New heterogeneous B(OEt)3-MCM-41 catalyst for preparation of α,β-unsaturated alcohols

Article abstract of DOI:10.1007/s11164-013-1498-0

Grafting of boron tri-ethoxide on mesoporous MCM-41 resulted in a highly active catalyst for the Meerwein-Ponndorf-Verley (MPV) reduction and the catalyst denoted as B(OEt)₃-MCM-41. Chemoselective reduction of α,β-unsaturated aldehydes and ketones to the corresponding α,β-unsaturated alcohols was achieved by MPV reduction reaction using a new B(OEt)₃-MCM-41 catalyst. The prepared new heterogeneous catalyst, B(OEt)₃-MCM-41, was characterized in detail by using XRD, ²⁹Si NMR-, ¹¹B NMR-, ¹³C NMR-, and TEM, N₂ adsorption, and ICP-OES. The results demonstrated the successful homogenous distribution of the B(OEt)₃ on the MCM-41 support. The heterogeneous B(OEt)₃-MCM-41 catalyst, in comparison with the homogeneous B(O ⁱ Pr)₃ and B(OEt)₃ catalysts, displayed similiar catalytic activity in the MPV reduction of α,β-unsaturated aldehydes and ketones with alcohols as reductants. Reduced reaction times and very high selectivities for the unsaturated alcohols were obtained with the heterogenous catalyst compared with the homogeneous catalysts. The B(OEt)₃-MCM-41 catalyst was found to be encouraging, as is is recyclable up to six cycles without any significant loss in its catalytic activity.

Full text of DOI:10.1007/s11164-013-1498-0

Res Chem Intermed
DOI 10.1007/s11164-013-1498-0
New heterogeneous B(OEt) -MCM-41 catalyst
3
for preparation of a,b-unsaturated alcohols
Burcu Uysal Birsen S. Oksal
•
Received: 30 March 2013 / Accepted: 26 November 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Grafting of boron tri-ethoxide on mesoporous MCM-41 resulted in a
highly active catalyst for the Meerwein–Ponndorf–Verley (MPV) reduction and the
catalyst denoted as B(OEt) -MCM-41. Chemoselective reduction of a,b-unsaturated
3
aldehydes and ketones to the corresponding a,b-unsaturated alcohols was achieved by
MPV reduction reaction using a new B(OEt) -MCM-41 catalyst. The prepared new
heterogeneous catalyst, B(OEt) -MCM-41, was characterized in detail by using XRD,
3
3
2
9
11 13
Si NMR-, B NMR-, C NMR-, and TEM, N adsorption, and ICP-OES. The
2
results demonstrated the successful homogenous distribution of the B(OEt) on the
3
MCM-41 support. The heterogeneous B(OEt) -MCM-41 catalyst, in comparison with
3
i
the homogeneous B(O Pr) and B(OEt) catalysts, displayed similiar catalytic activity
3
3
in the MPV reduction of a,b-unsaturated aldehydes and ketones with alcohols as
reductants. Reduced reaction times and very high selectivities for the unsaturated
alcohols were obtained with the heterogenous catalyst compared with the homoge-
neous catalysts. The B(OEt) -MCM-41 catalyst was found to be encouraging, as is is
3
recyclable up to six cycles without any significant loss in its catalytic activity.
Keywords Mesoporous MCM-41 ꢀ Boron tri-ethoxide ꢀ Chemoselectivity ꢀ
a,b-Unsaturated aldehydes and ketones ꢀ MPV reduction
Introduction
Allylic alcohols are important intermediates in the production of pharmaceuticals,
agrochemicals, and fragrances [1, 2]. They may be produced by oxidation of olefins
B. Uysal ꢀ B. S. Oksal (&)
Department of Chemistry, Akdeniz University, 07058 Antalya, Turkey
e-mail: bbirsen@akdeniz.edu.tr
B. Uysal
e-mail: buysal@akdeniz.edu.tr
123

