Organic Radical Chemistry Research Articles

Magnetic Properties of Self-Assemble Naphthalene Diimide Radical Aggregates.

The concept of creating room-temperature ferromagnets from organic radicals proposed nearly sixty years ago, has recently experienced a resurgence due to advances in organic radical chemistry and materials. However, the lack of definitive design paradigms for achieving stable long-range ferromagnetic coupling between organic radicals presents an uncertain future for this research. Here, an innovative strategy is presented to achieve room-temperature ferromagnets by assembling π-conjugated radicals into π-π stacking aggregates. These aggregates, with ultra-close π-π distances and optimal π-π overlap, provide a platform for strong ferromagnetic (FM) interaction. The planar aromatic naphthalene diimide (NDI) anion radicals form nanorod aggregates with a π-π distance of just 3.26 Å, shorter than typical van der Waals distances. The suppressed electron paramagnetic resonance (EPR) signal and emergent near-infrared (NIR) absorption of the aggregates confirm strong interactions between the radicals. Magnetic measurements of NDI anion radical aggregates demonstrate room-temperature ferromagnetism with a saturated magnetization of 1.1 emug-1, the highest among pure organic ferromagnets. Theoretical calculations reveal that π-stacks of NDI anion radicals with specific interlayer translational slippage favor ferromagnetic coupling over antiferromagnetic coupling.

Unveiling the environmental significance of acetylperoxyl radical: Reactivity quantification and kinetic modeling.

Acetylperoxyl radical (CH3C(O)OO•) is among highly reactive organic radicals which are known to play crucial roles in atmospheric chemistry, aqueous chemistry and, most recently, peracetic acid (PAA)-based advanced oxidation processes. However, fundamental knowledge for its reactivity is scarce and severely hampers the understanding of relevant environmental processes. Herein, three independent experimental approaches were exploited for revelation and quantification of the reaction rates of acetylperoxyl radical. First, we developed and verified laser flash photolysis of biacetyl, ultraviolet (UV) photolysis of biacetyl, and pulse radiolysis of acetaldehyde, each as a clean source of CH3C(O)OO•. Then, using competition kinetics and selection of suitable probe and competitor compounds, the rate constants between CH3C(O)OO• and compounds of diverse structures were determined. The three experimental approaches complemented in reaction time scale and ease of operation, and provided cross-validation of the rate constants. Moreover, the formation of CH3C(O)OO• was verified by spin-trapped electron paramagnetic resonance, and potential influence of other reactive species in the systems was assessed. Overall, CH3C(O)OO• displays distinctively high reactivity and selectivity, reacting especially favorably with naphthyl and diene compounds (k ∼ 107-108 M-1 s-1) but sluggishly with N- and S-containing groups. Significantly, we demonstrated that incorporating acetylperoxyl radical-oxidation reactions significantly improved the accuracy in modeling the degradation of environmental micropollutants by UV/PAA treatment. This study is among the most comprehensive investigation for peroxyl radical reactivity to date, and establishes a robust methodology for investigating organic radical chemistry. The determined rate constants strengthen kinetic databases and improve modeling accuracy for natural and engineered systems.

Stable Electron Spin Pan on Aromatic Oxalic Acid Radical

Comprehensive SummaryThe stability of organic radicals in ambient condition is important for their practical application. During the development of organic radical chemistry, the electron‐withdrawing and steric hindrance groups are usually introduced to improve the stability of radicals via reducing the reactivity of radicals with oxygen in air. Herein, the electron‐withdrawing carbonyl groups are introduced to construct a planar aromatic oxalic acid radical (IDF‐O8) with two‐dimensional electron spin pan structure. Interestingly, IDF‐O8 exhibited a low optical bandgap of 0.91 eV in film, however, the multiple quinone resonance structures between electron‐withdrawing ketone and phenol radicals contribute to the high stability of open‐shell radical IDF‐O8 without protection of large steric hindrance groups. Under the irradiation of 808 nm (1.2 W·cm–2), IDF‐O8 reaches 147 °C in powder state. This work provides an efficient synthesis route for the open‐shell electron spin pan system, which is different from the famous fullerene, carbon nanotube and graphene. The electron spin pan can be extended to spin tube or spin sphere system based on the design strategy of aromatic inorganic acid radicals in future.

