Enantioselective N-heterocyclic carbene-catalysed intermolecular crossed benzoin condensations: improved catalyst design and the role of racemisation.

The enantioselective intermolecular crossed-benzoin condensation mediated by novel chiral N-heterocyclic carbenes derived from pyroglutamic acid has been investigated. A small library of chiral triazolium ions were synthesised. Each possessed a tertiary alcohol H-bond donor and a variable N-aryl substituent.

Revisiting the Perfluorinated Trityl Cation.

Although ultimately not isolable for X-ray structural characterization, the free perfluorinated trityl cation was shown to be observable in neat triflic acid, which represents milder conditions than previous reports of this cation in "magic acid" or oleum. A triflate-bound species could be generated in organic solvents using stoichiometric amounts of triflic acid and was shown to be synthetically viable for hydride abstraction from Et SiH. It was demonstrated that the para-position on the -C F rings is the primary point of attack for decomposition of the cation.

Highly chemoselective intermolecular cross-benzoin reactions using an ad hoc designed novel N-heterocyclic carbene catalyst.

The design of a novel N-heterocyclic carbene catalyst incorporating a bulky yet highly electron-deficient N-aryl substituent has allowed the development of an efficient protocol for the first highly chemoselective intermolecular benzoin condensations between two non-identical aromatic aldehydes.

Aerobic oxidation of NHC-catalysed aldehyde esterifications with alcohols: benzoin, not the Breslow intermediate, undergoes oxidation.

Benzoin (and neither the Breslow intermediate nor the NHC-aldehyde tetrahedral adduct) has been unambiguously identified as the oxidised species in aerobic NHC-catalysed aldehyde esterifications.

NHC-catalysed aerobic aldehyde-esterifications with alcohols: no additives or cocatalysts required.

A highly efficient, broad scope, additive-free mild protocol for the oxidative carbene-catalysed esterification of aldehydes (including the related aqueous oxidation to acids) has been developed.