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The initial nucleophile attacks silicon, not the nitrile or an alkene carbon. Each trimethylsilyl group has an electrophilic Si center. A methyl group from methyllithium attacks Si, passing through a pentacoordinate silicon arrangement; Si–O cleavage releases $\mathrm{SiM}{\mathrm{e}}_4$ and the lithium salt of the oxygen-containing fragment. Cleaving the silyl enol ether generates a lithium enolate; cleaving the aryl silyl ether generates a phenoxide. Neither operation needs a new deprotonation at a carbon stereocenter.Here $R-O^{-}$ represents either the phenoxide or the oxygen-centered enolate contributor. Excess reagent permits both cleavages. The pre-existing methyl/nitrile stereocenter retains its configuration: it has no hydrogen to remove, and neither bond attached to that center is broken.

After HMPA is added, each anionic oxygen attacks the carbonyl carbon of allyl chloroformate. The C=O pi electrons move to the carbonyl oxygen, giving a tetrahedral intermediate; collapse expels chloride. The enolate becomes an allyl enol carbonate, and the phenoxide becomes an aryl allyl carbonate, each containing $\mathrm{O}-\mathrm{C}(\mathrm{O})-\mathrm{OC}{\mathrm{H}}_2\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$. oxygen acylation accounts for preservation of the enol double bond and for the two newly installed carbonate groups. carbon attack would give a different carbon skeleton. The two oxygens are trapped because they were selectively unmasked by desilylation, rather than because every ether oxygen in the molecule is equally reactive.

A is the kinetic, less-hindered-side silyl enol ether. Its double bond lies away from the methyl substituent. Forming B requires removal of a proton from the more hindered $\alpha$ position adjacent to the methyl-bearing carbon.

The base strengths alone do not determine the ratios. trimethylsilyl triflate is strongly electrophilic: oxygen silylation activates the ketone towards deprotonation, and irreversible trapping of the resulting silyl enol ether can make the product ratio reflect competing deprotonation rates. A weak amine therefore need not first produce a substantial equilibrium concentration of the free ketone enolate.

DBU is the strongest and comparatively accessible base, and distinguishes the two sites least strongly, giving the smallest A:B ratio. Triethylamine gives greater preference for the less-hindered site. The more sterically hindered diisopropylethylamine discriminates most strongly and gives the largest proportion of A. Its slightly higher conjugate-acid $p{K}_a$ than that of triethylamine does not reverse this trend, because steric accessibility and the coupled silylation/deprotonation mechanism matter alongside basicity. The data do not establish a simple thermodynamic enolate distribution.

A silyl enol ether has the connectivity $\mathrm{C}=\mathrm{C}-\mathrm{O}-\mathrm{Si}{\mathrm{R}}_3$. Nucleophilic attack at silicon can cleave the Si–O bond and generate an enolate without adding to the original carbonyl carbon.