Visible Light Photoredox-Catalyzed Decarboxylative Alkylation of 3-Aryl-Oxetanes and Azetidines via Benzylic Tertiary Radicals and Implications of Benzylic Radical Stability.

Four-membered heterocycles offer exciting potential as small polar motifs in medicinal chemistry but require further methods for incorporation. Photoredox catalysis is a powerful method for the mild generation of alkyl radicals for C-C bond formation. The effect of ring strain on radical reactivity is not well understood, with no studies that address this question systematically. Examples of reactions that involve benzylic radicals are rare, and their reactivity is challenging to harness. This work develops a radical functionalization of benzylic oxetanes and azetidines using visible light photoredox catalysis to prepare 3-aryl-3-alkyl substituted derivatives and assesses the influence of ring strain and heterosubstitution on the reactivity of small-ring radicals. 3-Aryl-3-carboxylic acid oxetanes and azetidines are suitable precursors to tertiary benzylic oxetane/azetidine radicals which undergo conjugate addition into activated alkenes. We compare the reactivity of oxetane radicals to other benzylic systems. Computational studies indicate that Giese additions of unstrained benzylic radicals into acrylates are reversible and result in low yields and radical dimerization. Benzylic radicals as part of a strained ring, however, are less stable and more π-delocalized, decreasing dimer and increasing Giese product formation. Oxetanes show high product yields due to ring strain and Bent's rule rendering the Giese addition irreversible.

Synthesis and anti-microbial activity of a new series of bis(diphosphine) rhenium(V) dioxo complexes.

Rhenium-based metallodrugs have recently been highlighted as promising candidates for new antibiotics to combat multi-drug resistant (MDR) pathogens. A new class of rhenium(V) dioxo complexes were prepared from readily accessible diphosphine ligands, and have been shown to possess potent activity against Staphylococcus aureus (S. aureus) and Candida albicans (C. albicans) alongside low human cell toxicity.

N-Centered Tripodal Phosphine Re(V) and Tc(V) Oxo Complexes: Revisiting a [3 + 2] Mixed-Ligand Approach.

N-Triphos derivatives (NP3R, R = alkyl, aryl) and asymmetric variants (NP2RXR', R' = alkyl, aryl, X = OH, NR2, NRR') are an underexplored class of tuneable, tripodal ligands in relation to the coordination chemistry of Re and Tc for biomedical applications. Mixed-ligand approaches are a flexible synthetic route to obtain Tc complexes of differing core structures and physicochemical properties. Reaction of the NP3Ph ligand with the Re(V) oxo precursor [ReOCl3(PPh3)2] generated the bidentate complex [ReOCl3(κ2-NP2PhOHAr)], which possesses an unusual AA'BB'XX' spin system with a characteristic second-order NMR lineshape that is sensitive to the bi- or tridentate nature of the coordinating diphosphine unit. The use of the asymmetric NP2PhOHAr ligand resulted in the formation of both bidentate and tridentate products depending on the presence of base. The tridentate Re(V) complex [ReOCl2(κ3-NP2PhOAr)] has provided the basis of a new reactive "metal-fragment" for further functionalization in [3 + 2] mixed-ligand complexes. The synthesis of [3 + 2] complexes with catechol-based π-donors could also be achieved under one-pot, single-step conditions from Re(V) oxo precursors. Analogous complexes can also be synthesized from suitable 99Tc(V) precursors, and these complexes have been shown to exhibit highly similar structural properties through spectroscopic and chromatographic analysis. However, a tendency for the {MVO}3+ core to undergo hydrolysis to the {MVO2}+ core has been observed both in the case of M = Re and markedly for M = 99Tc complexes. It is likely that controlling this pathway will be critical to the generation of further stable Tc(V) derivatives.

Synthesis of carbon-11 radiolabelled transition metal complexes using 11C-dithiocarbamates.

A novel radiolabelling method exploiting 11C-dithiocarbamate ligands has been used to generate 11C-labelled Au(I), Au(III), Pd(II) and Pt(II) complexes in high radiochemical yields (71-99%). Labelled complexes were prepared in a rapid one-pot procedure via the substitution reaction of 11C-dithiocarbamate ligands with appropriate transition metal chloride precursors.

On the Use of Differential Scanning Calorimetry for Thermal Hazard Assessment of New Chemistry: Avoiding Explosive Mistakes.

