Pd/C-catalyzed cross-coupling reaction of benzyloxysilanes with halosilanes for selective synthesis of unsymmetrical siloxanes

Abstract

A new protocol for the nonhydrolytic synthesis of unsymmetrical siloxanes has been developed. The cross-coupling reaction of benzyloxysilanes with halosilanes catalyzed by Pd/C afforded various unsymmetrical siloxanes with co-production of benzyl halides.

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Polysiloxanes (silicones) play important roles in a wide range of industries as irreplaceable materials due to their excellent properties such as thermal, light and oxidation stabilities, high electric insulation properties, high gas permeability, and so on.¹ Precise control of Si–O–Si bond formation is essential for the preparation of structurally well-defined siloxanes. In spite of siloxanes' importance, their synthetic methods are very limited.^1,2 The most important and widely used method is the hydrolysis of compounds containing Si–X (X = halogen, alkoxy, hydrogen, acyloxy, etc.) groups. However, this hydrolytic method is not suitable for selective synthesis of structurally well-defined siloxanes, because self-condensation of silanol intermediates spontaneously takes place under the hydrolytic reaction conditions and is difficult to control. Therefore, nonhydrolytic and highly selective reactions for siloxane synthesis has received significant attentions recently.

About half a century ago, colloidal Ni-, ZnCl₂- and H₂PtCl₆/6H₂O-catalyzed dehydrocoupling reaction of hydrosilanes with silanols to afford unsymmetrical siloxanes was discovered.³ Michalska reported that Rh complexes such as Wilkinson's catalyst were also useful for the dehydrocoupling of hydrosilanes with silanols.⁴ This Rh-catalyzed as well as Pd- and Pt-catalyzed reactions were applied to the synthesis of well-defined siloxane polymers.⁵ However, this method can be used only for stable silanols, such as tertiary silanols. During the last decade, homogeneous Lewis acid-catalyzed cross-coupling siloxane formation reactions under nonhydrolytic conditions have been developed. Piers–Rubinsztajn reaction is a catalytic unsymmetrical siloxane formation reaction taking place between hydrosilanes and alkoxysilanes in the presence of B(C₆F₅)₃ catalyst with simultaneous formation of alkanes.⁶ More recently, Kuroda and co-workers found that BiCl₃ efficiently catalyzes cross-coupling reaction of alkoxysilanes (t-butyloxysilanes and (diphenylmethoxy)silanes) with chlorosilanes to form siloxanes with simultaneous generation of chloroalkanes.⁷ Although these reactions have well applied to the synthesis of variety of siloxanes, a practical problem associated with these homogeneous catalysts is the difficulty of catalyst separation.

Herein we report a novel cross-coupling siloxane formation reaction, namely, nonhydrolytic siloxane synthesis by the reaction of benzyloxysilanes with halosilanes using simple heterogeneous palladium on carbon (Pd/C) catalysts which can be easily separated from the reaction mixture.

Very recently, we reported a Pd/C-catalyzed hydrogenolysis of benzyloxysilanes to the corresponding silanols under nonhydrolytic conditions.⁸ By taking advantage of the nonhydrolytic nature of the reaction, we tried to synthesize unsymmetrical siloxanes by performing the reaction in the presence of halosilanes. Commercially available Pd/Cs are generally sold as moistened with water for safety. Thus, to avoid hydrolysis of benzyloxysilanes and halosilanes, Pd/Cs used in this study were dried by heating at 120 °C for 48 h under vacuum before use.⁹ As expected, the reaction of Ph₃SiOBn (Bn = benzyl) (1) in the presence of 2 equivalents of Me₃SiCl and 10 mol% Pd/C (N. E. CHEMCAT, OH type) under H₂ atmosphere proceeded to give cross-coupled siloxane Ph₃SiOSiMe₃ (2) in almost quantitative yield (Scheme 1). However, during the course of detailed study on the reaction conditions, we unexpectedly found that the cross-coupling reaction of 1 with Me₃SiCl does proceed in the absence of H₂. Actually the reaction under Ar atmosphere quantitatively afforded PhCH₂Cl but not PhCH₃ as a co-product.

Scheme 1 Reaction of 1 in the presence of Me₃SiCl.

