Origin of Fast Catalysis in Allylic Amination Reactions Catalyzed by Pd鈥揟i Heterobimetallic Complexes

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• 作者：Whitney K. Walker ; Benjamin M. Kay ; Scott A. Michaelis ; Diana L. Anderson ; Stacey J. Smith ; Daniel H. Ess ; David J. Michaelis
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：June 17, 2015
• 年：2015
• 卷：137
• 期：23
• 页码：7371-7378
• 全文大小：480K
• ISSN：1520-5126

文摘

Experiments and density functional calculations were used to quantify the impact of the Pd鈥揟i interaction in the cationic heterobimetallic Cl₂Ti(N^tBuPPh₂)₂Pd(畏³-methallyl) catalyst 1 used for allylic aminations. The catalytic significance of the Pd鈥揟i interaction was evaluated computationally by examining the catalytic cycle for catalyst 1 with a conformation where the Pd鈥揟i interaction is intact versus one where the Pd鈥揟i interaction is severed. Studies were also performed on the relative reactivity of the cationic monometallic (CH₂)₂(N^tBuPPh₂)₂Pd(畏³-methallyl) catalyst 2 where the Ti from catalyst 1 was replaced by an ethylene group. These computational and experimental studies revealed that the Pd鈥揟i interaction lowers the activation barrier for turnover-limiting amine reductive addition and accelerates catalysis up to 10⁵. The Pd鈥揟i distance in 1 is the result of the N^tBu groups enforcing a boat conformation that brings the two metals into close proximity, especially in the transition state. The turnover frequency of classic Pd 蟺 allyl complexes was compared to that of 1 to determine the impact of P鈥揚d鈥揚 coordination angle and ligand electronic properties on catalysis. These experiments identified that cationic (PPh₃)₂Pd(畏³-CH₂C(CH₃)CH₂) catalyst 3 performs similarly to 1 for allylic aminations with diethylamine. However, computations and experiment reveal that the apparent similarity in reactivity is due to very fast reaction kinetics. The higher reactivity of 1 versus 3 was confirmed in the reaction of methallyl chloride and 2,2,6,6-tetramethylpiperidine (TMP). Overall, experiments and calculations demonstrate that the Pd鈥揟i interaction induces and is responsible for significantly lower barriers and faster catalysis for allylic aminations.