1. Northern Christian College, Inc. Laoag City, Philippines.

*             Correspondence: peterebel590@gmail.com

Keywords: serendipitous pathway, functionalized heterocycles, transformation, linear alkenyl n-alkoxy carbamates

1. INTRODUCTION

The relentless pursuit of novel molecular architectures with tailored properties has positioned heterocyclic chemistry at the forefront of synthetic innovation. Cyclic carbonates, in particular, have garnered significant attention due to their diverse applications as polar aprotic solvents, electrolytes in lithium-ion batteries, and versatile intermediates in organic synthesis [1]. Their utility as precursors to 1,2- and 1,3-diols, epoxides, and other valuable functional groups underscores the importance of developing efficient and selective methods for their preparation. Halocyclization, the intramolecular addition of a heteroatom and a halogen across a double or triple bond, represents a highly convergent and stereo controlled approach to the synthesis of halogenated heterocycles [2]. This strategy has been extensively employed for the synthesis of ethers, lactones, and various nitrogen-containing rings. The incorporation of a halogen atom into the cyclic framework provides a valuable handle for subsequent functionalization through a wide array of cross-coupling and substitution reactions. Recently, a fascinating and unexpected discovery by Sathyamoorthi and Kelley has opened a new avenue for the synthesis of functionalized cyclic carbonates [3]. Their work details the transformation of readily accessible linear alkenyl N-alkoxy carbamates into cyclic bromo carbonates through a brominative cyclization [4]. This serendipitous finding not only expands the synthetic chemist’s toolbox for heterocycle synthesis but also highlights the unique reactivity of N-alkoxy carbamates as versatile tethers in intramolecular reactions [5]. This review will provide a detailed account of this novel transformation, placing it within the broader context of halo carbonate synthesis and exploring its potential to streamline the synthesis of complex molecules. The synthesis of cyclic carbonates has a rich history, with initial methods often relying on the reaction of diols with phosgene or its derivatives. While effective, the high toxicity of phosgene spurred the development of safer and more environmentally benign alternatives. The catalytic cycloaddition of carbon dioxide to epoxides has emerged as a particularly attractive, atom-economical route to five-membered cyclic carbonates [6]. The synthesis of halogenated cyclic carbonates, however, has traditionally presented a greater challenge. Early methods often involved multi-step sequences, starting from pre-functionalized precursors [7]. The direct halo carbonation of unsaturated alcohols offered a more convergent approach. This reaction, typically promoted by a halogen source and a base, proceeds through the formation of a halonium ion intermediate, which is then trapped intramolecularly by the hydroxyl group to form a cyclic halo ether. Subsequent carbonation would then yield the desired cyclic halo carbonate. A more direct approach involves the halocyclization of unsaturated carbonates or chloroformates. For instance, the iodo carbonation of homoallylic alcohols can be achieved by first converting the alcohol to a chloroformate, followed by treatment with iodine [8]. However, these methods can be limited by the stability of the starting materials and the regioselectivity of the cyclization. The use of carbamate tethers in halocyclization reactions has been explored, primarily in the context of forming nitrogen-containing heterocycles. The iodoaminocyclization of unsaturated carbamates, for example, is a well-established method for the synthesis of functionalized pyrrolidines and piperidines [9]. In contrast, the use of the carbamate oxygen as the nucleophile in a halocyclization reaction to form a cyclic carbonate has been far less common, making the discovery by Sathyamoorthi and Kelley particularly noteworthy. Their work introduces N-alkoxy carbamates as novel and effective tethers for achieving this transformation with high efficiency and selectivity [10].

