Platinum-Based Catalysts for the Hydroamination of Olefins with Sulfonamides and Weakly Basic Anilines

摘要

Electrophilic Pt(Ⅱ) complexes catalyze efficient hydroaminations of olefins by sulfonamides and weakly basic anilines. Catalysts include the structurally characterized complex (COD)Pt(OTf)_2 (1) and the known dimer [PtCl_2(C_2H_4)]_2, activated by AgBF_4. Experiments with substituted anilines establish an empirical pK_a cutoff (conjugate acid pK_a < 1) for the participation of nitrogen-containing substrates in this catalysis. Arylsulfonamides (conjugate acid pK_a ≈ -6) with various para substituents hydroaminate olefins such as cyclohexene in yields greater than 95% at 90℃. Hydroamination of propylene by p-toluenesulfonamide proceeds with Markovnikov selectivity, suggesting a mechanism that involves olefin activation at Pt. With norbornene and p-toluenesulfonamide as the substrates and 1 as the catalyst, intermediate [(COD)Pt-(norbornene)_2][OTf]_2 (3) was identified and characterized by ~(19)F and ~(195)Pt NMR spectroscopies and mass spectrometry. Kinetic studies provide the empirical rate law, rate = k_(obs)[Pt][sulfonamide], and are consistent with a mechanism in which attack of a sulfonamide on the Pt-coordinated olefin is the rate-determining step.