Silver‐Catalyzed Transmetalation between Chlorosilanes and Aryl and Alkenyl Grignard Reagents for the Synthesis of Tetraorganosilanes (2008)

The silver savior: Nucleophilic substitution reactions of chlorosilanes with aryl Grignard reagents have been developed which take place under silver catalysis to afford tetraorganosilanes (see scheme). This transformation is likely to be promoted by diarylargentate reagents that are generated in situ. The nucleophilic substitution reaction of chlorosilanes with organometallic reagents is a fundamental method used for forming carbon–silicon bonds.1 The reactions of chlorotriorganosilanes with organolithium reagents generally proceed smoothly at low temperatures (−78 °C). On the other hand, reactions with the less reactive, yet readily available, organomagnesium reagents often require prolonged reaction times and high temperatures (solvent b.p.), and result in moderate yields of tetraorganosilanes.2 Herein we report that silver salts can catalyze the reactions of chlorotriorganosilanes with organomagnesium reagents to yield a variety of tetraorganosilanes efficiently; this reveals a new aspect of silver catalysis.3–5 Treatment of chlorodimethylphenylsilane (1 a) with p-tolylmagnesium bromide in the presence of a catalytic amount of silver nitrate in THF at 20 °C for 1.5 h provided the corresponding tetraorganosilane 2 a in 93 % yield (Table 1, entry 2).6 The transformation is regarded as a silver-catalyzed transmetalation reaction between chlorosilane and the Grignard reagent. Notably, 2 a was obtained in only 13 % yield in the absence of silver nitrate (Table 1, entry 1). Other silver salts, such as silver halides, acetate, and triflate, accelerated the carbon–silicon bond formation (Table 1, entries 3–7). Other Group 11 metal halides, such as copper(I) bromide and gold(I) chloride, also promoted the reaction with only slightly lower efficiency (Table 1, entries 8 and 9). Nickel and palladium salts failed to catalyze the reaction (Table 1, entries 10 and 11). Entry Catalyst Yield [%][a] Entry Catalyst Yield [%][a] 1 none 13 7 AgOTf 83 2 AgNO3 93 (92)[b] 8 CuBr 75 3 AgCl 86 9 AuCl 78 4 AgBr 91 10 NiCl2 24 5 AgI 86 11 Pd(OAc)2 9 6 AgOAc 91 The Grignard reagent scope was studied and the results are summarized in Table 2.7 The reactions with 2-naphthyl, p-fluorophenyl, p-methoxyphenyl, and p-(triisopropylsiloxy)phenyl Grignard reagents (Table 2, entries 1–4) proceeded as smoothly as those described in Table 1. Sterically hindered o-tolylmagnesium bromide was less reactive, and a prolonged reaction time was essential for the reaction to proceed to completion (Table 2, entry 5). An aryl Grignard reagent having an electron-withdrawing trifluoromethyl group reacted with chlorosilane 1 a slowly in the presence of silver nitrate (Table 2, entry 6). A bulky alkenylmagnesium reagent also participated in the reaction (Table 2, entry 7).8 Unfortunately, attempts to introduce an alkyl group failed because of the instability of the alkylsilver species (Table 2, entry 8). Entry R t [h] Product Yield [%][a] 1 2-naphthyl 1.5 2 b 71 2 p-FC6H4 1.5 2 c 92 3 p-MeOC6H4 1.5 2 d 97 4 p-(iPr3SiO)C6H4 1.5 2 e 96 5 o-MeC6H4 24 2 f 88 6 m-CF3C6H4 20 2 g 80 7 CH2C(SiMe3) 4.5 2 h 78 8 iPr 1.5 2 i 0 The reactions of bulkier chloromethyldiphenylsilane and chlorotriethylsilane with p-tolylmagnesium bromide under silver catalysis proceeded to completion after extended reaction times (Table 3, entries 1 and 2). Chlorosilanes having an olefinic moiety (Table 3, entries 3 and 4) or a chloromethyl moiety (Table 3, entry 5) reacted without any observable side reactions. Notably, the reaction could be performed on a scale as large as 50 mmol (with respect to 1 f). Sterically congested chlorotriisopropylsilane failed to react (Table 3, entry 6). Entry Chlorosilane, R t [h] Product Yield [%] 1 1 b, MePh2Si 10 3 a 87 2 1 c, Et3Si 9.5 3 b 73 3 1 d, (CH2CHCH2)Me2Si 11 3 c 80 4 1 e, (CH2CH)Ph2Si 18 3 d 74 5 1 f, (ClCH2)Me2Si 3 3 e[a] 74 (79)[b] 6 1 g, iPr3Si 10 3 f trace Based on our results [Eqs. (2)–(6)], we propose the reaction mechanism depicted in Scheme 1. Initially, diarylargentate species 8 would be generated in situ,11 followed by nucleophilic attack of the argentate complex upon the chlorosilane to afford silicate 9 or 9′ bearing a SiAgIII bond.12 Reductive elimination from 9 or 9′ would be slow12 and 9 or 9′ could undergo pseudorotation,9 resulting in the loss of the initial stereochemistry. After the scrambling, reductive elimination from 9′′ would occur to afford silicate 10 with concomitant formation of arylsilver 11. Silicate 10 would liberate a chloride ion to afford the product. By the action of the aryl Grignard reagent, 11 would be converted into the initial argentate 8. A proposed mechanism. Intermediates 9′′ and 10 should have trigonal-bipyramidal structures, although the position of each substituent is not clear. In summary, the protocol described here provides a mild and efficient method for the preparation of tetraorganosilanes, and will be applicable to the synthesis of organosilicon reagents and organosilicon-based advanced materials. There are many effective reactions that employ combinations of organomagnesium reagents and copper catalysts. On the other hand, little is known about the useful reactivity of organomagnesium reagents using silver catalysis.4a, 5 The present reaction will open up new possibilities for silver-catalyzed reactions with organometallic reagents. Typical procedure for silver-catalyzed reactions: Chlorodimethylphenylsilane (1 a, 85 mg, 0.50 mmol) in THF (5 mL) was added to AgNO3 (4.2 mg, 0.025 mmol) under argon, followed by 4-methylphenylmagnesium bromide (1.0 M solution in THF, 0.75 mL, 0.75 mmol). The reaction mixture was stirred at 20 °C for 1.5 h before a saturated aqueous solution of NH4Cl (2 mL) was added. The mixture was extracted with ethyl acetate and the combined organic extracts were dried over Na2SO4 and concentrated in vacuo. Purification of the residue by column chromatography on silica gel (n-hexane) afforded dimethyl(4-methylphenyl)phenylsilane (2 a, 104 mg, 0.46 mmol) in 92 % yield (Table 1, entry 2). Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2008/z801949_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.