Chain Transfer to Solvent and Monomer in Early Transition Metal Catalyzed Olefin Polymerization: Mechanisms and Implications for Catalysis

Even after several decades of intense research, mechanistic studies of olefin polymerization by early transition metal catalysts continue to reveal unexpected elementary reaction steps. In this mini-review, the recent discovery of two unprecedented chain termination processes is summarized: chain transfer to solvent (CTS) and chain transfer to monomer (CTM), leading to benzyl/tolyl and allyl type chain ends, respectively. Although similar transfer reactions are well-known in radical polymerization, only very recently they have been observed also in olefin insertion polymerization catalysis. In the latter context, these processes were first identified in Ti-catalyzed propene and ethene polymerization; more recently, CTS was also reported in Sc-catalyzed styrene polymerization. In the Ti case, these processes represent a unique combination of insertion polymerization, organic radical chemistry and reactivity of a M(IV)/M(III) redox couple. In the Sc case, CTS occurs via a σ-bond metathesis reactivity, and it is associated with a significant boost of catalytic activity and/or with tuning of polystyrene molecular weight and tacticity. The mechanistic studies that led to the understanding of these chain transfer reactions are summarized, highlighting their relevance in olefin polymerization catalysis and beyond.

Enhancing near-infrared photoluminescence from single-walled carbon nanotubes by defect-engineering using benzoyl peroxide

Single-walled carbon nanotubes (SWCNTs) have been modified with ester groups using typical organic radical chemistry. Consequently, traps for mobile excitons have been created, which enhanced the optical properties of the material. The proposed methodology combines the benefits of mainstream approaches to create luminescent defects in SWCNTs while it simultaneously avoids their limitations. A step change was achieved when the aqueous medium was abandoned. The selection of an appropriate organic solvent enabled much more facile modification of SWCNTs. The presented technique is quick and versatile as it can engage numerous reactants to tune the light emission capabilities of SWCNTs. Importantly, it can also utilize SWCNTs sorted by chirality using conjugated polymers to enhance their light emission capabilities. Such differentiation is conducted in organic solvents, so monochiral SWCNT can be directly functionalized using the demonstrated concept in the same medium without the need to redisperse the material in water.

Quantum chemical calculations for over 200,000 organic radical species and 40,000 associated closed-shell molecules

The stabilities of radicals play a central role in determining the thermodynamics and kinetics of many reactions in organic chemistry. In this data descriptor, we provide consistent and validated quantum chemical calculations for over 200,000 organic radical species and 40,000 associated closed-shell molecules containing C, H, N and O atoms. These data consist of optimized 3D geometries, enthalpies, Gibbs free energy, vibrational frequencies, Mulliken charges and spin densities calculated at the M06-2X/def2-TZVP level of theory, which was previously found to have a favorable trade-off between experimental accuracy and computational efficiency. We expect this data to be useful in the further development of machine learning techniques to predict reaction pathways, bond strengths, and other phenomena closely related to organic radical chemistry.

Happy Birthday to Bernd Giese

Happy Birthday to Bernd Giese

From Curiosity to Reliable Methodology – Organic Free Radical Chemistry

From Curiosity to Reliable Methodology – Organic Free Radical Chemistry

Special issue on organic free radical chemistry

Special issue on organic free radical chemistry

Organic peroxy radical chemistry in oxidation flow reactors and environmental chambers and their atmospheric relevance

