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Proton coupling across H-C-C≡C-H can resolve a small four-bond splitting. In a propargyl acetal this connects the acetal methine and terminal alkyne proton.

An anion formed by removing the terminal proton of an alkyne, $\mathrm{RC}\equiv {\mathrm{C}}^{-}$. It can act as a carbon nucleophile, for example in addition to a ketone.

Oxygen-directed anti addition of aluminium and hydrogen across an alkyne, giving a stereodefined vinylaluminium species.

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.

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.

The favoured insertion leaves nickel attached to the methyl-substituted alkyne carbon; the aryl group bonds to the SiMe${}_3$-substituted carbon. This is the connectivity leading to the illustrated ring-closed product: the carbon carrying methyl subsequently attacks the ester carbonyl.

The bulky trimethylsilyl substituent is kept away from the crowded phosphine-ligated nickel centre in the insertion arrangement. The alternative orientation would place Ni next to the more demanding SiMe${}_3$ end. regioselectivity can also reflect electronic polarization of a silyl-substituted alkyne and stabilization of the resulting alkenyl complex. The ortho ester helps capture the insertion product by oxygen coordination and subsequent ring closure. Steric discrimination at insertion, reinforced by the accessible intramolecular cyclization, accounts for the observed direction; the product drawing alone is not a quantitative separation of steric and electronic contributions.

B is oxidative addition; D is alkyne insertion into Ni–aryl followed by carbonyl-oxygen coordination; E is carbonyl insertion into Ni–alkenyl. The distinction is visible from the bonds made and from consistent electron counting.

For B the depicted Ni(0) starting complex has two phosphine and two MeCN donors, giving $10+4+4=18$ electrons. Addition across the aryl C–Br bond produces two X-type ligands and Ni(II). The drawn product has lost the two solvent donors, giving $8+4+2+2=16$ electrons. Intrinsic oxidative addition to the unsolvated 14-electron $\mathrm{N}{\mathrm{i}}^0(\mathrm{dppe})$ fragment raises its count to 16; the overall drawn $18\to 16$ change also includes loss of four solvent-donor electrons. A falling displayed count does not turn this oxidative addition into a reductive elimination.

Before D, the aryl–alkyne intermediate is Ni(I), $d^9$, with $9+4_{\mathrm{P},\mathrm{P}}+2_{\mathrm{aryl}}+2_{\mathrm{alkyne}}=17$ electrons. Syn migration couples aryl carbon to one alkyne carbon and leaves a Ni–alkenyl bond. Insertion alone removes the separate two-electron alkyne donor and would give a 15-electron alkenyl fragment. The ester carbonyl oxygen then binds, as drawn, restoring the count to $9+4+2_{\mathrm{alkenyl}}+2_{\mathrm{O}}=17$. Ni remains $+1$.

In E the Ni-bound alkenyl carbon attacks the coordinated ester carbonyl carbon, forming the new ring-closing C–C bond. The C=O pi electrons move to oxygen, and an Ni–O alkoxide bond replaces Ni–C. This is carbonyl migratory insertion, equivalently intramolecular addition to a coordinated carbonyl. A neutral O donor plus an alkenyl X ligand become one alkoxide X ligand, so the drawn count falls $17\to 15$, while Ni remains $+1$. A solvating molecule could fill the resulting vacancy, but it is not shown and should not be silently added to the stated count.

zinc reduction is required in A, C and F. Reading the drawn ligands with consistent ionic bookkeeping givesIn A the two chlorides of the precursor are removed and neutral solvent donors occupy the reduced centre. In C the bromide is removed while the aryl ligand remains and the neutral alkyne coordinates; the drawn neutral intermediate has Ni(I). In F an anionic alkoxide-derived product is released and the neutral solvated Ni(0) complex is regenerated. zinc supplies electrons by oxidation to Zn(II); halide and alkoxide ions can be taken up in zinc-containing products.

A is precatalyst activation. The two one-electron reductions C and F together require two electrons per catalytic turnover, corresponding to one Zn atom if its oxidation ends at $+2$. The diagram does not establish whether each formal reduction is a single elementary electron-transfer event or a sequence of surface reactions and ligand transfers.

The free propargylic OH first reacts with an aluminium hydride, releasing hydrogen and forming an O–Al bond. Coordination of the nearby alkyne to this aluminium center organizes a five-membered O–C–C–C–Al unit. Hydride addition from the opposite side of the activated alkyne gives a cyclic alkenyl aluminate: aluminium bonds to the distal alkyne carbon, while hydride is delivered to the carbon nearer $\mathrm{C}{\mathrm{H}}_2\mathrm{O}$.

The key step is directed trans hydroalumination. The new C–H and C–Al bonds are anti. Aqueous workup protonolyzes C–Al with retention of its alkene geometry and hydrolyzes the O–Al attachment. Replacing that C–Al bond by C–H therefore produces the E allylic alcohol, with $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ and the rest of the chain on opposite alkene sides. A generic syn hydroalumination drawing followed by simple protonation would predict the wrong geometry.

