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A hydrosilylation cycle involving Si–H oxidative addition, alkene insertion into the metal–hydride bond, and C–Si reductive elimination. This defines the conventional version; insertion into a metal–silyl bond describes a modified alternative.

In a standard oxidative addition, a bond X–Y is cleaved and both fragments become metal ligands, increasing formal metal oxidation state by two. A B–Br bond can give a metal–boryl bond and a bromide ligand without cleaving a borazine ring.

Replacement of an organic halide by hydrogen through a palladium catalytic cycle. oxidative addition, hydride delivery and reductive elimination can preserve the stereochemistry of an alkenyl fragment.

oxidative addition of an intact Si–H bond initially places H and SiR${}_3$ cis in the metal coordination sphere; this is the local arrangement generated by addition of a sigma bond. hydride and alkene must also attain adjacent sites for intramolecular insertion, and the eventual alkyl and silyl groups must be cis for direct reductive elimination. Subsequent rearrangements should not be confused with the stereochemistry of the bond-addition event itself.

For an alkyl-substituted terminal alkene, insertion puts Co on the terminal carbon and H on the substituted carbon, producing $\mathrm{Co}-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{R}$. It avoids the more crowded secondary metal–alkyl arrangement. Replacement of that Co–C bond by C–Si therefore gives the linear product $\mathrm{RC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{Si}{\mathrm{R}}_3$, an Anti-Markovnikov addition outcome. Hydrometallation is syn at the coordinated alkene, although this simple terminal substrate generally does not retain two substituents whose relative stereochemistry could diagnose the complete sequence.

Step 1 is oxidative addition of Si–H to Co(I), giving Co(III)–H and Co(III)–SiR${}_3$. It cleaves the Si–H bond and forms two metal–ligand bonds, increasing the formal metal oxidation state by two. The phosphine dissociation shown before this numbered step makes room for the reaction; alkene binding and phosphine exchange are ligand-substitution events rather than oxidative additions.

Step 2 is migratory insertion of the coordinated alkene into Co–H, equivalently intramolecular hydride migration to the alkene. One alkene carbon gains H and the other gains the Co–C bond. Co remains formally Co(III); a two-electron alkene ligand plus a hydride is replaced by one alkyl ligand, reducing the electron count by two before phosphine reassociation.

Step 3 is C–Si reductive elimination between the alkyl and silyl ligands. It releases the hydrosilylated alkane and regenerates Co(I). The electron count for the drawn sequence is consistent: the one-PMe${}_3$ alkene adduct is 16-electron; oxidative addition gives 18; insertion gives 16; PMe${}_3$ association gives 18; reductive elimination returns the 16-electron catalyst. Because insertion occurs into Co–H, this is the conventional Chalk-Harrod mechanism.

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.

oxidative addition inserts Pt into one B–Br bond. All three B–Br positions of the starting ring are equivalent, so there is no distinct choice among them before the first substitution. The product is the square-planar Pt(II) boryl bromide $\mathrm{trans}-[\mathrm{Pt}(\mathrm{PC}{\mathrm{y}}_3{)}_2(\mathrm{Br})({\mathrm{B}}_3\mathrm{B}{\mathrm{r}}_2{\mathrm{N}}_3{\mathrm{H}}_3)]$, with the two phosphines trans, the Pt–B bond trans to Pt–Br, and the six-membered ring intact. The third panel shows this connectivity. This is a metal–boron sigma bond, not attachment of the whole ring as an ${\eta }^6$ pi ligand. Formal electron counting gives Pt(II), $d^8$, with $8+4_{\mathrm{P}}+2_{\mathrm{Br}}+2_{\mathrm{boryl}}=16$ electrons, appropriate for square-planar coordination. The primary synthesis establishes this trans-bromo(borazinyl) structure.^[ref]

The electron-rich Pt(0) reacts at the polarized B–Br bond, whose electrophilic boron and departing bromide provide a favorable bond-activation pathway. This accounts for B functionalization without needing cleavage of a ring B–N bond or selection of a nitrogen-bound product. It does not imply that arbitrary N–H activation is impossible under every condition.

