N-Butyllithium Catalyzed Selective Hydroboration of Aldehydes and Ketones

Article abstract of DOI:10.1021/acs.joc.8b01495

Highly efficient and selective hydroboration of aldehydes and ketones with HBpin is achieved by using the simple and convenient n-BuLi as a catalyst. The reaction proceeds rapidly with low catalyst loading (0.1-0.5 mol %) under mild conditions. Key features include the high catalytic efficiency, exceptional functional group compatibility, ample substrate scope, and high selectivity for aldehydes over ketones. Computational studies were carried out to provide a mechanistic insight into the n-BuLi catalyzed hydroboration of aldehydes/ketones with HBpin.

Full text of DOI:10.1021/acs.joc.8b01495

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Note
n-Butyllithium Catalyzed Selective Hydroboration of Aldehydes and Ketones
Zhangye Zhu, Xueli Wu, Xiaojuan Xu, Zhenjie Wu,
Mingqiang Xue, Yingming Yao, Qi Shen, and Xiaoguang Bao
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01495 • Publication Date (Web): 02 Aug 2018
Downloaded from http://pubs.acs.org on August 2, 2018
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Page 1 of 8
The Journal of Organic Chemistry
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n-Butyllithium Catalyzed Selective Hydroboration of Aldehydes and Ketones
Zhangye Zhu, Xueli Wu, Xiaojuan Xu, Zhenjie Wu, Mingqiang Xue,* Yingming Yao, Qi Shen, and
Xiaoguang Bao*
1
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Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science,
Dushu Lake Campus, Soochow University, Suzhou 215123, People’s Republic of China
s Supporting Information
ABSTRACT: Highly efficient and selective hydroboration of aldehydes and
ketones with HBpin is achieved by using the simple and convenient n-BuLi as a
catalyst. The reaction proceeds rapidly with low catalyst loading (0.1-0.5 mol %)
under mild conditions. Key features include the high catalytic efficiency,
exceptional functional group compatibility, ample substrate scope and high
selectivity for aldehydes over ketones. Computational studies were carried out to
provide a mechanistic insight into the n-BuLi catalyzed hydroboration of
aldehydes/ketones with HBpin.
The development of inexpensive and environment-friendly
metal catalysts for valuable chemical transformations is of
great significance in modern organic synthesis. In contrast to
the intense attentions paid to the organic catalytic chemistry
driven by precious metals, the application of earth-abundant
and biocompatible main group metals is lagged and
and solvent-free method for the hydroboration of various
aldehydes with pinacolborane. Although this scheme was
27
applicable to aldehydes, the ketones were largely ineffective.
Besides, a higher temperature was required to get better
conversions for aldehyde substrates. Therefore, the
development of a general and highly selective hydroboration
method based on an easily accessible metal catalyst with
great efficiency is desirable to advance the synthetic toolkit.
Herein, we report the use of the inexpensive and
commercially available n-butyllithium (n-BuLi) and other
lithium anilides as catalysts for the hydroboration of
carbonyl compounds with pinacolborane (HBpin). The
excellent catalytic performance of n-BuLi for the
chemoselective hydroboration of aldehydes/ketones was
observed. More interestingly, this protocol maintains its high
catalytic efficiency when scaled up.
1
underdeveloped. Furthermore, the utilization of simple and
easily attainable metal catalysts remains limited as the
sphere is mostly dominated by complexes stabilized by
2
sterically hindered ligands. In addition, the synthesis of
these complexes is cumbersome and labor consuming.
Therefore, there is a critical need to develop more efficient
and sustainable main group metal based catalysts which
could essentially mitigate the environmental concerns and
over-exploitation of the less abundant earth elements.
Hydroboration reduction of carbonyl compounds is a vital
transformation in organic synthesis as the resultant borates
Benzaldehyde and HBpin were initially chosen as model
substrates to optimize reaction conditions and the progress
2a,3
provide an efficient approach to alcohols via hydrolysis. In
comparison to the traditional stoichiometric reduction by
using light metal hydrides, catalyzed hydroboration of
aldehydes and ketones has been realized by means of a vast
1
was monitored by H NMR. The reaction of a 1:1 molar ratio
of benzaldehyde and HBpin catalyzed by PhNHLi was carried
out at room temperature under neat conditions. A rapid
reaction was observed using as little as 0.01 mol % of catalyst
loading (Table 1, entry 1). An increase in catalyst loading
resulted in a corresponding increase in the yield of product,
giving a good yield of 96% with 0.1 mol % of catalyst loading
(entries 2 and 3). When a slight excess of HBpin was used
relative to benzaldehyde (1:1.1), full conversion was attained
1
b,c,4-16
number of catalysts ranging from transition metals,
main group elements
1
7-24
25,26
as well as lanthanide complexes.
In most cases, however, these compounds/complexes are
either expensive or difficult to be prepared. As noted earlier,
studies on the hydroboration reactions driven by simple
main group metal catalysts are rather limited. Recently,
Zhao’s group reported that powder NaOH could catalyze the
(>99%, entry 4). Other lithium compounds such as
24b
i
hydroboration of aldehydes and ketones.
