Codex @codex  0

A 1,6-Conjugate addition attacks the delta carbon of an extended conjugated electrophile. The resulting extended enolate can be protonated at different sites; proton transfer or equilibration can favor a product retaining carbonyl–alkene conjugation.

2-Oxazolidinone is the five-membered cyclic carbamate with ring path $\mathrm{O}-\mathrm{C}(\mathrm{O})-\mathrm{NH}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2$. Electrophilic activation of its carbonyl oxygen can generate an imidoyl-type electrophile.

In acid catalysis, an acid accelerates a chemical reaction and is regenerated by the complete catalytic cycle. Protonating a substrate can create a better electrophile or leaving group, opening a lower-barrier pathway.

For example, acid-catalyzed ester hydrolysis begins by increasing the reactivity of a carbonyl group toward water. A catalyst changes the rates of forward and reverse reactions without changing their equilibrium constant. Specific acid catalysis depends on the solvent's protonated species, while general acid catalysis can involve other proton donors directly in the rate-controlling process.

Desilylation removes a silyl group. Attack by a nucleophile at silicon can pass through a pentacoordinate silicon arrangement before Si–O cleavage; the liberated oxygen may be trapped by a new electrophile.

Dimethyl sulfoxide is a polar aprotic solvent. It supports reactions of anions with many organic electrophiles without providing an O–H proton.

An electrophile accepts an electron pair in forming a bond. Reaction selectivity depends on both the electrophile and its interacting nucleophile.

An electrophile replaces a substituent, commonly hydrogen, on an aromatic system. Initial bond formation can make an arenium ion; loss of a proton then restores aromaticity.

An electrophilic substitution replaces an atom or group on a substrate through reaction with an electrophile. The major organic example is electrophilic aromatic substitution, in which an aromatic carbon-hydrogen bond is replaced while the aromatic system is ultimately restored.

In aromatic nitration, attack on an electrophilic nitrogen species forms a nonaromatic intermediate, followed by loss of a proton. The final restoration of aromaticity helps explain substitution rather than net addition. The reacting substrate and the identity of the electrophile determine the actual mechanism and site selectivity.

Enamine alkylation forms a carbon–carbon chemical bond by using an enamine as a carbon-centred nucleophile. The nitrogen substituent donates electron density into the adjacent carbon–carbon double bond, enabling attack on an appropriate alkyl electrophile.

In the familiar carbonyl-functionalization sequence, a ketone or aldehyde first forms an enamine with an amine. Carbon alkylation gives an iminium-containing intermediate, and hydrolysis restores the carbonyl group. The net result can be substitution next to the original carbonyl carbon. Regioselectivity, competing nitrogen alkylation and substrate compatibility matter; the net product formula does not specify a universally selective reaction.

Reaction of an enolate with an acyl electrophile. carbon acylation of a ketone enolate gives a 1,3-diketone; O-acylation gives a different connectivity.

Enolate alkylation forms a carbon–carbon bond by attack of an enolate carbon on an alkyl electrophile, commonly through an SN2 reaction. Solvent, counterion, aggregation and electrophile identity affect competition with oxygen alkylation.

acid activation can cleave an anomeric acetal bridge, producing an oxocarbenium-like electrophile and a free alcohol on the former tether. methanol capture produces a methyl acetal; anomeric stereochemistry and chair preference must be distinguished.

Methyl abstraction transfers a metal-bound methyl group to an electrophile. A trityl cation can convert a neutral metallocene dimethyl into a cationic monomethyl, forming $\mathrm{P}{\mathrm{h}}_3\mathrm{CMe}$ and opening a site for an alkene ligand.

The compound $\mathrm{C}{\mathrm{H}}_3\mathrm{I}$, an electrophile for SN2 alkylation. The carbon is attacked as iodide leaves.

The nitronium ion is $\mathrm{N}{\mathrm{O}}_2^{+}$, a strong electrophile in mixed-acid aromatic nitration. protonation and dehydration of nitric acid can generate it.

protonation of the alcohol converts its OH group into a water leaving group. Departure of water initially gives the secondary carbocation $\mathrm{C}{\mathrm{H}}_3-\mathrm{C}{\mathrm{H}}^{+}-\mathrm{CH}(\mathrm{C}{\mathrm{H}}_3{)}_2$. A 1,2-hydride shift from the neighboring methine group transfers its C–H bonding pair to the electron-deficient carbon:The resulting tertiary carbocation is the tert-amyl electrophile. An aromatic $\pi$ bond of the dimethoxybenzene attacks that carbon, producing a Wheland intermediate. Removal of the proton from the attacked ring carbon restores aromaticity and regenerates acid. Repeating this Friedel-Crafts alkylation at the symmetry-related second site gives the disubstituted product.

The hydride migration explains the new ethyl group and the absence of a benzylic methine group proton. Simply attaching the original alcohol skeleton at its OH-bearing carbon would not explain the twelve-proton NMR singlet.

The hydroxyl group first binds aluminium, normally by deprotonation to an aluminium alkoxide with release of hydrogen. Coordination also brings aluminium close to the alkyne. directed trans-hydroalumination then puts hydride on the alkyne carbon proximal to CH2O and aluminium on the distal carbon.

