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Conjugate (1,4) addition to α,β-unsaturated carbonyls, soft vs hard nucleophiles, and the Robinson annulation ring-forming sequence
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1,2- vs 1,4-addition (HSAB) — hard nucleophiles (RLi, RMgBr, LiAlH₄) attack the carbonyl carbon (1,2); soft, stabilized nucleophiles (enolates, malonate, RCu, thiols, amines) attack the β-carbon of an α,β-unsaturated carbonyl (1,4 / Michael).
Michael donors — β-ketoesters, malonate esters, β-diketones, nitroalkanes, enamines.
Robinson annulation = Michael + intramolecular aldol condensation
This sequence is the workhorse for building 6-membered carbocycles in steroid and terpenoid total synthesis.
Predict the major product when (a) PhMgBr adds to 2-cyclohexenone, vs (b) Ph₂CuLi (Gilman reagent) adds to 2-cyclohexenone. Use HSAB to explain.
(a) PhMgBr → 1,2-addition — Grignards are hard nucleophiles (highly polarized, charge-localized, RDS is irreversible). They attack the harder electrophilic site, the carbonyl carbon, giving 3-phenylcyclohex-2-en-1-ol after aqueous workup.
(b) Ph₂CuLi → 1,4-addition (Michael) — Organocuprates are soft, polarizable nucleophiles. They prefer the softer site, the β-carbon (C3), giving the enolate, then 3-phenylcyclohexan-1-one after protonation. This selectivity is the textbook way to install an α,β-unsaturation product as a saturated β-substituted ketone.
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mol of reacts with mol of . How many grams of water are produced? Which is the limiting reagent? ()
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Show the complete Robinson annulation of 2-methyl-1,3-cyclohexanedione with methyl vinyl ketone (MVK) using KOH/EtOH. Provide the Michael adduct, the intramolecular aldol intermediate, and the final cyclohexenone-containing bicyclic product (the Wieland–Miescher ketone).
Step 1 — Michael addition. KOH deprotonates the doubly-activated α-carbon of 2-methyl-1,3-cyclohexanedione (pKa ≈ 5–6). The stabilized enolate attacks MVK in a 1,4 fashion, giving a 1,5-diketone with a new –CH₂CH₂COCH₃ side chain on the quaternary α-carbon.
Step 2 — Intramolecular aldol. A second equivalent of base forms the enolate at the methyl ketone's α-position. That enolate cyclizes onto one of the ring carbonyls (5-exo-trig), giving a β-hydroxy enone tethered onto the original 6-membered ring.
Step 3 — E1cb dehydration. Heat eliminates the β-hydroxide to install an α,β-unsaturation in the newly formed ring.
Product — (S)-(+)-Wieland–Miescher ketone, a cis-fused bicyclic 6-6 enedione with a quaternary methyl-bearing carbon at the ring junction. This compound is a foundational building block in steroid total synthesis (Hajos–Parrish/Wieland–Miescher ketones provide the AB rings).