Highly selective hydrosilylation of olefins and acetylenes by platinum(0) complexes bearing bulky N-heterocyclic carbene ligands

Article abstract of DOI:10.1039/c7dt04392a

Platinum complexes bearing bulky N-heterocyclic carbene (NHC) ligands, i.e., [Pt(IPr?)(dvtms)] (where, IPr? = 1,3-bis{2,6-bis(diphenylmethyl)-4-methylphenyl}imidazol-2-ylidene) and [Pt(IPr?OMe)(dvtms)] (where, IPr?OMe = 1,3-bis{2,6-bis(diphenylmethyl)-4-m

Full text of DOI:10.1039/c7dt04392a

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Highly selective hydrosilylation of olefins and acetylenes by
platinum(0) complexes bearing bulky N-heterocyclic carbene
ligands
Received 00th January 20xx,
Accepted 00th January 20xx
P. Żak, M. Bołt, M. Kubicki and C. Pietraszuk*
DOI: 10.1039/x0xx00000x
Platinum complexes bearing bulky N-heterocyclic carbene (NHC) ligands, i.e. [Pt(IPr*)(dvtms)] (where IPr*=1,3-bis{2,6-
www.rsc.org/
bis(diphenylmethyl)-4-methylphenyl}imidazol-2-ylidene)
and
[Pt(IPr*OMe)(dvtms)]
(where
IPr*OMe=1,3-bis{2,6-
bis(diphenylmethyl)-4-methoxyphenyl}imidazol-2-ylidene, dvtms
=
divinyltetramethyldisiloxane) catalyse nearly
quantitatively and highly or completely selective hydrosilylation of terminal olefins as well as terminal or internal
acetylenes.
This paper is dedicated to the memory of Professor Istvan E. Markó who pioneered the chemistry of
[Pt(NHC)(dvtms)] complexes.
Speier7 and Karstedt catalysts.1,8-10 Despite the high activity,
these catalytic systems have several drawbacks including low
regioselectivity and instability under reaction condition
Introduction
Hydrosilylation of olefins and acetylenes is the most
fundamental method for laboratory and industrial synthesis of
organosilicon compounds,1 widely used in both general
organic synthesis2 and special materials chemistry.3
causing the formation of colloidal platinum species which tend
to promote formation of undesired side-products.
In 2002 Markó reported new structural type of platinum
complexes of the general formula [Pt(NHC)(dvtms)] (where
dvtms = 1,3-divinyl-1,1,3,3-tetramethyldisiloxane).11 Compared
with the Karstedt catalyst, these complexes used as catalysts in
hydrosilylation of alkenes are characterized by slightly lower
catalytic activity, higher chemo- and regioselectivity, and
significantly improved stability. An important feature of these
catalysts is no tendency towards decomposition to platinum
colloids.11 Main factors which affect the catalytic activity and
regioselectivity of [Pt(NHC)(dvtms)] complexes are steric
hindrance imparted by the N-aryl groups, the rate of formation
of the monoligated platinum complex, and relative stability of
Pt(NHC) species.11 Development of the family of Markó
complexes resulted in the synthesis of new catalysts
permitting highly efficient and regioselective hydrosilylation of
a range of functional olefins.12
A
significant disadvantage of the hydrosilylation of olefins as a
synthetic method is the lack of selectivity. In the presence of
many catalytic systems, the reaction leads to the formation of
a mixture of regioisomers (Scheme 1). Moreover, the reaction
is often accompanied by side-processes, such as
dehydrogenative silylation, isomerization and hydrogenation
of olefins.1
Scheme 1 Hydrosilylation of olefins with trisubstituted silanes
Although hydrosilylation of olefins has been investigated using
various transition metal complexes as catalysts,1,4-6 industry
still relies largely on platinum-based systems such as the
Acetylenes are considered more demanding substrates for
hydrosilylation than olefins because the reaction usually
results in a mixture of regio- and stereoisomers (Scheme 2).
Adam Mickiewicz University in Poznań, Faculty of Chemistry, Umultowska 89b, 61-
614 Poznań, Poland
E-mail: pietrasz@amu.edu.pl
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: Analytical data of isolated
products and details of X-ray analyses. See DOI: 10.1039/x0xx00000x
Scheme 2 Hydrosilylation of acetylenes with trisubstituted silanes
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Literature provides reports on selective conversion of both
terminal and internal acetylenes in the presence of a variety of
transition metal complexes.1 Commonly used Karstedt catalyst
catalyzes cis addition of hydrosilane to acetylene, but with low
regioselectivity.1a,b The use of complexes with bulky phosphine
ligands has made significant improvement as far as
regioselectivity is concerned. Platinum(0) complexes of the
type [Pt(PR3)(dvtms)], where PR3 is a bulky phosphine ligand,
e.g. PtBu3, allow obtaining anti-Markovnikov product with
relatively high E selectivity in hydrosilylation of terminal
acetylenes.14
Results and Discussion
Our examination started with the testing of catalytic
properties of complexes and in olefin hydrosilylation. Allyl
1
2
glycidyl ether was chosen as a test olefin, as it is commonly
used in the studies of olefin hydrosilylation and it could
undergo undesirable processes of isomerization. Both
complexes
1 and 2 were tested as catalyst in hydrosilylation of
allyl glycidyl ether with dimethylphenylsilane under a range of
conditions in order to find the conditions for the efficient and
selective course of the reaction (Scheme 3). The results are
summarized in Table 1.
