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

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.