Photochemically Mediated Ring Expansion of Indoles and Pyrroles with Chlorodiazirines: Synthetic Methodology and Thermal Hazard Assessment

Single-atom molecular editing: transformative advances in carbocyclic and heterocyclic frameworks

Single-atom editing has emerged as a transformative strategy in organic synthesis, enabling precise modification of carbocyclic and heterocyclic frameworks by selectively targeting single atoms. These frameworks are crucial backbones of pharmaceuticals, agrochemicals, and advanced materials, making this approach powerful for organic chemists. In drug discovery and natural product synthesis, single-atom editing diversifies molecular scaffolds and tailors molecular properties to enhance pharmacological activity. In heterocyclic synthesis, this approach enables controlled heteroatom substitution, addition or deletion in an unprecedented and highly selective manner compared to traditional methods. Recent advances in transition-metal catalysis, organocatalysis, photoredox catalysis, and heterocycle-to-heterocycle metamorphosis have expanded the versatility of single-atom editing, enabling the synthesis of various carbocyclic and heterocyclic compounds. Principally, this approach has been exploited to design new architectures that are not easily accessible by other methods and to establish major improvements in the synthesis of known scaffolds, providing more efficient and sustainable routes towards large-scale chemical synthesis. This review overviews recent advances, focusing on carbocyclic and heterocyclic frameworks, and is organized by key single-atom editing strategies, such as ring contractions, atom deletions, ring expansions, and atom insertions. The review highlights key transformations like Favorskii and Wolff rearrangements, alongside modern photochemical and transition-metal-catalyzed processes, to provide a broad overview of synthetic applications and inspire further advancements in targeted molecular edits.

Sulfenylcarbene-Mediated Carbon Atom Insertion for the Late-Stage Functionalization of N-Heterocycles

Late-stage functionalization (LSF) is a crucial strategy in drug discovery, allowing the modification of complex molecules, including pharmaceuticals, to enhance chemical diversity in drug libraries. We harness the chemoselectivity of sulfenylcarbenes, which selectively react with alkenes even in the presence of more reactive functional groups such as alcohols, carboxylic acids, and amines. This reactivity allows sulfenylcarbenes to insert a single carbon atom bearing diverse functional groups, transforming pyrroles, indoles, and imidazoles into synthetically challenging pyridines, quinolines, and pyrimidines, respectively. Sulfenylcarbene precursors are easily synthesized in two steps from commercially available reagents. Our metal-free LSF approach employs benchtop-stable sulfenylcarbene precursors and enables late-stage modification of natural products, amino acids, pharmaceuticals, and C-glycosides. Mechanistic studies and density functional theory (DFT) calculations were conducted to investigate the regio- and chemoselectivity outcomes.

Indole-Quinoline Transmutation Enabled by a Formal Rhodium Carbynoid

Skeleton editing is an emerging and powerful tool in organic chemistry because it can simplify synthetic procedures towards complex molecules. Herein, we present an approach for indole-quinoline transmutation through rhodium-catalyzed single-carbon insertion into the C2(sp2)─C3(sp2) bond of an indole with an α-diazotrifluoroethyl sulfonium salt that we developed. This protocol involves a formal trifluoromethyl rhodium carbynoid (CF3C+ = Rh) resembling a trifluoromethyl cationic carbyne (CF3C+:), allowing facile access to an array of quinolines in moderate to high yields. Potential applications in the late-stage skeletal editing of pharmaceutical and natural product derivatives, preparation of adapalene analogs, scaled-up synthesis, and transformations of products are highlighted. Finally, a computational study was conducted to support the envisioned mechanism.

