Borylation of primary and secondary alkyl bromides catalyzed by Cu2O nanoparticles

Article abstract of DOI:10.1039/c5ra06631j

A Cu₂O nanoparticle catalyzed borylation of activated and unactivated alkyl bromides is developed, using bis(pinacolato)diboron as a boron source. To the best of our knowledge this is the first report of a heterogeneous Cu₂O nanocata

Full text of DOI:10.1039/c5ra06631j

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Borylation of Primary and Secondary Alkyl Bromides
Catalyzed by Cu O Nanoparticles
2
Cite this: DOI: 10.1039/x0xx00000x
a
a
a
b
b
XinꢀFeng Zhou, YaꢀDong Wu, JianꢀJun Dai, YongꢀJia Li, Yu Huang* and Huaꢀ
Jian Xu*
a
Received 00th January 2014,
Accepted 00th January 2014
DOI: 10.1039/x0xx00000x
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A Cu O nanoparticle catalyzed borylation of activated and
2
unactivated alkyl bromides is developed, using bis(pinacola-
-
to)diboron as boron source. To our best knowledge it is the
first report of heterogeneous Cu O nanocatalyst applied in
2
direct synthesis of primary and secondary alkylboronic
esters. Our catalytic system features in mild reaction
conditions, high yield (nearly 100%) and absence of ligands
which otherwise are essential in homogenous catalysis.
Alkylboron compound is an important synthetic intermediate in
medicinal chemistry due to its wide application in SuzukiꢀMiyaura
1
crossꢀcoupling reaction. Traditionally, alkylboron compounds are
synthesized via borylation of alkyllithium or alkylmagnesium
2
reagents with suitable boron compounds. These procedures suffer
from low shelf stability and poor compatibility with limited
3
functional groups. In order to develop more practical synthetic
routes, several protocols with transition metal based homogenous
catalyst have been studied lately, including Rhꢀ and Irꢀcatalyzed
4
olefin hydroboration, Ruꢀ, Rhꢀ, Irꢀ and Reꢀ catalyzed alkane CꢀH
5
borylation, as well as Pdꢀ, Niꢀ, Fe, Cuꢀ and so on catalyzed βꢀ
borylation of unsaturated carbonyl compounds. Very recently,
6ꢀ8
Scheme 1. Synthetic methods of alkylboronic esters from alkyl halides.
researchers successfully developed a new approach utilizing alkyl not require ligand. To our best knowledge, it is the first Cu O
2
halides or pseudohalides as substrates. Liu and coꢀworkers have nanocatalyst documented, in comparison to other homogenous
shown that CuI and Pd (dba) could both catalyze the borylation of catalysts for this reaction (Scheme 1).
2
3
9
alkyl halides (Scheme 1, a, c). Cu NPs exhibit good catalytic
We chose 1ꢀbromoꢀ2ꢀphenylethane (1a) as a model substrate and
9
activity in this reaction (Scheme 1, b). Tertiary alkyl electrophiles bis(pinacolato)diboron (B pin ) as a diboron reagent. CuI (10 mol%)
2
2
9
were first achieved by Fu and coꢀworkers (Scheme 1, d). Although was used as a starting catalyst. Limited product (4%) was found
this method is proved efficient, there are also disadvantages: the under ligandꢀfree condition, with dimethylformamide (DMF) as a
9
catalyst input dosage is relatively high; complicated experimental solvent (Table 1, entry 1). We then used bulk Cu O (10 mol%) in
setup or expensive ligand is required.
