Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp³)?C and C(sp³)?O bond‐forming reactions at high‐valent Ni as a function of oxidation state. Two Ni complexes are compared: a cationic octahedral Ni^IV complex ligated by tris(pyrazolyl)borate and a cationic octahedral Ni^III complex ligated by tris(pyrazolyl)methane. Key features of reactivity/selectivity are revealed: 1) C(sp³)?C(sp²) bond‐forming reductive elimination occurs from both centers, but the Ni^III complex reacts up to 300‐fold faster than the Ni^IV, depending on the reaction conditions. The relative reactivity is proposed to derive from ligand dissociation kinetics, which vary as a function of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO^?), the Ni^IV complex exclusively undergoes C(sp³)?OAc bond formation, while the Ni^III analogue forms the C(sp³)?C(sp²) coupled product selectively. This difference is rationalized based on the electrophilicity of the respective M?C(sp³) bonds, and thus their relative reactivity towards outer‐sphere S_N2‐type bond‐forming reactions.     

Borane‐Catalyzed Cross‐Metathesis Strategy for Facile Transformation of Cyclic (Alkyl)(Amino)Germylenes

A borane B(C₆F₅)₃‐catalyzed metathesis reaction between the Si?C bond in the cyclic (alkyl)(amino)germylene (CAAGe) 1 and the Si?H bond in a silane (R₃SiH; 2 ) is reported. Mechanistic studies propose that the initial step of the reaction involves Si?H bond activation to furnish an ionic species [ 1 ‐SiR₃]⁺[HB(C₆F₅)₃]^?, from which [Me₃Si]⁺[HB(C₆F₅)₃]^? and an azagermole intermediate are generated. The former yields Me₃SiH concomitant with the regeneration of B(C₆F₅)₃ whereas the latter undergoes isomerization to afford CAAGes bearing various silyl groups on the carbon atom next to the germylene center. This strategy allows the straightforward synthesis of eight new CAAGes starting from 1 .     

CO2‐Sourced α‐Alkylidene Cyclic Carbonates: A Step Forward in the Quest for Functional Regioregular Poly(urethane)s and Poly(carbonate)s

Described is a robust platform for the synthesis of a large diversity of novel functional CO₂‐sourced polymers by exploiting the regiocontrolled ring‐opening of α‐alkylidene carbonates by various nucleophiles. The reactivity of α‐alkylidene carbonates is dictated by the exocyclic olefinic group. The polyaddition of CO₂‐sourced bis(α‐alkylidene carbonate)s (bis‐αCCs) with primary and secondary diamines provides novel regioregular functional poly(urethane)s. The reactivity of bis‐αCCs is also exploited for producing new poly(β‐oxo‐carbonate)s by organocatalyzed polyaddition with a diol. This synthesis platform provides new functional variants of world‐class leading polymer families (polyurethanes, polycarbonates) and valorizes CO₂ as a chemical feedstock.     

κ3‐(N^C^C)Gold(III) Carboxylates: Evidence for Decarbonylation Processes

Gold(III) carboxylate species, stabilized by a κ³‐(N^C^C) ligand template, are presented herein. A η¹‐Au^III‐C(O)‐OH species has been characterized under cryogenic conditions as a result of the nucleophilic attack of an ammonium hydroxide onto a dinuclear μ‐CO₂‐κ³‐(N^C^C)Au^III precursor. Thermal decomposition for these species proceeds by an unusual decarbonylation process, in contrast to typical decarboxylation pathways observed in related metallocarboxylic acids.     

Sequential Oxidation and C−H Bond Activation at a Gallium(I) Center

In situ oxidation of the Ga^I compound NacNacGa by either N₂O or pyridine oxide results in the generation of a labile monomeric oxide, NacNacGa(O), which can easily cleave the C?H bonds of aliphatic and aromatic substrates featuring good donor sites. The products of this reaction are gallium organyl hydroxides. DFT calculations show that these reactions start with the formation of NacNac‐Ga(O)(L) adducts, the oxo ligand of which can easily abstract protons from nearby C?H bonds, even for sp²‐hybridized carbon centers. Aliphatic amines do not enter this reaction for kinetic reasons, presumably because of the unfavorable sterics.     

Propargylsilanes as Reagents for Synergistic Gold(I)‐Catalyzed Propargylation of Carbonyl Compounds: Isolation and Characterization of σ‐Gold(I) Allenyl Intermediates

Reported herein is the isolation and characterization, for the first time, of a σ‐gold allenyl complex as an intermediate in gold catalysis. This intermediate was captured during the study of a novel gold(I)‐catalyzed propargylation of carbonyl compounds with propargylsilanes. Notably, the gold‐catalyzed propargylation reaction, which proceeds with aldehydes and ketones, can be driven to the formation of either homopropargyl silyl ethers or the in situ synthesis of corresponding 2‐silyl‐4,5‐dihydrofurans.     

Reactivity of Highly Lewis‐Acidic Diborane(4) toward C≡N and N=N Bonds: Uncatalyzed Addition and N=N Bond‐Cleavage Reactions

The diboration of the C≡N bond in organic nitriles, and the N=N bond in azobenzene and pyridazine, by the highly Lewis‐acidic tetra(o‐tolyl)diborane(4) are reported. In the reactions with nitriles, azobenzene, and pyridazine, the addition of diborane(4) to the C≡N and N=N bonds was observed. Conversely, the N=N bond in phthalazine was cleaved by an addition/rearomatization sequence.