B. Uysal, B. S. Oksal
[
3], or by alkylation of unsaturated aldehydes [4], but a more general route is the
chemoselective reduction of a,b-unsaturated carbonyl compounds to allylic alcohols
3], and this reaction generally proceeds with special catalysts [4]. The stoichiom-
etric NaBH /CeCl system, and most conspicuously the ternary combination of Ru
[
4
3
(
phosphine) with a chiral diamine and a base, proved to be excellent for
chemoselective reductions [5–7]. However, this catalyst requires extra base for its
activation; it is not easily recuperated from the reaction mixture and may lose its
activity in recycling. Therefore, considerable effort has been done to develop
heterogeneous catalysts for this type of reduction [8]. Particularly, the combination
of group VIII metals (Pt, Rh, Ru) and some metal additives (Sn, Fe) has been found
to be suitable for the selective reduction of unsaturated aldehydes to allylic alcohols
[
9, 10]. However, when applied to the selective reduction of a,b-unsaturated
ketones, these catalysts often show high selectivity for the saturated ketone or the
alcohol [11]. As an alternative to metal-catalyzed hydrogenations, hydrogen transfer
reactions of the Meerwein–Ponndorf–Verley (MPV) type form an attractive
approach [12–14].
The reduction of carbonyl compounds by hydrogen transfer from an alcohol is
known as the MPV reaction in organic chemistry and can be performed under mild
conditions using Lewis acids as catalyst [15, 16]. The MPV reduction of carbonyl
compounds is a highly selective reaction since only the carbonyl group coordinates
with the Lewis acid center, while the double bond remains unreacted [17].
Therefore, the MPV reduction offers a facile access to unsaturated alcohols, many
of which are important starting materials for the production of fine chemicals [18].
Traditionally, the MPV reaction can be catalyzed using homogeneous catalysts such
as metal alkoxides. Heterogeneous catalysts for the MPV reactions include zeolites
[
19–21], grafted alkoxides [22, 23], and metal oxides such as magnesium oxide,
zirconia, silica, alumina [24], etc. A major advantage of heterogeneously over
homogeneously catalyzed MPV reactions is that the catalysts can easily be
separated from the liquid reaction mixture [19, 25].
The heterogenation of the MPV catalysts also offers the advantage of reusability
26]. In order to heterogenize the metal complexes used in MPV reductions, grafting
[
onto inorganic supports has been carried out. Recently, the mesoporous silicate
MCM-41 was explored as a versatile support material for metalorganic moieties [27,
2
8]. Since its discovery by Mobile in 1992 [29, 30], MCM-41 and related
mesoporous molecular sieves have attracted great attention. As is known, MCM-41
possesses unidirectional channel-like pores of rather uniform size which are
arranged in a regular hexagonal pattern. The pores diameters are adjustable from 15
˚
to 100 A depending on the synthesis conditions, such as temperature, amd the type
and size of the templating detergent cations [31].
Modification of the MCM-41 may provide an alternative strategy to obtain
suitable supports to maintain high activity in the heteregeneous catalyst system for
the reduction of carbonyl compounds. Several MPV reduction catalysts, such as
Al-alkoxides, Zr-alkoxides as well as Hf and Zr alkyl complexes, have been
anchored on MCM-41 or on amorphous SiO in order to generate potential catalytic
2
activity [32–35]. Many of these catalysts have been applied in reactions of saturated
ketones (e.g., substituted cyclohexanones), which are much easier to reduce than
1
23

Catalyst for preparation of a,b-unsaturated alcohols
unsaturated ketones. Only a few heterogeneous catalysts have been applied in the
MPV reduction of a,b-unsaturated ketones [26, 33, 36].
In the present study, we reported a new heterogeneous catalyst B(OEt) -MCM-41,
3
characterized its structure, and showed the selective reduction of a variety of a,b-
unsaturated aldehydes and ketones using heterogenised boron tri-ethoxide catalyst.
Heterogeneous MPV catalyst of similar type has just previously applied to the reduction
i
of unsaturated aldehydes and ketones using heterogenized B(O Pr) -MCM-41, and the
3
results have been very encouraging [37, 38]. Up to the present time, there has not
been any literature reporting on the boron tri-ethoxide grafted MCM-41 heteroge-
neous catalyst for the reduction of a,b-unsaturated ketones and aldehydes to a,b-
unsaturated alcohols.
Experimental
Preparation of mesoporous support
Pure siliceous MCM-41 was synthesized following a previously reported procedure
[37]. In a typical synthesis, MCM-41 was prepared by dissolving 19.43 g
(26.4 mmol) of tetraethylammonium hydroxide (TEAOH, 20 %) and 16.16 g
(12.6 mmol) cetyl trimethylammonium chloride (CTMACl, 25 %) in 20 mL of
deionized water with stirring (1,000 rpm) until a clear solution was obtained. Then,
9.27 g (128.3 mmol) of LUDOX AS-40 (Dupont) was added to the solution with
stirring. After 15 min, an additional amount of 32.33 g (25.3 mmol) CTMACl and
0 mL H O were added. The resulting mixture was vigorously stirred (1,000 rpm)
1
2
2
for another 1 h. The molar composition of the final gel mixture was
SiO :0.3CTMACl:0.2TEAOH:46.3H O.
2
2
The gel mixture was placed in a Teflon-lined stainless-steel autoclave (BERGHOF
BR-200 pressure reactor). The rotating autoclave was heated at 110 °C for 48 h. The
product was washed and calcined as in Refs. [37, 38].
Grafting of boron tri-ethoxide, B(OEt)3
Boron alkoxide-containing ordered mesoporous silica material was prepared by the
grafting method [38, 39]. Prior to grafting, the support (MCM-41) (2.0 g) was dried
at 250 °C for 2 h. It was then added to a solution of (1.5 mL; 5.8 mmol) B(OEt) (as
3
a 70 wt% solution in ethanol) in dry hexane (25 mL). The suspension was stirred
(
750 rpm) for 4 h at room temperature. The product was filtered under N2
atmosphere, washed three times with 10 mL of hexane, and finally dried under the
same inert gas flow. The material containing the grafted alkoxide contains 2.6 mmol
B per g final material and is denoted as B(OEt) -MCM-41.
3
Characterization
The surface area, average pore diameter, and pore size distribution of the MCM-41
support and the prepared heterogeneous catalyst were determined by nitrogen
123

