The Carbene Chemistry of N-Sulfonyl Hydrazones: The Past, Present, and Future

N-Sulfonyl hydrazones have been extensively used as operationally safe carbene precursors in modern organic synthesis due to their ready availability, facile functionalization, and environmental benignity. Over the past two decades, there has been tremendous progress in the carbene chemistry of N-sulfonyl hydrazones in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Many carbene transfer reactions of N-sulfonyl hydrazones are unique and cannot be achieved by any alternative methods. The discovery of novel N-sulfonyl hydrazones and the development of highly enantioselective new reactions and skeletal editing reactions represent the notable recent achievements in the carbene chemistry of N-sulfonyl hydrazones. This review describes the overall progress made in the carbene chemistry of N-sulfonyl hydrazones, organized based on reaction types, spotlighting the current state-of-the-art and remaining challenges to be addressed in the future. Special emphasis is devoted to identifying, describing, and comparing the scope and limitations of current methodologies, key mechanistic scenarios, and potential applications in the synthesis of complex molecules.

Synthesis of substituted benzylboronates by light promoted homologation of boronic acids with N-sulfonylhydrazones

The synthesis of benzylboronates by photochemical homologation of boronic acids with N-tosylhydrazones under basic conditions is described. The reaction involves the photolysis of the N-tosylhydrazone salt to give a diazoalkane followed by the geminal carboborylation of the diazoalkane. Under the mild reaction conditions, the protodeboronation of the unstable benzylboronic acid is circumvented and the pinacolboronates can be isolated after reaction of the benzylboronic acid with pinacol. The metholodogy has been applied to the reactions of alkylboronic acids with N-tosylhydrazones of aromatic aldehydes and ketones, and to the reactions of arylboronic acids with N-tosylhydrazones of aliphatic ketones. Moreover, the employment of the DBU/DIPEA bases combination allows for homogeneous reactions which have been adapted to photochemical continuous flow conditions. Additionally, the synthetic versatility of boronates enables their further transformation via Csp3-C or Csp3-X bond forming reactions converting this methodology into a novel method for the geminal difunctionalization of carbonyls via N-tosylhydrazones.

Inductive heating and flow chemistry - a perfect synergy of emerging enabling technologies

Inductive heating has developed into a powerful and rapid indirect heating technique used in various fields of chemistry, but also in medicine. Traditionally, inductive heating is used in industry, e.g., for heating large metallic objects including bending, bonding, and welding pipes. In addition, inductive heating has emerged as a partner for flow chemistry, both of which are enabling technologies for organic synthesis. This report reviews the combination of flow chemistry and inductive heating in industrial settings as well as academic research and demonstrates that the two technologies ideally complement each other.

Coupling Reactions of Anhydro-Aldose Tosylhydrazones with Boronic Acids

A catalyst-free coupling reaction between O-peracetylated, O-perbenzoylated, O-permethylated, and O-permethoxymethylated 2,6-anhydro-aldose tosylhydrazones (C-(β-d-glycopyranosyl)formaldehyde tosylhydrazones) and aromatic boronic acids is reported. The base-promoted reaction is operationally simple and exhibits a broad substrate scope. The main products in most of the transformations were open-chain 1-C-aryl-hept-1-enitol type compounds while the expected β-d-glycopyranosylmethyl arenes (benzyl C-glycosides) were formed in subordinate yields only. A mechanistic rationale is provided to explain how a complex substrate may change the well-established course of the reaction.

Transition-Metal-Free Valorization of Biomass-derived Levulinic Acid Derivatives: Synthesis of Curcumene and Xanthorrhizol

Levulinic acid (LA) is acknowledged one of the most promising biomass-derived platform molecules and can be transformed into various value-added chemicals. Here, we report a new reaction process for the valorization of LA derivatives under transition-metal-free condition. The protocol combined with the conversion of the levulinate to tosylhydrazone and base promoted arylation, acylation, and etherification cross-coupling. Moreover, our method was applied to synthesize three biologically active molecules, rac-curcumene, rac-xanthorrhizol and rac-4,7-dimethyl-l-tetralone. This reaction discloses a new avenue for the high-value utilization of platform molecules.

