Catechol-type ligand containing new modular design dioxaborinane compounds: Use in the transfer hydrogenation of various ketones

Article abstract of DOI:10.1016/j.catcom.2018.03.023

A novel class of tricoordinate dioxaborinane compounds, which have the general formula [B₁(L_1–5)] and [B₂(L_1–5)], were designed and synthesized by the corresponding catechol-type ligands (L₁–L₅) at ambient temperature. All the new compounds were fully characterized by NMR (¹H, ¹³C, and ¹¹B), FT-IR, UV–vis, LC-MS spectroscopy, and melting point analysis and microanalysis. The dioxaborinane [B₁(L_1–5)] and [B₂(L_1–5)] compounds were investigated as catalyst for the transfer hydrogenation of various ketones under suitable conditions. Particularly, it was proved that the ferrocene-based dioxaborinane [B₁(L_1–5)] molecules can afford an efficient catalytic conversion compared to corresponding 3,5-bis(trifluoromethyl)phenyl-based [B₂(L_1–5)] dioxaborinanes in transfer hydrogenation catalytic studies.

Full text of DOI:10.1016/j.catcom.2018.03.023

CatalysisCommunications111(2018)42–46
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Catechol-type ligand containing new modular design dioxaborinane
compounds: Use in the transfer hydrogenation of various ketones
Ahmet Kilica,⁎, İbrahim Halil Kayaa, Ismail Ozaslana, Murat Aydemirb,⁎⁎, Feyyaz Durapb
a
Harran University, Chemistry Department, Art and Science Faculty, 63290 Sanliurfa, Turkey
Dicle University, Chemistry Department, Science Faculty, 21280 Diyarbakir, Turkey
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
A novel class of tricoordinate dioxaborinane compounds, which have the general formula [B1(L1–5)] and
[B2(L1–5)], were designed and synthesized by the corresponding catechol-type ligands (L1–L5) at ambient
temperature. All the new compounds were fully characterized by NMR (1H, 13C, and 11B), FT-IR, UV–vis, LC-MS
spectroscopy, and melting point analysis and microanalysis. The dioxaborinane [B1(L1–5)] and [B2(L1–5)]
compounds were investigated as catalyst for the transfer hydrogenation of various ketones under suitable
conditions. Particularly, it was proved that the ferrocene-based dioxaborinane [B1(L1–5)] molecules can afford
an efficient catalytic conversion compared to corresponding 3,5-bis(trifluoromethyl)phenyl-based [B2(L1–5)]
dioxaborinanes in transfer hydrogenation catalytic studies.
Catechol-type ligands
Dioxaborinanes
Spectroscopy
Transfer hydrogenation
Ketones
1. Introduction
the absence of molecular hydrogen, the transfer hydrogenation method,
in which formic acid and its salts or secondary alcohols have been used
Boron compounds (reagents) have become an important part of
today's fine chemical industry for the preparation of highly complex
functionalized molecules. Commercially available boron compounds
are widely utilized in pharmaceutical polymer and electronics manu-
facturing [1]. Boron complexes are versatile compounds that are com-
monly used as regio-, chemo-, and stereoselective reducing agents of a
variety of group functionalities such as aldehydes, ketones, amides, and
olefins [2]. Over the past decades, environmentally friendly tri and
tetracoordinate boron compounds have attracted considerable attention
because of their low cytotoxicity, electronic structure, easy preparation,
low cost, and air-stable properties [3]. Furthermore, boron and related
compounds have important applications in many fields such as cata-
lysis; pharmacology; toxicology; agriculture; and textile, glass, ceramic,
cosmetic, energy, and construction industries, as well as in commu-
nication tool manufacturing and detergent production [4]. In contrast
to tetracoordinate boron compounds, tricoordinate dioxaborinanes
formed from the catechol-type ligands and boronic acid interactions
have an empty p-orbital, which makes them strong electrophiles and
Lewis acids, and the two covalent BeO bonds make the structure stable
in air. Because of their unique features, tricoordinate dioxaborinanes
are one of the most widely studied groups [5–7].