Markó has shown that [Pt(NHC)(dvtms)] complexes can be
used as selective catalysts of hydrosilylation of acetylenes15
and their selectivity depends strongly on the steric properties
of carbene ligands. Development of application of Markó type
complexes in acetylene hydrosilylation is a topic of several
recent reports.13f,16 The selectivity of hydrosilylation of 1-
Scheme 3 Hydrosilylation of allyl glycidyl ether with dimethylphenylsilane catalyzed by
1 or 2.
As shown in Table 1, the best results were achieved by
octyne expressed by molar ratio of isomers [
shown to increase with increasing steric parameter AH.
The highest selectivity ([ -E]/[ ] = 10.6) was obtained for a
β-E]/[α] was
15,17 conducting the reaction in toluene, at 80°C, under argon.
Under optimized conditions the reaction in the presence of
β
α
complexes 1 or 2 led to the exclusive formation of β-adduct.
complex containing a bulky carbene ligand, i.e. 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene (IPr).15 An analogous
complex [Pt(IPr)(AE)] (where AE = diallyl ether) with similar
steric parameters, but containing diallyl ether, a more labile
throw-away ligand than dvtms, enabled selective
hydrosilylation of terminal acetylenes with very high
Table 1 Hydrosilylation of allyl glycidyl ether with dimethylphenylsilane. Optimization
of the reaction conditions
Entry
Cat. (% mol) Time [h] Temp. [°C]
Conv.[a] [%]
β:
α [b]
1
2
3
4
5
6
7
1
1
1
2
1
2
1
(1
(1
(1
(1
(1
(1
(1
×
×
×
×
×
×
×
10-3)
10-4)
10-4)
10-4)
10-4)
10-4)
10-3)
5
2
100
100
80
90
99
99
98
92
90
75
94:6
95:5
β[c]
preference for the formation of E-β-adduct and different
6
regioselectivity in hydrosilylation of unsymmetrical internal
acetylenes.16b Relative, N-heterocyclic carbene platinum
bis(silyl) complexes [Pt(IPr)(SiR3)2],18 exhibit improved
regioselectivity of hydrosilylation of unsymmetrical internal
acetylenes giving the [β]/[α
] ratio generally exceeding 20/1.16a
Recently, we have reported the synthesis of two Markó type
6
80
β[c]
24
24
24
60
β[c]
60
β[c]
25
β[c]
Reaction condition: Toluene, argon; [3a]:[4a] = 1:1; [a] Determined by GC analysis;
[b]
Determined by GC analysis and confirmed by 1H NMR spectroscopy of the
crude reaction mixture; [c] no α addition product was detected
platinum(0) complexes bearing bulky N-heterocyclic carbene
ligands, e.i. [Pt(IPr*)(dvtms)]
bis(diphenylmethyl)-4-methylphenyl}imidazol-2-ylidene) and
[Pt(IPr*OMe)(dvtms)] IPr*OMe=1,3-bis{2,6-
(where
(1) (where IPr*=1,3-bis{2,6-
Both catalysts showed similar catalytic properties. Therefore,
in the following part of the publication, only the results
(2)
obtained for catalyst
1 were reported. The results obtained
bis(diphenylmethyl)-4-methoxyphenyl}imidazol-2-ylidene
(Figure 1).19 Because the values of AH angles in complexes
using catalyst 2 are included in Supporting Information. The
1
tests performed have shown that the process can be carried
out in solvent-free conditions, but due to the fact that many of
the potential substrates are solids, toluene is suggested as the
reaction medium.
and
surpass the corresponding values in the previously described
complexes,15 we decided to examine the complexes
and in
2 (equal to 201.6° and 213.4°, respectively) significantly
1
2
hydrosilylation of olefins and acetylenes and demonstrate the
impact of bulky yet flexible NHC ligands on chemo- regio- and
stereoselectivity. Herein, we report on the high productivity
Having an active and selective catalyst in hand, the range of
substrates was extended to determine the versatility of the
method (Scheme 4). The results were collected in Table 2.
and striking selectivity of complexes
1 and 2 in hydrosilylation
of terminal olefins as well as terminal and internal acetylenes.
