Palladium-catalyzed regioselective hydroboration of aryl alkenes with B2pin2

Article abstract of DOI:10.1039/c7cc09432a

A palladium(ii)-catalyzed hydroboration of aryl alkenes with stable and easy-to-handle (pinacolato)diboron (B₂pin₂) under mild conditions has been developed. Acetic acid acted as the solvent and the hydrogen source, which has been identified by deuterium experiments. Notably, isomerization-hydroboration of allyl benzene derivatives was observed. As a result, a series of benzyl boronic esters were obtained in moderate to excellent yields with exclusive regioselectivity.

Full text of DOI:10.1039/c7cc09432a

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Palladium-Catalyzed Regioselective Hydroboration of Aryl Alkenes
with B2pin2
Received 00th January 20xx,
Accepted 00th January 20xx
Jiuzhong Huang, Wuxin Yan, Chaowei Tan, Wanqing Wu*, and Huanfeng Jiang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
palladium(II)-catalyzed hydroboration of aryl alkenes with rhodium-catalyzed hydroboration with catecholborane, which
stable and easy-to-handle (pinacolato)diboron (B2pin2) under mild was the first catalytic hydroboration reaction.3a In the next
conditions has been developed. Acetic acid acted as the solvent decades, especially in recent years, many reports about
and the hydrogen source, which have been identified by rhodium-3, copper-4, iron-5, cobalt-6, manganese-7 catalyzed
deuterium experiments. Notably, isomerization−hydroboraꢀon of hydroboration of alkenes with borane derivatives (R1R2BH) and
allyl benzene derivatives was observed. As a result, a series of diboron reagents have been disclosed (Scheme 1)8. On the
benzyl boronic esters were obtained in moderate to excellent other
hand,
diboron
reagents,
represented
by
yields with exclusive regioselectivity.
bis(pinacolato)diboron (B2pin2), is more stable than borane
derivatives under air and moisture and a storable and easy-to-
handle reagent for a series of transition metal-catalyzed
reactions, including borylation reaction.9 Utilizing B2pin2 rather
than borane derivatives as boron source is safer and more
convenient choice. Despite some successful examples reported
in this field, a versatile palladium catalyzed hydroboration of
alkenes with B2pin2 still remains rare.10 One possible challenge
Organoboron derivatives are versatile building blocks in
pharmaceutical chemistry and materials chemistry field, which
could conveniently covert into more complex molecules via
cross-coupling and oxidation reaction etc..1 Due to the
hydroboration/oxidation sequence as the introduction in most
fundamental organic chemistry courses, the hydroboration of
alkenes is the unique method to construct the boron-
containing compounds.2 In contrast to the non-catalytic
hydroboration, transition metal-catalyzed hydroboration of
alkenes has obvious advantages in the high chem- and regio-
selectivity and broad functional group compatibility.
for this is that σ-alkyl-palladium(II) species facilely undergo β-H
elimination or protonation instead of transmetalation or
nucleophilic attack.11 It is supposed that the transformation of
σ
-palladium species into π-palladium species is the effective
means to overcome the key problem.12 Therefore, based on
our long-standing efforts and interest in palladium(II)-
catalyzed functionalization of alkenes,11d-e,13 we successfully
developed
the
regioselective
palladium(II)-catalyzed
hydroboration of aryl alkenes with B2pin2. Notably, the
isomerization−hydroboraꢀon of allyl benzene derivaꢀves was
demonstrated during the process (Scheme 2).
Scheme 1. Transition metal (TM)-catalyzed hydroboration of
alkenes
As early as thirty years ago, Männig and Nöth described the
Scheme 2. Palladium(II)-catalyzed hydroboration of aryl alkenes
with B2pin2
Key Laboratory of Functional Molecular Engineering of Guangdong
Province, School of Chemistry and Chemical Engineering, South
China University of Technology, Guangzhou 510640, China. E-mail:
cewuwq@scut.edu.cn; jianghf@scut.edu.cn
We initiated the investigation using 1b as substrate, and the
hydroboration product 2b was first detected with 73% yield in
the presence of 7.5 mol % of Pd(OAc)2 and terpy (terpyridine),
1.5 equiv of B2pin2 in AcOH/CH3CN at 90oC (Table 1, entry 1).