Differential scanning calorimetry (DSC) is increasingly used as evidence to support a favourable safety profile of novel chemistry, or to highlight the need for caution. DSC enables preliminary assessment of the thermal hazards of a potentially energetic compound. However, unlike other standard characterisation methods, which have well defined formats for reporting data, the current reporting of DSC results for thermal hazard assessment has shown concerning trends. Around half of all results in 2019 did not include experimental details required to replicate the procedure. Furthermore, analysis for thermal hazard assessment is often only conducted in unsealed crucibles, which could lead to misleading results and dangerously incorrect conclusions. We highlight the specific issues with DSC analysis of hazardous compounds currently in the organic chemistry literature and provide simple "best practice" guidelines which will give chemists confidence in reported DSC results and the conclusions drawn from them.

Thermal Stability and Explosive Hazard Assessment of Diazo Compounds and Diazo Transfer Reagents.

Despite their wide use in academia as metal-carbene precursors, diazo compounds are often avoided in industry owing to concerns over their instability, exothermic decomposition, and potential explosive behavior. The stability of sulfonyl azides and other diazo transfer reagents is relatively well understood, but there is little reliable data available for diazo compounds. This work first collates available sensitivity and thermal analysis data for diazo transfer reagents and diazo compounds to act as an accessible reference resource. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and accelerating rate calorimetry (ARC) data for the model donor/acceptor diazo compound ethyl (phenyl)diazoacetate are presented. We also present a rigorous DSC dataset with 43 other diazo compounds, enabling direct comparison to other energetic materials to provide a clear reference work to the academic and industrial chemistry communities. Interestingly, there is a wide range of onset temperatures (T onset) for this series of compounds, which varied between 75 and 160 °C. The thermal stability variation depends on the electronic effect of substituents and the amount of charge delocalization. A statistical model is demonstrated to predict the thermal stability of differently substituted phenyl diazoacetates. A maximum recommended process temperature (T D24) to avoid decomposition is estimated for selected diazo compounds. The average enthalpy of decomposition (ΔH D) for diazo compounds without other energetic functional groups is -102 kJ mol-1. Several diazo transfer reagents are analyzed using the same DSC protocol and found to have higher thermal stability, which is in general agreement with the reported values. For sulfonyl azide reagents, an average ΔH D of -201 kJ mol-1 is observed. High-quality thermal data from ARC experiments shows the initiation of decomposition for ethyl (phenyl)diazoacetate to be 60 °C, compared to that of 100 °C for the common diazo transfer reagent p-acetamidobenzenesulfonyl azide (p-ABSA). The Yoshida correlation is applied to DSC data for each diazo compound to provide an indication of both their impact sensitivity (IS) and explosivity. As a neat substance, none of the diazo compounds tested are predicted to be explosive, but many (particularly donor/acceptor diazo compounds) are predicted to be impact-sensitive. It is therefore recommended that manipulation, agitation, and other processing of neat diazo compounds are conducted with due care to avoid impacts, particularly in large quantities. The full dataset is presented to inform chemists of the nature and magnitude of hazards when using diazo compounds and diazo transfer reagents. Given the demonstrated potential for rapid heat generation and gas evolution, adequate temperature control and cautious addition of reagents that begin a reaction are strongly recommended when conducting reactions with diazo compounds.

Cobalt(-i) triphos dinitrogen complexes: activation and silyl-functionalisation of N2.

The cobalt dinitrogen complexes [{(EP3Ph)Co(µ-N2)}2Mg(THF)4], with triphos ligand scaffolds (EP3Ph, E = N or CMe), were prepared via two electron reductions of the Co(i) precursors [CoCl(EP3Ph)]. Both complexes showed high degrees of N2 activation owing to the formation of a rare M-NN-Mg-NN-M bridging-magnesium core. These systems showed further N2 functionalisation reactivity by silylation, forming silyldiazenido complexes [(EP3Ph)Co(NNSiMe3)].

Diazo-Transfer Reagent 2-Azido-4,6-dimethoxy-1,3,5-triazine Displays Highly Exothermic Decomposition Comparable to Tosyl Azide.

2-Azido-4,6-dimethoxy-1,3,5-triazine (ADT) was reported recently as a new "intrinsically safe" diazo-transfer reagent. This assessment was based on differential scanning calorimetry data indicating that ADT exhibits endothermic decomposition. We present DSC data on ADT that show exothermic decomposition with an initiation temperature ( Tinit) of 159 °C and an enthalpy of decomposition (Δ HD) of -1135 J g-1 (-207 kJ mol-1). We conclude that ADT is potentially explosive and must be treated with caution, being of comparable exothermic magnitude to tosyl azide (TsN3). A maximum recommended process temperature for ADT is 55 °C.

