available, stable to air and
moisture, and compatible with many functional groups.
In continuing our research on boron-selective reactions, we
envisioned that a chemoselective oxidative coupling reaction
between an arylboron and an alkylboron might be used to
generate a new C(sp )−C(sp ) bond (Scheme 1d). Thus, the
arylboron compound may selectively undergo two-electron
transmetalation on a transition-metal catalyst to form
while the alkylboron component might be seletively
oxidized to an alkyl radical (R ).
entry
metal
base
yield of 3a (%)
yield of 4 (%)
c
1
2
Cu(OAc)2
CuCl2
CH ONa
90 (86 )
6
3
CH ONa
67
50
28
56
30
30
61
trace
10
71
65
2
ND
ND
ND
ND
ND
15
3
3
Cu(acac)2
CoCl2
CH ONa
3
4
CH ONa
3
5
NiCl2
CH ONa
3
20
6
Fe(acac)3
Pd(OAc)2
Pd(OAc)2
PdCl2
CH ONa
3
7
-
8
CH ONa
31
3
2
3
9
CH ONa
10
3
10
11
12
13
14
15
-
CH ONa
ND
9
3
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
NaOAc
KOAc
2
1,22
[
A ],
ND
ND
ND
ND
ND
trace
Int
·
12,23
A single-electron
Int
Cs CO3
2
·
transmetalation process between R and [A ] then might
take place to form the high-valent intermediate [B ], which
should undergo facile reductive elimination to form the desired
K PO4
3
13
28
40
15
t-BuOK
Int
d
16
CH ONa
3
1
2−14,24
e
product.
This design would bypass the low reactivity of
17
CH ONa
3
a
alkylboronic acid derivatives in conventional transmetalation as
well as the undesired β-hydride elimination due to slow
reductive elimination. However, several key challenges must be
addressed in developing such a reaction. First, suitable
oxidant(s) need to be identified to selectively oxidize the
alkylboron compound and to regenerate the metal catalyst.
Second, the rates of generation of the highly reactive alkyl
Reaction conditions: 1a (0.4 mmol, 1.0 equiv), 2a (1.5 equiv),
catalyst (10 mol %), Ag O (2.0 equiv), base (1.0 equiv), solvent (2.0
2
b
mL), distilled water (40 μL), room temperature, 10 min. Determined
by GC analysis of the reaction mixture using tridecane as an internal
c
standard; the yield was based on 1a. Yield of the isolated product.
d
The reaction was conducted without addition of distilled water.
e
Reaction was run in air. ND: not determined.
radical and formation of [A ] must be balanced to avoid
Int
dimerization and/or hydrogenation of the radical, or second
transmetalation of arylboron reagents that will lead to a
homocoupling product. Third, the competitive C−X reductive
first-row transition-metal catalysts such as cobalt, nickel, iron
were all inferior (entries 4−6), while palladium acetate led to
low selectivity as significant amount of 4,4′-dimethoxy-1,1′-
biphenyl (4) as homocoupling product was observed (entries
7−9). In the absence of copper source, 10% of 3a was observed
(entry 10), presumably due to metal im-purities in commercial
elimination from [B ] should be minimized.
Int
With these considerations in mind, we commenced our
investigations toward a boron-selective oxidative cross coupling
using potassium 3-phenylpropyl trifluoroborate (1a) and 4-
methoxyphenylboronic acid (2a) as the model substrates
25
Ag O. Interestingly, the effect of sodium methoxide as a base
2
additive was dramatic, because other bases only resulted in
moderate to low yields (entries 11−15). Furthermore, addition
of small amount of distilled water was very important in this
reaction, otherwise the yield decreased dramatically (entry 16).
When 4-methoxyphenyl pinacol boronate, potassium trifluor-
oborate, or the corresponding boroxine was used in place of 2a,
little 3a was formed. These facts suggest that a possible role of
water might be keeping the arylboron species in the reactive
boronic acid form. In the absence of or using oxidants other
(
Table 1). After extensive experimental studies, we were able
to obtain the desired product 3a in high efficiency under the
standard” conditions. Thus, using copper(II) acetate (0.1
“
equiv) as the catalyst, silver(I) oxide (2.0 equiv) as the oxidant,
sodium methoxide (1.0 equiv) as a basic additive, a mixture of
1
a (1.0 equiv) and 2a (1.5 equiv) in toluene (0.2 M) was
stirred under nitrogen atmosphere at room temperature for just
0 min and 3a was formed in 90% GC yield. After filtration and
1
purification by column chromatography, 3a was isolated in 86%
yield (Entry 1). The rapid formation of a C−C bond under
exceptionally mild conditions is remarkable. The reaction was
not moisture-sensitive, and therefore, rigorous drying of the
solvent and reagents was not necessary. More importantly,
excellent chemoselectivities were achieved, because the
potential protodeboronation products of 1a and 2a, homocou-
pling product of 1a were only observed in trace amounts,
whereas the homocoupling product of 2a was formed in 6%
GC yield.
Variations from the “standard” conditions led to significantly
lower efficiency, and some representative results are shown in
Table 1. Other sources of copper catalyst resulted in lower
yields (entries 2 and 3). Under otherwise identical conditions,
than Ag O, no reaction was observed, indicating the unique
2
roles of Ag O in promoting this reaction. Attempt to run the
2
reaction in air atmosphere resulted in very low yield (entry 17).
Finally, the reactions were conducted at 1.0 and 5.0 mmol
scales, producing 3a in 85% and 75% isolated yields,
respectively (for more details, see SI).
