Oxidative Ring Expansion of Cyclobutanols: Access to Functionalized 1,2-Dioxanes

Cyclobutanols undergo an oxidative ring expansion into 1,2-dioxanols by using Co(acac)₂ and triplet oxygen (³O₂) as radical promoters. The formation of an alkoxy radical drives to the regioselective break of the strained ring with stabilization of a new radical on the most substituted side. The radical traps then oxygen to form 1,2-dioxanols. The reaction is particularly effective on secondary cyclobutanols but can work also on tertiary alcohols. Further acetylation generates peroxycarbenium species under catalytic Lewis acid conditions, which react with neutral nucleophiles. Many original 1,2-dioxanes, which would be difficult to prepare by another method, were then obtained with a preferred 3,6-<i>cis</i>-configuration. This method provides an interesting access to the total synthesis of many natural endoperoxides.

Oxidative Ring Expansion of Cyclobutanols: Access to Functionalized 1,2-Dioxanes

Cyclobutanols undergo an oxidative ring expansion into 1,2-dioxanols by using Co(acac)₂ and triplet oxygen (³O₂) as radical promoters. The formation of an alkoxy radical drives to the regioselective break of the strained ring with stabilization of a new radical on the most substituted side. The radical traps then oxygen to form 1,2-dioxanols. The reaction is particularly effective on secondary cyclobutanols but can work also on tertiary alcohols. Further acetylation generates peroxycarbenium species under catalytic Lewis acid conditions, which react with neutral nucleophiles. Many original 1,2-dioxanes, which would be difficult to prepare by another method, were then obtained with a preferred 3,6-<i>cis</i>-configuration. This method provides an interesting access to the total synthesis of many natural endoperoxides.

Formation and Emissions of Volatile Organic Compounds from Homo-PP and Co-PP Resins during Manufacturing Process and Accelerated Photoaging Degradation

Volatile organic compounds (VOCs) from polypropylene (PP) seriously restricts the application of PP in an automotive field. Herein, the traceability of VOCs from PP resins during manufacturing process and accelerated photoaging degradation was clarified on basis of an accurate characterization method of key VOCs. The influence of PP structures on changing the accelerated photoaging degradation on the VOCs was systematic. The VOCs were identified by means of Gas chromatography (GC) coupled with both a hydrogen flame ion detector (FID) and a mass spectrometry detector (MSD). Results showed that both the molecular structure of PP and the manufacturing process affected the species and contents of VOCs. In addition, the photoaging degradation of PP resulted in a large number of new emerged volatile carbonyl compounds. Our work proposed a possible VOC formation mechanism during the manufacturing and photoaging process. VOCs from PP resins were originated from oligomers and chain random scission during thermomechanical degradation. However, β scission of alkoxy radical and Norrish tape I reactions of ketones via intermediate transition were probably the main VOCs formation routes towards PP during photoaging degradation. This work could provide scientific knowledge on both the accurate traceability of VOCs emissions and new technology for development of low-VOCs PP composites for vehicle.

Pressure-Dependent Kinetics of Peroxy Radicals Formed in Isobutanol Combustion

Bio-derived isobutanol has been approved as a gasoline additive in the U.S., but our understanding of its combustion chemistry still has significant uncertainties. Detailed quantum calculations could improve model accuracy leading to better estimation of isobutanol’s combustion properties and its environmental impacts. This work examines 47 molecules and 38 reactions involved in the first oxygen addition to isobutanol’s three alkyl radicals located α, β, and γ to the hydroxide. Quantum calculations are mostly done at CCSD(T)-F12/cc-pVTZ-F12//B3LYP/CBSB7, with 1-D hindered rotor corrections obtained at B3LYP/6-31G(d). The resulting potential energy surfaces are the most comprehensive isobutanol peroxy networks published to date. Canonical transition state theory and a 1-D microcanonical master equation are used to derive high-pressure-limit and pressure-dependent rate coefficients, respectively. At all conditions studied, the recombination of α- isobutanol radical with O2 forms HO2 and isobutanal. The recombination of γ-isobutanol radical with O2 forms a stabilized hydroperoxy alkyl radical below 400 K, water and an alkoxy radical at higher temperatures, and HO2 and an alkene above 1200 K. The recombination of β-isobutanol radical with O2 results in a mixture of products between 700-1100 K, forming acetone, formaldehyde and OH at lower temperatures and forming HO2 and alkenes at higher temperatures. The barrier heights, high-pressure-limit rates, and pressure-dependent kinetics generally agree with the results from previous quantum chemistry calculations. Six reaction rates in this work deviate by over three orders of magnitude from kinetics in detailed models of isobutanol combustion, suggesting the rates calculated here can help improve modeling of isobutanol combustion and its environmental fate.

