The chiral boronate-catalyzed asymmetric transfer hydrogenation of various aromatic ketones to high-value alcohols: Preparation and spectroscopic studies

Article abstract of DOI:10.1016/j.jorganchem.2019.03.017

This work deals with the synthesis, spectroscopic studies and catalytic evaluation of the novel chiral salen (L₁H₂) and (L₂H₂) ligands and their chiral boronate [L₁(B_1-4)] and [L₂(B_1-4)] complexes. Initially, the reaction of 5-azidomethyl salicylaldehyde and (R)-(?)-2-amino-1-butanol in absolute ethanol afforded a new chiral salen ligand (L₁H₂). Then, a novel chiral salen ligand (L₂H₂) have been prepared from chiral salen ligand (L₁H₂) for the synthesis of boronate [L₂(B_1-4)] complexes through click reaction approach under ambient conditions. The reaction of chiral salen (L₁H₂) and (L₂H₂) ligands with various boronic acids afforded a new tetra-coordinated mononuclear chiral boronate [L₁(B_1-4)] and [L₂(B_1-4)] complexes. All the compounds are remarkably stable crystalline solids and were obtained in good yields. For the full characterization of newly synthesized chiral salen ligands and their boronate complexes, the FT-IR, UV–Vis, NMR (¹H, ¹³C, and ¹¹B), LC-MS, and elemental analysis techniques have been used. The well-shaped chiral boronate compounds were investigated as catalyst for the asymmetric transfer hydrogenation (ATH) of aromatic ketones under appropriate settings. Particularly, it was proved that the ferrocene-based boronate compounds can afford an efficient catalytic conversion compared to the other boronate complexes in the asymmetric transfer hydrogenation catalytic studies.

