Developments in transfer hydrogenations of aromatic ketones catalyzed by boron compounds

Article abstract of DOI:10.1080/00958972.2017.1300659

Boron complexes BL1 and BL2 were prepared from O-donor ligands, 2,2′-(1E,1′E)-(ethane-1,2-diylbis(azan-1-yl-1-ylidene))bis(methane-1-yl-1-ylidene)diphenol (L1) and 2,2′-(propane-1,3-diylbis(azan-1-yl-1-ylidene))bis(methane-1-yl-1-ylidene)diphenol (L2). The complexes were fully characterized by ¹H and ¹³C NMR, LC-MS/MS, TGA/DTA, UV-Vis, elemental analysis, SEM, and FTIR. The transfer hydrogenation of acetophenone derivatives was investigated by the boron complexes in the presence of isoPrOH, as the hydrogen source, under basic condition with NaOH. The results showed that the boron complexes were promising catalytic precursors for transfer hydrogenation of aromatic ketones in 0.1?M isoPrOH solution (up to 99%). Both steric and electronic factors of this class of molecules had a significant impact on the catalytic properties.

Full text of DOI:10.1080/00958972.2017.1300659

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Developments in transfer hydrogenations of
aromatic ketones catalyzed by boron compounds
Salih Pasa, Nedim Gürler, Hamdi Temel, Khadichakhan Rafikova & Murat
Aydemir
To cite this article: Salih Pasa, Nedim Gürler, Hamdi Temel, Khadichakhan Rafikova & Murat
Aydemir (2017): Developments in transfer hydrogenations of aromatic ketones catalyzed by boron
compounds, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2017.1300659
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Date: 29 March 2017, At: 02:06

Journal of Coordination Chemistry, 2017
http://dx.doi.org/10.1080/00958972.2017.1300659
Developments in transfer hydrogenations of aromatic
ketones catalyzed by boron compounds
Salih Pasaa, Nedim Gürlerb, Hamdi Temelc, Khadichakhan Rafikovad,f and
Murat Aydemire
afaculty of education, department of science, afyon Kocatepe university, afyonkarahisar, turkey; borganic
agriculture, Vocational schools, munzur university, tunceli, turkey; cdepartment of Pharmaceutical Chemistry,
d
faculty of Pharmacy, dicle university, diyarbakir, turkey; department of Chemical technology, institute of
e
Chemical engineering, Kazakh national research technical university, almaty, Kazakhstan; department of
Chemistry, faculty of science, university of dicle, diyarbakir, turkey; fschool of Chemical engineering, Kazakh-
British technical university (KBtu), almaty, Kazakhstan
ABSTRACT
ARTICLE HISTORY
received 30 november 2016
accepted 13 february 2017
Boron complexes BL1 and BL2 were prepared from O-donor ligands,
2,2′-(1E,1′E)-(ethane-1,2-diylbis(azan-1-yl-1-ylidene))bis(methane-1-
yl-1-ylidene)diphenol (L1) and 2,2′-(propane-1,3-diylbis(azan-1-yl-1-
ylidene))bis(methane-1-yl-1-ylidene)diphenol (L2). The complexes
were fully characterized by 1H and 13C NMR, LC-MS/MS, TGA/DTA, UV-
Vis, elemental analysis, SEM, and FTIR. The transfer hydrogenation of
acetophenone derivatives was investigated by the boron complexes
in the presence of isoPrOH, as the hydrogen source, under basic
condition with NaOH. The results showed that the boron complexes
were promising catalytic precursors for transfer hydrogenation of
aromatic ketones in 0.1 M isoPrOH solution (up to 99%). Both steric
and electronic factors of this class of molecules had a significant
impact on the catalytic properties.
KEYWORDS
homogeneous catalysis;
boron complexes;
spectroscopy; aromatic
ketones; catalysis; transfer
hydrogenation
1. Introduction
Boron and its derivatives are used in areas from agriculture to industry, such as glass, ceramic,
textile, automotive, electronics and nuclear industry, medicine, pharmaceutical and cosmetic
CONTACT hamdi temel
htemel@dicle.edu.tr
supplemental data for this article can be accessed at http://dx.doi.org/10.1080/00958972.2017.1300659.
© 2017 informa uK limited, trading as taylor & francis Group