Me
MeO
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Si
O
Si
N
N
Si
O
Si
N
N
Ph
Ph
Ph
Pt
Ph
Pt
Ph
Ph
Ph
Ph
Ph
Me
MeO
1
2
Scheme 4 Hydrosilylation of terminal olefins catalyzed by 1
Figure 1 Platinum complexes with bulky NHC ligands used in the presented studies
2 | J. Name., 2012, 00, 1-3
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alkenes were used as olefins (Entry 6-25) trace amounts (< 3%)
of the alkene isomerization products were observed. The
results contribute to expansion of current knowledge on the
reaction[12-13] by providing data on the reactivity of a wide
range of olefins and trisubstituted silanes. In each case, nearly
quantitative and fully selective hydrosilylation have been
achieved in the presence of extremely low concentration of
catalysts. Significant values of turnover numbers reaching TON
Table 2 Hydrosilylation of terminal alkenes in the presence of complex 1
Entry
Olefin
HSiR3
1
[mol%]
Time [h]
Yield[a] [%]
(isolated)
98[b]
98
1
2
3a
4b
4c
4d
4a
4e
4b
4c
4d
4a
4e
4b
4c
4d
5×
5×
5×
1×
1×
1×
5×
1×
1×
1×
1×
5×
1×
10-3
10-3
10-3
10-4
10-1
10-4
10-3
10-3
10-5
10-1
10-3
10-3
10-4
2
6
3
9
97[b]
4
6
99
5
24
8
99
=
107 were obtained with average values of
6
3b
3c
99[b]
99 (95)
98[b]
99
TON = ca 105.
7
24
24
18
24
5
8
Catalytic properties of complex 1 were also evaluated in the
9
hydrosilylation of a series of terminal arylacetylenes with a
variety of trisubstituted silanes and with the disubstituted one
(Scheme 5).
10
11
12
13
92
99[b]
2
98
99[b]
7
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
45
4a
4e
4b
4c
4d
4a
4e
4b
4c
4d
4a
4e
4b
4c
4d
4a
4e
4a
4e
4b
4c
4d
4a
4e
4b
4c
4d
4a
4e
4b
4c
4d
4a
4e
1
1
1
5
1
1
1
5
5
1
1
1
5
5
5
5
1
1
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
10-5
10-1
10-4
10-3
10-3
10-5
10-1
10-4
10-3
10-4
10-5
10-1
10-3
10-3
10-3
10-4
10-1
10-3
100
10-4
10-4
10-4
10-5
10-1
10-4
10-4
10-4
10-5
10-1
10-3
10-3
10-3
10-4
10-1
3
24
8
99
94
99[b]
3d
3e
3f
24
24
24
24
2
99
97[b]
99
95
99[b]
24
24
4
91
89[b]
99
Scheme 5 Hydrosilylation of terminal acetylenes catalyzed by complex 1
24
9
95
97[b]
Hydrosilylation was found to proceed easily in the presence of
0.1 – 0.05 mol% of complex 1, in toluene at 40 °C by using
9
96
equimolar ratio of reacting partners. Such conditions permit
quantitative or nearly quantitative conversion of reagents. The
results obtained are collected in Table 3. In each case
complete or excellent selectivity towards the formation of
24
5
99[b]
99
24
24
24
6
80
3g
3h
100
85
99[b]
99 (95)
97[b]
99
product β-(E) was achieved. For triphenylsilane as a reagent,
1
×
irrespectively of the acetylene used, the process was fully
regioselective. Similarly, fully selective hydrosilylation was
observed when diphenylsilane was used as reacting partner.
5
5
5
1
1
5
5
5
1
1
1
1
1
2
1
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
9
9
(Table 3, entries 4, 8, 12). Small amounts of β-(Z) and
isomers were observed when using HSiEt3 (4d) and even
smaller when using HSiMe2Ph 4a). Interestingly, the
hydrosilylation of 4-ethynylbiphenyl (7f) irrespective of the
silane used, led to the exclusive formation of -(E). To increase
α
8
24
2
99 (91)
99[b]
99
3i
3j
(
3
9
99[b]
β
9
98
the diversity of silanes tested, hydrosilylation of selected
acetylenes (7a-c) were performed with diphenylsilane (4f).
Reactions result in the chemo- regio- and stereoselective
24
4
96
97[b]
4
99
formation of single β-(E) products bearing SiPh2H group with
6
93[b]
98
one Si-H bond untouched (Table 3, entries 4,8 and 12).
