The Studies On Palladium(Ⅱ) Catalytic Cycles In Organic Reactions

Based on the previous work in our group, the thesis centered on the inhibition ofβ-H elimination and the use of cationic palladium complexes in Pd(â…¡)-catalyzed reactions. Combined the different initiation model (acetoxypalladation and carbopalladation) with different quenching methods (β-heteroatom elimination and protonolysis) , the research was focused on the reactions of carbocyclization of enyne, cyclization of aroylmethyl 2-alkynoates and Michael addition ofα,β-unsaturated carbonyl compounds. The thesis is constituted by four parts:(1) Pd(OAc)₂/iminopyridine catalyzed intermolecular carbocyclization of enyne initiated by acetoxypalladation.Based on the work of Dr. Zhang Qinghai, we disclosed another kind of ligand-iminopyridine which could help to realize the intramolecular carbocyclization of enyne initiated by acetoxypalladation. The presence of iminopyridine ligands made the Pd coordinatively saturated and theβ-hydride elimination not so feasible. Furthermore, the coordination of iminopyridine ligands to Pd increased the electron density of Pd, resulting in increasing stability of the C-Pd bond towards protonolysis. these roles had been confirmed by a stoichiometric reaction.(2) Pd(OAc)₂/bpy catalyzed intramolecular carbocyclization of enyne initiated by carbopalladation.Another sort of intramolecular carbocyclization of enyne which initiated by carbopalladation in the presence of Pd(OAc)₂ as catalyst and bipyridine as ligand was studied. The vinylpalladium intermediate was obtained by the addition of arylboronic acid to alkyne and quenched by.β-heteroatome elimination after the insertion to double bond. The reaction gave the target product in moderate yield (52%) after optimized. The reasons for unsatisfactory result were: (a) the substrate was not stable enough under the reaction condition. (b) the formation of Pd(0). (3) Cationic paIIadium(â…¡)-catalyzed enantioselective cyclization of aroylmethyl 2-alkynoates.We developed a cationic palladium(â…¡)-catalyzed addition of vinylpalladium species to ketones under mild conditions with excellent yield (up to 96%). The vinylpalladium intermediates was generated by the carbopalladation of alkynes triggered by the addition of arylboronic acids. Furthermore, the asymmetric version of this cascade cyclization was also realized using Pd(CF₃COO)₂/BDPP as catalyst. The ee values of the products were up to 92%. We found that the choice of substrates with electron-deficient triple bonds and cationic palladium catalysts were crucial for the reaction . This cascade reaction provided an efficient way for the construction of optically active hydroxylactones.(4) Pd(OAc)₂/iminopyridine catalyzed addition of arylboronic acid toα,β-unsaturated carbonyl compounds.The reaction gave moderate yield (62%) when Pd(OAc)₂ was used as catalyst and iminopyridine was used as ligand. The addition of acetic acid was necessary for the reaction system and the insufficient Lewis acidity of palladium complex might answer for the unsatisfactory result.