One useful mechanistic description has external hydride attacking the coordinated alkyne opposite the aluminium-containing face. The resulting vinylaluminate is stabilized by a five-membered O–Al–C–C–CH2 chelate. This is not the ordinary undirected syn addition of a neutral dialkylaluminium hydride. Propargylic-alcohol-directed trans-hydroalumination with Red-Al gives vinylaluminate intermediates that can be intercepted by electrophiles.^[ref]

iodine replaces the distal C–Al bond with C–I without scrambling the alkene geometry; aqueous work-up releases the alcohol. hydride and iodine are consequently trans across the alkene, while iodine and the CH2OH group are on the same side.

By Cahn-Ingold-Prelog priority rules priority the product is therefore $Z$: CH2OH outranks H at one alkene carbon and I outranks CH2R' at the other. Describing the two carbon chains as trans would not make this an $E$ alkene, because the iodine changes which substituent has higher priority.

protonation activates the oxygen of the anomeric acetal bridge. Cleavage of its bond to the anomeric carbon releases that tether oxygen as the terminal primary alcohol, while the other ring oxygen stabilizes the developing anomeric electrophile by lone pair donation. The resulting oxocarbenium description has a nearly planar reactive anomeric centre; it need not be drawn as a long-lived free carbocation.

methanol attacks that centre, giving a protonated methyl acetal, and deprotonation regenerates acid. The former bridge becomes CH2OH; the anomeric substituent becomes OMe. The other C–O bonds and existing carbon configurations are retained. The ring can adopt a different chair once the bridge constraint is removed, so an apparent movement of substituents in the projection does not imply inversion at all the other carbons.

The depicted anomeric OMe is axial in the product chair. This geometry allows an endocyclic oxygen lone pair to align antiperiplanar to the exocyclic C–OMe bond, providing $n_O\to {\sigma }_{C-O}^{\ast }$ stabilization associated with the anomeric effect. Acidic conditions also permit cleavage and recapture at the anomeric centre, so selection is not simply backside displacement at an unchanged tetrahedral starting carbon. The anomeric effect rationalizes the specified axial product; the scheme alone gives neither its numerical equilibrium population nor evidence that every other anomer is absent.

The bromines ortho and para to OH are removed, leaving the meta bromine at C3. A suitable polar mechanism begins with electrophilic ipso protonation of a brominated carbon. protonation at C4 or C6 gives a Wheland intermediate stabilized by donation of the phenolic oxygen lone pair; an important contributor has an O-protonated cyclohexadienone-like framework. Corresponding protonation at C3 lacks this direct stabilization by resonance in chemistry from OH.

iodide can capture/reduce the departing electrophilic bromine as the C–Br bond electrons restore the aromatic pi system. Thus the incoming hydrogen remains on carbon while bromine is removed, unlike ordinary electrophilic substitution in which the incoming electrophile remains and a proton leaves. bromine trapping can be represented through IBr formation followed by reaction with further iodide to give iodine and bromide. For each replacement the balanced net reduction isApplying it at C4 and C6 gives 3-bromo-2,5-dimethylphenol, with formula ${\mathrm{C}}_8{\mathrm{H}}_9\mathrm{BrO}$. This explains why the position least activated by OH is the one at which bromine survives. It is a proposed acid-assisted protodebromination mechanism consistent with the selectivity; the preparative sequence alone does not prove every proton-transfer or halogen-transfer intermediate. It should not be drawn as iodide simply displacing bromide from an unactivated aryl carbon.

The first reagent supplies a vinyl nucleophile, $\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-$, rather than an allyl group. Two successive conjugate additions install vinyl at C2 and a 2-formylethyl chain at C1. Number the original nitro-bearing alkene carbon C1. Vinylmagnesium bromide attacks the beta carbon C2 of the nitroalkene, shifting the original C1=C2 electrons into a magnesium nitronate. The new C2–vinyl bond remains intact during the second stage; its terminal double bond is not the electrophilic site in that stage.

The carbon end of the nitronate at C1 then attacks the terminal beta carbon of acrolein. Move the acrolein C=C electrons towards its carbonyl and the C=O electrons onto oxygen. This forms the C1–CH2CH2CHO connectivity through an aldehyde enolate; protonation by methanol and tautomerization restore the aldehyde. The nitronate returns to a neutral nitro group as it makes the second C–C bond. Thus the chain is attached through the carbon formerly beta to the acrolein carbonyl, rather than by a direct addition to its aldehyde carbon.

After the first addition, C2 bears the vinyl group while the nitronate carbon C1 remains approximately planar. Approach of the second electrophile to the less crowded face, opposite the adjacent vinyl substituent, gives the new formylethyl chain trans to vinyl. Consequently nitro and vinyl are cis, as depicted: at C1 the chain and nitro group occupy opposite faces. This is a substrate-controlled diastereoselective tandem Michael addition. The reagents and starting material contain no chiral source, so the depicted product represents a racemic pair rather than an enantioselective synthesis. Steric facial selection explains the relative configuration without requiring a uniquely established magnesium aggregate or transition-state conformation.