2
the reaction system (Table 1, entry 2), and small amount of desired
In our previous work, we found CuI nanoparticles have high product (12%) was detected. It inspired us to study further on the
catalytic activity in hydroxylation, amination, and thiolation of aryl activity of Cu O nanoparticles (NPs) prepared according to
2
11
halides. The reaction condition is environmentally benign without literature. Carbon black (CB) was used as a support material, in
1
0
12
ligand presence. It would be interesting to explore the catalytic order to prevent nanoparticles aggregation. Control experiment
performance of nanoparticle in direct borylation of alkyl halides. confirmed that no conversion occurred with only CB (Table 1, entry
13
Herein, we report a nanostructure Cu O catalyst for the synthesis of 3). When Cu O NPs/CB (0.8 mol% Cu O) was used as a catalyst,
2
2
2
activated or unactivated primary and secondary alkylboron the yield of product improved significantly to 56% (Table 1, entry 4),
compounds through borylation of the corresponding alkyl bromides. indicating this nanocatalyst substantially exhibits better catalytic
Our results demonstrate that it is effective in terms of reaction yield activity. Further, we tuned different experimental parameters to
with wide applicability for various alkyl functional groups, and does
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Journal Name
a
Table 1. Optimization of the Reaction Conditions.
bromide could be borylated (52%), while aryl bromide does not react
simultaneously. It reveals that aryl halides group in the substrate is
stable during the reaction. When two bromines are contained in the
substrate (3n), both of them can be borylated with 3.0 equivalents of
B pin .
2
2
b
Entry
Catalyst
CuI
Base
Solvent
Time yield [%]
h)
(
1
2
3
4
5
6
7
8
9
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
NaOH
DMF
DMF
DMF
DMF
THF
24
18
18
18
18
18
18
18
18
18
18
18
12
8
4
Cu
CB
2
O
12
NR
56
42
78
97
51
65
88
11
14
97
89
60
58
NR
56
95
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
2
O NPs /CB
O NPs /CB
O NPs /CB
O NPs /CB
O NPs /CB
O NPs /CB
O NPs /CB
O NPs /CB
O NPs /CB
2
2
2
2
2
2
2
2
MeOH
EtOH
tꢀBuOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
1
0
tꢀBuONa
1
1
K
K
3
2
CO
PO
3
1
1
1
1
1
1
1
2
3
4
5
6
7
8
9
4
Cu
2
O NPs /CB
O NPs /CB
O NPs /CB
O NPs /CB
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
Cu
Cu
Cu
2
c
2
2
12
12
12
12
12
d
CuO NPs /CB
e
f
Cu
Cu
2
O NPs /CB
O NPs /CB
1
2
a
Reaction conditions: 1ꢀbromoꢀ2ꢀphenylethane (0.25 mmol), B
O NPs/CB (0.8 mol % Cu
2
Pin
2
(0.375
O) under Ar. GC
b
mmol), base (0.5 mmol), Cu
2
2
c
d
yield using biphenyl as an internal standard. With 1 equiv base. With 3
e
f
2 2
mg Cu O NPs/CB. In air. 4.0 equiv of H O was added.
maximize catalytic efficiency. The solvent was changed to
tetrahydrofuran (THF), methanol (MeOH), ethanol (EtOH) and tertꢀ
butyl alcohol (tꢀBuOH). Highest yield (97%) was obtained in EtOH
Scheme 2. Substrate scope of the borylation reaction of primary alkyl
bromides. Reaction conditions: alkyl bromide (0.25 mmol), B Pin (0.375
mmol), MeOLi (0.5 mmol), EtOH (1.5 mL), and Cu O NPs/CB (0.8 mol %
O) at room temperature under Ar atmosphere for 12 h. All yields are
2
2
2
trial (Table 1, entries 5ꢀ8). In terms of base effect, we learnt sodium Cu
2
a
b
Isolated yields. 1e is 1ꢀiodopentane.
2 2
B Pin (0.75 mmol).
hydroxide (NaOH), sodium tertꢀbutoxide (tꢀBuONa), potassium
carbonate (K CO ) and potassium phosphate (K PO ) were inferior
2
3
3
4
to lithium methoxide (LiOMe) (Table 1, entries 9ꢀ12). The optimal
reation time was proved to be 12 h (Table 1, entries 13ꢀ14).
