Codex @codex  0

A Lewis acid and Lewis base whose direct neutralization is inhibited, allowing cooperative reaction with another molecule.

A Lewis acid whose acceptor interaction favours small, weakly polarizable donor atoms. High charge density favours hard character; Eu(III) typically favours oxygen and nitrogen donors over a softer sulfur donor.

A Lewis base donates an electron pair to a Lewis acid, forming a coordinate covalent bond in an adduct.

The nitrogen-centered highest occupied molecular orbital of ammonia supplies a lone pair; planar borane has an empty out-of-plane $p$-derived lowest unoccupied molecular orbital. Approach along this acceptor axis gives constructive orbital overlap and donation of the nitrogen pair into the boron-centered acceptor. The resulting N-to-B coordinate covalent bond forms the ammonia borane adduct.

The occupied molecular orbital formed by this interaction is bonding, while the corresponding antibonding combination remains empty. Borane pyramidalizes as coordination increases from three to four. Ammonia can use its existing outward-pointing lone pair; its inversion to the planar transition geometry is not a prerequisite. The Lewis base and Lewis acid descriptions summarize the donor and acceptor roles, while the frontier molecular orbitals explain their directional interaction.

The activated aromatic pi system polarizes bromine and attacks its electrophilic end while Br–Br cleaves. This makes a C–Br bond at C1 or C3 and gives a Wheland intermediate. Donation from phenolic oxygen stabilizes contributors with positive charge at its attached ring carbon. acetate or bromide then removes the proton from the attacked carbon; the C–H bond electrons restore the aromatic pi system. Repetition at the other activated ortho site gives the dibrominated product.

Each step replaces C–H by C–Br and restores aromaticity. No radical initiator or benzylic abstraction is needed, and no Lewis acid catalyst should be invented for this already activated phenolic substrate. The overall substitution balance is $\mathrm{Ar}{\mathrm{H}}_2+2B{\mathrm{r}}_2\to \mathrm{ArB}{\mathrm{r}}_2+2HBr$.

Number the phenolic carbon C1, with methyl groups at C2 and C5. All three remaining aromatic hydrogens are replaced: $H$ is 3,4,6-tribromo-2,5-dimethylphenol. The starting phenol is ${\mathrm{C}}_8{\mathrm{H}}_{10}\mathrm{O}$; replacing three hydrogens gives ${\mathrm{C}}_8{\mathrm{H}}_7\mathrm{B}{\mathrm{r}}_3\mathrm{O}$, agreeing with the intermediate's composition. Bromination of methyl side chains would not follow the stated Lewis acid-promoted aromatic pathway.

aluminium tribromide polarizes Br–Br. The aromatic ring attacks the electrophilic bromine end, forming a Wheland intermediate; removal of the proton on the attacked carbon restores aromaticity. Repetition fills C4 and C6, the available para and ortho positions activated by OH, and C3, which is activated by the methyl substituents when the other positions are occupied. The formula fixes the extent of substitution without requiring an experimentally unreported order for the individual brominations. The preparative perbromination/selective-debromination sequence is documented in the primary synthetic work.^[ref]

The zinc ion is coordinated by protein donors and a water oxygen placed near the reacting C6 center. Zinc acts as a Lewis acid that activates and positions water. Its positive charge withdraws electron density from oxygen, lowers the effective p$K_a$ of coordinated water and facilitates formation of the attacking hydroxide, potentially with assistance from a nearby base. The same metal–oxygen interaction can stabilize developing negative charge as the addition intermediate forms. Zinc stays Zn(II); this is metal ion catalysis, not a zinc redox cycle.

The structure identifies a plausible reaction geometry, but a static X-ray model does not by itself establish the water's protonation state, proton-transfer order or the rate-limiting step. Structural work with bound transition-state analogues independently supports a zinc-containing catalytic center.^[ref] An appropriate experimental test would compare native enzyme, a metal-depleted preparation and stoichiometrically zinc-reconstituted enzyme, checking retained fold and active enzyme concentration. Loss and recovery of activity with those controls distinguishes a catalytic metal requirement from nonspecific denaturation.