as hydrogen sources, is more attractive and safer than direct the hy-
drogenation method [8–10]. Nowadays, several transition metals such
as Au, Pt, Pd, Ir, Rh and Ru are often used as catalysts in transfer hy-
drogenation with high activity [11–13]. However, these metals are
usually not preferred for the transfer hydrogenation, because they are
expensive and cause environmental pollution. Instead of them, the
development of an effective and cheaper catalytst for the transfer hy-
drogenation of ketones is one of the main targets for the scientists [1,2].
Boron in its tricoordinate dioxaborinane molecules is mainly interesting
because it is considerably more effective and stable than tetra-
coordinate boron complexes for the transfer hydrogenation of ketones
as catalysts [1,2]. To the best of our knowledge, there is no report on
the use of tricoordinate dioxaborinane derivatives in the transfer hy-
drogenation. In view of the promising results, tricoordinate dioxabor-
inane derivatives were shown to be efficient homogeneous hydro-
genation catalysts toward various substrates, and these catalysts would
be a valuable material for the transfer hydrogenation. In this study,
various techniques such as NMR (1H, 13C, and 11B), FT-IR, UV–vis, LC-
MS spectroscopy, and melting point and elemental analyses were used
to characterize these compounds.
Two types of hydrogenation methods, namely the transfer hydro-
genation and direct hydrogenation, have been widely used. Because of
⁎
Corresponding author at: Harran University, Chemistry Department, TR-63190 Sanlıurfa, Turkey.
⁎⁎
Corresponding author.
E-mail addresses: kilica63@harran.edu.tr (A. Kilic), aydemir@dicle.edu.tr (M. Aydemir).
https://doi.org/10.1016/j.catcom.2018.03.023
Received 15 February 2018; Received in revised form 15 March 2018; Accepted 19 March 2018
Availableonline23March2018
1566-7367/©2018ElsevierB.V.Allrightsreserved.

A. Kilic et al.
CatalysisCommunications111(2018)42–46
Scheme 1. Synthesis of the catechol-type ligands (L1-L5).
2. Experimental
3.1. Characterization of the catechol-type ligands and their dioxaborinane
derivatives
The information about the chemicals and devices used in this study
is given in the supplementary information. The synthesis and spectro-
scopic results are also given as supplementary information for the new
catechol-type ligands (L1-L5) and their tricoordinate dioxaborinane
[B1(L1–5)] and [B2(L1–5)] compounds.
The bands in the range 3553–2329 cm−1 were due to the stretching
vibrations of intermolecular H-bond υ(OeH…OH) in 1,2-dihydroxy
groups of catechol-type ligands, which disappeared in the FT-IR spectra
of dioxaborinane [B1(L1–5)] and [B2(L1–5)] complexes (Fig. S1–S3), as
expected. These results show the deprotonation of the 1,2-dihydroxy
moiety owing to the coordination and confirm the formation of the
dioxaborinane derivatives (Fig. S1–S3). In addition, the bands at
1650–1641 cm−1 for catechol-type ligands and at 1659–1644 cm−1 for
dioxaborinane derivatives were due to the stretching vibrations of
azomethine groups (HC]N) [14]. Furthermore, compared to the ca-
techol-type ligands, the stretching peak of azomethine groups υ(HC]
N) of dioxaborinanes, [B1(L1–5)] and [B2(L1–5)], exhibited a higher
3. Results and discussion
The dark brown catechol-type ligands (L1-L5) were prepared in
EtOH with a yield in the range 83–78%, as shown in Scheme 1. The
dioxaborinane [B1(L1–5)] and [B2(L1–5)] compounds were obtained in
toluene and DMF at reflux temperature over 8 h with a yield up to 78%
(Scheme 2).
