Silicon in organic synthesis Silicio: Presenza e distribuzione Il suo nome è stato proposto da Thomson nel 1831 con la desinenza (inglese silicon) –on per sottolineare l’analogia con il carbonio (carbon) e il boro (boron). Silicio (27.2%): elemento più abbondante nella crosta terrestre dopo l’ossigeno (45.5%) questi due elementi costituiscono i 4/5 di tutti gli atomi presenti sulla superficie del globo. Nell’universo il silicio è “solo” al 7° posto come abbondanza. Ulteriore differenziazione in concentrazione sulla terra. core (31% circa della massa terrestre) sostanzialmente non contiene silicio mantello (68% circa della massa terrestre) contiene probabilmente silicati densi (es. olivina) crosta (0.4%) contiene minerali silicei leggeri. Silicon: Physical and Atomical Information Silicon is constituted aminly by 28Si (92.23%) together with 4. 67% of29Si and 3.10% of 30Si. No other stable isotopes. In ground state its electronic configuration is [Ne]3s23p2 Property Electronic configuration C [He]2s22p2 Si [Ne]3s23p2 Physical state (25°C, 1 atm) Appearence/Properties Melting point (°C) solid Diamond, graphite, fullerens/Non-metal non conductor 3550 (d) solid Solido lucente blugrigio/Metalloid semiconductor 1420 Boiling point (°C) 4827 (d) 3280 Density (g/cm3) 3.51 (diamond), 2.22 2.336 (graphite) Proprietà fisiche e atomiche Proprietà Atomic radius, pm C 77 Si 117 Electronegativity 2.5 1.8 E°(volt) Common oxidation states pH=0 CO2 -0.1 CO -0.52 IV II pH = 14 CO32-1.15 IV +4 Ionization energies (kJ/mol) M(g) → M+(g) + eM+(g) → M2+(g) + eM2+(g) → M3+(g) + eM3+(g) → M4+(g) + e- 1086 2354 4622 6512 pH=0 C SiO2 -091 0 IV pH = 14 C SiO32-1.7 0 IV (+2), +4 786 1577 3231 4355 Si 0 Si 0 Silicon: Preparation and Industrial uses Silicon (96-99% purity) is prepared by reduction of quartz or sand with high purity coke in electric oven (1500 °C); SiO2 + C Si + 2 CO Silicon at very high purity is prepared by conversion of Si in SiCl4 and reducing the cloride with Zn, Mg or H2 higly pure then melted and crystallized in a single crystal e successivamente fuso e cristallizzato in un cristallo singolo. Si + 2Cl2 → SiCl4 (p.e. = 57.6 °C) SiCl4 + 2Mg → Si + MgCl2 o SiCl4 + 2H2 → Si + 4HCl A recent process: reduction of Na2SiF6 with Na metal. So obtained silicon can be doped with B, Ga, P or As to produce electronic devices and for space industry. Silicon is an importand component of kinds of steel: Fe3Si alloys are used in metallurgy for production of stainless steel. Silicon Boule Top Full boules Chunk of 99.9999% crystal. wafers Anthophyllite asbestos Mg7Si8O22(OH)2 Tourmaline NaMg3Al6(BO3)3[Si6O18](OH)3(OH) Quartz SiO2 Aquamarine Beryl Be3Al2Si6O18 Silicon structure Silicon has a structure similar to that of diamond. Fullerene Grafite Chemical properties Silicon: Same group of carbon 4 valence electrons However they differ for many aspects. Silicon has also 3d orbitals in valence shell. carbon: maximum coordination number 4 silicon can give higher coordination (e.g. SiF62- ). Less tendency to form multiple bonds: Carbon has the peculiar property to form multiple bonds π-π as in C=C, C≡C, C=O, C=S and C≡N Silicon does not give multiple bonds π-π stable because the size of orbitals is too high and they are much too diffused. Because of its size ,Si shows more tendency to form chains and clusters of tetrahedra. As a consequence: carbon dioxide, CO2, is a gas containing discrete molecules silicon dioxide (quartz or o silica), SiO2, is a solid containing thetrahedral repeating units SiO4. Chemical properties Different bond energies (driving force = termodinamic) kJ/mole C-C 368 Si-Si 340 forms many and very long chains forms chains (up to Si6H14) but in a less extended fashion C-O 360 Si-O 452 a strong bond involving silicon C-F 453 Si-F 565 the strongest silicon bond C-H 435 Si-H 395 weaker than C-H Silicon compounds: silanes or silyl hydrides Silyl hydrides are analogues to hydrocarbons but, for many reasons are less stable. The only member stable undefinitely is silane (SiH4). The increase of complexity results in instability. SiH3 H3Si H3Si Si H2 H2Si Si H2 SiH3 SiH3 SiH4 H3Si SiH Si H2 SiH3 Silanes can be