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Highly Selective Synthesis of Hydrosiloxanes by Au-Catalyzed Dehydrogenative Cross-Coupling Reaction of Silanols with Hydrosilanes Yasushi Satoh, Masayasu Igarashi, Kazuhiko Sato, and Shigeru Shimada ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03560 • Publication Date (Web): 27 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017
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ACS Catalysis
Highly Selective Synthesis of Hydrosiloxanes by Au-Catalyzed Dehydrogenative Cross-Coupling Reaction of Silanols with Hydrosilanes Yasushi Satoh, Masayasu Igarashi, Kazuhiko Sato, and Shigeru Shimada* Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ABSTRACT: We report a highly selective synthesis of siloxane building blocks containing SiH2 or SiH functionalities. AuCl(PPh3)/PPh3 or AuCl(PPh3)/PnBu3 system catalyzed the reaction of trihydrosilanes with silanols giving SiH2containing siloxanes exclusively. On the other hand, a highly selective reaction of dihydrosilanes with silanols to afford SiH-containing siloxanes was achieved by simply changing the phosphine ligand to a bidentate one, xantphos. Usefulness of SiH2-containing siloxanes was demonstrated by the synthesis of a trisiloxane, Et3SiOSi(Ph)(H)OSitBuMe2, and a pentasiloxane, Ph2Si(OSiHPhOSiEt3)2, bearing SiH functionalities.
KEYWORDS : Hydrosiloxanes, Hydrosilanes, Silanols, Au-Catalyst, Cross-Coupling Siloxane materials (silicones) have a number of excellent properties including high thermal stability, light stability and transparency, high gas permeability, electrical insulation property, and constancy of properties over a wide temperature range.1,2 Therefore, oligo- and polysiloxanes are used as irreplaceable materials in a wide range of fields. For the further development of high performance siloxane materials, precise structural control of oligo- and polysiloxanes is indispensable. However, synthetic methods for siloxane compounds are surprisingly unexplored, and it is difficult to synthesize siloxane materials with well-defined structures by the conventional methods, hydrolytic condensation reaction of chlorosilanes or alkoxysilanes (sol-gel process) and base- or acid-catalyzed ring-opening polymerization of cyclic oligosiloxanes.3 Although some new synthetic methods for siloxane compounds have recently been developed, accessible structures are still very limited.4-12 For the construction of siloxanes with a well-defined structure, usage of siloxane building blocks with appropriate functional groups would be promising. Si-H bonds are among the most useful functional groups in silicon chemistry. A wide range of reactions has been developed for the transformation of the Si-H bonds.13,14 Therefore, siloxane compounds containing Si-H bonds (hydrosiloxanes) are highly useful building blocks for the synthesis of various siloxane compounds. Furthermore, hydrosiloxanes are useful for the incorporation of siloxane moieties in organic and inorganic compounds using the reactivity of the Si-H bonds.15,16 Recent reports showed that the incorporation of siloxane moiety improved the materials properties such as a hole mobility of thin film transistors,
lowering the melting points of triarylamines, and increasing the solubility of carbon nanotubes.16 Among hydrosiloxanes, those containing SiH2 moiety (dihydrosiloxanes) would be particularly useful for the construction of linear and branched oligo- and polysiloxanes with welldefined structures by the stepwise transformation of the two Si-H bonds. Selective transformation of one of the two Si-H bonds in dihydrosilanes has been well established.14 However, examples of dihydrosiloxanes are very limited because of the lack of simple and reliable synthetic procedures. Known examples are mostly symmetric siloxanes with two SiH2 groups, (RH2Si)2O, which can be synthesized by the hydrolytic condensation of RH2SiX (X = Cl or Br).17 The dehydogenative coupling reaction of silanols (R13SiOH) with trihydrosilanes (R2SiH3) would be a suitable reaction for the synthesis of dihydrosiloxanes R13SiOSiH2R2. However, there is no report for the selective synthesis of dihydrosiloxanes by this reaction. Michalska reported Rh complex-catalyzed reaction of silanols with hydrosilanes, in which the reaction of a trihydrosilane, PhSiH3, with silanols R3SiOH (R = Et or nPr) gave a mixture of dihydrosiloxanes PhSiH2OSiR3, monohydrosiloxanes PhSiH(OSiR3)2, and PhSi(OSiR3)3 even when the reaction was done with PhSiH3/R3SiOH = 1/1.4a In this paper, we report a highly selective synthesis of dihydrosiloxanes by dehydrogenative cross-coupling reaction of silanols with hydrosilanes catalyzed by Auphosphine complexes. Selective synthesis of monohydrosiloxanes was also achieved by simply changing the phosphine ligand.
