Synthesis and Properties of “Sandwich” Diimine-Coinage Metal Ethylene Complexes

Synthesis and full characterization of cationic isostructural “sandwich” diimine-coinage metal ethylene complexes are reported. Ethylene self-exchange kinetics proceeds by an associative exchange mechanism for Cu and Au complexes. The fastest ligand exchange was observed for Ag complex <b>8a</b>. The upper limit of Δ<i>G</i>^⧧, assuming associative ligand exchange, was found to be ca. 5.0 kcal/mol. Ethylene self-exchange in Cu complex <b>7b</b> proceeds with Δ<i>G</i>₂₉₈^⧧ = 12.9 ± 0.1 kcal/mol, while the exchange is the slowest in Au complex <b>9</b>, with Δ<i>G</i>₂₉₈^⧧ = 16.7 ± 0.1 kcal/mol. Copper complex <b>7b</b> is unusually stable and can survive in air for years

Synthesis and Properties of “Sandwich” Diimine-Coinage Metal Ethylene Complexes

Synthesis and full characterization of cationic isostructural “sandwich” diimine-coinage metal ethylene complexes are reported. Ethylene self-exchange kinetics proceeds by an associative exchange mechanism for Cu and Au complexes. The fastest ligand exchange was observed for Ag complex <b>8a</b>. The upper limit of Δ<i>G</i>^⧧, assuming associative ligand exchange, was found to be ca. 5.0 kcal/mol. Ethylene self-exchange in Cu complex <b>7b</b> proceeds with Δ<i>G</i>₂₉₈^⧧ = 12.9 ± 0.1 kcal/mol, while the exchange is the slowest in Au complex <b>9</b>, with Δ<i>G</i>₂₉₈^⧧ = 16.7 ± 0.1 kcal/mol. Copper complex <b>7b</b> is unusually stable and can survive in air for years

Copper-Catalyzed, Directing Group-Assisted Fluorination of Arene and Heteroarene C–H Bonds

We have developed a method for direct, copper-catalyzed, auxiliary-assisted fluorination of β-sp² C–H bonds of benzoic acid derivatives and γ-sp² C–H bonds of α,α-disubstituted benzylamine derivatives. The reaction employs a CuI catalyst, a AgF fluoride source, and DMF, pyridine, or DMPU solvent at moderately elevated temperatures. Selective mono- or difluorination can be achieved by simply changing reaction conditions. The method shows excellent functional group tolerance and provides a straightforward way for the preparation of ortho-fluorinated benzoic acids

2‑Vinyl Threoninol Derivatives via Acid-Catalyzed Allylic Substitution of Bisimidates

A diastereoselective synthesis of 4-vinyl oxazolines <i>syn-</i><b>2</b> was developed based on an acid-catalyzed cyclization of bistrichloroacetimidates (<i>E</i>)-<b>1</b>. The reaction likely involves an allyl carbenium ion intermediate in which the adjacent stereocenter directs the stereoselectivity for C–N bond formation. Oxazolines <i>syn-</i><b>2</b> were transformed to C-quaternary threoninol, threoninal, and threonine derivatives which can be further incorporated into complex natural compounds

Alkene Isomerization by “Sandwich” Diimine-Palladium Catalysts

In contrast to traditional diimine-palladium complexes, sterically hindered “sandwich” diimine-palladium adducts act as olefin isomerization catalysts. Terminal olefins are selectively converted to 2-olefins by a sequence of migratory insertion, β-hydride elimination, and olefin displacement. The reaction is performed at 0 °C with 1 mol % of an air-stable precatalyst and tolerates functional groups such as ketones, silyl ethers, and halogens. The isomerization may be used to produce silyl enol ethers from protected allylic alcohols

Secondary Alkene Insertion and Precision Chain-Walking: A New Route to Semicrystalline “Polyethylene” from α‑Olefins by Combining Two Rare Catalytic Events

While traditional polymerization of linear α-olefins (LAOs) typically provides amorphous, low <i>T</i>_g polymers, chain-straightening polymerization represents a route to semicrystalline materials. A series of α-diimine nickel catalysts were tested for the polymerization of various LAOs. Although known systems yielded amorphous or low-melting polymers, the “sandwich” α-diimines <b>3</b>–<b>6</b> yielded semicrystalline “polyethylene” comprised primarily of unbranched repeat units via a combination of uncommon regioselective 2,1-insertion and precision chain-walking events