Scheme 2. Synthesis of the dioxaborinane [B1(L1–5)] and [B2(L1–5)] complexes.
43

A. Kilic et al.
CatalysisCommunications111(2018)42–46
frequency shift, the supporting formation of the tricoordinate dioxa-
borinanes. New characteristics peaks appeared at 1194–1189 cm−1
υ(BeO), 893–890 cm−1 υ(BeC), and 484–480 cm−1 υ(FeeC) for
[B1(L1–5)] complexes and at 1164–1161 cm−1 υ(BeO) and
899–896 cm−1 υ(BeC) for [B2(L1–5)] complexes [15,16].
acetophenone were added to
a
solution of KOH in 2-propanol
(0.05 mmol of KOH in 10 mL 2-propanol) and refluxed at 82 °C, the
reaction being monitored by GC spectroscopy. At room temperature,
the transfer hydrogenation of acetophenone occurred very slowly, with
low conversion (up to 10%, 96 h) in the reactions. Additionally, as can
be inferred from the catalytic tests, the presence of base is necessary to
observe appreciable conversions. The base facilitateds the formation of
boron alkoxide by extracting the proton of the alcohol, and subse-
quently, the alkoxide undergoes β-elimination to give boron hydride,
which is an active species in this reaction. In our study, the use of the
base alone and the absence of tricoordinate dioxaborinane [B1(L1–5)]
or [B2(L1–5)] catalysts was not effective for the transfer hydrogenation
reaction, and the conversions are very low (> 3%), which conrims the
boron catalysts play a key role in catalyzing this reaction. In addition,
the choice of base, such as KOH or NaOH, had a remarkable influence
on the conversion, and with a catalyst/KOH ratio of 1/5, the dioxa-
borinane compounds were very active leading to a quantitative trans-
formation of the acetophenone (Table 1, entries 11–14,[d]). Reduction
of acetophenone into 1-phenylethanol could be achieved in high yields
by increasing the temperature up to 82 °C (Table 1, entries 1–10).
Furthermore, it is noteworthy that the reactivities of catalytic systems
the dioxaborinane [B1(L1–5)] and [B2(L1–5)] catalysts were different.
These results indicate that the structure difference of the dioxaborinane
compounds is a crucial factor for the acceleration of the reaction.
Therefore, from the results, the dioxaborinane [B1(L1–5)] and
[B2(L1–5)] were effective catalysts for the hydrogenation of acet-
ophenone, producing products with up to 99% conversion.
In general, the ferrocene-based [B1(L1–5)] catalysts showed a much
higher activity than other tricoordinate dioxaborinane catalysts
(Table 1). The higher catalytic activities (electron-rich ferrocene
moiety) could 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 could also play a crucial role in the
transfer hydrogenation reactions. These results obviously indicate that
the catalysts have ferrocene moiety that can possibly stabilize a cata-
lytic transition state when compared with other catalysts. Furthermore,
the observation that the electron-rich ferrocene ligands leading to a
more active catalyst is indeed interesting because one would expect that
an electron-withdrawing ligand would increase the Lewis acidity of the
boron center. Furthermore, it was also observed that the catalytic ac-
tivities in the studied hydrogen transfer reactions were generally much
higher for the catalysts having pyridine moiety, [B1(L4)] and [B2(L4)]
than for the dioxaborinane catalysts. In particular, the ferrocene-based
and pyridine-containing [B1(L4)] compound acted as a good catalyst
giving a good conversion of the corresponding alcohols up to 99%
(TOF ≤ 396 h−1). These results also show that the conversion rate of
the substrate can also be affected by the electronic factors and the steric
factors of the substituents on the ligand-attached dioxaborinane deri-
vatives. Moreover, the conversions gradually decreased with the in-
crease in mole ratios of [acetophenone]/[Cat] from 250/1 to 500/1 or
1000/1, and also the reaction time was increase (Table 2, entries 1–6).