Compared to earlier proposals,[10,13f] the presented procedure
permits highly selective hydrosilylation of a series of terminal
arylacetylenes with several substituted silanes under mild
conditions and low catalyst loadings. The obtained values of
8
24
99
[a]
Reaction conditions: toluene, 80°C, argon, [olefin]:[HSiR3] = 1:1; Determined
by GC analysis; [b] Reaction performed in closed system
turnover numbers were in the range of TON = 1
104. In order evaluate the possibility to reach even higher
TON values a reaction between (dimethylphenylsilyl)acetylene
7j) and dimethylphenylsilane (4a) in the presence of 0.01
mol% of catalyst were performed until complete conversion
×
103 and
Catalytic hydrosilylation of a range of terminal olefins bearing
a variety of functionalities studied in the presence of low
2×
loading of catalyst
1
(down to 10-5 mol% = 0.1 ppm) proceeds
(
with complete regioselectivity and gives excellent yields of
anti-Markovnikov products (Scheme 3). Hydrosilylation of
1
was obtained and then repeated without addition of the
catalyst. Each time the reaction was carried out to obtain a
olefins catalyzed by
1 is fully chemoselective. Only when 1-
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complete conversion. This procedure was repeated four times,
without the decrease in the conversion. Only during the fifth
repetition, significant drop in the conversion down to 56% was
observed. No decrease in selectivity was observed. This
experiment indicates that the actual productivity of the
Table 4 Hydrosilylation of terminal silylacetylenes in the presence of complex 1
Entry
R3SiC≡CH
HSiR’3
4a
Isolated yield [%]
1
2
3
4
5
6
7
7g
7g
7h
7h
7i
90
96
95
92
93
95
94
4e
4a
catalyst 1 can significantly exceed the values reported above.
4b
4a
7i
4b
Table 3 Hydrosilylation of terminal arylacetylenes in the presence of complex 1
7j
4a
Entry
R’C≡CH
HSiR3
1
Time
[h]
3
Yield[a]
(isol.)[%]
100
99
Selectivity [b]
β-(E):β-(Z):α
99:1:0
Reaction conditions: toluene, 40°C, [silane]:[acetylene] = 1:1, cat. 1 (10-2 mol%),
[mol%]
5×10-2
5×10-2
1×10-1
1×10-1
5×10-2
5×10-2
1×10-1
1×10-1
5×10-2
5×10-2
1×10-1
1×10-1
5×10-2
5×10-2
1×10-1
5×10-2
1×10-2
1×10-1
5×10-2
5×10-2
1×10-2
5×10-2
5×10-2
1×10-1
6-24 h
1
2
7a
4a
4d
4e
4f
8
97:2:1[c]
100:0:0
100:0:0
100:0:0
97:3:0[c]
100:0:0
100:0:0
98:1:1
96:3:1[c]
100:0:0
100:0:0
99:1:0
To investigate the scope of this reaction with respect to the
reagent type, internal acetylenes were used to afford the
corresponding unsaturated products. Hydrosilylation of
symmetrically substituted internal acetylenes in the presence
3
24
24
7
88
4
95 (90)
99
5
7b
7c
4a
4d
4e
4f
6
10
24
24
7
98
of catalyst
1 proceeds quantitatively or nearly quantitatively.
7
99
Only in the hydrosilylation of diphenylacetylene with
triphenylsilane, a reduced conversion of the reactants was
observed. Important advantage of the process is its complete
selectivity (Scheme 7). Characteristic of platinum complexes
selective cis-addition resulted in exclusive formation of single
stereoisomer irrespective of the nature of the substituents in
reacting partners. The results obtained are collected in Table 5.
8
96
9
4a
4d
4e
4f
99
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
8
99
24
24
7
99
97
7d
7e
7f
4a
4d
4e
4a
4d
4e
4a
4b
4c
4d
4e
4e
99
24
24
8
90
95:3:2[c]
100:0:0
98:1:1
98
99
10
24
10
12
12
10
24
24
96
96:2:2[c]
100:0:0
100:0:0
100:0:0[c]
100:0:0
100:0:0[c]
100:0:0
100:0:0
96
100
97
95
94[c]
Scheme 7 Hydrosilylation of symmetrically substituted internal acetylenes catalyzed by
complex 1
53
99
Table 5 Hydrosilylation of symmetrically substituted internal acetylenes in the presence
of complex 1
[a]
Reaction conditions: toluene, 40°C, [silane]:[acetylene] = 1:1; Determined by
GC analysis; [b] β-(E):β-(Z):α determined by GC analysis and confirmed by 1H NMR
[c]
Yield[a]
(isol.) [%]
100 (93)
98[b]
spectroscopy of the crude reaction mixture;
system
Reaction performed in closed
Entry
R’C≡CHR’
HSiR3
1 [mol%]
Time [h]
1
2
10a
4a
4b
4c
4d
4e
4e
4a
4b
4c
4d
4e
4a
4b
4c
4d
4e
5×10-2
5×10-2
5×10-2
5×10-2
1×10-1
1
7
10
8
To increase the applicability of the reaction we tested
hydrosilylation of selected silylacetylenes (Scheme 6). In each
case, near quantitative conversion and complete regio- and
stereoselectivity were observed (Table 4). High conversions
were observed upon preserving the equimolar ratio of the
reagents.