Secondary Alkene Insertion and Precision Chain-Walking: A New Route to Semicrystalline “Polyethylene” from α‑Olefins by Combining Two Rare Catalytic Events

While traditional polymerization of linear α-olefins (LAOs) typically provides amorphous, low <i>T</i>_g polymers, chain-straightening polymerization represents a route to semicrystalline materials. A series of α-diimine nickel catalysts were tested for the polymerization of various LAOs. Although known systems yielded amorphous or low-melting polymers, the “sandwich” α-diimines <b>3</b>–<b>6</b> yielded semicrystalline “polyethylene” comprised primarily of unbranched repeat units via a combination of uncommon regioselective 2,1-insertion and precision chain-walking events

Secondary Alkene Insertion and Precision Chain-Walking: A New Route to Semicrystalline “Polyethylene” from α‑Olefins by Combining Two Rare Catalytic Events

While traditional polymerization of linear α-olefins (LAOs) typically provides amorphous, low <i>T</i>_g polymers, chain-straightening polymerization represents a route to semicrystalline materials. A series of α-diimine nickel catalysts were tested for the polymerization of various LAOs. Although known systems yielded amorphous or low-melting polymers, the “sandwich” α-diimines <b>3</b>–<b>6</b> yielded semicrystalline “polyethylene” comprised primarily of unbranched repeat units via a combination of uncommon regioselective 2,1-insertion and precision chain-walking events

Understanding the Insertion Pathways and Chain Walking Mechanisms of α‑Diimine Nickel Catalysts for α‑Olefin Polymerization: A ¹³C NMR Spectroscopic Investigation

Nickel α-diimine catalysts have been previously shown to perform the chain straightening polymerization of α-olefins to produce materials with melting temperatures (<i>T</i>_m) similar to linear low density polyethylene (<i>T</i>_m = 100–113 °C). Branching defects due to mechanistic errors during the polymerization currently hinder access to high density polyethylene (<i>T</i>_m = 135 °C) from α-olefins. Understanding the intricacies of nickel α-diimine catalyzed α-olefin polymerization can lead to improved ligand designs that should allow production of chain-straightened polymers. We report a ¹³C NMR study of poly­(α-olefins) produced from monomers with ¹³C-labeled carbonsspecifically 1-decene with a ¹³C-label in the 2-position and 1-dodecene with a ¹³C-label in the ω-positionusing a series of α-diimine nickel catalysts. Furthermore, we developed a mathematical model capable of quantifying the resulting ¹³C NMR data into eight unique insertion pathways: 2,1- or 1,2- insertion from the primary chain end position (1°), the penultimate chain end position (2_p^°), secondary positions on the polymer backbone (2°), and previously installed methyl groups (1_m^°). With this model, we accurately determined overall regiochemistry of insertion and overall preference for primary versus secondary insertion pathways using nickel catalysts under various conditions. Beyond this, our model provides the tools necessary for determining how ligand structure and polymerization conditions affect catalyst behavior for α-olefin polymerizations

Understanding the Insertion Pathways and Chain Walking Mechanisms of α‑Diimine Nickel Catalysts for α‑Olefin Polymerization: A ¹³C NMR Spectroscopic Investigation

Nickel α-diimine catalysts have been previously shown to perform the chain straightening polymerization of α-olefins to produce materials with melting temperatures (<i>T</i>_m) similar to linear low density polyethylene (<i>T</i>_m = 100–113 °C). Branching defects due to mechanistic errors during the polymerization currently hinder access to high density polyethylene (<i>T</i>_m = 135 °C) from α-olefins. Understanding the intricacies of nickel α-diimine catalyzed α-olefin polymerization can lead to improved ligand designs that should allow production of chain-straightened polymers. We report a ¹³C NMR study of poly­(α-olefins) produced from monomers with ¹³C-labeled carbonsspecifically 1-decene with a ¹³C-label in the 2-position and 1-dodecene with a ¹³C-label in the ω-positionusing a series of α-diimine nickel catalysts. Furthermore, we developed a mathematical model capable of quantifying the resulting ¹³C NMR data into eight unique insertion pathways: 2,1- or 1,2- insertion from the primary chain end position (1°), the penultimate chain end position (2_p^°), secondary positions on the polymer backbone (2°), and previously installed methyl groups (1_m^°). With this model, we accurately determined overall regiochemistry of insertion and overall preference for primary versus secondary insertion pathways using nickel catalysts under various conditions. Beyond this, our model provides the tools necessary for determining how ligand structure and polymerization conditions affect catalyst behavior for α-olefin polymerizations