Microwave-assisted catalytic allylation of aldehydes promoted by a mesoporous silica-supported BINOL ligand in solid media

Article abstract of DOI:10.1039/b803111h

An efficient and operationally simple method for catalytic allylation has been developed and its application in the microwave-assisted catalytic allylation of aldehydes in solid media was investigated. The Royal Society of Chemistry.

Full text of DOI:10.1039/b803111h

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Microwave-assisted catalytic allylation of aldehydes promoted by a
mesoporous silica-supported BINOL ligand in solid mediaw
Guohua Liu,* Yan Gao, Xiaoquan Lu, Mouming Liu, Fang Zhang and Hexing Li*
Received (in Cambridge, UK) 22nd February 2008, Accepted 27th March 2008
First published as an Advance Article on the web 29th April 2008
DOI: 10.1039/b803111h
An efficient and operationally simple method for catalytic
allylation has been developed and its application in the micro-
wave-assisted catalytic allylation of aldehydes in solid media was
investigated.
of microwave-assisted catalysis employing mesoporous silica-
supported BINOL ligand or catalysts for catalytic allylation of
aldehydes in solid media has not yet been reported.
Recently, we reported a series of mesoporous catalysts and
9
their applications in green catalytic processes. We herein
The development of allylation under catalysis has attracted a
great deal of interest due to its extensive application in organic
develop a facile preparation of mesoporous silica-supported
ligand 6 by a post-grafting method based on SBA-15, and
apply this to microwave-assisted Ti-catalyzed allylation of
aromatic aldehydes in solid media. The research focuses on
the following: (1) construction of highly ordered mesoporous
silica-supported BINOL ligand; (2) investigation of different
catalytic performances through a comparison of ligand 6 with
the parent ligand BINOL; (3) exploration of its potential
application in industry.
1
chemistry. One of most successfully studied Lewis acid-
catalyzed allylation reactions employs titanium complexes of
1
,1-binaphthalene-2,2-diol (BINOL), with Ti(IV) Lewis acids
2
as catalysts. Although these catalysts show high catalytic
activities and selectivities for allylation of aldehydes, their
practical applications in industrial process are hindered due
to long reaction time and difficulty in recovery and reuse of
expensive titanium complexes. Microwave-assisted catalysis as
The mesoporous silica-supported BINOL ligand, abbre-
viated as SBA-15/BINOL (6), was prepared by a post-grafting
method. As shown in Scheme 1, the mesoporous material 4
3
a convenient method can accelerate transition-metal-cata-
lyzed reactions from hours or days to minutes, while immobi-
4
lization is well-known to solve the problem of transition-
was synthesized by the reaction of ClCH CH CH Si(OEt) 2
2 2 2 3
1
0
metal catalyst recycling. Furthermore, ‘‘solvent-free’’ catalytic
5
processes can be regarded as green processes. Thus, develop-
with 1 followed by anchoring onto SBA-15 (3). The silicon-
hydroxyl groups of 4 were then protected with HMDS
7a
ment of immobilization of homogeneous catalysts in micro-
wave-assisted catalysis under solvent-free condition is a
promising avenue in industry. Especially, in such a catalytic
allylation reaction with Ti(IV) Lewis acid as a catalyst, addi-
tion of molecular sieves in the BINOL/Ti(IV) system is found
to be extremely important for high reactivity and selec-
groups. Finally, 6 was obtained successfully via removal of
MOM protecting groups of hydroxyl groups at the binaphthyl
i
backbone by using HCl/ PrOH as deprotecting reagents. The
2
9
Si CP/MAS NMR measurement (Fig. 1(a)) showed clearly
4
3
that 6 presented a strong Q (À113 ppm), a medium T (À71
2
ppm) and a weak T (À63 ppm) peak in its spectrum. These
2a,b
tivity.
Thus, searching for suitable analogues or substitutes
values can be compared with typical isomer shift values of
1
2
3
3
for molecular sieves as supports is important in such catalytic
allylation reactions. Mesoporous materials possessing the
functions of molecular sieves have showed some salient fea-
À48.5, À58.5 and À67.5 ppm for T /T /T peaks (T {RSi-
2 1
(OSi)
}, T {R(HO)Si(OSi)
}, T {R(HO)
SiOSi}), and À91.5,
4
3
2
2
2
3
4
À101.5, À110 ppm for Q /Q /Q signals (Q {Si(OSi)
4
},
}). The strong Q peak
suggested that 6 possessed mainly a network structure of
6
3
2
11
4
tures, such as regular and adjustable pores, high surface area,
3 2 2
Q {(HO)Si(OSi) }, Q {(HO) Si(OSi)
remarkable thermal and mechanical stability and so on. These
features demonstrate that mesoporous materials may be po-
tentially ideal catalyst supports. Thus, a practical approach
through immobilizing BINOL ligand onto mesoporous mate-
2
3
{Si(OSi)
4
} and the relatively weak T and T peaks indicated
} and RSi(OSi) } (R =
the formation of {R(HO)Si(OSi)
2
3
organic ligand) as a part of wall in a mesoporous structure.