B. Uysal, B. S. Oksal
adsorption using a Micromeritics Gemini III 2375 Surface Area Analyzer. Prior to
measurements, the samples were degassed at 300 °C and 0.15 mbar for at least for
6
h. The surface areas were calculated by the method of Brunauer, Emmett, and
Teller (BET). The pore size distribution curves were obtained from the analysis of
desorption branch of the nitrogen adsorption–desorption isotherm by the Barrett–
Joyner–Halenda (BJH) method. The bulk crystalline phases of the samples was
determined by powder X-ray diffraction (XRD). This was conducted using a X’Pert
Pro MPD Diffractometer from PANalytical, with a CuKa1 radiation wavelength of
0
.154 nm.
Transmission electron microscopy (TEM) images were recorded on a tecnai
2
G 20 S-TWIN with an energy dispersive X-ray spectrometer (EDAX, r-TEM
SUTW) located at the ZELMI, TU Berlin.
The boron content of the grafted catalyst was determined by inductively coupled
plasma-optical emission spectroscopy (ICP-OES). The mesoporous material,
B(OEt) -MCM-41, was dissolved in a mixture solution of HCl and HF. An
3
aqueous HCl and HF solution containing boron was used as standard reference.
ICP-OES was performed using a Perkin Elmer Optima 4300DV.
The anchoring of boron tri-ethoxide species on MCM-41 was followed by
2
9
11
Si MAS NMR and B MAS NMR spectroscopies.
1
3
29
11
Solid state C MAS NMR, Si MAS NMR, and B MAS NMR spectra were
recorded on a Bruker Advance 400 NMR spectrometer with a resonance frequency
1
3
29
11
of 100.6, 79.46, and 128.3 MHz for C, Si, and B using a Bruker 4 mm double-
1
resonance probe head operating at a spinning rate of 10 kHz for C and 12 kHz for
3
2
9
11
13
Si and B, respectively. C spectra were collected with 70° rf pulses, 5 s delay,
2
9
while Si spectra were collected with 70° rf pulses, 30 s delay, and in both cases
with *6,000 scans. B MAS NMR spectra were accumulated for 1,024–4,096
scans with p/4 pulse width of 1.1 ls and 2 s recycle delay.
1
1
Catalytic activity of B(OEt) -MCM-41
3
The B(OEt) -MCM-41 catalyst was tested on the MPV reduction of a series of a,b-
3
unsaturated aldehydes and ketones.
The reaction mixture, containing 30 mmol of the aldehyde or ketone, 200 mmol
(
9.214 g; 11.7 mL) ethanol, and 500 mg of dried catalyst ,were placed in a 50-mL
two-mouthed flask with a side stopcock equipped with a 100-cm condenser. The
flask was immersed in an oil bath. The rigorously stirred reaction mixture was
gently heated to reflux at 80 °C. During the reduction reaction time, a slow stream
of dry nitrogen was passed just over the surface of reaction mixture. In this way, the
formed acetaldehyde was removed by nitrogen flow. Thus, the equilibrium reaction
was slipped to the right hand side. Aliquots were removed at different reaction times
and analyzed by GC/MS.
The reduction products were identified on the basis of their retention times by
comparing with authentic samples, and their mass spectral fragmentation patterns
with those stored in the data bank (Wiley/NIST library). Analyses were performed
on a Varian CP 3800 gas-chromatograph equipped with a Varian Saturn 2200 MS
1
23

