Ytterbium-Catalyzed Hydroboration of Aldehydes and Ketones

Article abstract of DOI:10.1021/acs.joc.7b02076

The well-defined heavy rare-earth ytterbium iodide complex 1 (L₂YbI) has been successfully employed as an efficient catalyst for the hydroboration of a wide range of aldehydes and ketones with pinacolborane (HBpin) at room temperature. The prot

Full text of DOI:10.1021/acs.joc.7b02076

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Article
Ytterbium-Catalyzed Hydroboration of Aldehydes and Ketones
Weifan Wang, Xingchao Shen, Fengyi Zhao, Huan Jiang,
Weiwei Yao, Sumod A Pullarkat, Li Xu, and Mengtao Ma
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b02076 • Publication Date (Web): 28 Nov 2017
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Ytterbium-Catalyzed Hydroboration of Aldehydes and Ketones
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Weifan Wang,† Xingchao Shen,† Fengyi Zhao,† Huan Jiang,† Weiwei Yao,‡ Sumod A. Pullarkat,§ Li
Xu,† and Mengtao Ma*,†
†College of Science, Nanjing Forestry University, Nanjing 210037, People's Republic of China
‡College of Pharmacy, Nanjing University of Chinese Medicine, Nanjing 210023, People's Republic of China
§Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological
University, Singapore 637371, Singapore
Supporting Information Placeholder
Yb Catalyst
Bpin
R'
O
O
O
O
0.1 to 0.5 mol %
BH
+
R
C6D6, rt
R
R'
H
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25 examples
ABSTRACT: The well-defined heavy rare earth ytterbium iodide complex 1 (L2YbI) has been successfully employed as an effi-
cient catalyst for the hydroboration of a wide range of aldehydes and ketones with pinacolborane (HBpin) at room temperature. The
protocol requires low catalyst loadings (0.1−0.5 mol %) and proceeds rapidly (>99% conversion in <10 min). Additionally, the
catalyst 1 shows good functional group tolerance even towards the hydroxyl and amino moieties and displays chemoselective hy-
droboration of aldehydes over ketones under mild conditions.
workers very recently.7 However, the reactivity toward alde-
hydes was higher than that of ketones in this instance. This
INTRODUCTION
Catalytic hydroboration of unsaturated organic compounds is a
trend is consistent with previously reported transition and
key transformation in commodity and fine chemical produc-
main group metal complexes but opposite to that reported by
tion, materials synthesis, and preparation of complex mole-
Marks et al.
cules. Among the hydroboration of unsaturated bonds such as
Schiff-base ligands are popular in coordination chemistry
C=C, C=N, and C=O double bonds, as well as C≡C triple
and catalytic chemistry by virtue of their ease of preparation,
bonds, the catalytic reduction of carbonyl compounds has been
high yield, and good complexing ability by reaction with metal
extensively studied.1,2 The common methods of hydroboration
ions.8,9 Recently, we used sterically bulky Schiff-base ligands
involve equimolar addition of a borane reagent to carbonyl
to synthesize a series of the corresponding magnesium com-
compounds to form borates, which can be subsequently hydro-
plexes and investigated their catalytic effect in the hydrosilyla-
lyzed to yield alcohols. Borates are excellent organic interme-
tion of ketones.10 While most catalysts are efficient for the
diates in organic synthesis. Thus, a number of catalytic meth-
reduction of aldehydes, they usually perform poorly with ke-
ods are employed for the synthesis of borate derivatives.
tones, requiring long reaction times, high catalyst loadings, or
Commonly, hydroboration reduction of carbonyl compounds
elevated temperatures. Compared to the above-mentioned light
is achieved by the use of transition metal complexes.3 During
rare earth metal (La),6,7 the catalytic hydroboration of heavy
the past several years, the main group metal and non-metal
rare earth metal remains unexplored and therefore deserves
catalyzed hydroboration has also attracted considerable atten-
attention. Herein, we report the facile preparation of the
tion.4 To our surprise, examples of rare earth metals catalyzed
Schiff-base heavy rare earth ytterbium halide complex (L2YbI)
hydroboration are very few.5-7 To the best of our knowledge,
which can catalyze the hydroboration of various aldehydes and
there are only two reports about hydroboration of carbonyl
ketones with pinacolborane with low catalyst loadings and
compounds catalyzed by rare earth metal complexes to date.
excellent reaction rates at room temperature to afford alkoxy-
One is the rapid, clean hydroboration of ketones and aldehydes
pinacolboronate esters.