However, the
2,6-Me
2
PhNHLi, 2,6- Pr
2
PhNHLi and n-BuLi could also
relatively high catalytic demand and the use of deuterated
solvents to achieve a better solubility of the powdered alkali
hydroxide were obvious drawbacks. During the preparation
of this manuscript, Hreczycho's group reported a catalyst-
effectively catalyze the hydroboration of benzaldehyde with
HBpin (entries 5-7). It was recorded that all lithium anilides
and n-BuLi exhibited excellent catalytic activity: complete
conversions were accomplished under solvent-free condition
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Table 1. Hydroboration of Benzaldehyde Catalyzed by
Lithium Anilides and n-BuLi
in 10 min at ambient temperature. Pleasingly, it was observed
that these lithium compounds possess comparable catalytic
activity to the rare-earth metal catalytic systems. Compared
with some known lithium catalysts, the catalytic activity of
a
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
25
lithium anilides and n-BuLi is found to be better than that of
substrate
ratiob
c
entry
cat. (mol %)
yield (%)
24d-f
the latest published work,
most active lithium hydridotriphenylborate.
although lower than that of the
24a
Considering
1
1:1
52
78
PhNHLi (0.01)
PhNHLi (0.05)
PhNHLi(0.1)
PhNHLi(0.1)
the readily availability and commercial attainability, n-BuLi
as an ideal catalyst was served for this study.
1:1
2
3
1:1
96
With the afore-defined reaction conditions in hand, the
adaptability of n-BuLi with a wide range of aldehydes was
explored. As shown in Table 2, almost all of the aldehydes
achieved full conversions within 10 min with 0.1 mol %
catalyst loading. The presence of halogens had no obvious
effect as demonstrated by the clean hydroboration of the
p-fluoro and m-chloro-benzaldehyde substrates (entries 2
1
:1.1
:1.1
>99
>99
4
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2,6-Me
2
PhNHLi (0.1)
PhNHLi(0.1)
i
1:1.1
1:1.1
>99
>99
2,6- Pr
2
7
n-BuLi (0.1)
a
Benzaldehyde, HBpin, and lithium compounds at rt.
Benzaldehyde, HBpin. Yield was determined by 1H NMR
b
c
and
3).
Electron
donating
aldehydes
including
spectroscopy.
o-tolualdehyde, p-tolualdehyde, 2,4,6-trimethylbenzaldehyde,
and p-methoxybenzafdehyde all afforded the corresponding
borate esters in excellent yields (entries 4-7). Moreover, there
was no obvious steric effect with substrates bearing ortho
substituents (entries 4 and 6). A higher catalyst loading and
longer time were required for the full conversion of
cinnamaldehyde and ferrocene carboxaldehyde (entries 8
and 18). Naphthalene-2-carbaldehyde and heterocyclic
Table 2. Hydroboration of Aldehydes Catalyzed byn-BuLia
yieldsc
entr
y
aldehyde
s
cat.
(mol %)
t
product
(min)
(%)
aldehydes
such
as
2-acetyl
thiophene
and
1
0.1
10
>99
2-pyridinecarboxaldehyde were also compatible with the
hydroboration protocol giving full conversions (entries 9-11).
Interestingly, an increase in catalyst loading was not required
in order to achieve complete conversion for a dialdehyde
substrate m-phthalaldehyde (entry 12). For aliphatic
aldehydes, including cyclic substrate (1-formylcyclohexane),
short straight chain (propionaldehyde) and long straight
chain (n-heptanal), the corresponding borate esters were
delivered in excellent yields (entries 13-15). Pleasingly, n-BuLi
also displayed high chemoselectivity and catalytic efficiency
towards the hydroboration of aldehydic substrates
containing sensitive unsaturated functional groups such as C
＝C, C≡C, and C≡N. Notably, these functional groups were
left intact after completion of reaction (entries 8, 16 and 17).
Encouraged by the satisfactory results obtained with
aldehydes, the scope was expanded to ketone substrates. As
expected, a longer time was required for the hydroboration
of ketones compared with aldehydes. This is analogous to
previously described methods in which aldehydes were
2
0.1
10
>99
3
0.1
10
>99
4
5
0.1
0.1
0.1
10
10
10
>99
>99
>99
6
7
0.1
10
>99
0.3
>99
(73)
8
30
10
(0.1)
9b
10
0.1
>99
0.1
0.1
0.1
10
10
10
>99
>99
>99
6
c,18b,19,20,26
generally more reactive than ketones.
A catalyst
1
1
loading of 0.1 mol % was sufficient for achieving the full
conversions of acetophenone and p-fluoroacetophenone in
1
2d
20 min (Table 3, entries 1 and 2). Similarly, full conversions of
13
0.1
10
>99
most aromatic ketones were achieved within 20 min by
employing a catalyst loading of 0.1 mol % (entries 1-9). This
showcased better reactivity in comparison with the NaOH
catalytic system where the catalyst loading of 5 mol % was
1
4
0.1
0.1
10
10
>99
>99
15
24b
required for ketone substrates. It is worth noting that full
conversions were achieved with substrates having either
electron-withdrawing or electron-donating substituents such
1
6b
0.1
0.1
0.2
10
10
10
>99
>99
>99
2 3 3
as p-F, p-Cl, p-Br, p-NO , p-CH and p-OCH groups. Again,
17b
no obvious effect due to steric hindrance was found in this
reaction. Furthermore, 3-methyl-2-butanone required 0.2
mol % catalyst loading to achieve full conversion into the
corresponding borate ester (entry 10).
1
8b
a
Aldehydes (1 mmol), HBpin (1.1 mmol) and n-BuLi at rt. bTHF (0.2
c
In
the
beginning,
the
reaction
involving
mL) was added as the aldehyde is a solid. Yield of aldehydes based
1
d
on H NMR analysis. HBpin (2.2 mmol) was used.