Without adding terpy, 4-tert-butylstyrene 1b could not be
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Electronic
supplementary
information
(ESI)
available:
Experimental section, characterization of all compounds, and
copies of 1H and 13C NMR spectra for selected compounds. For ESI
and other electronic format See DOI:10.1039/x0xx00000x
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Table 1. Optimization of reaction conditionsa
electron-withdrawing groups, such as t-butyl, methoxyl,
acetoxyl, phenyl, fluoro-, chloro-, iodo-, were well tolerated to
afford the corresponding hydroboration product in good to
excellent yields (2a
substituted styrenes also exhibited good reactivities to deliver
the target products 2i 2q in moderate to good yields. Both 1-
and 2-vinylnaphthalene gave the excellent yields owing to the
weak aromaticity of naphthalene ring (2r
2s).9c,14 To our
delight, nitrogen-containing heteroaromatic ethenes could
convert into the desired products in satisfactory yields (2t 2u).
-2h). In addition, meta-, ortho- and multi-
-
entry
1
ligand
terpy
ꢀ
conversion (%)
2b yield (%)
3b yield (%)
>99
7
73
N.D.
34
11.6
5
,
2
3
bpy
>99
>99
86
26
19
6
,
4
1,10ꢀphen
bathophen
bc
80
Gratifyingly, the styrene 1v derived from estrone was
compatible under the standard conditions and transformed to
the target product 2v in 85% yield. Furthermore, we tested the
disubstituted terminal styrene 1w under the standard
conditions, and the product 2w was formed in acceptable yield.
5
77
6
84
77
5
7
DAF
dpk
16
15
N.D.
7
8
80
71
9
L1
95
93(90)
13
N.D.
40
45
2
10
11
12
13
14
15
16
L2
54
L3
46
trace
88
L4
95
L5
25
12
13
12
8
L6
80
34
L7
>99
93
90(86)
27
L8
53
Conditions: substrate 1b (0.3 mmol), Pd(OAc)2 (7.5 mol %), ligand (7.5
mol %), B2pin2 (1.5 equiv), AcOH (0.5 mL), CH3CN (1.0 mL) under N2
atmosphere at 90 °C for 24 h; Yields determined by GC-MS with n-
dodecane as internal standard; N.D. = not detected.
Selected ligands:
O
O
O
O
O
O
O
O
O
N
N
N
N
N
N
N
N
N
N
N
N
Ph
Ph iPr
iPr tBu
tBu Bn
Bn
L1
L2
L3
L4
F3C
O
O
N
N
O
N
O
N
N
N
N
N
N
L5
L6
L7
L8
completely converted, and was recycled in 90% yield (Table 1,
entry 2). Next, we further examined the effect of various
bidentate nitrogen-containing ligands on the efficiency of this
reaction. When bpy (2,2'-bipyridin) was used as ligand, the
reaction gave higher conversion rate, but afforded 2b in low
yield (Table 1, entry 3). 1,10-Phen (1,10-phenanthroline) and
its derivatives led to the predominant hydroboration product
in moderate yield (Table 1, entries 4-6). Electron-deficient
ligands such as DAF (diazafluorenone) and dpk (dipyridinyl
ketone) showed different results for the space distance of two
nitrogen atoms (Table 1, entries 7 and 8). Considering the
similarity of chelating site between terpy and pyridine-
Scheme 3. Pd(II)-catalyzed hydroboration of styrene
derivativesa
a
Conditions: substrate 1 (0.3 mmol), Pd(OAc)2 (7.5 mol %), L1 (7.5 mol %),
B2pin2 (1.5 equiv), AcOH (0.5 mL), CH3CN (1.0 mL) under N2 atmosphere at
90 °C for 24 h; Isolated yields. b d. r. = 18: 1.
Subsequently, allyl benzene derivatives were subjected to the
optimized conditions, benzyl boronic esters (Scheme 4, 2aa-2ae)
rather than phenethyl boronic esters were afforded in moderate to
good yields. Interestingly, when the carbon chain was extended to
4-phenyl-1-buten,
the
isomerization-hydroboration
oxazoline ligands, L1-L5 were applied to the hydroboration
product−phenethyl boronic ester 2af could still be formed. However,
the target product 2ag was failed to be obtained due to the longer
distance between phenyl and the double bond.
process (Table 1, entries 9-13). To our delight, 2b could be
obtained in 90% isolated yield with L1 as the partner. Other
pyridine-oxazoline ligands L6-L8 did not show higher efficiency
Next, in view of the practicability of method and the
importance of organoboron derivatives in organic synthesis,
than that of L1 (Table 1, entries 14-16).