with HBpin achieved using the homoleptic lanthanum amido
catalyst La[N(SiMe3)2]3 reported by Lohr, Marks and co-
RESULTS AND DISCUSSION
workers in early 2017.6 The reaction rates of ketones in the
The reaction of the extremely bulky ligand 2-((2,6-dibenzh
ydryl-4-isopropylphenylimino)methyl)-4,6-di-tert-butylphenol
(LH) with one equiv KH in THF produced the potassium salt
of Schiff-base ligand. Subsequently, the resultant potassium
salt was treated with YbI3 in THF to generate the correspond-
ing ytterbium iodide complex 1 (L2YbI) as yellow crystals in
moderate yield (Scheme 1). The formation and molecular
hydroboration were generally faster than for aldehydes. This
observation was contrary to what has been observed previous-
ly, where aldehydes are more reactive than ketones. The other
report involves a series of tri(cyclopentadienyl)lanthanide
complexes Cp3Ln (Ln = La, Nd, Sm, Yb, and Y) that can be
employed as excellent catalysts for the hydroboration of vari-
ous carbonyl compounds reported by Xue, Yao, Shen and co-
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structure of 1 was confirmed by single-crystal X-ray diffrac-
Table 1. Hydroboration of Aldehydes Catalyzed by 1
tion (Figure S1). In complex 1, the ytterbium atom is in a five
coordinate distorted trigonal bipyramid environment in which
one iodine atom and two nitrogen atoms define the equatorial
plane and the two oxygen atoms of Schiff-base ligands occupy
the apical position.
Time
(min)
Yield
(%)a
Entry
1
Product
Scheme 1. Synthesis of Complex 1
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2a
2b
<10
<10
>99
>99
2
3
2c
120
>99
It well known that the rare earth metal atom radius has a
considerable effect on the catalytic activity.11 In the aforemen-
tioned two examples of rare earth catalyzed hydroboration of
carbonyl compounds, the lanthanum metal of both
La[N(SiMe3)2]3 and Cp3La catalysts is light rare earth ele-
ment.6,7 The heavy rare earth metal catalyzed hydroboration of
aldehydes and ketones is not introduced in detail in literature
although Shen et al. briefly mentioned the test of Cp3Yb dur-
ing the optimization of hydroboration reaction conditions.7 We
found that the heavy rare earth ytterbium iodide complex 1
was a very efficient catalyst for the hydroboration of alde-
hydes and ketones. This is contrary to that observed with the
precursor of the manganese alkyl complexes reported by
Zhang et al., namely the manganese chloride, which is not an
active catalyst for the hydroboration of carbonyl compounds.3g
Therefore, we used the heavy rare earth ytterbium halide com-
plex 1 as catalyst to investigate the hydroboration of aldehydes
and ketones in detail.
4
5
2d
2e
80
>99
>99
<10
6
2f
120
>99
7
8
2g
2h
50
>99
>99
As expected, the catalyst-free hydroboration of benzalde-
hyde displayed little conversion.6 In contrast, when we began
our investigation with the addition of only 0.5 mol % of cata-
lyst 1 to the mixture of benzaldehyde and HBpin, we observed
the formation of PhCH2OBpin in quantitative yield within less
than 10 min at room temperature. Encouraged by this we then
decreased the catalyst loading to 0.1 mol %, and noted that the
catalyst could still give full conversion in the same reaction
time (Table 1, entry 1). Therefore, we proceeded to explore
the hydroboration of a wide range of aldehydes by using 0.1
mol % catalyst loading at room temperature as optimized con-
dition. Progress of the hydroboration reaction was monitored
<10
9
2i
2j
<10
<10
<10
>99
>99
>99
10
11
2k
1
by H NMR of the reaction mixture, which confirmed the
quantitative formation of boronate esters. Table 1 summarizes
the full scope of aldehydes substrates investigated in this re-
port. It should be noted that the reactivity of catalyst 1 toward
aldehydes is higher than that of the reported La[N(TMS)2]3
complex and in some instances are indeed better than that of
Cp3La.6,7 The position of halogen on the phenyl ring has an
important effect on the reactivity, as demonstrated by the clean
hydroboration of 2/3/4-chlorobenzaldehyde or 2/3/4-
fluorobenzaldehyde (Table 1, entries 3-7). It can be seen that
the presence of the chloride or fluoride groups in the ortho
position of benzaldehyde led to full conversion in less than 10
min of the parent benzaldehyde. However, the meta/para-
halide-substituted benzaldehyde needs a slightly longer time to
achieve full conversion when compared to the ortho substitut-
ed analogue. This substituent effect is different from that ob-
served for the Cp3La based protocol wherein the substituent
position (o, m, p) on the benzene ring have no significant
12
13
14
2l
2m
2n
<10
<10
<10
>99
>99
>99
15
16
2o
2p
60
>99
>99
<10
a Yield was determined by 1H NMR spectroscopy.