Attributable to their efficiency in the transfer hydrogenation of
acetophenone, dioxaborinane [B1(L4)] and [B2(L4)] catalysts were
further investigated in the transfer hydrogenation of acetophenone
derivatives to be converted into the corresponding secondary alcohols
under the optimal conditions as shown in Table 1S. Because the cata-
lytic activities in the studied hydrogen transfer reactions were generally
much higher for the dioxaborinane [B1(L4)] and [B2(L4)] catalysts
than those for the other compounds, only these two catalysts were
extensively investigated with acetophenone derivatives. The verifica-
tion of the results indicates clearly that among all the tested dioxa-
borinane compounds, the best yield was achieved in the reduction of
acetophenone derivatives when dioxaborinane compound that has not
only ferrocene moiety but also pyridine group, [B1(L4)], was used as
the catalyst precursor. The results showed that a range of acetophenone
derivatives can be hydrogenated with high conversions. As already
The NMR spectra of the ligands and their dioxaborinane derivatives
are given in the Supporting Information (Fig. S4–S18). The resonances
of azomethine (CH]N) region were observed in the range
8.41–8.29 ppm [17] as singlet, and this region showed little variation
between catechol-type ligands. The azomethine (CH]N) signals of
[B1(L1–5)] complexes were shifted to lower frequencies (in the range
δ = 8.23–7.95 ppm) with respect to the ligands, whereas the proton
signals of the azomethine groups in [B2(L1–5)] complexes (in the range
δ = 8.73–8.42 ppm) significantly shifted to the high field compared to
the ligands because of the nature of ferrocene or 3,5-bis(tri-
fluoromethyl)phenyl moiety. The peaks of 1, 2-dihydroxy proton (OH)
region of ligands (L1–L5) were observed in the range 8.04–7.96 ppm as
singlet or doublets, and the disappearance of the OH signals in the di-
oxaborinane compounds indicates the binding of the boron atoms to the
two oxygen atoms. The ferrocene region could show from three to four
signals (around δ = 4.45–4.08 ppm) that correspond to the non-sub-
stituted cyclopentadiene of [B1(L1–5)] complexes [18], as expected.
When the 13C NMR was examined (Fig. S4–S18), the chemical shifts
observed in the range δ = 168.23–162.18 ppm were assigned to azo-
methine
(HC]N)
carbons
and
those
in
the
range
δ = 162.43–105.58 ppm were assigned to aromatic (AreCH) carbons.
In addition, the ferrocene unit carbon signals were observed in the
range 73.35–67.68 ppm [18], which confirms that the two cyclo-
pentadiene rings containing ferrocene unit is connected to the boron
center in the ferrocene-based [B1(L1–5)] compounds. In the 13C NMR
spectra of all compounds, other characteristic resonances appeared in
the range 66.18–17.47 ppm, which provides another evidence for the
formation of all compounds. In the 11B NMR spectra of dioxaborinane
[B1(L1–5)] and [B2(L1–5)] derivatives, the boron resonances observed
in the range δ = 14.12–14.78 ppm for the [B1(L1–5)] complexes and in
the range δ = 9.02–10.04 ppm for the [B2(L1–5)] complexes due to the
same chemical environment for the boron atoms of each compound
(Fig. S19-S22), which confirms the formation of tricoordinate dioxa-
borinane compounds.
The UV–vis absorption spectra of the ligands (L1–L5) and their di-
oxaborinane [B1(L1–5)] and [B2(L1–5)] derivatives were evaluated in
dilute C2H5OH and DMF (2.10−5–2.10−7 M) at room temperature (Fig.
S23–S26). In the absorption spectra of the ligands (L1–L5) and their
dioxaborinane derivatives [B1(L1–5)] and [B2(L1–5)], similar absor-
bance wavelengths were observed because of the π → π* or n → π*
electronic transitions arising from the donor atoms and aromatic rings
[19]. These obvious absorption bands were observed in the range
218–506 and 264–501 nm in C2H5OH and DMF, respectively.