With the optimal conditions in hand, we investigated a
variety of substituted styrene derivatives (Scheme 3). Various
substitutions in para-position including electron-donating and
we subjected the mixture of three allylbenzene isomers (1aa
,
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double bond (Scheme 6, eq. 7).16 At last, when 1-phenyl-1,3-
butadiene was submitted to the optimal conditions, the
hydroboration product of terminal bond was detected by GC-
MS in 83% yield, and the hydroboration/oxidation product
(Z, E)-1ai) to the standard conditions, and only 2aa was formed
8
in 69% isolated yield (Scheme 5, eq 1), which demonstrated
n
Pd(OAc)2 (7.5 mol %)
O
L1 (7.5 mol %)
B2pin2 (1.5 equiv)
AcOH / CH3CN
9
Ar
B
n
Ar
O
was isolated by flash column chromatography in 75% yield
(Scheme 6, eq. 8), suggesting that the π-palladium species
could be formed under the current conditions.
1
2
n = 1, 2, 3
Et
Et
Et
O
B
O
B
O
B
O
O
O
F
O
2aa, 75%
2ab, 67%
2ac, 81%
Et
Et
n
O
B
B
O
B
O
O
O
O
O
n = 2, 2afb, 55%
n = 3, 2agc, 0%
2ad, 73%
2ae, 74%
Scheme 4. Pd(II)-catalyzed isomerization−hydroboraꢀon of
long chain alkenesa
a
Conditions: substrate 1 (0.3 mmol), Pd(OAc)2 (7.5 mol %), L1 (7.5 mol %),
B2pin2 (1.5 equiv), AcOH (0.5 mL), CH3CN (1.0 mL) under N2 atmosphere at
90 °C for 24 h; Isolated yields. b 4-Phenyl-1-buten as substrate. c 5-Phenyl-1- Scheme 6. Control experiments
penten as substrate.
Base on the abovementioned experimental results as well as
the potential utilization in the transformation of raw
petrochemical materials. Then the further transformations of
literature precedents,17 we proposed a tentative mechanism
for the hydroboration reaction of alkene, which is shown in
Scheme 7. Initially, palladium acetate coordinates with ligand
L1. The Pd(OAc)2/L1 complex undergoes transmetalation with
product
2
were carried out. 2e and 2t efficiently converted
under mild oxidative conditions in 88% and 76%
into 4 and
5
yields, respectively (Scheme 5, eq 2 and 3). Moreover, a
variety of potentially bioactive 1,1-diaryalkane scaffolds could
be prepared from 2r and 2u via Pd-catalyzed carbon-carbon
bond formation with aryl iodides (Scheme 5, eqs 4 and 5).
B2pin2 to generate Int-A
. The intermediate Int-A might
facilitate the hydrogen atom transfer from AcOH to palladium,
affording [PdH] species Int-B. Next, a sequence of migratory
insertion to alkene 1 provided σ-benzy palladium species Int-C,
which could further convert into π-benzyl palladium
intermediate Int-D. Last, the Int-D was attracted by B2pin2 to
afford the target hydroboration product
2 and released Int-A.
In the meanwhile, β-H elimination and protonation of
intermediate Int-C are almost forbidden with the assistance of
ligand L1
.
Scheme 5. The application of method and the transformation of 2
To better understand the reaction mechanism, we began to
gain more insight into the Pd(II)-catalyzed hydroboration
process (Scheme 6). First, deuterium labeling experiments
were carried out. When acetic acid was replaced with d1-AcOD,
the hydroboration product was obtained in 75% yield and d1-
styrene (d1-1a) was detected by GC-MS in 17% yield, which
indicated the acetic acid was hydrogen source (Scheme 6, eq.
Scheme 7. Proposed mechanism
In conclusion, we have developed the first example of
palladium(II)-catalyzed hydroboration of aryl alkenes with
bis(pinacolato)diboron and exclusive regioselectivity was
exhibited in the products. Isomerization−hydroboraꢀon of allyl
benzene derivatives was reported in this process. In addition,
6). When the substrate 1ai containing vinyl and allyl group was acetic acid played the role of hydrogen source according to
deuterium experiments. Efforts for detailed mechanism
subjected to the standard condition, the vinyl group occurred
hydroboration and the allyl group was isomerized into internal
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Ibrahim, S. W. Entsminger, A. R. Fout, ACS Catal., 2017, 7,
3730−3734.
research and the asymmetric version of hydroboration process
are currently underway in our laboratory.
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The authors are grateful to the National Key Research and
Development Program of China (2016YFA0602900), the
National Natural Science Foundation of China (21420102003)
and the Fundamental Research Funds for the Central
Universities (2015ZY001) for financial support.
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