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Addition of Si–H across an unsaturated bond. For an alkene, the net products acquire one C–H and one C–Si bond. Alternative catalytic mechanisms can insert the alkene into either a metal–hydride or a metal–silyl bond; these alternatives should not be conflated.

In hydroxyl-directed epoxidation, hydrogen bonding between an allylic hydroxyl and the oxidant favors oxygen delivery from a particular alkene face. This influences diastereoselectivity without requiring an unchanged preference under every solvent or protecting-group condition.

Addition of a zirconium–hydrogen bond across an alkene or alkyne. Schwartz reagent commonly gives a syn addition and places zirconium at the less substituted terminus; reversible insertion can influence regioselectivity.

An alkene bearing two thioalkoxy groups on one carbon, ${\mathrm{R}}_2\mathrm{C}=\mathrm{C}(SR\prime {)}_2$. pi conjugation to a carbonyl can allow addition–elimination at that beta carbon with thiolate departure.

oxidation of an enol or enolate can generate a carbon-centred radical that adds intramolecularly to an alkene. Further oxidation or oxygen capture determines the final functional group.

The conventional addition orientation in which the incoming hydrogen goes to the alkene carbon already bearing more hydrogens, in the ordinary applicable unsymmetrical-alkene setting.

A bridged cationic intermediate formed when an alkene coordinates an electrophilic mercury species. Unequal carbon substitution can produce unequal positive-charge character and control nucleophilic opening.

Metachloroperbenzoic acid is a peroxycarboxylic acid used for concerted oxygen transfer to alkenes. A neighboring allylic hydroxyl can direct its approach through hydrogen bonding.

In migratory insertion, adjacent ligands combine while remaining attached to the metal in the product. In alkene insertion into M–R, R forms a bond to one alkene carbon while the other carbon acquires the M–C bond. metal oxidation state need not change.

Primary experimental proposal of an NHC/silyl nickel hydride and insertion/elimination mechanism. Full published title: Modular Ni(0)/Silane Catalytic System for the Isomerization of Alkenes.

$\mathrm{C}{\mathrm{H}}_2=\mathrm{CHN}{\mathrm{O}}_2$, an electron-poor alkene that can serve as a dienophile. Conversion of a resulting secondary nitro group to a ketone can connect a Diels–Alder reaction to a Nef reaction.

A 1,3-dipolar cycloaddition joining a nitrone to an alkene. Intramolecular tethering controls regioselectivity and relative approach; the nitrone nitrogen–oxygen bond can later be cleaved reductively.

A nucleophile adds reversibly at the beta carbon to form an enolate, which is functionalized before elimination restores the alkene.

Osmium tetroxide adds to an alkene to form a cyclic osmate ester. Hydrolysis releases a vicinal diol; regeneration of Os(VIII) requires a reoxidant in a catalytic cycle.

electrophilic activation of an alkene by mercury(II), followed by nucleophilic opening of a bridged mercurinium ion. water usually attacks the more substituted carbon; subsequent demercuration replaces C–Hg with C–H.

The new ring is formed by intramolecular Friedel-Crafts addition to the ketone. titanium tetrachloride coordinates the ketone oxygen of the cyclic beta-ketoester, increasing electrophilicity at that carbonyl carbon. The nearby electron-rich aryl ring attacks this activated ketone at the available position ortho to its ethyl tether.

The electron-flow sequence is: an aromatic $\pi$ bond forms the new aryl–carbonyl-carbon bond, the ketone $\pi$ bond moves onto oxygen, and loss of the aromatic proton restores aromaticity. The two-carbon tether and the shared quaternary carbon make the newly closed carbocycle six-membered. Hydrolytic work-up converts the titanium-bound alkoxide into the tertiary alcohol.

The lactone carbonyl survives. This is an addition to the ketone with retention of its oxygen as OH, rather than an acyl substitution that removes that oxygen. The pendant terminal alkene is not involved. These bond changes account for the fused aromatic ring system and the alcohol in C without needing to assign the new stereochemistry.

Let $\mathrm{Ar}$ denote the 2,5-dimethoxyphenyl group. A is the doubly C-alkylated beta-ketoesterThe central carbon has no hydrogen after the second step. The alkene, ester and tert-butyl ether remain intact.

carbonate reversibly removes a proton from the methylene group between the two carbonyl groups. The resulting resonance-stabilized enolate attacks the primary iodoethyl carbon of the aryl-containing reagent by an $S_{\mathrm{N}}2$ displacement, releasing iodide. The monoalkylated intermediate still has one proton at the same activated carbon. A second deprotonation followed by $S_{\mathrm{N}}2$ attack on the primary bromide introduces the but-3-en-1-yl group.

