Small Molecule Activation and Catalysis by Low Valent Group 14 Compounds
Abstract
The work described in this thesis focuses on three main themes: small molecule activation by ditetrelenes, the Lewis acidity assessment of low valent germanium(II) and tin(II) crown ether complexes and catalysis by low valent germanium(II) and tin(II) crown ether complexes. The activation of ammonia and amines using tetramesityldisilene and -digermene was explored synthetically and the lowest energy path to ammonia activation by tetramesityldisilene was computed. The results of the mechanistic analysis have implications for the reactions of disilenes and digermenes with nucleophiles. Subsequent functionalization of the ditetrelene-amine adducts with electrophilic alkynes was attempted, highlighting the significant challenges in the area of secondary functionalization using ditetrelenes.
The Lewis acidities of selected germanium(II) and tin(II) crown ether complexes were examined both synthetically and computationally. In the addition of triethylphosphine oxide to the germanium(II) and tin(II) crown ether complexes following the Gutmann-Beckett method, mixtures of species derived from the coordination of multiple phosphine oxide equivalents and displacement of the crown ether were observed. These results have implications for the standard procedure of the Gutmann-Beckett method in which exclusively one equivalent of phosphine oxide is assumed to coordinate. The Lewis acidity of Ge2+ was found to be higher than that of Sn2+.
Lewis acid-catalyzed hydrosilylation of carbonyl compounds was explored using the germanium(II) and tin(II) crown ether complexes as catalysts. Only the germanium(II) crown ether complexes were found to selectively reduce aldehydes efficiently and selectively to one product. The scope of aldehyde reduction was explored using [Ge([12]crown-4)2][OTf]2 revealing tolerance of the reaction for a number of functional groups, excluding those containing nitrogen. Lower conversions to reduced products was observed in ketone hydrosilylation using [Ge([12]crown-4)2][OTf]2 as a catalyst. Preliminary studies on the mechanism of addition revealed the first step is consistent with hydrosilylation and that the catalysis is derived from the germanium(II) centre. This study includes the first extensive substrate scope of hydrosilylation by a cationic germanium(II) compound.
The mechanism of aldehyde reduction catalyzed by [Ge([12]crown-4)2][OTf]2 was probed using a number of experimental techniques including the stoichiometric addition of silane and aldehyde, deuterium labelling studies, a Hammett analysis and Variable Time Normalization Analysis. The results of the experiments were consistent with carbonyl-activation and is the first study to implement Linear Free Energy Relationships to examine the reduction of aldehydes catalyzed by low valent Group 14 compound.