13
The C CP/MAS NMR spectrum displayed peaks around
7
rials for microwave-assisted catalysis in solvent-free system
may combine these advantages in this allylation reaction.
8
Although some mesoporous materials, such as MCM-41
128.7 ppm for benzene rings, and 0.1, 10.0, 26.3, 45.6 and 50.5
ppm for the other atoms, which were marked in Fig. 1(b). All
these observations confirmed the successful post-grafting of
ligands onto the mesoporous support. The amount of grafted
and SBA-15, have been successfully used as supports to
immobilize homogeneous ligands or catalysts and some of
them have exhibited high catalytic activities, the effectiveness
Department of Chemistry, College of Life and Environmental Science,
Shanghai Normal University, Shanghai, China.
E-mail: ghliu@shnu.edu.cn. E-mail: HeXing-Li@shnu.edu.cn;
Fax: + 86 21 64322511; Tel: +86 21 64321819
w Electronic supplementary information (ESI) available: Experimental
procedures for the synthesis and characterization of 4 and 6, and
analytical data for obtained homoallylic alcohols. See DOI: 10.1039/
b803111h
Scheme 1 Preparation of SBA-15/BINOL (6).
3
184 | Chem. Commun., 2008, 3184–3186
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29
13
Fig. 1 The Si CP/MAS NMR (1a) and C CP/MAS NMR (1b)
of 6.
Fig. 3 TEM images of 6 viewed along [100] and [001] directions.
BINOL was found to be 24.5 mg/100 mg of solid support
calculated from elemental analysis and thermal gravimetric
data. The pore sizes of SBA-15 (3) and SBA-15/BINOL (6)
were measured to be 7.6 and 3.5 nm.
13
Meerwein–Ponndorf–Verley–Oppenauer process. For exam-
ple, combining benzaldehyde with tetraallyltin and 10 mol%
of catalyst provided 1-phenyl-3-buten-l-ol in more than 99%
conversion and 99.4% selectivity (entry 2), which was the
nearly same as that obtained using the parent BINOL ligand
The powder XRD patterns (Fig. 2) revealed that SBA-15/
BINOL (6) showed one intense peak and two very weak peaks
indicative of (100), (110) and (200) reflections, suggesting that
the dimensional-hexagonal pore structure (p6mm) could be
(
entry 1). Additionally, both reactions could be completed in a
short reaction time (15 min), much faster than that without
titanium(IV) isopropoxide (entry 3), or even than those using
isopropanol and water as solvents in 12 h (entries 4 and 5). Of
particular note was that the reaction could be run at 5% mol
of catalysts without obviously affecting the selectivity, as
exemplified by the allylation of 7a (entry 6).
1
2
preserved after the post-grafting. The reduced peak intensity
in 6 indicated that the incorporation of the organic ligands
onto the SBA-15 might disturb the ordered dimensional-
hexagonal mesostructure by a certain degree. The TEM
morphology further confirmed that 6 had a well-ordered
mesostructure with the dimensional-hexagonal arrangement
as shown in Fig. 3. From the structural parameters listed in
Fig. 2, it was found that 6 obtained by a post-grafting method
resulted in a decrease of nanopore size, surface area, and pore
volume of 6 relative to SBA-15. This could be attributed to
occupation of the ligands in the pore channels to make the
pores narrower and coverage of the ligands on the channel
a
Table 1 Microwave-assisted catalytic allylation of aromatic aldehydes
9a–d
surface resulting in an increase of the wall thickness.
