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The first equivalent of titanocene methylidene methylenates the ester: the carbonyl oxygen and titanium form an oxatitanacyclobutane, whose fragmentation gives $\mathrm{PhC}(\mathrm{OMe})=\mathrm{C}{\mathrm{H}}_2$ and a titanium–oxygen product. The second equivalent adds across this electron-rich double bond. The initial adduct is a titanacyclobutane with cyclic connectivity $\mathrm{C}{\mathrm{p}}_2\mathrm{Ti}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}(\mathrm{Ph})(\mathrm{OMe})-\mathrm{C}{\mathrm{H}}_2-$, 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, $\mathrm{C}{\mathrm{p}}_2\mathrm{Ti}(\mathrm{OMe})[\mathrm{C}{\mathrm{H}}_2\mathrm{C}(\mathrm{Ph})=\mathrm{C}{\mathrm{H}}_2]$, 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 $\mathrm{PhC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{O}-\mathrm{Ti})\mathrm{C}{\mathrm{H}}_2\mathrm{C}(\mathrm{Ph})=\mathrm{C}{\mathrm{H}}_2$. Acidic aqueous workup cleaves O–Ti and protonates oxygen, giving $\mathrm{PhC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{OH})\mathrm{C}{\mathrm{H}}_2\mathrm{C}(\mathrm{Ph})=\mathrm{C}{\mathrm{H}}_2$. A six-membered allyl-transfer arrangement is a convenient mechanistic model. No enantioselectivity is imposed by these achiral reagents.

A phosphonate with an adjacent ester-bearing carbon that can supply the carbon nucleophile in a Horner–Wadsworth–Emmons olefination.

Potassium hexamethyldisilazide is the potassium salt of the anion $\mathrm{N}(\mathrm{SiM}{\mathrm{e}}_3{)}_2^{-}$. The bulky silyl groups make it useful as a strong, sterically hindered base.

It can remove an acidic proton to form an enolate while competing less readily in some addition reactions than a smaller nucleophile. Counterion, solvent, and aggregation still influence the observed selectivity.

RNA degradation is the chemical or enzymatic breakdown of RNA, including cleavage of its phosphodiester bonds into shorter strands. Fragmentation can remove the continuity needed for an RNA molecule's function even while much of its nucleotide sequence remains present. Ribonucleases catalyze RNA cleavage; chemical cleavage also occurs without an enzyme.

A chemical route exploits the hydroxyl group at the $2^{\prime }$ position of ribose. Its oxygen can act as a nucleophile toward the adjacent backbone phosphorus, producing an internal transesterification that breaks the chain. In their 1999 primary study, Yingfu Li and Ronald R. Breaker used chimeric DNA/RNA substrates to measure hydroxide-dependent cleavage of RNA phosphodiester bonds. Such a substrate isolates a susceptible linkage within a defined molecular context, allowing chemical instability to be measured separately from a complex sample's overall fragmentation pattern.^[ref]

An illustrative kinetic model follows the number $N$ of molecules that remain intact under constant conditions. If each intact molecule has a constant probability per unit time $k$ of undergoing its first destructive cleavage, thenThe last quantity is an RNA half-life in this model. With stipulated $k=0.020\mathrm{mi}{\mathrm{n}}^{-1}$, half the starting molecules remain intact after about $34.7\min$, and $e^{-1.2}\approx 30.1\%$ after an hour. The numerical rate is an example, not a universal value for RNA or a measurement from the cited experiment.

A cleavage need not remove material from the tube: breaking one long RNA into two pieces initially preserves its total number of nucleotides. Consequently a concentration measurement can stay almost unchanged while RNA integrity declines. An electropherogram can reveal redistribution toward shorter fragments, while a short target region may remain detectable by reverse transcription and polymerase chain reaction. The relevant question is which molecular continuity the intended assay requires.

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.

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.

A substitution reaction replaces an atom or group in a molecular entity with another. The replacement can occur through one elementary step or several; the definition specifies the net chemical change rather than a unique mechanism.^[ref]

For example, in $R-X+N{u}^{-}\to R-Nu+X^{-}$, a nucleophile replaces a leaving group. Concerted displacement and a pathway through a carbocation can have different concentration dependences and stereochemical outcomes even when the written products match. The reacting functional group, solvent and competing elimination reactions help determine which pathway is observed.

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 is $\mathrm{PhSH}$. Its conjugate base is thiophenolate, $\mathrm{Ph}{\mathrm{S}}^{-}$, a sulfur-centered nucleophile.

Tomasz Glinka coauthored research on vicarious nucleophilic substitution.^[ref]

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.

Trimethylsilyl chloride is $\mathrm{M}{\mathrm{e}}_3\mathrm{SiCl}$. Reaction of an oxygen nucleophile at silicon can replace chloride and form a trimethylsilyl ether.

Trapping an enolate on oxygen forms a silyl enol ether. This preserves an enol-derived carbon-carbon double bond while changing the species' charge and handling properties.

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.

William T. Booth Jr. coauthored research on grignard reagent.^[ref]

An organomagnesium reagent commonly written $\mathrm{RMgX}$, with organic group R and halide X. Its carbon group can act as a nucleophile; actual solution structures depend on solvent and aggregation.