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impact on the reactivity.7 Under the same reaction conditions,
Table 2. Hydroboration of Ketones Catalyzed by 1
both electron withdrawing and electron donating groups such
as Me2N-, O2N-, Me- and MeO-substituted benzaldehydes
underwent the reaction rapidly with equal efficiency (Table 1,
entries 8-11). Aliphatic aldehyde, cinnamaldehyde and 9-
anthraldehyde were also hydroborated in full conversion with
0.1 mol % catalyst loading in less than 10 min (Table 1, en-
tries 12-14). It needs to be noted that for α, β-unsaturated cin-
namaldehyde, the catalytic activity of the ytterbium complex 1
is higher than that of observed when employing La[N(TMS)2]3
and Cp3La catalysts. Furthermore, those procotols need 1.0
mol % catalyst, 15 min reaction time and 0.1 mol % catalyst,
120 min reaction time, respectively.6,7 No dearomatization of
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Time
(min)
Yield
Entry
1
Product
(%)a
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50
>99
2
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3c
3d
3e
3f
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>99
>99
92
the
thiophene
ring
was
obtained
when
2-
thiophenecarboxaldehyde was used, underscoring the catalyst
selectivity towards the C=O functionality (Table 1, entry 15).
Surprisingly, when 3-hydroxybenzaldehyde was used, hy-
droboration occurred at the aldehyde functional group instead
of hydroxylborane dehydrocoupling (Table 1, entry 16). How-
ever, in the absence of catalyst, the dehydrocoupling of HBpin
was achieved albeit with low yield when the same reaction is
carried out as reported by Bertrand and co-workers.12 No rare
earth metal catalyst has so far been reported to show tolerance
towards the phenolic OH group.
180
60
>99
>99
As expected, the Yb-catalyzed hydroboration of more steri-
cally bulky ketonic carbonyl functions normally required
slightly higher catalyst loadings than that of aldehydes to
achieve similar results (Table 2). This observation is consistent
with the aforementioned Cp3La based protocol, but contrary to
that reported for the La[N(TMS)2]3 catalyst.6,7 Initial investiga-
tion on the hydroboration of acetophenone catalyzed by 1 with
pinacolborane at room temperature under different catalyst
loading was carried out. We observed that the acetophenone
was clearly converted into the corresponding borate ester in 30
min at 1 mol % catalyst loading while the 0.5 mol % catalyst
loading in 50 min gave the same conversion (Table 2, entry 1).
Encouraged by this result, a wide range of aromatic and ali-
phatic ketones were investigated for hydroboration activity
with HBpin at room temperature using catalyst loadings as
low as 0.5 mol % (Table 2). We chose the 0.5 mol % catalyst
loading for the conversion since the reaction time is almost the
same and the catalyst loading is relatively lower compared to
1.0 mol %. In most cases, the reaction was completed within 1
h in quantitative yields. For the hydroboration of aromatic
ketones with electron withdrawing or electron donating group
(Table 2, entries 2−8), quantitative conversion was obtained.
4-Acetylbenzonitrile and 4-aminoacetophenone (Table 2, en-
tries 4, 11) need a slightly longer time since it will react with
HBpin leading to the uncatalyzed amine-borane blocking cata-
lytic C=O hydroboration. Comparison of the reactivity of ace-
tophenone and 2,4,6-trimethylacetophenone once again high-
lighted the im portance of steric factors in these hydroboration
reactions (Table 2, entry 8). Under the same reaction condi-
tions, dialkyl ketones were independently reacted with an
equimolar amount of HBpin in full yields in a very short time
(Table 2, entries 9, 10). The reaction showed excellent func-
tional group tolerance toward NH functionality: for 4-
aminoacetophenone, the reaction with HBpin led to the corre-
sponding borate ester instead of dehydrocoupling product13
(Table 2, entry 11).
<10
7
8
9
3g
3h
3i
50
>99
99
48*60
<10
>99
10
11
3j
<10
>99
>99
3k
4.5*60
a Yield was determined by 1H NMR spectroscopy.
were treated with 0.1 mol % 1. It resulted in 99 % conversion
of benzaldehyde in 20 min. The acetophenone remained al-
most intact. 1H NMR analysis indicated the presence of more
than 99 % of unreacted acetophenone in the reaction mixture.
Such functional group selectivity for aldehyde versus ketone
hydroboration with pinacolborane is similar to that previously
reported using lanthanum, ruthenium and aluminum cata-
lysts.3a,5e,6,7 Similar intermolecular chemoselectivity is also
observed in competitive catalytic hydroboration reactions of 2-
fluorobenzaldehyde over 2-fluoroacetophenone and cyclohex-
anecarbaldehyde over cyclohexyl methyl ketone (Scheme 2).