The catalytic allylations of aromatic aldehydes under micro-
wave irradiation in solid media were carried out through
employing only a small excess of tetraallyltin as nucleophilic
reagent based on consideration of green chemistry. This was
not only useful to afford almost pure allylated products, but
also prevented pollution from large amounts of hydrolyzed
inorganic tin compounds during the work-up stage. As can be
seen from Table 1, a variety of aromatic aldehydes 7a–11a
were cleanly reacted with 0.28 equiv. of tetraallyltin to give the
corresponding homoallylic alcohols 7b–11b in excellent yields,
with a small amount of by-products 7c–11c produced via the
b
Run Conv. (%) Sel. (%)
b
Entry Substrate Ligand
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
7a
7a
7a
7a
7a
7a
8a
9a
10a
11a
7a
7a
7a
7a
7a
7a
BINOL
6
6
6
6
6
6
6
6
1
1
1
1
1
1
1
1
1
1
1
98.6
499
499.9
99.4
c
41.8
67.8
44.2
78.7
96.7
85.1
85.7
499
499
499
499.9
d
99.6e
88.7
99.9
f
98.6
92.5
99.7
95.4
90.6
95.0
0
1
2
3
4
5
6
6
SBA-15
BINOL + TM-SBA-15 1
g
6
6
6
6
2
3
4
5
98.4
499.9
g
98.0
98.7
97.9
499.9
98.4
94.7
g
g
a
Reactions were carried out using a MAS-1 single mode cavity
microwave (continuous irradiation: 2.45 GHz, power: 700 W). Reac-
tion conditions: ligands (0.015 mmol), titanium(IV) isopropoxide (0.015
mmol), tetraallyltin (0.041 mmol), aldehydes (0.15 mmol), reaction
b
time (15 min). Conversion of products on the basis of starting
material (aldehydes) and selectivity determined by HPLC peak area
c
integration at 210 nm. Data were obtained without titanium(IV)
d
isopropoxide. Data were obtained using isopropanol as solvent;
e
reaction time (12 h). Data were obtained using water as solvent;
f
reaction time (12 h). Data were obtained using 5% mol of catalyst.
g
Recovered ligands were used.
Fig. 2 Powder XRD patterns of SBA-15 and 6.
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Because pure SBA-15 contains large amounts of Si–OH
groups, it is not only a simple medium but also a mild acid
catalyst or an accelerator. In order to demonstrate the effect of
Si–OH groups and to compare catalytic performance, two
control experiments were carried out using pure SBA-15 and
BINOL plus TM-SBA-15 (SBA-15 with trimethylsilylated
end-capping of Si–OH groups) as ligands under similar reac-
tion conditions. It was found that the former afforded the
corresponding alcohol in 90.6% selectivity, while the latter
gave the corresponding alcohol in 95.0% selectivity (entries 11
and 12). Apparently, the latter was a better alkylating reagent
than the former, suggesting that BINOL was more efficient in
such catalysis. In comparing both cases with the mesoporous
silica-supported BINOL ligand 6, the high chemoselectivity of
serious loss of its activity and selectivity, showing good
potential in industrial application.
We are grateful to China National Natural Science Foun-
dation (20673072), 973 Pre-program (2005CCA01100), Shang-
hai Sciences and Technologies Development Fund (071005119
and 06JC14060), and Shanghai Municipal Education
Commission (No. 08YZ71) for financial support
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indicated the advantage of regularly organizing BINOL
(
ligands on a highly ordered mesoporous material (entry 2 vs.
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dispersed arrangement of the catalytic species. This kind of
arrangement did not only offer reasonable space for recogni-
tion of the substrates, but also restricted aggregation of the
catalytic species, resulting in a high efficiency in catalytic
allylation reaction.
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An important feature of the design of ligand 6 is the easy
and reliable separation via simple filtration. For example,
upon completion of the reaction, the catalyst was quantita-
tively recovered via filtration. The recycled catalyst still af-
forded high catalytic activity and selectivity (entries 13–16).
Therefore, employing the mesoporous silica-supported
BINOL ligand 6 to promote the catalytic allylation under
microwave irradiation in solid media, the following conclu-
sions can be drawn. First, the microwave radiation can accel-
erate obviously the catalytic allylation reaction, which is not
only convenient but also environmentally friendly. Second,
the regularly immobilized ligands on the highly ordered
mesoporous materials can enhance effectively chemoselectivity
due to the regularly dispersed arrangement of the catalytic
species. Third, the highly ordered mesoporous catalyst can be
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In conclusion, we describe an efficient and operationally
simple method for the catalytic allylation of aromatic alde-
hydes under microwave irradiation in solid media. The cata-
lyst derived from a mesoporous silica-supported ligand 6
showed excellent catalytic activities and high selectivities for
the allylation of aromatic aldehydes. Furthermore, such
catalysts could be recovered and reused five times without
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186 | Chem. Commun., 2008, 3184–3186
This journal is ꢀc The Royal Society of Chemistry 2008