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Table 1. Screening of Catalysts and Reaction Conditions for the Cross-Coupling Reaction of Silanols 1 with Phenylsilane 2aa
R3Si-OH
+
1a: R = Et 1b: R = Me
H H Si Ph H
Catalyst (mol%) Ligand (mol%) Solvent rt, 13 h
2a
R3Si
O
H Ph Si H
+
R3Si
3aa: R = Et 3ba: R = Me
O
Ph O Si SiR3 H
+
4aa: R = Et 4ba: R = Me
R3Si
O
5a: R = Et 5b: R = Me
Yield (%) entry c
1
SiR3
b
1
Catalyst (mol%)
Ligand (mol%)
Solvent
3
4
5
1a
RhCl(PPh3)3 (2.5)
none
THF
6
51
0
d ,e
1a
Pt( Bu)2 (2.5)
none
THF
11
63
0
d
1a
[RuCl2(p-cymene)]2 (2.5)
none
THF
7
73
0
2 3
4
f
t
1a
BiCl3 (2.5)
none
THF
14
0
0
5
1a
B(C6F5)3 (2.5)
none
THF
15
0
82
6
1a
AuCl(PPh3) (2.5)
none
THF
36
0
0
7
1a
AuCl(PPh3) (2.5)
PPh3 (2.5)
THF
97(84)
0
0
8
d
1a
none
PPh3 (2.5)
THF
0
0
0
9
g,
1b
AuCl(PPh3) (2.5)
PPh3 (2.5)
THF
83
0
17
10
g,
n
THF
85
0
15
n
THF
91(76)
0
3
n
Toluene
92
0
5
n
CH2Cl2
69
0
8
n
DMAc
71
6
22
n
DMSO
42
0
48
n
THF
99
0
0
1b
AuCl(PPh3) (2.5)
P Bu3 (2.5)
g
1b
AuCl(PPh3) (2.5)
P Bu3 (7.5)
12
g
1b
AuCl(PPh3) (2.5)
P Bu3 (7.5)
13
g
1b
AuCl(PPh3) (2.5)
P Bu3 (7.5)
14
g,
1b
AuCl(PPh3) (2.5)
P Bu3 (7.5)
15
g
1b
AuCl(PPh3) (2.5)
P Bu3 (7.5)
16
h
1a
AuCl(PPh3) (0.025)
P Bu3 (0.5)
11
a
Reaction conditions: 1 (0.5 mmol), 2a (0.5 mmol), and catalyst (2.5 mol% based on 1) in THF (1 mL) at 25 ºC for 13 h under Ar. 29 Yields are based on 1 and were determined by integral values of Si NMR signals using PhSiMe3 as an internal standard. The c d values in parentheses are isolated yields. Ph2SiH2 (14%), PhSi(OSiEt3)3 (4%), and Et3SiCl (2%) were also obtained. Reaction e f g h time, 4 h. Ph2SiH2 (9%) was also obtained. Et3SiCl (10%) was also obtained. 2a (1 mmol) was used. Reaction conditions: 1a (2 n mmol) and 2a (2 mmol), AuCl(PPh3) (0.025 mol%), P Bu3 (0.5 mol%) in THF at 25 ºC for 48 h under Ar. b
A model reaction between triethylsilanol (1a) and phenylsilane (2a) was examined with 1a/2a = 1/1 ratio using various catalysts in THF at room temperature. Representative results are shown in Table 1. As reported, RhCl(PPh3)34a catalyzed the reaction unselectively giving PhSiH2OSiEt3 (3aa), PhSiH(OSiEt3)2 (4aa) and PhSi(OSiEt3)3 as well as a redistribution product of 2a, Ph2SiH2, and Et3SiCl (Table 1, entry 1). Other late transition metal complexes Pt(tBu)2 and [RuCl2(p-cymene)]2, the latter is active for the dehydrogenative silylation of alcohols and carboxylic acids with hydrosilanes,18 similarly catalyzed the reaction to give 4aa as a major product together with 3aa as a minor product (Table 1, entries 23). In the case of Pt(tBu)2, a small amount of Ph2SiH2 was also produced. BiCl3 also catalyzed this reaction to give 3aa in a low yield together with Et3SiCl as a side product (Table 1, entry 4). B(C6F5)3, which is known to catalyze the dehydrogenative coupling of silanols with hydrosilanes,6 mainly catalyzed the self-condensation of 1a to give 5a in 82% yield (Table 1, entry 5). Then we examined Au complexes; Ito and co-workers first reported the Au/phosphine catalysis for hydrosilyla-
tion and alcohol silylation reaction with hydrosilanes.19,20 This chemistry was also perused by other researchers.21 Fortunately we found that AuCl(PPh3) selectively gave 3aa in 36% yield without any by-product (Table 1, entry 6). Significant improvement of the yield was attained by the addition of PPh3 (1 equiv to AuCl(PPh3)) resulting in the exclusive formation of 3aa in 97% yield (Table 1, entry 7).22 PPh3 alone without AuCl(PPh3) did not catalyze the reaction at all (Table 1, entry 8). On the other hand, the reaction of 2a with Me3SiOH (1b), which is less sterically bulky and easier to self-condense than 1a, afforded not only the