Preparation of Secondary Phosphine Oxide Ligands through Nucleophilic Attack on Imines and Their Applications in Palladium-Catalyzed Catellani Reactions

摘要

Several new amino-type secondary phosphine oxide (SPO) pre-ligands (3a-3h) that contain P-N bonds were synthesized and characterized. SPOs 3a-3h can tautomerize to phosphinous acids (PA, 3a-3h) as genuine ligands. The formation of SPOs 3a-3h occurred first through nucleophilic attack on the imine carbon atom, then by the addition of RPCl2 (R = Ph, Cy, tBu, or iPr), and work-up under acidic conditions. The P-N bond in the newly prepared SPOs is evident from the crystal structures of SPOs 3d and 3h. Reactions of SPOs 3f, 3g, or 3h with Pd(COD)Cl-2 (COD = cyclooctadiene) yielded palladium complexes 6f, 6g, or 6h. In these crystal structures, PAs 3f, 3g, and 3h act as didentate ligands through P and N donors. Intriguingly, the reaction of SPO 3f with Pd(COD)Cl-2 also gave rise to palladium complexes 7fa, 7fb, and 8f. The crystal structures of 7fa and 7fb show that the palladium atom is chelated by PA 3f and coordinated by a phosphine-like ligand fragmented from SPO 3f through C-N bond dissociation. Finally, the syntheses of carbazole derivatives were pursued in Catellani reactions with SPOs 3g and 3h as pre-ligands. A mechanism is proposed to account for the catalytic reaction (see the Supporting Information). The optimized conditions for Suzuki reactions using selected SPO ligands are also reported.