Effects of Bases and Halides on the Amination of Chloroarenes Catalyzed by Pd(PtBu3)2
journal contributionposted on 15.01.2007 by Shashank Shekhar, John F. Hartwig
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We report a detailed study of the effects of anions on the rates of the amination of aryl halides catalyzed by palladium complexes of the hindered alkylmonophosphine PtBu3. These reactions occur by turnover-limiting oxidative addition. The extent of the dependence of the rates on the concentration and identity of bases was found to depend on the electronic properties of the haloarenes. The rates of reactions of electron-rich and electron-neutral chloroarenes were independent of the concentration of the bulky alkoxide base OCEt3-, but they were dependent on the concentrations of the less hindered OtBu- and the softer 2,4,6-tri-tert-butylphenoxide bases. The reactions were fastest when 2,4,6-tri-tert-butylphenoxide was used as base and slowest when NaOCEt3 was used as base. A concurrent reaction pathway involving simultaneous oxidative addition of chloroarenes to [Pd(PtBu3)] and [(PtBu3)Pd(OR)]- may explain the dependence of the rates of reactions of electron-rich and electron-neutral chloroarenes on the identity and concentration of bases. The rates of reactions of electron-poor chloroarenes and the reactions of bromoarenes were independent of the concentration of OCEt3-, OtBu-, or 2,4,6-tri-tert-butylphenoxide bases but were dependent on the identity of bases, even though an adduct of the base did not accumulate in any observable amounts. Further, long induction periods were observed for the reactions of electron-poor chloroarenes. These long induction periods suggest that the complex kinetic behavior could result from the generation of several catalytically active species.