Oxodealkenylative Cleavage of Alkene C(sp3 )-C(sp2 ) Bonds: A Practical Method for Introducing Carbonyls into Chiral Pool Materials

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.

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.

Oxoammonium-Catalyzed Oxidation of N-Substituted Amines

We report the development of oxoammonium-catalyzed oxidation of N-substituted amines via a hydride transfer mechanism. Steric and electronic tuning of catalyst led to complementary sets of conditions that can oxidize a broad scope of carbamates, sulfonamides, ureas, and amides into the corresponding imides. The reaction was further demonstrated on a 100-g scale using a continuous flow setup.

Photoinduced Deconstructive Alkylation Approach Enabled by Oxy-Radicals from Alcohols

Alcohols are the most commercially abundant, synthetically versatile and operationally convenient functional groups in organic chemistry. Therefore, a strategy that utilizes hydroxy-containing compounds to develop novel bond disconnection and formation process would achieve molecular diversity. Herein, a deconstructive strategy for the generation of quinoxalin-2(1H)-one derivatives has been developed from alcohol precursors via oxy-radical-induced β-fragmentation. Additionally, 1,5-HAT and deoxygenation by P(III) along with oxy-radical were demonstrated as alternative pathways for this transformation. Furthermore, with the deep-seated reorganization of a few terpenes carbon framework, a unique activity with inhibition against the growth of pathogenic fungi was observed.

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.

Carbon-Carbon Bond Cleavage for Late-Stage Functionalization

Late-stage functionalization (LSF) introduces functional group or structural modification at the final stage of the synthesis of natural products, drugs, and complex compounds. It is anticipated that late-stage functionalization would improve drug discovery's effectiveness and efficiency and hasten the creation of various chemical libraries. Consequently, late-stage functionalization of natural products is a productive technique to produce natural product derivatives, which significantly impacts chemical biology and drug development. Carbon-carbon bonds make up the fundamental framework of organic molecules. Compared with the carbon-carbon bond construction, the carbon-carbon bond activation can directly enable molecular editing (deletion, insertion, or modification of atoms or groups of atoms) and provide a more efficient and accurate synthetic strategy. However, the efficient and selective activation of unstrained carbon-carbon bonds is still one of the most challenging projects in organic synthesis. This review encompasses the strategies employed in recent years for carbon-carbon bond cleavage by explicitly focusing on their applicability in late-stage functionalization. This review expands the current discourse on carbon-carbon bond cleavage in late-stage functionalization reactions by providing a comprehensive overview of the selective cleavage of various types of carbon-carbon bonds. This includes C-C(sp), C-C(sp2), and C-C(sp3) single bonds; carbon-carbon double bonds; and carbon-carbon triple bonds, with a focus on catalysis by transition metals or organocatalysts. Additionally, specific topics, such as ring-opening processes involving carbon-carbon bond cleavage in three-, four-, five-, and six-membered rings, are discussed, and exemplar applications of these techniques are showcased in the context of complex bioactive molecules or drug discovery. This review aims to shed light on recent advancements in the field and propose potential avenues for future research in the realm of late-stage carbon-carbon bond functionalization.

When all C-C breaks LO-Ose

Organic peroxides are becoming popular intermediates for novel chemical transformations. The weak O-O bond is readily reduced by transition metals, including iron and copper, to initiate a radical cascade process that breaks C-C bonds. Great potential exists for the rapid generation of complexity, originating from the ability to couple the resulting free radicals with a wide range of partners. First, this review article discusses the history and synthesis of organic peroxides, providing the context necessary to understand this methodology. Then, it highlights 91 examples of recent applications of the radical functionalization of C-C bonds accessed through the transition metal-mediated reduction of organic peroxides. Finally, we provide some comments about safety when working with organic peroxides.

Dealkenylative Alkynylation Using Catalytic FeII and Vitamin C

In this paper, we report the synthesis of alkyl-tethered alkynes through ozone-mediated and FeII-catalyzed dealkenylative alkynylation of unactivated alkenes in the presence of alkynyl sulfones. This one-pot reaction, which employs a combination of a catalytic FeII salt and l-ascorbic acid, proceeds under mild conditions with good efficiency, high stereoselectivity, and broad functional group compatibility. In contrast to our previous FeII-mediated reductive fragmentation of α-methoxyhydroperoxides, the FeII-catalyzed process was devised through a thorough kinetic analysis of the multiple competing radical (redox) pathways. We highlight the potential of this dealkenylative alkynylation through multiple post-synthetic transformations and late-stage diversifications of complex molecules, including natural products and pharmaceuticals.

