Discussion
The most suitable reagent for the following transformation is
Answer: A
The most suitable reagent is Li / Liq. NH3.
Only the carbon-carbon double bond conjugated with the ketone is reduced; the isolated ring double bond and the C=O group survive. A dissolving-metal reduction does exactly this: lithium in liquid ammonia adds an electron to the enone, the radical anion is protonated and reduced again, and the resulting enolate is protonated on work-up, so the α,β-double bond is reduced while isolated alkenes, which cannot accept an electron, are left alone.
PtO2 / H2 would hydrogenate both double bonds. LiAlH4 attacks the carbonyl group, giving an alcohol. B2H6 hydroborates the more accessible isolated alkene and reduces ketones, the opposite of the selectivity needed.
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