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Acetonitrile is suitable: it dissolves the organic reagents, is polar enough to stabilize the developing ionic products and is an aprotic, relatively unreactive medium. It does not strongly hydrogen-bond-solvate the amine lone pair or compete efficiently as the methylating nucleophile. Sufficient solubility and chemical compatibility matter as well as polarity.
All comparisons assume the same temperature and comparable reagent thermodynamic activities in methanol.
- Iodomethane: faster, because iodide is a better leaving group and the C-I bond is more readily broken than C-Cl.
- 1-Chloropropane: slower than methyl chloride for SN2, because its ethyl substituent crowds backside approach and raises the substitution barrier. It is still a primary substrate, so SN2 remains plausible.
- MeSNa: generally faster substitution with methanethiolate than methoxide in this protic solvent. Sulfur is more polarizable and its larger anion is less strongly hydrogen-bond-solvated, making it a better nucleophile here. This must not be inferred simply from basicity: methanethiolate is the weaker base.
The ordering is allyl chloride 1-chloropropane 2-chloropropane. Iodide is a good nucleophile but a weak base, and polar aprotic solvent acetone favours its SN2 reaction. SN2 reaction and C–Cl cleavage occur in one step; precipitation of sodium chloride can also favour halide exchange without changing the basic steric comparison.
The primary saturated chloride supplies the reference rate. Allyl chloride is also primary, but its neighbouring pi system stabilizes the developing delocalized electronic structure in the substitution transition state. It can therefore react about 64 times faster without requiring a free allylic carbocation. The secondary chloride is sterically crowded at the attacked carbon, making backside approach and the crowded transition state less favourable; its rate is about 0.02 times the primary value. The leaving group and nucleophile are the same in the comparison, so assigning the ordering to different halide leaving groups would be incorrect. If prefactor differences are neglected, at 333 K gives roughly and for the allylic and secondary substrates respectively: modest barrier changes produce large rate ratios.
Use the nitro group as a masked carbonyl and a carbon nucleophile. The most informative disconnection reverses the six-membered aldol annulation. Before that closure, the cyclopentanone beta carbon bearsThe carbon carrying will become the lower carbonyl of the final six-membered ring by a Nef reaction. It comes from the specified nitromethane. The remote methyl ketone supplies the upper methyl-substituted alkene carbon after aldol condensation.
First, use a mild base to generate a nitromethane-derived nitronate and add it conjugatively to the indicated cyclopent-2-en-1-one. Protonation gives 3-(nitromethyl)cyclopentanone. Deprotonate its nitromethyl group and perform a second Michael addition to methyl vinyl ketone, giving . The nitro-stabilized carbon is appreciably more acidic than a simple ketone alpha carbon, permitting selective nitronate formation; control the acceptor stoichiometry to avoid a further Michael alkylation of the remaining nitro-bearing H.
Form the enolate/enamine at the cyclopentanone carbon between its carbonyl and the side-chain-bearing carbon. Intramolecular attack on the remote methyl ketone closes a six-membered ring; the two shared atoms are the original enone alpha and beta carbons. Hydrolysis if required and dehydration create the alkene between the original alpha carbon and the newly formed tertiary carbinol carbon. That carbinol carbon retains its methyl substituent, placing the methyl exactly as required. The original cyclopentanone carbonyl remains the carbonyl of the five-membered ring.
Finally, form the secondary nitronate and carry out controlled acidic Nef hydrolysis to replace its unit by . The resulting fused product contains the two required ketones. Carrying out the annulation before exposing this second ketone reduces competing enolate/aldol pathways that would be available in the corresponding triketone. The nitro intermediate's competing deprotonation means that the aldol step needs conditions designed to access the ring carbonyl enolate/enamine; simply treating every carbonyl position as equally reactive would not establish the route.
The carbon ledger is from cyclopentenone, from nitromethane and from methyl vinyl ketone, giving the nine-carbon fused skeleton plus its methyl substituent. A primary-nitro Nef reaction performed before the second carbon alkylation would instead give an aldehyde and would lose this useful protected-carbonyl construction.