Abstract. Oxidation flow reactors (OFRs) are a promising complement to environmental chambers for investigating atmospheric oxidation processes and secondary aerosol formation. However, questions have been raised about how representative the chemistry within OFRs is of that in the troposphere. We investigate the fates of organic peroxy radicals (RO2), which play a central role in atmospheric organic chemistry, in OFRs and environmental chambers by chemical kinetic modeling and compare to a variety of ambient conditions to help define a range of atmospherically relevant OFR operating conditions. For most types of RO2, their bimolecular fates in OFRs are mainly RO2+HO2 and RO2+NO, similar to chambers and atmospheric studies. For substituted primary RO2 and acyl RO2, RO2+RO2 can make a significant contribution to the fate of RO2 in OFRs, chambers and the atmosphere, but RO2+RO2 in OFRs is in general somewhat less important than in the atmosphere. At high NO, RO2+NO dominates RO2 fate in OFRs, as in the atmosphere. At a high UV lamp setting in OFRs, RO2+OH can be a major RO2 fate and RO2 isomerization can be negligible for common multifunctional RO2, both of which deviate from common atmospheric conditions. In the OFR254 operation mode (for which OH is generated only from the photolysis of added O3), we cannot identify any conditions that can simultaneously avoid significant organic photolysis at 254 nm and lead to RO2 lifetimes long enough (∼ 10 s) to allow atmospherically relevant RO2 isomerization. In the OFR185 mode (for which OH is generated from reactions initiated by 185 nm photons), high relative humidity, low UV intensity and low precursor concentrations are recommended for the atmospherically relevant gas-phase chemistry of both stable species and RO2. These conditions ensure minor or negligible RO2+OH and a relative importance of RO2 isomerization in RO2 fate in OFRs within ∼×2 of that in the atmosphere. Under these conditions, the photochemical age within OFR185 systems can reach a few equivalent days at most, encompassing the typical ages for maximum secondary organic aerosol (SOA) production. A small increase in OFR temperature may allow the relative importance of RO2 isomerization to approach the ambient values. To study the heterogeneous oxidation of SOA formed under atmospherically relevant OFR conditions, a different UV source with higher intensity is needed after the SOA formation stage, which can be done with another reactor in series. Finally, we recommend evaluating the atmospheric relevance of RO2 chemistry by always reporting measured and/or estimated OH, HO2, NO, NO2 and OH reactivity (or at least precursor composition and concentration) in all chamber and flow reactor experiments. An easy-to-use RO2 fate estimator program is included with this paper to facilitate the investigation of this topic in future studies.

Semi-solid and solid frustrated Lewis pair catalysts.

Recently discovered homogeneous frustrated Lewis pairs (FLPs) have attracted much attention for metal-free catalysis due to their promising potential for the activation of small molecules (e.g., H2, CO, CO2, NOx and many others). Hence, a wide range of these homogeneous FLPs have been extensively explored for many advanced organic syntheses, radical chemistry and polymerizations. In particular, these FLPs are efficiently utilized for the hydrogenation of various unsaturated substrates (e.g., olefins, alkynes, esters and ketones). Inspired by the substantial progress in these homogeneous catalytic systems, heterogeneous FLP catalysts, including semi-solid and all-solid catalysts, have also emerged as an exciting and evolving field. In this review, we highlight the recent advances made in heterogeneous FLP-like catalysts and the strategies to construct tailorable interfacial FLP-like active sites on semi-solid and all-solid FLP catalysts. Challenges and outlook for the further development of these catalysts in synthetic chemistry will be discussed.

ChemInform Abstract: Combination of Organotrifluoroborates with Photoredox Catalysis Marking a New Phase in Organic Radical Chemistry

AbstractReview: [18 refs.

Combination of organotrifluoroborates with photoredox catalysis marking a new phase in organic radical chemistry.

Combination of organotrifluoroborates and visible-light-driven photoredox catalysis, both of which have attracted the attention of synthetic chemists, marks a new phase in the field of organic radical chemistry. We have developed photoredox-catalyzed radical reactions with organotrifluoroborates, which turn out to serve not only as a source of organic radicals but also as radical acceptors. The first part of this Perspective deals with the generation of organic radicals from organotrifluoroborates, and the latter part describes addition of the CF3 radical to alkenyltrifluoroborates. The good chemistry between organoborates and photoredox catalysis and its future will be discussed.

Biosynthesis of the [FeFe] Hydrogenase H Cluster: A Central Role for the Radical SAM Enzyme HydG.

