A modular approach to structurally diverse bidentate chelate ligands for transition metal catalysis

A modular approach to a new class of structurally diverse bidentate P/N, P/ P, P/S, and P/Se chelate ligands has been developed. Starting from hydroqui none, various ligands were synthesized in a divergent manner via orthogonal ly bis-protected bromohydroquinones as the central building block. The firs t donor functionality (L-1) is introduced to the aromatic (hydroquinone) li gand backbone either by Pd-catalyzed cross-coupling (Suzuki coupling) with hetero-aryl bromides, by Pd-catalyzed amination, or by lithiation and subse quent treatment with electrophiles (e.g., chlorophosphanes, disulfides, dis elenides, or carbamoyl chlorides). After selective deprotection, the second ligand tooth (L-2) is attached by reaction of the phenolic OH functionalit y with a chlorophosphane, a chlorophosphite, or a related reagent. Some of the resulting chelate ligands were converted into the respective PdX, compl exes (X = Cl, I), two of which were characterized by X-ray crystallography. The methodology developed opens an access to a broad variety of new chiral and achiral transition metal complexes and is generally suited for the sol idphase synthesis of combinatorial libraries, as will be reported separatel y.