The challenging chemoselective hydroboration of aldehydes
over the ketones using catalyst 1 was also explored. Equimolar
amounts of benzaldehyde, acetophenone, and pinacolborane
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Scheme 2. Chemoselective Hydroboration of Aldehyde /
the solution at −80 °C. The suspension was warmed to room
temperature and stirred overnight. The resultant solution was
filtered, all volatiles were removed in vacuo, and the residue
was extracted with toluene (20 mL). The solution was concen-
trated to ca. 5 mL. Complex 1 was obtained as yellow crystals
at −30 °C after several days (0.82 g, 60%). M.p. 282−284 °C.
Anal Calcd for C100H104IN2O2Yb: C, 72.10; H, 6.29; N, 1.68.
Found C, 72.58; H, 6.63; N, 1.32. IR (KBr, cm-1): 3429, 3061,
3028, 2960, 2867, 1626 (C=N), 1602, 1571, 1535, 1445, 1400,
1253, 1162, 1117, 1076, 1022, 803, 758, 699, 604. Note: NMR
data for 1 could not be obtained due to paramagnetic character.
General Procedure for Catalytic Hydroboration of Alde-
hydes. In a glove box, catalyst 1 (0.1 mol %) was added to a
solution of aldehyde (1 mmol) and pinacolborane (1.2 mmol)
in a J. Young NMR tube equipped with a Teflon screw cap,
which was charged with C6D6 (0.5 mL). The progress of the
reaction was monitored by 1H NMR, 13C NMR and 11B NMR,
which indicated the completion of the reaction by the disap-
pearance of aldehyde (RCHO) proton and appearance of a new
CH2 resonance.
Ketone Catalyzed by 1
CHO
O
OBpin
O
0.1 mol% 1
+
+ HBpin
+
20 min, C6D6, rt
99%
O
O
OBpin
F
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CHO
0.1 mol% 1
F
F
F
+
+ HBpin
+
20 min,
C6D6, rt
98%
OBpin
O
O
CHO
0.1 mol% 1
20 min,
+
+
+
HBpin
C6D6, rt
99%
Spectroscopic Data for Aldehyde Hydroboration Prod-
CONCLUSION
ucts. 2-(Benzyloxy)pinacolborane (2a).4 1H NMR (600 MHz,
In summary, we have demonstrated that Schiff-base heavy
rare earth ytterbium halide complex 1 was an efficient catalyst
for the hydroboration of ketones and aldehydes with HBpin. It
demonstrated higher reactivity than that reported for the simi-
lar amidinato calcium iodide complex that typically needed
higher catalyst loading (0.5-3 mol %) and longer reaction time
(40 min to 5 h).5j The newly developed protocol also shows
good functional group tolerance even towards OH and NH
groups. High chemoselectivity for aldehyde hydroboration
over ketones and C=C double bond is also observed. As
known, the lanthanide halide is also a very useful precursor for
other lanthanide derivatives. Hence further investigations of
the potential of the ytterbium iodide 1, such as the preparation
of the corresponding ytterbium alkyl and amide complexes etc.
and their catalytic behavior, are currently in progress.
3
C6D6): δ 7.30 (d, JHH = 7.2 Hz, 2 H), 7.13 (t, 3JHH = 7.2 Hz, 2
H), 7.05 (t, 3JHH = 7.2 Hz, 1 H), 4.95 (s, 2 H), 1.04 (s, 12 H).
13C{1H} NMR (151 MHz, C6D6): δ 140.1, 128.6, 127.6, 127.1,
82.7, 67.0, 24.7. 11B{1H} NMR (193 MHz, C6D6): δ 22.82.
2-(2-Chlorobenzyloxy)pinacolborane (2b).7 1H NMR (600
MHz, C6D6): δ 7.55 (d, 3JHH = 7.2 Hz, 1 H), 7.08 (d, 3JHH = 7.2
Hz, 1 H), 6.92 (t, 3JHH = 7.2 Hz, 1 H), 6.77 (t, 3JHH = 7.2 Hz, 1
H), 5.14 (s, 2 H), 1.04 (s, 12 H). 13C{1H} NMR (151 MHz,
C6D6): δ 137.6, 132.2, 129.3, 128.1, 127.9, 127.0, 82.9, 64.4,
24.7. 11B{1H} NMR (193 MHz, C6D6): δ 22.80.