Additionally, lower yields were obtained when the amount of base or
Cu O nanoparticles was reduced (Table 1, entries 15ꢀ16), indicating
2
conversion rate is proportional to both base concentration and
catalyst dosage. Since CuO may grow when Cu O nanoparticles are
2
exposed in air, we also examined the catalytic effect of pure CuO
nanoparticles, which shows no catalytic activity (Table 1, entry 17).
To quantitatively assess the air influence particularly during batch
processing, we tested our catalytic reaction in air without argon
protection, which indeed gave negative impact (yield drops to 56%)
(Table 1, entry 18). Lastly, we discovered this catalysis system was
fairly insensitive to moisture, because adding 4 equivalents of water
only reduces the yield to 95% (Table 1, entry 19).
In view of this efficient catalytic system in hand, we next
extended the scope of the substrate to various primary alkyl
bromides (Scheme 2). The isolated yield of 3a is 92%. Linear chain
Scheme 3. Substrate scope of the borylation reaction of secondary alkyl
bromides. Reaction conditions: alkyl bromide (0.25 mmol), B Pin (0.375
2
2
or branched chain alkyl bromides give high yield (3b, 3c, 3d, 3e). mmol), MeOLi (0.5 mmol), EtOH (1.5 mL), and Cu O NPs/CB (0.8 mol %
2
2
Alkyl iodide (3f) gives a higher conversion rate while alkyl chloride Cu O) at room temperature under Ar atmosphere for 12 h. All yields are
Isolated yields.
do not react at all. This reactivity distinction can be used to
selectively borylate the CꢀBr bond when CꢀCl (3g) bond coexist in
the reaction system. Other useful functional groups such as ester
In addition to primary alkyl borylation, secondary and tertiary
alkyl group borylation is an emerging research focus in this field. In
Pd based catalysis systems, secondary alkyl bromide cannot be
(
3h), ketal (3i), ether (3j) and cyano (3k) groups are well tolerated in
9
borylated. Nonetheless, Cu based catalysts helps borylate primary
the current catalytic system, since isolated yields of desired
alkylboronates range from 50% to 92%. Benzyl bromide (3l) as a
reactive electrophile which can be hardly borylated starting from
alkyllithium or alkylmagnesium reagents, gives 54% yield in our
catalytic reaction. In further studies, we found if aryl bromide and
alkyl bromide were both present in the substrate (3m), only alkyl
electrophile, but secondary and tertiary electrophiles have seldom
9,14,15
been active.
Therefore, we also would like to examine the effect
of the catalyst on secondary and tertiary electrophiles (Scheme 3).
Cyclic secondary alkyl bromides with different ring sizes (5a–5d)
can be successfully borylated: cycloheptyl bromide yields 71%,
2
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cyclohexyl 68%, cyclopentyl 75% and cyclobutyl 50%, respectively. observed and two additional peaks at 934.0 eV and 953.8 eV could
Other substrates, such as bicyclic (5e, 5f) and acyclic (5g) bromides be indexed to Cu(II) 2p3/2 and Cu(II) 2p1/2 of CuO, respectively. It is
can also be converted. When functional group such as cyano (5h) possible attributed to partial oxidation of Cu O to CuO. The Xꢀray
2
exists in the substrate, high yield can be obtained (85%). Still, diffraction (XRD) pattern is shown in Figure 1d. The diffraction
o
tertiary electrophiles were unsuccessful (not shown in Scheme 3).
peak of 2θ = 25 can be assigned to CB. All other diffraction peaks
Furthermore, other diboron reagents were used in our reaction could be perfectly indexed to Cu O (JCPDS, No. 78ꢀ2076). The
2
system to evaluate the tolerance of our Cu O nanocatalyst under recovered catalyst was characterized by TEM and XRD (Figure 1e,
2
different boron sources. We found in the presence of 1f). It can be seen from TEM image that most nanoparticles still
bis(neopentylglyꢀcolato)diboron (B (neop) ), primary alkyl (7a), maintain integrity. The average size increases a bit, probably due to
2
2
secondary alkyl (7b) and secondary alkyl with cyano group (7c) Ostwald ripening effect during catalytic process. The XRD pattern
o
substrates can be catalyzed successfully, although yields of these shows a new peak around 39 between Cu O (111) and (200) peaks
2
reactions were lower than those of the corresponding pinacol after recovering. It is highly possible the CuO (111) peak, which is
alkylboronate esters (Scheme 4).
consistent with our rationale from XPS result. A Cu content of 5.0
wt% in the catalyst was determined by atomic absorption
spectroscopy (AAS).