2
S. PASA ET AL.
industry, energy, construction industry, and communication tools and detergent [1, 2]. Due
to the widespread use of boron compounds, the synthesis and application of these systems
in science and technology have attracted attention. Boron complexes are stable and can be
handled in air due to the presence of B←N and B–O bonds in the structures. There are three
main types of BF2/BPh2 chelated boron complexes, which can be classified as N,N-bidentate,
N,O-bidentate, and O,O-bidentate compounds [3–6].
Catalytic transfer hydrogenation with a stable hydrogen donor is a useful alternative
method for catalytic hydrogenation employed with molecular hydrogen for reduction of
ketones [7–10]. In the process of transfer hydrogenation, molecules such as secondary alco-
hols [11, 12] or formic acid and its salts [13] have been used as hydrogen sources. The hydro-
gen donor has some advantages over use of molecular hydrogen, preventing the risks and
constraints associated with hydrogen gas as well as the necessity for pressure vessels and
other equipment [14–17]. Although a number of transition metals have been used for hydro-
genation reactions, there have been continuous studies to explore new metals for transfer
hydrogenation reactions with excellent performance as well as cost advantages compared
to the other hydrogenation metals, such as ruthenium, rhodium, and iridium [18–21].
We have reported on the synthesis and applications of a number of modified boron
catalysts for transfer hydrogenation [6, 7]. Boron complexes were shown to be efficient
homogeneous hydrogenation catalysts towards various substrates. Derivatization of boron-
based catalysts would be important for further development of boron-catalyzed transfer
hydrogenation reactions. It is our particular interest to design new ligand systems with dif-
ferent spacers, control the electronic attributes at boron [22–24] and investigate their coor-
dination chemistry. In the present work, we report the synthesis and characterization of
boron complexes by 1H NMR (Bruker AV-400 MHz), 13C NMR (Bruker AV-400 MHz), FTIR (Perkin
Elmer Spectrum 100)), UV-Vis (Perkin Elmer Lambda 25), elemental analysis (Costech ECS
4010), LC-MS (Shimadzu LC/MS 8040), and their catalytic evaluation in transfer hydrogenation
for aromatic ketones.
2. Experimental
2.1. Synthesis of L1 and L2
2,2′-(1E,1′E)-(ethane-1,2-diylbis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)diphenol (L1)
and 2,2′-(propane-1,3-diylbis(azan-1-yl-1-ylidene))bis(methane-1-yl-1-ylidene)diphenol (L2)
were synthesized according to literature procedures [8, 9, 25–27] (figure 1).
2.2. Synthesis of BL1 and BL2
2.2.1. General procedure
Ligand (1 mmol) was dissolved in 20 mL toluene in a round-bottom flask. Then an equivalent
amount of the boron derivative which approximately corresponds to 1 mmol was added,
after which the ligand solution turned completely clear. The reaction mixture was stirred at
110°C for 5 h . Different gradations of yellow precipitates were filtered, washed with excess
solvent and dried in a ventilated oven. The proposed reactions for formation of BL1 and BL2
are depicted in figures 2 and 3 [28].

JOURNAL OF COORDINATION CHEMISTRY
3
Figure 1. the synthesis of 2,2′-(1e,1′e)-(ethane-1,2-diylbis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)
diphenol (L1) and 2,2′-(propane-1,3-diyl bis(azan-1-yl-1-ylidene)) bis(methane-1-yl-1-ylidene)diphenol
(L2).
Figure 2. Proposed reactions of boron derivatives with BL1.
2.2.1.1. Synthesis of BL1a, BL1b, BL1c, BL1d, and BL1e. The compounds were synthesized
according to the procedure in the previous section. All reactions were conducted under an
atmosphere of argon. The structures were determined by various instrumentation techniques
1
(figure 2). BL1a: H NMR (ppm, DMSO-d6): δ = 8.57 (br, 2H, CH=N), δ = 6.84–7.58 (m, 13H,
Ar–H), and δ = 3.72 (t, 4H, J = 4.6 Hz, –CH2–CH2–), FTIR-ATR (cm−1): 1316 v(B–O), 1058 and
1027 v(B–C), 876 v(B–Ph), 3046 v(Ar–CH), 2936 v(Aliph–CH), 1636 v(Ar–CH=CH), 1608 v(C=N),