4-aminoacetophenone led to a dehydrogenative byproduct
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coupling of the amino group with HBpin.28 However, by
Scheme 1. Chemoselective Hydroboration of
Aldehydes Catalyzed by n-BuLi
1
2
3
4
5
6
7
8
9
controlling the order of addition of substrates, the complete
and favorable formation of the desired borate ester was
realized (entry 11). A drug molecule spironolactone was also
selectively hydroborated in excellent yield, thus indicating
the potential application of the present method in medicinal
chemistry (entry 12).
Furthermore, the chemoselective hydroboration of
aldehydes over ketones was explored. The reaction of
equimolar amounts of benzaldehyde, acetophenone, and
HBpin were carried out in the presence of n-BuLi (0.1 mol %)
under neat conditions at room temperature. This resulted in
the preferential conversion of the benzaldehyde into the
corresponding boronate ester (96% conversion). A similar
trend was also observed with the reactions involving other
aldehydes and ketones, but was more pronounced for an
aliphatic carbonyl compound where complete selectivity was
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
shown towards the aliphatic aldehyde (Scheme 1a).
Table 3. Hydroboration of Ketones Catalyzed by n-BuLia
Both hydroboration reactions of benzaldehyde and
acetophenone with HBpin (1:1.02) catalyzed by 0.1 mol % of
n-BuLi under neat conditions were carried out, respectively,
on a large scale. Remarkably, complete conversions were
obtained in both cases giving 99% yields (Scheme 2).
cat.
mol %)
t
yieldc
(%)
entry
ketones
product
(
(min)
20
>99
(96)
0
.1
1
2
(
10)
Computational studies were carried out to explore a
mechanistic understanding of the n-BuLi catalyzed
20
>99
(90)
0.1
0.1
(
10)
Scheme
2.
Large-scale
Reaction
of
3
20
>99
Benzaldehyde/Acetophenone with HBpin
4b
5b
6
0.1
0.1
0.1
20
20
20
>99
>99
>99
7b
8b
9
0.1
0.1
20
20
>99
>99
hydroboration of aldehydes with HBpin.29 Initially, the
lithium atom of the catalyst could bind with an oxygen atom
0.1
0.2
20
20
>99
>99
10
of HBpin to form a complex INT
the alkyl group of the catalyst may attack HBpin to afford the
Next
zwitterionic intermediate INT2 via TS1 INT2 could
bind with substrate benzaldehyde to form a complex INT3
(Scheme 3) Subsequently,
1 .
1
1
1b
0.5
0.5
20
20
>99
.
,
,
2b
from which the hydride migration to the carbonyl carbon
could occur to yield the intermediate INT4.30 After a ligand
exchange step with HBpin, the alkoxy group in the formed
intermediate INT5 could nucleophilically attack the HBpin
to generate the zwitterionic intermediate INT6. Similarly,
another molecule of benzaldehyde could bind with INT6 to
give INT7, and a hydride transfer to the carbonyl carbon
could follow to form the intermediate INT8. Thus, the final
product would be obtained after a ligand exchange step with
>99
a
b
Ketones (1 mmol), pinacolborane (1.1 mmol) and n-BuLi at rt. THF
c
1
(
0.2 mL) was added as ketones are solid. Yield of ketones based on H
NMR analysis.
Importantly, the chemoselectivity towards aldehyde was very
noticeable in the hydroboration of 4-acetylbenzaldehyde to
give pure 2-(1-(4-acetylphenyl)ethoxy)pinacol borane in a
yield of 99% (Scheme 1b).
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Scheme 3. Energy Profiles (in kcal/mol) for the
-BuLi Catalyzed Hydroboration of PhCHO with
HBpin.
under mild conditions, a wide range of substrates, excellent
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
chemical selectivity, good functional group tolerance,
solvent-free, and a controllable manner to obtain full
conversion of an amino-containing substrate into target
product. Also, the high catalytic efficiency was maintained
when the reaction was scaled up.
n
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 4. Proposed Mechanistic Pathway
EXPERIMENTAL SECTION
General Information
All reactions were performed in
nitrogen-atmosphere. CDCl was purchased from TCI
chemicals and stored over activated 4Å molecular sieves.
HBpin was purchased from Accela and used without further
purification. All the liquid ketones and aldehydes were
a glove box under
3
distilled, dried over CaH
2
and degassed prior to use. All the
1
solid ketones and aldehydes were degassed prior to use. H,
13
11
1
C and B spectra were recorded on Bruker AV-400 ( H: 400
13
11
MHz, C: 101 MHz, B: 128 MHz). NMR chemical shifts were
reported in ppm with respect to tetramethylsilane.
General Procedure for the Hydroboration of
Aldehydes/Ketones Catalyzed by n-BuLi
Aldehyde/ketone (1 mmol, THF (200 ul) for solid substrates),
HBpin (1.1 mmol) and a stock solution containing n-BuLi
(
0.05 M) were injected into a pre-dried vial equipped with a
magnetic bar. The resultant mixture was stirred at room
temperature and the reaction progress was monitored by H
NMR. For aldehydes, the disappearance of aldehydic proton
2
signal and appearance of a new -CH - proton signal indicate
completion of reaction. For ketones, the completion of the
reaction was indicated by the appearance of a new -CH-
proton signal. After completion of reaction, the solvent and
excess HBpin were removed from the mixture by vacuum
drying in an oven to afford the corresponding boronate ester
without further purification (Note: the catalytic
1
HBpin and the yieled INT5 is ready for the next catalytic
cycle. According to the DFT study, a possible mechanistic
pathway is proposed in Scheme 4
.
