Allylsilanes

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In some cases, especiallywith 3,3-disubstituted allylsilanes, the problem can be obviated by the generation of allyl Grignard reagents with high stereochemical purity, by direct insertion of highlyreactive Rieke magnesium into the allylic carbon-halogen bonds at verylow temperature (±958C); this is shown, for example, bythe formation of (E)- and (Z)-11 with remarkable regio- (á/ã > 99:1) and stereoselectivity(Scheme 6). [54] However, for monosubstituted allylsilanes, such as dec-2-enyltrimethylsilane, this method is not suitable, and use of the corresponding allyllithium intermediate is recommended for high regio- and stereoselectivity. [54] Scheme 6 <strong>Allylsilanes</strong> from Allylmagnesium Halides and Chlorotrimethylsilane [50,52,54] Cl FOR PERSONAL USE ONLY 840 Science of Synthesis 4.4 Silicon Compounds 9 SiMe3 1. Mg Cl SiMe 2. TMSCl 3 10 1. Rieke Mg, −95 o Cl C 2. TMSCl 80% SiMe3 (E)-11 (E/Z) >99:1:
pressure. Flash chromatography(silica gel, pentane/Et 2O 20:1) gave the product as a colorless oil; yield: 5.33 g (97%); bp 134±1358C/0.22 Torr. 4.4.40.1.2 Variation 2: Silylation of an In Situ Generated Grignard Reagent The addition of an allyl halide to a suspension of magnesium metal and chlorotrimethylsilane in tetrahydrofuran yields the corresponding allylsilane, for example, in the preparation of 13 and 14 (Scheme 8). [56,57] This reaction is believed to involve the in situ generation of the allylmagnesium halide, which reacts with the trialkylhalosilane by displacement. Scheme 8 Silylation of an In Situ Generated Grignard Reagent [56,57] Cl ( )n X TMSCl, Mg Cl Me3Si SiMe3 TMSCl, Mg, THF X = Cl; n = 1 94% X = Br; n = 2 54% FOR PERSONAL USE ONLY 4.4.40 <strong>Allylsilanes</strong> 841 Cyclopent-2-enyltrimethylsilane (14, n = 1); Typical Procedure: [56] A mixture of TMSCl (55.33 g, 0.51 mol), Mg (18.5 g, 0.77 mol), and THF (250 mL) was cooled to 58C in an ice bath. An addition funnel was charged with a soln of 3-chlorocyclopentene (52.27 g, 0.51 mol) in THF (500 mL), and cooled with a dry-ice jacket. The resulting cooled soln was added dropwise to the stirred mixture over several hours. The mixture was allowed to warm to rt, and then stirred overnight. It was washed with H 2O (5 ” 100 mL), and the aqueous washings were back-extracted with pentane (50 mL). The combined organic phases were washed with brine (100 mL), dried, and concentrated under reduced pressure; yield: 67.1 g (94%); bp 140±1468C/760 Torr. ( ) n 14 13 SiMe 3 4.4.40.2 Method 2: From Allylmagnesium Halides by Transition-Metal-Catalyzed Coupling with Hydrosilanes E-But-2-enylsilanes are available by nickel-catalyzed coupling of but-2-enylmagnesium bromide with hydrosilanes. The most effective catalyst for this reaction is [1,1¢-bis(diphenylphosphino)ferrocene]dichloronickel(II) [NiCl 2(dppf)], which is used in the preparation of 15 (Scheme 9). [1,58] Of note is the requirement of a large excess of the Grignard reagent for the yield to be good and for the regio- and stereoselectivity to be high. For some simple allylsilanes, such as allyltris(4-methoxyphenyl)silane, the yield of product by this protocol is higher than bythe Grignard method. [55] Scheme 9 Preparation of E-But-2-enylsilanes [1,58] R 1 R 2 2SiH MgBr SiR 15 1R2 NiCl2(dppf) (cat.) R 2 1 = Me; R2 = Ph 85%; (E/Z) 98:2 R1 = Ph; R2 = Me 72%; (E/Z) 98:2 [(E)-But-2-enyl]dimethylphenylsilane (15,R 1 = Ph; R 2 = Me); Typical Procedure: [1,58] [NiCl 2(dppf)] (13.6 mg, 0.02 mmol) was placed in a two-necked flask equipped with a magnetic stirring bar and a three-waystopcock. After evacuation, the flask was filled with N 2 and was cooled to 08C. To it was added a soln of 0.54 M but-2-enylmagnesium bromide in Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG for references see p 920
Page 1 and 2: 4.4.40 Product Subclass 40: Allylsi
Page 3: matching of both partners is approp
Page 7 and 8: Scheme 11 From Allylic Halides by E
Page 9 and 10: similar protocol starting with but-
Page 11 and 12: + PhMe2Si MgCl Cl Br (R,S)-PPFA = +
Page 13 and 14: Scheme 18 Allylsilanes from Vinyl T
Page 15 and 16: Trimethyl(non-2-enyl)silane (44); T
Page 17 and 18: complex which reacts with the carbo
Page 19 and 20: (2-Cyclohexylidenepropyl)trimethyls
Page 21 and 22: (2 ” 10 3 kPa)±depressurization
Page 23 and 24: Scheme 31 Allylsilanes from Allyl A
Page 25 and 26: mended for the synthesis of Z-allyl
Page 27 and 28: Allylsilanes 80; General Procedure:
Page 29 and 30: Scheme 39 Allylsilanes from Allyl C
Page 31 and 32: Scheme 42 2-Substituted Allylsilane
Page 33 and 34: FOR PERSONAL USE ONLY 4.4.40 Allyls
Page 35 and 36: Conversion of â-Hydroxy Acids 99 i
Page 37 and 38: Scheme 48 Allylsilanes by Carbocupr
Page 39 and 40: 119. [183,185] The trick for succes
Page 41 and 42: the mixture was washed with sat. aq
Page 43 and 44: (+)-(3R,4E)-N,N-Dimethyl-3-(trimeth
Page 45 and 46: Scheme 57 From Alkenyl Fischer Carb
Page 47 and 48: 4.4.40.37 Method 37: From Allyl Sul
Page 49 and 50: Allyltris(trimethylsilyl)silane (15
Page 51 and 52: the Z-isomer; very little, if any,
Page 53 and 54: Pd2(dba)3 CHCl3 (cat.) (R)-MOP-phen
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Scheme 70 An Alk-2-enylsilane by Ca
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(E)-Trimethyl[3-(4-tolyl)allyl]sila
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FOR PERSONAL USE ONLY 4.4.40 Allyls
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Scheme 77 2-Substituted Allylsilane
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stream of argon, which was passed t
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Z-Vinylsilanes carrying a (tributyl
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1-tert-Butyl-1-vinylcyclohexane (21
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4.4.40.59 Method 59: Allylation of
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(5SR,6SR,9RS,10RS)-6,9-Divinyltetra
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Scheme 97 Synthesis of Homoallylic
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Scheme 102 Synthesis of Enantiomeri
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Scheme 106 Diastereoselective Synth
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stirred for 18 h at ±78 8C, and 5
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shaken with some solid K 2CO 3 and
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FOR PERSONAL USE ONLY 4.4.40 Allyls
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FOR PERSONAL USE ONLY References 92
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Page 89 and 90:
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Allylsilanes

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