Recoverable platinum bis(fluoro-ponytailed) bipyridine complex as catalyst for hydrosilylation of alkynes under thermomorphic condition

Article abstract of DOI:10.1016/j.tet.2016.11.020

The fluorous PtCl₂[5,5′-bis-(n-C₁₁F₂₃CH₂OCH₂)-2,2′-bpy] complex (2) was prepared and employed as recoverable catalyst for the hydrosilylation of alkynes. Pt-catalyst 2 with F-content 0.53 was recovered under the thermomorphic method by design and the catalyst was demonstrated capable of re-usage for 8 times without loss of activity. The leaching of Pt in product mixture was monitored, with ICP-MS, to be at as low as 10^?6level per cycle in the 5-decyne reactions and at 10^?3level per cycle in the (HO)CMe₂C[tbnd]CH reaction. The reaction-catalyst fulfills the principle of green chemistry, and with its addition nature the hydrosilylation delivers 100% atom economy. The easily prepared polyfluorinated (2,2′-bpy)Pt complex 2 is the robust and thermally stable catalyst, which doesn't require specific handling for utilization and storage in laboratory.

Full text of DOI:10.1016/j.tet.2016.11.020

Accepted Manuscript
Recoverable platinum bis(fluoro-ponytailed) bipyridine complex as catalyst for
hydrosilylation of alkynes under thermomorphic condition
Norman Lu, Wei-Cheng Chung, Hsing-Fang Chiang, Yung-Cheng Fang, Ling-Kang
Liu
PII:
S0040-4020(16)31164-4
10.1016/j.tet.2016.11.020
TET 28234
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 4 October 2016
Revised Date: 4 November 2016
Accepted Date: 7 November 2016
Please cite this article as: Lu N, Chung W-C, Chiang H-F, Fang Y-C, Liu L-K, Recoverable platinum
bis(fluoro-ponytailed) bipyridine complex as catalyst for hydrosilylation of alkynes under thermomorphic
condition, Tetrahedron (2016), doi: 10.1016/j.tet.2016.11.020.
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ACCEPTED MANUSCRIPT
Graphic Abstract
Recoverable Platinum Bis(fluoro-ponytailed) Bipyridine Complex as Catalyst for
Hydrosilylation of Alkynes Under Thermomorphic Condition
Pt catalyst 2 easily recoverable at 25 °C after rxn.
(low Pt leaching level: ca 0.001%)

ACCEPTED MANUSCRIPT
Recoverable Platinum Bis(fluoro-ponytailed) Bipyridine Complex as Catalyst for
Hydrosilylation of Alkynes Under Thermomorphic Condition
Norman Lu,a,* Wei-Cheng Chung,a Hsing-Fang Chiang,a Yung-Cheng Fang,a and
Ling-Kang Liua,b
aInstitute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei
106, Taiwan
bInstitute of Chemistry, Academia Sinica, Taipei 115, Taiwan
Correspondence
Norman Lu
Institute of Organic and Polymeric Materials &
Department of Molecular Science and Engineering
National Taipei University of Technology
Taipei 106, TAIWAN
Phone: (+886)2-2771-2171 ext. 2417
Fax: (+886)2-2731-7174
E-mail: normanlu@mail.ntut.edu.tw
1

ACCEPTED MANUSCRIPT
Recoverable Platinum Bis(fluoro-ponytailed) Bipyridine Complex as Catalyst for
Hydrosilylation of Alkynes Under Thermomorphic Condition
Norman Lu,a,* Wei-Cheng Chung,a Hsing-Fang Chiang,a Yung-Cheng Fang,a and
Ling-Kang Liua,b
aInstitute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei
106, Taiwan
bInstitute of Chemistry, Academia Sinica, Taipei 115, Taiwan
Abstract
The fluorous PtCl2[5,5′-bis-(n-C11F23CH2OCH2)-2,2′-bpy] complex (2) was prepared and
employed as recoverable catalyst for the hydrosilylation of alkynes. Pt-catalyst 2 with F-content
0.53 was recovered under the thermomorphic method by design and the catalyst was
demonstrated capable of re-usage for 8 times without loss of activity. The leaching of Pt in
product mixture was monitored, with ICP-MS, to be at as low as 10-6 level per cycle in the
5-decyne reactions and at 10-3 level per cycle in the (HO)CMe2C≡CH reaction. The
reaction-catalyst fulfills the principle of green chemistry, and with its addition nature the
hydrosilylation delivers 100% atom economy. The easily prepared polyfluorinated (2,2′-bpy)Pt
complex 2 is the robust and thermally stable catalyst, which doesn’t require specific handling for
utilization and storage in laboratory.
