When all C-C breaks LO-Ose

Synthesis through C(sp3)-C(sp2) Bond Scission in Alkenes and Ketones

ConspectusThe homolytic cleavage of C-C bonds adjacent to functional groups has recently become a popular strategy for restructuring the skeletons of complex organic molecules. In contrast to the traditional reactivity profiles of polar bond disconnections, homolytic scission furnishes carbon-centered free radicals primed for controlled termination with a diverse range of radicophiles. Beyond standard radical capture, transition-metal catalysis facilitates sophisticated C-C and C-heteroatom bond-forming reactions. Intensive efforts have been focused over many years into the cleavage of the neighboring C-C bonds of carboxylic acids and alcohols. Despite the ubiquity of alkenes and ketones in natural products, feedstock chemicals, and common synthetic intermediates, much less attention has been paid to exploiting their potential in diversifying chiral pool materials, such as terpenes and terpenoids. Defunctionalization in this manner is a powerful approach for synthesizing high-value chemicals and advanced synthetic intermediates because of the possibility to reconstruct and further decorate chirality-bearing carbon skeletons. Motivated by synthetic necessity, since 2018 our group has focused on developing ozonolysis-based dealkenylative molecular diversification, and we expanded into deacylation in 2025. In this Account, we chronicle our initial motivation, describe the historical background, and summarize our research into dealkenylative and deacylative synthesis. Our dealkenylative approach capitalizes on the ozonolysis of alkenes in MeOH to generate α-methoxyhydroperoxides primed for a reaction with reducing agents. Their reduction through single electron transfer, mediated by a transition metal, leads to the formation of an alkoxyl radical that undergoes rapid β-scission, furnishing both a carbon-centered free radical and an ester group derived from the acetal carbon atom. The produced free radical can be strategically terminated by radicophiles, thereby delivering remodeled chiral molecules. Using this concept, we have developed hydrodealkenylation (through hydrogen atom transfer from benzenethiol), dealkenylative thiylation (through thiyl group transfer from diaryl disulfides), alkenylation (through addition/elimination with nitrostyrenes), and oxodealkenylation (through treatment with TEMPO followed by oxidation). Furthermore, kinetic analysis has enabled the development of a catalytic FeII/vitamin C system for dealkenylative alkynylation and halodealkenylation. Synergizing ozonolysis and copper catalysis has recently enabled aminodealkenylation through net-redox-neutral C-C cleavage followed by C-N bond formation. Although the high oxidation potential of ozone relative to organic compounds makes alkene-to-peroxide conversion possible, it also limits the applicability of dealkenylative techniques for substrates featuring ozone-sensitive functional groups. We recently overcame this constraint by first applying Isayama-Mukayiama peroxidation to olefins and then using a novel catalytic system─catalytic FeIII and PhSH with stoichiometric γ-terpinene─for ozone-free hydrodealkenylation. Beyond alkenes, we have developed a straightforward methodology for the homolytic deacylative cleavage of ketones as well, including cycloalkanones. This process is applicable in total syntheses and in the late-stage modifications of complex ketone-containing natural products.

Hydrodealkenylative C(sp3)-C(sp2) Bond Fragmentation Using Isayama-Mukaiyama Peroxidation

Advancements in radical capture strategies have expanded the range of products accessible from alkenes through dealkenylative synthesis. These methods, however, are still limited, as they rely on ozonolysis to generate the key peroxide intermediates from alkenes. Ozonolysis has several limitations. It is not compatible with alkenes containing electron-rich aromatics. It is also inapplicable to certain alkene substitution patterns in the context of dealkenylative synthesis. Additionally, it struggles with sterically hindered alkenes, internal nucleophiles and electrophiles, and allylic alcohols. In this paper, using Isayama-Mukaiyama peroxidation (IMP), we address the limitations of ozonolysis to rescue previously inaccessible alkene substrates and broaden the applicability of dealkenylative functionalization. In particular, we apply IMP in hydrodealkenylation and describe a novel radical hydrogenation condition─employing catalytic [FeIII], catalytic benzenethiol, and γ-terpinene in refluxing methanol─to resolve β-scission issues associated with IMP-generated alkyl silylperoxides.

Thermocontrolled Radical Nucleophilicity vs Radicophilicity in Regiodivergent C-H Functionalization

The temperature-dependent switching behavior of the saccharin radical is demonstrated, enabling the regiodivergent C3-H and C7-H functionalization of quinoxalin-2(1H)-ones. The saccharin radical was generated through N-Br bond cleavage in N-bromosaccharin (NBSA) and was observed to transition between radical and radicophile roles. At -10 °C, it was utilized as a radicophile, resulting in 100% C3-amination, while at +35 °C, it acted as a radical, leading to exclusive C7-bromination. Radical nucleophilicity was controlled by temperature modulation.