Catalyst for preparation of a,b-unsaturated alcohols
detector (Walnut Creek, CA, USA) and a VF-5 ms capillary column (30 m length
and 0.25 mm I.D., 0.25 lm film thickness) (Palo Alto, CA, USA).
The same procedure was used to reduce other a,b-unsaturated aldehydes and
ketones in the presence of B(OEt) -MCM-41. The selectivity towards the a,b-
3
unsaturated alcohol (SUOL) at variable conversion levels was calculated from the
following expression [16]:
SUOL ¼ ðmmol unsaturated alcohol=mmol a;b-unsaturated
carbonyl compound convertedÞ ꢁ 100:
To test for any leaching, about 500 mg of the grafted catalyst was refluxed in
2
00 mmol of ethanol at 80 °C for 6 h. The solution was filtered and the filtrate was
tested for activity in the MPV reduction of cinnamaldehyde. The reuse of catalyst
was also tested for the MPV reduction of cinnamaldehyde. The yield was practically
unaffected up to five cycles. The filtrate and quenched solutions of the catalyst did
not show any catalytic activity. This confirms that the activity observed is due only
to the solid catalyst and not partly due to the leached active species. Character-
ization of reused catalyst was determined by XRD.
Results and discussion
Characterization of materials
The as-obtained heterogenous catalyst systems were investigated with various
1
analytical methods including N adsorption/desorption measurements BET, C MAS
3
2
2
NMR and Si MAS NMR-spectroscopies, and XRD and TEM techniques.
9
The N adsorption–desorption isotherms and the corresponding BJH pore size
2
distributions based on the desorption branch for the MCM-41 and B(OEt) -MCM-41
3
samples are shown in Fig. 1a, b, respectively. The N isotherm for the MCM-41
2
sample is shown in Fig. 1a. It is a typical type IV adsorption isotherm as defined by
IUPAC nomenclature without any hysteresis, and shows a sharp capillary
condensation step at a relative pressure between 0.2 and 0.4 [40–42]. A sharp
inflection between relative pressure P/P = 0.2 and 0.4 corresponds to capillary
0
condensation within uniform mesopores. The sharpness of this step reflects the
uniform pore size [43]. Compared to the untreated MCM-41 sample, the sharp
capillary condensation step in the case of incorporated MCM-41 samples shifts
toward lower P/P region. This suggests that the introduction of B(OEt) into the
0
3
channel causes changes in the pore structure of the support during the incorporation
process. Also, B(OEt) -MCM-41 still displays a type IV adsorption isotherm
3
according to the IUPAC without any hysteresis between the adsorption and the
desorption curves (Fig. 1a). This is an indication for uniform pore sizes arranged in
networks.
As we can see from Table 1, before functionalization with B(OEt) , the calcined
3
2
MCM-41 has a surface area of 1,100 m /g, with an average pore diameter of 25.1 A
˚
(
Fig. 1b) as deduced using the BJH method. The specific surface decreases to
123

B. Uysal, B. S. Oksal
2
˚
62 m /g, with an average pore diameter of 20.8 A. This implies that the surface
7
functionalization to some extent decreases the available pore volume and diameter,
but the material still has all the characteristics of an ordered, mesoporous support.
The low-angle XRD patterns of MCM-41 and B(OEt) -MCM-41 are shown in
3
Fig. 2a, b, respectively. X-ray diffractograms of the MCM-41 and B(OEt) -MCM-41
3
materials were characterized by three distinct Bragg reflexes at low angles indexed
to (1 0 0), (1 1 0), and (2 0 0), representative of ordered materials with hexagonal
arrangement of mesopores (Fig. 2a, b). The XRD pattern of the untreated MCM-41
exhibits a high intensity (100) and two low intensity reflections (110) and (200)
which are characteristic of the hexagonal mesoporous MCM-41 [44]. After the
incorporation of the boron alkoxide complex into the framework, the main peak
intensity decreases. This result indicates that the incorporation of boron alkoxide
complexes in the channel of MCM-41 leads to a substantial loss in the scattering
contrast between the channel and the wall, and reduces the intensity of the scattered
X-ray in the powder diffraction experiment [45]. Furthermore, the intensity of the
two less intense reflections, (110) and (200), decreases much more. This observation
is supported by a drop in the pore volume and the surface area (Table 1).
The reused catalyst was also characterized by XRD measurement. The low-angle
XRD pattern of reused B(OEt) -MCM-41 catalyst is shown in Fig. 2c. After using
3
the catalyst for five cycles, the XRD pattern of the reused B(OEt) -MCM-41
3
catalyst still exhibits a high intensity, (100), and two low intensity reflections, (110)
and (200), which are characteristic of the hexagonal mesoporous MCM-41 material
[
46]. In comparision with the XRD pattern of the unused B(OEt) -MCM-41
3
catalyst, the intensity of the reflections of the catalyst reused for five cycles was
almost the same.
2
Fig. 1 a N adsorption/desorption isotherms and b BJH pore size distribution curves for MCM-41 and
B(OEt) -MCM-41
3
1
23