Introduction
Although homogeneous catalysis is a powerful tool in highly active and stereoselective organic
transformations,1 the majority of catalytic processes in industry are still conducted under
heterogeneous conditions, because the separation of the catalysts from the product mixture is
easier than under homogeneous conditions. Thus besides developing homogeneous catalysts that
can be used in very low concentrations (for which the catalyst quantity is below the ppm level),
efforts have been made to provide heterogeneous reactions, which need the catalysts to be fixed
to a solid support.2,3 In other words, it is important to find a recoverable catalyst to be easily
separated from the product mixture after reaction.4
One of the new approaches to solve the problems of separation and catalytic efficiency is the
thermomorphic catalysis2,3,5,6 with or without the fluorous solvents. Bergbreiter et al. used a
polymer-based thermomorphic system whose polymeric support was not totally dissolved at high
temperature.2,3 Gladysz et al. published the results of fluorinated compounds to catalyze addition
2

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reaction on unsaturated substrates without the use of expensive fluorous solvents.5,6 Fan et al.
demonstrated the use of the thermomorphic system with a non-fluorous catalyst for asymmetric
hydrogenation.7 Yamamoto et al. prepared recyclable fluorous catalysts by using liquid/solid
phase separation without fluorous solvents.8,9 Progress in our laboratory was mainly a recyclable
polyfluorinated bipyridine metal complex system in the application on cross-coupling C-C bond
formation reactions.10
Recovery and reuse with fluorous catalysts may have its strength if compared to that with the
non-fluorous ones, because additional fluorous interactions could be utilized as controlling tool.
A unique feature in ideal thermomorphic catalysis is that the reaction mixture is homogeneous at
high temperature during reaction and the product mixture becomes phase separated when being
cooled after reaction, taking advantages of the thermomorphic property of compounds carrying
high fluorine content.11,12
The high price of platinum raw material makes recovery of platinum-catalysts more valuable, on
top of its homogeneous catalysis that offers advantages such as high efficiency, better tuning of
chemoselectivity, regioselectivity and/or stereoselectivity. Ramon et al. used impregnated Pt on
magnetite as the recyclable catalyst for the hydrosilylation of alkynes.13 The hydrosilylation of
alkynes produces olefinic silanes that are potential precursors of trisubstituted alkenes, i.e., the
resulting organosilicon reagents are versatile building blocks in a number of synthetic processes.
The transition metal-catalyzed hydrosilylation of alkynes represents the most straightforward and
convenient route for the preparation of olefinic silanes with 100% atom efficiency.14 In this
research, we selected for demonstration the Pt-catalyzed reactions on hydrosilylation of alkyne
using specifically a dichloro Pt-diimine complex that carries long fluorous-ponytails at periphery
of 2,2′-bpy for recycling studies. Such Pt complexes are known to be soluble in organic solvent
only at elevated temperatures, e. g. in Bu2O, yet insoluble at all at room temperature. The use of
expensive fluorinated solvents is thus avoided, the homogeneous catalysis achieved, and the
Pt-catalysts recovered and reused using a simple procedure. To our knowledge, this is the first
time Pt-catalyzed hydrosilylation of alkynes has been successfully tested for the feasibility of
recycling usage under the thermomorphic conditions.