obtained in different ways, the most common being the reduction of corresponding halides with LiAlH4. SinCl2n+2 + LiAlH4 → SinH2n+2 Silicon compounds: Silicones Compounds containing Si-O and Si-C bonds. Silicon oils: used as lubricants, additives, diathermic oils, in cosmetics etc. O Si O Si O Si O Si O Si O Si O Si Si Elastomers: like the previous ones but with longer chains And loaded with ultrafine silica Silicon resins: Si Si O Si O Si O Si O Si O O O Si O Si Si O O O Si Si O O Si O Si O Si O Si Si Organosilicon compounds in synthesis Silicon is electropositive with respect to carbon, making possible nucleophilic attack tat silicon For this reason silicon forms strong bonds with electronegative elements such as oxygen or fluorine Moreover, silicon can stabilize a negative charge on the adjacent carbon atom. This and susceptibility to nucleophilic attack are ascribed to the presence of empty 3d orbitals. Finally, it is able to stabilize a positive charge on the b carbon atom, with respect to silicon Organosilicon compounds in synthesis: silyl enol ethers Silyl enol ethers can be generated in a number of ways either under kinetic or thermodinamic control, wich give rise to different silyl enol ethers in unsimmetrical ketones. Brownbridge, P. Synthesis, 1983, 1. Organosilicon compounds in synthesis: silyl enol ethers Silyl enol ethers can be used for alkylation or acylation of ketones, for directed aldol condensation and for Michael addition: Mukaiyama, T, Angew. Chem. Int. Ed., 1977, 16, 817. Organosilicon compounds in synthesis: vinyl silanes Vinyl silanes are useful in synthesis because they react with electrophiles regio- and stereospecifically, and their epoxides serve as masked carbonyl compounds: tandem Chan, TH, Fleming, I. Synthesis, 1979, 761. Organosilicon compounds in synthesis: vinyl silanes Vinyl silanes can be used to in a tandem or in silicon directed Nazarov cyclization: tandem silicon directed Chan, TH, Fleming, I. Synthesis, 1979, 761. Organosilicon compounds in synthesis: epoxysilanes Epoxysilanes can be easily converted to a variety of compounds; ketones, vinyl halides, dicarbonyl compounds Chan, TH, Fleming, I. Synthesis, 1979, 761. Organosilicon compounds in synthesis: allylsilanes Allylsilanes also react with electrophiles. Here reaction takes place at the g-carbon with migration of the double bond and displacement of silicon from a-carbon. Again, reaction is facilitated by silicon stabilization of a transient carbonium ion in b position. Chan, TH, Fleming, I. Synthesis, 1979, 761. Organosilicon compounds in synthesis: a-silyl carbanions Because of (p-d) p back-bonding between silicon and carbon, silicon can stabilize an adjacent carbanion. Usually, the carbanion is flanked by another electron withdrawing group. An example is the Peterson olefination: Another interesting reaction is the additon of chlorotrimethylsilylmethyl lithium to carbonyl compound resulting in epoxysilanes Burford, C, Cooke, F, Roy, G, Magnus, P. Tetrahedron 1983, 39,867. Organosilicon reagents: trimethylsilyl cyanide Trimethylsilyl cyanide is a useful reagent which reacts readily with aldehydes and ketones to give silylated cyanidrins., which can be further elaborated. Evans, DA, Carroll, GL, Truesdale, LK. J. Org. Chem. 1974, 39, 914. Hunig, S, Wehner, G. Chem. Ber. 1979, 112, 2062. Organosilicon reagents : trimethylsilyl iodide and triflate Other two useful silicon reagents are trimethylsilyl iodide and trimethylsilyl trifluoromethanesulfonate (triflate). The iodide is commercially available but it is expensive and very sensitive to light and moisture and is probably best prepared in situ from chlorotrimethylsilane and sodium iodode in acetonitrile. It is an excellent reagent for the cleavage of ethers, (methyl) esters and acetals. Olah, GA, Narang, SC, Gupta, BG., Malhrotra , R. Synthesis. 1979, 61. Olah, GA, Husain, A, Singh, BP., Mehrotra , AK. J. Org: Chem. 1983, 48, 3667. Trimethylsilyl triflate is a powerful silylating agent and for organic compound and acts either as Brönsted or Lewis acid in many reactions. Schmidt, RR, Michel, J., Roos, M. Liebigs Ann. Chem. 1984, 1343 – 1357.