Table 1 shows the comparison of catalytic activity of several Pd/Cs purchased from N. E. CHEMCAT toward the cross-coupling reaction of 1 with Me₃SiCl (2 equivalents) in the absence of H₂. Initial attempts by using ASCA-2 type Pd/C, which showed the best result for the deprotection of benzyloxysilanes in the previous studies, proved that it was moderately selective catalyst for this reaction (Table 1, Entry 1).⁸ NX type was much less effective (Table 1, Entry 2), while OH type (Pd(OH)₂ on carbon), so-called Pearlman's catalyst, almost quantitatively afforded the cross-coupling product 2 (Table 1, Entry 3). Simultaneous generation of approximately equimolar amounts of PhCH₂Cl (96%) was confirmed by gas chromatographic analysis of the reaction mixture.

Table 1 Cross-coupling reaction of 1 with Me₃SiCl using various types of Pd/Cs^a

Entry	Pd/C	Conv.^b (%)	Yield^c (%)	Selectivity^d (%)
a Reaction conditions: 1 (0.550 mmol), Me3SiCl (1.10 mmol) and Pd/C (10 mol% Pd metal) in EtOAc (6 mL) were stirred at room temperature for 9 h under Ar.b Conversion of compound 1.c ^1H NMR yield by using hexamethylbenzene as an internal standard.d Selectivity = yield/conversion (%).
1	ASCA-2 (Pd 4.5 wt%, Pt 0.5 wt%)	96	61	64
2	NX type (Pd 5 wt%)	16	2	13
3	OH type (Pd 10 wt%)	100	96	96

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In order to clarify the difference in reactivity depending of the difference of halogens, we examined the reaction of 1 with Me₃SiX (X = Cl, Br and I) (Table 2). Reaction times required for complete consumption of 1 were determined by monitoring the reaction progress by gas chromatography. As expected, the reactivity is in the order Me₃SiCl < Me₃SiBr < Me₃SiI, while Me₃SiI afforded 2 in lower yield (76%) than Me₃SiBr and Me₃SiCl (Table 2, Entries 1–3). Generation of the corresponding benzyl halides was also confirmed.

Table 2 Cross-coupling reaction of 1 with Me₃SiX (X = Cl, Br and I) to produce 2^a

Entry	Me3SiX	Time (h)	Me3SiX/1	Yield^c (%)
a Reaction conditions: 1 (0.550 mmol), Me3SiX (1.10 mmol) and Pd/C (OH type, 10 mol% Pd metal) in EtOAc were stirred at room temperature under Ar.b Me3SiI (1.65 mmol) was used.c Yields were determined by ^1H NMR analysis by using hexamethylbenzene as an internal standard.
1	Me3SiCl	9	2	96
2	Me3SiBr	6	2	98
3	Me3SiI	1	2	76
4^b	Me3SiI	1	3	95

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In this reaction, substituent exchange reaction between the two reactants can take place as a competitive reaction as observed in the related Lewis acid-catalyzed reactions.^7,10 Such exchange reaction did occur in the reaction of 1 with Me₃SiI to give Ph₃SiI and Me₃SiOBn, which cause the formation of homo-coupling by-products, Ph₃SiOSiPh₃ and Me₃SiOSiMe₃ (Table 2, Entry 3). Thus, increasing the amount of Me₃SiI from 2 to 3 equivalents improved the yield of 1 to 95% (Table 2, Entry 4). Similar exchange reaction also took place in the reaction of Me₃SiCl and Me₃SiBr to some extent, but did not affect the yield of 2 (Table 2, Entries 1–2).

It is known that Me₃SiI can cleave alkoxysilanes to form iodoalkanes and disiloxanes in the absence of a catalyst.¹¹ Therefore, necessity of the catalyst was checked by performing the reaction of 1 with Me₃SiX under the reaction conditions of Table 2 without Pd/C (reaction time, 1 day). In the case of Me₃SiI, disiloxane 2 was obtained in 10% yield, while no reaction took place at all for Me₃SiCl and Me₃SiBr. These results imply that Pd/C is indispensable for the siloxane formation in the case of Me₃SiCl and Me₃SiBr.