Results and discussion
1. Catalyst synthesis
The preparation of ligand bis-5,5′-(n-C11F23CH2OCH2)-2,2′-bpy (1) followed a literature
procedure.15 The ligand has a high fluorine content, F% = 0.64. The reaction of fluorinated
bipyridine ligand 1 with K2PtCl4 in DMF, as shown in Scheme 1, resulted in the Pt complex
3

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PtCl2[5,5′-bis-(n-C11F23CH2OCH2)-2,2′-bpy] (2) as red solids. Compound 2 exhibits a fluorine
content at F% = 0.53 and has a slight solubility in Bu2O at 120 ºC. The color of solution becomes
red. Compound 2 is insoluble in Bu2O at 25 ºC, however.
Rf
Rf
Rf
O
N
O
O
K2PtCl6
DMF
N
O
N
N
Rf
Pt
Cl
Cl
Rf = C11F23
2
1
Scheme 1
2. General procedure for recovery and reuse of 2 in hydrosilylation reaction
The Pt complex 2 was then tested for the subsequent catalytic experiments to examine the
addition of HSiEt3 to (a) symmetrical internal alkyne 5-decyne (3a), (b) terminal alkyne 1-hexyne
(3b), and (c) functionalized terminal propargylic alkyne (3c), in the hydrosilylation reactions.16
The Pt-catalyzed hydrosilylation reactions of 3a-c with HSiEt3 were carried out that successfully
demonstrated the feasibility of recycling usage with 2 as the catalyst using Bu2O as the solvent
under the thermomorphic condition. The catalytic reaction was conducted at ca 120 °C for about
4 h in each run. At the end of each cycle, the product mixtures were cooled to room temperature
then centrifuged for precipitation of the Pt catalyst 2. After the Pt catalyst was recovered by
decantation, the collected 2 was again supplied with the same amounts of Bu2O solvent, 3a-c, and
1
HSiEt3 to proceed to the next cycle. The products were quantified with GC/MS and H NMR
analysis, using NMP as internal standard.
3. Recovery and reuse of 2 in hydrosilylation reaction of 5-decyne
Product analysis of the hydrosilylation reaction indicated only one product of [(5E)‐dec‐5‐en‐5‐
yl]triethylsilane (4a), in quantitative yields, as shown in Scheme 2, with the (5Z)‐form isomer
4

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un-observed. Thus it became a good model reaction to study the recovery and reuse of
polyfluorinated Pt-catalyst 2, using 3a and HSiEt3 in the hydrosilylation reaction. The products
1
were quantified with GC/MS and H NMR analysis, using NMP as internal standard. Very good
recycling results (Table 1), for a total of 8 times were achieved for Pt-catalyst 2-catalyzed
hydrosilylation without difficulty under the thermomorphic mode.
Table 1. Recycling results of 2-catalyzed hydrosilylation reaction of symmetrical internal alkyne
R−≡−Ra
Cycle No.
Reactant
(R =)
Time (h)
Yield[b] (%)
TON
Temp (°C)
1
2
3
4
5
6
7
8
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
4
4
4
4
4
4
4
4
120
120
120
120
120
120
120
120
>97
>97
>97
>97
>97
>97
>97
>97
80.8
80.8
80.8
80.8
80.8
80.8
80.8
80.8
a Reaction conditions: Bu2O as solvent (2 mL), 5-decyne 3a (1.55 mmol, 214 mg), HSiEt3 (2
mmol, 233 mg), 1.2 mol % Pt catalyst 2 (0.0186 mmol; 30.6 mg). b Yields mainly determined by
1H NMR area integration on product solution.
Ramón et al. studied the hydrosilylation reaction with recyclable PtO/PtO2-Fe3O4 catalyst in the
absence of any solvent on 3a and HSiEt3 and yielded just 4a quantitatively.13 Shafir and Pleixats
et al. studied tris-imidazolium salt-stabilized Pd nanoparticles and reported the hydrosilylation of
3a with HSiEt3 resulted also in only 4a, at 60% yield with a different recyclable metal.17 In this
manuscript, the 2-catalyzed hydrosilylation reaction of 3a with HSiEt3 resulted in also one
product 4a quantitatively.
