Some studies of syntheses and reactions of silicon-functionalized silyl enol ethers



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Texas Tech University


One goal of this research project was to develop a synthesis of silyl enol ethers bearing a stereogenic silicon atom and to study the stereochemical outcome of chemical reactions of such silyl enol ethers. We envisioned that the diastereomers of an alkoxysilyl enol ether bearing a chiral alkoxy group and stereogenic at die silicon atom could be separated to yield enantiomerically pure chiral silyl enol ethers. Very few studies have been published on the effects of chiral silicon atoms upon the stereochemical outcomes of reactions of silanes. We envisioned that it was reasonable to expect the chirality on the silicon atom in a chiral silyl enol ether to influence, stericaUy, the approach of a reagent engaging with one face versus the other of the alkenyloxy ligand's double bond. To explore this possibility we studied the aldol reaction in the presence of Lewis acids, epoxidation reactions with meta-chloroperbenzoic acid (mcpba), cyclopropanation of the 16 enoxy ligand, halogenation of the enoxy ligand in the presence of N-bromosuccinimide, and reduction of the enoxy ligand.

Another goal of this research was to study tiie effects of ligands attached to silicon upon the 6-endo-ttig radical cyclization of 2-IoroethylsiIyl enol ethers. As discussed above, the majority of observed free radical cyclizations feature the 5-exo-trig cyclization of a radical center 5-to an alkene to form five-membered carbocyclic or heterocyclic rings. The alternative 6-endo-trig mode of cyclization has been encountered less often, especially when the radical center is a carbon atom.41 The net conversion of 6-chloro-3-oxa-4-siIa-lhexenes to 1-oxa -2-silacyclohexanes is unprecedented and holds a great promise as a general means for extending carbon chains selectively and under mild conditions.