4
S. PASA ET AL.
Figure 3. Proposed reactions of boron derivatives with BL2.
and 1151 v(Ar–O). m/z: 355 [M + H+] C22H19N2O2B (MW: 354.15 g mol−1 (exact mass)).
UV-Vis (nm): λ1 = 216 (ε = 2.16 × 103), λ2 = 279 (ε = 2.79 × 103), λ3 = 330 (ε = 3.30 × 103), and
λ4 = 376 (ε = 3.76 × 103) (in acetone:ethanol (1 : 1)). Elemental analysis: Found (Calculated)
C 74.92 (75.02), H 5.67 (5.75), and N 7.52 (7.61) %. BL1b: 1H NMR (ppm, DMSO-d6): δ = 8.57
(br, 2H, CH=N), δ = 6.52–7.60 (m, 11H, Ar–H), δ = 3.71 (t, 4H, J = 4.5 Hz, –CH2–CH2–), δ = 3.64
(s, 3H, Ar–OCH3), and δ = 1.74 (br, 3H, Ar–CH3). FTIR-ATR (cm-1): 1312 v(B–O), 1059 and 1027
v(B–C), 852 v(B–Ph), 3001 v(Ar–CH), 2941 v(Aliph-CH), 1460 v(Ar–CH3), 1608 v(C=N), 1163,
and 1152 v(Ar–O). m/z: 399 [M + H+] C24H23N2O3B (MW: 398.18 g mol−1 (exact mass)).
UV-Vis (nm): λ1 = 308 (ε = 3.08 × 103), λ2 = 314 (ε = 3.14 × 103), λ3 = 328 (ε = 3.28 × 103), and
λ4 = 368 (ε = 3.68 × 103) (in acetone). Elemental analysis: Found (Calculated) C 71.29 (71.38),
H 4.05 (4.12), and N 5.98 (6.03) %. BL1c: 1H NMR (ppm, DMSO-d6): δ = 8.62 (br, 2H, CH=N),
δ = 6.52–7.58 (m, 12H, Ar–H), δ = 3.77 (s, 3H, Ar–OCH3), and δ = 3.40 (t, 4H, J = 4.5 Hz, –CH2–
CH2–). FTIR-ATR (cm−1): 1363 v(B–O), 1061 and 1024 v(B–C), 758 v(B–Ph), 3047 v(Ar–CH),
2990 v(–CH2–CH2–), 1636 v(Ar–CH=CH–), 1607 v(C=N), and 1151 v(Ar–O). m/z: 385 [M + H+]
C23H21N2O3B (MW: 384.16 g mol−1 (exact mass)). UV-Vis (nm): λ1 = 216 (ε = 2.16 × 103),
λ2 = 290 (ε = 2.90 × 103), and λ3 = 339 (ε = 3.39 × 103) (in acetone:methanol (1 : 1)). Elemental
1
analysis: Found (Calculated) C 69.92 (70.09), H 4.07 (4.11), and N 5.51 (5.59) %. BL1d: H
NMR (ppm, DMSO-d6): δ = 8.61 (br, 2H, CH=N), δ = 6.92–7.68 (m, 14H, Ar–H), δ = 3.81 (s, 3H,
Naph–OCH3), and δ = 3.57 (t, 4H, J = 4.6 Hz, –CH2–CH2–). FTIR-ATR (cm−1): 1382 v(B–O), 1061
and 1027 v(B–C), 752 v(B–Ph), 3042 v(Ar–CH), 2993 v(–CH2–CH2–), 1632 v(Ar–CH=CH–), 1607
v(C=N), and 1150 v(Ar–O). m/z: 435 [M + H+] C27H23N2O3B (MW: 434.18 g mol−1 (exact mass)).
UV-Vis (nm): λ1 = 311 (ε = 3.11 × 103), λ2 = 315 (ε = 3.15 × 103), λ3 = 323 (ε = 3.23 × 103), and
λ4 = 346 (ε = 3.46 × 103) (in acetone). Elemental analysis: Found (Calculated) C 74.60 (74.67),
H 4.30 (4.34), and N 4.98 (5.04) %. BL1e: 1H NMR (ppm, DMSO-d6): δ = 8.64 (br, 2H, CH=N),
δ = 6.67–7.97 (m, 11H, Ar–H), δ = 3.70 (t, 4H, J = 4.5 Hz, –CH2–CH2–), and δ = 2.34 (br, 3H,
Ar–CH3). FTIR-ATR (cm−1): 1349 v(B–O), 1074 and 1027 v(B–C), 751 v(B–Ph), 3046 v(Ar–CH),
2960 v(–CH2–CH2–), 1611 v(C=N), 1633 v(Ar–CH=CH–), and 1152 v(Ar–O). m/z: 414 [M + H+]
C23H20N3O4B (MW: 413.15 g mol−1 (exact mass)). UV-Vis (nm): λ1 = 310 (ε = 3.10 × 103),
λ2 = 319 (ε = 3.19 × 103), λ3 = 322 (ε = 3.22 × 103), and λ4 = 355 (ε = 3.55 × 103) (in acetone).
Elemental analysis: Found (Calculated) C 63.77 (63.85), H 3.13 (3.18), and N 10.03 (10.17) %.
2.2.1.2. Synthesis of BL2a and BL2b. The process mentioned in Section 2.2.1 was applied
for the synthesis. The obtained spectral information is summarized below:

JOURNAL OF COORDINATION CHEMISTRY
5
1
BL2a: H NMR (ppm, DMSO-d6): δ = 8.54 (br, 2H, CH=N), δ = 6.35–7.69 (m, 13H, Ar–H),
δ = 3.78 (m, –CH2–CH2–CH2–), and δ = 3.51 (m, 2H, –CH2–CH2–CH2–). FTIR-ATR (cm−1): 1346
v(B–O), 1065 and 1028 v(B–C), 753 v(B–Ph), 3064, 3046 v(Ar–CH), 2932 v(–CH2–CH2–CH2-),
1608 v(C=N), 1643 v(Ar–CH=CH–), and 1152 v(Ar–O). m/z: 369 [M + H+] C23H21N2O2B (MW:
368.17 g mol−1 (exact mass)). UV-Vis (nm): λ1 = 313 (ε = 3.13 × 103), λ2 = 320 (ε = 3.20 × 103),
λ3 = 325 (ε = 3.25 × 103), and λ4 = 370 (ε = 3.7 × 103) (in acetone). Elemental analysis: Found
(Calculated) C 74.97 (75.02), H 5.69 (5.75), and N 7.55 (7.61) %. BL2b: 1H NMR (ppm, DMSO-d6):
δ = 8.49 (br, 2H, CH=N), δ = 6.33–7.89 (m, 12H, Ar–H), 3.79 (m, 4H, –CH2–CH2–CH2–), δ = 3.70
(s, 3H, Ar–OCH3), and δ = 3.53 (m, 2H, –CH2–CH2–CH2–). FTIR-ATR (cm−1): 1359 v(B–O), 1068,
1028 v(B–C), 746 v(B–Ph), 3054 v(Ar–CH), 2949 v(–CH2–CH2–CH2–), 1612 v(C=N), 1641 v(Ar–
CH=CH–), and 1150 v(Ar–O). m/z: 399 [M + H+] C24H23N2O3B (MW: 398.18 g mol−1 (exact
mass)). UV-Vis (nm): λ1 = 253 (ε = 2.53 × 103), λ2 = 277 (ε = 2.77 × 103), λ3 = 352 (ε = 3.52 × 103)
(solvent: DMSO). Elemental analysis: Found (Calculated) C 71.27 (71.38), H 4.60 (4.68), and N
5.95 (6.03) % [29].
2.3. Transfer hydrogenation of acetophenone by BL1 and BL2
The common procedure for transfer hydrogenation: particular amount of (0.04 mmol) boron
complex solutions, NaOH (0.05 mmol), and acetophenone (5 mmol) in degassed isoPrOH
(5 mL) were refluxed at 82 °C. Samples from the mixture were taken, diluted with acetone
and checked for conversion at different time periods. The products were compared with the
unreacted acetophenone. GC analyses were applied with a Shimadzu 2010 Plus Gas
Chromatograph having a capillary column (95% dimethyl siloxane and 5% biphenyl)
(30 m × 0.32 mm × 0.25 μm). The adjusted GC parameters were: initial temperature, 50 °C;
solvent delay, 4.48 min; initial time, hold 1 min; final temperature, 270 °C; hold 5 min; tem-
perature ramp 15 °C min−1; injector port temperature, 200 °C; detector temperature, 200 °C,
final time, 20.67 min, and injection volume, 1.5 μL.
3. Results and discussion
3.1. Spectral identification
The molecular structures of L1 and L2 were reported by previous studies [8, 9, 30–32] which
are consistent with what we find (figure 1).
The boron complexes have been synthesized by the condensation of L1 and L2 and boron
precursors, and they were found to be stable in air and not hygroscopic. 1H NMR spectra of
the compounds showed the presence of the resonances between 8.08 and 8.64 ppm corre-
sponding to the azomethine (CH=N) proton [27, 33–36]. In addition to this, disappearance
of resonances due to OH was evidence of binding of boron to oxygen. Also, formation of
the desired compounds was proved by the presence of the stretching vibrations of boron
with oxygen species observed from 1312 cm−1 to 1378 cm−1 and B–Ph interaction at 751–
852 cm−1. The exact mass spectra of all compounds applied in positive charged scan indicated
the meaningful values [27, 37]. UV-Vis analysis of the boron-based molecules demonstrated
n → π* and π → π* electronic transitions due to donor and aromatic rings [6, 38-40]. The
surface images of the BL1 and BL2 series were demonstrated as bent fibers, ground rocks,