Pressure-Dependent Kinetics of Peroxy Radicals Formed in Isobutanol Combustion

Bio-derived isobutanol has been approved as a gasoline additive in the U.S., but our understanding of its combustion chemistry still has significant uncertainties. Detailed quantum calculations could improve model accuracy leading to better estimation of isobutanol’s combustion properties and its environmental impacts. This work examines 47 molecules and 38 reactions involved in the first oxygen addition to isobutanol’s three alkyl radicals located α, β, and γ to the hydroxide. Quantum calculations were mostly done at CCSD(T)-F12/cc-pVTZ-F12//B3LYP/CBSB7, with 1-D hindered rotor corrections obtained at B3LYP/6-31G(d). The resulting potential energy surfaces are the most comprehensive isobutanol peroxy networks published to date. Canonical transition state theory and a 1-D microcanonical master equation are used to derive high-pressure-limit and pressure-dependent rate coefficients, respectively. At all conditions studied, the recombination of α- isobutanol radical with O2 forms HO2 and isobutanal. The recombination of γ-isobutanol radical with O2 forms a stabilized hydroperoxy alkyl radical below 400 K, water and an alkoxy radical at higher temperatures, and HO2 and an alkene above 1200 K. The recombination of β-isobutanol radical with O2 results in a mixture of products between 700-1100 K, forming acetone, formaldehyde and OH at lower temperatures and forming HO2 and alkenes at higher temperatures. The barrier heights, high-pressure-limit rates, and pressure-dependent kinetics generally agree with the results from previous quantum chemistry calculations. Six reaction rates in this work deviate by over three orders of magnitude from kinetics in detailed models of isobutanol combustion, suggesting the rates calculated here can help improve modeling of isobutanol combustion and its environmental fate.

Biomimetic Synthesis of Hitorins A and B via an Intermolecular Alkoxy Radical-Olefin Coupling Cascade

A biomimetic synthesis of hitorins A and B was achieved based on our modified biosynthetic proposal. In our synthesis, a radical cascade reaction between an alkoxy radical, generated from a hydroperoxide, and a monoterpene (+)-sabinene renders the tetrahydrofuran ring of hitorins A and B. In addition, experimental results supported that the oxidative cleavage of the tetrasubstituted olefin in a key intermediate is via a radical oxidation cascade followed by a Grob fragmentation.<br>

Biomimetic Synthesis of Hitorins A and B via an Intermolecular Alkoxy Radical-Olefin Coupling Cascade

A biomimetic synthesis of hitorins A and B was achieved based on our modified biosynthetic proposal. In our synthesis, a radical cascade reaction between an alkoxy radical, generated from a hydroperoxide, and a monoterpene (+)-sabinene renders the tetrahydrofuran ring of hitorins A and B. In addition, experimental results supported that the oxidative cleavage of the tetrasubstituted olefin in a key intermediate is via a radical oxidation cascade followed by a Grob fragmentation.<br>

Light-Driven Depolymerization of Native Lignin Enabled by Proton- Coupled Electron Transfer

<div><p>Here we report a catalytic, light-driven method for the redox-neutral depolymerization of native lignin biomass at ambient temperature. This transformation proceeds via a proton-coupled electron-transfer (PCET) activation of an alcohol O–H bond to generate a key alkoxy radical intermediate, which then drives the <i>β</i>-scission of a vicinal C–C bond. Notably, this depolymerization is driven solely by visible light irradiation, requiring no stoichiometric chemical reagents and producing no stoichiometric waste. This method exhibits good efficiency and excellent selectivity for the activation and fragmentation of <i>β</i>-O-4 linkages in the polymer backbone, even in the presence of numerous other PCET-active functional groups. DFT analysis suggests that the key C–C bond cleavage reactions produce non-equilibrium product distributions, driven by excited-state redox events. These results provide further evidence that visible-light photocatalysis can serve as a viable method for the direct conversion of lignin biomass into valuable arene feedstocks.</p></div>