The beta-dicarbonyl position is much more acidic than the methylene group next to the ketone and ether at the other end, explaining the alkylation site. Counting the retained carbon skeleton gives ${\mathrm{C}}_{24}{\mathrm{H}}_{36}{\mathrm{O}}_6$, including the two carbonyl groups and one terminal alkene. The second panel of the original product drawing specifies this connectivity.

Number the enone carbonyl carbon C1, the adjacent alkene carbon C2 and the ethoxy-bearing alkene carbon C3. In the bromine-first pathway, the enol-ether-like $C2=C3-\mathrm{OEt}$ unit attacks $\mathrm{B}{\mathrm{r}}_2$ at C2, breaking the Br–Br bond. The intermediate has a new C2–Br bond and positive charge at C3, stabilized as an oxonium chemical resonance contributor by donation from the ethoxy oxygen.

triethylamine removes the remaining C2 hydrogen. Its C–H bonding pair reforms C2=C3 and neutralizes the oxonium system. The result is replacement of the vinylic C2–H by C2–Br, with the original enone and ethoxy group retained. If bromide first captures the cationic intermediate, the equivalent vicinal dibromide can eliminate HBr to reach the same product.

The ethoxy group makes this alkene sufficiently electron-rich for the electrophilic-addition description. It is important to place bromine at C2 and restore the double bond, rather than stop at a saturated addition product.

A is a fused five-/six-membered bicyclic diketone containing sulfur. To specify its structure unambiguously, retain the cyclohexanone numbering C1=O, C2 and C3 for the original alkene. Add the five-membered ringThe original C1 ketone remains, and the former methyl-ester carbonyl becomes the second ketone. The product is drawn in the first panel below.

methoxide first deprotonates the thiol. The soft thiolate attacks the beta carbon C3 in a Michael addition, shifting the enone electrons onto oxygen. The resulting enolate is nucleophilic at C2. It then attacks its tethered ester carbonyl intramolecularly, forming a five-membered ring through a tetrahedral intermediate. Collapse expels methoxide. proton transfers and work-up give the neutral bicyclic beta-diketone.

The initial 1,4 addition followed by intramolecular Claisen acylation explains the regiochemistry: it connects C3 to sulfur and C2 to the ester carbonyl, giving a favorable five-membered closure. Addition to the original ketone would not give this same enolate-and-ester arrangement. The net loss of methanol from the combined starting materials gives the observed ${\mathrm{C}}_8{\mathrm{H}}_{10}{\mathrm{O}}_2\mathrm{S}$ formula.

Label the enamine carbon bonded to nitrogen C1 and its alkene partner C2; C6 is the other carbon next to C1. B is the C-alkylated iminium ion bromide. Its new substituent at C2 is $-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}(\mathrm{C}{\mathrm{O}}_2\mathrm{Et})=\mathrm{C}{\mathrm{H}}_2$, while C1 is double-bonded to the pyrrolidine nitrogen, which bears positive charge. bromide is the counterion, accounting for Br in the empirical formula without requiring a retained C–Br bond.

The C2 end of the enamine attacks the bromomethyl carbon by $S_{\mathrm{N}}2$ displacement. Simultaneously, donation of the nitrogen lone pair creates the C1=N bond. This is ordinary enamine C-alkylation, not N-alkylation.

proton transfer removes an alpha proton at C6, converting the iminium ion into the enamine regioisomer C1=C6. That enamine attacks the terminal $\mathrm{C}{\mathrm{H}}_2$ of the tethered acrylate in an intramolecular Michael addition. nitrogen donation restores the iminium ion, and protonation of the ester enolate completes the sequence.

The new bridge is $C2-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}(\mathrm{C}{\mathrm{O}}_2\mathrm{Et})-\mathrm{C}{\mathrm{H}}_2-C6$. Together with the original ring and its C1 bridge this produces the bicyclo$[\mathrm{3.3.1}]$nonane iminium ion skeleton in C. It retains the pyrrolidine group and the ester; hydrolysis to a ketone is not part of the displayed reaction. The second panel of the original annulation figure shows the chemical atom connectivity and distinguishes ionic bromide in B from a covalent bromine substituent.