4. Recovery and reuse of 2 in hydrosilylation reaction of 1-hexyne
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1
In the second Pt-catalyzed hydrosilylation reaction for simple terminal 1-hexyne (3b), the H
NMR spectrum of hydrosilylation product mixtures clearly indicated two products
(E)-triethyl(hex-1-en-1-yl)silane (4b) and triethyl(hex-1-en-2-yl)silane (5b) as shown in Scheme
3. In Table 2 the hydrosilylation reaction of 3b with HSiEt3 catalyzed by 2 showed very good
recycling results for a total of 8 times. The two isomeric products were obtained with >97% total
yield all the time.
Table 2. Recycling results of 2-catalyzed hydrosilylation reaction of terminal R1−≡
−Ha
Cycle No.
Reactant
(R1=)
Time (h)
Yield[b] (%)
TON
Temp (°C)
1
2
3
4
5
6
7
8
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
4
4
4
4
4
4
4
4
120
120
120
120
120
120
120
120
>97
>97
>97
>97
98
80.8
80.8
80.8
80.8
81.7
80.8
80.8
80.8
>97
>97
>97
a Reaction conditions: Bu2O (2mL) as solvent, 1-hexyne 3b (1.52 mmol, 125 mg), HSiEt3 (1.92
mmol, 223 mg), 1.2 mol % Pt catalyst 2 (0.0186 mmol, 30 mg). b Yields determined by 1H NMR
area integration on product solution.
Figure 1 details the 1H NMR spectrum of the product mixture for the 3b reaction, focusing on the
double bond proton region, which indicates clearly the presence of 4b and 5b without the
presence of third isomer (Z)-triethyl(hex-1-en-1-yl)silane, nonetheless. The 1H NMR spectrum of
4b shows the alkenyl protons at δ 5.96 (dt, J = 18.6, 6.2 Hz, 1 H), 5.46 (dt, J = 18.6, 1.2 Hz, 1 H)
with typical coupling constant of 18.6 Hz for the alkenyl protons at trans positions; that of 5b
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shows the alkenyl protons at δ 5.55 (dt, J = 3.1, 1.5 Hz, 1 H), 5.20 (dt, J = 3.1, 0.6 Hz, 1 H) with
typical coupling constant of 3.1 Hz for the germinal ones. Concerning the
(Z)-triethyl(hex-1-en-1-yl)silane isomer, a coupling constant of 6-12 Hz is expected, yet not seen,
in Figure 1.
Figure 1. 1H-NMR spectrum (CDCl3, δ 5.0-6.1) of the product mixtures from Scheme 3.
The GC/MS method was employed to look into the products. After stirring for 4 h, the product
mixtures were analyzed (see Figure 2). The GC peaks at retention times of 3.2 min and 6.0 min
are the signals for reagent (HSiEt3, slight excess) and high boiling solvent (Bu2O), respectively.
The peak at retention time of 1.8 min for 3b disappeared and two new peaks at retention times of
about 12.0 and 12.3 min appeared, which were subject to MS identification (Figure 2b-c). Both
mass spectra of species at 12.0 min and 12.3 min showed weak peak at m/e =198 [M+] and major
fragmentation peaks at m/e= 169 [M+−Et] and 141 [M+−Bu], in different ratios. The MS results
clearly demonstrated that only two isomeric products resulted from the Pt-catalyzed
hydrosilylation of terminal 1-hexyne. The ratio of peak area integrations by GC is 56:44,
consistent to the 1H NMR area integration ratio of the two products 4b and 5b, 1:0.84.
7

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Figure 2. The GC/MS results for the products in Scheme 3. (a) the GC chromatogram, with (b)
the MS data for the peak at retention time 12.0 min (5b) and (c) that at 12.5 min (4b).