Site-Specific and Degree-Controlled Alkyl Deuteration via Cu-Catalyzed Redox-Neutral Deacylation

Deuterated organic compounds have become increasingly important in many areas; however, it remains challenging to install deuterium site-selectively to unactivated aliphatic positions with control of the degree of deuteration. Here, we report a Cu-catalyzed degree-controlled deacylative deuteration of diverse alkyl groups with the methylketone (acetyl) moiety as a traceless activating group. The use of N-methylpicolino-hydrazonamide (MPHA) promotes efficient aromatization-driven C-C cleavage. Mono-, di-, and trideuteration at specific sites can be selectively achieved. The reaction is redox-neutral with broad functional group tolerance. The utility of this method has been demonstrated in forming a complete set of deuterated ethyl groups, merging with the Diels-Alder reaction, a net devinylative deuteration, and the synthesis of the d₂-analogue of Austedo.

Site-selective amination and/or nitrilation via metal-free C(sp2)-C(sp3) cleavage of benzylic and allylic alcohols

Benzylic/allylic alcohols are converted via site-selective C(sp²)-C(sp³) cleavage to value-added nitrogenous motifs, viz., anilines and/or nitriles as well as N-heterocycles, utilizing commercial hydroxylamine-O-sulfonic acid (HOSA) and Et₃N in an operationally simple, one-pot process. Notably, cyclic benzylic/allylic alcohols undergo bis-functionalization with attendant increases in architectural complexity and step-economy.

C-H Insertion via Ruthenium Catalyzed gem-Hydrogenation of 1,3-Enynes

gem-Hydrogenation of an internal alkyne with the aid of [Cp*RuCl]₄ as the precatalyst is a highly unorthodox transformation, in which one C atom of the triple bond is transformed into a methylene group, whereas the second C atom gets converted into a ruthenium carbene. In the case of 1,3-enynes bearing a propargylic steering substituent as the substrates, the reaction occurs regioselectively, giving rise to vinyl carbene complexes that adopt interconverting η¹/η³-binding modes in solution; a prototypical example of such a reactive intermediate was characterized in detail by spectroscopic means. Although both forms are similarly stable, only the η³-vinyl carbene proved kinetically competent to insert into primary, secondary, or tertiary C–H bonds on the steering group itself or another suitably placed ether, acetal, orthoester, or (sulfon)amide substituent. The ensuing net hydrogenative C–H insertion reaction is highly enabling in that it gives ready access to spirocyclic as well as bridged ring systems of immediate relevance as building blocks for medicinal chemistry. Moreover, the reaction scales well and lends itself to the formation of partly or fully deuterated isotopologues. Labeling experiments in combination with PHIP NMR spectroscopy (PHIP = parahydrogen induced polarization) confirmed that the reactions are indeed triggered by gem-hydrogenation, whereas kinetic data provided valuable insights into the very nature of the turnover-limiting transition state of the actual C–H insertion step.

Co-catalyzed C(sp3)−C(sp2) bond cleavage via hydrogen atom transfer

The discovery of a new Co-catalyzed hydrogen atom transfer (HAT) C(sp3)-C(sp2) bond cleavage method to access ketones from alkenes is reported. This unprecedented transformation features mild reaction conditions and good...

Dealkenylative Ni-Catalyzed Cross-Coupling Enabled by Tetrazine and Photoexcitation

A new and general method to functionalize the C(sp³)-C(sp²) bond of alkyl and alkene linkages has been developed, leading to the dealkenylative generation of carbon-centered radicals that can be intercepted to undergo Ni-catalyzed C(sp³)-C(sp²) cross-coupling. This one-pot protocol leverages the easily procured alkene feedstocks for organic synthesis with excellent functional group compatibility without the need for a photoredox catalyst.

Photochemical Synthesis of 1,4-Dicarbonyl Bifluorene Compounds via Oxidative Radical Coupling Using TEMPO as the Oxygen Atom Donor

A visible-light-induced metal-free synthesis of 1,4-dicarbonyl compounds from alkyne-containing aryl iodides via photochemical C-I bond cleavage, intramolecular cyclization, oxidation, and intermolecular radical coupling sequence is reported. TEMPO was employed as the oxygen atom donor in this transformation. This protocol provided a new strategy for the synthesis of 1,4-dicarbonyl bifluorene compounds.

Dealkenylative Alkenylation: Formal σ-Bond Metathesis of Olefins

The dealkenylative alkenylation of alkene C(sp3 )-C(sp2 ) bonds has been an unexplored area for C-C bond formation. Herein 64 examples of β-alkylated styrene derivatives, synthesized through the reactions of readily accessible feedstock olefins with β-nitrostyrenes by ozone/FeII -mediated radical substitutions, are reported. These reactions proceed with good efficiencies and high stereoselectivities under mild reaction conditions and tolerate an array of functional groups. Also demonstrated is the applicability of the strategy through several synthetic transformations of the products, as well as the syntheses of the natural product iso-moracin and the drug (E)-metanicotine.