Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 8 / b

Trialkylaluminium compounds commonly form dimers with two bridging alkyl groups and four terminal alkyl groups. Each electron-deficient bridge is a three-centre two-electron bond spanning Al–C–Al; the carbon–metal bond electrons are shared with the second aluminium center. A localized ordinary two-center bond to each Al would overcount the electrons.

For $[{\mathrm{R}}_2\mathrm{AlH}{]}_2$, hydride bridges are expected to be especially favorable, giving a four-membered Al–H–Al–H core with two terminal alkyl groups on each Al. The Al–H–Al links are again three-centre two-electron bonds. Hydride is small and can bridge without the steric cost of a bulky alkyl bridge. Aggregation still depends on substituent size, donor solvent and concentration; this is the natural dimeric model, not a statement that all dialkylaluminium hydrides are dimers under every condition.

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 frustrated Lewis pair captures carbon dioxide cooperatively: the phosphine lone pair attacks its carbon while a carbonyl oxygen coordinates to aluminium. A five-membered ring closes alongThe new ${\mathrm{C}}_{\mathrm{C}{\mathrm{O}}_2}$ atom is bonded to P, double-bonded to one O and single-bonded to the O bound to Al. P becomes phosphonium-like and the O–Al end completes a zwitterionic carboxylate-like adduct; the original alkene remains intact.

A direct P–C bond to the captured CO${}_2$ carbon accounts for its large ${}^{13}$C–${}^{31}$P coupling. The carbonyl-like chemical shift is consistent with a captured, bent CO${}_2$ unit. Reversibility follows from cleavage of the new P–C and Al–O contacts to regenerate the separated donor and acceptor.