2-(3-Chlorobenzyloxy)pinacolborane (2c).7 1H NMR (600
MHz, C6D6): δ 7.30 (s, 1 H), 7.00 (t, 3JHH = 6.6 Hz, 2 H), 6.82
3
(t, JHH = 7.8 Hz, 1 H), 4.74 (s, 1 H), 1.03 (s, 12 H). 13C{1H}
NMR (151 MHz, C6D6): δ 142.1, 134.5, 129.8, 127.6, 127.1,
124.9, 82.9, 66.0, 24.6. 11B{1H} NMR (193 MHz, C6D6): δ
22.76
EXPERIMENTAL SECTION
2-(4-Chlorobenzyloxy)pinacolborane (2d).5e 1H NMR (600
MHz, C6D6): δ 7.07 (d, 3JHH = 8.4 Hz, 2 H), 7.00 (d, 3JHH = 8.4
Hz, 2 H), 4.76 (s, 2 H), 1.03 (s, 12 H). 13C{1H} NMR (151
MHz, C6D6): δ 138.5, 133.4, 128.7, 128.4, 82.9, 66.1, 24.7.
11B{1H} NMR (193 MHz, C6D6): δ 22.73.
General Information. All air-sensitive compounds were
carried out using standard Schlenk-line or glove box tech-
niques under high-purity argon. Toluene, THF and hexane
were dried and distilled from molten sodium. C6D6 was dried
1
2-(2-Fluorobenzyloxy)pinacolborane (2e).5i 1H NMR (600
MHz, C6D6): δ 7.41−7.44 (m, 1 H), 6.85−6.81 (m, 2 H),
6.76−6.73 (m, 1 H), 5.09 (s, 2 H), 1.04 (s, 12 H). 13C{1H}
NMR (151 MHz, C6D6): δ 161.5, 159.9, 129.2, 124.3, 115.3,
115.1, 82.9, 61.0, 24.7. 11B{1H} NMR (193 MHz, C6D6): δ
22.81.
over sodium mirror and freeze–thawed twice prior to use. H,
13C{1H}, and 11B{1H} NMR spectra were recorded at 25 °C on
Bruker Avance III 600 MHz spectrometer in deuterated sol-
vents and were referenced to the resonances of the solvent
used. IR Spectrum for complex 1 was recorded in Perkin–
Elmer FT–IR spectrometer. Microanalyses were performed by
the Elemental Analysis Laboratory of the Advanced Analysis
and Testing Center at Nanjing Forestry University. Melting
points were determined in sealed capillaries under dinitrogen
and are uncorrected. Ligand was prepared according to a liter-
ature procedure.14 Chemicals were purchased from Sigma–
Aldrich, Alfa–Aesar, and Acros and used without further puri-
fication.
2-(3-Fluorobenzyloxy)pinacolborane (2f).5i 1H NMR (600
MHz, C6D6): δ 7.05−6.68 (m, 5 H), 4.79 (s, 2 H), 1.03 (s, 12
H). 13C{1H} NMR (151 MHz, C6D6): δ 130.1, 130.0, 122.24,
122.21, 114.4, 114.1, 113.9, 113.6, 82.8, 66.0, 24.6. 11B{1H}
NMR (193 MHz, C6D6): δ 22.74.
2-(4-Fluorobenzyloxy)pinacolborane (2g).6 1H NMR (600
MHz, C6D6): δ 7.09−7.06 (m, 2 H), 6.78−6.75 (m, 2 H), 4.79
(s, 2 H), 1.04 (s, 12 H). 13C{1H} NMR (151 MHz, C6D6): δ
163.4, 161.8, 135.8, 128.9, 115.4, 115.3, 82.8, 66.2, 24.7.
11B{1H} NMR (193 MHz, C6D6): δ 22.73.
Experimental Procedure. Synthesis of Complex 1. KH
(0.07 g, 1.75 mmol) in THF (5 mL) was added dropwise to a
solution of ligand 2-((2,6-dibenzhydryl-4-isopropylphenyl
imino)methyl)-4,6-di-tert-butylphenol (1.00 g, 1.46 mmol) in
THF (40 mL) at −80 °C. The suspension was warmed to room
temperature and stirred for 3 hours. Then the solution of YbI3
(0.81 g, 1.46 mmol) in THF (20 mL) was added dropwise to
2-(4-N,N-Dimethylaminobenzyloxy)pinacolborane (2h).5e 1
H
NMR (600 MHz, C6D6): δ 7.34 (d, 3JHH = 8.4 Hz, 2 H), 6.56 (d,
3JHH = 8.4 Hz, 2 H), 5.00 (s, 2 H), 2.50 (s, 6 H), 1.07 (s, 12 H).
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13C{1H} NMR (151 MHz, C6D6): δ 150.6, 129.0, 112.8, 82.5,
2-(1-(2-Fluorophenyl)ethoxy)pinacolborane (3b).5k,7 1H
67.2, 40.3, 24.8. 11B{1H} NMR (193 MHz, C6D6): δ 22.80.
