Clark R. Landis

We report a new stopped‐flow instrument designed for initiation and analysis of rapid reactions using standard NMR spectrometers and probes. The instrument is capable of kinetic measurements on mixing‐initiated reactions with half‐lives as short as 5 milliseconds. Design features of the instrument are discussed, with high‐speed video footage, pressure and NMR line‐shape data used to characterise mixing and stopping of reagent flows. The reactants are thermally and magnetically equilibrated prior to initiation of mixing and transport of the nascent reaction to the active volume for application of the NMR pulse sequence, ensuring near‐quantitative results. NMR data acquisition and processing require special considerations for fast reactions, and simulations were carried out to investigate the effect of composite instrument dead times on the observed rate constant. The kinetics of base‐mediated hydrolysis of methyl formate ( ¹ H) and pentafluorophenyl boronic acid ( ¹⁹ F) are employed to demonstrate the capability of the stopped‐flow instrument for the reproducible generation of NMR data from fast reactions. Full details of the design and construction of the instrument, together with engineering and electronics drawings, are provided as supporting information. The system requires no permanent modifications to the spectrometer or console.

Waste plastics are an abundant feedstock for the production of renewable chemicals. Pyrolysis of waste plastics produces pyrolysis oils with high concentrations of olefins (>50 weight %). The traditional petrochemical industry uses several energy-intensive steps to produce olefins from fossil feedstocks such as naphtha, natural gas, and crude oil. In this work, we demonstrate that pyrolysis oil can be used to produce aldehydes through hydroformylation, taking advantage of the olefin functionality. These aldehydes can then be reduced to mono- and dialcohols, oxidized to mono- and dicarboxylic acids, or aminated to mono- and diamines by using homogeneous and heterogeneous catalysis. This route produces high-value oxygenated chemicals from low-value postconsumer recycled polyethylene. We project that the chemicals produced by this route could lower greenhouse gas emissions ~60% compared with their production through petroleum feedstocks.

Palladium(II)-catalyzed allylic acetoxylation has been the focus of extensive development and investigation. Methods that use molecular oxygen (O₂) as the terminal oxidant typically benefit from the use of benzoquinone (BQ) and a transition-metal (TM) cocatalyst, such as Co(salophen), to support oxidation of Pd⁰ during catalytic turnover. We previously showed that Pd(OAc)₂ and 4,5-diazafluoren-9-one (DAF) as an ancillary ligand catalyze allylic oxidation with O₂ in the absence of cocatalysts. Herein, we show that BQ enhances DAF/Pd(OAc)₂ catalytic activity, nearly matching the performance of reactions that include both BQ and Co(salophen). These observations are complemented by mechanistic studies of DAF/Pd(OAc)₂ catalyst systems under three different oxidation conditions: (1) O₂ alone, (2) O₂ with cocatalytic BQ, and (3) O₂ with cocatalytic BQ and Co(salophen). The beneficial effect of BQ in the absence of Co(salophen) is traced to the synergistic roles of O₂ and BQ, both of which are capable of oxidizing Pd⁰ to Pd^II. The reaction of O₂ generates H₂O₂ as a byproduct, which can oxidize hydroquinone to quinone in the presence of Pd^II. NMR spectroscopic studies, however, show that hydroquinone is the predominant redox state of the quinone cocatalyst in the absence of Co(salophen), while inclusion of Co(salophen) maintains oxidized quinone throughout the reaction, resulting in better reaction performance.

Wu et al . (Reports, 31 August 2018, p. 912) claim that recently characterized octacarbonyls of Ca, Sr, and Ba mimic the classical Dewar-Chatt-Duncanson bonding motif of transition metals. This claim, which contradicts known chemistry and computed electron density distributions, originates in the assumption of a flawed reference state for energy decomposition analyses.

The rational development of homogeneous catalytic systems for selective aerobic oxidations of organics has been hampered by the limited available knowledge of how oxygen reacts with important organometallic intermediates. Recently, several mechanisms for oxygen insertion into late transition metal-hydride bonds have been described. Contributing to this nascent understanding of how oxygen reacts with metal-hydrides, a detailed mechanistic study of the reaction of oxygen with the Ir^III hydride complex (^dmPhebox)Ir(OAc)(H) (1) in the presence of acetic acid, which proceeds to form the Ir^III complex (^dmPhebox)Ir(OAc)₂(OH₂) (2), is described. The evidence supports a multifaceted mechanism wherein a small amount of an initially formed metal hydroperoxide proceeds to generate a metal-oxyl species that then initiates a radical chain reaction to rapidly convert the remaining Ir^III–H. Insight into the initiation step was gained through kinetic and mechanistic studies of the radical chain inhibition by BHT (butylated hydroxytoluene). Computational studies were employed to contribute to a further understanding of initiation and propagation in this system.

Allylic C–H acetoxylations are among the most widely studied palladium(II)-catalyzed C–H oxidation reactions. While the principal reaction steps are well established, key features of the catalytic mechanisms are poorly characterized, including the identity of the turnover-limiting step and the catalyst resting state. Here, we report a mechanistic study of aerobic allylic acetoxylation of allylbenzene with a catalyst system composed of Pd(OAc)₂ and 4,5-diazafluoren-9-one (DAF). The DAF ligand is unique in its ability to support aerobic catalytic turnover, even in the absence of benzoquinone or other co-catalysts. Herein, we describe operando spectroscopic analysis of the catalytic reaction using X-ray absorption and NMR spectroscopic methods that allow direct observation of the formation and decay of a palladium(I) species during the reaction. Kinetic studies reveal the presence of two distinct kinetic phases: (1) a burst phase, involving rapid formation of the allylic acetoxylation product and formation of the dimeric Pd^I complex [Pd^I(DAF)(OAc)]₂, followed by (2) a post-burst phase that coincides with evolution of the catalyst resting state from the Pd^I dimer into a π-allyl-Pd^II species. The data provide unprecedented insights into the role of ancillary ligands in supporting catalytic turnover with O₂ as the stoichiometric oxidant and establish an important foundation for the development of improved catalysts for allylic oxidation reactions.