3
100 (95)
100[b]
65
4
9
5
24
24
12
18
7
6
100
7
10b
10c
5×10-2
5×10-2
5×10-2
5×10-2
1×10-1
5×10-2
5×10-2
5×10-2
5×10-2
1×10-1
99
99[b]
8
9
99
99 (95)[b]
10
11
12
13
14
15
16
8
24
8
99
100
6
99 (95)[b]
99 (94)
95 (90)[b]
92
5
8
Scheme 6 Hydrosilylation of silylacetylenes catalyzed by 1
24
[a]
Reaction conditions: toluene, 90°C, [silane]:[acetylene] = 1:1; Determined by
GC analysis; [b] Reaction performed in closed system
4 | J. Name., 2012, 00, 1-3
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The structures of the obtained products were proposed on the
basis of the NMR spectroscopic characterization of selected
products.
Table 6 Hydrosilylation of 1-aryl-2-trimethylsilylacetylene with trisubstituted silanes in
the presence of complex 1
Entry
ArC≡CSiMe3
HSiR3
1 [mol%]
Time [h]
Yield[a]
(isol.) [%]
99[b]
99 (91)
95[b]
much more challenging unsymmetrically substituted
internal acetylenes. For this purpose, acetylenes (12a) and
1
2
12a
4a
4b
4c
4d
4e
4a
4b
4c
4d
4e
1×10-1
1×10-1
1×10-1
1×10-1
1×10-1
5×10-2
5×10-2
5×10-2
5×10-2
5×10-2
24
24
24
18
24
20
18
12
18
24
(
12b) having substituents at carbon-carbon triple bond
differing in steric hindrance were subjected to reactions with a
series of trisubstituted silanes. In each case complete or nearly
complete conversion of reacting partners was achieved under
conditions optimized for hydrosilylation of the internal
acetylenes. Regardless of silane used, formation of a single
isomer was observed (Table 6). The structure of the
hydrosilylation product of 12a with triphenylsilane was
determined by X-ray structure analysis methods (Figure 2). The
use of acetylenes containing a strongly electron-withdrawing
trifluoromethyl group in the phenyl ring at para position (12b),
did not affect the reaction selectivity but permitted a two-fold
reduction in the catalyst concentration.
3
4
99 (92)
5
97
99[b]
6
12b
7
99
98[b]
8
9
100 (92)
98
10
[a]
Reaction conditions: toluene, 90°C, [silane]:[acetylene] = 1:1; Determined by
GC analysis; [b] Reaction performed in closed system
The results of the reported catalytic tests demonstrate the
ability of all tertiary silanes (4a-e) to undergo efficient and
selective hydrosilylation with olefins and acetylenes. Under
optimized reaction conditions, the effect of substituents at
silicon atoms can be observed. In the hydrosilylation of olefins,
the highest efficiency was observed for dimethylphenylsilane
(
4a). Hydrosilylation this reagent, irrespective of the olefin
used, required catalyst loading of ca. 10-4-10-5 mol%, i.e. ten
times less than the one required for the other silanes (Table 2).
The lowest process efficiency was observed for the reactions
involving triphenylsilane (4e), where it was necessary to use
the catalyst in an amount of 10-1 mol% and to extend the
reaction time up to 24 hours in order to reach quantitative
conversion. A similar effect of substituents at silicon on the
reaction efficiency was observed in hydrosilylation of terminal
acetylenes (Table 3).
Scheme 8 Hydrosilylation of unsymmetrically substituted internal acetylenes catalyzed
by 1
Postulated dependence of reaction selectivity on the
stereoelectronic environment of Pt site15 was checked by
performing hydrosilylation of phenylacetylene
(7a) with
dimethylphenylsilane (4a) in the presence of a series of
complexes of the type [Pt(NHC)(dvtms)] differing in steric
properties, characterized by AH and %Vbur values17. The
obtained results are collected in Table 7. Under optimized
conditions higher selectivities from those described in the
literature [16a] were observed for catalysts containing IMes
(IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IPr
and SIPr (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-
Figure 2. A perspective view of the molecule 13ae. Ellipsoids are drawn at the
30% probability level, hydrogen atoms are represented by spheres of arbitrary
radii.
ylidene) ligands. In the presence of catalysts
1 and 2
characterized by higher values of the AH and %Vbur parameters
slightly higher selectivities were observed.
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terminal and internal acetylenes, the proposed procedures
were scaled up to gram-scale without decrease in
Table 7 Hydrosilylation of phenylacetylene with dimethylphenylsilane in the
presence of [Pt(IMes)(dvtms)] (14), [Pt(IPr)(dvtms)] (15), [Pt(SIPr)(dvtms)] (16),
a
[Pt(IPr*)(dvtms)] (
1 2)
) and [Pt(IPr*OMe)(dvtms)] (
conversions and selectivities (See Supporting Information).
Cat
AH
%Vbur
Reaction
Conv.