Diversity-Generating Skeletal Editing Transformations

ConspectusSkeletal editing, as a synthetic tool, offers the unique potential to selectively and efficiently modify the core skeleton of a target molecule at a late-stage. The main benefit of such transformations is the rapid exploration of the chemical space around lead compounds without necessitating a de novo synthesis for each new molecule. However, many skeletal editing transformations are inherently restricted to generating a single product from a single starting compound, limiting the potential for diversification, a concept central to expediting structure-activity relationship (SAR) investigations. In this Account, we describe our efforts to develop novel skeletal editing transformations in which a modification to the central motif of a molecule is performed simultaneously with the incorporation of additional functionality that can be easily varied through a judicious choice of the reagents. Specifically, we successfully developed an α-iodonium diazo-based carbynyl radical equivalent reagent that, under photoredox conditions, could facilitate the ring-expansion of indene scaffolds while enabling the insertion of over ten different functionalized carbon atoms into the corresponding naphthalene products. This concept was later extended to the design of an atomic carbon equivalent reagent that could promote mild and selective Ciamician-Dennstedt-type indole ring-expansion reactions, while simultaneously installing an oxime ester handle that could undergo further functionalization. Furthermore, we highlight recent work from our group on multiple-atom insertion reactions, namely, the development of a photocatalyzed De Mayo reaction for the ring-expansion of cyclic ketones and a photocatalyzed dearomative ring-expansion of thiophenes via small-ring insertion. In both of these cases, multiple products can be potentially accessed from a single starting material upon variation of the insertion reagent. The diversity-generating skeletal editing strategy could also be applied to single-atom transmutation, as demonstrated by the development of a nitrogen-to-functionalized carbon atom transmutation reaction to convert pyridine to benzene rings. Here, the desired transformation was achieved via a sequence of pyridine ring-opening, Horner-Wadsworth-Emmons (HWE) olefination, and ring-closure, with a judicious choice of the HWE reagent allowing the installation of a wide variety of versatile functional groups. Finally, an energy transfer-mediated quinoline ring-contraction is discussed, specifically with reference to the ways in which it does and does not fit the criteria of a skeletal editing reaction. Although formal atom deletion transformations are typically restricted to single products from each discrete substrate, this [2 + 2] cycloaddition/rearrangement cascade also involves the incorporation of an alkene into the molecule and introduces a point of variation that can be exploited for diversity generation. We hope to not only highlight the transformations reported herein but also inspire further research into this synthetic strategy to access new classes of skeletal editing transformations that, through rapid diversity generation, provide the potential to expedite SAR investigations.

Oxenoid Reactivity Enabled by Targeted Photoactivation of Periodate

The chemistry of low-valent intermediates continues to inspire new modes of reactivity across synthetic chemistry. But while the generation and reactivity of both carbenes and nitrenes are well-established, difficulties in accessing oxene, their oxygen-based congener, has severely hampered its application in synthesis. Here, we report a conceptually novel approach towards oxenoid reactivity through the violet-light photolysis of tetrabutylammonium periodate. Computational studies reveal an unexpected geometric change upon periodate photoexcitation that facilitates intersystem crossing and near-barrierless dissociation of triplet periodate into oxene. Under these operationally simple conditions, we have demonstrated the epoxidation of a wide range of substituted olefins, revealing unprecedented functional group compatibility. By overcoming the historic challenges associated with employing oxene as an intermediate in organic chemistry, we believe that this platform will inspire the development of new reactive oxygen-based methodologies across industry and academia.

Asymmetric dearomative single-atom skeletal editing of indoles and pyrroles

Heterocycle skeletal editing has recently emerged as a powerful tactic for achieving heterocycle-to-heterocycle transmutation without the need for multistep de novo heterocycle synthesis. However, the enantioselective skeletal editing of heteroarenes through single-atom logic remains challenging. Here we report the enantiodivergent dearomative skeletal editing of indoles and pyrroles via an asymmetric carbon-atom insertion, using trifluoromethyl N-triftosylhydrazones as carbene precursors. This strategy provides a straightforward methodology to access enantiomerically enriched six-membered N-heterocycles containing a trifluoromethylated quaternary stereocentre from planar N-heteroarenes. The synthetic utility of this enantiodivergent methodology was demonstrated by a broad evaluation of reaction scope, product derivatization and concise syntheses of drug analogues. Mechanistic studies reveal that the excellent asymmetric induction arises from the initial cyclopropanation step. The asymmetric single-atom insertion strategy is expected to have a broad impact on the field of single-atom skeletal editing and catalytic asymmetric dearomatization of aromatic compounds.

Violet-Light-Induced Ring-Opening of Anthranils with Chlorodiazirines

A violet-light-induced ring-opening of anthranils with chlorodiazirines has been developed. The metal-free protocol provides a rapid and efficient approach to N-(2-carbonylaryl)benzamides in moderate to good yields under mild conditions. The reaction appears to involve α-chlorocarbenes, which trigger the ring-opening of anthranils.

