A Dynamic Kinetic C–P Cross–Coupling for the Asymmetric Synthesis of Axially Chiral P,N Ligands

The
Pd-catalyzed enantioselective C–P cross-coupling between
racemic, configurationally stable heterobiaryl triflates and trialkylsilyldiaryl­(dialkyl)­phosphines
has been used for the synthesis of several families of enantiomerically
enriched heterobiaryl phosphines including QUINAP, PINAP, and QUINAZOLINAP
analogues, which can be performed with good yields and enantioselectivities
using JOSIPHOS-type bidentate phosphines. The strategy relies on two
key assumptions: (I) The N atom of the heterocycle is a better ligand
than triflate, and upon oxidative addition, it incorporates into the
coordination sphere of the PdII center to form cationic
cyclic intermediates. (II) The geometry of the palladacycle results
in a widening of the angles involved in the stabilization of the stereogenic
axis, facilitating a fast interconversion of diastereomeric structures
and, hence, a dynamic kinetic C–P cross-coupling reaction.
These starting hypotheses are supported by experimental and computational
studies.