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1,2-Bis(dimethylphosphino)ethane has the connectivity $\mathrm{M}{\mathrm{e}}_2\mathrm{PC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{PM}{\mathrm{e}}_2$. Chelation through both phosphorus donors contributes four electrons in ionic electron counting.

Chelation binds multiple donor sites of one ligand to the same central atom, forming a chelate ring. It can retain a donor near a metal even when one attachment temporarily opens.

B is the major product: the newly formed OH is the dashed bond in the printed zigzag structure. Retain the existing solid-wedge OH and the configuration at the carbamate-bearing carbon.

zinc coordinates the ketone oxygen and the adjacent hydroxyl oxygen, forming a five-membered O–Zn–O chelate. This constrains rotation around the carbonyl–alpha-carbon bond. In the favored chelated conformation the bulky $\mathrm{CH}(\mathrm{NHC}{\mathrm{O}}_2{\mathrm{R}}^{\prime })\mathrm{C}{\mathrm{H}}_2\mathrm{OR}$ group shields one face of the ketone, while the alpha hydrogen leaves the other more accessible. hydride attacks from that less crowded face, the carbonyl $\pi$ bond opens onto oxygen, and aqueous work-up gives the second alcohol.

Mapping this Cram chelation model back onto the printed zigzag gives B, not A. Merely placing a new OH “on the same side” of an unrotated page drawing is not a valid chelation argument: the reactive chelate and that drawing use different conformations. Zinc-borohydride reductions of alpha-hydroxy ketones are experimentally known to favor the corresponding anti-1,2-diol family under chelation control.^[ref]

The reagent combination can generate zinc borohydride and related zinc-associated reducing species. The stereochemical justification is coordination-controlled facial selectivity; it does not require identifying a unique solution aggregate from the reagent list. The quoted yield is not itself a measurement of a 95:5 diastereomeric ratio.

Use ionic electron counting, counting each ordinary donor atom once and keeping ligand charge distinct from denticity. A is 16-electron, B is 14-electron and C is 16-electron, so none is an 18-electron complex under this convention.

A contains a modified, monoanionic PNP pincer: the exocyclic double bond and altered pyridine-ring bond pattern in the drawing matter. Its two phosphines and its central N donor provide six electrons altogether, while neutral N${}_2$ provides two. Neutrality then gives Co(I), $d^8$, and $8+6+2=16$. Treating the modified pincer as a neutral intact pyridine ligand would incorrectly produce Co(0) and a 17-electron count. A four-coordinate approximately square planar molecular geometry in a $d^8$ complex can leave the strongly sigma-antibonding orbital empty and be stable at 16 electrons. The underlying modified-pincer chemistry has been examined using structure, spectroscopy and electronic-structure calculations.^[ref]

B has a neutral tertiary amine donor at the centre of its tripod and three anionic amido arms. Their combined charge is $-3$, so Re is Re(III), $d^4$. The four N donors give eight electrons and N${}_2$ gives two: $4+8+2=14$. Such an electron count is not inherently unexpected in an amido-supported complex: strong amido pi donation and the imposed five-coordinate geometry alter the orbital energies, and the empty orbitals are not nine equivalent slots that must all be filled. Steric protection and chelation also help stabilize this coordinatively restricted structure. The 14-electron figure uses the usual two-electron ionic count for each amido donor; pi bonding should not be appended to it inconsistently.

C has an anionic acyl carbon donor and a neutral phosphine in the chelate, together with two neutral PMe${}_3$ donors. Thus Co(I), $d^8$, receives $2+2+4=8$ ligand electrons, giving $8+8=16$. This is again a reasonable four-coordinate $d^8$ structure. The acyl C=O is part of a carbon-bound acyl ligand, not an additional free CO donor.

Write $L^{-}$ for the cyclopentadienyl ligand bearing its $\mathrm{C}{\mathrm{H}}_2$–2-pyridyl arm; ${\eta }^5:\kappa N$ means that its ring and pyridyl nitrogen both bind the same metal. $D=[\mathrm{Mo}({\eta }^5:\kappa N\text{-}L)(\mathrm{CO}{)}_2(\mathrm{NCMe}){]}^{+}$. The base removes the pyridinium N–H proton in $C$, restoring a pyridine lone pair. Intramolecular nitrogen coordination then replaces one acetonitrile ligand and closes a chelate ring. Both CO ligands are retained; the molybdenum remains Mo(II), $d^4$.

This is an proton transfer followed by chelation, with metal electron count $4+6+2_N+4_{\mathrm{CO}}+2_{\mathrm{NCMe}}=18$. The chelate effect, favorable formation of the Mo–N bond, and release of a solvent molecule explain the driving force. Merely invoking a 16-to-18-electron change would be misleading because $C$ is already 18-electron. In the preceding acid treatment, protonation of the ${\eta }^3$ allyl ligand allows release of propene; the other proton resides on the pendant pyridine, accounting for $C$'s overall $2+$ charge. One proton is removed here; the cyclopentadienyl tether is not deprotonated or detached.

One valid asymmetric route uses N-Boc-(R)-proline as a chiral starting material. Convert its acid to the Weinreb amide with N,O-dimethylhydroxylamine and a coupling reagent such as EDC, then add phenylmagnesium bromide at low temperature. chelation of the magnesium tetrahedral intermediate prevents a second addition; aqueous workup releases the phenyl ketone. Reduce it with sodium borohydride, separate the resulting alcohol diastereomers, and retain the one with $R$ configuration at both pyrrolidine C2 and the benzylic carbinol. This is an asymmetric synthesis through a pre-existing enantiopure centre, even though the final reduction need not be perfectly diastereoselective. If the other protected alcohol is isolated, Mitsunobu esterification with a carboxylic acid followed by ester hydrolysis inverts just the carbinol centre. Keeping nitrogen Boc-protected during that inversion avoids neighbouring-amine participation. Mild Boc removal followed by reductive benzylation with benzaldehyde and sodium triacetoxyborohydride gives D, the $(R,R)$ amino alcohol. Use conditions that avoid epimerizing the amino ketone.

For ring expansion, activate D's alcohol with methanesulfonyl chloride and a tertiary amine base. Its own tertiary nitrogen attacks the benzylic carbon from the back, displacing mesylate with inversion and making a fused aziridinium ion. Acetate, supplied as a soluble acetate salt, attacks the original pyrrolidine C2 from the opposite face and breaks the old C2–N bond. The new N–benzylic-C bond is retained, incorporating that side-chain carbon into the six-membered ring. Inversion at the two successively attacked carbons gives the cis acetate/phenyl piperidine E in the displayed orientation. Attack at the benzylic carbon instead would simply open the three-membered ion back to a five-membered-ring derivative; specifying the attacked carbon is the crucial regiochemical step.

The six diphosphines bind the six Pd centres, occupying two adjacent sites on each and displacing two pyridines per Pd. Reassembly therefore favours the two-connected Pd nodes of the bipyramid instead of the cube's four-connected nodes. The balanced transformation isThus two cages of type 4 and four free metalloligands of type 3 form per cage 5. iron, palladium, ligand and charge balances all close. Strong phosphine chelation drives the donor replacement, while the release of additional discrete species can favour entropy. The analogous conversion is observed experimentally in the cited heterobimetallic system.^[ref]

A pyridine ring bearing carboxylic acid substituents at positions 2 and 5. Removing both acidic protons gives a dianion with a neighbouring N,O chelation site and a remote carboxylate site.