Robyn Ovans - University of Victoria
Placement: McMaster University
Supervisor: Dr. Willie Leigh
The Base-catalyzed Hydrolysis of 1,1-Diphenylgermene
Germacyclobutanes have been shown to be convenient photochemical precursors to compounds containing a Ge=C bond (germenes). The reactivity of germenes is similar to that of silenes, which have been extensively studied. For example, germenes react rapidly with alcohols to yield the corresponding alkoxygermane due to ROH addition across the Ge=C double bond. Although the mechanisms of many silene reactions are well understood, little work has been done to investigate the kinetics and mechanisms of germene reactions.
Laser flash photolysis of 1,1-diphenylgermacyclobutane allows the detection of the transient 1,1-diphenylgermene. The germene is quenched by water, but with a rate law that is strictly second order in H2O. It was proposed that the reaction occurred by initial nucleophilic attack by water at germanium, leading to the formation of a zwitterionic complex, followed by rate determining proton transfer by a second molecule of water. This led to the investigation of tertiary amines as catalysts for the proton transfer step. More recent work indicates that, in the presence of base, the reaction more likely involves attack by hydroxide ion in the rate-determining step, preceded by an acid-base equilibrium.
Rate constants were determined for the reaction of diphenylgermene with water in the presence of several amines of varying pKb. As expected, increasing the basicity of the amines lead to an increase in the rate constant. The rate constant in the presence of hydroxide was also investigated, as well as buffered amine solutions, in order to determine which of the two reaction mechanisms was taking place.
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