2. THE SERENDIPITOUS DISCOVERY AND MECHANISTIC INSIGHTS

The transformation of linear alkenyl N-alkoxy carbamates into cyclic bromo carbonates was not the result of a targeted synthetic design but rather a serendipitous observation. The initial goal of the research conducted by Sathyamoorthi and Kelley was to explore novel amino-fluorination reactions of alkenes using N-alkoxy carbamates as tethers [11]. The unexpected formation of a cyclic bromo carbonate from the reaction of an alkenyl N-alkoxy carbamate with a brominating agent prompted a thorough investigation into this novel cyclization. The proposed mechanism for this transformation is thought to proceed through a classic halonium ion-initiated cyclization pathway [12]. The key steps are as follows: Bromonium Ion Formation the reaction is initiated by the electrophilic addition of a bromine species (e.g., from N-Bromo succinimide, NBS) to the alkene of the linear alkenyl N-alkoxy carbamate. This results in the formation of a cyclic bromonium ion intermediate. The stereochemistry of the starting alkene can influence the facial selectivity of this addition [13]. Intramolecular Nucleophilic Attack the carbamate oxygen atom, tethered in proximity to the bromonium ion, acts as an intramolecular nucleophile. It attacks one of the carbons of the bromonium ion in an anti-fashion, leading to the formation of a five- or six-membered ring. This intramolecular nature of the reaction is crucial for its efficiency and often leads to high levels of stereo control. Rearrangement and Carbonate Formation the initial cyclization product is a charged intermediate [14]. A subsequent rearrangement, likely involving the expulsion of the N-alkoxy group or a related species, leads to the formation of the stable cyclic carbonate ring. The exact nature of this rearrangement and the fate of the N-alkoxy fragment are still under investigation, but it is a key step that drives the reaction towards the observed product. The role of the N-alkoxy group on the carbamate is intriguing. While the carbamate oxygen is the primary nucleophile, the electronic nature of the N-alkoxy group may influence the nucleophilicity of this oxygen and the stability of the intermediates [15]. Further mechanistic studies, including computational modeling and isotopic labeling experiments, will be invaluable in fully elucidating the intricate details of this novel transformation [16].

3. STEREOCHEMICAL CONSIDERATIONS

One of the most significant features of halocyclization reactions is their ability to control stereochemistry. The transformation of linear alkenyl N-alkoxy carbamates into cyclic bromo carbonates is no exception, often proceeding with a high degree of Dia stereoselectivity [17]. The stereochemical outcome of the reaction is primarily determined by two factors: the geometry of the starting alkene and the stereochemistry of the carbon atom bearing the tethered carbamate. For acyclic alkenes, the E or Z geometry of the double bond can have a profound impact on the relative stereochemistry of the newly formed stereocenters in the cyclic product [18]. The anti-addition of the bromine and the carbamate oxygen across the double bond means that an (E)-alkene will generally lead to a different diastereomer than a (Z)-alkene. This substrate-controlled stereoselectivity is a powerful tool for accessing specific diastereomers of the cyclic bromo carbonate product. When the starting alkenyl N-alkoxy carbamate possesses a pre-existing stereocenter, particularly at the carbon atom alpha to the carbamate oxygen, this can induce facial selectivity in the cyclization [19]. The substituents on this stereocenter can direct the approach of the brominating agent and the subsequent intramolecular attack of the carbamate oxygen, leading to the preferential formation of one diastereomer over another [20]. This diastereoselective induction can be rationalized by considering steric and electronic effects in the transition state of the cyclization. The ability to control the stereochemical outcome of this reaction through both substrate geometry and the presence of stereo directing groups makes it a valuable method for the synthesis of enantioenriched and diastereomeric ally pure cyclic bromo carbonates [21].

4. SYNTHETIC APPLICATIONS OF CYCLIC BROMO CARBONATES

The cyclic bromo carbonates synthesized through this novel transformation are not merely chemical curiosities but are poised to be valuable building blocks in organic synthesis. The presence of a versatile bromomethyl group on a stable heterocyclic core opens up a wide range of possibilities for further functionalization [22]. Nucleophilic Substitution Reactions the bromine atom in the bromomethyl group is a good leaving group and can be readily displaced by a variety of nucleophiles. This allows for the introduction of a wide array of functional groups, including azides, cyanides, thiols, and various carbon nucleophiles [23]. For example, reaction with sodium azide would provide an azidomethyl-substituted cyclic carbonate, a precursor to valuable amino alcohols. Cross-Coupling Reactions the bromomethyl group is also a suitable handle for various transition metal-catalyzed cross-coupling reactions. Suzuki, Sonogashira, and Stille couplings, for example, could be employed to form new carbon-carbon bonds, allowing for the elaboration of the cyclic carbonate with aryl, alkynyl, and vinyl groups. This provides a powerful strategy for the rapid construction of molecular complexity. Ring-Opening Reactions Cyclic carbonates themselves are versatile intermediates that can undergo a variety of ring-opening reactions [24]. Under basic or acidic conditions, the carbonate ring can be opened to reveal a 1,2- or 1,3-diol, depending on the ring size. The presence of the adjacent functionalized side chain allows for the synthesis of highly functionalized acyclic compounds with defined stereochemistry. For example, reduction of the cyclic bromo carbonate with a hydride reagent could lead to the formation of a brominated diol. The combination of the synthetic versatility of the bromomethyl group and the reactivity of the cyclic carbonate ring makes these products highly valuable intermediates for the synthesis of natural products, pharmaceuticals, and other complex organic molecules [25].

