Chiral ion enantioselective catalysis Flashcards
What is ACDC?
ACDC = asymmetric counterion-directed catalysis
induction of enantioselectivity in a reaction proceeding through a cationic intermediate by ion pairing with a chiral, enantiomerically pure anion provided by the catalyst.
What modes of activation can chiral ions use to act as sources of stereochemical information?
Coordinative interaction (Lewis acid catalysis), double hydrogen-bonding interaction (H-bond catalysis), single hydrogen bonding interaction (Bronsted acid catalysis), electrostatic interaction (chiral anion catalysis)
How does tethered ACDC work?
The chiral anion is tethered onto the phosphine ligand of a gold(I) chloride complex.
Can you describe the concepts of tethered ACDC?
Activation by coordination to gold complex. Gold complex bound to the phosphine ligand, which includes a tether to a chiral anion. Electrostatic interactions between chiral anion and positive charge generated on substrate brings the stereochemical information in close proximity, directing the attack of a nucleophile by shielding one face.
How does ACDC work with strong Bronsted acids?
Chiral anion positioning via H-bonding interactions tethered on phosphine ligand of gold complex. Removal of counterion by hydrogen bonding to ligand gives stabilisation.
How can ACDC and metal catalysts work together?
Reactivity - render the Au(I) centre catalytically active/activate the double/triple bond
Selectivity - place chiral information close to the substrate
What strategies can be used to position chiral ions?
Apolar solvents for promotion of H-bonding
Non-basic stand-alone counterion to increase ligand acidity and reduce coord ability
Position of urea group and identity of 3,3’ substituents on anion for enantioselectivity
Identifying where H-bond is occurring can be done via proton NMR titrations via deshielding of signals
Give a brief outline of the case study covering supramolecular formation of chiral ions.
Achiral anion made supramolecularly chiral via induction with asymmetric H-bond donor.
Concepts explored: solvent effects, H-bonding, stoichiometry of binding, catalytic activity and transition state stability.
Techniques used: proton NR titrations, Job plot, ROESY, kinetic studies, SAR, DFT