Catalyst for preparation of a,b-unsaturated alcohols
3
Table 1 Surface properties of MCM-41 and B(OEt) -MCM-41
2
BET surface area (m /g)
3
Pore volume (cm /g)
˚
Pore diameter (A)
Sample
MCM-41
1,100
762
0.38
0.12
25.1
20.8
3
B(OEt) -MCM-41
ICP-OES analysis showed 2.81 wt% of boron content in the catalyst. To
determine boron content, B(OEt) -MCM-41 was dissolved in a mixture solution of
3
HCl and HF. An aqueous HCl and HF solution containing boron was used as
standard reference, and 2.6 mmol B was found per g of B(OEt) -MCM-41.
3
Figure 3 shows the representative TEM images of MCM-41 and B(OEt) -MCM-41.
3
The highly ordered hexagonal arrays of mesoporous channels confirm the existence
of a hexagonal mesostructure for B(OEt) -MCM-41. These images reveal that the
3
hexagonally ordered mesostructure of the MCM-41 host material was unaffected by
grafting of boron tri-ethoxide.
The grafting of the B(OEt) on the calcined MCM-41 material was followed with
3
2
9
29
Si solid state NMR. Si MAS NMR spectra of calcined mesoporous MCM-41 and
B(OEt) -MCM-41 samples are shown in Fig. 4. The presence of broad resonance
peaks from -90 to -110 ppm is indicative for a range of Si–O–Si bond angles and
3
2
the formation of more tetrahedral silicon environments. The Si MAS NMR
9
4
spectrum of the calcined MCM-41 shows signals of Q species (Si(4OSi)) at
3
2
-
110 ppm, Q species (Si(3OSi,1OH)) at -100 ppm, and Q species (Si(2O-
Si,2OH)) at -92 ppm. Compared to MCM-41, the spectrum of B(OEt) -MCM-41
3
3
shows a marked decrease in the intensity of the Q and Q signals, hence due to the
2
2
substitution of the hydroxyl groups belonging to Q or Q with the boron–alkoxide
3
1
1
complex during the incorporation process. In addition, the B NMR of the
B(OEt) -MCM-41 reveals the presence of threefold coordinated boron with a very
3
characteristic peak around 21 ppm as shown in Fig. 5, indicating the successful
modification of MCM-41 with the boron ethoxide.
1
Solid state C NMR spectra of MCM-41 and B(OEt) -MCM-41 are shown in
3
3
Fig. 6. The spectrum of unmodified MCM-41 exhibits no carbonaceous compound
as impurities on the unmodified MCM-41. The signals at 18.8 and 60.2 ppm in the
1
solid state C NMR spectrum of B(OEt) -MCM-41 are characteristic for the ethoxy
3
3
1
3
group: C NMR (300 MHz, CDCl , 25 °C): d = 18.8 (CH3), 60.2 (–CH CH ).
3
2
3
1
3
C NMR of B(OEt) -MCM-41 shows the successful modification of MCM-41
material with the desired molecule.
3
Activity of heterogenized B(OEt) -MCM-41 catalyst
3
The MPV reaction have been catalyzed by various metal alkoxides, such as
aluminum 2-propoxide, zirconium 1-propoxide, etc. [47]. Also, different boron
alkoxides, such as boron 2-propoxide, boron tri-ethoxide, etc., have been used in the
MPV reductions [48, 49]. In this study, we aimed to show the applicability of the
new material as a heterogenous catalyst in the MPV reduction and to study the
possible advantages with respect to the unsupported analogues. For this purpose,
123

B. Uysal, B. S. Oksal
3 3
Fig. 2 Low-angle XRD patterns of a unmodified MCM-41, b B(OEt) -MCM-41, and c reused B(OEt) -
MCM-41 catalyst
B(OEt) -MCM-41 was tested for the reduction of a,b-unsaturated aldehydes and
3
i
ketones. Also, this new catalyst was compared with homogeneous B(O Pr) and
3
B(OEt) analogues.
3
It was found that, for the heterogeneously catalyzed MPV reduction of
unsaturated aldehydes and ketones, B(OEt) -MCM-41 has a similiar activity with
3
homogeneous boron alkoxide catalysts. The studied carbonyl compounds, the yields
of the reduction product, reaction times, and corresponding yields in the presence of
heterogeneous catalyst are summarized in Table 2, where it can be seen that
heterogenization of the boron tri-ethoxide leads to a decrease in the reaction time
and an increase in the reaction yield when compared with the homogeneous
catalysts. Also, in the presence of B(OEt) -MCM-41, the desired allylic alcohols
3
were obtained with high selectivity at high aldehyde and ketone conversion
(
Table 2).
High surface area mesoporous materials like MCM-41 provide a useful support
for the heterogenisation of boron alkoxides [37, 38]. Surface hydroxyl groups are
involved in the grafting reaction, probably allowing the boron tri-ethoxide to build
up to a monolayer. Therefore, MCM-41 with the hexagonal pore structure favors
easy access of the reactants to the B centers, and thus increases the reaction rate
1
23