Full text of DOI:10.1016/j.jorganchem.2019.03.017

Accepted Manuscript
The chiral boronate-catalyzed asymmetric transfer hydrogenation of various aromatic
ketones to high-value alcohols: Preparation and spectroscopic studies
Ahmet Kilic, Mustafa Durgun, Feyyaz Durap, Murat Aydemir
PII:
S0022-328X(19)30110-X
DOI:
https://doi.org/10.1016/j.jorganchem.2019.03.017
JOM 20745
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 9 January 2019
Revised Date: 30 January 2019
Accepted Date: 22 March 2019
Please cite this article as: A. Kilic, M. Durgun, F. Durap, M. Aydemir, The chiral boronate-catalyzed
asymmetric transfer hydrogenation of various aromatic ketones to high-value alcohols: Preparation
and spectroscopic studies, Journal of Organometallic Chemistry (2019), doi: https://doi.org/10.1016/
j.jorganchem.2019.03.017.
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ACCEPTED MANUSCRIPT
The Chiral Boronate-Catalyzed Asymmetric Transfer Hydrogenation of Various
Aromatic Ketones to High-Value Alcohols: Preparation and Spectroscopic Studies
Ahmet KILICa*, Mustafa DURGUNa, Feyyaz DURAPb, Murat AYDEMİRb#
a Harran University, Chemistry Department, Art and Science Faculty, 63290, Sanliurfa, Turkey
b DicleUniversity, Chemistry Department, Science Faculty, 21280, Diyarbakır, Turkey
Abstract
This work deals with the synthesis, spectroscopic studies and catalytic evaluation of the novel
chiral salen (L1H2) and (L2H2) ligands and their chiral boronate [L1(B1-4)] and [L2(B1-4)]
complexes. Initially, the reaction of 5-azidomethyl salicylaldehyde and (R)-(-)-2-amino-1-
butanol in absolute ethanol afforded a new chiral salen ligand (L1H2). Then, a novel chiral
salen ligand (L2H2) have been prepared from chiral salen ligand (L1H2) for the synthesis of
boronate [L2(B1-4)] complexes through click reaction approach under ambient conditions. The
reaction of chiral salen (L1H2) and (L2H2) ligands with various boronic acids afforded a new
tetra-coordinated mononuclear chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes. All the
compounds are remarkably stable crystalline solids and were obtained in good yields. For the
full characterization of newly synthesized chiral salen ligands and their boronate complexes,
the FT-IR, UV-Vis, NMR (1H, 13C, and 11B), LC-MS, and elemental analysis techniques have
been used. The well-shaped chiral boronate compounds were investigated as catalyst for the
asymmetric transfer hydrogenation (ATH) of aromatic ketones under appropriate settings.
Particularly, it was proved that the ferrocene-based boronate compounds can afford an
efficient catalytic conversion compared to the other boronate complexes in the asymmetric
transfer hydrogenation catalytic studies.
Keywords: Chiral salen ligands, Boronate complexes, Spectroscopy, Catalyst, Asymmetric
Transfer Hydrogenation, Ketone
To whom correspondence should be addressed: Harran University, Chemistry Department, TR-63290
Sanlıurfa, Turkey. Tel: +90 414 318 3587, Fax: +90 414 318 3541, e-mail: kilica63@harran.edu.tr;
http://orcid.org/0000-0001-9073-4339 (A. Kilic)
#
Dicle University, Chemistry Department, Science Faculty, 21280, Diyarbakır, Turkey, Tel: +90 412
2488550/3028; aydemir@dicle.edu.tr (M. Aydemir)
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1. Introduction
In the last decades, environmentally friendly tri and tetra-coordinate boron compounds
and related different boronate derivatives have found a wide range of application fields in
synthetic chemistry, chemical industry, materials science, energy research, pharmacology,
cytotoxicity and bioimaging cells, catalysis and other application areas [1-5]. In addition,
highly Lewis acidic organoboron compounds continue to attract tremendous interest and
current applications range from Lewis acid catalysis of organic transformations and
polymerization reactions to small molecule activation, sensors for anions, and the
development of new and unusual electronic materials [6]. Besides, significant advances for
developing a wide range of new chiral catalysts and improvement studies toward high
enantioselectivity and practicability of boron-based catalytic transfer hydrogenation of
various ketones have been accomplished [7-10]. In light of those researches, the development
of an effective and new a stable metal-free catalytst for the asymmetric transfer hydrogenation
of aromatic ketones is one of the main aim for the many scientists due to the expensive and
causing environmental pollution of metal-based homogeneous catalysts [10-12]. Therefore,
the production of the high-value alcohols through the asymmetric transfer hydrogenation of
various ketones was the factor for which the research of the effective, robust, cheap, easy-to-
handle, air-stable and different structure catalyst has been continued.
The asymmetric transfer hydrogenation of unsaturated carbonyl functionalities in the
presence of isoPrOH as a hydrogen source is one of the most important chemical
transformations to precious alcohols. The transfer hydrogenation process, which involves
hydrogen transfer from a donor to an acceptor, has become one of the most important
synthetic routes for producing highly active and enantioselective alcohols from different
ketones [13]. Symetric or asymmetric transfer hydrogenation is the addition of hydrogen to an
unsaturated molecule using a reagent other than hydrogen gas. Although the direct
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hydrogenation of aromatic ketones is more widely applied, transfer hydrogenation is an
attractive alternative [14-16]. Numerous various catalysts were developed for transfer
hydrogenation of various aromatic ketones to high-value alcohols, which includes
homogeneous metal complexes [17, 18], metal-free homogeneous organocatalysts such as
boron, phosphorus and chiral auxiliaries [10, 19-21], and apart from transition metal
complexes and metal-free organocatalysts, zeolites, oxides of cerium and metal-organic
frameworks have also been used for transfer hydrogenation [22, 23].
To the best of our knowledge, the litareture demonstrates that the use of boronate
complexes for the asymmetric transfer hydrogenation of various ketones to high-value
alcohols is very rare. However, asymmetric transfer hydrogenation is also an operationally
simple method to synthesize highly enantioselective chiral alcohols with the use of hydrogen
donors in the presence of new chiral boronate catalysts. Recently, our research group reported
the synthesis and catalytic properties of a series of novel salen/salan boron compounds
because the salicylaldimine boron complexes have gained increasing interest. In this work, we
describe the synthesis and spectroscopic properties of new chiral salen ligands and their
metal-free one-pot synthesis of four-coordinate chiral boronate complexes using boronic acids
and chiral salen ligands. Then, these chiral boronate complexes were investigated as catalysts
for the asymmetric transfer hydrogenation of acetophenone derivatives under suitable
conditions. The probable structures of all the newly synthesized compounds were assigned by
13
11
the FT-IR, UV-Vis, NMR (1H, C, and B) spectroscopy, LC-MS, melting point analysis,
and elemental analysis techniques. The obtained spectroscopic results of all newly chiral salen
ligands and their chiral boronate complexes are consistent with proposed structure.
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2. Experimental Section
2. 1. General Considerations
All organic solvents and starting materials used for the synthesis of the novel chiral salen
ligands and their boronate complexes were purchased from Aldrich Inc. or Fluka Inc. and
used without further purification. All reactions requiring a dry atmosphere were performed
under a argon (Ar) gas atmosphere in standard Schlenk techniques. FT-IR spectra were
performed with a Perkin-Elmer Two UATR-FT spectrophotometer in the range of 4000 to
400 cm-1 at 25 oC equipped with ATR accessory. 1H, 13C and 11B NMR spectra were obtained
13
with an Agilent Technologies or a Bruker Avance instrument (1H: 400 MHz, C: 100 MHz
and 11B: 192.5 MHz) in DMSO-d6 with TMS as internal standard. All the NMR spectra were
measured at 298 K unless otherwise mentioned. Chemical shifts are reported in delta (δ) units
in ppm downfield from TMS, and J values are given in Hertz. Elemental analyses
measurements of the novel ligands and their boronate complexes were carried out on a LECO
CHNS 932 model elemental analyzer. Mass spectra were determined with an Agilent LC-MS
spectrometer (LC-MS/MS). UV-Vis spectra were determined using a Perkin-Elmer model
Lambda 25 spectrometer in the scan range of 200 to 1100 nm using quartz cuvettes at room
temperature and studied in EtOH. Melting points were determined using an Electrothermal
9100 and uncorrected. GC analyses were performed on a Shimadzu 2010 Plus Gas
Chromatograph equipped with a capillary column (5% biphenyl, 95% dimethylsiloxane) (30
m x 0.32 mm x 0.25 µm). The GC parameters for the transfer hydrogenation of ketones were
o
as follows: initial temperature, 110 C; initial time, 1 min; solvent delay, 4.48 min;
temperature ramp 80 ºC/min; final temperature, 200 ºC; final time, 21.13 min; injector port
temperature, 200 ºC; detector temperature, 200 ºC, injection volume, 2.0 µL. The key starting
material 5-chloromethyl salicylaldehyde was synthesised according to literature procedures
[24, 25].
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2. 2. Synthesis of chiral ligand (L1H2)
Firstly, a solution of 5-chloromethyl salicylaldehyde (3.30 g, 19.2 mmol) and NaN3
o
(1.25 g, 19.2 mmol) in DMF (20 mL) was heated at 100 C during overnight stirring
vigorously (Scheme 1). When TLC analysis showed complete comsumption of the starting
material the reaction was cooled to room temperature and then the crude product was added to
the water and the target product was extracted into the chloroform (3 x 100 mL). Organic
phase dried over Na2SO4, evaporated and residue was used without further purification,
affording dark red crystals of the product [26]. After 5-azidomethyl salicylaldehyde was
synthesized, a mixture of 5-azidomethyl salicylaldehyde (2.0 g, 11.30 mmol) and (R)-(-)-2-
amino-1-butanol (1.0 g, 11.30 mmol) were placed into a two-necked-flask, equipped with a
argon (Ar) connection in ethanol (40 mL), which was stirred and refluxed for 6 h, in the
presence of two drop formic acid as catalyst. After this time, the mixture was allowed to cool
o
to 25 C. All solvent was removed and the solid was washed three times with diethyl ether
and n-hexane, successively, and dried under vacuum. Finally, re-crystallization from
CH3Cl/C2H5OH by slow evaporation afforded pure chiral salen ligand (L1H2), successfully.
o
Color: red, yield (%): 81, M.p.: 145 C, Elemental Analysis (calculated for C12H16N4O2)
(F.W: 248.3 g/mol) (%): C, 58.05; H, 6.50; N, 22.57. Found: C, 58.01; H, 6.46; N, 22.53. LC-
MS (Scan ES+): m/z = 249.3 [M+H]+ and 294.3 [M+2Na]+. FT-IR (ATR, υmax-cm-1): 3520-
2438 υ(OH····N=CH), 3048 υ(Ar-CH), 2965-2874 υ(Aliph-CH), 2103 υ(N3), 1658 υ(C=N),
1533-1495 υ(C=C), and 1122 υ(C-O). 1H-NMR (DMSO-d6): δ (ppm) = 13.42 (s, 1H, OH, D-
exchangeable), 8.37 (s, 1H, HC=N), 7.92 (s, 3H, Ar-CH), 4.34 (s, 1H, CH2-OH), 3.64 (q, 2H,
CH2-OH), 3.34 (q, 1H, CH-N=CH), 2.85 (s, 2H, Ph-CH2-N3), 1.62 (m, 2H, CH3-CH2-CH),
and 0.88 (t, 3H, J = 7.4 Hz, CH3-CH2-CH). 13C-NMR (DMSO-d6): δ (ppm) = 165.82
(HC=N), 161.27, 132.18, 131.70, 128.40, 118.57 and 116.56 (Ar-CH), 67.78 (-CH2-OH),
5

