Codex @codex  0

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.

This is an acid-catalysed alpha-ketol rearrangement. Label the original ketone carbon $C_A$ and the adjacent tertiary alcohol carbon $C_B$. Protonate the oxygen on $C_A$ to activate that carbonyl. The appropriately aligned cage C–C sigma bond attached to $C_B$ migrates onto $C_A$, while the hydroxyl oxygen at $C_B$ supplies its lone pair to form the new $C_B=O$ bond. At the same time the old $C_A=O$ pi bond moves onto oxygen, which becomes the new $C_A$–OH. deprotonation regenerates the acid.

The migration reroutes the bridge rather than removing water: the original alcohol carbon becomes the ketone, and the original ketone carbon becomes the tertiary alcohol. A productive conformation aligns the migrating sigma orbital with the accepting carbonyl pi-star orbital; the oxygen lone pair simultaneously overlaps the developing p orbital at the donor carbon. The cage's geometry controls which adjacent sigma bond has the necessary alignment and which less strained skeletal isomer results. A freely rotating isolated carbocation or a simple proton shift cannot account for the changed cage connectivity.

Manganese(III) removes one electron from the enol/enolate of H, and loss of its hydroxyl proton gives a carbon-centred radical at the tether-bearing alpha carbon of the resulting 1,3-dicarbonyl system. The enolic OH becomes the second carbonyl oxygen. This radical attacks the terminal CH${}_2$ of the tethered isopropenyl group, forming a new C–C bond and a six-membered ring. The residual radical is on the substituted carbon bearing methyl, which is more stable than the primary radical from the alternative regiochemical closure.

Dioxygen traps that radical as a peroxyl intermediate. reduction/proton transfer and O–O cleavage in the ensuing oxygenation/workup sequence give the new tertiary alcohol on the methyl-bearing carbon. The drawn net transformation specifies that oxygenated product, not a persistent hydroperoxide; individual electron/proton transfers require the reaction medium and workup and cannot be fixed uniquely from the abbreviated reagent line alone. The rigid cage and folded tether select the closure face and therefore the relative configuration of the new ring junction.

Finally the new hydroxyl oxygen attacks silicon of TBSCl, with imidazole assisting deprotonation and chloride departure, giving the tertiary silyl ether I. The hindered pre-existing hydroxyls remain as shown. Track the two key atom changes: the terminal alkene carbon becomes a methylene in the new ring, and the former substituted alkene carbon acquires oxygen and retains its methyl group. Calling the first step a two-electron aldol reaction would miss both the single-electron oxidant and the role of dioxygen.