2-((4-Methylbenzyl)oxy)pinacolborane (2i).6,7 1H NMR (600
MHz, C6D6): δ 7.26 (d, 3JHH = 7.8 Hz, 2 H), 6.97 (d, 3JHH = 7.8
Hz, 2 H), 4.96 (s, 2 H), 2.08 (s, 3 H), 1.05 (s, 12 H). 13C{1H}
NMR (151 MHz, C6D6): δ 137.2, 137.0, 129.3, 127.3, 82.7,
66.9, 24.7, 21.1. 11B{1H} NMR (193 MHz, C6D6): δ 22.82.
2-(4-Nitrophenyl) pinacolborane (2j).5j 1H NMR (600 MHz,
C6D6): δ 7.80 (d, 3JHH = 8.4 Hz, 2 H), 6.93 (d, 3JHH = 7.8 Hz, 2
H), 4.69 (s, 1 H), 1.05 (s, 12 H). 13C{1H} NMR (151 MHz,
C6D6): δ 147.6, 146.6, 126.8, 123.6, 83.2, 65.6, 24.7. 11B{1H}
NMR (193 MHz, C6D6): δ 22.71.
NMR (600 MHz, C6D6): δ 7.63−7.60 (m, 1 H), 6.87−6.84 (m,
3
2 H), 6.77−6.74 (m, 1 H), 5.86 (q, JHH = 6.6, 1 H), 1.49 (d,
3JHH = 6.6 Hz, 3 H), 1.02 (s, 6 H), 1.00 (s, 6 H). 13C{1H} NMR
(151 MHz, C6D6): δ 160.5, 132.5, 128.7, 127.1, 124.5, 115.3,
82.7, 67.0, 25.0, 24.6, 24.6. 11B{1H} NMR (193 MHz, C6D6): δ
22.48.
2-(1-(4-Nitrophenyl)ethoxy)pinacolborane (3c).5j,7 1H NMR
(600 MHz, C6D6): δ 7.81 (d, 3JHH = 8.4 Hz, 2 H), 7.02 (d, 3JHH
= 8.4 Hz, 2 H), 5.19 (q, 3JHH = 6.6 Hz, 1 H), 1.26 (d, 3JHH = 6.6
Hz, 3 H), 1.04 (s, 6 H), 1.01 (s, 6 H). 13C{1H} NMR (151
MHz, C6D6): δ 151.8, 147.5, 126.1, 123.6, 82.9, 71.6, 25.3,
24.6. 11B{1H} NMR (193 MHz, C6D6): δ 22.39.
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2-(4-Methoxybenzyloxy)pinacolborane (2k).6 1H NMR (600
MHz, C6D6): δ 7.26 (d, 3JHH = 7.8 Hz, 2 H), 6.75 (d, 3JHH = 7.8
Hz, 2 H), 4.93 (s, 1 H), 3.30 (s, 3 H), 1.05 (s, 12 H). 13C{1H}
NMR (151 MHz, C6D6): δ 159.7, 132.2, 128.9, 114.1, 82.7,
66.8, 54.8, 24.7. 11B{1H} NMR (193 MHz, C6D6): δ 22.80.
2-(Cyclohexylmethoxy)pinacolborane (2l).5e 1H NMR (600
2-(1-(4-Benzonitrile)ethoxy)pinacolborane (3d).15 1H NMR
(600 MHz, C6D6): δ 7.03 (d, 3JHH = 8.4 Hz, 2 H), 6.97 (d, 3JHH
= 8.4 Hz, 2 H), 5.15 (q, 3JHH = 6.6 Hz, 1 H), 1.24 (d, 3JHH = 6.6
Hz, 3 H), 1.03 (s, 6 H), 1.00 (s, 6 H). 13C{1H} NMR (151
MHz, C6D6): δ 149.8, 132.2, 126.0, 118.9, 111.6, 82.5, 72.1,
25.3, 24.6, 24.6. 11B{1H} NMR (193 MHz, C6D6): δ 22.41.
2-(Diphenylmethoxy)pinacolborane (3e).7 1H NMR (600
MHz, C6D6): δ 7.45−7.44 (m, 4 H), 7.11−7.08 (m, 4 H),
7.02−6.99 (m, 2 H), 6.43 (s, 1 H), 0.98 (s, 12 H). 13C{1H}
NMR (151 MHz, C6D6): δ 143.9, 128.6, 127.6, 127.0, 82.9,
78.5, 24.2. 11B{1H} NMR (193 MHz, C6D6): δ 22.84.
3
MHz, C6D6): δ 3.77 (d, JHH = 6.6 Hz, 2 H), 1.73−1.59 (m, 4
H), 1.55−1.51 (m, 2 H), 1.15−1.09 (m, 3 H), 1.07 (s, 12 H),
0.92−0.90 (m, 2 H). 13C{1H} NMR (151 MHz, C6D6): δ 82.4,
70.6, 39.9, 29.7, 26.9, 26.2, 24.8. 11B{1H} NMR (193 MHz,
C6D6): δ 22.55.
