SolutionsThe Peterson reaction and related processes
Exercise 1
A single diastereomer of a β-hydroxysilane is split into two portions. One is treated with KH in THF and the other with H2SO4 in THF. What is the relationship between the alkenes obtained? Explain.
They are the two stereoisomers of the alkene (one E and the other Z). With KH, the alkoxide attacks silicon, the four-membered O–Si ring forms and elimination is syn. Under acidic conditions the OH is protonated and leaves as water with the silyl group antiperiplanar: elimination is anti. Going from the anti to the syn conformation requires a 180° rotation about the C–C bond, which swaps the relative positions of the substituents in the alkene.
Exercise 2
Why can the adduct of (trimethylsilyl)methylmagnesium chloride with a ketone be isolated, whereas that of (trimethylsilyl)methyllithium and, especially, that of a potassium reagent eliminate spontaneously?
Basic elimination requires the alkoxide oxygen to attack silicon. The more covalent the O–metal bond (Mg > Li > Na > K), the less nucleophilic the oxygen: the magnesium alkoxide does not eliminate at room temperature, the lithium one does so slowly and the potassium one, almost a “free” alkoxide, eliminates at once. That is why KH or NaH is used to trigger elimination from the isolated β-hydroxysilane.