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62.13 (CH=N-CH), 54.33 (-CH2-N3), 36.22 (CH3-CH2), and 11.48 (CH2-CH3). UV-Vis
(λmax/(nm), * = shoulder peak): 231, 253, 268, 286* and 324 (C2H5OH).
2. 3. Synthesis of chiral ligand (L2H2)
The click reaction to form 1, 2, 3-triazole-containing ligand (L2H2) was carried out by
modification of a published procedure [27, 28] via click chemistry approach (Scheme 1). To
the solution of the azido-functionalized ligand (L1H2) (0.50 g, 2.01 mmol) in a 1:1 (25:25
mL) mixture of THF and water in a round bottom flask, an excess quantity of propargyl
alcohol was added. After this time, CuSO4 (0.16 g, dissolved in 3 mL water) and freshly
prepared sodium ascorbate solution (0.12 g, dissolved in 2 mL water) were added to the
continuously stirred reaction mixture at room temperature overnight. After THF solvent was
o
removed in vacuo, 10 mL CH2Cl2 and 3 mL conc. NH3 were added to the solution at 25 C
and then organic layer was dried with MgSO4. The pure ligand (L1H2) was collected after
evaporating all solvent. Finally, the novel ligand (L1H2) was recrystallized from CH3Cl and n-
o
hexane. Color: green, yield (%): 78, M.p.: 172 C, Elemental Analysis (calculated for
C15H20N4O3) (F.W: 304.3 g/mol) (%): C, 59.20; H, 6.62; N, 18.41. Found: C, 59.16; H, 6.57;
N, 18.38. LC-MS (Scan ES+): m/z = 305.3 [M+H]+ and 327.2 [M+Na]+. FT-IR (ATR, υmax
-
cm-1): 3647 υ(triazole-OH), 3478-2493 υ(OH····N=CH), 3048 υ(Ar-CH), 2962-2870 υ(Aliph-
CH), 1637 υ(C=N), 1540 υ(triazole-C=C), 1476 υ(triazole-N=N), and 1117 υ(C-O). 1H-NMR
(DMSO-d6): δ (ppm) = 13.30 (s, 1H, OH, D-exchangeable), 8.42 (s, 1H, HC=N), 7.99 (s, 1H,
triazole-CH), 7.93 (s, 3H, Ar-CH), 5.69 (s, 1H, triazole-CH2-OH), 4.13 (s, 2H, triazole-N-
CH2), 3.94 (s, 2H, triazole-CH2-OH), 2.87 (q, 2H, CH2-OH), 2.71 (q, 1H, CH-N=CH), 1.53
(s, 1H, CH2-OH), 1.21 (m, 2H, CH3-CH2-CH), and 0.86 (s, 3H, Hz, CH3-CH2-CH). 13C-NMR
(DMSO-d6): δ (ppm) = 162.78 (HC=N), 161.26, 136.59, 133.56, 132.74, 124.47 and 117.66
(Ar-CH), 143.12 (triazole-CH), 122.47 (triazole-C-CH2OH), 67.05 (-CH2-OH), 61.74
6

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(CH=N-CH), 56.09 (N-CH2-Ph), 52.15 (triazole-CH2-OH), 32.28 (CH3-CH2), and 11.42
(CH2-CH3). UV-Vis (λmax/(nm), * = shoulder peak): 233, 254, 269, 315* and 382 (C2H5OH).
2. 4. Synthesis of chiral boronate [L1(B1-4)] complexes
At first, chiral salen ligand (L1H2) (0.30 g, 1.21 mmol) was dissolved in 30 mL
toluene in a round-bottomed flask under N2 atmosphere and the mixtures were stirred for 30
min. Under stirring, an equivalent amount ferrocene boronic acid (0.28 g, 1.21 mmol) for
[L1B1] complex, butylboronic acid (0.12 g, 1.21 mmol) for [L1B2] complex, 3,5-
bis(trifluoromethyl)phenyl boronic acid (0.32 g, 1.21 mmol) for [L1B3] complex or
phenylboronic acid (0.15 g, 1.21 mmol) for [L1B4] complex was added, and then, stirring was
continued for overnight at reflux temperature. Upon cooling to room temperature, stirring was
maintained, and the solutions turned from red to brown for chiral boronate [L1(B1-4)]
complexes. All solvent was removed and the solid was washed three times with diethyl ether
with n-hexane and dried under vacuum to get pure products.
o
[L1B1] complex: Color: brown, yield (%): 72, M.p. > 300 C, Elemental Analysis
(calculated for C22H23BFeN4O2) (F.W: 442.1 g/mol) (%): C, 59.77; H, 5.24; N, 12.67. Found:
C, 59.72; H, 5.20; N, 12.61. LC-MS (Scan ES+): m/z = 442.2 [M]+ and 488.1 [M+2Na]+. FT-
IR (ATR, υmax-cm-1): 3086 υ(Ar-CH), 2968-2877 υ(Aliph-CH), 2113 υ(N3), 1624 υ(C=N),
1
1547-1462 υ(C=C), 1380 υ(Fer C-C), 1122 υ(B-O), 1109 υ(C-O) and 891 υ(B-C). H-NMR
(DMSO-d6): δ (ppm) = 8.42 (s, 1H, HC=N), 7.48 (s, 3H, Ar-CH), 4.44 (s, 2H, Fer-CH), 4.29
(s, 2H, Fer-CH), 4.08 (s, 5H, Fer-CH), 2.73 (q, 2H, CH2-O-B), 2.34 (s, 1H, CH-N=CH), 2.08
(s, 2H, Ph-CH2-N3), 1.23 (m, 2H, CH3-CH2-CH), and 0.87 (t, 3H, J = 7.4 Hz, CH3-CH2-CH).
13C-NMR (DMSO-d6): δ (ppm) = 164.42 (HC=N), 160.98, 148.03, 134.52, 122.08, 119.49
and 118.21 (Ar-CH), 73.83, 72.91, 71.49 and 69.99 (Fer-CH), 68.53 (-CH2-O-), 59.77
11
(CH=N-CH), 53.96 (-CH2-N3), 37.28 (CH3-CH2), and 11.55 (CH2-CH3). B NMR (DMSO-
7