Selectivity [b]
-(E): -(Z):α
Experimental Section
time [h]
β
β
14
15
16
71
97
99
40.8
-
6
5
7
3
3
100
99
97:3:0
97:3:0
97:3:0
99:1:0
98:2:0
General methods and reagents
Unless otherwise indicated, all operations were carried out
under dry argon, using standard Schlenk techniques. All
syntheses and catalytic tests were performed in an open
system. Closed system was used only for carrying out the
reaction at temperatures exceeding the boiling point of
substrates. Reagents were purchased from commercial
sources and, unless otherwise indicated, were used without
further purification. 1H NMR and 13C NMR spectra were
recorded in CDCl3 on a Varian 400 operating at 402.6 and
101.2 MHz, respectively. GC analyses were carried out on a
Bruker Scion 436-GC (column: DB-5 30 m I.D. 0.53mm)
equipped with a TCD. The GC/MS analyses were performed on
a Varian Saturn 2100T equipped with DB-5, 30m capillary
column and Ion Trap detector. Thin layer chromatography
(TLC) was recorded on plates coated with 250 mm thick silica
gel and column chromatography was performed on silica gel
60 (70–230 mesh) using hexane/dichloromethane. Synthesis
46.6
50.9
52.8
99
1
2
201.6
213.4
100
100
Reaction conditions: toluene, 40 °C, [silane]:[acetylene] = 1:1; cat. 0.05
mol%; [a] Determined by GC analysis; [b] β-(E):β-(Z):α determined by GC analysis
Moreover, catalytic tests carried out under the conditions
described in ref. 13f for exemplary hydrosilylation of terminal
and internal acetylene in the presence of complexes 1, 2 and
15 indicate similar dependence of selectivity on steric
properties of the catalysts (Tables S6 and S7).
In all the tests performed, under the reaction conditions
used, no generation of platinum nanoparticles was observed.
The heterogeneity problems of the hydrosilylation conducted
in the presence of N-heterocyclic carbene platinum complexes
has not been studied. Therefore, the mercury poisoning
experiment [20] has been performed for the hydrosilylation of
and characterization of complexes [Pt(IPr*)(dvtms)] (
[Pt(IPr*OMe)(dvtms)]
1) and
phenyl propargyl ether (3j) with dimethylphenylsilane (4a
)
(2)
was reported in Supporting
occurring in the presence of complex 1. The obtained result
Information. All solvents used for the synthesis of the
complexes were dried prior to use over CaH2 and stored under
argon. DCM was additionally passed through a column with
alumina, and after that it was degassed by repeated
freeze−pump−thaw cycles.
unexpectedly indicates a partial inhibition of the reaction by
mercury excess (Figure S1). Hot filtration test [21] performed
for the same reaction indicates retaining the catalytic activity
of the filtrate and lack of any catalytic activity of the residue
(Figure S2). Due to the known limitations of methods used for
resolving the heterogeneity problems in homogeneous
catalysis [22] any unambiguous conclusion cannot be drawn on
the basis of performed tests. Results do not exclude the
possibility of formation of soluble oligonuclear platinum(0)
species and their contribution to catalysis. However, our
attempts to confirm formation of such species by mass
spectrometry have failed.
General procedure for hydrosilylation
A 5 mL glass reactor (open system) or 10 mL high-pressure
Schlenk vessel (closed system) was filled successively with
toluene (1 mL), appropriate olefin (0.19 mmol) or acetylene
(0.19 mmol), silane (0.19 mmol) and internal standard (decane
or dodecane, 20 μL). Then, catalyst
1 was added to solution
(10−5–100 mol%) of Pt (detailed amounts were indicated in the
footnotes below the tables). The reaction mixture was heated
and stirred at 40 °C (hydrosil. of terminal acetylenes), 80 °C
(hydrosil. of olefin) or 90 °C (hydrosil. of internal acetylenes)
until full conversion of Si–H was detected. Conversion of the
substrates and yield of hydrosilylated products was
determined by gas chromatography (GC). Reaction selectivities
were calculated on the basis of the 1H NMR spectra of the
reaction mixture.