Hydrogenase enzymes catalyze the rapid and reversible interconversion of H2 with protons and electrons. The active site of the [FeFe] hydrogenase is the H cluster, which consists of a [4Fe-4S]H subcluster linked to an organometallic [2Fe]H subcluster. Understanding the biosynthesis and catalytic mechanism of this structurally unusual active site will aid in the development of synthetic and biological hydrogenase catalysts for applications in solar fuel generation. The [2Fe]H subcluster is synthesized and inserted by three maturase enzymes-HydE, HydF, and HydG-in a complex process that involves inorganic, organometallic, and organic radical chemistry. HydG is a member of the radical S-adenosyl-l-methionine (SAM) family of enzymes and is thought to play a prominent role in [2Fe]H subcluster biosynthesis by converting inorganic Fe(2+), l-cysteine (Cys), and l-tyrosine (Tyr) into an organometallic [(Cys)Fe(CO)2(CN)](-) intermediate that is eventually incorporated into the [2Fe]H subcluster. In this Forum Article, the mechanism of [2Fe]H subcluster biosynthesis is discussed with a focus on how this key [(Cys)Fe(CO)2(CN)](-) species is formed. Particular attention is given to the initial metallocluster composition of HydG, the modes of substrate binding (Fe(2+), Cys, Tyr, and SAM), the mechanism of SAM-mediated Tyr cleavage to CO and CN(-), and the identification of the final organometallic products of the reaction.

ChemInform Abstract: Reaction Mechanisms: Radical and Radical Ion Reactions

AbstractReview: 66 refs.

ChemInform Abstract: The Renaissance of Organic Radical Chemistry — Deja Vu All over Again

AbstractReview: 33 refs.

The renaissance of organic radical chemistry – deja vu all over again

The renaissance of organic radical chemistry – deja vu all over again

Reaction mechanisms: radical and radical ion reactions

Radical reactions endear themselves to organic chemists due to their ability to mediate otherwise difficult transformations (e.g. functionalization of unactivated hydrocarbons), and the associated challenges with predicting and/or controlling their reactivity. This manuscript provides an overview of the most significant developments in organic radical chemistry in 2012, with particular attention paid to the mechanistic attributes identified in these contributions. It is loosely structured to provide the broadest possible coverage, and is organized in sections highlighting (1) radical reactions that are either initiated or mediated by transition metals, (2) reactions not involving any transition metals, and (3) reactions of biological molecules or models thereof that provide insight on the roles of organic radicals in biological processes.

Guidelines for radical reactions: some thirty years on

In this viewpoint article we reflect on the state of play of organic free radical chemistry before the contributions of Beckwith et al. to our understanding of the factors that control intramolecular homolytic addition chemistry, and the rapid rise in the use of this chemistry once the impact of the "guidelines for radical reactions" became fully appreciated.

Laboratory studies of organic peroxy radical chemistry: an overview with emphasis on recent issues of atmospheric significance

Organic peroxy radicals (often abbreviated RO(2)) play a central role in the chemistry of the Earth's lower atmosphere. Formed in the atmospheric oxidation of essentially every organic species emitted, their chemistry is part of the radical cycles that control the oxidative capacity of the atmosphere and lead to the formation of ozone, organic nitrates, organic acids, particulate matter and other so-called secondary pollutants. In this review, laboratory studies of this peroxy radical chemistry are detailed, as they pertain to the chemistry of the atmosphere. First, a brief discussion of methods used to detect the peroxy radicals in the laboratory is presented. Then, the basic reaction pathways - involving RO(2) unimolecular reactions and bimolecular reactions with atmospheric constituents such as NO, NO(2), NO(3), O(3), halogen oxides, HO(2), and other RO(2) species - are discussed. For each of these reaction pathways, basic reaction rates are presented, along with trends in reactivity with radical structure. Focus is placed on recent advances in detection methods and on recent advances in our understanding of radical cycling processes, particularly pertaining to the complex chemistry associated with the atmospheric oxidation of biogenic hydrocarbons.