Emerging porous materials in confined spaces: from chromatographic applications to flow chemistry.

Searching for porous materials that can be employed as solid stationary phases for chromatographic separations, porous membrane matrixes and solid supports for catalysis has become an active research area. Strategies for embedding emerging porous materials in columnar systems and their subsequent applications (separation and catalysis) have been developed, which benefit from the remarkable progress in the discovery and development of porous materials based on metal-organic coordination or dynamic covalent bonding such as metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), porous organic cages, and porous organic polymers. In this review, porous materials that have been confined within capillary columns as packed, monolithic and open tubular columns are discussed. Progress in chromatographic separation and continuous flow catalytic synthesis is reviewed according to three major strategies. Applications of porous materials in membrane-separation fibre membrane systems and microfluidic devices with various functions are also highlighted.

Antibody Fragment and Affibody ImmunoPET Imaging Agents: Radiolabelling Strategies and Applications.

Antibodies have long been recognised as potent vectors for carrying diagnostic medical radionuclides, contrast agents and optical probes to diseased tissue for imaging. The area of ImmunoPET combines the use of positron emission tomography (PET) imaging with antibodies to improve the diagnosis, staging and monitoring of diseases. Recent developments in antibody engineering and PET radiochemistry have led to a new wave of experimental ImmunoPET imaging agents that are based on a range of antibody fragments and affibodies. In contrast to full antibodies, engineered affibody proteins and antibody fragments such as minibodies, diabodies, single-chain variable region fragments (scFvs), and nanobodies are much smaller but retain the essential specificities and affinities of full antibodies in addition to more desirable pharmacokinetics for imaging. Herein, recent key developments in the PET radiolabelling strategies of antibody fragments and related affibody molecules are highlighted, along with the main PET imaging applications of overexpressed antigen-associated tumours and immune cells.

Ammonium [11C]thiocyanate: revised preparation and reactivity studies of a versatile nucleophile for carbon-11 radiolabelling.

Herein we report the preparation of ammonium [11C]thiocyanate via the reaction of [11C]CS2 with ammonia. The [11C]SCN- ion is demonstrated as a potent nucleophile that can be used to readily generate a range of 11C-labelled thiocyanate molecules in high conversions. Furthermore, novel 11C-labelled thiazolone molecules can be easily prepared from the intermediate α-thiocyanatophenones via an acid mediated cyclisation reaction.

Synthesis and reactivity of an N-triphos Mo(0) dinitrogen complex.

The preparation and reactivity of a novel molybdenum dinitrogen complex supported by a nitrogen-centred tripodal phosphine ligand (N-triphos, N(CH2PPh2)3, NP3Ph) are reported. Reaction of N-triphos with [MoX3(THF)3] (X = Cl, Br, I) gave the Mo(iii) complex [MoX3(κ2-NP3Ph)(THF)] (1), where bidentate N-triphos coordination was observed. Reduction of this complex in the presence of dppm (bis(diphenylphosphino)methane) gave the dinitrogen complex [Mo(N2)(dppm)(κ3-NP3Ph)] (2), which exhibits moderate dinitrogen activation. An additional hydride complex, [Mo(H)2(dppm)(κ3-NP3Ph)] (4), was produced either as a minor side product during the reduction step, or as a major product by direct hydrogenation of the dinitrogen complex 2. The reactivity of the dinitrogen complex 2 with a range of Lewis acids was also investigated. At low temperatures, protic or borane Lewis acids (H+, BBr3 and tris(pentafluorophenyl)borane (BCF)) were found to coordinate to the apical nitrogen atom of the N-triphos ligand, with no conclusive evidence of any functionalisation of the dinitrogen ligand. Alkali metal Lewis acid addition to 2 resulted in the unexpected rearrangement of the N-triphos ligand to form [Mo(dppm)(PMePh2)(PCP)][B(C6F5)4] (7), where PCP, [Ph2PCNHCH2PPh2] is the carbenic ligand formed upon rearrangement from the reaction of 2 with M[B(C6F5)4] (M = Li, Na or K). Single crystal X-ray diffraction of complexes 1, 2, 4 and 7 provided structural confirmation of the N-triphos molybdenum complexes described.

