Gas-phase chemistry of pentacoordinate silicon hydride ions
Abstract
The reactivity and thermochemistry of gas-phase pentacoordinate silicon hydride ions were examined in a flowing afterglow-triple quadrupole apparatus. The hydride affinity of four different silanes and the reactivity of alkylhydridosiliconates and monoalkoxysiliconate ions were studied. The reactivity and thermochemistry of silacyclobutane was also studied. In addition, the diastereo- and the regioselectivities for gas-phase reduction reactions of carbonyl compounds with monoalkoxysiliconate ion were investigated. The hydride affinities of ${\rm BuSiH\sb3,\ Et\sb2SiH\sb2},$ and ${\rm Et\sb3SiH}$ are determined to be 20 $\pm$ 4 kcal/mol, which is inconsistent with HA (SiH$\sb4)$ = 19 $\pm$ 4 kcal/mol. Up to 4 H/D exchanges were observed in the reaction of BuSiH$\sbsp{4}{-}$ with SiD$\sb4.$ A carbanion transfer with H$\sb2$ loss was also found in the reaction of ${\rm R\sb3SiH\sbsp{2}{-}}$ with ${\rm R\sp\prime\sb3SiH}.$ Moreover, silyl anions and $\alpha$-silylcarbanions are generated by loss of H$\sb2$ from ${\rm R\sb3SiH\sbsp{2}{-}}$ ions. Dissociation of hydride ion presumably occurs when ${\rm R\sb3SiH\sbsp{2}{-}}$ is collisionally activated. Mixtures of diastereomers produced by reduction of 2-methylcyclohexanone, 4-tert-butylcyclohexanone, 3,3,5-trimethylcyclohexanone, norcamphor, and 2-tert-butyl-1,3-dioxan-5-one are determined quantitatively. Diastereoselectivities are obtained by comparing the alkoxide fragment yield ratios produced from dialkoxysiliconate ion derivatives possessing pure cis or trans alkoxide isomers with the ratio obtained from fragmentation of the dialkoxysiliconate ion formed by reduction of the corresponding ketones. Using analogous procedures, the regioselectivities for 1,2- and 1,4 reduction of acrolein, methyl vinyl ketone, and 2-cyclohexen-1-one with alkoxysiliconate ions are also determined. In comparison with acyclic alkylsilanes, c-$\rm (CH\sb2)\sb3SiH\sb2,$ was found to behave differently. The acidity of c-${\rm (CH\sb2)\sb3SiH\sb2}$ was found to be much lower (ca. 10-13 kcal/mol) than acyclic alkylsilanes. Moreover, the hydride affinity of c-${\rm (CH\sb2)\sb3SiH\sb2}$ is found to be higher than HA(benzene) = 22.8 kcal/mol. The reaction of c-${\rm (CH\sb2)\sb3SiH\sb2F\sp{-}}$ with acetaldehyde produces no hydride adduct, which is different than the reactions of acetaldehyde with ${\rm Et\sb2 SiH\sb2F\sp{-}}$ and c-${\rm (CH\sb2)\sb3SiH\sb3\sp{-}}.$
Degree
Ph.D.
Advisors
Squires, Purdue University.
Subject Area
Analytical chemistry|Organic chemistry
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