Silenes as novel synthetic reagents

Abstract

Several syntheses of aryltris(trimethylsilyl)silanes were investigated and finally a novel route was developed starting from tetrakis(trimethylsilyl)silane, the mechanism for which was investigated. This new robust and reproducible synthesis involved a halogen-metal exchange and exploited a Schlenk equilibrium and was used to produce phenyltris(trimethylsilyl)silane in high yield (-70 %) on scales of up to 20 g. Phenyltris(trimethylsilyl)silane was used as the starting material for the synthesis of silene precursors; 1-hydroxyalkylphenyltrisilanes (silyl alcohols), involving KO'Bu-mediated formation of phenyltris(trimethylsilyl)silylpotassium, transmetallation to the silylmagnesium bromide and addition of an aldehyde. Following optimisation and mechanistic studies on this reaction, it could be employed in the synthesis of several alkyl-analogues. Silenes were generated from the silyl alcohols by the modified Peterson olefination. Extensive optimisation and mechanistic studies led to a procedure that involved the use of "BuLi as a base to cause a 1,3-Si, O TMS migration which was followed by the addition of catalytic amounts of lithium bromide to induce elimination of MegSiOLi to give the silene. Silene generation in the presence of a diene afforded [4+2] silacycloadducts containing an alkene in the β-position to silicon, along with small amounts of ene products and silene dimers. The silacycles were formed with satisfactory diastereoselectivity with the major isomer constituting 70-80 % of the mixture. A range of dienes were screened for reaction with various silenes. The resulting silacycles were shown to be useful synthetic intermediates by their conversion into lactones. This involved hydrogenation of the carbon-carbon double bond followed by a Fleming-Tamao oxidation to afford diols, initiated by protodesilylation of the Si-phenyl group. Subsequent TPAP, NMO oxidation of the diols produced the desired lactones from which NOESY and other NMR experiments were used to deduce the stereochemistry. In addition, silacycles without substitution on the carbon-carbon double bond could be converted into bishomoallyhc alcohols by omission of the hydrogenation and direct Fleming-Tamao oxidation, thus exploiting the latent reactivity of the allylic silane.