2-(Cinnamyloxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(2m).5e 1H NMR (600 MHz, C6D6): δ 7.18−7.17 (m, 2 H),
7.10−7.07 (m, 2 H), 7.03−7.00 (m, 1 H), 6.62−6.59 (m, 1 H),
2-(2-Methyl-1-phenylpropoxy)pinacolborane (3f).3a 1H
NMR (600 MHz, C6D6): δ 7.33 (d, 3JHH = 7.2 Hz, 2 H), 7.14 (t,
3JHH = 7.8 Hz, 2 H), 7.05 (t, 3JHH = 7.8 Hz, 1 H), 5.03 (d, 3JHH
= 6.6 Hz, 1 H), 1.97 (q, 3JHH = 6.6 Hz, 1 H), 1.01 (s, 6 H), 0.98
(s, 6 H), 0.97 (d, 3JHH = 3.6 Hz, 3 H), 0.84 (d, 3JHH = 6.6 Hz, 3
H). 13C{1H} NMR (151 MHz, C6D6): δ 143.2, 127.4, 126.9,
82.1, 81.9, 35.8, 24.62, 24.59, 19.1, 17.8. 11B{1H} NMR (193
MHz, C6D6): δ 22.66.
3
6.19−6.15 (m, 1 H), 4.53 (d, JHH = 5.4 Hz, 2 H), 1.07 (s, 12
H). 13C{1H} NMR (151 MHz, C6D6): δ 137.4, 130.9, 128.8,
127.7, 127.5, 126.9, 82.7, 65.5, 24.7. 11B{1H} NMR (193 MHz,
C6D6): δ 22.74.
2-(Anthracen-9-ylmethoxy)pinacolborane (2n).6 1H NMR
(600 MHz, C6D6): δ 8.58 (d, 3JHH = 9.0 Hz, 2 H), 8.12 (s, 1 H),
7.75 (d, 3JHH = 7.2 Hz, 2 H), 7.33 (t, 3JHH = 7.2 Hz, 2 H), 7.23
2-(1-(4-Methoxyphenyl)ethoxy)pinacolborane (3g).7 1H
NMR (600 MHz, C6D6): δ 7.30 (d, 3JHH = 8.4 Hz, 2 H), 6.76 (d,
3JHH = 8.4 Hz, 2 H), 5.41 (q, 3JHH = 6.6 Hz, 1 H), 3.30 (s, 3H),
3
(t, JHH = 7.2 Hz, 2 H), 5.90 (s, 2 H), 1.01 (s, 12 H). 13C{1H}
NMR (151 MHz, C6D6): δ 132.0, 131.2, 130.5, 129.3, 128.8,
126.2, 125.2, 125.1, 82.8, 59.7, 24.7. 11B{1H} NMR (193 MHz,
C6D6): δ 22.86.
3
1.48 (d, JHH = 6.6 Hz, 3 H), 1.04 (s, 6 H), 1.02 (s, 6 H).
13C{1H} NMR (151 MHz, C6D6): δ 159.4, 137.5, 127.0, 114.0,
82.5, 72.7, 54.8, 25.7, 24.7, 24.6. 11B{1H} NMR (193 MHz,
C6D6): δ 22.56.
2-(Thiophen)pinacolborane (2o).6 1H NMR (600 MHz,
C6D6): δ 6.86-6.66 (m, 3 H), 5.00 (s, 2 H), 1.04 (s, 12 H).
13C{1H} NMR (151 MHz, C6D6): δ 142.8, 126.8, 126.1, 125.7,
82.9, 61.8, 24.7. 11B{1H} NMR (193 MHz, C6D6): δ 22.78.
2-(Phenol)pinacolborane (2p).5j 1H NMR (600 MHz, C6D6):
δ 7.12−7.03 (m, 2 H), 6.96−6.75 (m, 2 H), 5.86 (s, 1 H), 4.91
(s, 2 H), 1.03 (s, 12 H). 13C{1H} NMR (151 MHz, C6D6): δ
157.1, 141.5, 129.8, 118.7, 114.9, 114.1, 82.1, 66.9, 24.6.
11B{1H} NMR (193 MHz, C6D6): δ 21.98.
2-(Mesitylmethoxy)pinacolborane (3h).7 1H NMR (600
MHz, C6D6): δ 6.72 (s, 2 H), 5.85 (q, 3JHH = 7.2 Hz, 1 H), 2.47
(s, 6 H), 2.09 (s, 3 H), 1.53 (d, 3JHH = 7.2 Hz, 3 H), 0.99 (s, 6
H), 0.96 (s, 6 H). 13C{1H} NMR (151 MHz, C6D6): δ 137.4,
136.1, 135.8, 130.3, 82.4, 70.3, 24.7, 24.5, 22.0, 20.8. 11B{1H}
NMR (193 MHz, C6D6): δ 22.51.
