Gold-catalysed reactions Flashcards
What are the benefits and challenges in using Au(I) catalysts?
Relativistic effects are at a maximum for gold which leads to linear geometry, high electronegativity and soft Lewis acid character of Au (I).
Challenges (for alkenes as prochiral): linear geometry (ligand and substrate far away), outer-sphere enantiodetermining step, enantiofacial control of approaching alkenes, Au-L and Au-alkyne bonds maintain free rotation and intermolecular transformation.
What types of ligands are used for Au(I) catalysis? How are they prepared?
Generally bulky phosphines. Prepared by ligand exchange with (dimethylsulfide)gold(I) chloride to give LAuCl. More electron withdrawing phosphines will make Au more electrophilic.
What is needed for precatalyst activation and why is it required?
LAuCl is coordinatively saturated, and Cl bound strongly by electrostatics. Require removal of Cl by chloride scavenging, often done in situ using silver salts AgX, where X is a poorly coordinating counterion.
What types of reactions can Au(I) complexes catalyse and why? With what substrates?
Au(I) complexes can catalyse 1,6-enyne reactions, retro-Buchner (type of ring contraction reaction), or NuH addition to alkynes where NuH can be ArH for e-rich Ar, ROH or RR’NH) through activation of multiple C-C bonds (alkenes/alkynes)
Can you give a general outline of a catalytic cycle for nucleophilic additions to Au(I)-activated alkynes?
Coordination of alkyne to gold. Nucleophile attacks anti to gold to form a vinyl Au(I) complex. Protodeauration to form functionalised alkene and regenerate catalyst.
What do you need to take into account with ligand design for enantioselective Au(I) catalysis?
Depending on identity of L, will affect chemistry and bonding situation of Au - can range between carbocation like and carbene like bonding situation.
How do chiral Au (III) complexes compare?
Square planar geometry which makes transfer of stereochemical information from ligands more easy. And activated alkene terminal reacts in similar manner to Au(I).
How do bifunctional Buchwald ligands work with gold as a catalyst?
Ligand-directed anti-nucleophilic attack. Rigid structure minimises entropy and introduces directed group onto activated alkyne bond.
How can we use these catalysts for cyclobutene formation?
Tend to use dinuclear Au complexes with Josiphos-type ligands. Chloride scavenge one side and complex to alkyne starting material. React with alkene to give cyclopropene and double bond between C-Au, ring expansion to give cyclobutene, followed by protodeauration/ligand dissociation to give free cyclobutene.