Catalyst for preparation of a,b-unsaturated alcohols
3
Fig. 3 TEM of a MCM-41 and b B(OEt) -MCM-41
2
9
3
Fig. 4 Si solid-state MAS NMR spectra of a pure MCM-41 and b B(OEt) -MCM-41
1
1
3
Fig. 5 B NMR spectra of B(OEt) -MCM-41
123

B. Uysal, B. S. Oksal
1
3
3
Fig. 6 C MAS NMR spectra of a MCM-41 and b B(OEt) -MCM-41
[
38]. This is consistent with the tendency of B-alkoxides to form aggregates in a
homogeneous reaction medium. So, compared with homogeneous catalysts,
heterogeneous boron alkoxide catalysts, which are dispersed over the siliceous
surface by reaction with the surface silanol groups, have more available B centers
for MPV reaction [50].
As can be seen in Table 2, some differences were observed in reduction yields
and reduction times for the selected unsaturated aldehydes and ketones. These
differences were similar for heterogeneous and homogeneous catalysts. Within the
group of cyclohexenones, it is clear that an increasing number of alkyl substituents
leads to a decrease in reactivity, partially due to electronic effects. In comparision
with 2-cyclohexen-1-one and 3-methly-2-cyclohexen-1-one, the smallest yield for
3,5-dimethyl-2-cylohexene-1-one can be attributed to its steric hindrance due to the
presence of two methyl groups.
Reuse of grafted catalysts
Unmodified MCM-41 was not active in the MPV reduction of a,b-unsaturated
aldehydes and ketones. However, once boron tri-ethoxide was grafted onto MCM-41,
the heterogenized material showed good activity and selectivity to the correspond-
ing alcohols. To test the reused catalyst, after each round of MPV reaction, it was
washed with ethanol and tested in another MPV reaction of cinnamaldehyde.
Table 3 shows that, in the MPV reduction of cinnamaldehyde, the activity of the
catalyst, B(OEt) -MCM-41, remained high even after six rounds of reaction. The
3
selectivity to the corresponding alcohol was 100 % in each round. The catalyst
recuperating from a first run considerably conserves its activity until the sixth cycle
(
Table 3). The filtrate obtained after refluxing the grafted catalyst in ethanol was
1
23

Catalyst for preparation of a,b-unsaturated alcohols
123

B. Uysal, B. S. Oksal
1
23

Catalyst for preparation of a,b-unsaturated alcohols
123

B. Uysal, B. S. Oksal
inactive for the MPV reduction of unsaturated aldehydes and ketones, showing the
absence of leaching into the reaction medium.
In comparison with the CoHMA catalyst, the B(OEt) -MCM-41 heterogeneous
3
catalyst shows higher cinnamyl alcohol yield after the sixth run. As shown in
Table 3, cinnamyl alcohol yield was 73 % for CoHMA and 80.2 % for the
B(OEt) -MCM-41 catalyst after the sixth run. Therefore, B(OEt) -MCM-41
3
3
catalyst is as good as the CoHMA catalyst with respect to reusability for the
reduction of cinnamaldehyde [51].
Chemoselectivity of heterogenized B(OEt) -MCM-41 catalyst
3
In Table 2 are summarized the results of the catalytic MPV reduction of several
i
unsaturated carbonyls in the presence of homogeneous B(O Pr) , B(OEt) and
3
3
heterogeneous B(OEt) -MCM-41 catalysts. As can be seen, the catalysts exhibited
3
excellent chemoselectivity towards a,b-unsaturated alcohols.
Catalysis in the perfumery industry is one of the new research fields using
hydrogenation catalysts. There are numerous hydrogenation processes both in gas
and liquid phases, but most of them are not selective to target unsaturated alcohols.
Selective hydrogenation of unsaturated aldehydes and ketones such as cinnamal-
dehyde, 2-cyclohexen-1-one, b-ionone, a-ionone, and benzalacetone to their
corresponding unsaturated alcohols is highly desirable due to the corresponding
unsaturated alcohols being useful as intermediates, pharmaceuticals, and as flavor
chemicals [52]. The bond energy of C=C bond is smaller (615 kcal/mol) than that of
the C=O bond (715 kcal/mol), which makes the hydrogenation of C=O bond
difficult [52]. In this instance, the selective MPV reduction of the C=O bond using
MPV catalysts will be a promising process for perfumery industry.
3-Methyl-2-butenal, citral, 2-cyclohexene-1-one, 3-methyl-2-cyclohexen-1-one,
3
,5-dimethyl-2-cyclohexen-1-one, a-ionone, b-ionone, and cinnamaldehyde are
important a,b-unsaturated aldehydes and ketones. These important carbonyl
compounds possess conjugated C=C and C=O bonds. The C=C bonds are more
readily hydrogenated than the C=O bonds in most usual techniques [53]. Although
reduction of the C=O bonds without affecting the C=C bonds is very difficult, the
i
C=O bonds of these compounds were reduced using homogeneous B(O Pr) ,
3
B(OEt) catalysts. Notably, not only the C=C double bond in the enone group of a-
3
ionone and b-ionone but also the endocyclic C=C bond are left totally intact. Same
catalytic activity was observed with the B(OEt) -MCM-41 catalyst. The catalyst
3
selectively reduces the carbonyl functions without affecting the olefinic bonds.
Conversion yield of unsaturated aldehydes and ketones to corresponding unsatu-
rated alcohols range from 59.2 to 87.03 % within 5–10 h of reaction time with
B(OEt) -MCM-41.
3
Citral occurs as cis- and trans-isomers, known as neral and geranial, respectively.
Scheme 1 outlines the reduction of citral to the corresponding alcohols using
ethanol as reductant. In the conversions of citral, the isolated double bond was left
intact, and a mixture of the allylic alcohols geraniol and nerol was obtained. The
preferential formation of geraniol as compared to nerol can be explained by steric
1
23

