Origin of Fast Catalysis in Allylic Amination Reactions Catalyzed by Pd–Ti Heterobimetallic Complexes

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Abstract

Experiments and density functional calculations were used to quantify the impact of the Pd–Ti interaction in the cationic heterobimetallic Cl2Ti(NtBuPPh2)2Pd(η3-methallyl) catalyst 1 used for allylic aminations. The catalytic significance of the Pd–Ti interaction was evaluated computationally by examining the catalytic cycle for catalyst 1 with a conformation where the Pd–Ti interaction is intact versus one where the Pd–Ti interaction is severed. Studies were also performed on the relative reactivity of the cationic monometallic (CH2)2(NtBuPPh2)2Pd(η3-methallyl) catalyst 2 where the Ti from catalyst 1 was replaced by an ethylene group. These computational and experimental studies revealed that the Pd–Ti interaction lowers the activation barrier for turnover-limiting amine reductive addition and accelerates catalysis up to 105. The Pd–Ti distance in 1 is the result of the NtBu groups enforcing a boat conformation that brings the two metals into close proximity, especially in the transition state. The turnover frequency of classic Pd π allyl complexes was compared to that of 1 to determine the impact of P–Pd–P coordination angle and ligand electronic properties on catalysis. These experiments identified that cationic (PPh3)2Pd(η3-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–Ti interaction induces and is responsible for significantly lower barriers and faster catalysis for allylic aminations.

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