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d6, 192.5 MHz, 23 oC, δ ppm): 29.41. UV-Vis (λmax/(nm), * = shoulder peak): 234, 269, 288*
and 329 (C2H5OH).
o
[L1B2] complex: Color: brown, yield (%): 70, M.p. > 300 C, Elemental Analysis
(calculated for C16H23BN4O2) (F.W: 314.2 g/mol) (%): C, 61.16; H, 7.38; N, 17.83. Found: C,
61.11; H, 7.33; N, 17.79. LC-MS (Scan ES+): m/z = 315.2 [M+H]+ and 337.2 [M+Na]+. FT-
IR (ATR, υmax-cm-1): 3045 υ(Ar-CH), 2960-2871 υ(Aliph-CH), 2110 υ(N3), 1633 υ(C=N),
1
1569-1459 υ(C=C), 1155 υ(B-O), 1112 υ(C-O) and 896 υ(B-C). H-NMR (DMSO-d6): δ
(ppm) = 8.39 (s, 1H, HC=N), 7.38 (s, 3H, Ar-CH), 4.62 (q, 2H, CH2-O-B), 3.04 (q, 1H, CH-
N=CH), 2.11 (s, 2H, Ph-CH2-N3), 1.58 (m, 2H, CH3-CH2-CH), 1.48 (m, 4H, CH3-CH2-CH2-
CH2-B), 1.21 (t, 3H, J = 7.1 Hz, CH3-CH2-CH), 0.88 (t, 3H, J = 7.1 Hz, CH3-CH2-CH2-CH2-
13
B), and 0.56 (t, 2H, J = 7.4 Hz, B-CH2-). C-NMR (DMSO-d6): δ (ppm) = 162.70 (HC=N),
158.93, 132.21, 131.52, 130.08, 124.49 and 118.60 (Ar-CH), 68.51 (CH=N-CH), 66.10 (-
CH2-O-), 56.37 (-CH2-N3), 34.72 (B-CH2), 27.11 (CH3-CH2-CH2-), 26.92 (-N-CH-CH2),
15.42 (CH3-CH2-CH2-),14.32 (CH3-CH2-CH2-), and 11.53 (CH-CH2-CH3). 11B NMR (DMSO-
d6, 192.5 MHz, 23 oC, δ ppm): 32.44. UV-Vis (λmax/(nm), * = shoulder peak): 244, 268, 286*
and 326 (C2H5OH).
o
[L1B3] complex: Color: brown, yield (%): 76, M.p. > 300 C, Elemental Analysis
(calculated for C20H17BF6N4O2) (F.W: 470.2 g/mol) (%): C, 51.09; H, 3.64; N, 11.92. Found:
C, 51.05; H, 3.60; N, 11.89. LC-MS (Scan ES+): m/z = 471.2 [M+H]+ and 493.2 [M+Na]+.
FT-IR (ATR, υmax-cm-1): 3038 υ(Ar-CH), 2973-2883 υ(Aliph-CH), 2114 υ(N3), 1637 υ(C=N),
1
1566-1463 υ(C=C), 1161 υ(B-O), 1122 υ(C-O) and 897 υ(B-C). H-NMR (DMSO-d6): δ
(ppm) = 8.83 (s, 1H, HC=N), 8.33 (s, 1H, Ar-CH), 8.27 (s, 1H, Ar-CH), 8.15 (s, 1H, Ar-CH),
7.90 (s, 1H, Ar-CH), 7.37 (d, 1H, J = 6.4 Hz, Ar-CH), 7.29 (d, 1H, J = 6.4 Hz, Ar-CH), 3.55
(q, 2H, CH2-O-B), 3.10 (q, 1H, CH-N=CH), 2.68 (s, 2H, Ph-CH2-N3), 1.49 (m, 2H, CH3-CH2-
13
CH), and 1.02 (t, 3H, J = 7.0 Hz, CH3-CH2-CH). C-NMR (DMSO-d6): δ (ppm) = 164.86
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(HC=N), 162.72, 160.27, 141.97, 134.35, 132.77, 132.04, 129.59, 121.75, 120.24, 118.65, and
108.53 (Ar-CH), 124.54 (-CF3), 65.33 (CH=N-CH), 60.68 (-CH2-O-), 54.22 (-CH2-N3), 22.54
(CH-CH2-CH3), and 15.29 (CH-CH2-CH3). 11B NMR (DMSO-d6, 192.5 MHz, 23 oC, δ ppm):
23.32. UV-Vis (λmax/(nm), * = shoulder peak): 250, 270, 291* and 364 (C2H5OH).
o
[L1B4] complex: Color: brown, yield (%): 72, M.p. > 300 C, Elemental Analysis
(calculated for C18H19BN4O2) (F.W: 334.2 g/mol) (%): C, 64.69; H, 5.73; N, 16.77. Found: C,
64.65; H, 5.68; N, 16.74. LC-MS (Scan ES+): m/z = 335.2 [M+H]+ and 381.2 [M+Na]+. FT-
IR (ATR, υmax-cm-1): 3064 and 3008 υ(Ar-CH), 2970-2878 υ(Aliph-CH), 2111 υ(N3), 1635
1
υ(C=N), 1560-1459 υ(C=C), 1123 υ(B-O), 1119 υ(C-O) and 896 υ(B-C). H-NMR (DMSO-
d6): δ (ppm) = 8.40 (s, 1H, HC=N), 8.19 (s, 5H, Ar-CH), 7.75 (s, 1H, Ar-CH), 7.33 (d, 2H, J
= 7.5 Hz, Ar-CH), 4.34 (q, 2H, CH2-O-B), 3.49 (m, 1H, CH-N=CH), 2.71 (s, 2H, Ph-CH2-
N3), 1.43 (m, 2H, CH3-CH2-CH), and 0.86 (t, 3H, J = 7.1 Hz, CH3-CH2-CH). 13C-NMR
(DMSO-d6): δ (ppm) = 165.42 (HC=N), 162.78, 153.68, 134.48, 131.98, 130.37, 127.75,
122.75, 114.05, and 108.15 (Ar-CH), 68.16 (CH=N-CH), 63.22 (-CH2-O-), 55.32 (-CH2-N3),
25.67 (CH-CH2-CH3), and 16.29 (CH-CH2-CH3). 11B NMR (DMSO-d6, 192.5 MHz, 23 oC, δ
ppm): 28.87. UV-Vis (λmax/(nm), * = shoulder peak): 256, 271, 284, 331 and 370* (C2H5OH).
2. 5. Synthesis of chiral boronate [L2(B1-4)] complexes
At first, chiral salen ligand (L2H2) (0.30 g, 0.99 mmol) was dissolved in 25 mL
toluene in each a round-bottomed flask under N2 atmosphere and the mixtures were stirred for
45 min. Under stirring, an equivalent amount ferrocene boronic acid (0.23 g, 0.99 mmol) for
[L2B1] complex, butylboronic acid (0.10 g, 0.99 mmol) for [L2B2] complex, 3,5-
bis(trifluoromethyl)phenyl boronic acid (0.26 g, 0.99 mmol) for [L2B3] complex or
phenylboronic acid (0.12 g, 1.21 mmol) for [L2B4] complex was added, and then, stirring was
continued for overnight at reflux temperature. Upon cooling to room temperature, stirring was
maintained, and the solutions turned from green to dark green for chiral boronate [L2(B1-4)]
9

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complexes. All solvent was removed and the solid was washed three times with diethyl ether
and n-hexane and dried under vacuum to get pure products.
o
[L2B1] complex: Color: dark green, yield (%): 70, M.p. > 300 C, Elemental Analysis
(calculated for C25H27BFeN4O3) (F.W: 498.2 g/mol) (%): C, 60.28; H, 5.46; N, 11.25. Found:
C, 60.21; H, 5.41; N, 11.21. LC-MS (Scan ES+): m/z = 499.2 [M+H]+ and 521.2 [M+Na]+.
FT-IR (ATR, υmax-cm-1): 3655 υ(triazole-OH), 3084 υ(Ar-CH), 2963-2874 υ(Aliph-CH),
1628 υ(C=N), 1538 υ(N=N), 1465 υ(C=C), 1382 υ(Fer C-C), 1158 υ(B-O), 1106 υ(C-O) and
1
890 υ(B-C). H-NMR (DMSO-d6): δ (ppm) = 8.21 (s, 1H, HC=N), 7.93 (d, 2H, J = 6.4 Hz,
Ar-CH), 7.68 (s, 1H, Ar-CH), 7.32 (s, 1H, triazole-CH), 5.48 (s, 1H, triazole-CH2-OH), 4.49
(s, 2H, triazole-CH2-OH), 4.24 (s, 2H, Fer-CH), 4.16 (s, 2H, Fer-CH), 4.06 (s, 5H, Fer-CH),
2.87 (s, 2H, CH2-O-B), 2.71 (s, 1H, CH-N=CH), 2.34 (s, 2H, Ph-CH2-), 1.22-1.06 (m, 2H,
CH3-CH2-CH), and 0.81 (s, 3H, -CH3-CH2-CH). 13C-NMR (DMSO-d6): δ (ppm) = 162.74
(HC=N), 160.82, 148.08, 135.02, 121.98, 119.43 and 118.17 (Ar-CH), 148.31 (triazole-CH),
127.90 (triazole-C-CH2-OH), 73.92, 72.83, 70.54 and 69.32 (Fer-CH), 66.59 (-CH2-O-B),
59.46 (CH=N-CH), 55.76 (triazole-CH2-OH), 54.40 (Ph-CH2-triazole), 36.24 (CH3-CH2), and
o
11.63 (CH2-CH3). 11B NMR (DMSO-d6, 192.5 MHz, 23 C, δ ppm): 28.82. UV-Vis
(λmax/(nm), * = shoulder peak): 245, 271, 311* and 378 (C2H5OH).
o
[L2B2] complex: Color: dark green, yield (%): 74, M.p. > 300 C, Elemental Analysis
(calculated for C19H27BN4O3) (F.W: 370.2 g/mol) (%): C, 61.63; H, 7.35; N, 15.13. Found: C,
61.59; H, 7.32; N, 15.09. LC-MS (Scan ES+): m/z = 371.2 [M+H]+ and 409.2 [M+K]+. FT-IR
(ATR, υmax-cm-1): 3653 υ(triazole-OH), 3064 υ(Ar-CH), 2963-2878 υ(Aliph-CH), 1630
1
υ(C=N), 1536 υ(N=N), 1468 υ(C=C), 1161 υ(B-O), 1115 υ(C-O) and 895 υ(B-C). H-NMR
(DMSO-d6): δ (ppm) = 8.08 (s, 1H, HC=N), 7.71 (s, 1H, triazole-CH), 7.34 (s, 1H, Ar-CH),
7.13 (d, 1H, J = 4.8 Hz, Ar-CH), 6.66 (d, 1H, J = 4.8 Hz, Ar-CH), 5.68 (s, 2H, triazole-CH2-
Ph), 5.32 (s, 1H, triazole-CH2-OH), 4.68 (s, 2H, triazole-CH2-OH), 4.46 (q, 2H, CH2-O-B),
10