desired product 3ba but also 17% of hexamethyldisiloxane (5b) even with 2 equiv of 2a (Table 1, entry 9). Therefore, further optimization of the catalyst was performed; the addition of trialkylphosphines such as PnBu3 instead of PPh3 slightly improved the selectivity for 3ba over 5b (Table1, entry 10), and increasing the amount of PnBu3 to 3 equiv (to Au) further improved the yield of 3ba to 91% with only 3% of 5b (Table 1, entry 11). Reaction solvents significantly affected the selectivity between 3ba and 5b; THF and toluene gave almost similar results (Table 1, entries 11, 12), while other solvents such as CH2Cl2,
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N,N-dimethylacetamide (DMAc), and dimethylsulfoxide (DMSO) considerably decreased the yield of 3ba with increased amounts of 5a (Table 1, entries 13-15). PnBu3 is also effective for the reaction of 1a with 2a; with 20 equiv (to Au) of PnBu3, the catalyst loading could be reduced to 0.025 mol% giving 3aa almost quantitatively after 48 h, and the TON reached ca. 4000 (Table 1, entry 16). The increase of PnBu3/Au ratio probably prevents the catalyst decomposition and metal aggregation.19,21c,22 Table 2. Substrate Scope of Au-Catalyzed CrossCoupling Reaction of Silanols with Trihydrosilanesa
[Si](OH)n
+
n RSiH3
1 n = 1-3 O
Et3Si
H Si
Au Complex PPh3 or PnBu3
Ph
Me3Si
O
H Si
H
O
Ph3Si
Ph
i
Pr3Si
H Si
H Si
Ph
3
Ph
BuMe2Si
O
H
H Si
H
Ph
O
Ph3Si
H
O Si Si Me H
H Si
Si O
Ph
3da 85% H
n
Si
Hex
H
3fa 80%
Me
Si
H
(tBuO)3Si
Me
H
O
t
3ca 76%
3ea 89%
H
O
H
3bab 76%
3aa 84%
[Si](OSiH2R)n
THF, rt – 70oC, 13 h
2
3eb 76% n
Hex
H
H Si
O
H
Ph Si Ph
Me
O
Hn Si
Hex
H
3hbc 90%
3gcc 80% OSiH2Ph OSiH2Ph R O Si Si O Si R O O O OSiH2Ph O Si O Si Si Si O O R R R R R
H nHex Si H H O Hn n O O Hex Hex Si Si Si H
Ph
3ibd 95%
H
R=
3jad 93%
N
H Ph O Si Si Me2 H
phenylenebis(dimethylsilanol) (1g) and diphenylsilanediol (1h) were cleanly silylated respectively with cyclopentylsilane (2c) and 2b giving 3gc and 3hb in high yields. Furthermore, all three silanol groups in phenylsilanetriol (1i) as well as in 1,3,5,7,9,11,14- heptaisobutyltricyclo[7.3.3.15,11]heptasiloxane-endo-3,7,14-triol (1j) were successfully reacted with 2b and 2a, respectively, to give the desired silylated products 3ib and 3ja in excellent isolated yields. It is noteworthy that silanol 1k having (Ntert-butoxycarbonylpyrrolyl) group was also participated in this reaction, and the corresponding dihydrosiloxane 3ka was obtained in 75% isolated yield as a sole product. We further investigated the potential of Au catalysts for the selective transformation of dihydrosilanes to monohydrosiloxanes. A model reaction of 1a with diphenylsilane (6a) giving monohydrosiloxane 7aa was examined using various phosphine ligands in the presence of AuCl(PPh3). Representative results are summarized in Table 3. As expected from the above results, in which further reaction of dihydrosiloxanes 3 with silanols did not proceed, the AuCl(PPh3)/PR3 catalyst system did not catalyze the reaction of 1a with 6a (Table 3, entries 1-2). Changing the phosphine ligand from a monodentate PR3 to a bidentate ligand such as 1,2bis(diphenylphosphino)ethane (dppe) or 1,1’bis(diphenylphosphino)ferrocene (dppf) was not effective at all (Table 3, entries 3-4). Ito and Sawamura reported that AuCl(xantphos) complex (xantphos = 4,5bis(diphenylphosphino)-9,9-dimethylxanthene) efficiently catalyzed alcohol silylation with monohydrosilanes.20a,20b Table 3. Screening of Catalysts and Reaction Conditions for the Cross-Coupling Reaction of 1a with Diphenylsilane 6aa
Boc
3kae,f 75% b
yield (%)
a