Single-Carbon Insertion into Single C-C Bonds with Diazirines

A novel platform for the skeletal editing of single C-C bonds via a single-carbon insertion has been developed using diazirines. This strategy involves the photogeneration of arylchlorocarbenes as carbynoid species that undergo site-selective carbene insertion into tertiary C-H bonds and a subsequent Wagner-Meerwein rearrangement promoted by a silver salt. Our skeletal editing strategy based on a formal selective carbyne C-C bond insertion has been demonstrated in six core-to-core conversions, including linear and cyclic benzylic substrates, alkanes and late-stage functionalizations.

Triftosylhydrazone in Single-Atom Skeletal Editing

ConspectusIn the past decade, single-atom skeletal editing, which involves the precise insertion, deletion, or exchange of single atoms in the core skeleton of a molecule, has emerged as a promising synthetic strategy for the rapid construction or diversification of complex molecules without laborious de novo synthetic processes. Among them, carbene-initiated skeletal editing is particularly appealing due to the ready availability and diverse reactivities of carbene species. The initial endeavors to modify the core skeleton of heteroarenes through carbon-atom insertion could date back to 1881, when Ciamician and Denstedt described the conversion of pyrroles to pyridines by trapping haloform-derived free carbene. Despite its potential synthetic value, the general applicability of this one-carbon insertion has seen limited progress due to poor yields and harsh reaction conditions. Significant advances in skeletal editing via carbene insertion were achieved only in the past 3 years by Levin, Ball, Xu, Song, Glorius, and others. The hallmark of these approaches is facile halocyclopropanation followed by regioselective ring opening facilitated by the expulsion of the halide ion. Consequently, only specially designed α-halocarbene precursors, such as haloform derivatives, α-halodiazoacetates, chlorodiazirines, and α-chlorodiazo oxime esters, can be employed to achieve Ciamician-Denstedt-type skeletal editing. This not only limits the types of functional groups installed on the ring expansion products but also prevents their widespread adoption, especially in late-stage contexts. The enduring quest to develop environmentally friendly and versatile carbene precursors, superior functional group compatibility, and potential application in late-stage diversifications and the investigation of mechanistic insights into carbon insertion reactions remain a fundamental objective.In our endeavors over the past 5 years, we have developed o-trifluoromethylbenzenesulfonylhydrazones (named Triftosylhydrazones) as operationally safe and easily decomposable diazo surrogates and explored their application in various challenging catalytic carbene transfer reactions. Recently, our group has put great efforts into expanding the application scope and unlocking the potential of triftosylhydrazones as carbene precursors in single-atom skeletal editing reactions. Since 2018, we have realized a range of skeletal editing of acyclic 1,3-dicarbonyls with silver carbenes to access 1,4-dicarbonyls, proceeding through a cyclopropanation/ring-opening process. Inspired by these results, we recently demonstrated a series of transition-metal-catalyzed highly selective single-atom skeletal editing of medicinally interesting heteroarenes like pyrroles, indoles, and 1,2-diazoles via carbenic carbon insertion. We then achieved the skeletal editing of strained three-membered nitrogen- and oxygen-containing heterocycles through the insertion or exchange of single-carbon atoms. In this Account, we present an overview of our achievements in the single-atom skeletal editing of heterocycles, organized based on three types of in situ-generated key intermediates, such as cyclopropane, N-ylide, and O-ylide from triftosylhydrazones and heterocycles, with a focus on reaction scopes, mechanistic features, and synthetic applications. We hope that this Account will provide valuable insights and contribute to the development of new methodologies in both the skeletal editing and carbene chemistry fields.

Ultraviolet-Light-Induced P-H Insertion of α-Halocarbenes

An intriguing P-H insertion of arylhalodiazirines with H-phosphorus oxides under ultraviolet-light irradiation is described. This methodology provides an efficient and straightforward route to the construction of a variety of α-halophosphorus oxides in good yields (≤95%), which represents a unique example of P-H insertion of α-halocarbenes for C-P bond formation. The metal-free protocol features the advantages of mild reaction conditions, high atom economy, and environmental friendliness.