Scheme 4. Borylation of Alkyl Bromides Using Bis(neopentylglyꢀ
colato)diboron. Alkyl bromide (0.25 mmol), B
2
(neop)
2
(0.375 mmol),
O) at
MeOLi (0.5 mmol), EtOH (1.5 mL), and Cu O NPs/CB (0.8 mol % Cu
2
2
room temperature under Ar atmosphere for 12 h. All yields are Isolated
yields.
a
Table 2. Gramꢀscale Reaction and Recyclability of Cu
2
O NPs/CB.
[
b]
run
1
catalyst recovery (%)
product yield (%)
92
90
84
90
88
81
2
3
a
Reaction conditions: alkyl bromide (10 mmol), B
2
Pin
2
(15 mmol), MeOLi
O) at room
(20 mmol), EtOH (15 mL), and Cu O NPs/CB (0.8 mol % Cu
2
2
b
temperature under Ar atmosphere for 12 h. All yields are Isolated yields.
Activity and reusability are also key factors to test catalyst
2
Figure 1. a) TEM before the reaction, b) the size distribution of Cu O NPs,
feasibility in real industry application. To this point, a gramꢀscale c) XPS, d) XRD before the reaction, e) TEM after third cycle, f) XRD after
reaction was attempted. In the presence of Cu O NPs/CB (0.8 third cycle.
2
mol%), 1a reacts with B pin at room temperature for 12 h to
2
2
In conclusion, we have developed an efficient route for the
borylation of primary and secondary alkyl bromides catalyzed by
produce 3a in 90% yield (Table 2). This proves our nanocatalyst has
potential for industry process. As well, the catalyst is recovered from
the reaction mixture by centrifugation and reused for the fresh
Cu O nanoparticles. Our approach gives high yield of borylation of
2
priamry and secondary alkyls with wide range of functional groups,
with either B pin or B (neop) as boron donor. It also demonstrates
16
reaction. Only a slight decrease in catalytic activity after three
cycles (81% of third cycle vs. 90% of first cycle) is observed, which
demonstrates an excellent recyclability.
2
2
2
2
outstanding recyclability and hence has enormous potential in largeꢀ
scale production.
Finally, the Cu O NPs/CB catalyst was characterized by different
2
techniques. Figure 1a shows transmission electron microscopy
(
TEM) image of an asꢀobtained sample. It suggests Cu O
2
Notes and references
School of Chemistry and Chemical Engineering, School of Medical
nanoparticles with average diameter of 137.5 nm (Figure 1b). The Xꢀ
ray photoelectron spectroscopy (XPS) spectrum in Figure 1c is
a
performed to detect the valence of catalysts. 932.7 eV peak was Engineering, Hefei University of Technology, Hefei, P. R. China. Eꢀmail:
assigned for Cu(I) 2P3/2, 952.4 eV peak for Cu(I) 2P1/2. But the
satellite shakeup feature characteristic of Cu(II) species were
hjxu@hfut.edu.cn
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Department of Materials Science and Engineering, Henry Samueli
School of Engineering and Applied Sciences, University of California,
Los Angeles, USA. Eꢀmail: yhuang@seas.ucla.edu
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Entry for the Table of Contents
A Cu O nanoparticle catalyzed borylation of activated and unactivated alkyl bromides is developed, using bis(pinacolato)diboron as boron
2
source. To our best knowledge it is the first report of heterogeneous Cu O nanocatalyst applied in direct synthesis of primary and secondary
2
alkylboronic esters. Our catalytic system features in mild reaction conditions, high yield and absence of ligands which otherwise are essential
in homogenous catalysis.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5