The Tsipis’ PtCl2-catalyzed version of 3b with HSiEt3 yielded 4b and 5b (96:4, 93% total yield).
13
Tsipis had noted that the magnitude of difference in C chemical shifts for terminal alkynes is
rather large (~15−20 δ units) compared to that for internal alkynes (~3−5 δ units).18 In our study,
the 2-catalyzed hydrosilylation of 1-hexyne 3b with HSiEt3 resulted in 4b and 5b (56:44, >97%
total yield). A diminishing regioselectivity was observed in current study, likely because the
coordination sphere of (2,2′-bpy)PtCl2 remains largely unaltered, which might have caused
enough electronic difference from that of a loose coordination sphere of PtCl2.
5. Recovery and reuse of 2 in hydrosilylation reaction of 2‐methylbut‐3‐yn‐2‐ol
In the third Pt-catalyzed hydrosilylation reaction (Scheme 4), for more complicated terminal
alkyne (HO)CMe2C≡CH (3c) with HSiEt3, the propargyl alkyne 3c has a hydroxyl group as well
as a sterically crowded C-atom next to the triple bond. The recycling experiments using
Pt-catalyst 2 proceeded without difficulty for 8 times (Table 3), giving yields > 97 % for total
isomeric products.
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The Pt-catalyzed hydrosilylation of 3c with HSiEt3 was noticed that reactivity was seemingly less
than that of 1-hexyne 3b with HSiEt3, although the rates were not strictly measured in this study.
1
The reaction was monitored with GC/MS and H NMR. Figure 3 shows a typical gas
chromatogram of the product mixtures after catalysis. No peak could be attributed to the starting
3c (if present, it should be located at retention time 1.8 min). The HSiEt3 peak (slight excess in
reaction) was observed at retention time 3.2 min and the Bu2O peak (solvent) was observed at
retention time 5.9 min. Two new peaks were recorded at 12.3 and 12.5 min, which were assigned
to the products (see later for 1H NMR analysis).
Table 3. Recycling results of 2-catalyzed hydrosilylation reaction of terminal R2−
≡−H
Cycle No. Reactant (R2 =)
Time (h)
Yield[b] (%)
TON
Temp (°C)
1
2
3
4
5
6
7
8
4
4
4
4
4
4
4
4
120
120
120
120
120
120
120
120
>97
>97
>97
>97
>.97
>97
>97
>97
80.8
HOC(Me)2
HOC(Me)2
HOC(Me)2
HOC(Me)2
HOC(Me)2
HOC(Me)2
HOC(Me)2
HOC(Me)2
80.8
80.8
80.8
80.8
80.8
80.8
80.8
a Reaction conditions: Bu2O (2 mL) as solvent, 2‐methylbut‐3‐yn‐2‐ol 3c (1.52 mmol, 128 mg),
HSiEt3 (1.92 mmol, 223 mg), 1.2 mol% Pt catalyst 2 (0.0186 mmol, 30 mg). b Yields determined
by 1H NMR area integration on product solution.
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The HSiEt3 addition to (HO)CMe2C≡CH 3c would generate three possible product isomers, all
with molecular mass of 200. The two isolated products were found to be
(E)-2-methyl-4-(triethylsilyl)but-3-en-2-ol 4c with retention time of 12.3 min and
2-methyl-3-(triethylsilyl)but-3-en-2-ol 5c with retention time of 12.5 min. The ratio from area
integration for 4c to 5c in the gas
Figure 3. The GC/MS results for the products in Scheme 4. (a) the GC chromatogram, with (b)
the MS data for peak at retention time 12.3 min (4c) and (c) that at 12.5 min (5c).
chromatogram was about 67:33. For 4c the MS data indicated only a small signal for M+ (m/e=
200) but intense signal for M+−Et (m/e= 171). For 5c the MS data showed even less intensity for
both
M+
(m/e=
200)
and
M+−Et
(m/e=
171)
peaks.