Synthesis of aryl fluorocyclopropanes from aryl fluorodiazirines and alkenes in continuous flow

Photochemically induced generation of aryl fluorocarbenes from aryl fluorodiazirines and their subsequent [2+1] cycloaddition with alkenes was developed in continuous flow. The in situ generated electrophilic aryl fluorocarbene reacted with a range of alkenes enabling the synthesis of the corresponding 3-fluoro-3-aryl-cyclopropanes in a 5-minute residence time under 380-nm LED irradiation in continuous flow (20 examples). The scaled-up reaction of 3-fluoro-3-(4-chlorophenyl)-3H-diazirine with styrene under irradiation at 380 nm led to the fluorocyclopropane with a 77% yield, providing a throughput yield of 0.945 g h-1.

Halogencarbene-free Ciamician-Dennstedt single-atom skeletal editing

Single-atom skeletal editing is an increasingly powerful tool for scaffold hopping-based drug discovery. However, the insertion of a functionalized carbon atom into heteroarenes remains rare, especially when performed in complex chemical settings. Despite more than a century of research, Ciamician-Dennstedt (C-D) rearrangement remains limited to halocarbene precursors. Herein, we report a general methodology for the Ciamician-Dennstedt reaction using α-halogen-free carbenes generated in situ from N-triftosylhydrazones. This one-pot, two-step protocol enables the insertion of various carbenes, including those previously unexplored in C-D skeletal editing chemistry, into indoles/pyrroles scaffolds to access 3-functionalized quinolines/pyridines. Mechanistic studies reveal a pathway involving the intermediacy of a 1,4-dihydroquinoline intermediate, which could undergo oxidative aromatization or defluorinative aromatization to form different carbon-atom insertion products.

Direct oxygen insertion into C-C bond of styrenes with air

Skeletal editing of single-atom insertion to basic chemicals has been demonstrated as an efficient strategy for the discovery of structurally diversified compounds. Previous endeavors in skeletal editing have successfully facilitated the insertion of boron, nitrogen, and carbon atoms. Given the prevalence of oxygen atoms in biologically active molecules, the direct oxygenation of C-C bonds through single-oxygen-atom insertion like Baeyer-Villiger reaction is of particular significance. Herein, we present an approach for the skeletal modification of styrenes using O2 via oxygen insertion, resulting in the formation of aryl ether frameworks under mild reaction conditions. The broad functional-group tolerance and the excellent chemo- and regioselectivity are demonstrated in this protocol. A preliminary mechanistic study indicates the potential involvement of 1,2-aryl radical migration in this reaction.

Asymmetric one-carbon ring expansion of diverse N-heterocycles via copper-catalyzed diyne cyclization

One-carbon ring expansion reaction of N-heterocycles has gained particular attention in the past decade because this method allows for the conversion of readily available N-heterocycles into potentially useful complex ring-expanded N-heterocycles, which are inaccessible by traditional methods. However, the catalytic asymmetric variant of this reaction has been rarely reported to date. Herein, we disclose an enantioselective one-carbon ring expansion reaction through chiral copper-catalyzed diyne cyclization, leading to the practical, atom-economic and divergent assembly of an array of valuable chiral N-heterocycles bearing a quaternary stereocenter in generally good to excellent yields with excellent enantioselectivities (up to >99% ee). This protocol represents the first example of asymmetric one-carbon ring expansion reaction of N-heterocycles based on alkynes.

Rhodium-Catalyzed Asymmetric Cyclopropanation of Indoles with N-Triftosylhydrazones

Herein we report a general rhodium-catalyzed asymmetric intermolecular dearomative cyclopropanation of indoles using trifluoromethyl N-triftosylhydrazones as carbene precursors. The reaction enables the rapid construction of diverse cyclopropane-fused indolines bearing a trifluoromethylated quaternary stereocenter with high enantioselectivity (up to 99 % ee). This mild method exhibits broad substrate scope, tolerating various functional groups, and can even be utilized for the late-stage diversification of complex bioactive molecules. DFT calculations suggest that the formation of a key zwitterionic intermediate is responsible for the chiral induction. Overall, this approach opens up new possibilities for asymmetric cyclopropanation of challenging aromatic heterocyclic compounds.

Silver-Catalyzed Single-Carbon Insertion of Indoles with Acetophenone N-Triftosylhydrazones

Here, we report a silver carbene-enabled single-carbon insertion reaction of indoles via a one-pot, two-step sequence to deliver a dearomative quaternary center quinoline scaffold in a modular fashion. Specifically, we used N-triftosylhydrazones as masked donor-donor carbene precursors that facilitate the insertion of carbon atoms bearing various functional groups to the library of functionalized quinoline. Experimental and DFT evidence support the transient presence of a cyclopropane species and removal of protecting groups.