Further confirmation of the formation of the catechol-type ligands
(L1-L5) and their dioxaborinane [B1(L1–5)] and [B2(L1–5)] compounds
was also provided by LC-MS spectroscopy (Fig. S27–S32). Their LC-MS
calculated and obtained spectral results showed the expected molecular
and fragmentation ions, with appropriate isotope distribution (Fig.
S23–S28).
3.2. Catalytic properties
We preferred to examine the transfer hydrogenation of simple ke-
tones by using a 2-propanol/base system in the presence of the tri-
coordinate dioxaborinane [B1(L1–5)] or [B2(L1–5)] molecules as cata-
lysts. In a preliminary study, the ferrocene-based [B1(L1–5)] or 3,5-bis
(trifluoromethyl)phenyl-based [B2(L1–5)] dioxaborinane compounds
were tested as precursors for the catalytic transfer hydrogenation of
acetophenone and the results are summarized in Table 1. A typical
procedure using acetophenone as substrate was performed as follows.
First, 0.01 mmol of dioxaborinane catalysts and 1.0 mmol of
44

A. Kilic et al.
CatalysisCommunications111(2018)42–46
Table 1
Transfer hydrogenation of acetophenone with 2-propanol catalyzed by dioxaborinane [B1(L1–5)] and [B2(L1–5)] compounds.
[d]
[e]
Entry
Catalysts
S/C/base
Time
Conv.%
TOF (h−1
)
1
2
3
4
5
6
7
8
[B1L1][a]
[B1L2][a]
[B1L3][a]
[B1L4][a]
[B1L5][a]
[B2L1][a]
[B2L2][a]
[B2L3][a]
[B2L4][a]
[B2L5][a]
[B1L4][c]
[B1L4][c]
[B1L4][c]
[B1L4][c]
100:1:5
100:1:5
100:1:5
100:1:5
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
30 min
30 min
30 min
15 min (20 min)[b]
30 min
2 h
98
97
98
196
198
196
99(96)[b]
396 (288)[b]
98
98
97
99
196
49
49
2 h
2 h
50
9
1 h (3/2 h)[b]
2 h
99(97)[b]
97
99 (65)[b]
49
10
11
12
13
14
15 min
15 min
15 min
15 min
93
99
94
87
372
396
376
348
Reaction conditions [a] Refluxing in 2-propanol; acetophenone/Cat/KOH, 100:1:5; [b] Refluxing in 2-propanol; acetophenone/Cat/NaOH, 100:1:5; [c] Refluxing in 2-
[d]
[e]
propanol; acetophenone/Cat/KOH, 100:1:x;
Determined by GC (three independent catalytic experiments);
Referred at the reaction time indicated in column;
TOF = (mol product/mol Cat.) × h−1
.
in the studied hydrogen transfer reactions than that of dioxaborinane
[B2(L4)] compound. Because of their efficiency in the transfer hydro-
genation of acetophenone derivatives, complexes [B1(L4)] and
[B2(L4)] were further investigated in the transfer hydrogenation of a
variety of ketones to be converted to the corresponding alcohols under
the optimization conditions. Table 3S shows several examples for the
asymmetric reductions performed using this method. The reaction of
methyl/alkyl and methyl/aryl ketones gave an acceptable chemical
yield of their corresponding products. Thus, a variety of simple alkyl/
alkyl and aryl/alkyl ketones were transformed to their corresponding
secondary alcohols and it was found that the activities were highly
dependent on the structure of the alkyl or alkyl group. Reaction of
ketones possessing a bulky alkyl substituent proceeded rather sluggish
and decreased the conversion rate. When the size of the alkyl group
increased, the activity decreased. In addition, as shown in Table 3S, the
highest activity was observed with 1-naphtyl methyl ketone (up to 99%
conversion, TOF: 392). Furthermore, the hydrogenation of the ketones
having the cyclohexyl group was very slow (Table 3S, entries 1–16).