In conclusion, the use of simple n-BuLi as an efficient
catalyst for the selective hydroboration of aldehydes and
ketones is described for the first time. Notable features
encompass high catalytic efficiency with low catalyst loading
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hydroboration of carbonyl compounds by n-BuLi is an
exothermic reaction. Large-scale reactions should be carried
out under cooling in an ice bath, and n-BuLi should be added
in a slow dropwise manner).
Ar-H), 7.49-7.41 (m, 2H, Ar-H), 5.10 (s, 2H, OCH
12H, CH ); C NMR (101 MHz, CDCl
3 3
2
), 1.32 (s,
) δ 159.8 (Ar-C), 143.7
(Ar-C), 139.6 (Ar-C), 123.4 (Ar-C), 120.1 (Ar-C), 81.0 (OC), 66.5
(OCH ), 25.5 (CH ).
-(thiophen)pinacolborane (1k). Brown oil; H NMR (400
MHz, CDCl ) δ 7.25 (d, J = 4.3 Hz, 1H, Ar-H), 7.02-6.94 (m,
H, Ar-H), 5.04 (s, 2H, OCH
(101 MHz, CDCl ) δ 142.0 (Ar-C), 126.6 (Ar-C), 125.9 (Ar-C),
25.5 (Ar-C), 83.1 (OC), 61.6 (OCH ), 24.6 (CH ).
-(benzyloxy)dipinacolborane (1l). Colorless oil; 1H NMR
) δ 7.34-7.23 (m, 4H, Ar-H), 4.91 (s, 4H,
1
3
1
2
3
4
5
6
7
8
9
2
3
25b
1
2
Spectral Data for Boronate Esters
25a
Colorless oil; 1H NMR
3
2-(benzyloxy)pinacolborane (1a).
(400 MHz, CDCl ) δ 7.36-7.23 (m, 5H, Ar-H), 4.92 (s, 2H,
OCH ), 1.26 (s, 12H, CH
1
3
2
2 3
), 1.27 (s, 12H, CH ); C NMR
3
1
3
3
2
3
); C NMR (101 MHz, CDCl
3
) δ 138.8
1
2
2
3
(
Ar-C), 127.8 (Ar-C), 126.9 (Ar-C), 126.2 (Ar-C), 82.5 (OC),
4
6
2 3
6.2 (OCH ), 24.1 (CH ).
(
400 MHz, CDCl
3
1
2
-(4-fluorobenzyloxy)pinacolborane (1b).25b Colorless oil; H
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
13
OCH
2
), 1.26 (s, 24H, CH
3
); C NMR (101 MHz, CDCl
3
) δ 138.8
NMR (400 MHz, CDCl
(
3
) δ 7.34-7.29 (m, 2H, Ar-H), 7.04-6.98
(
Ar-C), 127.8 (Ar-C), 125.3 (Ar-C), 124.6 (Ar-C), 82.5 (OC), 66.1
m, 2H, Ar-H), 4.87 (s, 2H, OCH2), 1.26 (s, 12H, CH3); 13
C
2 3
(OCH ), 24.1 (CH ).
2-(cyclohexylmethoxy)pinacolborane (1m). Colorless oil; H
3
NMR (101 MHz, CDCl ) δ 161.7 (ds, Ar-C), 134.5 (d, J = 3.2 Hz,
25a
1
Ar-C), 128.1 (d, J = 8.1 Hz, Ar-C), 114.6 (ds, Ar-C), 82.5 (OC),
5.6 (OCH ), 24.1 (CH ).
-(3-chlorobenzyloxy)pinacolborane (1c).25b Colorless oil; H
NMR (400 MHz, CDCl ) δ 7.36 (s, 1H, Ar-H), 7.28-7.19 (m, 3H,
Ar-H), 4.89 (s, 2H, OCH
MHz, CDCl ) δ 140.8 (Ar-C), 133.7 (Ar-C), 129.1 (Ar-C), 127.0
Ar-C), 126.3 (Ar-C), 124.2 (Ar-C), 82.6 (OC), 65.4 (OCH
4.1 (CH ).
NMR (400 MHz, CDCl
.57-1.47 (m, 6H, C 11), 1.57-1.47 (m, 1H, C
CH ), 1.18-1.11 (m, 2H, C), 0.99-0.87 (m, 2H, C
101 MHz, CDCl
3
) δ 3.65 (d, J = 6.4 Hz, 2H, OCH
2
),
6
2
3
1
6
H
6
H11), 1.25 (s, 12H,
1
2
13
3
6
H11); C NMR
3
(
) δ 82.0 (OC), 69.8 (OCH
), 24.0 (CH ).
), 38.8, 28.8, 26.0,
3
2
1
3
2
3
), 1.27 (s, 12H,CH ); C NMR (101
2
5.2 (C
6
3
25a
1
3
2
-(ethylmethoxy)pinacolborane (1n). Colorless oil; H NMR
(
2
),
(
(
13
400 MHz, CDCl
m, 2H, CH ), 1.25 (s, 12H, CH
C NMR (101 MHz, CDCl ) δ 82.0 (OC), 66.0 (OCH
CH CH ), 24.1 (CH ), 9.6 (CH CH ).