Nitrogen-to-functionalized carbon atom transmutation of pyridine

The targeted and selective replacement of a single atom in an aromatic system represents a powerful strategy for the rapid interconversion of molecular scaffolds. Herein, we report a pyridine-to-benzene transformation via nitrogen-to-carbon skeletal editing. This approach proceeds via a sequence of pyridine ring-opening, imine hydrolysis, olefination, electrocyclization, and aromatization to achieve the desired transmutation. The most notable features of this transformation are the ability to directly install a wide variety of versatile functional groups in the benzene scaffolding, including ester, ketone, amide, nitrile, and phosphate ester fragments, as well as the inclusion of meta-substituted pyridines which have thus far been elusive for related strategies.

Carbene-Assisted Arene Ring-Opening

Despite the significant achievements in dearomatization and C-H functionalization of arenes, the arene ring-opening remains a largely unmet challenge and is underdeveloped due to the high bond dissociation energy and strong resonance stabilization energy inherent in aromatic compounds. Herein, we demonstrate a novel carbene assisted strategy for arene ring-opening. The understanding of the mechanism by our DFT calculations will stimulate wide application of bulk arene chemicals for the synthesis of value-added polyconjugated chain molecules. Various aryl azide derivatives now can be directly converted into valuable polyconjugated enynes, avoiding traditional synthesis including multistep unsaturated precursors, poor selectivity control, and subsequent transition-metal catalyzed cross-coupling reactions. The simple conditions required were demonstrated in the late-stage modification of complex molecules and fused ring compounds. This chemistry expands the horizons of carbene chemistry and provides a novel pathway for arene ring-opening.

Controllable Skeletal and Peripheral Editing of Pyrroles with Vinylcarbenes

The skeletal editing of azaarenes through insertion, deletion, or swapping of single atoms has recently gained considerable momentum in chemical synthesis. Here, we describe a practical skeletal editing strategy using vinylcarbenes in situ generated from trifluoromethyl vinyl N-triftosylhydrazones, leading to the first dearomative skeletal editing of pyrroles through carbon-atom insertion. Furthermore, depending on the used catalyst and substrate, three types of peripheral editing reactions of pyrroles are also disclosed: α- or γ-selective C-H insertion, and [3+2] cycloaddition. These controllable molecular editing reactions provide a powerful platform for accessing medicinally relevant CF3-containing N-heterocyclic frameworks, such as 2,5-dihydropyridines, piperidines, azabicyclo[3.3.0]octadienes, and allylated pyrroles from readily available pyrroles. Mechanistic insights from experiments and density functional theory (DFT) calculations shed light on the origin of substrate- or catalyst-controlled chemo- and regioselectivity as well as the reaction mechanism.

Cyclopropanation of Alkenes with Halodiazirines as Halocarbene Precursors in Continuous Flow

The cyclopropanation reaction of alkenes with photolytically-generated chlorocarbenes from chlorodiazirines is reported as an effective way to prepare substituted 3-chloro-3-aryl-cyclopropanes. This practical and efficient approach allows the synthesis of various 3-chloro-3-aryl-cyclopropanes (32 examples) in continuous flow in 5-minute residence time under light-emitting diode (LED) irradiation. The conditions using 380 nm LED irradiation were successfully extended to the synthesis of substituted 3-bromo-3-aryl-cyclopropanes (3 examples).

Cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles

This Feature Article discusses recent advances in the development of cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles. Cascade ring expansion reactions have much potential for use in the synthesis of biologically important medium-sized rings and macrocycles, most notably as they don't require high dilution conditions, which are commonly used in established end-to-end macrocyclisation methods. Operation by cascade ring expansion method can allow large ring products to be accessed via rearrangements that proceed exclusively by normal-sized ring cyclisation steps. Ensuring that there is adequate thermodynamic driving force for ring expansion is a key challenge when designing such methods, especially for the expansion of normal-sized rings into medium-sized rings. This Article is predominantly focused on methods developed in our own laboratory, with selected works by other groups also discussed. Thermodynamic considerations, mechanism, reaction design, route planning and future perspective for this field are all covered.