Facile Preparation of Drug-Loaded Tristearin Encapsulated Superparamagnetic Iron Oxide Nanoparticles Using Coaxial Electrospray Processing.

Naturally occurring polymers are promising biocompatible materials that have many applications for emerging therapies, drug delivery systems, and diagnostic agents. The handling and processing of such materials still constitutes a major challenge, which can limit the full exploitation of their properties. This study explores an ambient environment processing technique: coaxial electrospray (CO-ES) to encapsulate genistein (an isoflavonoid and model drug), superparamagnetic iron oxide nanoparticles (SPIONs, 10-15 nm), and a fluorophore (BODIPY) into a layered (triglyceride tristearin shell) particulate system, with a view to constructing a theranostic agent. Mode mapping of CO-ES led to an optimized atomization engineering window for stable jetting, leading to encapsulation of SPIONs within particles of diameter 0.65-1.2 µm and drug encapsulation efficiencies of around 92%. Electron microscopy was used to image the encapsulated SPIONs and confirm core-shell triglyceride encapsulation in addition to further physicochemical characterization (AFM, FTIR, DSC, and TGA). Cell viability assays (MTT, HeLa cells) were used to determine optimal SPION loaded particles (∼1 mg/mL), while in vitro release profile experiments (PBS, pH = 7.4) demonstrate a triphasic release profile. Further cell studies confirmed cell uptake and internalization at selected time points (t = 1, 2, and 4 h). The results suggest potential for using the CO-ES technique as an efficient way to encapsulate SPIONs together with sensitive drugs for the development of multimodal particles that have potential application for combined imaging and therapy.

Homogeneous Catalyzed Reactions of Levulinic Acid: To γ-Valerolactone and Beyond.

Platform chemicals derived from lignocellulosic plant biomass are viewed as a sustainable replacement for crude oil-based feedstocks. Levulinic acid (LA) is one such biomass-derived chemical that has been widely studied for further catalytic transformation to γ-valerolactone (GVL), an important 'green' fuel additive, solvent, and fine chemical intermediate. Although the transformation of LA to GVL can be achieved using heterogeneous catalysis, homogeneous catalytic systems that operate under milder reactions, give higher selectivities and can be recycled continuously are attracting considerable attention. A range of new homogeneous catalysts have now been demonstrated to efficiently convert LA to GVL and to transform LA directly to other value-added chemicals such as 1,4-pentanediol (1,4-PDO) and 2-methyltetrahydrofuran (2-MTHF). This Minireview covers recent advances in the area of homogeneous catalysis for the conversion of levulinic acid and levulinic ester derivatives to GVL and chemicals beyond GVL.

Insight into the stereoelectronic parameters of N-triphos ligands via coordination to tungsten(0).

A series of new N-triphos tungsten complexes have been synthesised and structurally characterised. The coordination behaviour of a range of N-triphos (N(CH2PR2)3, NP3(R)) ligands, and a mixed-arm diphosphine-pyridyl (PPN(Cyh)) ligand were explored. The steric and electronic parameters of five N-triphos ligands: NP3(Ph), NP3(iPr), NP3(Cyp), NP3(Cyh) and NP3(PhF2), and the carbon-centred triphos ligand, CH3C(CH2PPh2)3 (MeCP3(Ph)), were established. Steric parameters were evaluated by analysing the cone angles calculated from X-ray crystal structures, whilst the electron-donating ability of the ligands was determined from (31)P-(77)Se NMR coupling constants of selenium derivatives and the IR carbonyl stretching frequencies across a series of tungsten-carbonyl complexes. In general, electron-rich phosphines formed bidentate complexes while less electron-rich ligands coordinated in a tridentate mode, regardless of steric bulk. An indirect interaction between the apical nitrogen of the ligand and metal centre is implicated for tridentate complexes and is supported through DFT calculations and analysis of N-protonated complexes. Complexes 1, 3, 4, 6-8 and 10 were characterised by single-crystal X-ray crystallography.

Carbon-11 radiolabelling of organosulfur compounds: (11) C synthesis of the progesterone receptor agonist tanaproget.

Herein a new (11) C radiolabelling strategy for the fast and efficient synthesis of thioureas and related derivatives using the novel synthon, (11) CS2 , is reported. This approach has enabled the facile labelling of a potent progesterone receptor (PR) agonist, [(11) C]Tanaproget, by the intramolecular reaction of the acyclic aminohydroxyl precursor with (11) CS2 , which has potential applications as a positron emission tomography radioligand for cancer imaging.