2-(hexylmethoxy)pinacolborane (1o). Colorless oil; H NMR
(400 MHz, CDCl ) δ 3.82 (t, J = 8 Hz, 2H, OCH ), 1.52-1.58 (m,
H, CH ), 1.27-1.34 (m, 8H, CH ), 1.24 (s, 12H, CH ), 0.87 (t, J =
Hz, 3H, CH ); C NMR (100 MHz, CDCl ) δ 82.0 (OC), 64.4
OCH ), 31.3 (CH ), 30.9 (CH ), 28.4 (CH ), 25.0 (CH ), 24.0
CH ), 22.1 (CH ), 13.5 (CH ).
-(2-ethynylbenzyloxy)pinacolborane (1p).
3
) δ 3.80 (t, J = 6.6 Hz, 2H, OCH
), 0.91 (t, J = 7.4 Hz, 3H, CH
), 24.1
2
), 1.63-1.54
2
3
2
3
3
);
26a
1
2-((2-methylbenzyl)oxy)pinacolborane (1d). Colorless oil; H
3
2
NMR (400 MHz, CDCl
3
) δ 7.39 (dq, J = 7.2, 3.6 Hz, 1H,Ar-H),
), 2.30 (s, 3H,
); C NMR (101 MHz, CDCl ) δ 136.7
(
3
2
3
2
3
7.18-7.11 (m, 3H, Ar-H), 4.92 (s, 2H, OCH
2
26c
1
1
3
Ar-CH
3
), 1.26 (s, 12H, CH
3
3
3
2
(
Ar-C), 135.1 (Ar-C), 129.5 (Ar-C), 126.9 (d, J = 25.25, Ar-C),
25.4 (Ar-C), 82.4 (OC), 64.5 (OCH ), 24.2 (CH ), 18.2 (CH ).
-((4-methylbenzyl)oxy)pinacolborane (1e).25b Colorless oil; H
) δ 7.24-7.22 (m, 2H,Ar-H), 7.13-7.11 (m,
H, Ar-H), 4.87 (s, 2H, OCH ), 2.32 (s, 3H, Ar-CH ), 1.24 (s,
2H, CH ) δ 136.5 (Ar-C), 135.8
); 13C NMR (101 MHz, CDCl
Ar-C), 128.5 (Ar-C), 126.4 (Ar-C), 82.4 (OC), 66.1 (OCH ),
), 20.7 (CH ).
2-(mesitylmethoxy)pinacolborane (1f).25a Colorless oil; 1H
NMR (400 MHz, CDCl ) δ 6.83 (s, 2H,Ar-H), 4.96 (s, 2H,
OCH ), 2.38 (s, 6H, Ar-CH ), 2.25 (s, 3H, Ar-CH ), 1.26 (s, 12H,
CH ) δ 137.2 (Ar-C), 137.1 (Ar-C),
); 13C NMR (101 MHz, CDCl
28.7 (Ar-C), 128.4 (Ar-C), 82.3 (OC), 60.7 (OCH ), 24.1 (CH ),
2
8
(
(
2
2
2
3
1
2
2
3
3
13
3
3
1
2
2
2
2
2
NMR (400 MHz, CDCl
3
3
2
3
2
2
3
25a
Brown oil; 1H
1
3
3
NMR (400 MHz, CDCl
3
) δ 7.49 (dd, J = 19.8, 7.6 Hz, 2H,
(
2
Ar-H), 7.35 (m, 1H, Ar-H), 7.23 (dd, J = 15.9, 8.4 Hz, 1H, Ar-H),
24.1 (CH
3
3
13
5
.12 (s, 2H, OCH
NMR (101 MHz, CDCl
Ar-C), 126.4 (Ar-C), 125.6 (Ar-C), 119.0 (Ar-C), 82.5 (OC), 81.7
(CHC), 80.4 (CCH), 64.4 (OCH ), 24.1 (CH ).
2-(4-cyanobenzyloxy)pinacolborane (1q). White solid; mp
2
), 3.30 (s, 1H, CH), 1.27 (s, 12H, CH
3
); C
3
) δ 141.2 (Ar-C), 131.9 (Ar-C), 128.5
3
(
2
3
3
2
3
25a
3
3
1
2
3
o
1
9
2-94 C; H NMR (400 MHz, CDCl
3
) δ 7.64-7.62 (m, 2H,
), 1.27 (s,
2
2
0.5 (CH
-(4-methoxybenzyloxy)pinacolborane (1g). Colorless oil; H
) δ 7.29-7.25 (m, 2H,Ar-H), 6.87-6.84
m, 2H, Ar-H), 4.84 (s, 2H, OCH ), 3.78 (s, 3H, Ar-CH ), 1.21
); C NMR (101 MHz, CDCl ) δ 158.6 (Ar-C), 131.0
Ar-C), 128.0 (Ar-C), 113.2 (Ar-C), 82.4 (OC), 65.9 (OCH ),
), 24.1 (CH ).
2-(cinnamyloxy)pinacolborane (1h). Yellowish oil; H NMR
(400 MHz, CDCl ) δ 7.38-7.36 (m, 2H, Ar-H), 7.29 (t, J = 7.5
Hz, 2H, Ar-H), 7.24-7.20 (m, 1H, Ar-H), 6.62 (d, J = 15.9 Hz,
3 3
), 18.9 (CH ).