Catalyst for preparation of a,b-unsaturated alcohols
Table 3 Reduction of cinnamaldehyde to cinnamyl alcohol under different reaction conditions: com-
parison with CoHMA catalyst
Entry
System
Yield (%)
System [51]
CoHMA
Yield (%)
a
a
81
1
2
3
4
5
6
B(OEt)
3
3
-MCM-41
-MCM-41
86.8
b
b
73
B(OEt)
80.2
c
c
c
c
c
c
c
No catalyst
Filtrate
Quenched solution
3
B(OEt) free MCM-41
Reaction condition: Cinnamaldehyde (30 mmol), catalyst (500 mg) in ethanol (200 mmol, 15, 4 mL)
refluxed at 353 K for 6 h
a
Yield on single experiment
b
Yield after sixth run
c
No reaction
hindrance of the bulky chain [54]. The carbonyl group of the geranial is trans to the
bulky chain of the molecule, minimizing any steric hindrance for the binding of
C=O to the boron center. Hence, the rate of reaction for geranial is faster than for
neral.
The reduction product, geraniol, is a monoterpenoid and an alcohol. It is the
primary part of rose oil, palmarosa oil, and citronella oil. It appears as a clear to
pale-yellow oil that is insoluble in water, but soluble in most common organic
solvents. It has a rose-like odor and is commonly used in perfumes. Geraniol has
also been suggested to help prevent cancer. Carnesecchi et al. [55] demonstrated in
his study ‘‘Geraniol inhibits growth and polyamine biosyntehsis in human colon
cancer cells’’, that geraniol causes a 50 % decrease of ornithine decarboxylase
activity, a key enzyme of polyamine biosynthesis, which is enhanced in cancer
growth.
Scheme 1 MPV reduction of citral to geraniol and nerol
123

B. Uysal, B. S. Oksal
MPV reaction mechanism
The reaction mechanism for the homogeneous MPV reaction involves a cyclic six-
membered transition state [56, 57]. Recent studies by Creyghton et al. [19] proposed
that, over heterogeneous catalysts like zeolite beta, the MPV mechanism also
involves a six-membered transition state where both the alcohol and the ketone
coordinate to the same Lewis acid site. In this regard, the reaction mechanism for
the reactions under study can be presumed to be similar to the classical MPV
reduction of carbonyls reported for aluminium isopropoxide [56]. Scheme 2 shows
the proposed mechanism for the MPV reduction of carbonyl compounds in the
presence of B(OEt) -MCM-41. First, the carbonyl compound is coordinated to the
3
boron of the boron alkoxide. The reaction proceeds by hydride-transfer to the
carbonyl compound from the alcohol, which is bound to the boron center as an
alkoxide. Since the reduction is reversible, the acetaldehyde was removed from the
medium by a slow stream of nitrogen. The removal of the acetaldehyde from the
reaction solution leads to the progress of the reaction to the right hand side (see
Scheme 2).
Catalytic MPV reduction reactions: comparison of homogeneous
and heterogeneous catalysts
The heterogeneous solid supported catalyst offers further advantages like the easy
catalysis and product recovery as well as a possible reuse of the catalyst systems.
Hence, a mesoporous material, MCM-41, was selected for the grafting of boron
alkoxides due to its potential for incorporating higher amounts of heteroatoms in the
frame wall positions, as well as due to the presence of higher amounts of silanol
groups, for the stabilization of boron tri-ethoxide.
The synthesized boron tri-ethoxide-containing B(OEt) -MCM-41 was first
3
applied as catalyst for the MPV reduction reaction of unsaturated aldehydes and
3
Scheme 2 Proposed mechanism of MPV reduction over B(OEt) -MCM-41
1
23