Silver-Catalyzed Dearomative Skeletal Editing of Indazoles by Donor Carbene Insertion

Given the prevalence of heterocyclic scaffolds in drug-related molecules, converting these highly modular heterocyclic scaffolds into structural diversified and dearomatized analogs is an ideal strategy for improving their physicochemical and pharmacokinetic properties. Here, we described an efficient method for silver carbene-mediated dearomative N-N bond cleavage leading to skeletal hopping between indazole and 1,2-dihydroquinazoline via a highly selective single-carbon insertion procedure. Using this methodology, a series of dihydroquinazoline analogues with diarylmethylene-substituted quaternary carbon centers were constructed with excellent yields and good functional group compatibility, which was further illustrated by the late-stage diversification of important pharmaceutically active ingredients. DFT calculations indicated that the silver catalyst not only induces the formation of the silver carbene, but also activates the diazahexatriene intermediate, which plays a crucial role in the formation of the C-N bond.

Dearomatization of Pyridines: Photochemical Skeletal Enlargement for the Synthesis of 1,2-Diazepines

In this report, we developed a unified and standardized one-pot sequence that converts pyridine derivatives into 1,2-diazepines by inserting a nitrogen atom. This skeletal transformation capitalizes on the in situ generation of 1-aminopyridinium ylides, which rearrange under UV light irradiation. A thorough evaluation of the key parameters (wavelength, reaction conditions, activating agent) allowed us to elaborate on a simple, mild, and user-friendly protocol. The model reaction was extrapolated to more than 40 examples, including drug derivatives, affording unique 7-membered structures. Mechanistic evidence supports the transient presence of a diazanorcaradiene species. Finally, pertinent transformations of the products, including ring contraction reactions to form pyrazoles, were conducted and paved the way to a broad application of the developed protocol.

One-Carbon Ring Expansion of Indoles and Pyrroles: A Straightforward Access to 3-Fluorinated Quinolines and Pyridines

3-Fluorinated quinolines and pyridines are prevalent pharmacophores, yet their synthesis is often challenging. Herein, we demonstrate that dibromofluoromethane as bromofluorocarbene source enables the one-carbon ring expansion of readily available indoles and pyrroles to structurally diverse 3-fluorinated quinolines and pyridines. This straightforward protocol requires only a short reaction time of ten minutes and can be performed under air atmosphere. Preliminary investigations reveal that this strategy can also be applied to the synthesis of other valuable azines by using different 1,1-dibromoalkanes as bromocarbene sources.

Direct Access to Quinazolines and Pyrimidines from Unprotected Indoles and Pyrroles through Nitrogen Atom Insertion

Recent advances in single-atom insertion reactions have opened up new synthetic approaches for molecular diversification. Developing innovative strategies to directly transform biologically relevant molecules, without any prefunctionalization, is key to further expanding the scope and utility of such transformations. Herein, the direct access to quinazolines and pyrimidines from the corresponding unprotected 1H-indoles and 1H-pyrroles is reported, relying on the implementation of lithium bis(trimethylsilyl)amide (LiHMDS) as a novel nitrogen atom source in combination with commercially available hypervalent iodine reagents. Further application of this strategy in late-stage settings demonstrates its potential in lead structure diversification campaigns.

Photochemically Mediated Ring Expansion of Indoles and Pyrroles with Chlorodiazirines: Synthetic Methodology and Thermal Hazard Assessment

We demonstrate that arylchlorodiazirines serve as photo-activated halocarbene precursors for the selective one-carbon ring expansion of N-substituted pyrroles and indoles to the corresponding pyridinium and quinolinium salts. Preliminary investigations indicate that the same strategy also enables the conversion of N-substituted pyrazoles to pyrimidinium salts. The N-substituent of the substrate plays an essential role in: (1) increasing substrate scope by preventing product degradation, (2) enhancing yields by suppressing co-product inhibition, and (3) activating the azinium products towards subsequent synthetic manipulations. This latter point is illustrated by subjecting the quinolinium salts to four complementary partial reductions, which provide concise access to ring-expanded products with different degrees of increased C(sp3 ) character. Thermal analysis of the diazirines by differential scanning calorimetry (DSC) provides detailed insight into their energetic properties, and highlights the safety benefits of photolyzing-rather than thermolyzing-these reagents.