Ar-H), 7.46-7.44 (m, 2H, Ar-H), 4.98 (s, 2H, OCH
2
25b
1
13
12H, CH
3
); C NMR (101 MHz, CDCl
3
) δ 144.1 (Ar-C), 131.6
NMR (400 MHz, CDCl
3
(
Ar-C), 126.3 (Ar-C), 118.3 (NC), 110.6 (Ar-C), 82.8 (OC), 65.2
(
(
(
2
3
(
2
2 3
OCH ), 24.1 (CH ).
13
s, 12H, CH
3
3
25a
-(ferrocenylmethoxy)pinacolborane (1r). Brown solid; mp
o
1
2
7
4
13
0-72 C; H NMR (400 MHz, CDCl
.24 (s, 2H, OCH ), 4.14-4.11 (m, 5H, Cp-H), 1.26 (s, 12H, CH
C NMR (101 MHz, CDCl ) δ 84.9 (OC), 82.3, 67.9, 67.7, 62.7
Cp-C), 24.2 (CH ).
2-(phenylethoxy)pinacolborane (2a). Colorless oil; H NMR
(400 MHz, CDCl ) δ 7.38-7.21 (m, 5H, Ar-H), 5.24 (q, J = 6.5
Hz, 1H, OCH), 1.49 (d, J = 6.5 Hz, 3H, CH ), 1.22 (d, J = 11.9 Hz,
) δ 144.1 (Ar-C), 127.7
) δ 4.64 (s, 3H, Cp-H),
3
54.7 (OCH
3
3
2
3
);
25a
1
3
3
(
3
25a
1
1
1
1
(
2
H, CH), 6.28 (dt, J = 15.9, 5.3 Hz, 1H, CH), 4.52 (s, 2H, OCH
.26 (s, 12H, CH ); C NMR (101 MHz, CDCl
3 3
2
),
) δ 136.4 (CH),
30.1 (Ar-C), 128.0 (Ar-C), 127.0 (Ar-C), 126.3 (Ar-C), 126.0
CH), 82.4 (OC), 64.8 (OCH ), 24.2 (CH ).
-(2-naphthylmethoxy)pinacolborane (1i). White solid; mp
3
1
3
3
13
3 3
12H, CH ); C NMR (101 MHz, CDCl
2
3
(
(
2
Ar-C), 126.6 (Ar-C), 124.9 (Ar-C), 82.3 (OC), 72.1 (OCH), 25.0
1
8b
CH
3
), 24.0 (d, J = 4.5 Hz, CH
3
).
o
1
8
2-84 C; H NMR (400 MHz, CDCl
3
) δ 7.83-7.81 (m, 2H,
), 1.29 (s,
) δ 136.2 (Ar-C), 132.9
-(1-(4-fluorophenyl)ethoxy)pinacolborane (2b).26a Colorless
Ar-H), 7.49-7.44 (m, 2H, Ar-H), 5.10 (s, 2H, OCH
2
1
oil; H NMR (400 MHz, CDCl
3
) δ 7.35-7.30 (m, 2H, Ar-H),
13
12H, CH
3
); C NMR (100 MHz, CDCl
3
7.02-6.96 (m, 2H, Ar-H), 5.22 (q, J = 6.4 Hz, 1H, OCH), 1.47 (d,
(Ar-C), 132.3 (Ar-C), 127.5 (Ar-C), 127.4 (Ar-C), 127.2 (Ar-C),
125.5 (Ar-C), 125.2 (Ar-C), 124.7 (Ar-C), 124.4 (Ar-C), 82.6
13
J = 6.5 Hz, 3H, CH
3
3
), 1.22 (d, J = 11.6 Hz, 12H, CH ); C NMR
(
1
101 MHz, CDCl
27.0 (d, J = 8.0 Hz, Ar-C), 114.9 (ds, Ar-C), 82.8 (OC), 72.0
(OCH), 25.4 (CH ), 24.5 (d, J = 6.1 Hz, CH ).
3
) δ 161.9 (ds, Ar-C), 140.3 (d, J = 3.1 Hz, Ar-C),
2 3
(OC), 66.3 (OCH ), 24.2 (CH ).
27
1
2
-(pyridine)pinacolborane (1j). Yellow oil; H NMR (400 MHz,
3
3
3
CDCl ) δ 8.61 (d, J = 5.4 Hz, 1H, Ar-H), 7.91 (t, J = 7.7 Hz, 1H,
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The Journal of Organic Chemistry
Page 6 of 8
2
6a
2
-(1-(4-chlorophenyl)ethoxy)pinacolborane (2c).
Colorless
) δ 7.31-7.26 (m, 4H, Ar-H), 5.21
), 1.22 (d,
) δ 142.6
Ar-C), 132.3 (Ar-C), 127.8 (Ar-C), 126.3 (ds, Ar-C), 82.4 (OC),
Ar-H), 6.63-6.61 (m, 2H, Ar-H), 5.14 (q, J = 6.4 Hz, 1H, OCH),
3.62 (s, 2H, NH ), 1.45 (d, J = 6.4 Hz, 3H, CH ), 1.22 (d, J = 9.2
) δ 145.1 (Ar-C), 134.2
(Ar-C), 126.1 (Ar-C), 114.3 (Ar-C), 82.1 (OC), 71.9 (OCH), 24.7
1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
oil; H NMR (400 MHz, CDCl
3
2
3
1
3
(q, J = 6.4 Hz, 1H, OCH), 1.46 (d, J = 6.5 Hz, 3H, CH
J = 11.6 Hz, 12H, CH ); C NMR (101 MHz, CDCl
3 3
3
3 3
Hz, 12H, CH ); C NMR (101 MHz, CDCl
1
3
1
1
(
7
2
(CH
22.15.
