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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

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posted on 01.10.2010 by Lensey L. Hill, Jason L. Crowell, Strudwick L. Tutwiler, Nicholas L. Massie, C. Corey Hines, Scott T. Griffin, Robin D. Rogers, Kevin H. Shaughnessy, Gabriela A. Grasa, Carin C. C. Johansson Seechurn, Hongbo Li, Thomas J. Colacot, Joe Chou, Christopher J. Woltermann
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.