Reaction conditions: 1 (3 mmol), 2 (3 mmol), AuCl(PPh3) n or AuCl (1 - 2.5 mol% based on 1), and PPh3 or P Bu3 (2.5 - 7.5 mol% based on 1) in THF at 25 - 70 ºC for 13 h under Ar. Detailed reaction conditions are shown in the supporting inb c formation. 2 (6 mmol) was used. 1 (2 mmol) and 2 (12 d e mmol) were used. 1 (2 mmol) and 2 (20 mmol) were used. 1 f (2 mmol) and 2 (4 mmol) were used. Boc = tertbutoxycarbonyl.
The scope of the Au-catalyzed dehydrogenative crosscoupling reaction of silanols 1 with trihydrosilanes 2 is shown in Table 2. Various monosilanols, including trialkylsilanols 1a, 1b, iPr3SiOH 1c, and tBuMe2SiOH 1d, triarylsilanol Ph3SiOH 1e, and trialkoxysilanol (tBuO)3SiOH 1f, successfully reacted with 2a to give dihydrosiloxanes 3aa-3fa in 76-89% isolated yields. Hexylsilane (2b) also reacted with 1e to give dihydrosiloxane 3eb in 76% isolated yield. Two silanol groups in 1,4-
entry
Au-Complex
ligand
7aa
5a
1
AuCl(PPh3)
none
0
0
2
AuCl(PPh3)
PPh3
3
0
3
AuCl(PPh3)
dppe
0
0
4
AuCl(PPh3)
dppf
trace
0
5
AuCl(PPh3)
xantphos
97(82)
3
6
AuCl(PCy3)
xantphos
68
12
7
AuCl
xantphos
57
4
a
Reaction conditions: 1a (0.5 mmol), 6a (0.5 mmol), AuCl(PPh3) (0.0125mmol), and a ligand (0.0125 mmol) in THF b (1 mL) at 25 ºC for 13 h under Ar. Yields were determined by 29 integral values of Si NMR signals using PhSiMe3 as an internal standard. The value in parentheses is an isolated yield.
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We found that the AuCl(PPh3)/xantphos system is active for the reaction of 1a with 6a to selectively give 7aa with a small amount of 5a (Table 3, entry 5). As the catalyst precursor, usage of AuCl(PCy3) or AuCl instead of AuCl(PPh3) considerably decreased the yield of 7aa (Table 3, entries 6-7). Table 4 shows the scope of the AuCl(PPh3)/xantphos system for the selective synthesis of monohydrosiloxanes. Silanols 1c and 1d respectively reacted with diethylsilane (6b) and methylphenylsilane (6c) to give monohydrosiloxanes 7cb and 7dc in high yields. Silanediol 1h and triol 1i also efficiently reacted with 6b to selectively afford 7hb and 7ib in high yields. Table 4. Substrate Scope of the Au-Catalyzed CrossCoupling Reaction of Silanols with Dihydrosilanesa
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perature for 8 h (Scheme 2, eq.2). Both compounds 8 and 9 possess SiH functionalities and are useful building blocks for further transformations. So far there are very limited examples for the selective synthesis of functionalized oligosiloxanes.11,23 As demonstrated by the synthesis of 8 and 9, dihydrosiloxanes 3 are highly useful building blocks for the synthesis of a variety of organosilicon compounds including oligo- and polysiloxanes.24 Ito and co-workers reported a mechanistic study for the AuCl(xantphos)-catalyzed silylation of alcohols, where chlorosilanes produced by the reaction of AuCl(xantphos) and hydrosilanes reacted with alcohols to form alkoxysilanes.20b In order to confirm the possibility of the similar mechanism in our system, we performed the reaction of 2a with AuCl(PPh3)/PnBu3. However, almost no reaction occurred and the formation of chlorophenylsilane was not observed at all by 29Si NMR. Therefore, a different mechanism should be operated at least in the AuCl(PPh3)/PnBu3-catalyzed reactions. Scheme 1. A gram-scale synthesis of dihydrosilane 3aa
a
Reaction conditions: 1 (3 mmol), 6 (3 mmol), AuCl(PPh3) (2.5 mol% based on 1), and xantphos (2.5 mol% based on 1) in THF at 25 - 60 ºC for 13 h under Ar. Detailed reaction condib tions are shown in the supporting information. 6b (6 mmol), AuCl(PPh3) (2 mol%), and xantphos (2 mol%) were c used. 1i (1.5 mmol), 6b (15 mmol), AuCl(PPh3) (4 mol%), and xantphos (4 mol%) were used.