Deacylative Homolysis of Ketone C(sp3)-C(sp2) Bonds: Streamlining Natural Product Transformations

The homolytic cleavage of C-C bonds adjacent to specific functional groups has lately emerged as a versatile approach for molecular diversification. Despite the ubiquity and synthetic utility of ketones, radical fragmentation of their α-C-C bonds has proven to be a formidable challenge. Here, we present a broadly applicable deacylative strategy designed to homolytically cleave aliphatic ketones of various complexities, including transformations of cycloalkanones into carboxylic acids tethered to C-centered free radicals that can be engaged in diverse radical-based processes. The method involves ketone activation through treatment with hydrogen peroxide, yielding gem-dihydroperoxides. Subsequent single-electron-transfer reduction mediated by a low-valent metal complex generates alkyl radicals that can be captured selectively with a radicophile of choice, including through catalytic cross-coupling. The logic of our deacylative functionalization is exemplified by the total synthesis of 14 natural products, one analogue, and two drugs starting from readily available natural products, showcasing its transformative power in complex settings. This approach obviates the need for complex reagents and allows the controlled conversion of ketones to reconstructed products, making the process highly applicable across a spectrum of domains.

FeCl2-Mediated Rearrangement of Aminoperoxides into Functionalized Tetrahydrofurans: Dynamic Non-innocence of O-Ligands at an Fe Center Coordinates a Radical Cascade

The selective reaction of cyclic aminoperoxides with FeCl2 proceeds through a sequence of O-O and C-C bond cleavages, followed by intramolecular cyclization, yielding functionalized tetrahydrofurans in 44-82% yields. Replacing the peroxyacetal group in the peroxide structure with a peroxyaminal fragment fundamentally alters the reaction pathway. Instead of producing linear functionalized ketones, this modification leads to the formation of hard-to-access substituted tetrahydrofurans. Although the aminoperoxide cores undergo multiple bond scissions, this cascade is atom-economical. Computational analysis shows that the O-ligands at the Fe center have enough radical character to promote C-C bond fragmentation and subsequent cyclization. The stereoelectronic flexibility of oxygen, combined with iron's capacity to stabilize multiple reactive intermediates during the multistep cascade, explains the efficiency of this new atom-economic peroxide rearrangement.

Halodealkenylation: Ozonolysis and Catalytic FeII with Vitamin C Convert C(sp3)-C(sp2) Bonds to C(sp3)-Halide Bonds

As part of our investigations into C-C bond scission and functionalization, we report a halodealkenylation in which the C(sp3)-C(sp2) bonds of alkenes are cleaved and C(sp3)-halide bonds are formed, via a radical intermediate. These transformations occur through Criegee ozonolysis and FeII-catalyzed reductive coupling assisted by vitamin C as a stoichiometric reductant. We applied this strategy to the formal synthesis of (R,R,R)-γ-tocopherol.

Advances in radical peroxidation with hydroperoxides

Organic peroxides have become sought-after functionalities, particularly following the multi-tone consumption in polymer production and success in medicinal chemistry. The selective introduction of a peroxide fragment at different positions on the target molecule is a priority in the modern reaction design. The pioneering Kharasch-Sosnovsky peroxidation became the basic universal platform for the development of peroxidation methods, with its great potential for rapid generation of complexity due to the ability to couple the resulting free radicals with a wide range of partners. This review discusses the recent advances in the radical Kharasch-type functionalization of organic molecules with OOR fragment including free-component radical couplings. The discussion has been structured by the type of the substrate of radical peroxidation: C(sp 3 )-H substrates; aromatic systems; compounds with unsaturated C-C or C-Het bonds.

Chemical Diversity, Pharmacology, Synthesis and Detection of Naturally Occurring Peroxides

Natural occurring peroxides are interesting bioprospecting targets due to their molecular structural diversity and the wide range of pharmacological activities. In this systematic review, a total of 123 peroxide compounds were analysed from 99 published papers with the compounds distributed in 31 plants, 18 animals and 41 microorganisms living in land and water ecosystems. The peroxide moiety exists as both cyclic and acyclic entities and can include 1,2-dioxolanes, 1,2-dioxane rings and common secondary metabolites with a peroxo group. These peroxides possessed diverse bioactivities including anticancer, antimalarial, antimicrobial, anti-inflammatory, neuroprotective, adipogenic suppressor, antituberculosis, anti-melanogenic and anti-coagulant agents. Biosynthetic pathways and mechanisms of most endoperoxides have not been well established. Method development in peroxide detection has been a challenging task requiring multidisciplinary investigation and exploration on peroxy-containing secondary metabolites are necessary.