From the point of view of the utility in organic and
organometallic synthesis, the results show a number of
advantages. The simple and convenient protocols presented
permit nearly quantitative conversions and highly or fully
selective syntheses of the desired products using low loadings
of catalyst
1 or 2. These catalysts are highly productive (TON
reaches 107 for olefins and 104 for terminal and internal
acetylenes). No generation of platinum colloids was observed
under the reaction conditions used, which helps avoiding the
undesired side reactions. Precatalysts
1 and 2 are insensitive
General procedure for the synthesis of hydrosilylation
products
towards air and moisture and can be stored as solids for
extended periods of time without loss of activity. In contrast to
the less bulky structural analogues, for catalyst
1 and 2 there is
A 25 mL glass reactor equipped with a reflux condenser (open
system) or 10 mL high-pressure Schlenk vessel (closed system)
was charged with toluene (4 mL), silane (2 mmol) and
appropriate alkene (2 mmol) or alkyne (2 mmol). Then
no need to substitute the dvtms ligand by the more labile
diallyl ether to achieve high activity.[16a] In contrast to the
catalysts described by Marko,[13f] complexes
1 and 2 do not
platinum catalyst 1
was added in the amount of 10−5–100 mol%
need preliminary generation of active species. For each of the
reactions tested, i.e. for hydrosilylation of terminal olefin,
of platinum complex, depending on the experiment. The
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Brase, M. Oestreich), Wiley-VCH, 2014, pp. 475; b) K. Miura
and A. Hosomi, Silicon in Organic Synthesis, in Main Group
Metals in Organic Synthesis, Vol. 2 (Eds.: H. Yamamoto, K.
Oshima), Wiley-VCH, 2004, pp. 409; c) M. A. Brook, Silicon in
Organic, Organometallic, and Polymer Chemistry, Wiley,
2000; d) W. P. Weber, Silicon Reagents for Organic Synthesis,
Springer, Berlin, 1983; e) E. W. Colvin, Silicon Reagents in
Organic Synthesis, Academic Press: London, 1988.
a) Organosilicon Materials, (Ed. Grish Chandra), Springer,
2013; b) Silicon Polymers, (Ed. A. M. Muzafarov), Springer
Science & Business Media, 2010.
M. Zaranek, B. Marciniec and P. Pawluć, Org. Chem. Front.,
2016, 3, 1337.
J. Sun and L. Deng, ACS Catal., 2016, 6, 290.
reaction mixture was heated to appropriate temperature
(40°C, 80°C or 90°C) and stirred until full conversion of Si–H
was detected. Then, the solvent was evaporated under
vacuum. The crude product was dissolved in petroleum ether
or dichloromethane (depending on product polarity) and
filtered through silica gel. After evaporation of solvents the
product was characterized by spectroscopic methods.
3
4
Synthesis of (E)-9ia in a preparative scale
A 10 mL high-pressure Schlenk vessel was charged with
toluene (2 mL), (triisopropylsilyl)acetylene (0.43 ml, 1.92×10-3
mol) and dimethylphenylsilane (0.29 ml, 1.92×10-3 mol). The
reaction mixture was warmed up to 40 °C in an oil bath and
5
6
a) M. D. Greenhalgh, D. J. Frank and S. P. Thomas, Adv.
Synth. Catal., 2014, 356, 584; b) M. D. Greenhalgh, A. S.
Jones and S. P. Thomas, ChemCatChem, 2015, 7, 190.
platinum complex
1
(0.25 mg, 1.92×10-7 mol) was added. The
7
8
9
a) J. L. Speier, J. A. Webster and G. H. Barnes, J. Am. Chem.
Soc., 1957, 79, 974; b) J. L. Speier, Adv. Organomet. Chem.,
1979, 407.
a) B. D. Karstedt, US Patent 226928, 1972; b) P. B. Hitchcock,
M. F. Lappert and N. J. W. Warhurst, Angew. Chem. Int., Ed.
Engl., 1991, 30, 438.
reaction was carried out for 24 h. Then the solvent was
evaporated under vacuum and the residue was purified using
column chromatography (silica gel 60/petroleum ether : DCM
= 5 : 1). Evaporation of the solvent gave the analytically pure
product (oil, 0.553 g, 90 %).
L. N. Lewis, J. Stein, Y. Gao, R. E. Colborn and G. Hutchings,
Platinum Met. Rev., 1997, 41, 66.
10 Ch. Dong, Y. Yuan, Y.-M. Cui, Z.-J. Zheng, J. Cao, Z. Xu and L.-
W. Xu, Appl. Organometal. Chem., 2017, DOI:
10.1002/aoc.4037.
11 I. E Markó, S. Sterin, O. Buisine, G. Mignani, P. Branlard, B.
Tinant and J.-P. Declercq, Science, 2002, 298, 204.
Conclusions
By using platinum(0) complexes [Pt(IPr*)(dvtms)] (
[Pt(IPr*OMe)(dvtms)] (
1) and
2
) as catalysts of hydrosilylation of olefins
and acetylenes it was demonstrated that postulated
dependence of selectivities of hydrosilylation processes on
steric bulk of NHC ligand remains true for complexes
containing N-heterocyclic carbene ligands significantly bulkier
12 G. Berthon-Gelloz and I. E. Markó, Efficient and Selective
Hydrosilylation of Alkenes and Alkynes Catalyzed by Novel N-
Heterocyclic Carbene Pt0 Complexes in N-heterocyclic
Carbenes in Synthesis, (Ed. S. P. Nolan), Wiley-VCH, 2006, pp.
119.