ACCEPTED MANUSCRIPT
2.89 (s, 1H, CH-N=CH), 1.98 (m, 2H, CH3-CH2-CH), 1.47 (m, 4H, CH3-CH2-CH2-CH2-B),
1.23 (t, 3H, J = 7.0 Hz, CH3-CH2-CH), 1.04 (t, 3H, J = 7.1 Hz, CH3-CH2-CH2-CH2-B), and
13
0.86 (t, 2H, J = 7.3 Hz, B-CH2-). C-NMR (DMSO-d6): δ (ppm) = 162.65 (HC=N), 157.83,
132.71, 131.80, 128.98, 124.53 and 116.68 (Ar-CH), 148.30 (triazole-CH), 128.91 (triazole-
C-CH2-OH), 66.18 (-CH2-O-B), 63.50 (CH=N-CH), 57.43 (Ph-CH2-triazole), 54.46 (triazole-
CH2-OH), 34.76 (B-CH2), 27.12 (CH3-CH2-CH2-), 26.91 (-N-CH-CH2), 15.32 (CH3-CH2-
CH2-), 14.28 (CH3-CH2-CH2-), and 11.52 (CH-CH2-CH3). 11B NMR (DMSO-d6, 192.5 MHz,
o
23 C, δ ppm): 32.38. UV-Vis (λmax/(nm), * = shoulder peak): 243, 271, 311* and 382
(C2H5OH).
o
[L2B3] complex: Color: dark green, yield (%): 68, M.p. > 300 C, Elemental Analysis
(calculated for C23H21BF6N4O3) (F.W: 526.2 g/mol) (%): C, 52.49; H, 4.02; N, 10.65. Found:
C, 52.45; H, 3.97; N, 10.63. LC-MS (Scan ES+): m/z = 526.2 [M]+, and 549.2 [M+Na]+. FT-
IR (ATR, υmax-cm-1): 3659 υ(triazole-OH), 3045 υ(Ar-CH), 2971-2880 υ(Aliph-CH), 1627
1
υ(C=N), 1534 υ(N=N), 1464 υ(C=C), 1164 υ(B-O), 1131 υ(C-O) and 894 υ(B-C). H-NMR
(DMSO-d6): δ (ppm) = 8.27 (s, 1H, HC=N), 7.94 (s, 1H, triazole-CH and 3H, Ar-CH), 7.68
(s, 1H, Ar-CH), 7.37 (d, 1H, J = 6.4 Hz, Ar-CH), 7.29 (d, 1H, J = 6.4 Hz, Ar-CH), 5.54 (s,
1H, triazole-CH2-OH), 4.64 (s, 2H, triazole-CH2-Ph), 4.48 (s, 2H, triazole-CH2-OH), 3.69 (q,
2H, CH2-O-B), 2.88 (q, 1H, CH-N=CH), 1.22 (m, 2H, CH3-CH2-CH), and 0.83 (t, 3H, J = 7.2
Hz, CH3-CH2-CH). 13C-NMR (DMSO-d6): δ (ppm) = 165.05 (HC=N), 162.63, 141.27,
134.45, 133.59, 132.06, 130.36, 127.73, 121.82, 120.20, 118.43, and 115.53 (Ar-CH), 124.83
(-CF3), 142.28 (triazole-CH), 128.86 (triazole-C-CH2-OH), 68.19 (CH=N-CH), 61.28 (-CH2-
O-), 59.27 (Ph-CH2-triazole), 54.41 (triazole-CH2-OH), 23.46 (CH-CH2-CH3), and 14.70
(CH-CH2-CH3). 11B NMR (DMSO-d6, 192.5 MHz, 23 oC, δ ppm): 20.42. UV-Vis (λmax/(nm),
* = shoulder peak): 251, 275, 311* and 384 (C2H5OH).
11

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o
[L2B4] complex: Color: dark green, yield (%): 71, M.p. > 300 C, Elemental Analysis
(calculated for C21H23BN4O3) (F.W: 390.2 g/mol) (%): C, 64.63; H, 5.94; N, 14.36. Found: C,
64.58; H, 5.90; N, 14.33. LC-MS (Scan ES+): m/z = 391.2 [M+H]+ and 429.2 [M+K]+. FT-IR
(ATR, υmax-cm-1): 3658 υ(triazole-OH), 3070 and 3017 υ(Ar-CH), 2963-2875 υ(Aliph-CH),
1
1629 υ(C=N), 1537 υ(N=N), 1465 υ(C=C), 1161 υ(B-O), 1114 υ(C-O) and 897 υ(B-C). H-
NMR (DMSO-d6): δ (ppm) = 8.15 (s, 1H, HC=N), 7.96 (s, 1H, triazole-CH), 7.87 (s, 1H, Ar-
CH), 7.69 (d, 2H, J = 6.8 Hz, Ar-CH), 7.26-7.20 (m, 3H, Ar-CH), 6.65 (d, 2H, J = 6.8 Hz, Ar-
CH), 5.28 (s, 1H, triazole-CH2-OH), 4.58 (s, 2H, triazole-CH2-Ph), 4.41 (s, 2H, triazole-CH2-
OH), 4.30 (q, 2H, CH2-O-B), 2.85 (s, 1H, CH-N=CH), 1.15 (m, 2H, CH3-CH2-CH), and 0.80
(t, 3H, J = 7.0 Hz, CH3-CH2-CH). 13C-NMR (DMSO-d6): δ (ppm) = 162.38 (HC=N), 161.22,
153.46, 134.33, 131.87, 130.06, 127.42, 122.78, 115.48, and 106.24 (Ar-CH), 143.38
(triazole-CH), 129.41 (triazole-C-CH2-OH), 68.67 (CH=N-CH), 63.41 (-CH2-O-), 56.43 (Ph-
CH2-triazole), 53.98 (triazole-CH2-OH), 31.21 (CH-CH2-CH3), and 10.63 (CH-CH2-CH3). 11B
NMR (DMSO-d6, 192.5 MHz, 23 oC, δ ppm): 28.62. UV-Vis (λmax/(nm), * = shoulder peak):
252, 271, 284, 312* and 380 (C2H5OH).
2. 6. General procedure for the asymmetric transfer hydrogenation of ketones
Typical procedure for the catalytic hydrogen transfer reaction: a solution of chiral
boronate [L1(B1-4)] and [L2(B1-4)] complexes (0.01 mmol), NaOH (0.05 mmol) and the
corresponding ketone (1.00 mmol) in degassed 2 propanol (10 ml) was refluxed until the
reaction was completed. Followed by a sample of the reaction mixture was taken off, diluted
with acetone and analysed immediately by GC. Conversion rates were calculated based on
unreacted ketone.
12