6
S. PASA ET AL.
and crumbled ice pieces (figures 2 and 3), in agreement with literature data under 16,000
times magnification [41–43] (figures 2 and 3).
3.2. SEM and EDX results of compounds
Surface morphologies of boron compounds were determined under reduced pressure at
different magnifications (Supplemental Material). The EDX analyses were also taken simul-
taneously. Qualitative elemental analyses, especially for boron, are as expected. Existence
of boron supports the complexation reaction with oxygen on ligand. BL1a and BL1b have
a homogenous dispersion in image. It looks like skewed fibers and single phase. BL1c and
BL1d seemed like crumbled rocks with homogenous single phase features. BL2b has thin
and long prism shapes with 5–10 μm length. BL1e includes a mixture of needle and ground
stone shapes. However, the clearness of images for boron complexes depends on the amor-
phous and crystallinity percentages and also the conductivity of the materials. If the amor-
phous content is more than crystallinity, the sharpness of SEM images will be excellent.
Furthermore, having a conductive material also improves images.
3.3. Thermal analysis
The thermal stabilities of the compounds used for catalytic investigations were investigated.
The experiment was designed by TGA/DTA under nitrogen by increasing the heat temper-
ature at 10 °C per minute. Thermograms of boron compounds showed that they are stable
up to 300 °C, since there are no weight losses till that temperature. The weight losses and
the corresponding heats were evaluated for BL1 and BL2 with endothermic and exothermic
peaks in thermograms. Acetate (CH3COO−), water, and small amount of humidity remaining
in the solid sample give endothermic peaks; combustion or decomposition of organic struc
-
tures causes exothermic curves in the thermogram (Supplemental Material). All BL1 and
BL2 series have two-step decomposition. Thermal decompositions can be summarized as:
65.78 and 9.85% (BL1a); 62.23 and 10.34% (BL1b); 51.01 and 23.99% (BL1c); 60.22 and 5.53%
(BL1d); 27.88 and 23.52% (BL1e); 61.90 and 13.81% (BL2a); 74.15 and 8.10% (BL2b); 81.73%.
These results show that they can be favorable for higher temperature catalytic reactions
[28–30, 44–46].
3.4. Catalytic transfer hydrogenation of ketones
The catalytic activities of boron complexes in transfer hydrogenation of aromatic ketones
by isoPrOH are summarized in table 1. Transfer hydrogenation reaction chart for catalytic
conversions is also depicted in figure 4. The conversions to isopropyl alcohol during the
reaction showed different yields due to the different substituents. BL1a, BL1b, BL1c, BL1d,
BL1e, and BL2a and BL2b were used as catalysts for the transfer hydrogenation reaction.
Recently, we reported that boron hybrid complexes are active catalysts in the reduction of
aromatic ketones [47, 48]. In a typical experiment, 0.005 mmol of the boron complex and
0.5 mmol of acetophenone were added to a solution of NaOH in isoPrOH (0.025 mmol of
NaOH in 5 mL isoPrOH), refluxed at 82 °C and the reaction was monitored by gas
chromatography.