The
possible
(Z)-2-methyl-4-(triethylsilyl)but-3-en-2-ol isomer was not detected in GC/MS. Besides catalyst
2, another catalyst, PtCl2[5,5′-bis-(n-C9F19CH2OCH2)-2,2′-bpy] (2’), was also used to test the
different fluorous ponytailed Pt-catalyzed hydrosilylation reaction of 2‐methylbut‐3‐yn‐2‐ol. The
results also gave a mixture of two hydrosilylated isomers with about the same ratio.
The 1H NMR spectrum of the product solution is shown in Figure 4. There are two sets of peaks
in the C=C double bond region. One set of olefinic protons shows signals at δ 6.11 (d, J = 19 Hz,
1H), δ 5.70 (d, J = 19 Hz, 1H), assigned to the trans positioned protons of 4c. The second set of
vinyl protons are at δ 5.69 and 5.24 with J = 1.8Hz, assigned to germinal protons of 5c. The peak
area integration ratio between 4c and 5c is approximately 2:1, similar to the GC area integration
ratio..
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Figure 4. 1H-NMR of product solution of Scheme 4 (CDCl
3, δ 5.1-6.2).
Cook et al. found that the hydrosilylation of 3c and HSiEt3 using PtCl2 and XPhos ligand limited
the products to just 4c (89% yield) as a colorless oil.19 In early Russian literatures, 3c and HSiEt3
catalyzed by H2PtCl6 gave 4c and 5c with no reported ratio.20 The impregnated Pt on magnetite
PtO/PtO2-Fe3O4 recyclable catalyst used by Ramón et al. in the hydrosilylation of 3c with HSiEt3
(neat) gave 4c and 5c (78:22, 99% total yield).13 Tsipis in his PtCl2 catalyzed hydrosilylation
reaction of 3c produced 4c 93% and 5c 7% (88% total yield).19 The similar HSiEt3 addition to 3c
in this study produced 4c and 5c (67:33, > 97% total yield). The propargylic alcohol functional
group may have influence in addition to steric and electronic effects (vide infra).
6. Analysis of residual Pt level in product solution by ICP-MS
As solid the Pt-catalyst 2 was separated from the reaction mixtures after centrifugation. The
recovered catalyst 2 was then ready for reuse in the next cycle of hydrosilylation reaction. The
quantity of residual Pt metal present in the solution was also analyzed, taking random samples
among the product solutions resulting from the thermomorphic hydrosilylation process. The
polyfluorinated (2,2′-bpy)PtCl2 complexes are robust. Only very low Pt leaching was observed in
solution from the complex 2 after the thermomorphic, homogeneous catalysis procedure (Table
4). The first three entries of Table 4 (1−1, 1−3, 1−6) are samples taken from product mixtures
after cycles 1, 3, and 6 of the hydrosilylation of 3a, and the Pt leaching levels by ICP-MS are
6.7×10-5, 6.1×10-5, and 1.6×10-5, respectively, averaging at 5.5×10-5. It provided strong evidence
that the molecular catalyst 2 studied in this research possesses great thermostability and is indeed
a robust, reliable and recoverable Pt catalyst under the thermomorphic conditions. In the bottom
entries of Table 4 (3−2, 3−4), the hydrosilylation of (HO)CMe2C≡CH 3c exhibits a greater Pt
leaching level in the range of 1.4-1.5×10-3. The increase of the Pt leaching level for the
11

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hydrosilylation of 3c could be attributed to the tertiary propargyl −OH functionality as an extra
site for coordination to the Pt metal ion center during the catalytic cycle. There is no such −OH
functionality in 3a and 3b.