A catalytic chiral gel microfluidic reactor assembled via dynamic covalent chemistry.

A novel dynamic covalent gel strategy is reported to immobilize an asymmetric catalyst within the channels of a microfluidic flow reactor. A layer of a catalytically active Mn-salen dynamic covalent imine gel matrix was coated onto a functionalized capillary. Mn-salen active moiety was incorporated into dynamic covalent imine gel matrix via the reaction of a chiral Mn-salen dialdehyde unit with a tetraamine linker. The catalytic activity of the capillary reactor has been demonstrated in enantioselective kinetic resolution of secondary alcohols.

A dynamic covalent imine gel as a luminescent sensor.

A novel dynamic covalent gel has been prepared via the imine bond formation reaction of tetraamine building blocks with luminescent tetraphenylethene tetraaldehyde units. The gel shows high hierarchical porosity (SBET = 512 m(2) g(-1)) and remarkable aggregation-induced emission enhancement, which has been demonstrated to be a selective sensor.

Synthesis, characterization, and reactivity of ruthenium hydride complexes of N-centered triphosphine ligands.

The reactivity of the novel tridentate phosphine ligand N(CH2PCyp2)3 (N-triphos(Cyp), 2; Cyp = cyclopentyl) with various ruthenium complexes was investigated and compared that of to the less sterically bulky and less electron donating phenyl derivative N(CH2PPh2)3 (N-triphos(Ph), 1). One of these complexes was subsequently investigated for reactivity toward levulinic acid, a potentially important biorenewable feedstock. Reaction of ligands 1 and 2 with the precursors [Ru(COD)(methylallyl)2] (COD = 1,5-cycloocatadiene) and [RuH2(PPh3)4] gave the tridentate coordination complexes [Ru(tmm){N(CH2PR2)3-κ(3)P}] (R = Ph (3), Cyp (4); tmm = trimethylenemethane) and [RuH2(PPh3){N(CH2PR2)3-κ(3)P}] (R = Ph (5), Cyp (6)), respectively. Ligands 1 and 2 displayed different reactivities with [Ru3(CO)12]. Ligand 1 gave the tridentate dicarbonyl complex [Ru(CO)2{N(CH2PPh2)3-κ(3)P}] (7), while 2 gave the bidentate, tricarbonyl [Ru(CO)3{N(CH2PCyp2)3-κ(2)P}] (8). This was attributed to the greater electron-donating characteristics of 2, requiring further stabilization on coordination to the electron-rich Ru(0) center by more CO ligands. Complex 7 was activated via oxidation using AgOTf and O2, giving the Ru(II) complexes [Ru(CO)2(OTf){N(CH2PPh2)3-κ(3)P}](OTf) (9) and [Ru(CO3)(CO){N(CH2PPh2)3-κ(3)P}] (11), respectively. Hydrogenation of these complexes under hydrogen pressures of 3-15 bar gave the monohydride and dihydride complexes [RuH(CO)2{N(CH2PPh2)3-κ(3)P}] (10) and [RuH2(CO){N(CH2PPh2)3-κ(3)P}] (12), respectively. Complex 12 was found to be unreactive toward levulinic acid (LA) unless activated by reaction with NH4PF6 in acetonitrile, forming [RuH(CO)(MeCN){N(CH2PPh2)3-κ(3)P}](PF6) (13), which reacted cleanly with LA to form [Ru(CO){N(CH2PPh2)3-κ(3)P}{CH3CO(CH2)2CO2H-κ(2)O}](PF6) (14). Complexes 3, 5, 7, 8, 11, and 12 were characterized by single-crystal X-ray crystallography.

Transition metal mediated [(11) C]carbonylation reactions: recent advances and applications.

[(11) C]Carbon monoxide is undoubtedly a highly versatile radiolabelling synthon with many potential applications for the synthesis of positron emission tomography (PET) tracer molecules and functional groups, but why has it not found more applications in the PET radiolabelling arena? Today, (11) CO radiolabelling is still primarily viewed as a niche area; however, there are signs that this is beginning to change as some of the technical and chemistry challenges of producing, handling and reacting (11) CO are overcome. This mini review covers the more recent developments of (11) CO-labelling chemistry and is focused on palladium and rhodium-mediated carbonylation reactions that are growing in importance and finding wider application for carbon-11 PET radiotracer development.