Cyclic Hypervalent Iodine Reagents for Azidation: Safer Reagents and Photoredox-Catalyzed Ring Expansion

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.

Rhenium-catalyzed alkylarylation of alkenes with PhI(O2CR)2via decarboxylation to access indolinones and dihydroquinolinones

Rhenium-catalyzed alkylarylation of alkenes with hypervalent iodine(iii) reagents (HIRs) via decarboxylation to access various 3,3-disubstituted indolinones and trans-3,4-dihydroquinolinones is described.

Design and application of diimine-based copper(i) complexes in photoredox catalysis

Structurally different bis(imino)copper(i) complexes were prepared in a highly modular manner and utilized as copper-based photocatalysts in the ATRA reactions of styrenes and alkyl halides. The new photocatalysts showed good catalytic activity and ensured efficient chemical transformations.

Thermal stability of N-heterocycle-stabilized iodanes - a systematic investigation

The thermal stability of pseudocyclic and cyclic N-heterocycle-stabilized (hydroxy)aryl- and mesityl(aryl)-λ³-iodanes (NHIs) through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) is investigated. Peak decomposition temperatures (T_peak) were observed within a wide range between 120 and 270 °C. Decomposition enthalpies (ΔH_dec) varied from −29.81 to 141.13 kJ/mol. A direct comparison between pseudocyclic and cyclic NHIs revealed high T_peak but also higher ΔH_dec values for the latter ones. NHIs bearing N-heterocycles with a high N/C-ratio such as triazoles show among the lowest T_peak and the highest ΔH_dec values. A comparison of NHIs with known (pseudo)cyclic benziodoxolones is made and we further correlated their thermal stability with reactivity in a model oxygenation.

Selenocyanobenziodoxolone: a practical electrophilic selenocyanation reagent and its application for solid-state synthesis of α-carbonyl selenocyanates

A solvent- and oxidant-free solid state synthetic methodology for the synthesis of α-carbonyl selenocyanates is developed.

Recent Applications of the Zhdankin Reagent in Organic Synthesis

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Methods for direct C(sp2)–H bonds azidation

Direct functionalization of C–H bonds has attracted great attention in recent years from the perspectives of atom and step economy.

Cyclic Hypervalent Iodine Reagents: Enabling Tools for Bond Disconnection via Reactivity Umpolung

The efficient synthesis of organic compounds is an important field of research, which sets the basis for numerous applications in medicine or materials science. Based on the polarity induced by functional groups, logical bond disconnections can be deduced for the elaboration of organic compounds. Nevertheless, this classical approach makes synthesis rigid, as not all bond disconnections are possible. The concept of Umpolung has been therefore introduced: by inverting the normal polarity of functional groups, new disconnections become possible. Among the tools for achieving Umpolung, hypervalent iodine reagents occupy a privileged position. The electrophilicity of the iodine atom and the reactivity of the hypervalent bond allow access to electrophilic synthons starting from nucleophiles. Nevertheless, some classes of hypervalent iodine reagents can be too unstable for many applications, in particular involving metal catalysis. In this context, cyclic hypervalent iodine reagents, especially benziodoxolones (BXs), have been known for a long time to be more stable than their acyclic counterparts, yet their synthetic potential had not been fully exploited. In this Account, we report our efforts since 2008 on the use of BX reagents in the development of new transformations in organic synthesis, which showed for the first time their versatility as synthetic tools. Our work started with electrophilic alkynylation, as alkynes are one of the most important functional groups in organic chemistry, but are usually introduced as nucleophiles. We used ethynylbenziodoxolones (EBXs) in the direct alkynylation of nucleophiles, such as keto esters, thiols, or phosphines. The reagents could then be applied to the gold- and palladium-catalyzed alkynylation of C-H bonds on (hetero)arenes, leading to a more efficient alternative to the Sonogashira reaction. More complex reactions were then developed with formations of several bonds in a single transformation. Gold- and platinum-catalyzed cyclization/alkynylation domino processes gave access to new types of alkynylated heterocycles. Multifunctionalization of olefins became possible through intramolecular oxy- and amino-alkynylations. (Enantioselective) copper-catalyzed oxy-alkynylation of diazo compounds led to stereocenters with perfect atom economy. Finally, EBXs were also used for the alkynylation of radicals generated under photoredox conditions. Since 2013, we then extended the use of BX reagents to other transformations. Azidobenziodoxol(on)ess (ABXs) were used in the azidation of keto esters, enol silanes, and styrenes. New more stable derivatives were introduced. Cyanobenziodoxolones (CBXs) enabled the cyanation of stabilized enolates, thiols, and radicals. Finally, new BX reagents were developed for the Umpolung of indoles and pyrroles. They could be used in metal-catalyzed directed C-H functionalizations, as well as in Lewis acid mediated oxidative coupling to give functionalized bi(hetero)arenes. In the past decade, our group and others have shown that BX reagents are not only "structural beauties", but also extremely useful reagents in synthetic chemistry. A toolbox of cyclic hypervalent iodine reagents is now available to achieve Umpolung-based disconnections. We are convinced that the field is still in its infancy, and many new reagents and transformations still remain to be discovered.