In borazine, each B and N is approximately trigonal planar. The alternating ring contains a sigma framework with perpendicular p orbitals; the nitrogen lone pairs can donate into the boron acceptor orbitals. Six pi electrons can therefore be distributed around the ring, giving partial B–N multiple-bond character rather than six ordinary isolated single bonds. Equal ring distances are compatible with this symmetric delocalization.

The electronegativity difference makes each B–N bond strongly polar, ${\mathrm{B}}^{\delta +}-{\mathrm{N}}^{\delta -}$. The occupied pi orbitals are weighted toward N, and the empty acceptors toward B. In benzene, equivalent carbon centers give much more even pi distribution and strong resistance to addition that destroys the aromatic system. borazine resembles benzene geometrically but has substantially more polarized, more reactive bonding. It presents electrophilic boron sites to nucleophiles and nitrogen donors to Lewis acids. Polar reagents can add across B–N, and water can cleave the ring; benzene ordinarily favors substitution under suitable conditions rather than facile polar addition or hydrolysis. The label “inorganic benzene” should therefore not be interpreted as identical aromatic stabilization or reactivity. nitrogen-bound Lewis-acid adducts and hydrogen-halide additions demonstrate this contrast experimentally.^[ref]

$B$ is a 1:1 nitrogen-bound adduct, with $\mathrm{N}\to \mathrm{AlB}{\mathrm{r}}_3$ at one ring nitrogen. Its lone pair is diverted from N-to-B pi donation into the new coordinate bond. That nitrogen becomes approximately tetrahedral and the ring can pucker. The two B–N bonds adjacent to it lose pi character and lengthen; the neighboring unbound nitrogen atoms can donate more strongly into those borons, shortening the next pair of B–N bonds. The remaining pair forms a third class. The second structural panel marks these three two-bond sets; it represents connectivity and bond classes, not a measured geometry.

In the slow-exchange structure, two boron atoms flank the bound nitrogen and are equivalent to one another, while the third boron is unique. This gives two ${}^{11}$B environments in a $2:1$ ratio. At room temperature, rapid exchange of the Lewis acid among the three equivalent nitrogen sites averages every boron over the same environments, giving one boron-11 nuclear magnetic resonance signal. Cooling slows exchange relative to the resonance-frequency difference and resolves the two environments.

This is Chemical exchange in NMR, not restoration of equal instantaneous B–N bonds. A spectrum alone cannot distinguish intramolecular migration from reversible dissociation and reassociation; concentration dependence and crossover experiments would test that distinction. Related N-bound borazine adducts have been characterized by diffraction and variable-temperature NMR, supporting the structural logic while not supplying the exact temperature dependence of this particular hexamethyl compound.^[ref]

The dimer has four terminal chlorides and two bridging chloride ligands. Each aluminium is approximately tetrahedrally surrounded by two terminal and two bridging chlorines. A chlorine lone pair from one $\mathrm{AlC}{\mathrm{l}}_3$ fragment donates into an empty orbital of the other Lewis acid centre; reciprocal donation closes the four-membered $\mathrm{A}{\mathrm{l}}_2\mathrm{C}{\mathrm{l}}_2$ ring.

The localized coordinate covalent bond model has single Al–Cl bonds, formal bond order one, for both terminal and bridging connections. Donation describes the origin of a pair, not half an electron-pair bond. A bridge chlorine has two bonded pairs and two remaining lone pairs in this all-single-bond Lewis accounting; the original terminal chlorines have three lone pairs. Bridging contacts are longer and weaker than terminal ones, so equal formal order does not imply equal strength or equal numerical quantum-chemical bond indices. In particular, these chloride bridges should not be assigned the electron-deficient methyl-bridge half-bonds of (b).