Metal-Free Denitrogenative C-C Couplings of Pyridotriazoles with Boronic Acids To Afford α-Secondary and α-Tertiary Pyridines

Pyridotriazoles are utilized as robust building blocks to access α-secondary and α-tertiary pyridines via the development of a simple yet practically useful metal-free denitrogenative C-C cross-coupling with boronic acids. The reaction shows a high level of functional tolerance, broad substrate scope, and facile scalability. The synthetic potential of the method is demonstrated by the strurctural modification of a bioactive molecule and concise synthesis of pheniramine analogs.

A Versatile Route to Unstable Diazo Compounds via Oxadiazolines and their Use in Aryl-Alkyl Cross-Coupling Reactions

Coupling of readily available boronic acids and diazo compounds has emerged recently as a powerful metal-free carbon-carbon bond forming method. However, the difficulty in forming the unstable diazo compound partner in a mild fashion has hitherto limited their general use and the scope of the transformation. Here, we report the application of oxadiazolines as precursors for the generation of an unstable family of diazo compounds using flow UV photolysis and their first use in divergent protodeboronative and oxidative C(sp2 )-C(sp3 ) cross-coupling processes, with excellent functional-group tolerance.

Taming hazardous chemistry by continuous flow technology

Over the last two decades, flow technologies have become increasingly popular in the field of organic chemistry, offering solutions for engineering and/or chemical problems. Flow reactors enhance the mass and heat transfer, resulting in rapid reaction mixing, and enable a precise control over the reaction parameters, increasing the overall process selectivity, efficiency and safety. These features allow chemists to tackle unexploited challenges in their work, with the ultimate objective making chemistry more accessible for laboratory and industrial applications, avoiding the need to store and handle toxic, reactive and explosive reagents. This review covers some of the latest and most relevant developments in the field of continuous flow chemistry with the focus on hazardous reactions.

Taming tosyl azide: the development of a scalable continuous diazo transfer process

In situ generation and use of tosyl azide in flow enables enhanced safety and ready scale-up in diazo transfer processes.

Machine-Assisted Organic Synthesis

In this Review we describe how the advent of machines is impacting on organic synthesis programs, with particular emphasis on the practical issues associated with the design of chemical reactors. In the rapidly changing, multivariant environment of the research laboratory, equipment needs to be modular to accommodate high and low temperatures and pressures, enzymes, multiphase systems, slurries, gases, and organometallic compounds. Additional technologies have been developed to facilitate more specialized reaction techniques such as electrochemical and photochemical methods. All of these areas create both opportunities and challenges during adoption as enabling technologies.

Diazo compounds in continuous-flow technology

Diazo compounds are very versatile reagents in organic chemistry and meet the challenge of selective assembly of structurally complex molecules. Their leaving group is dinitrogen; therefore, they are very clean and atom-efficient reagents. However, diazo compounds are potentially explosive and extremely difficult to handle on an industrial scale. In this review, it is discussed how continuous flow technology can help to make these powerful reagents accessible on large scale. Microstructured devices can improve heat transfer greatly and help with the handling of dangerous reagents safely. The in situ formation and subsequent consumption of diazo compounds are discussed along with advances in handling diazomethane and ethyl diazoacetate. The potential large-scale applications of a given methodology is emphasized.

Flow chemistry as a discovery tool to access sp2-sp3 cross-coupling reactions via diazo compounds

The room temperature sp²–sp³ cross-coupling of flow-generated diazo compounds with boronic acids is reported.

The work takes advantage of an important feature of flow chemistry, whereby the generation of a transient species (or reactive intermediate) can be followed by a transfer step into another chemical environment, before the intermediate is reacted with a coupling partner. This concept is successfully applied to achieve a room temperature sp²–sp³ cross coupling of boronic acids with diazo compounds, these latter species being generated from hydrazones under flow conditions using MnO₂ as the oxidant.

A three step continuous flow synthesis of the biaryl unit of the HIV protease inhibitor Atazanavir

The development of multistep continuous flow reactions for the synthesis of important intermediates for the pharmaceutical industry is still a significant challenge. In the present contribution the biaryl-hydrazine unit of Atazanavir, an important HIV protease inhibitor, was prepared in a three-step continuous flow sequence in 74% overall yield. The synthesis involved Pd-catalyzed Suzuki–Miyaura cross-coupling, followed by hydrazone formation and a subsequent hydrogenation step, and additionally incorporates a liquid–liquid extraction step.

Transition-metal-free C–C bond forming reactions of aryl, alkenyl and alkynylboronic acids and their derivatives

Transition-metal-free C–C bond forming reactions of boronic acids are new emerging tools in organic synthesis which complement metal-based procedures.