2-(Cyclohexyloxy)pinacolborane (3i).5f 1H NMR (600 MHz,
C6D6): δ 4.20 (sept, 3JHH = 4.2 Hz, 1 H), 1.89−1.87 (m, 2 H),
1.62−1.59 (m, 2 H), 1.50−1.44 (m, 2 H), 1.30−1.27 (m, 2 H),
1.16−1.10 (m, 2 H), 1.07 (s, 12 H). 13C{1H} NMR (151 MHz,
C6D6): δ 82.2, 72.7, 34.8, 25.8, 24.7, 24.1. 11B{1H} NMR (193
MHz, C6D6): δ 22.32.
General Procedure for Catalytic Hydroboration of Ke-
tones. In a glove box, catalyst 1 (0.5 mol %) was added to a
solution of ketone (1 mmol) and pinacolborane (1.2 mmol) in
a J. Young NMR tube equipped with a Teflon screw cap,
which was charged with C6D6 (0.5 mL). The progress of the
reaction was monitored by 1H NMR, 13C NMR and 11B NMR,
which indicated the completion of the reaction by the appear-
ance of a new CH resonance.
2-(Cyclohexylmethoxy)pinacolborane (3j).5f 1H NMR (600
MHz, C6D6): δ 4.12 (q, 3JHH = 6.0 Hz, 1 H), 1.88−1.86 (m, 1
H), 1.68−1.63 (m, 3 H), 1.57−1.55 (m, 1 H), 1.33−1.29 (m, 1
3
Spectroscopic Data for Ketone Hydroboration Products.
2-(Phenylethoxy)pinacolborane (3a).4 1H NMR (600 MHz,
C6D6): δ 7.36 (d, 3JHH = 7.2 Hz, 2 H), 7.14 (t, 3JHH = 7.2 Hz, 2
H), 7.05 (t, 3JHH = 7.2 Hz, 1 H), 5.40 (q, 3JHH = 6.6 Hz, 1 H),
H), 1.18 (d, JHH = 6.0 Hz, 3 H), 1.15−1.11 (m, 2 H), 1.08 (s,
12 H), 1.06−1.03 (m, 2 H), 0.98−0.96 (m, 1 H). 13C{1H} NMR
(151 MHz, C6D6): δ 82.2, 75.0, 44.7, 29.1, 28.6, 26.9, 26.6,
26.5, 24.75, 24.70, 20.1. 11B{1H} NMR (193 MHz, C6D6): δ
22.41.
3
1.45 (d, JHH = 6.6 Hz, 3 H), 1.03 (s, 6 H), 1.00 (s, 6 H).
13C{1H} NMR (151 MHz, C6D6): δ 145.4, 128.5, 127.4, 125.7,
82.5, 73.0, 25.8, 24.7, 24.6. 11B{1H} NMR (193 MHz, C6D6): δ
22.51.
2-(1-(4-Aminophenyl)ethoxy)pinacolborane (3k).5j 1H NMR
(600 MHz, C6D6): δ 7.22 (d, 3JHH = 8.4 Hz, 2 H), 6.35 (d, 3JHH
= 8.4 Hz, 2 H), 5.38 (q, 3JHH = 6.6 Hz, 1 H), 2.93 (s, 2 H), 1.51
3
(d, JHH = 6.6 Hz, 3 H), 1.03 (s, 6 H), 1.01 (s, 6 H). 13C{1H}
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NMR (151 MHz, C6D6): δ 146.5, 134.8, 126.9, 126.6, 117.9,
(14) Zhang, W.; Wang, Y.; Redshaw, C.; Hao, X.; Sun, W. H.
114.9, 82.4, 72.9, 25.7, 24.7, 24.6. 11B{1H} NMR (193 MHz,
C6D6): δ 22.49.
J. Organomet. Chem. 2012, 715, 119.
(15) Xuan, Q.; Zhao, C.; Song, Q. Org. Biomol. Chem. 2017,
15, 5140.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
NMR spectra of hydroboration products (PDF)
Crystallographic data for the complex 1 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*Email: mengtao@njfu.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank financial supports from the National Natural Science
Foundation of China (no. 21372117, 21772093), the Natural Sci-
ence Foundation of Jiangsu Province, China (no. BK20141468),
PAPD, the Doctorate Fellowship Foundation of Nanjing Forestry
University (no. 163360767), and the Qing Lan Project of Jiangsu
Province.
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