The developed Au-catalyzed cross-coupling reaction could easily be performed on a gram-scale as demonstrated by the synthesis of 3aa (Scheme 1). In this case, PnOct3 was used instead of PnBu3 for easier separation of the product 3aa from the phosphine ligand by vacuum distillation. With 1 mol% Au-catalyst, 20 mmol of 1a was reacted with 2a to produce 4 g of 3aa. In order to demonstrate the usefulness of dihydrosiloxane products 3, synthesis of a trisiloxane 8 and a pentasiloxane 9 from 3aa was examined using AuCl(PPh3)/xantphos- or B(C6F5)3-catalyzed reaction (Scheme 2). Trisiloxane 8 was selectively obtained in 86% yield by the reaction of 3aa (1 mmol) and 1d (1 mmol) in the presence of AuCl(PPh3) (0.025 mmol) and xantphos (0.025 mmol) in THF at 70 oC (Scheme 2, eq. 1). On the other hand, pentasiloxane 9 was obtained in 96% yield through a selective mono-substitution of one of the two Si-H bonds in two 3aa molecules by the reaction of 3aa (1 mmol) and dimethoxydiphenylsilane (0.5 mmol) in the presence of B(C6F5)3 (0.05 mmol) in toluene at room tem-
Scheme 2. Synthesis of trisiloxane 8 and pentasiloxane 9 from dihydrosiloxane 3aa
In summary, Au-catalyzed dehydrogenative crosscoupling reaction of silanols with hydrosilanes has been developed to give dihydrosiloxanes as well as monohydrosiloxanes with excellent selectivities. The resulting hydrosiloxanes are highly useful building blocks for the synthesis of a variety of organosilicon compounds as demonstrated by the synthesis of the functionalized trisiloxane and pentasiloxane. Further investigation with regard to the reaction mechanism and applications of this reaction is currently in progress.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:
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AUTHOR INFORMATION Corresponding Author *E-mail for S.S.:
[email protected] ACKNOWLEDGMENT This work was supported by the "Development of Innovative Catalytic Processes for Organosilicon Functional Materials" project (PL: K.S.) from the New Energy and Industrial Technology Development Organization (NEDO).
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Table 1. Screening of Catalysts and Reaction Conditions for the Cross-Coupling Reaction of Silanols 1 with Phenylsilane 2a 179x27mm (300 x 300 DPI)
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Table 2. Substrate Scope of Au-Catalyzed Cross-Coupling Reaction of Silanols with Trihydrosilanes 116x160mm (300 x 300 DPI)
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Table 3. Screening of Catalysts and Reaction Condi-tions for the Cross-Coupling Reaction of 1a with Diphenylsilane 6a 108x27mm (300 x 300 DPI)
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Table 4. Substrate Scope of the Au-Catalyzed Cross-Coupling Reaction of Silanols with Dihydrosilanes 114x67mm (300 x 300 DPI)
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Scheme 1. A gram-scale synthesis of dihydrosilane 3aa 111x28mm (300 x 300 DPI)
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Scheme 2. Synthesis of trisiloxane 8 and pentasilox-ane 9 from dihydrosiloxane 3aa 112x59mm (300 x 300 DPI)
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83x44mm (300 x 300 DPI)
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