Larger scale Stahl oxidation with instant Cu removal in convenient synthesis of chiral bidentate N–heterocyclic carbene precursor

Commercially available N-Boc protected L-proline can be efficiently converted into a chiral, bidentate, aminoalkyl N-heterocyclic carbene ligand precursor in high yield (50% total after 4 steps) without column chromatography purification at any moment. The developed synthetic path includes: (1) redox step leading to an aldehyde, (2) imine condensation and in situ reduction to 1,2-diamine; (3) heterocyclization; (3) removal of the protecting group. Instant separation of Cu traces after the key Stahl oxidation at gram-scale was facilitated by the use of bis(isocyanide) scavenger, SnatchCat, forming insoluble, easy to remove RNC:→[Cu] complexes that allowed usto obtain crude intermediate suitably pure for next steps without tedious purification.