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

Abstract

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. Ph2PfcPCg (1) and Ph2PfcCH2PCg (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.

Synthesis and Characterization of Pd(0) Complexes with Electronically Differentiated Ferrocene Diphosphane Ligands

“…The structure of 5 ⋅ PhMe (Figure 3) reveals a pseudotrigonal coordination environment around the palladium atom, constituted by P,P′‐chelating diphosphane and side‐on, η 2 ‐coordinated ma. The Pd1‐P1 and Pd1‐P2 distances, 2.3282(6) and 2.3360(6) Å, are comparable with the values determined for analogous complexes obtained from dppf, B and C [6a] (see Scheme 1) and even for [PdCl 2 ( 1 ‐κ 2 P , P ′)] (≈2.36 and 2.33 Å) [5a] . The ligand bite angle, P1‐Pd1‐P2, of 108.96(2)°, is similar to that in [Pd(dppf‐κ 2 P , P ′)(η 2 ‐ma)] ( 8 ) and, expectedly, wider than in the mentioned, tetracoordinate complex [PdCl 2 ( 1 ‐κ 2 P , P ′)] (96.31(3)°).…”

“…Complexes 5 – 7 and their dppf analog [Pd(dppf‐κ 2 P , P ′)(η 2 ‐ma)] ( 8 ) [6a] were employed as precatalysts for the C−H phenylation of benzoxazole with chlorobenzene (1.5 equiv.) to afford 2‐phenylbenzoxazole ( 9 ; Scheme 4).…”

“…to afford 2‐phenylbenzoxazole ( 9 ; Scheme 4). The reactions were performed at 125 °C in n ‐butanol for 18 h using 1.5 mol % of the Pd catalyst and K 3 PO 4 as the base [6a] . The results collected in Table 4 indicate that the catalytic activity of the newly prepared compounds was generally lower than that of the previous complex [Pd( C ‐κ 2 P , P ′)(η 2 ‐ma)], which ensued in quantitative yield of arylation product 9 under similar conditions at only 0.5 mol % Pd loading.…”

“…Materials and methods : All syntheses were performed in a nitrogen atmosphere using standard Schlenk techniques and oven‐dried glassware. Compounds 1 , [5a] 4 [5d] and [Pd(ma)(N^N)] [6a,15] were prepared according to the literature procedures. Anhydrous methanol, dichloromethane and tetrahydrofuran were obtained from a PureSolv MD5 solvent purification system (Innovative Technology, USA).…”

Beyond phosphorus: synthesis, reactivity, coordination behaviour and catalytic properties of 1,1′-bis(diphenylstibino)ferrocene

“…Lastly, the Pd(0)– 1 complex [Pd(η 2 -ma)( 1 -κ 2 Sb , Sb ′)] ( 18 ) was obtained by replacing the diimine ligand ( viz ., N , N ′-di- t -butyl-ethanedialdimine or N ^ N ) in [Pd(η 2 -ma)( N ^ N )] with 1 (Scheme 7) 39 and was isolated as an air-stable, orange crystalline solid in good yield (84%). Even in this case, the formulation of all complexes was supported by the NMR and MS spectra and by elemental analysis and was further corroborated using X-ray diffraction analysis.…”