The simplest assignment is two phenylethynyl bridges and four equivalent terminal methyl groups, represented as $\mathrm{A}{\mathrm{l}}_2\mathrm{M}{\mathrm{e}}_4(\mu -\mathrm{C}\equiv \mathrm{CPh}{)}_2$. All 12 methyl protons then occupy the same class of environment, accounting for the single nonaromatic singlet without requiring methyl bridge/terminal averaging. The phenyl protons account for the aromatic-region signals; the metal-bound acetylide has no terminal acetylenic hydrogen.

An alkynyl bridge has electronic possibilities unavailable to a saturated methyl: in addition to sigma attachment from its carbon end to one Lewis acid centre, its occupied C≡C pi system can donate to the other aluminium. A delocalized sigma/pi description stabilizes the phenylethynyl-bridged arrangement while keeping the methyl groups terminal. One must not draw an unchanged localized triple bond plus two independent full ordinary Al–C bonds at the same carbon and count all of them as separate two-electron bonds. The exact distribution of sigma and pi donation requires structural or orbital evidence; this NMR observation alone does not supply a numerical bridge bond order.

The absence of concentration or temperature dependence is consistent with this preferred arrangement throughout the measured range. It is not proof that the molecule has no motion: exchange between symmetry-equivalent terminal positions, or exchange remaining fast throughout that range, could be spectroscopically invisible. Phenylethynyl bridging in this particular dimethylaluminium dimer has independent primary NMR precedent.^[ref]

Amine nitrogen binds the carbon end of CO${}_2$; Mg${}^{2+}$ binds through oxygen. The polarized C=O bonds make carbon electrophilic and oxygen electron rich. An amine nitrogen lone pair therefore donates toward carbon, potentially giving a carbamate-like adduct after proton transfer. The hard Lewis acid Mg${}^{2+}$ accepts an oxygen lone pair, either from coordinated CO${}_2$ or from an oxygen of a captured carbamate species. Simply reversing these assignments because nitrogen and oxygen both have lone pairs would put two electron-pair donors together without accounting for the electrophilic site.

The local binding pattern does not by itself determine the full proton-transfer state or whether cooperative chains form; those depend on the framework and amine arrangement. A documented diamine-appended framework shows cooperative carbon-dioxide insertion and ammonium carbamate chain formation, illustrating one possible implementation.^[ref]

To regenerate the material, lower the CO${}_2$ chemical potential using reduced pressure or a CO${}_2$-free purge, and, where binding is exothermic, heat the framework to favour desorption. The temperature must remain compatible with framework stability. This reverses adsorption or reversible chemisorption; it does not require permanently destroying the metal–organic framework.

Write the alkyne as $\mathrm{Me}{\mathrm{s}}_2\mathrm{P}-{\mathrm{C}}_{\alpha }\equiv {\mathrm{C}}_{\beta }-\mathrm{Ph}$. The product places Al at $C_{\alpha }$ and H at $C_{\beta }$:A useful Lewis acid-assisted model begins with a side-on or bridged alkyne–Al interaction. As in the unsymmetrical bonding of aryl-bridged aluminium aggregates, the electrophilic Al center can interact with more than one carbon while a donating group biases the contact. The phosphine can additionally associate transiently with Al, organizing the alkyne near an Al–H bond. Transfer of hydride to the distal carbon while the proximal carbon retains Al gives the observed regiochemistry.

The drawing requires syn addition of H and Al. In the printed alkene, $\mathrm{Me}{\mathrm{s}}_2\mathrm{P}$ and Ph are opposite; consequently Al and the newly added H are on the same side. A cyclic hydroalumination transition state produces this relationship. Steric congestion subsequently inhibits a persistent P$\to$Al bond, leaving the two sites available as a frustrated Lewis pair. A long-lived free vinyl cation or an anti hydride attack is unnecessary, and would not naturally explain the stereochemistry.

The molecule combines three chemically different binding regions.

These assignments identify the preferred local sites, rather than guaranteeing three independent binding constants: occupancy can alter conformation, electrostatics and the affinity of neighboring sites.

A molecular titanium(IV) chloride used as a Lewis acid.

A zinc salt used as a Lewis acid and coordination reagent.