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

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journal contribution
posted on 13.05.2016, 00:00 by Pedro Ramírez-López, Abel Ros, Beatriz Estepa, Rosario Fernández, Béla Fiser, Enrique Gómez-Bengoa, José M. Lassaletta
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.