2-(spironolactone)pinacolborane (2l).24b White solid; mp
3 3 3
), 24.0 (d, J = 7.1 Hz, CH ); B NMR (128 MHz, CDCl ) δ
3 3
1.4 (OCH), 24.9 (CH ), 24.0 (d, J = 4.7 Hz, CH ).
4
-(1-(4-bromophenyl)ethoxy)pinacolborane (2d). Colorless oil;
1
o
1
H NMR (400 MHz, CDCl
3
) δ 7.45-7.42 (m, 2H, Ar-H),
3
135-137 C; H NMR (400 MHz, CDCl ) δ 5.22 (d, J = 8.9 Hz,
7
.26-7.23 (m, 2H, Ar-H), 5.19 (q, J = 6.4 Hz, 1H, OCH), 1.46 (d,
1H), 4.60-4.56 (m, 1H), 3.92-3.88 (m, 1H), 2.69-2.65 (m, 1H),
2.58-2.45 (m, 2H), 2.43-2.34 (m, 1H), 2.33 (s, 3H), 2.18 (ddd, J =
10.6, 9.1, 3.1 Hz, 1H), 2.07 (dd, J = 14.3, 2.6 Hz, 1H), 1.96-1.88
(m, 3H), 1.81-1.69 (m, 3H), 1.61-1.49 (m, 4H), 1.44-1.29 (m, 4H),
1
3
J = 6.5 Hz, 3H, CH
101 MHz, CDCl ) δ 143.1 (Ar-C), 130.8 (Ar-C), 126.7 (Ar-C),
20.4 (Ar-C), 82.4 (OC), 71.5 (OCH), 24.9 (CH ), 24.0 (d, J =
.4 Hz, CH ).
2-(1-(4-nitrophenyl)ethoxy)pinacolborane (2e).
3 3
), 1.22 (d, J = 11.3 Hz, 12H, CH ); C NMR
(
1
4
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
3
1.25 (s, 12H), 1.08 (s, 3H), 0.95 (s, 3H), 0.78 (td, J = 11.7, 4.3 Hz,
25a
13
Yellow oil;
) δ 8.20-8.18 (m, 2H, Ar-H),
), 5.33 (q, J = 6.5 Hz, 1H, OCH), 1.51 (d, J
1H); C NMR (101 MHz, CDCl
3
) δ 194.3 (s), 176.2 (s), 141.3 (s),
1
H NMR (400 MHz, CDCl
7.55-7.52 (m, 2H, CH
6.5 Hz, 3H, CH ), 1.24 (d, J = 12.7 Hz, 12H, CH
101 MHz, CDCl ) δ 151.4 (Ar-C), 146.6 (Ar-C), 125.6 (Ar-C),
23.1 (Ar-C), 82.6 (OC), 71.3 (OCH), 24.8 (CH ), 24.0 (d, J = 1.1
Hz, CH ).
-(1-(4-methylphenyl)ethoxy)pinacolborane (2f). Colorless oil;
3
125.9 (s), 95.3 (s), 82.1 (s), 69.4 (s), 67.4 (s), 49.2 (s), 45.5, 45.4,
45.0 (s), 38.6 (s), 36.6 (s), 34.7 (s), 34.5 (s), 30.8, 30.7, 28.7 (s),
26.9 (s), 25.1 (s), 24.0 (d, J = 13.8 Hz), 21.9 (s), 19.8 (s), 18.4 (s),
3
1
3
=
(
1
3
3
); C NMR
1
1
3
14.0 (s); B NMR (128 MHz, CDCl
3
) δ 22.09.
4
3
2-(1-(4-acetylphenyl)ethoxy)pinacolborane (3). Colorless oil;
1
3
H NMR (400 MHz, CDCl
3
) δ 7.93 (d, J = 8.3 Hz, 2H, Ar-H),
), 2.59 (s, 3H,
); C NMR (101 MHz, CDCl ) δ 197.3
(Ar-CO), 144.1 (Ar-C), 135.7 (Ar-C), 127.9 (Ar-C), 125.9 (Ar-C),
1
9
2
1
7.43 (d, J = 8.3 Hz, 2H, Ar-H), 4.99 (s, 2H, OCH
COCH
2
13
H NMR (400 MHz, CDCl
.11-7.13 (m, 2H, Ar-H), 5.21 (q, J = 6.4 Hz, 1H, OCH), 2.32 (s,
H, Ar-CH ), 1.47 (d, J = 6.5 Hz, 3H, CH ), 1.22 (d, J = 10.4 Hz,
); 13C NMR (101 MHz, CDCl
) δ 141.2 (Ar-C), 136.2
3
) δ 7.26-7.24 (m, 2H, Ar-H),
3
), 1.27 (s, 12H, CH
3
3
7
3
11
3
3
82.7 (OC), 65.5 (OCH
2 3 3
), 26.1 (COCH ), 24.1 (CH ); B NMR
12H, CH
3
3
(128 MHz, CDCl ) δ 22.44.
3
(Ar-C), 128.4 (Ar-C), 124.8 (Ar-C), 82.2 (OC), 72.0 (OCH),
25.0 (CH
3
), 24.1 (d, J = 3.7 Hz, CH
3
), 20.6 (s, Ar-CH
3
).