Replacing an electronegative bromine substituent by a metal fragment changes the boron's electron demand. Relative to the B–Br sites, the metal-bound boron is less effective at accepting N-to-B pi donation, so its adjacent B–N bonds are expected to have less multiple-bond character and be longer; donation toward the remaining B–Br sites produces compensating unequal ring bonds. The original threefold equivalence is lost. The detailed lengths depend on the full electronic structure; a localized drawing with three rigid alternating double bonds would conceal the redistribution that the unequal lengths reveal.

Write $L$ for the imidazolylidene N-heterocyclic carbene bearing the two mesityl groups shown. The proposed active unit is $\mathrm{LNi}(\mathrm{H})(\mathrm{SiP}{\mathrm{h}}_3)$, with H and SiPh${}_3$ both bonded to nickel, rather than a nickel-free hydrosilane or a bare NHC-Ni fragment.

Displacement of coordinated styrene and formal oxidative addition of Si-H to Ni(0) give a low-coordinate nickel(II) hydride-silyl unit; substrate binding fills a vacant site. The productive cycle is alkene coordination, migratory insertion into Ni-H, then beta-hydride elimination from the other beta carbon, releasing the shifted alkene and regenerating Ni-H.

The strongly donating NHC stabilizes a reactive low-coordinate metal and assists Si-H activation. Its steric bulk discourages unproductive saturated bis-alkene species and promotes product departure, but excessive crowding can hinder substrate binding/insertion. The silyl ligand also tunes the metal electronically: changing its aryl substituents changes donation and the barriers for hydride transfer/elimination. Product release and reversible insertion determine whether further double-bond migration occurs. Bulky ligands can suppress competing alkyl-silyl reductive elimination, which would instead consume substrate by hydrosilylation.

The hydride-silyl formulation and insertion/elimination interpretation are the experimentally proposed mechanism; the individual catalyst resting states need not all be uniquely fixed by the activation recipe.^[ref]

A coherent mechanism is an allene cyclization–norbornene insertion–cyclopropane closure cascade. Label the propargyl chain $\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}\equiv \mathrm{CH}$ as $D-E-F$. Base-assisted propargyl-to-allenyl isomerization gives $\mathrm{N}-\mathrm{CH}=\mathrm{C}=\mathrm{C}{\mathrm{H}}_2$. The hydrogen relocation is important: the terminal carbon must become a methylene before the final C–H substitution.

Reduction of the palladium precursor gives a phosphine-ligated Pd(0) catalyst. Oxidative addition into the aryl C–I bond supplies an aryl–Pd(II) intermediate. Intramolecular carbopalladation of the allene forms the aryl–$E$ bond and the six-membered lactam ring, retaining $D=E$ and placing Pd on the terminal carbon $F$. This allylic organopalladium connectivity may be represented by its interconverting sigma and pi forms.

Norbornene approaches from its less hindered exo face and inserts into the $F$–Pd bond. Carbon $F$ bonds to one former norbornene alkene carbon, while Pd bonds to the adjacent one. The resulting intermediate is geometrically unable to undergo an ordinary favorable beta-hydride elimination: the relevant syn arrangement is blocked, or would produce a bridgehead alkene.

Base-assisted activation of an $F$–H bond gives a cyclopropane-forming dialkylpalladium connectivity, represented as a four-membered Pd–C–C–$F$ ring. Reductive elimination closes the bond between $F$ and the Pd-bearing norbornene carbon, releases Pd(0), and gives the cyclopropane. The lactam alkene $D=E$ remains intact. Carbonate removes the acid equivalent generated in the overall arylation.

This mapping accounts for the six-membered heterocycle, the new three-membered ring and exo addition to norbornene. The catalyst and substrates are achiral, so the drawn relative stereochemistry represents a racemic product. This is a proposed organopalladium pathway; the question's product alone does not uniquely establish the lifetime or order of every palladium intermediate.

Only one bromine is replaced by hydrogen. Writing the unchanged protected chain as $R=\mathrm{THPOC}{\mathrm{H}}_2\mathrm{CH}(\mathrm{Me})$, the product is $R\mathrm{CH}=\mathrm{C}(\mathrm{Me})\mathrm{CH}=\mathrm{CHBr}$, retaining the original internal alkene geometry and forming predominantly the $Z$ terminal bromoalkene: bromine and the carbon chain lie on the same side of that new stereodefined double bond. The formula is consequently ${\mathrm{C}}_{13}{\mathrm{H}}_{21}\mathrm{Br}{\mathrm{O}}_2$.