The free OH provides the directing tether; the silyl-protected oxygen is not equivalently available. Primary electrophile-trapping studies of Red-Al-treated propargylic alcohols support the alkenyl-alanate description.^[ref]

A has a strong ester C=O band at roughly $1715$–$1730\mathrm{c}{\mathrm{m}}^{-1}$, lowered by conjugation with C=C. B's unconjugated ester carbonyl is nearer $1735$–$1750\mathrm{c}{\mathrm{m}}^{-1}$; it also has terminal-alkene features, including the approximately $990/910\mathrm{c}{\mathrm{m}}^{-1}$ pair.

C has no carbonyl band. Its terminal alkyne gives a relatively sharp sp C-H stretch near $3300\mathrm{c}{\mathrm{m}}^{-1}$ and a usually weaker C≡C feature near $2100$–$2150\mathrm{c}{\mathrm{m}}^{-1}$. C-O stretching alone is less diagnostic because esters and the acetal all contain C-O bonds. The numbers are characteristic ranges, not exact frequencies inferred from an unprinted IR trace.

Carbon-13 NMR spectra: 1 = C, 2 = A, 3 = B. Approximate shifts, read from the printed axes, are shown in the annotated structures.

The two methyls in A are chemically distinct because one is cis and the other trans to each substituent at the other alkene carbon. Their two measured shifts are secure; assigning an individual methyl to a particular side of the drawn double bond requires an additional stereospecific shift argument or experiment, so their spatial interchange is left explicit.

The assignments are A → spectrum 3, B → spectrum 4, C → spectrum 1, and D → spectrum 2. The useful infrared spectroscopy evidence is the combination of functional-group absorptions, rather than every feature of the fingerprint region.

A is a saturated diketone: spectrum 3 has a strong ketone C=O band near $1710\mathrm{c}{\mathrm{m}}^{-1}$ without the very broad acid O–H absorption. Its two symmetry-related carbonyls do not require two well-separated diagnostic bands.

B is a conjugated dienoic acid: spectrum 4 combines a broad carboxylic acid O–H envelope, approximately $2500$–$3300\mathrm{c}{\mathrm{m}}^{-1}$, with a conjugated C=O band near $1690$–$1700\mathrm{c}{\mathrm{m}}^{-1}$ and C=C absorptions around $1600$–$1650\mathrm{c}{\mathrm{m}}^{-1}$.

C is an alkynoic acid: spectrum 1 contains the broad acid O–H absorption, a carbonyl near $1700\mathrm{c}{\mathrm{m}}^{-1}$, and a conspicuous C≡C stretch near $2250\mathrm{c}{\mathrm{m}}^{-1}$. Its alkyne is internal, so no terminal ≡C–H stretch is expected.

D is a conjugated vinyl ester: spectrum 2 has a strong ester C=O absorption around $1730$–$1740\mathrm{c}{\mathrm{m}}^{-1}$, alkene features near $1640\mathrm{c}{\mathrm{m}}^{-1}$, and strong C–O fingerprint absorptions, but no broad acid O–H envelope. These are approximate readings of the original axes, not exact assignments of every vibrational mode.

Disconnect the tertiary alcohol at its bond to the alkenyl group. The required carbonyl partner is $\mathrm{MeCOCH}(\mathrm{Me})\mathrm{Et}$; the other partner is an E-configured $\mathrm{PhC}{\mathrm{H}}_2\mathrm{CH}=\mathrm{CH}$ organometallic reagent. This disconnection fixes the alkene geometry before the alcohol is made.

First form the ethyl-side enolate of butan-2-one under equilibrating conditions, then trap it with ethyl iodide and isolate the required monoalkylated ketone:The incoming group is Et: methylation would give a carbon skeleton one carbon too short. Because an unsymmetrical ketone can form two enolates and undergo repeated alkylation, regioisomer separation and controlled conversion are part of this step.

Independently perform a halo-Wittig reaction on phenylacetaldehyde using $\mathrm{P}{\mathrm{h}}_3\mathrm{P}=\mathrm{CHBr}$ and isolate E-$\mathrm{PhC}{\mathrm{H}}_2\mathrm{CH}=\mathrm{CHBr}$ from any geometrical mixture. magnesium insertion in dry ether gives the corresponding vinylmagnesium halide; add the isolated ketone at low temperature and protonate the resulting alkoxide. Addition to C=O preserves the pre-existing C=C geometry:The E alkene and the complete fifteen-carbon skeleton are obtained. No absolute configuration is specified at the newly formed alcohol centre, so an unselected stereoisomeric mixture there is acceptable. An alternative carbon–carbon disconnection is acetylide addition to the same ketone followed by a controlled trans-selective alkyne reduction; the vinyl-reagent route avoids relying on reduction of a tertiary propargylic alcohol.

A propargylic alcohol has its hydroxyl-bearing carbon adjacent to a carbon–carbon triple bond. Its alkoxide can coordinate a metal hydride and direct alkyne reduction.

$\mathrm{C}{\mathrm{H}}_3-\mathrm{C}\equiv \mathrm{CH}$, a terminal alkyne.