Having identified the optimal reaction conditions, we next
investigated the substrate scope of this transformation. We first
varied the arylboronic acid under the standard reaction
conditions, and the results are shown in Scheme 2. Various
arylboronic acids with electron-donating or -withdrawing
substituents at para- or meta-positions are good substrates,
cleanly forming the desired oxidative cross-coupling products in
good to high yields. Chlorine and bromine substituents are
1
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Letter
Scheme 2. Scope of the Arylboronic Acid Component
5-hexenyltrifluoroborate was reacted with 2a under the
standard conditions, a mixture of the normal linear product
3
ad and a cyclized product 3ae (1:1.5) was obtained,
suggesting the intermediacy of primary alkyl radical. Fur-
thermore, secondary alkyltrifluoroborates were also viable
substrates under the current conditions, although the yields
are moderate (3af−3ah).
In order to probe the reaction mechanism, 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO, 1.1 equiv) was added
as a radical scavenger to the reaction mixture of 1a and 2a
under the standard conditions. The formation of 3a (14%
yield) was significantly suppressed and the alkyl-TEMPO
adduct 5 could be isolated in 70% yield (Scheme 4a). No aryl-
Scheme 4. Preliminary Mechanistic Studies
a
Yield of the isolated product.
compatible (3c and 3d), thus representing orthogonal reactivity
with conventional palladium-catalyzed cross-coupling. Alcohol
and ester groups, which may not be used in reactions of
organomagnesium or organozinc reagents, are also compatible
(3h, 3i, 3q). Furanyl and thiophenyl boronic acids were also
found to successfully paticipate the current reaction (3k−3n).
Finally, an alkenylboronic acid also served as a viable substrate,
producing the coupling product (3t) in 55% yield.
Next, we briefly investigated the scope of alkyl trifluor-
oborates (Scheme 3). Primary alkyl trifluoroborates bearing
Scheme 3. Scope of the Potassium Alkyltrifluoroborate
Component
TEMPO adduct was observed. The independent oxidation of
1
a using either Ag O (2.0 equiv) or Cu(OAc) (2.0 equiv) as
2
2
the sole oxidant in the presence of TEMPO was also conducted
Scheme 4b). Interestingly, while Ag O was a very effective
(
2
oxidant to produce 5 in high yield, Cu(OAc) was totally
2
unreactive under the current conditions. This is remarkable
because Cu(OAc) had been used before in different conditions
2
23
as effective oxidant of alkyltrifluoroborate.
On the basis of the experimental results, we propose a
catalytic cycle for the present oxidative cross-coupling reaction,
as depicted in Scheme 4c. Transmetalation of the arylboronic
acid onto the Cu(II) catalyst forms ArCu(II)X species, in
analogy to the commonly accepted process in Chan−Lam−
22
Evans reaction. An alkyl radical, generated from Ag O-
2
mediated oxidation of the potassium alkyltrifluoroborate,
undergoes a single-electron transmetalation onto ArCu(II)X
to form a three-valent copper species ArCu(III)RX which then
undergoes C−C reductive elimination to produce the desired
product ArR and Cu(I)X. Oxidation of Cu(I)X again by Ag O
2
regenerates Cu(II)X catalyst to close the catalytic cycle.
2
In summary, we have discovered a copper-catalyzed boron-
selective oxidative cross-coupling reaction between readily
available arylboronic acids and potassium alkyltrifluoroborates.
This reaction features excellent product selectivity, short
reaction time, mild reaction conditions, simple operation, and
broad functional group tolerance. In comparison with the
conventional Suzuki−Miyaura coupling, the current reaction
requires one additional step for the preparation of an
organoboron reactant from the corresponding halide. However,
considering that many shelf-stable aryl and alkyl organoboron
compounds are already commercially available, this reaction
should be useful in facilitating small-molecule array synthesis
due to its characteristics mentioned above. Notably, due to the
a
Yield of the isolated product.
functional groups such as ester (3w, 3x), amide (3y), and even
ketone (3v) were all suitable coupling partners. Additionally,
alkyl trifluoroborates with a thienyl group was compatible (3z).
When branched alkyl trifluoroborates were used, moderate
yields were obtained presumably due to steric hindrance (3x,
3
ab). This notion was also consistent with the relatively low
yield of 3r or 3s where ortho-methoxylphenyl boronic acid and
ortho-methylphenyl boronic acid was used (Scheme 2). When
1
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Letter
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alkyltrifluoroborate and a single-electron transmetalation of
the resulting alkyl radical onto copper species are likely
involved as the key steps. Future studies on the mechanism and
methods for controlled generation of the alkyl radical may
further improve this coupling reaction.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b02524.
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AUTHOR INFORMATION
Corresponding Author
E-mail: lipengfei@mail.xjtu.edu.cn.
■
*
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful for financial support by the National Science
Foundation of China (No. 21472146), the Ministry of Science
and Technology of PRC (973 program for young scientists, No.
014CB548200), the Department of Science and Technology
of Shaanxi Province (No. 2015KJXX-02) and Xi’an Jiaotong
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25) Ag O used in the present study was purchased from Alfa
(
2
3
(
2
(
2
(
>99.99%), and the copper content was determined to be 10 ppm by
inductively coupled plasma-atomic (Optical) emission spectrometry
ICP-AES). For roles of trace metal impurities in catalysis, see:
Thome, I.; Nijs, A.; Bolm, C. Chem. Soc. Rev. 2012, 41, 979−981.
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1
333
DOI: 10.1021/acscatal.5b02524
ACS Catal. 2016, 6, 1329−1333