Phosphine dissociation exposes Pd(0), which undergoes preferential oxidative addition into the less hindered C–Br bond, the bromine trans to the carbon chain in the gem-dibromoalkene. hydride transfer from tributyltin hydride replaces the palladium bromide ligand, producing tributyltin bromide and an alkenyl–Pd–H intermediate. reductive elimination joins the alkenyl carbon and hydride, regenerating Pd(0). Retention of the alkenyl geometry through this sequence puts the surviving bromine cis to the chain. The conjugated carbon skeleton and protecting group remain intact; this is not addition of tin hydride across either alkene. The stereoselective monohydrogenolysis is established for this reagent combination.^[ref]

Let $\mathrm{Ar}=2,6$-dimethylphenyl and $\mathrm{A}{\mathrm{r}}^{\prime }=2$-methoxyphenyl. After conversion of the Pd(OAc)${}_2$ precatalyst into an active phosphine–Pd(0) species, oxidative addition of $\mathrm{A}{\mathrm{r}}^{\prime }\mathrm{Cl}$ gives ${\mathrm{L}}_{\mathrm{n}}\mathrm{P}{\mathrm{d}}^{\mathrm{II}}(\mathrm{A}{\mathrm{r}}^{\prime })\mathrm{Cl}$. Carbonate-assisted replacement of chloride by the alcohol's alkoxide produces ${\mathrm{L}}_{\mathrm{n}}\mathrm{P}{\mathrm{d}}^{\mathrm{II}}(\mathrm{A}{\mathrm{r}}^{\prime })[\mathrm{OC}(\mathrm{Me}{)}_2\mathrm{Ar}]$.

In the key beta-carbon elimination, the alkoxide oxygen forms a C=O bond while the bond between its carbinol carbon and $\mathrm{Ar}$ cleaves, transferring $\mathrm{Ar}$ to palladium. This gives acetone and a diarylpalladium(II) complex, ${\mathrm{L}}_{\mathrm{n}}\mathrm{P}{\mathrm{d}}^{\mathrm{II}}(\mathrm{A}{\mathrm{r}}^{\prime })(\mathrm{Ar})$. reductive elimination creates the new biaryl bond and regenerates Pd(0). Thus the tertiary alcohol functions as an aryl donor by carbon–carbon bond cleavage, rather than being incorporated as a carbinol substituent. The overall organic products are the cross-coupled biaryl and acetone. The carbinol carbon has no hydrogen, so ordinary beta-hydride elimination from this alkoxide cannot explain the observed aryl transfer. The bulky electron-rich phosphine helps access the aryl-chloride oxidative addition and the sterically congested biaryl-forming elimination.

An unsaturated Rh(I)–BINAP complex undergoes oxidative addition of the aldehydic C–H bond, giving a Rh(III) acyl hydride. Coordination of the tethered alkene brings it into the chiral coordination sphere. hydride migration to the internal, tert-butyl-substituted alkene carbon places Rh on the terminal methylene. The resulting acyl–alkyl complex has the connectivity required for a five-membered ring. reductive elimination between its acyl and alkyl carbons closes the C–C bond, releases the cyclopentanone and regenerates Rh(I).

The aldehyde carbon becomes the ketone carbonyl carbon; its hydrogen ultimately reaches the alkene-derived internal carbon. The original tert-butyl substituent remains attached to the internal alkene carbon, which becomes the beta stereocentre of the cyclopentanone. alkene coordination and insertion favour the chiral pocket arrangement with the bulky tert-butyl group in the less crowded region of the BINAP complex. For the stated (S)-BINAP system the observed product is the $R$ beta-tert-butylcyclopentanone, equivalently the product with tert-butyl on the solid-wedge face in the supplied ring orientation. Its priorities are tert-butyl, the shorter carbonyl-side ring path, the other ring path, and hydrogen. The ligand's S label alone is not a general rule that every product stereocentre must be S; the substrate orientation in the catalyst must also be specified. No decarbonylation is needed in the productive hydroacylation cycle.

Dalton Transactions, 3531–3534. B–Br oxidative addition gives trans-bromo(borazinyl) palladium and platinum complexes.