A cationic NCN pincer platinum(II) aquo complex with a bis(imidazolinyl) phenyl ligand: Studies toward its synthesis and asymmetric Friedel-Crafts alkylation of indoles with nitroalkenes

摘要

Studies toward the synthesis and characterization of a cationic NCN pincer platinum(II) aquo complex with a chiral bis(imidazolinyl)phenyl (Phebim) ligand are reported. During these studies, the new symmetrical Phebim-H ligands 1c,d and unsymmetrical hybrid oxazoline-imidazoline ligands 6a-c, as well as the related neutral symmetrical NCN pincer Pt(II) complexes 2b-d and 3a-d and unsymmetrical NCN′ pincers 7a,b and 8c have been isolated and characterized. Despite considerable attempts, only one of the expected cationic chiral NCN pincer Pt(II) complexes (4), which contains water as an exchangeable neutral ligand for the Pt(II) center, was obtained in pure form from the reaction of the neutral Pt-I complex 3a with AgOTf in wet CH _2Cl _2 at room temperature. In contrast, the cationic Pt(II) pyridine complexes 5a,c could be easily prepared in high yields from the reaction of the neutral Pt-Cl complexes 2a,c with AgOTf in the presence of pyridine in CH _2Cl _2 at room temperature. The molecular structures of the neutral symmetrical Pt(II) complexes 2c,d and 3c,d and the unsymmetrical Pt(II) complex 7b as well as the cationic Pt(II) aquo complex 4 have been determined by X-ray single-crystal analysis. The cationic complexes 4 and 5a,c have been applied to the asymmetric Friedel-Crafts alkylation of indoles with nitroalkenes. Among them, the aquo complex 4 was found to be effective under the given reaction conditions. With a catalyst loading of 5 mol %, a variety of the nitroalkylated indoles were produced in good yields with moderate to good enantioselectivities (up to 83% ee).