Synthesis of N-Boc-α-amino Acids from Carbon Dioxide by Electrochemical Carboxylation of N-Boc-α-aminosulfones

Recent trends in incorporation of CO2 into organosulfur compounds via C-S bond cleavage

Desulfurative functionalization of organosulfur compounds to form various carbon-carbon and carbon-heteroatom bonds has become established as a powerful tool in organic chemistry. In this context, desulfurative carboxylation of this class of compounds using carbon dioxide (CO2) as a sustainable and renewable source of carboxyl has recently been developed as an efficient option for the synthesis of carboxylic acid derivatives. The aim of this Focus Review is to summarize the major progress in this appealing research field with particular emphasis on the mechanistic features of the reactions. Literature has been surveyed until the end of February 2024, according to the data collected using SciFinder and Google Scholar engines.

Mild Method for Deprotection of the N-Benzyloxycarbonyl (N-Cbz) Group by the Combination of AlCl3 and HFIP

Herein, we report our findings on the novel ability of aluminum chloride (AlCl3) in fluorinated solvent 1,1,1,3,3,3-hexafluoroisopropanol [HFIP] to selectively deprotect the N-benzyloxycarbonyl group (N-Cbz). The salient features of this method are good functional group tolerance including other reducible groups, cost-effectiveness, easy-to-handle, safe protocol, amenable to scale-up, high yields, and ambient temperature reactions. The methodology would serve as an excellent alternative to the use of pyrophoric hydrogen gas and metal catalyst reagents that pose severe safety and environmental concerns. The most notable feature of this methodology is the orthogonal deprotection of the N-Cbz group in the presence of O- and N-Bn protecting groups, hence, expanding the scope for designing synthetic routes to target compounds requiring multiple functional group transformations.

Magnesium-Mediated Umpolung Carboxylation of p-Quinone Methides with CO2

Magnesium-mediated reductive carboxylation of p-QMs with CO2 via an Umpolung strategy has been developed, which can be used for the preparation of various aryl acetic acids. This protocol featured high atom economy, mild conditions, and operational simplicity. The creation of this Umpolung carboxylation of p-QMs will unprecedentedly extend the application of p-QMs to nucleophilic reagents.

Electroreductive Desulfurative Transformations with Thioethers as Alkyl Radical Precursors

Thioethers are highly prevalent functional groups in organic compounds of natural and synthetic origin but remain remarkably underexplored as starting materials in desulfurative transformations. As such, new synthetic methods are highly desirable to unlock the potential of the compound class. In this vein, electrochemistry is an ideal tool to enable new reactivity and selectivity under mild conditions. Herein, we demonstrate the efficient use of aryl alkyl thioethers as alkyl radical precursors in electroreductive transformations, along with mechanistic details. The transformations proceed with complete selectivity for C(sp3 )-S bond cleavage, orthogonal to that of established transition metal-catalyzed two-electron routes. We showcase a hydrodesulfurization protocol with broad functional group tolerance, the first example of desulfurative C(sp3 )-C(sp3 ) bond formation in Giese-type cross-coupling and the first protocol for electrocarboxylation of synthetic relevance with thioethers as starting materials. Finally, the compound class is shown to outcompete their well-established sulfone analogues as alkyl radical precursors, demonstrating their synthetic potential for future desulfurative transformations in a one-electron manifold.

A guide to organic electroreduction using sacrificial anodes

Organic electrosynthesis is a green strategy for the synthesis of valuable molecules. Electrochemical reactions using sacrificial metal anodes enable new reactivity to be uncovered that could not be achieved with traditional non-electrochemical methods. Compared with reactions using metal powder as the reducing reagent, the mild electroreduction protocols usually exhibit diverse reactivity and excellent selectivity. The inexpensive metal anodes possess low oxidation potential, which could prevent undesired overoxidation of substrates, active intermediates and products. The in situ generated metal ions from sacrificial anodes could not only serve as Lewis acids to activate the reactants but also as a promoter or mediator. This tutorial review highlights the recent achievements in this rapidly growing area within the past five years. The sacrificial anode-enabled electroreductions are discussed according to the reaction type.