JOURNAL OF COORDINATION CHEMISTRY
7
Table 1. transfer hydrogenation results of acetophenone with isoProh catalyzed by BL1a, BL1b, BL1c,
BL1d, BL1e, and BL2a and BL2b.
Entry
Catalyst
S/C/NaOH
Time
Conversion (%)[d]
TOF (h−1)[e]
1
2
3
4
5
6
7
8
[BL1a][a]
[BL1b][a]
[BL1c][a]
[BL1d][a]
[BL1e][a]
[BL2a][a]
[BL2b][a]
[BL1a][b]
[BL1b][b]
[BL1c][b]
[BL1d][b]
[BL1e][b]
[BL2a][b]
[BL2b][b]
[BL1a][c]
[BL1b][c]
[BL1c][c]
[BL1d][c]
[BL1e][c]
[BL2a][c]
[BL2b][c]
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1
100 : 1
100 : 1
100 : 1
100 : 1
48 h
48 h
48 h
48 h
48 h
48 h
48 h
72 h
72 h
72 h
72 h
72 h
72 h
72 h
12 h
24 h
24 h
24 h
16 h
24 h
48 h
<3
<3
<3
<3
<3
<3
<3
<5
<5
<5
<5
<5
<5
<5
98
99
97
99
99
98
99
…..
…..
…..
…..
…..
…..
…..
…..
…..
…..
…..
…..
…..
…..
<10
<5
9
10
11
12
13
14
15
16
17
18
19
20
21
100 : 1
100 : 1
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
<5
<5
<10
<5
<5
reaction conditions:
[a]at room temperature; acetophenone/Cat./naoh, 100 : 1 : 5.
[b]refluxing in iso-Proh, acetophenone/Cat, 100 : 1, in the absence of base.
[c]refluxing in iso-Proh; acetophenone/Cat/naoh, 100 : 1 : 5.
[d]determined by GC (three independent catalytic experiments).
[e]referred at the reaction time indicated in column; tof = (mol product/mol Cat.) × h−1
.
Figure 4. General reaction of transfer hydrogenation.
These boron hybrid complexes catalyzed the reduction of ketones to the corresponding
alcohols by hydrogen transfer from isoPrOH. Furthermore, with a complex/NaOH ratio of
1/5, the boron complexes are active, leading to quantitative transformation of the acetophe-
none. The results are summarized in table 1. At room temperature no appreciable formation
of 1-phenylethanol was observed (table 1, entries 1–7). As can be inferred from table 1
(entries 8–14), the precatalysts as well as NaOH are necessary to observe appreciable con-
versions. The base facilitates formation of boron alkoxide by abstracting proton of the alcohol
and subsequently alkoxide undergoes β-elimination to give boron hydride, which is an active
species in this reaction. As table 1 shows, high conversions can be achieved with BL1a and
BL1e catalytic systems. The results obtained from optimization studies showed clearly that
excellent conversions were achieved in reduction of acetophenone to 1-phenylethanol when
BL1a and BL1e were used as catalytic precursors, with a substrate-catalyst molar ratio