Table 2
Transfer hydrogenation of acetophenone with 2-propanol catalyzed by different
mole ratios of [acetophenone]/[Cat] (Cat: [B1(L4)] and [B2(L4)]).
[c]
Entry
Catalysts
S/C/KOH
Time
Conv. (%)[b]
TOF (h−1
)
1
2
3
4
5
6
[B1(L4)][a]
[B2(L4)][a]
[B1(L4)][a]
[B2(L4)][a]
[B1(L4)][a]
[B2(L4)][a]
250:1:5
250:1:5
500:1:5
500:1:5
1000:1:5
1000:1:5
30 min
2 h
1 h
5 h
2 h
98
99
99
99
98
98
196
45
99
20
49
12
8 h
Reaction conditions: [a] Refluxing in 2-propanol; acetophenone/Cat/KOH, 250,
500 or 1000:1:5;
ments);
[b]
Determined by GC (three independent catalytic experi-
TOF = (mol product/mol Cat.) × h−1
.
[c]
stated, the electronic properties (the nature and position) of the sub-
stituents on the phenyl ring of the ketone affected the changes in the
reduction rate. As shown in Table 1S, the introduction of electron-
withdrawing substituents to the ortho-, meta-, or para positions of the
aryl ring of the ketone decreased the electron density of the C]O bond
so that the activity was improved giving rise to easier hydrogenation
[20,21]. Thus, the introduction of electron-withdrawing substituents
such as F- or NO2- resulted in an improved activity with a good yield
(Table 1S, entries 1–8). On the contrary, the introduction of an electron-
donating group such as methoxy group decelerated the reaction re-
gardless of the position of substitution (Table 1S, entries 9–17). Overall,
the catalytic efficiency attained a very high level in the reduction of
aromatic ketones and compared with the recently discovered similar
catalyst systems [22,23]. We carried out further experiments to study
the influence of bulkiness of the alkyl groups on the catalytic activity
and selectivity (Table 2S, entries 1–8), and the results are presented in
Table 2S. A variety of simple aryl alkyl ketones were transformed to
their corresponding secondary alcohols. It was observed that the ac-
tivity was significantly dependent on the steric hindrance of the alkyl
group [24–28]. The reactivity gradually reduced with the increase in
the bulkiness of the alkyl groups [29,30]. For example, the hydro-
genations of isopropyl phenyl ketone could be achieved approximately
in 2 and 5 h by dioxaborinane [B1(L4)] and [B2(L4)] catalysts, re-
spectively. Consequently, one can easily conclude that the catalytic
activity of dioxaborinane [B1(L4)] complex was generally much higher
4. Conclusion
In summary, the catechol-type ligands and their dioxaborinane de-
rivatives obtained in this work were characterized by NMR spectro-
scopy (1H, 13C and 11B), FT-IR spectroscopy, UV–vis spectroscopy, LC-
MS spectroscopy, and melting point and elemental analyses. Then, the
dioxaborinane [B1(L1–5)] and [B2(L1–5)] compounds were evaluated
for the transfer hydrogenation of the reduction of ketones as catalysts.
The catalytic activities in the studied hydrogen transfer reactions were
generally much higher for the dioxaborinane [B1(L4)] and [B2(L4)]
catalysts than those for the other compounds. However, the catalytic
results indicate that among all the tested dioxaborinane compounds,
the best yield was achieved in the reduction of acetophenone deriva-
tives when [B1(L4)] was used as the catalyst precursor, which has not
only ferrocene moiety but also pyridine group. Furthermore, the sim-
plicity and efficiency clearly make it an excellent choice of catalyst for
the practical preparation of high-value alcohols by the catalytic asym-
metric transfer hydrogenation of ketones.
45

A. Kilic et al.
CatalysisCommunications111(2018)42–46
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.catcom.2018.03.023.
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