Electroreductive Ring-Opening Carboxylation of Cycloketone Oxime Esters with Carbon Dioxide

Electroreductive ring-opening carboxylation of cycloketone oxime esters with atmospheric carbon dioxide is reported. This reaction proceeded under simple constant current conditions in an undivided cell using glassy carbon as the cathode and magnesium as the sacrificial anode, providing substituted γ- and δ-cyanocarboxylic acids in moderate to good yields. Electrochemically generated cyanoalkyl radicals and cyanoalkyl anion are proposed as the key intermediates.

Direct and Selective Electrocarboxylation of Styrene Oxides with CO2 for Accessing β-Hydroxy Acids

Highly selective and direct electroreductive ring-opening carboxylation of epoxides with CO2 in an undivided cell is reported. This reaction shows broad substrate scopes within styrene oxides under mild conditions, providing practical and scalable access to important synthetic intermediate β-hydroxy acids. Mechanistic studies show that CO2 functions not only as a carboxylative reagent in this reaction but also as a promoter to enable efficient and chemoselective transformation of epoxides under additive-free electrochemical conditions. Cathodically generated α-radical and α-carbanion intermediates lead to the regioselective formation of α-carboxylation products.

Recent Advances in C(sp3)-C(sp3) and C(sp3)-C(sp2) Bond Formation through Cathodic Reactions: Reductive and Convergent Paired Electrolyses

The formation of C(sp³)-C(sp³) and C(sp³)-C(sp²) bonds is one of the major research goals of synthetic chemists. Electrochemistry is commonly considered to be an appealing means to drive redox reactions in a safe and sustainable fashion and has been utilized for C-C bond-forming reactions. Compared to anodic oxidative methods, which have been extensively explored, cathodic processes are much less investigated, whereas it can pave the way to alternative retrosynthetic disconnections of target molecules and to the discovery of new transformations. This review provides an overview on the recent achievements in the construction of C(sp³)-C(sp³) and C(sp³)-C(sp²) bonds via cathodic reactions since 2017. It includes electrochemical reductions and convergent paired electrolyses.

Electrochemical Dearomative Dicarboxylation of Heterocycles with Highly Negative Reduction Potentials

The dearomative dicarboxylation of stable heteroaromatics using CO₂ is highly challenging but represents a very powerful method for producing synthetically useful dicarboxylic acids, which can potentially be employed as intermediates of biologically active molecules such as natural products and drug leads. However, these types of transformations are still underdeveloped, and concise methodologies with high efficiency (e.g., high yield and high selectivity for dicarboxylations) have not been reported. We herein describe a new electrochemical protocol using the CO₂ radical anion (E_1/2 of CO₂ = -2.2 V in DMF and -2.3 V in CH₃CN vs SCE) that produces unprecedented trans-oriented 2,3-dicarboxylic acids from N-Ac-, Boc-, and Ph-protected indoles that exhibit highly negative reduction potentials (-2.50 to -2.94 V). On the basis of the calculated reduction potentials, N-protected indoles with reduction potentials up to -3 V smoothly undergo the desired dicarboxylation. Other heteroaromatics, including benzofuran, benzothiophene, electron-deficient furans, thiophenes, 1,3-diphenylisobenzofuran, and N-Boc-pyrazole, also exhibit reduction potentials more positive than -3 V and served as effective substrates for such dicarboxylations. The dicarboxylated products thus obtained can be derivatized into useful synthetic intermediates for biologically active compounds in few steps. We also show how the dearomative monocarboxylation can be achieved selectively by choice of the electrolyte, solvent, and protic additive; this strategy was then applied to the synthesis of an octahydroindole-2-carboxylic acid (Oic) derivative, which is a useful proline analogue.