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148 Science of Synthesis 4.4 Silicon Compounds position is stabilized. It is therefore not surprising that the method has found use for the preparation of Æ-aryl-Æ-haloalkylsilanes. There are no examples of the synthesis of Æ-iodoalkylsilanes by direct combination of a silane with iodine. There is one example of the synthesis of Æ-fluoromethylsilanes from the reaction of tetramethylsilane and fluorine. [12] This is unlikely to be a useful method with any general applicability. However, the Grignard reagents derived from chloromethyl-substituted silanes react with the electrophilic fluorinating agent N-fluoropyridinium triflate to give fluoromethyl-substituted silanes. [13] 4.4.27.1.1 Variation 1: Chlorination of Alkylsilanes There are many reports of the chlorination of simple alkylsilanes. For example, the transformation of tetramethylsilane (1) into (chloromethyl)trimethylsilane (2) is effected by the action of chlorine under photochemical conditions. [14] The yield of the reaction in carbon tetrachloride is not high, in common with other examples of this type of reaction. However, significantly higher yields are obtained when the photochemical reaction is performed in the gas phase. [15] The reaction has been used for the synthesis of (Æ-chloroethyl)triethylsilane from tetraethylsilane. [16] The radical chlorination of alkylsilanes is also effected by sulfuryl chloride in the presence of a peroxide initiator (usually dibenzoyl peroxide). The reaction is mostly limited to tetraalkylsilanes and triarylalkylsilanes or those <strong>only</strong> bearing a methyl group. [17] Disilanes also undergo the reaction (e.g., 3 fi 4) (Scheme 2). The reaction of trihaloalkylsilanes [14] and dihaloalkylsilanes [18] is complicated by substitution at other positions in the alkyl chain. Indeed, trichloroalkylsilanes undergo substitution preferentially at the â-position. As expected for a radical reaction, an Æ-aryl group accelerates the reaction and allows selective Æ-chlorination of benzylic silanes. [14,19] Scheme 2 Chlorination of Alkylsilanes [11,14] Me4Si Cl2, hν 74% Me3Si Cl 1 2 Cl Me2Si SiMe2 SO2Cl2, (PhCO)2O2 23% Cl Si Cl 3 4 Cl Me SiMe2 Cl 1,2-Dichloro-1-(chloromethyl)-1,2,2-trimethyldisilane (4); Typical Procedure: [11] 1,2-Dichloro-1,1,2,2-tetramethyldisilane (3; 40 g, 214 mmol) and benzoyl peroxide (0.1 g) were placed in a 100-mL three-necked flask equipped with a reflux condenser and pressure-equalizing dropping funnel. The mixture was heated to 80–85 8C and SO 2Cl 2 (43.5 g, 320 mmol) added dropwise through the funnel over a period of 30 min. The mixture was heated for a further 3 h. Additional SO 2Cl 2 (20 g, 150 mmol) was added and the mixture was heated for another 5 h. After flash distillation, fractionation through a short column packed with glass helices gave, in addition to unchanged starting material 3 (yield: 14 g; bp 68–69 8C/50 Torr), the Æ-chlorodisilane 4; yield: 11 g (23%); bp 84–858C/20 Torr. 4.4.27.1.2 Variation 2: Bromination of Alkylsilanes The bromination of alkylsilanes has not found widespread use as a general route to Æ-bromoalkylsilanes. A rare example is the efficient bromination of (isopropyl)trimethylsilane (5 fi 6). [20] The related bromination of (isopropyl)dimethylsilane clearly shows that a hy-
4.4.27 Æ-Haloalkylsilanes 149 drosilyl group is oxidized to silyl bromide under the same conditions (7 fi 8) (Scheme 3). [21] Difluorosilanes undergo the reaction without complication. [22] An Æ-methoxymethyl group is cleanly transformed into a Æ-bromo-Æ-methoxymethyl group. [23,24] The bromination of benzyltrialkylsilanes is synthetically useful. [25–28] Numerous reports detail the efficient reaction of bromine or N-bromosuccinimide and a benzylsilane (e.g., 9 fi 10) under irradiation or in the presence of an appropriate radical initiator. This is often 2,2¢-azobisisobutyronitrile or benzoyl peroxide. The reaction can sometimes be complicated by competing dibromination; [19] an excess of N-bromosuccinimide has been used deliberately in the preparation of the Æ,Æ-dibromosilane 11. [29,30] Scheme 3 Bromination of Alkylsilanes [19–21,28] R Me2Si 1 5 R 1 = Me 7 R 1 = H Br2, 60 o C R Me2Si 1 Br 6 R 1 = Me 64% (from 5) 8 R 1 = Br 55% (from 7) Me3Si Ph NBS, CCl4 reflux, 10 h Me3Si Ph Br 9 10 60% O 12 NPr i 2 SiMe 3 NBS, (PhCO 2)O 2, CCl 4 reflux, 10 h 79% + Me 3Si Ph Br 11 21% O NPri 2 Br 2-[Bromo(trimethylsilyl)methyl]-N,N-diisopropylbenzamide (13); Typical Procedure: [28] N,N-Diisopropyl-2-[(trimethylsilyl)methyl]benzamide (12; 1.14 g, 3.9 mmol) in CCl 4 (200 mL) (CAUTION) was added to NBS (1.01 g, 5.0 mmol) and benzoyl peroxide (10 mg). The mixture was heated under reflux and irradiated by a UV sun lamp for 90 min. The mixture was cooled