Table 4. The ICP-MS results of Pt leaching levels
Leaching levelb
Recovery (%)
Table no.−Run no.a
Pt (in ppb)
Detected from ICP-MS
3.35x10-5
3.05×10-5
8×10-6
99.997
99.997
99.999
99.995
99.996
99.998
99.925
99.930
60.8
55.6
15.9
87.8
69.3
36.1
2630
2570
1−1
1−3
1−6
2−2
2−4
2−7
3−2
3−4
4.85×10-5
3.80×10-5
2.00×10-5
0.75×10-3
0.70×10-3
a For example, 1−1 stands for the entry 1 from Table 1. The Pt leaching level is calculated as
follows, using entry 1−1 as example: (60.8 ppb × 2 mL) / (0.0186 mmol Pt) = 60.8 × (mgPt/103L)
× 2 × (10-3L) / (1.86 × 10-2 × 195 mgPt) = 3.35 × 10-5.
b
In literature, the recyclable impregnated Pt on magnetite PtO/PtO2-Fe3O4 catalyst by Ramón et al.
in the hydrosilylation reaction had been reported about the Pt leaching level, at 0.01% of the
initial amount of Pt (ICP-MS analysis of the resulting product mixture, and 0.0008% reportedly
for Fe).13 Thus, the recycling Pt catalyst 2 performs really great with the lowest leaching level of
only 0.001% (entry 1-6) while utilizing the thermomorphic method.
7. Mechanistic insights
Overall, the hydrosilylation with Pt complex 2 is believed to follow the Chalk–Harrod
mechanism. The Et3SiH is oxidatively added to [Pt], followed by coordination of the alkyne and
insertion to [Pt]−H bond. The reductive elimination results in the cis addition of Si−H onto an
alkyne triple bond,16 as shown in Scheme 5. The electronic effect that the polarization of an
alkyne in hydrosilylation reaction would direct the hydride addition to the more electropositive
C-atom, was first summarized by Tsipis in a PtCl2 catalyzed hydrosilylation.19 This electronic
effect leaves the steric C-atom directing force only a secondary factor to affect regioselectivity.
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At the molecular level, the coordination sphere of 2 with a (2,2′-bpy)PtCl2 core remains largely
unaltered, which might have caused enough electronic difference from that of a loose
coordination sphere of PtCl2. Concerning the propargylic alcohol 3c, Ferreira et al. had analyzed
the influences dictating regioselectivity in Pt-catalyzed hydrosilylation reactions of internal
alkynes,21,22 and suggested that the impact of the propargylic alcohol functional group on
hydrosilylation regioselectivity cannot be fully explained with simple steric and electronic
arguments, and had hypothesized that the −OH functional group is participating in an interaction
with the Pt catalytic complex, perhaps through hydrogen bonding.
As a summary, we have demonstrated, to our knowledge, the first example of Pt-catalyzed
hydrosilylation reaction of symmetric internal alkyne and terminal alkynes under the
thermomorphic conditions, all being with good turnover numbers on hydrosilylation catalysis and
easy recovery and re-usage for 8 cycles without loss of reactivity. The preparation of fluorous
PtCl2[5,5′-bis-(n-C11F23CH2OCH2)-2,2′-bpy] catalyst was simple, with built-in molecular designs.
The leaching level of Pt in the product mixture was only as low as 0.001%.
8. Experimental section
8.1. General procedures
Gas chromatographic/mass spectrometric data were obtained using an Agilent 6890 Series gas
chromatograph with a series 5973 mass selective detector. The reaction was monitored with a HP
6890 GC using a 30 m 0.250 mm HP-1 capillary column with a 0.25 µm stationary phase film
thickness. The flow rate was 1 mL/min and splitless. Samples analyzed by fast atom
bombardment (FAB) mass spectroscopy were done by the staff of the National Central University
(Taiwan) mass spectrometry laboratory. The amounts of residual Pt samples were analyzed by
ICP-MS. Infra-red spectra were obtained on a Perkin–Elmer RX I FT-IR Spectrometer. NMR
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spectra were recorded on Bruker AM 500 and Joel AM 200 using 5 mm sample tubes. CD3OD,
CD2Cl2, CDCl3, deuterated DMF and deuterated Me2SO were the references for both 1H and 13C
NMR spectra; and Freon® 11 (CFCl3) was the reference for 19F NMR spectra.
