Generation of Tosyl Azide in Continuous Flow Using an Azide Resin, and Telescoping with Diazo Transfer and Rhodium Acetate-Catalyzed O-H Insertion

Poly(hydrazinophosphine diazide)s (PHPDs): Hybrid Organic-Inorganic Polymers via Polycondensation between PN Cages and Organic Diazides

Organic polymers generally feature 1-dimensional chains or 2-dimensional rings in their backbones since synthetic challenges limit the availability of 3-dimensional monomers. Inorganic cages are less strained and more accessible, offering an alternative route to explore this parameter space. However, only two families─carboranes and polyhedral oligomeric silsesquioxanes (POSS)─have been well-studied, revealing materials with valuable mechanical and thermal properties. Further exploration of this frontier requires the development of new inorganic cages that are accessible, stable, and polymerizable. Here we report that an easily assembled, bench-stable PN cage, P(NMeNMe)3P, undergoes Staudinger polycondensation with organic diazides to yield robust, solution-processable, and film-forming linear poly(trihydrazino-diphosphine diazide)s─PHPDs─as a new family of hybrid organic-inorganic polymers. Their solubility can be controlled by diazide choice and backbone architecture, which we rationally modify to access alternating or multiblock copolymers. We also show how a tetraphosphorus cage, P4(NMe)6, can be used to cross-link PHPDs. The Tg values for PHPDs are comparable to those of rigid π-conjugated polymers (>150 °C), and, despite a high nitrogen content (up to 32%) and three N-N σ-bonds per repeat unit, they show decomposition temperatures >200 °C with char yields up to 60%. These data support hypotheses of high stability arising from the presence of 3-dimensional backbone units. We further show that PHPDs may be leveraged for halogen-free flame retardancy. Collectively, the results debut new low-carbon polymers with an unusual backbone topology, reveal the design rules for controlling their microstructures and properties, and lay the foundation for future applied studies.

Heterogeneous Fe-N-C Catalyst for Aerobic Dehydrogenation of Hydrazones to Diazo Compounds Used for Carbene Transfer

Organic diazo compounds are versatile reagents in chemical synthesis and would benefit from improved synthetic accessibility, especially for larger scale applications. Here, we report a mild method for the synthesis of diazo compounds from hydrazones using a heterogeneous Fe-N-C catalyst, which has Fe ions dispersed within a graphitic nitrogen-doped carbon support. The reactions proceed readily at room temperature using O2 (1 atm) as the oxidant. Aryl diazoesters, ketones, and amides are accessible, in addition to less stable diaryl diazo compounds. Initial-rate data show that the Fe-N-C catalyst achieves faster rates than a heterogeneous Pt/C catalyst. The oxidative dehydrogenation of hydrazones may be performed in tandem with Rh-catalyzed enantioselective C-H insertion and cyclopropanation of alkenes, without requiring isolation of the diazo intermediate. This sequence is showcased by using a flow reactor for continuous synthesis of diazo compounds.

Will the next generation of chemical plants be in miniaturized flow reactors?

For decades, a production paradigm based on centralized, stepwise, large scale processes has dominated the chemical industry horizon. While effective to meet an ever increasing demand for high value-added chemicals, the so-called macroscopic batch reactors are also associated with inherent weaknesses and threats; some of the most obvious ones were tragically illustrated over the past decades with major industrial disasters and impactful disruptions of advanced chemical supplies. The COVID pandemic has further emphasized that a change in paradigm was necessary to sustain chemical production with an increased safety, reliable supply chains and adaptable productivities. More than a decade of research and technology development has led to alternative and effective chemical processes relying on miniaturised flow reactors (a.k.a. micro and mesofluidic reactors). Such miniaturised reactors bear the potential to solve safety concerns and to improve the reliability of chemical supply chains. Will they initiate a new paradigm for a more localized, safe and reliable chemical production?

Continuous Flow Generation of Acylketene Intermediates via Nitrogen Extrusion

A flow chemistry process for the generation and use of acylketene precursors through extrusion of nitrogen gas is reported. Key to the development of a suitable continuous protocol is the balance of reaction concentration against pressure in the flow reactor. The resulting process enables access to intercepted acylketene scaffolds using volatile amine nucleophiles and has been demonstrated on the gram scale. Thermal gravimetric analysis was used to guide the temperature set point of the reactor coils for a variety of acyl ketene precursors. The simultaneous generation and reaction of two reactive intermediates (both derived from nitrogen extrusion) is demonstrated.