19
ASSOCIATED CONTENT
Supporting Information
2
-(1-(4-methoxyphenyl)ethoxy)pinacolborane (2g). Colorless
1
oil; H NMR (400 MHz, CDCl
.86-6.83 (m, 2H, Ar-H), 5.20 (q, J = 6.4 Hz, 1H, OCH), 3.77 (s,
H, OCH ), 1.47 (d, J = 6.4 Hz, 3H, CH ), 1.22 (d, J = 10.4 Hz,
2H, CH ) δ 158.2 (Ar-C), 136.3
); 13C NMR (101 MHz, CDCl
Ar-C), 126.1 (Ar-C), 113.1 (Ar-C), 82.2 (OC), 71.7 (OCH), 54.7
OCH ), 24.8 (s, CH ), 24.0 (d, J = 6.2 Hz, CH ).
-(diphenylmethoxy)pinacolborane (2h).
3
) δ 7.29-7.26 (m, 2H, Ar-H),
6
3
1
(
(
The Supporting Information is available free of charge on the
ACS Publications website.
3
3
3
3
Copies of NMR spectra for all the compounds; DFT
computational methods (PDF)
3
3
3
25a
2
White solid; mp
o
1
1
39-141 C; H NMR (400 MHz, CDCl
Ar-C), 6.17 (s, 1H, OCH), 1.19 (s, 12H, CH
CDCl ) δ 143.1 (Ar-C), 128.2 (Ar-C), 127.3 (Ar-C), 126.5 (Ar-C),
83.0 (OC), 77.9 (OCH), 24.5 (CH ).
-(1-(thiophen-2-yl)ethoxy)pinacolborane (2i). Colorless oil;
3
) δ 7.39-7.20 (m, 10H,
AUTHOR INFORMATION
Corresponding Author
1
3
3
). C NMR (101 MHz,
3
*E-mail: xuemingqiang@suda.edu.cn.
*E-mail: xgbao@suda.edu.cn.
3
26a
2
1
H NMR (400 MHz, CDCl
3
) δ 7.20-7.18 (m, 1H, Ar-H),
6
.97-6.91 (m, 2H, Ar-H), 5.48 (q, J = 6.4 Hz, 1H, OCH), 1.60 (d,
ORCID
1
3
J = 6.4 Hz, 3H, CH
101 MHz, CDCl ) δ 147.7 (Ar-C), 125.9 (Ar-C), 123.6 (Ar-C),
22.8 (Ar-C), 82.4 (OC), 68.1 (OCH), 24.6 (CH ), 24.0 (d, J =
.8 Hz, CH ).
-( isopropylethoxy)pinacolborane (2j). Colorless oil; 1H
NMR (400 MHz, CDCl ) δ 3.94 (p, J = 6.2 Hz, 1H, OCH), 1.66
(dq, J = 13.5, 6.8 Hz, 1H, CH CH), 1.25 (s, 12H, CH ), 1.14 (d, J =
6.3 Hz, 3H, CH ), 0.91-0.85 (m, 6H, CHCH
MHz, CDCl ) δ 82.3 (OC), 75.4 (OCH), 34.2 (OCHCH), 24.5
d, J = 7.4 Hz, CH ), 19.3 (CH ), 18.1 (CH ), 17.7 (CH ).
-(1-(4-aminophenyl)ethoxy)pinacolborane (2k).
3 3
), 1.24 (d, J = 4.9 Hz, 12H, CH ); C NMR
Mingqiang Xue: 0000-0001-6939-240X
Yingming Yao: 0000-0001-9841-316
Qi Shen: 0000-0002-0223-3591
(
1
9
2
3
3
3
Xiaoguang Bao: 0000-0001-7190-8866
19
3
Notes
3
3
The authors declare no competing financial interest.
1
3
3
3
); C NMR (101
3
ACKNOWLEDGMENT
(
2
3
3
3
3
1
9
At the
We gratefully acknowledge financial support from the
National Natural Science Foundation of China (Grant Nos.
outset, 4-aminoacetophenone (1 mmol, THF (200 ul)) and a
stock solution containing n-BuLi (0.05 M) were injected into a
pre-dried vial equipped with a magnetic bar. Then, the
resultant mixture was stirred at room temperature followed
by the slow addition of HBpin (1.1 mmol). The reaction
progress was monitored by H NMR. After completion of
reaction, the solvent and excess HBpin were removed from
the mixture by vacuum drying in an oven to afford the
21372171，21642004 and 21674070) and PAPD.
REFERENCES
1
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corresponding boronate ester without further purification as
a yellowish oil; H NMR (400 MHz, CDCl
1
3
) δ 7.17-7.14 (m, 2H,
ACS Paragon Plus Environment

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The Journal of Organic Chemistry
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7
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（ ）
（
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（29）See SI for computational details. In order to reduce the
computational cost, the n-butyl group in the n-BuLi catalyst was
replaced by the ethyl group.
（30）One may wonder if the alkoxy group in INT4 could
nucleophilically attack the boron site via TS5 to afford the
zwitterionic intermediate INT9, from which the desired product
would be afforded and the catalyst is regenerated via TS6 (Figure
S1). Although the formation of INT9 is not difficult, the
subsequent conversion from INT9 to the final product has a
significantly high energy barrier, suggesting the pathway via
INT9 is not favorable.
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