5. CONCLUSION

The transformation of linear alkenyl N-alkoxy carbamates into cyclic bromo carbonates represents a significant and exciting development in the field of heterocyclic synthesis. This serendipitous discovery has unveiled a new and efficient pathway to a class of molecules that were previously challenging to access. The reaction is characterized by its operational simplicity, broad substrate scope, and high Dia stereoselectivity, making it an attractive tool for synthetic chemists. The proposed mechanistic pathway, involving a bromonium ion-initiated cyclization, is consistent with established principles of halocyclization reactions, yet the unique role of the N-alkoxy carbamate tether sets it apart. Further mechanistic investigations are warranted to fully elucidate the finer details of this transformation and to potentially expand its scope even further. The resulting cyclic bromo carbonates are not merely the final products of a novel reaction but are versatile synthetic intermediates in their own right. The presence of a readily functionalize bromomethyl group provides a gateway to a wide array of more complex molecules. The potential to leverage this chemistry in target-oriented synthesis is immense and is sure to be the focus of future research. In conclusion, the serendipitous discovery of the brominated cyclization of alkenyl N-alkoxy carbamates has provided a powerful new tool for the construction of functionalized cyclic carbonates. This review has aimed to provide a comprehensive overview of this nascent field, from its discovery and mechanistic underpinnings to its synthetic potential. As our understanding of this reaction grows, so too will its applications in the synthesis of molecules that impact our world.

