Enantioselective reactions Flashcards
Give a detailed description of the Tsuji-Trost reaction and its mechanism.
Pd-catalysed allylic substitution. Coordination of alkene to Pd, followed by Sn2 to coordinate allyl and kick out leaving group. Attack of Pd/C by nucleophile, followed by decomplexation if attacked at C or reductive elimination if attacked at Pd.
How does the mechanism of the Tsuji-Trost reaction affect the stereochemistry of the final product?
Pd-catalysed allylic substitution, via Pd-allyl complex. From this key intermediate, HSAB determines whether attack occurs at carbon (soft) or at Pd (hard) to give the final product. Soft gives overall retention of configuration whereas hard gives overall inversion.
Give a brief overview of the Sharpless asymmetric epoxidation.
Conversion of allylic alcohols into hydroxy epoxides. Metal centre to coordinate all reactants (Ti), chiral ligand to control enantioselectivity (tartrate esters), oxidant (peroxide) and a substrate capable of binding (hence alcohol). Stereochemical outcome (top/bottom) predicted by S/R configuration of allylic alcohol. Can be affected by matched and mismatched catalyst-substrate combinations.
Give a brief overview of the Noyori asymmetric hydrogenation.
Example of dynamic kinetic resolution. Racemisation between species via enol formation and sufficiently faster than hydrogenation. Hydrogenation catalysed by chiral BINAP-Ru with cyclic structures of substrates differentiating between syn and anti transition states.