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3. Results and discussion
3.1. Synthesis and characterization
Scheme 1 and 2 shows the synthetic path and structures of the chiral salen (L1H2) and
(L2H2) ligands and their boronate [L1(B1-4)] and [L2(B1-4)] complexes. Firstly, the chiral salen
(L1H2) ligand was synthesized in 81 % yield via the straightforward condensation of 5-
azidomethyl salicylaldehyde and (R)-(-)-2-amino-1-butanol in ethanol, which was stirred and
refluxed for 6 h in the presence of two drop formic acid as catalyst (Scheme 1). Then, 1, 2, 3-
triazole-containing a chiral ligand (L2H2) was synthesized from the azido-functionalized
ligand (L1H2) via click chemistry approach in 78 % yield in the presence of propargyl
alcohol, CuSO4 and freshly prepared sodium ascorbate (Scheme 1). Finally, as shown in
Scheme 2, the chiral salen (L1H2) and (L2H2) ligands was reacted with various boronic acid
(ferrocene boronic acid, butylboronic acid, 3,5-bis(trifluoromethyl)phenyl boronic acid or
phenylboronic acid) in refluxing toluene in a 1:1 molar ratio to create targeted tetra-
coordinated chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes in the range 76-68 % yield,
respectively. The proposed structures of chiral salen ligands with their boronate complexes
13
11
were confirmed by spectroscopic and analytical techniques such as NMR (1H, C, and B),
FT-IR, UV-Vis, LC-MS spectroscopy, and elemental analysis. The spectroscopic results of
the salen ligands and their boronate complexes are consistent with proposed structure.
Moreover, the elemental compositions of the chiral salen-based ligands (L1H2) and (L2H2)
and their chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes were determined and the results
confirmed the formation of the desired products.
13

ACCEPTED MANUSCRIPT
O
C
O
C
CHO
Cl
H
N3
NaN3
DMF
H
CH2O, HCl
H2O, 24 h
OH
OH
OH
5-Chloromethyl
salicylaldehyde
Salicylaldehyde
5-Azidomethyl
salicylaldehyde
O
C
H
N3
HO
OH
OH
OH
EtOH, 6h
HCO2H
H
+
HC
+
C
N
OH
propargyl
alcohol
H3C
H3C
NH2
N3
Chiral (L1H2) ligand
HO
OH
H
C
N
H3C
N
N
N
OH
Chiral (L2H2) ligand
Scheme 1 Synthesis of the proposed chiral salen (L1H2) and (L2H2) ligands.
14

ACCEPTED MANUSCRIPT
N3
N3
CH3
CH3
N3
N3
N
C
CH3
CH3
H
N
C
H
B
O
O
N
C
B
N
C
O
O
H
H
B
O
B
O
O
O
Fe
CH3
F3C
CF3
[L1B2]
[L1B4]
[L1B3]
[L1B1]
CH3
HO
N3
N
CH3
HC
N
N
C
HO
N
H
N
OH
HO
OH
(L2H2)
(L1H2)
OH
OH
OH
OH
N
N
N
N
N
N
N
N
CH3
CH3
N
N
N
N
CH3
CH3
N
C
N
C
H
H
N
C
N
C
B
B
O
O
H
O
O
H
B
O
B
O
O
O
Fe
H3C
F3C
CF3
[L2B2]
[L2B1]
[L2B3]
[L2B4]
Scheme 2 Synthesis of the chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes.
15

ACCEPTED MANUSCRIPT
3.2. Spectroscopic studies
FT-IR spectroscopy were used to characterize the structure of the synthesized two
chiral salen (L1H2) and (L2H2) ligands and their chiral boronate [L1(B1-4)] and [L2(B1-4)]
complexes. These chiral salen ligands and corresponding chiral boronate complexes exhibited
many prominent infrared peaks and the obtained results are given in the experimental part.
The most important characteristic FT-IR peaks of the chiral salen (L1H2) and (L2H2) ligands
were in the range 3520-2348 cm-1, which are ascribed to intermolecular H-bond υ(OH····N-
CH) stretching, which disappeared in the FT-IR spectra of the chiral boronate [L1(B1-4)] and
[L2(B1-4)] complexes due to the deprotonation of the OH group, as expected [29, 30]. While
another important FT-IR peak of free salen ligands are found in the range 1658-1637 cm-1,
corresponding stretching vibrations of the chiral boronate complexes are found in the region
of 1637-1624 cm-1 towards lower wavenumber as compared to that of the free ligands [31].
These two results indicates the coordination of imine group to boron center through the
nitrogen atom with phenolic oxygen and confirm the formation of the chiral boronate [L1(B1-
4)] and [L2(B1-4)] complexes. In the FT-IR spectra of the azido-functionalized chiral salen
(L1H2) ligand and its chiral boronate [L1(B1-4)] complexes, characteristic stretching peak of
the azido groups were observed in the range 2114-2103 cm-1 as very strong peaks owing to
the transformation of chlorine to an azido group in these compounds [26-28, 32]. This band
was disappeared in the FT-IR spectra of the chiral salen (L2H2) ligand and its chiral boronate
[L2(B1-4)] complexes because of the click reaction between the azido-functionalized
compounds with propargyl alcohol and the presence of different new peaks in the final FT-IR
spectra of triazole-containing compounds. In addition, in the FT-IR spectrum of each chiral
boronate complexes were observed new characteristic peaks in the range 1123-1112 cm-1 and
897-891 cm-1 for [L1(B1-4)] and 1164-1158 cm-1 and 897-890 cm-1 for [L2(B1-4)] complexes
16

ACCEPTED MANUSCRIPT
indicative of the presence of υ(B-O) and υ(B-C) bonds, respectively, thus corroborating the
coordination of the chiral salen ligands to the boron center of boronate complexes [33-36].
For the full characterization of the new chiral salen (L1H2) and (L2H2) ligands and
1
their chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes, the H, 13C, and 11B NMR
spectroscopy have been used. The NMR spectra results were in good agreement with the
proposed structures in d6-DMSO and all chemical shifts were in the expected ranges (Figure
1, 2 and S1-S8) The absence of the signal was attributed to the D-exchangeable phenolic-OH
1
resonance in the H NMR spectra of the chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes,
which is present at 13.42 and 13.30 ppm as singlet in the spectra of free chiral salen ligands,
indicating the successful formation of boronate complexes and the deprotonation of the -OH
1
group during complexation [37-39]. The compared with the H NMR spectra of chiral salen
(L1H2) and (L2H2) ligands and their chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes, the
chemical shift of the azomethine (CH=N) protons appears at 8.37-8.42 ppm for ligands and
8.83-8.03 ppm for boronate complexes [4, 40]. These results clearly indicates the coordination
of azomethine nitrogen atom to boron center and provides further evidence for succesfull
boronate complexation. Another decisive event, the appearance of new signals at aromatic
region and presence of two new shielded signals at 5.69-5.28 and 4.68-3.94 ppm for (L2H2)
ligand and chiral boronate [L2(B1-4)] complexes as singlet signals corresponding to the -CH2-
1
OH and the CH2-OH protons of 1,2,3-triazole group, respectively in their H-NMR spectra.
Also, these results obviously indicates the success of the click cyclization reaction and the
1
formation of the 1, 2, 3-triazole-containing compounds [41]. In addition, the H NMR
chemical shift values of the other aromatic and aliphatic groups have been supported the
formation of all compounds in DMSO-d6, which gave suitable signals that are in good
agreement with the proposed structures. This indicates the coordination of atom to boron
center through azomethine nitrogen atom, phenolic oxygen, and deprotonation of the -OH
17