Although Me₃SiBr and Me₃SiI also led to acceptable yields of product 2, further study was conducted by using Me₃SiCl, because it is more stable, cheaper and readily available than Me₃SiBr and Me₃SiI. As shown in Table 3, scope and limitation of this reaction were examined by using various benzyloxysilanes. Introduction of a bulkier substituent, t-butyl group, to 1 considerably decreased the yield even after longer reaction time (72 h, 91% conversion and 65% yield, Table 3, Entry 2). On the other hand, replacement of one or two Ph groups of 1 with Me groups considerably affected the selectivities partly because of the easiness of homo-coupling reaction following the substituent exchange reaction between benzyloxysilanes and Me₃SiCl (Table 3, Entries 3 and 4). This reaction is well applicable to substrates having two to four benzyloxy groups. High selectivity (92%) was observed in the reaction of Ph₂Si(OBn)₂ to produce Ph₂Si(OSiMe₃)₂ (Table 3, Entry 5). In the case of Me₂Si(OBn)₂, substituent exchange reaction giving Me₂SiCl(OBn), Me₂SiCl₂ and Me₃SiOBn considerably affected the selectivity and afforded the desired product Me₂Si(OSiMe₃)₂ in lower yield (54%, Table 3, Entry 6). Although the reaction of tribenzyloxysilanes were expected to be more complicated, corresponding cross-coupled siloxanes were obtained in good yields irrespective of the size of substituents; PhSi(OBn)₃ and MeSi(OBn)₃ respectively afforded the desired products in 89% and 88% yields (Table 3, Entries 7 and 8). The reaction of Si(OBn)₄ also proceeded smoothly to give Si(OSiMe₃)₄ in high yield (93%, Table 3, Entry 9).

Table 3 Syntheses of cross-coupled siloxanes from various benzyloxysilanes, R_4−nSi(OBn)_n, with Me₃SiCl^a

Entry	Benzyloxysilane	Time (h)	Conv.^b (%)	Yield^c (%)	Selectivity^d (%)
a Reaction conditions: benzyloxysilane (0.550 mmol), Me3SiCl (2n × 0.550 mmol) and Pd/C (OH type, n × 10 mol% Pd metal) in EtOAc were stirred at room temperature (n = numbers of BnO groups in benzyloxysilanes).b Conversion of benzyloxysilanes.c ^1H NMR yield by using hexamethylbenzene as an internal standard. The value in parentheses shows the yield of isolated product.d Selectivity = yield of R4−nSi(OSiMe3)n/conversion (%).e Ph3SiOBn (10.0 g, 27.3 mmol) was used.f Conversion and yield based on benzyloxysilanes were determined by integral value of ^29Si NMR analysis using inverse-gated decoupling pulse sequence with 1,4-bis(trimethylsilyl)benzene as an internal standard.
1	Ph3SiOBn	9	100	Ph3SiOSiMe3 96 (94)^e	96
2	Ph2^tBuSiOBn	72	91	Ph2^tBuSiOSiMe3 65	71
3	Ph2MeSiOBn	9	100	Ph2MeSiOSiMe3 74	74
4	PhMe2SiOBn	9	100	PhMe2SiOSiMe3 54	54
5	Ph2Si(OBn)2	5	100	Ph2Si(OSiMe3)2 92	92
6^f	Me2Si(OBn)2	9	100	Me2Si(OSiMe3)2 54	54
7	PhSi(OBn)3	9	100	PhSi(OSiMe3)3 89	89
8^f	MeSi(OBn)3	3	100	MeSi(OSiMe3)3 88	88
9	Si(OBn)4	6	100	Si(OSiMe3)4 93	93

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The present method can be carried out on a 10 g scale (Scheme 2). That is, with 10.0 g of 1 as a starting material, 8.93 g of 2 was obtained by Kugelrohr distillation of the crude mixture after the removal of the catalyst by simple filtration through Celite (94% yield), although longer reaction time was needed.

Scheme 2 A large-scale synthesis of compound 2.

In conclusion, we developed a nonhydrolytic Pd/C-catalyzed cross-coupling siloxane formation reaction. Various cross-coupled siloxanes were successfully synthesized by the reaction of benzyloxysilanes with Me₃SiCl. Easiness of catalyst removal from the reaction mixture by centrifugation or filtration is one of advantageous features of this reaction over the previously reported cross-coupling type siloxane synthesis catalyzed by homogeneous catalysts. This feature would be particularly important in the case of polymeric materials. Application of this reaction to polymer synthesis as well as mechanistic studies are underway in this laboratory.

This work was supported by the Future Pioneering Projects “Development of Innovative Catalytic Processes for Organosilicon Functional Materials” from the Minister of Economy, Trade and Industry, Japan (METI).

Notes and references

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9. Caution! Dried Pd/Cs are potentially flammable in air. Keep the reaction mixture under an inert atmosphere and moisten Pd/Cs with water for safety after the reaction. Ignoring safety precautions can lead to fire!.
10. (a) J. Chojnowski, W. Fortuniak, J. Kurjata, S. Rubinsztajn and J. A. Cella, Macromolecules, 2006, 39, 3802; (b) J. Chojnowski, S. Rubinsztajn, W. Fortuniak and J. Kurjata, Macromolecules, 2008, 41, 7352.
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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra02126f

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