C═N Bond Ozonolysis and β-Scission: A Breakthrough Approach to the Synthesis of ω-Functionalized Compounds from Carbonyl Derivatives

This work discloses a two-step, one-pot approach to ω-functionalized esters via cleavage of the alicyclic fragment of cycloalkanone semicarbazones. This approach is based on a combination of the synthesis of various alkoxyhydroperoxides via cycloalkanone semicarbazone ozonolysis and in situ interaction of these peroxides with transition metal salts, leading to cleavage of the aliphatic cycle and subsequent ω-functionalized ester formation. A broad series of ω-halogen or pseudohalogen esters have been successfully synthesized in yields ranging from 23 to 73% per starting semicarbazone. A major advantage of the approach is the ability to use different cycloalkanone semicarbazones, including those with large cycles and substituents in them. The possibility of carrying out ozonolysis in the presence of various alcohols makes it possible to obtain the corresponding esters of ω-substituted carboxylic acids.

Recent advances in C(sp3)-N bond formation via metallaphoto-redox catalysis

The C(sp3)-N bond is ubiquitous in natural products, pharmaceuticals, biologically active molecules and functional materials. Consequently, the development of practical and efficient methods for C(sp3)-N bond formation has attracted more and more attention. Compared to the conventional ionic pathway-based thermal methods, photochemical processes that proceed through radical mechanisms by merging photoredox and transition-metal catalyses have emerged as powerful and alternative tools for C(sp3)-N bond formation. In this review, recent advances in the burgeoning field of C(sp3)-N bond formation via metallaphotoredox catalysis have been highlighted. The contents of this review are categorized according to the transition metals used (copper, nickel, cobalt, palladium, and iron) together with photocatalysis. Emphasis is placed on methodology achievements and mechanistic insight, aiming to inspire chemists to invent more efficient radical-involved C(sp3)-N bond-forming reactions.

Interrupted Dance of Five Heteroatoms: Reinventing Ozonolysis to Make Geminal Alkoxyhydroperoxides from C═N Bonds

Four heteroatoms dance in the cascade of four pericyclic reactions initiated by ozonolysis of C═N bonds. Switching from imines to semicarbazones introduces the fifth heteroatom that slows this dance, delays reaching the thermodynamically favorable escape path, and allows efficient interception of carbonyl oxides (Criegee intermediates, CIs) by an external nucleophile. The new three-component reaction of alcohols, ozone, and oximes/semicarbazones greatly facilitates synthetic access to monoperoxyacetals (alkoxyhydroperoxides).

Copper-catalyzed oxidative cyclization of 2-(1H-pyrrol-1-yl)aniline and alkylsilyl peroxides: a route to pyrrolo[1,2-a]quinoxalines

An efficient method was developed for the one-pot construction of pyrrolo[1,2-a]quinoxalines via a Cu(II)-catalyzed domino reaction between 2-(1H-pyrrol-1-yl)anilines and alkylsilyl peroxides. This reaction proceeds through C-C bond cleavage and new C-C and C-N bond formation. A mechanistic study suggests that alkyl radical species participate in the cascade reaction.

Dealkenylative Functionalizations: Conversion of Alkene C(sp3)-C(sp2) Bonds into C(sp3)-X Bonds via Redox-Based Radical Processes

This review highlights the history and recent advances in dealkenylative functionalization. Through this deconstructive strategy, radical functionalizations occur under mild, robust conditions. The reactions described proceed with high efficiency, good stereoselectivity, tolerate many functional groups, and are completed within a matter of minutes. By cleaving the C(sp3)-C(sp2) bond of terpenes and terpenoid-derived precursors, rapid diversification of natural products is possible.

Aminodealkenylation: Ozonolysis and copper catalysis convert C(sp3)-C(sp2) bonds to C(sp3)-N bonds

Great efforts have been directed toward alkene π bond amination. In contrast, analogous functionalization of the adjacent C(sp3)-C(sp2) σ bonds is much rarer. Here we report how ozonolysis and copper catalysis under mild reaction conditions enable alkene C(sp3)-C(sp2) σ bond-rupturing cross-coupling reactions for the construction of new C(sp3)-N bonds. We have used this unconventional transformation for late-stage modification of hormones, pharmaceutical reagents, peptides, and nucleosides. Furthermore, we have coupled abundantly available terpenes and terpenoids with nitrogen nucleophiles to access artificial terpenoid alkaloids and complex chiral amines. In addition, we applied a commodity chemical, α-methylstyrene, as a methylation reagent to prepare methylated nucleosides directly from canonical nucleosides in one synthetic step. Our mechanistic investigation implicates an unusual copper ion pair cooperative process.