Selenoether ligand assisted Heck catalysis

Selenoether ligands, 2,2′-methylenebis(selanediyl)bis(2,1-phenylene)dimethanol (5), (2,2′-(ethane-1,2-diylbis(selanediyl))bis(2,1-phenylene))dimethanol (6) and (2-(benzylselanyl)phenyl)methanol (7) have been synthesized by reducing di-o-formylphenyl diselenide and reacting the in situ generated selenolate with dibromomethane, 1,2-dibromoethane and benzyl chloride, respectively. The ligands, bis(2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)phenylselanyl)methane (8) and 1,2-bis(2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)phenylselanyl)ethane (9) have been synthesized similarly from bis[2-(4,4-dimethyl-2-oxazolinyl)phenyl] diselenide using electrophiles dibromomethane and 1,2-dibromoethane, respectively. Activity of ligands 5–9 along with 2-(2-(benzylselanyl)phenyl)-4,4-dimethyl-4,5-dihydrooxazole (10) and 1-(2-(benzylselanyl)phenyl)-N,N-dimethylmethanamine (11) were examined for the Heck reaction of aryl halides with olefins. Bidentate Se,N ligand 11 was found to be the best one in the series and constitutes an efficient phosphine-free catalytic system with PdCl2. The catalytic system showed moderate activity for the coupling of activated aryl chlorides in the presence of tetra-n-butyl ammonium bromide (TBAB). Complexes [10-PdCl2] (12) and [11-PdCl2] (13) have shown marginally better activity in comparison to the in situ generated catalysts from PdCl2 and 10 and 11, respectively in the coupling of 4-bromoacetophenone with n-butylacrylate. Ligand 9 and complex 13 have been characterized by single crystal X-ray diffraction analysis.

Graphical abstractA set of selenoether ligands synthesized from di-o-formylphenyl diselenide, bis[2-(4,4-dimethyl-2-oxazolinyl)phenyl] diselenide and bis[2-(N,N-dimethylaminomethyl)phenyl] diselenide have shown good catalytic activity in the Heck coupling of aryl bromides under aerobic condition.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The efficacy of neutral bidentate selenium-oxygen/selenium-nitrogen ligands has been evaluated in Heck cross coupling reactions. ► The isolated PdCl2 complexes with six-membered chelate rings afforded high yields of the coupled products. ► Single crystal structures of a tetradentate ligand, 1,2-bis(2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)phenylselanyl)ethane (9) and 1:1 complex (13) of 1-(2-(benzylselanyl)phenyl)-N,N-dimethylmethanamine with PdCl2 are reported.