Mechanism of palladium complex-catalyzed enantioselective Mannich-type reaction: Characterization of a novel binuclear palladium enolate complex

Article abstract of DOI:10.1021/ja9902827

Studies on the enantioselective addition of enol silyl ethers to imines catalyzed by optically active palladium diaquo complexes 3 or binuclear palladium μ-hydroxo complex 4 are described, with particular focus on the mechanistic aspects. Asymmetric induction in the reaction using [Pd((R)-binap)(H₂O)₂]²⁺(BP₄ ^-)₂ (3a) was quite sensitive to the reaction conditions, suggesting unfavorable effects of HBF₄ generated from 3a in situ. Novel optically active binuclear μ-hydroxo complexes [{Pd((R)-binap)(μ-OH)}₂]²⁺(BF₄ ^-)₂ (4a), [{Pd-((R)-tol-binap)(μ-OH)}₂]²⁺(BF₄ ^-)₂ (4b), [{Pd((R)-binap)(μ-OH)}₂]²⁺(TfO^-)₂ (4c), and [{Pd((R)-tol-binap)(μ-OH)}₂]²⁺(TfO^-) ₂ (4d) were prepared and were found to be better catalysts for the asymmetric Mannich-type reaction. Benzoylalanine derivatives 5 were obtained in excellent chemical and optical yields (up to 90% ee). Mechanistic studies using ¹H NMR and electrospray ionization mass spectrometry indicated that a unique binuclear palladium-sandwiched enolate 12 was involved in the reaction of enol silyl ether 1 with imine 2 catalyzed by 4.