Catalyst for preparation of a,b-unsaturated alcohols
ketones. The results showed that the catalyst showed good conversion and indicated
that the Lewis acidity is strong enough, and the amount of framework boron is big
enough, to proceed the reaction effectively. Hence, B(OEt) -grafted mesoporous
3
MCM-41 have potantial for application in the MPV reduction of different a,b-
unsaturated aldehydes and ketones.
The MPV reduction times for a,b-unsaturated aldehydes and ketones were found
to be higher with homogeneous catalysts than heterogeneous. An interesting result
was that the selectivity towards the reduced product was C98 % in the presence of
the heterogeneous catalyst, showing that the supported boron alkoxide catalyst is
highly selective. The resulting improved conversion for the B(OEt) -MCM-41
3
catalyst shows that B(OEt) is grafted as a monolayer over the support surface and
3
the higher concentration of boron helps to increase the number of Lewis acid sites
for the conversion of unsaturated aldehydes and ketones [52].
Comparing these results with the Al-beta, Zr-beta, and Sn-beta zeolites, it was
inferred that the zeolite catalysts enhance undesired side reactions due to the
presence of Lewis and Br o¨ nsted acidity, generated after the calcination process [54,
5
8]. However, the absence of undesired side products, even after 10 h, for the boron
alkoxide-grafted mesoporous materials suggests that polarization of carbonyl groups
gets enhanced over the Lewis acid sites or Lewis acid sites be the active catalytic
sites. Moreover, as pointed out by Anwander et al. [23] the surface confinement
effects in mesoporous hosts may prevent the boron alkoxide groups from self-
association.
In comparison with homogeneous boron alkoxide catalysts, the structure as well
as the pore dimensions added to its particular Lewis acid properties: B(OEt) -MCM-41
3
shows a higher activity and better selectivity. Also, the catalyst can be recycled
several times and can be used without important loss in its activities, this being of
clear interest for a potential industrial application of the catalyst.
To show catalytic efficiency of boron tri-ethoxide catalysts according to different
MPV catalysts, boron tri-ethoxide catalysts was also compared with various MPV
catalysts (Table 2). In comparison with the heterogeneous 11Zr-SBA-15 catalyst,
for cinnamaldehyde and citral (geranial and neral), heterogeneous and homogeneous
boron tri-ethoxide catalysts show higher activity (yield) and similiar selectivity [59].
Also, comparing boron tri-ethoxide catalysts with the Zr(O-n-Pr)n-MCM-41
catalyst, it was found that, in the presence of B(OEt) -MCM-41 and homogeneous
3
boron tri-ethoxide catalysts, 2-cyclohexen-1-one, 3-methyl-2-cyclohexen-1-one,
3
,5-dimethyl-2-cyclohexen-1-one, a-ionone, and b-ionone were reduced with better
chemoselectivity and similiar catalytic activity [39]. Furthermore, in the presence of
boron tri-ethoxide catalysts, 3-methyl-2-butenal was reduced with similar selectivity
and catalytic activity compared with Zr-zeolite beta (Table 2) [36].
Conclusion
In conclusion, we have devised an essentially new boron alkoxide-based hetero-
geneous B(OEt) -MCM-41 catalyst that exerts high catalytic activity with EtOH as
3
a hydride donor in the MPV reduction of various a,b-unsaturated carbonyls, having
123

B. Uysal, B. S. Oksal
importance for fine chemicals and drugs. This is the most reactive boron-based
catalyst reported so far, and it has remarkable potential for practical use. Also, the
yield and reaction time are highly encouraging. The catalyst is recyclable up to six
cycles without any significant loss in catalytic activity.
In comparison with different heterogeneous catalysts, the advantages of ease of
separation and reusability added to its particular catalytic activity and selectivity
properties, we can say that B(OEt) -MCM-41 will be of interest in potential
3
industrial applications using similar types of catalysts.
Acknowledgment Scientific ResearchProjects Unit ofAkdenizUniversity(GrantNo. 2009.03.0121.005)
is gratefully acknowledged.
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