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152 Science of Synthesis 4.4 Silicon Compounds ture was extracted with Et 2O (3 ” 30 mL). The combined Et 2O extracts were washed with sat. aq NH 4Cl and brine, dried (MgSO 4), and evaporated. The residue was purified by flash chromatography (silica gel, hexanes) followed by Kugelrohr distillation (1108C, 0.7 Torr) to give the stable Æ-iodoalkylsilane 24; yield 1.36 g (96%). 4.4.27.3 Method 3: Haloalkylation of Halosilanes The haloalkylation of chlorosilanes by the reaction of a (Æ-haloalkyl)metal species is a useful and reasonably general method for the preparation of Æ-haloalkylsilanes (disconnection d, Scheme 1). Generally, the (chloroalkyl)lithium reagents are prepared in situ by the deprotonation of the corresponding alkyl halide. [45] For example, (chloromethyl)lithium is conveniently prepared in situ by the treatment of bromochloromethane with butyllithium at temperatures between –70 and –60 8C, in the presence of the chlorosilane (25 fi 26). [45] Under these conditions butyllithium does not react with the chlorosilane. The method is applicable to the synthesis of a wide variety of silanes including disilanes and allylsilanes. Bromomethyl and iodomethyl derivatives can be prepared with increasing difficulty by the use of dibromomethane and diiodomethane. 3-Haloprop-1-enes react in the same way with chlorotrimethylsilane and lithium dicyclohexylamide efficiently producing Æ-haloallylsilanes (allyl bromide fi 27, Scheme 8). [46] (Halomethyl)aryl compounds react in the same way when lithium diisopropylamide is used as the base. [47–49] Several heterocyclic derivatives have been prepared by this method. [Chloro(pyridin-3yl)methyl]lithium [prepared from lithium diisopropylamide and 3-(chloromethyl)pyridine] reacts well with chlorotrimethylsilane. [50] Similar processes can be used to silylate the lithio derivatives of 4-(chloromethyl)pyridine, [51] 4-(fluoromethyl)pyridine, [51] and 2- (chloromethyl)benzothiazole. [52] (Dihalomethyl)trialkylsilanes are prepared efficiently from dihalomethanes, lithium diisopropylamide, and chlorosilanes. [53] Deprotonation of trihalomethanes with butyllithium at low temperature, followed by reaction with chlorosilanes, generates trihalomethylsilanes. [54] Similar processes can be effected by formation of the Grignard reagent derived from polybromo- or polyiodomethane. [55] 1-Bromo-1-(trimethylsilyl)cyclopropanes [56] can be prepared by the treatment of 1,1-dibromocyclopropanes, magnesium, and chlorotrimethylsilane under ultrasonic irradiation. [57] Scheme 8 Reaction of Halosilanes with (Æ-Haloalkyl)lithium Reagents [45,46,60] Cl Me2Si Br Bu t O BrCH2Cl, BuLi THF, −60 oC 71% Me2Si 25 26 O 28 F 1. Cy2NLi, TMSCl, −60 oC 2. H + /H2O 55% 71% Cl Me3Si Br 27 1. LDA (4 equiv), TMSCl (6 equiv), −78 oC 2. H + /H2O Bu O t O F SiMe3 29
4.4.27 Æ-Haloalkylsilanes 153 In general, the approach works when electron-withdrawing groups are present at the Æposition. For example, Æ-haloesters are easily deprotonated with lithium diisopropylamide and silylated to give Æ-haloalkylsilanes. [58,59] The reactions are often complicated by competing O-silylation. An excess of both lithium diisopropylamide and chlorosilane ensures a good yield of tert-butyl Æ-fluoro-Æ-(trimethylsilyl)acetate (29) from tert-butyl Æfluoroacetate (28). [60] The treatment of silyl ethers of type 30 yields Æ-chloroalkylsilanes effectively by an in situ method of (Æ-haloalkyl)metal formation (Scheme 9). In this case the silyl group undergoes intramolecular transfer from the nearby silyl ether group. [61] Elimination of hydrochloric acid from the Æ-chloroalkylsilane 31 is a competing reaction producing alkenylsilanes as byproducts. Scheme 9 Intramolecular Reaction of a Silyl Ether with a (Haloalkyl)lithium Species [61] O SiMe 2Bu t Cl 1. BuLi, t-BuOK, −70 oC, 10 min 2. H2O 72% OH 30 31 SiMe 2Bu t Allyl(chloromethyl)dimethylsilane (26); Typical Procedure: [45] Allyl(chloro)dimethylsilane (25; 9.7 g, 70 mmol), BrCH 2Cl (9.3 g, 70 mmol) and dry THF (150 mL) were added to a 500-mL three-necked flask equipped with a magnetic stirrer bar, N 2 inlet tube, and thermometer. The mixture was cooled to –70 8C and 1.6 M BuLi in hexanes (45 mL, 70 mmol) added down the cold wall of the flask via a syringe over 40 min. During the addition the temperature of the mixture was maintained between –70 and –60 8C. The soln was then warmed to rt and H 2O (50 mL) was added. The mixture was extracted with hexanes (3 ” 100 mL). The combined extracts were dried (CaCl 2) and evaporated under reduced pressure. The residue was distilled to give the Æ-chloromethylsilane 26; yield: 7.6 g (71%); bp 778C/80 Torr. 4.4.27.4 Method 4: Reaction of Halosilanes with Diazomethane Halosilanes react with diazoalkanes to form Æ-haloalkylsilanes (32 fi 33, Scheme 10). The reaction is facilitated by copper powder or copper(II) salts. [62–64] Fluorosilanes are the least reactive of the halides in this type of reaction and effectively do not react in this manner. The reaction is most efficient with di-, tri-, and tetrachlorosilanes. The reaction between tetrachlorosilane and diazomethane is violent at room temperature, but can be controlled at low temperature. In polyhalosilanes, the introduction of one halomethyl group generally retards the introduction of another. Nevertheless, the method is useful for the synthesis of dichloro[bis(chloromethyl)]silane from trichloro(chloromethyl)silane. [65] Bromosilanes are more reactive than chlorosilanes. The method is applicable to halodisilanes. [66] The reaction of iodotrimethylsilane or trimethylsilyl trifluoromethanesulfonate with diazomethane does not require catalysis, whereas that of bromotrimethylsilane proceeds conveniently in the presence of zinc dibromide. [67] Scheme 10 Reaction of Halosilanes with Diazomethane [64] CH2N2, Cu, Et2O, −60 45% 32 33 o HSiCl3 C Cl H Cl Si Cl Cl for references see p 159
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Science of Synthesis Houben-Weyl Me
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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 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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1.1.4 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 69 Sc
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1.1.4 Nickel-Alkene Complexes 71 [5
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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 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
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