Allylsilanes

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Scheme 2 Diastereoselective Reactions of E- and Z-But-2-enylsilanes [36] (E)-4 SiMe 3 SiMe3 (Z)-4 + Bu t CHO TiCl4 (syn-5/anti-5) >99:1 OH t Bu + TiCl4 + ButCHO syn-5 (syn-5/anti-5) 65:35 Titanium(IV) chloride has also been used widelyas the Lewis acid of choice to promote the reactions of allylsilanes with other electrophiles including epoxides, acetals, iminium ions, and acid chlorides. An allylsilane with silicon-centered chiralityshows a low level of chiralitytransfer in its Lewis acid mediated reactions with aldehydes and acetals. [37] However, when a stereogenic center is in one of the substituents on the silyl group, good asymmetric induction has been observed, for example, the formation of 6 in 47% ee (Scheme 3). [38] Scheme 3 Asymmetric Induction in a Lewis Acid Mediated Allylation Reaction [38] MeO 2C N SiMe 2 + PrCHO TiCl 4 OH 6 47% ee Besides titanium(IV) chloride, other catalysts such as tin(IV) chloride, the boron trifluoride±diethyl ether complex, ethylaluminum dichloride, trimethylsilyl trifluoromethanesulfonate, iron(III) chloride, bismuth(III) bromide, [39] Amberlyst 15, and so forth have also found use in the reactions of allylsilanes. The allylation reaction of aldehydes with allylsilanes generally requires stoichiometric or greater amounts of Lewis acid because of tight binding of the product homoallyl alcohol to the Lewis acid catalyst, as well as poor efficiency of silicon transfer to oxygen. A catalytic version of this reaction is possible if 2±10 mol% of scandium(III) triflate is used; thus, if a stoichiometric amount of acetic anhydride is used in the allylation reaction, homoallylic acetates can be prepared in an essentially one-pot reaction, for example, the formation of 7 (Scheme 4). [40] Scheme 4 Catalytic Allylation of Aldehydes with a Scandium(III) Triflate Catalyst [40] 1 SiMe3 + PhCHO FOR PERSONAL USE ONLY 838 Science of Synthesis 4.4 Silicon Compounds Sc(OTf) 3 (cat.) Ac2O (2 equiv) 74% Allylation reactions of allylsilanes can also be carried out under nucleophilic conditions (TBAF, KF, or NaOMe). Fluoride ion induced reaction of carbonyl compounds with allylsilanes generallytakes place readilyat room temperature. With á,â-unsaturated esters, nitriles, and amides as substrates, reactions of allyltrimethylsilane in the presence of fluoride ion occur exclusively in a 1,4-fashion, while Lewis acid catalyzed processes yield no addition products at all. [20] Allyltris(trimethylsilyl)silanes react by a free-radical chain mechanism with organic halides, therebyeffecting a net replacement of the halide bya propenyl group if the polar Ph OAc Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG 7 Bu t OH anti-5
matching of both partners is appropriate [41] [see Section 4.4.40.57 and Science of Synthesis, Vol. 47 (Alkenes)]. The reactivityof allylsilanes can be suitablytuned byvariation of the ligands on the silicon atom. The use of a large number of examples of allylsilanes containing pentacoordinated silicon have also been described in the literature. [22,27,42] The principle of strain-induced Lewis acidity is beautifully demonstrated in the stereoselective reactions of allylsilane (E)-8 and (Z)-8 with an aldehyde (Scheme 5); no catalytic activation is needed here, where the stereoselection implicates a cyclic transition structure. [43] Scheme 5 But-3-enols from Thermal Allylation of Aldehydes [43] Pr Pr (E)-8 (Z)-8 Si Si Ph Ph + PhCHO + PhCHO FOR PERSONAL USE ONLY 4.4.40 Allylsilanes 839 1. 130 oC 2. H + 68% 1. 130 oC 2. H + 66% Lewis acid promoted [3 +2] and [2+2] annulation of stericallyhindered allylsilanes and subsequent oxidative cleavage of the carbon-silicon bond provide hydroxycyclopentanes and hydroxymethylcyclobutanes [44,45] (see Section 4.4.40.69). In general, allylsilanes are thermally stable and relatively inert to moisture and air. Theyare storable and can be handled without special precautions. Onlya few thermally labile allylsilanes, which require storage at low temperature, are known. [46] Allylsilanes are stable to chromatographyon silica gel or Florisil; it is rare to see the addition of a little triethylamine to the eluent to avoid acid-catalyzed decomposition on the column being recommended. [47] On the other hand, allylhalosilanes are moisture sensitive and therefore care should be taken to prevent hydrolysis during storage. Allyltrimethylsilane is nontoxic; other allylsilanes are also expected to be so, with the exception of allylhalosilanes. The main dangers with the latter may arise from hydrolysis products such as hydrogen halides. Allylsilanes have a C=C stretching band which appears in the approximate range of í = 1600±1660 cm ±1 in their infrared spectra; the E configuration of 3-substituted allylsilanes can be ascertained from a band of medium intensityat approximatelyí = 965± 990 cm ±1 . 29 Si NMR is a powerful technique for determining the structure of silicon-containing compounds. [48] Synthesis of Product Subclass 40 4.4.40.1 Method 1: From Allylmagnesium Halides and Trialkylhalosilanes Allylsilanes are available from allyl halides by silylation of the corresponding Grignard reagents with an appropriate chlorosilane. This route works best for simple and symmetrical alk-2-enylsilanes, for example, 9, or those having a considerable steric and/or electronic bias, for example, 10 (Scheme 6). [49±52] Regio- and stereocontrolled synthesis of allylsilanes bythis conventional method is not, however, an easytask in view of well-known stereorandomization of the allylmetallic intermediates. [53] Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG Ph Ph OH Pr OH Pr + 5:95 + 95:5 Ph Ph OH Pr OH Pr for references see p 920
Page 1: 4.4.40 Product Subclass 40: Allylsi
Page 5 and 6: pressure. Flash chromatography(sili
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,
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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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