Synthesis and X-ray Structure Determination of Highly Active Pd(II), Pd(I), and Pd(0) Complexes of Di(tert-butyl)neopentylphosphine (DTBNpP) in the Arylation of Amines and Ketones

The air-stable complex Pd(η3-allyl)(DTBNpP)Cl (DTBNpP = di(tert-butyl)neopentylphosphine) serves as a highly efficient precatalyst for the arylation of amines and enolates using aryl bromides and chlorides under mild conditions with yields ranging from 74% to 98%. Amination reactions of aryl bromides were carried out using 1−2 mol % Pd(η3-allyl)(DTBNpP)Cl at 23−50 °C without the need to exclude oxygen or moisture. The C−N coupling of the aryl chlorides occurred at relatively lower temperature (80−100 °C) and catalyst loading (1 mol %) using the Pd(η3-allyl)(DTBNpP)Cl precatalyst than the catalyst generated in situ from DTBNpP and Pd2(dba)3 (100−140 °C, 2−5 mol % Pd). Other Pd(DTBNpP)2-based complexes, (Pd(DTBNpP)2 and Pd(DTBNpP)2Cl2) were ineffective precatalysts under identical conditions for the amination reactions. Both Pd(DTBNpP)2 and Pd(DTBNpP)2Cl2 precatalysts gave nearly quantitative conversions to the product in the α-arylation of propiophenone with p-chlorotoluene and p-bromoanisole at a substrate/catalyst loading of 100/1. At lower substrate/catalyst loading (1000/1), the conversions were lower but comparable to that of Pd(t-Bu3P)2. In many cases, the tri-tert-butylphosphine (TTBP) based Pd(I) dimer, [Pd(μ-Br)(TTBP)]2, stood out to be the most reactive catalyst under identical conditions for the enolate arylation. Interestingly, the air-stable Pd(I) dimer, Pd2(DTBNpP)2(μ-Cl)(μ-allyl), was less active in comparison to [Pd(μ-Br)(TTBP)]2 and Pd(η3-allyl)(DTBNpP)Cl. The X-ray crystal structures of Pd(η3-allyl)(DTBNpP)Cl, Pd(DTBNpP)2Cl2, Pd(DTBNpP)2, and Pd2(DTBNpP)2(μ-Cl)(μ-allyl) are reported in this paper along with initial studies on the catalyst activation of the Pd(η3-allyl)(DTBNpP)Cl precatalyst.