Open the cyclic ester at its acyl–oxygen bond. The resulting delta-hydroxy acid isOxidation-state reversal at the benzylic alcohol gives . This identifies the key Michael disconnection between the central and groups:The nucleophile is the enolate or enamine of the phenyl ketone. The electrophile's propyl substituent must be alpha to its ester carbonyl; using a beta-propyl acrylate would move the substituent to the wrong ring position.
Disconnect further to the indicated acetophenone. Crossed aldol addition to acetone followed by dehydration gives ; selective conjugate reduction or controlled alkene hydrogenation then gives . This avoids requiring an efficient SN2 alkylation with a secondary isopropyl halide. The alpha-propyl acrylate can be obtained by alpha-hydroxymethylation/dehydration of ethyl pentanoate, , using formaldehyde.
In the forward direction, perform the Michael coupling, reduce the phenyl ketone selectively with sodium borohydride while retaining the ordinary ester, hydrolyse the ester, and cyclize the delta-hydroxy acid by controlled acid-catalysed lactonization or an activated-acid coupling. The six-membered ring then has the order carbonyl–––––O. This atom order checks all three substituent positions. No relative configuration is specified, so the scheme does not imply a single diastereomer from the Michael addition or reduction.
Amide base removes a proton alpha to the nitrile, giving a resonance-stabilized carbon nucleophile. Removal of the available aryl proton ortho to chlorine, together with chloride elimination, generates a benzyne between the chlorine-bearing carbon and its unsubstituted ortho neighbour. The side-chain nitrile carbanion attacks the former chlorine-bearing aryne carbon intramolecularly; one aryne pi bond shifts to the neighbouring aryl carbon. Protonation of that aryl anion by ammonia restores the aromatic ring. The newly formed C–C bond closes the four-membered ring of the cyanobenzocyclobutane product. Two deprotonation processes are required to generate both the nitrile nucleophile and the aryne. The nitrile group is retained, and the product is not formed by simple replacement of aryl chlorine with an external amide nucleophile.
The first equivalent of titanocene methylidene methylenates the ester: the carbonyl oxygen and titanium form an oxatitanacyclobutane, whose fragmentation gives and a titanium–oxygen product. The second equivalent adds across this electron-rich double bond. The initial adduct is a titanacyclobutane with cyclic connectivity , closing from the last methylene to titanium. The ester's phenyl-substituted carbon lies opposite Ti; the two new methylenes originate from the two methylidene equivalents.
Beta-methoxy elimination opens that metallacycle to B, , the allyltitanium reagent. The C–OMe bond breaks, OMe transfers to Ti, one Ti–C bond becomes the new C=C bond, and the other Ti–C bond remains as the allylic nucleophile. The cyclic adduct is therefore a precursor to the opened reagent B; treating B as merely the one-equivalent enol ether loses the second reagent carbon.
The aldehyde oxygen coordinates to titanium. Allylic carbon transfer to its carbonyl carbon, accompanied by movement of the C=O pi electrons to oxygen, gives the titanium alkoxide . Acidic aqueous workup cleaves O–Ti and protonates oxygen, giving . A six-membered allyl-transfer arrangement is a convenient mechanistic model. No enantioselectivity is imposed by these achiral reagents.
Phosphonoacetate 2026-10-20
A phosphonate with an adjacent ester-bearing carbon that can supply the carbon nucleophile in a Horner–Wadsworth–Emmons olefination.
Serine residue 2026-10-20
A serine unit within a polypeptide. Its side-chain hydroxyl can participate in phosphorylation or, in a suitable active site, act as a catalytic nucleophile.
Sigma-H adduct 2026-10-20
A sigma-H adduct forms by nucleophilic addition at an aromatic carbon that initially bears hydrogen. Both the original hydrogen and the incoming nucleophile remain attached at that carbon before the rearomatization steps.
Thiol Michael addition 2026-10-20
Conjugate addition of a thiol-derived sulfur nucleophile to an activated alkene. The sulfur substituent is installed at the beta carbon of an enone, not at its carbonyl carbon.
Thiophenol 2026-10-20
Vicarious nucleophilic substitution 2026-10-20
Vicarious nucleophilic substitution replaces aromatic hydrogen using a nucleophile that carries its own leaving group. In nitroarenes, reversible carbanion addition gives a sigma-H adduct; base-induced elimination of the hydrogen and the nucleophile-borne leaving group, followed by protonation, gives substitution without expelling hydride.