.
Angewandte
Communications
Table 1: Elemental analysis of silica-supported Pd catalysts.
Catalyst
C
N
Cl
Pd
[mmolgÀ1
]
[mmolgÀ1
]
[mmolgÀ1
]
[mmolgÀ1
]
SiO2/diamine/Pd
SiO2/diamine/Pd/
NEt2 (4)
3.1
7.2
1.1
1.7
1.3
0.8
0.60
0.47
tion of organic amines. To confirm the formation of covalent
Si O Si C bonds on the silica surface, solid-state 29Si magic-
À À À
angle spinning (MAS) NMR spectroscopy was conducted.
The 29Si MAS NMR spectrum of the parent SiO2 showed
three signals at À113, À104, and À94 ppm corresponding to
Q4, Q3, and Q2 species of the silica framework, respectively
(Qn = Si(OSi)n(OH)4Àn). The Q3/(Q2 + Q3 + Q4) ratio
decreased from 0.18 to 0.13 and the Q2/(Q2 + Q3 + Q4)
ratio decreased from 0.07 to < 0.01 after immobilization of
diaminopalladium and the tertiary amine (see the Supporting
Information, Figure S1). The signals corresponding to the T3
and T2 sites appeared at À67 and À59 ppm, respectively [Tm =
RSi(OH)3Àm(OSi)m], in the 29Si MAS NMR after immobiliza-
tion of diaminopalladium and the amines (see the Supporting
Information, Figure S1 (B)). These 29Si NMR spectra indicate
that organic reagents were immobilized on the SiO2 surface
Figure 2. Liquid-state 13C NMR spectra of A) 3-diethylaminopropyl-
trimethoxysilane (2), and B) 3-(2-aminoethylamino)propyl-
trimethoxysilane (1), and solid-state 13C CP/MAS NMR spectra of
C) SiO2/diamine/NEt2 (3), D) SiO2/diamine/Pd, and E) SiO2/diamine/
À
after the silane coupling reaction between the surface Si OH
and Si(OMe)3 groups.[8] To determine the structure of the
immobilized diaminopalladium and amine systems 3 and 4,
solid-state 13C cross-polarization magic-angle spinning (CP/
MAS) NMR spectroscopy was conducted. Figure 2A–C
shows the 13C NMR spectra of the amines before and after
immobilization on SiO2. After immobilization of the diamine
(1) and tertiary amine (2), no significant change in the
13C NMR chemical shift was observed except for the chemical
shift corresponding to the carbon atom connected to the Si
atom (Figure 2A–C). This result indicates the diamine and
tertiary amine were immobilized on the SiO2 surface while
maintaining their carbon skeletons. Figure 2D shows the
13C NMR spectrum of SiO2/diamine/Pd. The signals assigned
to the carbon atoms that are connected to nitrogen atoms (42
~
&
Pd/NEt2 (4). Definitions of symbols; and : carbon atoms con-
*
nected to nitrogen atoms of the diamine functional group, : central
~
carbon atom in the propyl chain of the diamine functional group,
^
and : carbon atoms of the tertiary amine functional group, &:
methoxy groups.
Pd/Cl ratio (Table 1). Overall, the proposed surface structure
of 4 is shown in Figure 2E. The average distance between the
diaminopalladium and tertiary amine anchoring site was
calculated to be approximately 6 ꢀ. This value was calculated
using the surface area of silica (300 m2 gÀ1), amount of the Pd
complex (0.47 mmolgÀ1), and amount of the tertiary amine
(0.76 mmolgÀ1). This value is reasonable for cooperative
catalysis because of the flexibility of the propyl chain
(approximately 2.5 ꢀ) of the anchored Pd complex and
tertiary amine.
The Tsuji–Trost reaction between ethyl 3-oxobutanate (5)
and allylmethylcarbonate (6) in the presence of the SiO2-
supported Pd catalysts was examined (Table 2). Mono-
allylated product 7 and diallylated product 8 were obtained.
The use of 4 resulted in > 99% yield of the total allylated
products (Table 2; entry 1). The yield of allylated products
was 26% for SiO2-supported diaminopalladium without the
tertiary amine (SiO2/diamine/Pd; Table 2; entry 2). The
Tsuji–Trost reaction using 3 did not proceed (Table 2;
entry 3). These results indicate that the tertiary amine on 4
accelerates the Pd-catalyzed Tsuji–Trost reaction. After hot
filtration of the solid catalyst at approximately 75% con-
version, no further reaction proceeded in the filtrate (see the
Supporting Information, Figure S3), thus indicating that the
catalytic reaction occurred at the solid surface. To examine
the effect of immobilization of a tertiary amine on catalytic
activity, the reaction using SiO2/diamine/Pd with a free
~
&
( ) and 51 ppm ( )) were shifted to lower field (46 and
55 ppm) after treatment with PdCl2(PhCN)2. The 13C NMR
signal corresponding to ethylenediamine (44 ppm) showed
a similar downfield shift upon the complexation with PdCl2
(48 ppm). The signal for the central carbon atom in the propyl
*
chain of 1 (23 ppm ( )) was shifted to 21 ppm. These changes
in 13C NMR chemical shifts indicate complexation of the Pd
species with the ethylenediamine function on the SiO2
surface. Figure 2E represents the 13C NMR spectrum of
SiO2/diamine/Pd/NEt2. Signals corresponding to the diami-
nopalladium species, observed in SiO2/diamine/Pd (Fig-
ure 2D), were detected and are shown in Figure 2E. In
addition, signals assigned to a tertiary amine were observed,
~
^
for example at 21 ( ) and 56 ( ) ppm. These NMR results
indicate that 4 possesses both diaminopalladium and tertiary
amine functions.
The XPS analysis of Pd3d5/2 showed a peak at 337.5 eV,
which was assigned to the PdII species (see the Supporting
Information, Figure S2). Elemental analysis of 4 showed a 1:2
2
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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