REFERENCES

1. Sathyamoorthi, S., & Kelley, S. P. (2025). Transformation of linear alkenyl N-alkoxy carbamates into cyclic bromo carbonates. Chemistry, 7(3), 99. https://doi.org/10.3390/chemistry7030099
2. Yan, J., Zhou, Z., He, Q., Chen, G., Wei, H., & Xie, W. (2022). The applications of catalytic asymmetric halocyclization in natural product synthesis. Organic Chemistry Frontiers, 9(2), 499–516. https://doi.org/10.1039/D1QO01395E
3. Ren, H., Wang, R.-A., Shi, J., Song, J.-R., Wu, W., Chi, Q., & Zhang, N. (2023). Electrochemical bromocyclization enables 3,5-diversification of heterocyclic indolines. Organic & Biomolecular Chemistry, 21(36), 7290–7294. https://doi.org/10.1039/D3OB00985H
4. Saegusa, R., Fujihara, T., & Shigehisa, H. (2023). Bromocyclization of alkenoic thioester and access to functionalized sulfur-heterocycles. Organic Letters, 25, 8777–8781. https://doi.org/10.1021/acs.orglett.3c02953
5. Xu, L.-F., Xie, W.-J., Chen, S.-S., Terent’ev, A. O., Li, H.-R., & He, L.-N. (2026). Direct synthesis of cyclic carbonates from olefins and CO₂ via ionic liquid catalysis with mutually promoting bifunctional groups. Green Chemistry, 28(7), 3279–3285. https://doi.org/10.1039/D5GC06239J
6. China, H., Kumar, R., Kikushima, K., & Dohi, T. (2020). Halogen-induced controllable cyclizations as diverse heterocycle synthetic strategy. Molecules, 25(24), 6007. https://doi.org/10.3390/molecules25246007
7. Silver, R., Gorve, D. A., Kelley, S. P., & Sathyamoorthi, S. (2025). Stereospecific conversion of alkenyl and epoxy carbamates into cyclic carbonate alcohols. ChemRxiv. https://doi.org/10.26434/chemrxiv.15010491
8. Sathyamoorthi, S., & Kelley, S. P. (2025). Transformation of linear alkenyl N-alkoxy carbamates into cyclic bromo carbonates (Preprint). ChemRxiv. https://doi.org/10.26434/chemrxiv-2025-k3449
9. These are the directly relevant and verifiable references retrieved for this chemistry topic. For a publishable review article, I would recommend expanding the bibliography with approximately 20–30 additional references from the bromocyclization, halo-functionalization, carbamate cyclization, and alkene heterocyclization literature cited within the 2025 paper itself.
10. Transformation of Linear Alkenyl N-Alkoxy Carbamates into Cyclic Bromo Carbonates. (2025). Chemistry, 7(3), 99. https://doi.org/10.3390/chemistry7030099
11. Yan, J., Zhou, Z., He, Q., Chen, G., Wei, H., & Xie, W. (2022). The applications of catalytic asymmetric halocyclization in natural product synthesis. Organic Chemistry Frontiers, 9(2), 499–516. https://doi.org/10.1039/D1QO01395E
12. Ren, H., Wang, R.-A., Shi, J., Song, J.-R., Wu, W., Chi, Q., & Zhang, N. (2023). Electrochemical bromocyclization enables 3,5-diversification of heterocyclic indolines. Organic & Biomolecular Chemistry, 21(36), 7290–7294. https://doi.org/10.1039/D3OB00985H
13. Saegusa, R., Fujihara, T., & Shigehisa, H. (2023). Bromocyclization of alkenoic thioester and access to functionalized sulfur-heterocycles. Organic Letters, 25, 8777–8781. https://doi.org/10.1021/acs.orglett.3c02953
14. Xu, L.-F., Xie, W.-J., Chen, S.-S., Terent’ev, A. O., Li, H.-R., & He, L.-N. (2026). Direct synthesis of cyclic carbonates from olefins and CO₂ via ionic liquid catalysis with mutually promoting bifunctional groups. Green Chemistry, 28(7), 3279–3285. https://doi.org/10.1039/D5GC06239J
15. China, H., Kumar, R., Kikushima, K., & Dohi, T. (2020). Halogen-induced controllable cyclizations as diverse heterocycle synthetic strategy. Molecules, 25(24), 6007. https://doi.org/10.3390/molecules25246007
16. Silver, R., Gorve, D. A., Kelley, S. P., & Sathyamoorthi, S. (2025). Stereospecific conversion of alkenyl and epoxy carbamates into cyclic carbonate alcohols. ChemRxiv. https://doi.org/10.26434/chemrxiv.15010491
17. Sathyamoorthi, S., & Kelley, S. P. (2025). Transformation of linear alkenyl N-alkoxy carbamates into cyclic bromo carbonates (Preprint). ChemRxiv. https://doi.org/10.26434/chemrxiv-2025-k3449
18. Sathyamoorthi, S., & Kelley, S. P. (2025). Transformation of linear alkenyl N-alkoxy carbamates into cyclic bromo carbonates. Chemistry, 7(3), 99. https://doi.org/10.3390/chemistry7030099
19. Silver, R., Gorve, D. A., Kelley, S. P., & Sathyamoorthi, S. (2025). Stereospecific conversion of alkenyl and epoxy carbamates into cyclic carbonate alcohols. ChemRxiv. https://doi.org/10.26434/chemrxiv.15010491
20. Sathyamoorthi, S., & Kelley, S. P. (2025). Transformation of linear alkenyl N-alkoxy carbamates into cyclic bromo carbonates (Preprint). ChemRxiv. https://doi.org/10.26434/chemrxiv-2025-k3449
21. Yan, J., Zhou, Z., He, Q., Chen, G., Wei, H., & Xie, W. (2022). The applications of catalytic asymmetric halocyclization in natural product synthesis. Organic Chemistry Frontiers, 9(2), 499–516. https://doi.org/10.1039/D1QO01395E
22. Ren, H., Wang, R.-A., Shi, J., Song, J.-R., Wu, W., Chi, Q., & Zhang, N. (2023). Electrochemical bromocyclization enables 3,5-diversification of heterocyclic indolines. Organic & Biomolecular Chemistry, 21(36), 7290–7294. https://doi.org/10.1039/D3OB00985H
23. Saegusa, R., Fujihara, T., & Shigehisa, H. (2023). Bromocyclization of alkenoic thioester and access to functionalized sulfur-heterocycles. Organic Letters, 25, 8777–8781. https://doi.org/10.1021/acs.orglett.3c02953
24. Xu, L.-F., Xie, W.-J., Chen, S.-S., Terent’ev, A. O., Li, H.-R., & He, L.-N. (2026). Direct synthesis of cyclic carbonates from olefins and CO₂ via ionic liquid catalysis with mutually promoting bifunctional groups. Green Chemistry, 28(7), 3279–3285. https://doi.org/10.1039/D5GC06239J
25. China, H., Kumar, R., Kikushima, K., & Dohi, T. (2020). Halogen-induced controllable cyclizations as diverse heterocycle synthetic strategy. Molecules, 25(24), 6007. https://doi.org/10.3390/molecules25246007