Stable Pd(0) Complexes with Ferrocene Bisphosphanes Bearing Phosphatrioxaadamantyl Substituents Efficiently Catalyze Selective C-H Arylation of Benzoxazoles by Aryl Chlorides

Versatile applications and unique performance of 1,1'-bis(diphenylphosphanyl)ferrocene (dppf) in coordination chemistry and catalysis prompted the search for its analogs. This contribution describes the synthesis of the first donor-unsymmetric dppf congeners bearing bulky and rigid 1,3,5,7-tetramethyl-2,4,6-trioxa-8-phosphaadamantyl (CgP) donor groups, viz. Ph(2)PfcPCg (1) and Ph(2)PfcCH(2)PCg (2; fc=ferrocene-1,1'-diyl). Bis-phosphanes 1 and 2 were converted into air-stable Pd(0) complexes, [Pd(ma)(L<^>L)] (L<^>L=1 and 2; ma=maleic anhydride). Together with [Pd(ma)(dppf)], these complexes were applied as catalysts in Pd-catalyzed C-H arylation of benzoxazoles with aryl chlorides in n-butanol as an environmentally benign solvent. Among all catalysts tested in this study, complex [Pd(ma)(2)] performed the best, providing a high-yield and selective synthesis of 2-arylbenzoxazoles from a range of the generally less reactive chloroarenes at low catalyst loading (typically 1 mol.%). Under similar conditions, the structurally related heterocycles (e. g., 1-methylbenzimidazole and benzothiazole) did not react.

Automated summary

This study described the synthesis of new donor-unsymmetric dppf congeners and their conversion into air-stable Pd(0) complexes, which showed high catalytic activity and selectivity in the Pd-catalyzed C-H arylation reaction. The complex [Pd(ma)(2)] performed the best in synthesizing 2-arylbenzoxazoles with high yield and selectivity under environmentally friendly conditions.