ACCEPTED MANUSCRIPT
group on the other side of ligands, as expected. Moreover, the 13C NMR spectra results can be
13
analyzed in an analogous manner to that of the proton spectra and the chemical shift of C
NMR resonances supports the formation of the chiral salen (L1H2) and (L2H2) ligands and
their chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes. The signals for the azomethine
group carbon (HC=N) was observed in the range 165.42-162.38 ppm for boronate complexes
with an low-field shift in its position as compared to that of the salen chiral ligands, which
provides further evidence for the formation of the chiral boronate compounds. Besides, the
appearance of new signals at δ = 148.30-142.28 ppm, at δ = 129.41-122.47 ppm, and at δ =
55.76-52.15 ppm corresponding to the triazole-CH, triazole-C-CH2-OH, and triazole-CH2-OH
carbons of 1,2,3-triazole group, respectively for the chiral salen (L2H2) ligand and its chiral
13
boronate [L2(B1-4)] complexes in their C-NMR spectra obviously shows the success of the
click cyclization reaction and the formation of the 1,2,3-triazole-containing (L2H2) and
[L2(B1-4)] compounds [28, 41]. Also, the chemical shift of aromatic (Ar-CH) and other group
carbons were in the expected range, which support the formation of the proposed structures
(Figure 1, 2 and S1-S8). Additionally, in order to demonstrate the formation of the chiral
boronate [L1(B1-4)] and [L2(B1-4)] complexes, spectroscopic analysis of the 11B NMR data
was also performed (Figure 3, 4 and S9-S13). The 11B NMR spectroscopic results are of
special interest because the chemical shift of the 11B NMR spectroscopic signal gives a strong
indication of whether or not the boronate compounds is trigonal planar or tetrahedral [42, 43].
In the 11B NMR spectra of the chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes, the
characteristic boron signals located in the range δ = 32.44-20.42 ppm as broad singlets due to
the similar chemical environment for the boron center of each boronate complexes [44-46],
which confirms the formation of tetra-coordinate boronate compounds. However, these values
suggest that tetra-coordinated boronate [L1(B1-4)] and [L2(B1-4)] complexes exist
predominantly as monomers in solution.
18

ACCEPTED MANUSCRIPT
Figure 1. 1H NMR spectra of the chiral salen (L1H2) ligand
Figure 2. 13C NMR spectra of the chiral boronate [L1B1] complex
19

ACCEPTED MANUSCRIPT
Figure 3. 11B NMR spectra of the chiral boronate [L1B2] complex
Figure 4. 11B NMR spectra of the chiral boronate [L2B4] complex
20

ACCEPTED MANUSCRIPT
To understand the electronic transitions of our compounds, we studied the UV-Vis
absorption spectra of the chiral salen (L1H2) and (L2H2) ligands and the corresponding chiral
boronate [L1(B1-4)] and [L2(B1-4)] complexes under the same conditions for comparison
(Figure S20-23). The UV-vis results of the chiral salen ligands and their boronate complexes
show very similar features in EtOH at room temperature, probably the electronic effects
resulting upon coordination of the boron atom and the planarization of the π-conjugated salen
ligands entail a narrowing of the energy gap between the ground state and the first singlet
excited state [47]. Another reason for the showing similar absorption bands in ethanol,
probably it was dependent on the electron-withdrawing or electron-releasing group at
different positions in the ligand groups or the boronate complexes. The electronic spectrum of
the chiral salen (L1H2) and (L2H2) ligands exhibits an absorption band at range 382-231 nm,
while the corresponding chiral boronate [L1(B1-4)] complexes at range 370-234 nm and
[L2(B1-4)] complexes at range 384-243 nm, respectively. These absorption bands can be
assigned to π → π* transitions or the n→π* transition of non-bonded electrons of the phenolic
chromophores and azomethine (-CH=N) unit in the structure of all the compounds [48].
Further confirmation of the formation of the chiral salen (L1H2) and (L2H2) ligands
and the corresponding chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes was also provided
by LC-MS (Figure 5-8 and S14-S19). The LC-MS spectra indicated the formation of a variety
of fragments during the course of its decomposition and the existence of [M]+ or [M+H]+
peaks, with the base peak were in the proposed nature of the compounds. The LC-MS mass
peaks showed the expected molecular and fragmentation ions, with appropriate isotope
distribution. As seen in Figure 3 and 4, the molecular ion peak for the the chiral salen (L1H2)
and (L2H2) ligands were observed at m/z = 249.3 amu for [M+H]+ and 184.0 amu for [M+H]+
consecutively, whereas those of the chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes were
observed at m/z = 442.2 amu for [M]+, 315.2 amu for [M+H]+, 471.2 amu for [M+H]+, 335.2
21

ACCEPTED MANUSCRIPT
amu for [M+H]+, 499.2 amu for [M+H]+, 371.2 amu for [M+H]+, 526.2 amu for [M]+, and
391.2 amu for [M+H]+ [1, 4, 49] (Figure 5-6 and S14-S19). In light of those LC-MS data, the
existence of the molecular ion and other peaks confirms the formation of the chiral salen
(L1H2) and (L2H2) ligands and the corresponding chiral boronate [L1(B1-4)] and [L2(B1-4)]
complexes.
Figure 5. LC-MS spectra of the chiral salen (L1H2) ligand
22

ACCEPTED MANUSCRIPT
Figure 6. LC-MS spectra of the chiral salen (L2H2) ligand
Figure 7. LC-MS spectra of the chiral boronate [L1B2] complex
23

ACCEPTED MANUSCRIPT
Figure 8. LC-MS spectra of the chiral boronate [L2B3] complex
3.3. Catalytic performances of boron complexes
In a preliminary study, the chiral compounds, [L1(B1-4)] and [L2(B1-4)] have been
examined as catalyst precursors for the asymmetric transfer hydrogenation of acetophenone
under variable conditions. A typical procedure using acetophenone as substrate was as
follows: 0.01 mmol of the complex and 1.00 mmol of acetophenone were added to a solution
of NaOH in 2-propanol (0.05 mmol of NaOH in 10 mL 2-propanol) and refluxed at 82 ºC,
while the reaction was watched by GC. In all reactions, these compounds catalyzed the
reduction of ketones to the corresponding alcohols via hydrogen transfer from 2-propanol. A
comparison of chiral boronate complexes [L1(B1-4)] and [L2(B1-4)] as precatalysts for the
24

ACCEPTED MANUSCRIPT
asymmetric hydrogenation of acetophenone by 2-propanol is given in Table 1. All Catalytic
experiments were completed under inert (Ar) atmosphere using typical Schlenk-line methods.
These systems catalyzed the reduction of acetophenone to corresponding alcohol ((S)-, (R)-1-
phenylethanol) in the existence of NaOH. Transfer hydrogenation of acetophenone occurred
very slowly at room temperature [50], with low conversions (up to 30 %, 48 h) and low to
moderate enantioselectivities (up to 52 % ee) in the reactions (Table 1, entries 1-8). Because
of the reversibility at room temperature (rt), the prolonging the reaction time (96 h) led to a
slight reducing of enantioselectivity, as indicated by the catalytic results collected with chiral
catalyst, [L2B1] (entry 5, [b]) [51, 52]. Moreover, as can be inferred from Table 1 (entry 9), the
existence of base is essential to detect appreciable conversions [53-56]. Furthermore, the
select of base, such as KOH and NaOH, had little influence on the conversion and
enantioselectivity (Table 1, entry 14,[e]) and with a catalyst/NaOH ratio of 1/5, the boranate
complexes are very active leading to a quantifiable transformation of acetophenone (Table 1,
entries 18-21,[f]).
25