8.1.1. Starting materials
Chemicals, reagents, and solvents employed were commercially available and used as received.
C11F23CH2OH were purchased from Aldrich and SynQuest.
8.2. Preparation of Pt complex 2
The
reaction
of
K2PtCl4
with
fluorinated
bipyridine
derivatives,
5,5 -bis(n-C11F23CH2OCH2)-2,2
′
′-bpy (1), resulted in PtCl2[5,5′-bis-(n-C11F23CH2OCH2)-2,2′-bpy]
(2), as red solids. The reaction was stirred under nitrogen for 4 h, and the complex 2 would then
precipitate from the reaction mixture. After collection of the precipitates, water and Et2O were
used to wash the red solids for several times, and the yield was 87%.
8.2.1. Analytical data of 2
1H NMR (500 MHz, DMF-d6, at 90 °C,
δ
ppm, J Hz; very weak sample): 8.72 (d, 2H, H6), 8.48
(s, 2H, H3), 7.32 (d, 2H, H5) , 4.88 (4H, s, bpy-CH ), 4.12 (t, 4H, C11F23-CH2); 19F NMR (470.5
2
MHz, DMF-d6,
δ ppm, J Hz; very weak sample) -81.7 (t, 6F, –CF3), 119.3 ~125.7 (broad peaks,
20 F, -C10F20); 13C NMR spectrum was not obtained due to the extremely weak signals at 90 °C.
FT-IR: 1601, 1559 (νbpy, w), 1197, 1148 (C−F stretch, s).
HR−MS (FAB):
[M+] C36H1435Cl2F46N2O2Pt m/e calc: 1644.9346, found: 1644.9327; C36H1435Cl37ClF46N2O2Pt
m/e calc: 1646.9328, found: 1646.9321; C36H1437Cl2F46N2O2Pt m/e calc: 1648.9287, found:
1648.9280.
8.3. Recycling studies
A tube was charged with Bu2O (2 mL) as solvent, alkyne substrate (1.52 mmol), HSiEt3 (1.92
mmol, 223 mg), and 1.2 mol% Pt catalyst 2 (0.0186 mmol, 30 mg) for heating to 120 °C and then
maintained stirring at 120 °C under N2 atmosphere. During the reaction the reaction mixture
became colored (Figure 5b) and the mixture was periodically monitored by GC/MS for
completion. After 4h, the temperature was lowered to 25 °C, followed by centrifugation to
precipitate the Pt catalyst 2 (Figure 5c). The product mixtures were withdrawn using a syringe for
analysis. The Pt catalyst 2 was rinsed with Bu2O for 3 times before next charging of the solvent
and reactants for catalytic study. Total number of recycling is 8 times.
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Over the time, the Pt-catalyzed hydrosilylation reaction products were also isolated for
characterization. Since the products (4a, 4b, 4c, 5b, and 5c) were known, they showed correct
GC/MS and 1H NMR data, and m.p. for solids.13,19,20,23
Figure 5. (a) Graphic representation of Pt catalyst recycle under thermomorphic condition; (b)
The colored reaction mixtures during hydrosilylation reaction; (c) The product mixture separated
from Pt catalyst 2 after centrifugation.
Acknowledgements
The authors thank the Ministry of Science and Technology of Taiwan (MOST
104-2113-M-027-004-MY2), Taiwan, for the financial support.
Supplementary data
Supplementary material which is associated with this article to show the pictures of the process in
recovering the catalyst can be found in the online version, at
References and notes
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Highlights
ꢀ
ꢀ
ꢀ
ꢀ
Pt-catalyst 2 with F-content 0.53 was easily recoverable under the thermomorphic method.
Pt complex 2 is the robust and thermally stable catalyst.
The leaching of Pt was monitored by ICP-MS to be as low as 10-6 level per cycle.
The reaction-catalyst fulfills the principle of green chemistry, and the hydrosilylation delivers
100% atom economy.