8
S. PASA ET AL.
Table 2. transfer hydrogenation results for substituted acetophenones with the catalyst systems, [BL1a]
and [BL1e].[a]
Conversion(%)[b]
TOF(h−1)[c]
Entry
R
Time
Cat: [BL1a]
1
2
3
4
5
6
4-f
4-no2
4-Cl
4-Br
2-meo
4-meo
8 h
8 h
10 h
12 h
24 h
18 h
98
99
98
99
98
96
12
12
10
<10
<10
<10
Cat: [BL1e]
7
8
9
10
11
12
4-f
4-no2
4-Cl
4-Br
2-meo
4-meo
10 h
12 h
15 h
20 h
30 h
24 h
99
99
98
97
98
95
10
<10
<10
<10
<10
<10
[a]Catalyst (0.005 mmol), substrate (0.5 mmol), isoProh (5 ml), naoh (0.025 mmol), 82 °C, respectively, the concentration of
acetophenone derivatives is 0.1 m.
[b]Purity of compounds is checked by 1h nmr and GC (three independent catalytic experiments), yields are based on methyl
aryl ketone.
[c]tof = (mol product/mol Cat.) × h−1
.
Table 3. transfer hydrogenation results for substituted alkyl phenyl ketones with the catalyst systems,
[BL1a] and [BL1e].[a]
Conversion(%)[b]
TOF(h−1)[c]
Entry
Catalyst
Time
Substrate
Product
O
OH
1
7
18 h
98
<10
2
3
8
7
26 h
24 h
98
99
<10
<10
O
O
O
OH
OH
OH
4
5
8
7
36 h
36 h
98
97
<10
<5
6
7
8
7
48 h
42 h
98
98
<5
<5
8
8
56 h
99
<5
[a]Catalyst (0.005 mmol), substrate (0.5 mmol), iso-Proh (5 ml), naoh (0.025 mmol), 82 °C, respectively, the concentration
of alkyl phenyl ketones is 0.1 m.
[b]Purity of compounds is checked by 1h nmr and GC (three independent catalytic experiments), yields are based on methyl
aryl ketone.
[c]tof = (mol product/mol Cat.) × h−1
.
(100 : 1) in isoPrOH at 82 °C (table 1, entries 15–21). As seen from table 1, the catalytic activ-
ities in the studied hydrogen transfer reactions were generally much higher for BL1a and
BL1e than the other catalytic systems. For example, under identical conditions, transfer
hydrogenation of acetophenone derivatives with BL1a led to 98% conversions within 12 h
whereas with BL1e as the auxiliary, 99% conversions were achieved only after 16 h period
(table 1, entries 15 and 19).

JOURNAL OF COORDINATION CHEMISTRY
9
Due to its efficiency in transfer hydrogenation of acetophenone, BL1a and BL1e were
further investigated in transfer hydrogenation of substituted acetophenone derivatives. The
catalytic reductions of acetophenone derivatives were all tested with the conditions opti-
mized for acetophenone and the results are summarized in table 2, which illustrates con-
versions of the reduction performed in a 0.1 M of isoPrOH solution containing BL1a and
BL1e and NaOH (Ketone : Cat. : NaOH = 100 : 1 : 5). Electronic properties of the substituents
on the phenyl ring of the ketone caused changes in the reduction rate. An ortho- or para-sub-
stituted acetophenone with an electron-donor substituent, i.e. 2-methoxy or 4-methoxy is
reduced more slowly than acetophenone (table 2, entries 5, 6, 11, and 12) [49]. Additionally,
the introduction of electron-withdrawing substituents, such as F, NO2, Cl, and Br to the
para-position of the aryl ring of the ketone decreased the electron density of the C=O bond
so that the activity was improved, giving easier hydrogenation [50, 51]. The examination of
the results indicates clearly that the best yield was achieved in the reduction of acetophe-
none derivatives when BL1a was used as the catalyst precursor (table 2, entries 1 and 2).
We also carried out experiments to explore the effect of bulkiness of the alkyl groups on
the catalytic activity and the results are given in table 3 (entries 1–12). A variety of simple
aryl alkyl ketones were transformed to the corresponding secondary alcohols. Catalytic activ-
ity is highly dependent on the steric bulk of the alkyl group. The reactivity regularly decreased
by increasing the bulkiness of the alkyl groups [52–55]. Catalytic activity of BL1a was gen-
erally much higher in the studied hydrogen transfer reactions than that of BL1e.
4. Conclusion
In the present study, we have synthesized ligands including O donors and their boron com-
plexes. The compounds were characterized with various techniques. All complexes can be
implemented for conversions of aromatic ketone to corresponding alcohols. The boron
compounds are catalytically active and preferred instead of expensive catalysts like rhodium,
ruthenium, and iridium. The construction of these catalysts and their flexibility towards
transfer hydrogenation make these systems for further study. Further work and possible use
of asymmetric versions of these complexes for catalytic purposes are in progress.
Acknowledgment
Partial supports of this work by Turkish Academy of Sciences and Dicle University (Project numbers:
FEN.16.010 (NMR), DÜBAP FEN.15.021, FEN.15.022, DÜAPK 05-FF-27, and 14-EZF-14), and the analysis
conducted by the Dicle University Science and Technology Research Center (DUBTAM) is gratefully
acknowledged.
Disclosure statement
No potential conflict of interest was reported by the authors.
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