13 a) J. W. Sprengers, M. J. Mars, M. A. Duin, K. J. Cavell and C.
J. Elsevier, J. Organomet. Chem., 2003, 679, 149; b) I. E
Markó, G. Michaud, G. Berthon-Gelloz, O. Buisine and S.
Sterin, Adv. Synth. Catal., 2004, 346, 1429; c) O. Buisine, G.
Berthon-Gelloz, J.-F. Briere, S. Sterin, G. Mignani, P. Branlard,
B. Tinant, J.-P. Declercq and I. E. Markó, Chem. Commun.,
2005, 3856; d) G. Berthon-Gelloz, O. Buisine, J.-F. Briere, G.
Michaud, S. Stérin, G. Mignani, B. Tinant, J.-P. Declercq, D.
(Nolan height parameter AH > 200 °) than the previously
described complexes. Complexes efficiently catalyze the highly
regio- and chemoselective hydrosilylation of a wide range of
functionalized terminal olefins, as well as terminal and internal
acetylenes. Moreover, the complexes tested are characterized
by particularly high values of TON reaching 107 for
hydrosilylation of olefins and 104 for hydrosilylation of
acetylenes.
Chapon and I. E. Markó , J. Organomet. Chem., 2005, 690
6156; e) J. J. Dunsford, K. J. Cavell and B. Kariuki, J.
Organomet. Chem., 2011, 696 188; f) S. Dierick, E.
,
,
Vercruysse, G. Berthon-Gelloz and I. E. Markó, Chem. Eur. J.,
2015, 21, 17073.
14 a) P. J. Murphy, J. L. Spencer and G. Procter, Tetrahedron
Lett., 1990, 31, 1051; b) S. E. Denmark and Z. Wang, Org.
Lett., 2001, 3, 1073; c) H. Aneetha, W. Wu and J. G. Verkade,
Acknowledgements
The authors gratefully acknowledge the financial support from
the National Centre for Research and Development (Poland)
(Project No. PBS2/A5/40/2014). P. Żak acknowledges the
financial support from the National Science Centre (Poland)
(SONATA Project No. UMO-2016/23/D/ST5/00417).
Organometallics, 2005, 24, 2590; d) W. Wu, X. Zhang, S. Kang
and Y. Gao, Chinese Chem. Lett., 2010, 21, 312; e) L. Ortega-
Moreno, R. Peloso, C. Maya, A. Suarez and E. Carmona,
Chem. Commun., 2015, 51, 17008.
15 G. De Bo, G. Berthon-Gelloz, B. Tinant and I. E. Markó,
Organometallics, 2006, 25, 1881.
16 a) G. Berthon-Gelloz, J.-M. Schumers, G. De Bo and I. E.
Marko, J. Org. Chem., 2008, 73, 4190; b) M. Ruiz-Varilla, E. A.
Baquero, G. F. Silbestri, C. Gonzalez-Arellano, E. de Jesús and
J. C. Flores, Dalton Trans., 2015, 44, 18360.
17 for characterization of steric properties of NHC ligands see H.
Clavier and S. P. Nolan, Chem. Commun., 2010, 46, 841.
18 G. Berthon-Gelloz, B. de Bruin, B. Tinant and I E. Markó,
Angew. Chem. Int. Ed., 2009, 48, 3161.
19 P. Żak, M. Bołt, J. Lorkowski, M. Kubicki and C. Pietraszuk,
ChemCatChem, 2017, 9, 3627.
Notes and references
1
a) B. Marciniec, J. Gulinski, W. Urbaniak and Z. W. Kornetka,
Comprehensive Handbook on Hydrosilylation, (Ed. B.
Marciniec), Pergamon Press, Oxford, 1992; b) B. Marciniec,
H. Maciejewski, C. Pietraszuk and P. Pawluć, Hydrosilylation:
A
Comprehensive Review on Recent Advances, (Ed. B.
Marciniec), Springer, 2009 c) D. Troegel and J. Stohrer,
Coord. Chem. Rev., 2011, 255, 1440; d) Y. Nakajima and S.
Shimada, RSC Adv., 2015, , 20603.
;
5
2
a) S. E. Denmark and R. F. Sweis, Organosilicon Compounds
in Cross-Coupling Reactions in Metal-Catalyzed Cross-
Coupling Reactions and More, Vol 2 (Eds.: A. de Meijere, S.
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20 P. Foley, R. Dicosimo, G. M. Whitesides, J. Am. Chem. Soc.
1980, 102, 6713.
21 R. A. Sheldon, M. Wallau, I. W. C. E. Arends, U. Schuchardt,
Acc. Chem. Res. 1998, 31, 485.
22 R. H. Crabtree, Chem. Rev. 2012, 112, 1536.
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Platinum(0) complexes bearing bulky N-heterocyclic
carbene ligands, catalyse selective hydrosilylation of
terminal olefins as well as terminal and internal
acetylenes.