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

Abstract

Experiments and density functional calculations were used to quantify the impact of the Pd-Ti interaction in the cationic heterobimetallic Cl2Ti(N(t)BuPPh2)2Pd(η(3)-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(N(t)BuPPh2)2Pd(η(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-Ti interaction lowers the activation barrier for turnover-limiting amine reductive addition and accelerates catalysis up to 10(5). The Pd-Ti distance in 1 is the result of the N(t)Bu 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(η(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-Ti interaction induces and is responsible for significantly lower barriers and faster catalysis for allylic aminations.

Selectivity Effects in Bimetallic Catalysis

“…An emerging area of homogeneous catalysis is the use of bimetallic catalysts featuring two closely associated metal sites . In addition to the “knobs” mentioned above for optimizing single‐site catalysts, these bimetallic systems present, in principle, additional structural elements that can be manipulated to “tune” reactions (Figure ).…”

Reactivity of a Dititanium Bis(pentalene) Complex toward Heteroallenes and Main-Group Element–Element Bonds

“…Total yield: 60 mg (92% with respect to 1). 1 (4). To a solution of 1 (70 mg, 0.076 mmol) in pentane (1 mL) was added a solution of PhNCO (9 mg, 0.076 mmol) in pentane (2 mL), dropwise at −35 °C.…”

“…1 There are a growing number of reports of compounds in which group 4 metal atoms are directly bonded to late transition metals or p-block metals in heteronuclear di-and polynuclear complexes. [2][3][4] Despite this, isolable compounds featuring a metalmetal bonding interaction between two group 4 metals are limited to a handful of examples. 5 We have recently reported the synthesis and isolation of the dititanium bis(pentalene) complex (μ:η 5 ,η 5 -Pn † )2Ti2 (1, Pn † = 1,4-{Si i Pr3}2C8H4), 6 which density functional theory (DFT) studies revealed to have two metalmetal bonds and a low lying metal-based empty orbital.…”

Synthesis and Characterization of Heterobimetallic Tantalum–Rhodium and Tantalum–Iridium Complexes Connected by a Tantalacyclopentadiene Fragment

“…Among them, heterobimetallic complexes containing early‐ and late‐transition metals (early‐late heterobimetallic complexes, ELHBs) have attracted special interest because they provide vastly different reaction sites that comprised of a hard Lewis acidic early‐transition metal and a soft Lewis basic late‐transition metal, respectively. [2] In addition, the unique nature of ELHBs is highlighted to the covalent bonding[3] and dative bonding[3c][4] character of the metal–metal interaction in ELHBs, and the metal–metal interaction has been widely applied for activating small molecules. [3i][4e – o][5] Such ELHBs involving a metal–metal interaction were categorized into two types of structures: one is a metal–metal bond without any bridging ligand, while the other is a ligand‐bridged metal–metal bonded structure.…”