to rt and the succinimide removed by filtration through Celite. The filtrate was concentrated under reduced pressure to give a crude oil. The crude product was purified by chromatography (silica gel) to give the Æ-bromosilane 13 as a white solid after recrystallization (pentane); yield: 1.14 g (79%); mp 89–90 8C. 4.4.27.2 Method 2: Substitution of Æ-Hydroxyalkylsilanes Æ-Haloalkylsilanes are most conveniently prepared by the nucleophilic substitution by halide of an Æ-nucleofuge, often derived from a hydroxy group. In very general terms, the method is applicable to the synthesis of Æ-fluoro- and Æ-iodoalkylsilanes, in addition to the more common Æ-chloro- and Æ-bromoalkylsilanes. The method is mostly limited by the availability of the silyl electrophile. Æ-Silyl alcohols (see Section 4.4.28) are often used as the source of this electrophile, which is formed in situ by a variety of methods (Sections 4.4.27.2.1–4.4.27.2.3). However, other derivatives are sometimes used. For example, (fluoromethyl)trimethylsilane (15) has been prepared in 38% yield by the reaction of the tosylate 14 [derived from (hydroxymethyl)trimethylsilane] with dry potassium fluoride in diethylene glycol. [31] A small amount (ca. 10%) of the ethylsilane 16 (see Applications 13 SiMe 3 Br for references see p 159
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Science of Synthesis Houben-Weyl Me
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8 Science of Synthesis Category 1:
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Volume 1: Compounds withTransition
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Table of Contents 17 4.4.27 Product
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2001 Georg Thieme Verlag Rüdigerst
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1.1 Product Class 1: Organometallic
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1.1 Product Class 1: Organometallic
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1.1.1 Nickel Complexes of 1,3-Diene
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1.1.1 Nickel Complexes of 1,3-Diene
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1.1.2 Nickel-Allyl Complexes 37 tra
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1.1.2 Nickel-Allyl Complexes 39 Bis
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1.1.2 Nickel-Allyl Complexes 41 App
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1.1.2 Nickel-Allyl Complexes 43 NiC
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1.1.2 Nickel-Allyl Complexes 45 Sch
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1.1.2 Nickel-Allyl Complexes 47 Sch
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1.1.3 Nickel-Alkyne Complexes 49 Ca
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1.1.3 Nickel-Alkyne Complexes 51 (2
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1.1.3 Nickel-Alkyne Complexes 53 Sc
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1.1.3 Nickel-Alkyne Complexes 55 at
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1.1.3 Nickel-Alkyne Complexes 57 Sc
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1.1.3 Nickel-Alkyne Complexes 59 en
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1.1.3 Nickel-Alkyne Complexes 61 of
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1.1.4 Nickel-Alkene Complexes 63 Bi
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1.1.4 Nickel-Alkene Complexes 65 Sc
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1.1.4 Nickel-Alkene Complexes 67 1.
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1.1.4 Nickel-Alkene Complexes 69 Sc
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1.1.4 Nickel-Alkene Complexes 71 [5
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1.1.4 Nickel-Alkene Complexes 75 4-
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References 79 References [1] Chetcu
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References 81 [98] Tsuda, T.; Mizun
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86 Biographical Sketches Rinaldo Po
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88 2.6.4.2.2 Variation 2: Two-Elect
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90 2.6 Product Class 6: Organometal
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92 Science of Synthesis 2.6 Complex
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Page 166 and 167: 166 Biographical Sketches Alan Spiv
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Page 170 and 171: 170 Science of Synthesis 5.1 German
Page 176 and 177: 176 References [43] Eaborn, C.; Var
Page 178 and 179: 178 Abbreviations Chemical (cont.)
Page 180 and 181: 180 Abbreviations Ligands (cont.) 2
Page 182: 182 Abbreviations General (cont.) q
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