d0 Metal-Catalyzed Alkyl–Alkyl Cross-Coupling Enabled by a Redox-Active Ligand

Elucidating the Impact of Rare Earth or Transition Metal Identity on the Physical and Electronic Structural Properties of a Series of Redox-Active Tris(amido) Complexes

The use of redox-active ligands with the f-block elements has been employed to promote unique chemical transformations and explore their unique emergent electronic properties for a myriad of applications. In this study, we report eight new tris(amido) metal complexes: 1-Ln (Ln = Tb3+, Dy3+, Ho3+, Er3+, Tm3+, and Yb3+), 1-La, and 1-Ti (an early transition metal analogue). The one-electron oxidation of the tris(amido) ligand was conducted to generate semi-iminato complexes 2-Ln, 2-La, and 2-Ti, and these complexes were studied using EPR. Tris(amido) complexes 1-Ln, 1-La, and 1-Ti were fully characterized using a range of spectroscopic (NMR and UV-vis/NIR) and physical techniques (X-ray diffraction and cyclic voltammetry, with the exception of 1-La). Computational methods were employed to further elucidate the electronic structures of these complexes. Lastly, complexes 1-Ln, 1-La, and 1-Ti were probed as catalysts for alkyl-alkyl cross-coupling, and the initial rate of the reaction was measured to explore the influence of the metal ion.

Access to Stereoblock Polyesters via Irreversible Chain-Transfer Ring-Opening Polymerization (ICT-ROP)

Precise control over polymer microstructure can enable the molecular tunability of material properties and represents a significant challenge in polymer chemistry. Stereoblock copolymers are some of the simplest stereosequenced polymers, yet the synthesis of stereoblock polyesters from prochiral or racemic monomers outside of "simple" isotactic stereoblocks remains limited. Herein, we report the development of irreversible chain-transfer ring-opening polymerization (ICT-ROP), which overcomes the fundamental limitations of single catalyst approaches by using transmetalation (e.g., alkoxide-chloride exchange) between two catalysts with distinct stereoselectivities as a means to embed temporally controlled multicatalysis in ROP. Our combined small-molecule model and catalytic polymerization studies lay out a clear molecular basis for ICT-ROP and are exploited to access the first examples of atactic-syndiotactic stereoblock (at-sb-st) polyesters, at-sb-st polyhydroxyalkanoates (PHAs). We achieve high levels of control over molecular weight, tacticity, monomer composition, and block structures in a temporally controlled manner and demonstrate that stereosequence control leads to polymer tensile properties that are independent of thermal properties.

Multielectron Redox Afforded by a Pincer Ligand Promoting Kumada Cross-Coupling Reactions

The redox-active nature of a pincer has been exploited to conduct C-C cross-coupling reactions under mild conditions. A nickel complex with a NNN pincer was dimeric in the solid state, and the structure displayed a Ni2 N2 diamond core. In the dimeric structure, both ligand backbones house an electron, in the iminosemiquinonate form, to keep the metal's oxidation state at +2. In the presence of an aryl Grignard reagent, only 3 mol % loading the nickel complex generates a Kumada cross-coupled product in good yield from a wide variety of aryl-X (X= I, Br, Cl) substrates. That the ligand-based radical remains responsible for promoting such a coupling reaction following a radical pathway is suggested by TEMPO quenching. Furthermore, a radical-clock experiment along with tracing product distribution unambiguously supported the radical's involvement through the catalytic cycle. A series of thorough mechanistic probation, including computational DFT analysis, disclosed the cooperative action of both redox-active pincer ligand and the metal centre to drive the reaction.

Lanthanum-Catalyzed Stereospecific Cross-Coupling of Propargylic Substrates with Grignard Reagents

Transition-metal-catalyzed cross-coupling of propargylic electrophiles and Grignard reagents provides densely functionalized products that are extremely useful synthetic intermediates. However, examples of conversion of propargylic derivatives to form propargyl compounds remain limited due to the challenging regioselectivity. We use LaCl3·2LiCl to catalyze propargylation of Grignard reagents in the absence of ligand in high regioselectivity and stereospecificity. The approach shows a wide substrate scope using alkyl or (hetero)aryl Grignard reagents and alkynyl electrophiles with different leaving groups. Our protocol was further applied for the formal synthesis of frondosin B. It is worth exploring methodologies utilizing the naturally abundant and relatively nontoxic lanthanum catalysts.

Redox-Neutral Decarboxylative and Desulfonylative C(sp3) Trifluoromethylation: Method Development and Mechanistic Inquiry

Sodium triflinate (CF₃SO₂Na) is an inexpensive bench-stable radical CF₃ source that is often activated by external oxidants such as peroxides. However, despite the commercial accessibility of CF₃SO₂Na, the salt has never been applied to decarboxylative trifluoromethylation due to challenges in controlled cross-radical coupling. We report a redox-neutral approach to decarboxylative C(sp³) trifluoromethylation of carboxylic acid derivatives. Mechanistic inquiry is performed to address the limitations in scope.

Dioxygen Splitting by a Tantalum(V) Complex Ligated by a Rigid, Redox Non-Innocent Pincer Ligand

The reaction of TaMe₃ Cl₂ with the rigid acridane-derived trisamine H₃ NNN yields the tantalum(V) complex [TaCl₂ (NNN^cat )]. Subsequent reaction with dioxygen results in the full four-electron reduction of O₂ yielding the oxido-bridged bimetallic complex [{TaCl₂ (NNN^sq )}₂ O]. This dinuclear complex features an open-shell ground state due to partial ligand oxidation and was comprehensively characterized by single crystal X-ray diffraction, LIFDI mass spectrometry, NMR, EPR, IR and UV/VIS/NIR spectroscopy. The mechanism of O₂ activation was investigated by DFT calculations revealing initial binding of O₂ to the tantalum(V) center followed by complete O₂ scission to produce a terminal oxido-complex.

Cobalt-catalyzed cross-coupling of Umpolung carbonyls with alkyl halides under mild conditions

While classical cross-couplings are dominated by palladium and nickel catalysts, cobalt-based catalysts have shown unique advantages for such cross-coupling reactions in terms of higher catalytic activity and lower toxicity. Herein, we describe a novel cobalt-catalyzed alkyl-alkyl cross-coupling reaction of hydrazone with alkyl halides under mild reaction conditions, where the use of a PNP-type pincer ligand is essential for catalysis. Both aldehyde and ketone hydrazones are compatible with this reaction, giving a series of C(sp³)-C(sp³) coupling products in moderate to good yields.