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H
O
HO
Cat.
*
PrOH
acetone
iso
Entry
Complex
[L1B1][a]
[L1B2][a]
[L1B3][a]
[L1B4][a]
S/C/Base
100:1:5
100:1:5
100:1:5
100:1:5
Time
48 h
48 h
48 h
48 h
48 h
(96 h)[b]
48 h
48 h
48 h
72 h
7 h
Conv.(%)[h]
% ee[i]
33
Config.[k] TOF(h-1)[l]
15
30
17
18
R
R
R
R
<3
<3
<3
<3
1
2
3
4
16
14
17
[L2B1][a]
100:1:5
10(28)[b]
52(48)[b]
R
<3
5
[L2B2][a]
[L2B3][a]
[L2B4][a]
[L2B1] [c]
[L1B1][d]
[L1B2][d]
[L1B3][d]
[L1B4][d]
[L2B1][d]
[L2B2][d]
[L2B3][d]
[L2B4][d]
[L2B1][f]
[L2B1][f]
[L2B1][f]
[L2B1][f]
[L2B1][g]
[L2B1][g]
[L2B1][g]
100:1:5
100:1:5
100:1:5
100:1
19
12
11
<5
99
98
31
28
30
-
R
R
R
-
<3
<3
6
7
<3
8
trace
14
9
100:1:5
100:1:5
37
19
R
R
R
R
R
R
R
R
R
R
R
R
10
11
1 h
98
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:3
100:1:5
100:1:7
100:1:9
250:1:5
500:1:5
1000:1:5
7 h
8 h
4 h (4 h)[e]
1/2 h
5 h
98
99
99 (98)[e]
99
17
18
61 (58)[e]
24
14
12
25
198
20
33
-
12
13
14
15
16
17
18
19
20
21
22
23
24
97
27
3 h
98
26
4 h
97
57
4 h
99
61
-
4 h
98
56
-
4 h
6 h
12 h
24 h
98
96
95
52
56
52
-
R
R
-
-
R
97
51
-
26

ACCEPTED MANUSCRIPT
Reduction of acetophenone into (S)- or (R)-1-phenylethanol could be reached in high
o
yield by increasing the temperature up to 82 C (Table 1, entries 10-17). Moreover, the
conversions gradually declined with increasing the mole ratios of [acetophenone]/[Cat.] from
250/1 to 500/1 or 1000/1, although the enantioselectivities were still high (up to 56% ee,
Table 1, entries 22-24). These outcomes clearly indicate that the structural variance of the
ligands is a critical factor for catalytic activity and it is noteworthy that the catalytic systems,
chiral boronate [L1(B1-4)] and [L2(B1-4)] catalysts show the differences in reactivity. It has
also been shown that the catalytic activities in the studied hydrogen transfer reactions were
mostly much higher for chiral boronate [L2(B1-4)] complexes than those for the [L1(B1-4)]
complexes (Table 1, entries 10-17). Furthermore, it was seen that when ligands incorporate
ferrocene moiety, the catalytic activity is low, but the % enantioselectivities is high compared
to the other boronate complexes (Table 1, entries 10 and 14). So, from the outcomes, one can
easily say that chiral boronate [L1B1] and [L2B1] complexes were effective catalysts for the
hydrogenation of acetophenone. Particularly, chiral boronate [L2B1] complex afforded good
ee % in the asymmetric transfer hydrogenation of acetophonone (up to 61% ee). As shown in
Table 1, these results have also presented that enantiomeric purity of the product can be
affected by electronic issues as well as steric factors of the substituents on the ligands.
27

ACCEPTED MANUSCRIPT
OH
O
OH
Cat
R
O
*
R
+
+
Entry
Catalyst
[L1B1]
[L2B1]
R
F
F
Time Conv.(%)[b] % ee[c] TOF (h-1)[d]
Config.[e]
4 h
2 h
99
99
35
60
25
50
R
R
1
2
R
R
[L1B1]
[L2B1]
Cl
Cl
5 h
3 h
99
99
38
62
20
33
3
4
R
R
[L1B1]
[L2B1]
Br
Br
6 h
4 h
98
98
40
65
16
25
5
6
R
R
[L1B1]
[L2B1]
NO2
NO2
4 h
2 h
99
97
39
63
25
49
7
8
R
R
[L1B1]
[L2B1]
o-CH3O
o-CH3O
20 h
12 h
99
98
51
78
<5
<8
9
10
R
R
[L1B1]
[L2B1]
m-CH3O
m-CH3O
15 h
9 h
98
99
40
66
6
11
11
12
R
R
[L1B1]
[L2B1]
p-CH3O
p-CH3O
12 h
7 h
98
99
25
47
12
14
13
14
In order to explore the request range of the boron-catalyzed transfer hydrogenation,
acetophenone derivatives were subjected to asymmetric reduction using the conditions
optimized for acetophenone. Table 2 demonstrates conversions of the reduction performed in
a 0.1 M of isoPrOH solution containing chiral boronate [L1B1] or [L2B1] catalysts and sodium
28

ACCEPTED MANUSCRIPT
hydroxide (Ketone/Cat./NaOH = 100/1/5). The outcomes obviously exhibit that a range of
acetophenone derivatives can be hydrogenated with low and good enantioselectivities.
Substitution of the phenyl ring of the acetophenone substrate led to a clear decrease or
increase in the catalytic activity and enantiselectivity. That’s to say, electronic properties (the
nature and position) of the substituents on the phenyl ring of the corbonil compound caused
the important changes in the reduction rate. It is eminent that the introduction of electron
withdrawing substituents to the aryl ring of the ketone decreases the electron density of the
C=O bond so that the activity was enriched resulting in easier hydrogenation (Table 2, entries
1-14) [57, 58]. Therefore, the introduction of electron-withdrawing substituents resulted in
improved activity with good ee % (Table 2, entries 1-8) [59, 60]. The introduction of an
electron-donating group such as -CH3 or -OCH3 group decelerates the reaction degree with
various enantioselectivities, whatever the location of substitution (Table 2, entries 9-14).
Additionally, it can be seen in Table 2, ortho-substituted acetophenones can dramatically
increase ee %, while meta- and para- substitution to acetophenones have detrimental effect
[61, 62]. The investigation of the results indicates obviously that with each of the tested
complexes, the highest enantioselectivity was found for o-methoxyacetophenone (78% ee)
when chiral boronate [L2B1] complex was used as the catalyst.
It is well-known that transition-metal-catalyzed hydrogen transfer involves metal
hydrides as key intermediates. All boron complexes, most likely follow the well-established
mechanism via a boron alkoxide intermediate and β-elimination [63-65]. The importance of
this study is that the use of inexpensive boron instead of expensive Ruthenium, Rhodium and
Iridium. These higher catalytic activities can be explained by the nature of the ligands which
can generate an open coordination site at boron more easily. It thus appears that the nature of
the ligands can also play a crucial role in the transfer hdrogenation reactions. These results
indicate the catalysts possibly can stabilize a catalytic transtion state.
29