KBr-Mediated Electrochemical Dihydroxylation of Alkenes Using H2O as the Hydroxyl Source

Dihydroxylation of alkenes provides direct access to vicinal diols. Herein, a new electrochemical strategy for dihydroxylation of alkenes in only the presence of KBr is disclosed. Water serves as a green and sustainable hydroxyl source. Cheap KBr acts as both an electrolyte and a catalyst. Both styrenes and unactivated alkenes proceed in the dihydroxylation reactions smoothly to furnish vicinal diols in good yields. The successful synthesis of Cyclandelate, DTD derivative precursors, and a key intermediate for the synthesis of herbicide Metamitron highlights its synthetic utility.

Metal-free electrochemical dihydroxylation of unactivated alkenes

Herein, a metal-free electrochemical dihydroxylation of unactivated alkenes is described. The transformation proceeds smoothly under mild conditions with a broad range of unactivated alkenes, providing valuable and versatile dihydroxylated products in moderate to good yields without the addition of costly transition metals and stoichiometric amounts of chemical oxidants. Moreover, this method can be applied to a range of natural products and pharmaceutical derivatives, further demonstrating its synthetic utility. Mechanistic studies have revealed that iodohydrin and epoxide intermediate are formed during the reaction process.

Photoredox catalysis harvesting multiple photon or electrochemical energies

Photoredox catalysis (PRC) is a cutting-edge frontier for single electron-transfer (SET) reactions, enabling the generation of reactive intermediates for both oxidative and reductive processes via photon activation of a catalyst. Although this represents a significant step towards chemoselective and, more generally, sustainable chemistry, its efficacy is limited by the energy of visible light photons. Nowadays, excellent alternative conditions are available to overcome these limitations, harvesting two different but correlated concepts: the use of multi-photon processes such as consecutive photoinduced electron transfer (conPET) and the combination of photo- and electrochemistry in synthetic photoelectrochemistry (PEC). Herein, we review the most recent contributions to these fields in both oxidative and reductive activations of organic functional groups. New opportunities for organic chemists are captured, such as selective reactions employing super-oxidants and super-reductants to engage unactivated chemical feedstocks, and scalability up to gram scales in continuous flow. This review provides comparisons between the two techniques (multi-photon photoredox catalysis and PEC) to help the reader to fully understand their similarities, differences and potential applications and to therefore choose which method is the most appropriate for a given reaction, scale and purpose of a project.

Oxidative transformations of olefins employing persulfate/visible light irradiation in water

We present a green and economical approach for the photooxidation of diverse olefins through the use of ammonium persulfate and blue light irradiation, resulting in the formation of vicinal diols from styrenes and aliphatic alkenes, and vinyl esters and diacids from α,β-unsaturated ketones. The involvement of sulfate radicals in the reaction medium was established as the primary species responsible for the selective generation of the products. A significant advantage of the method lies in its broad substrate scope and economic feasibility, making it a promising alternative to conventional transition metal photocatalysis.

Olefin Dihydroxylation Using Nitroarenes as Photoresponsive Oxidants

Vicinal diols are abundant among natural and synthetic molecules, and also represent valuable intermediates throughout organic synthesis. Olefin dihydroxylation is an effective strategy to access these derivatives owing to the broad range and availability of alkene feedstocks. OsO₄ is among the most used reagents to achieve this transformation, yet its high toxicity and cost remain concerning. Herein, we present a mechanistically distinct strategy for olefin dihydroxylation using nitroarenes as photoresponsive oxidants. Upon purple LEDs irradiation, these species undergo a [3+2]-photocycloaddition with a wide range of olefins to give stable 1,3,2-dioxazolidine intermediates. These species can be accumulated in solution and then reduced in situ to the desired diols, utilising readily accessible and easy to handle solid reagents as H₂ surrogates.

Electro-/photocatalytic alkene-derived radical cation chemistry: recent advances in synthetic applications

Alkene-derived radical cations are versatile reactive intermediates and have been widely applied in the construction of complex functionalized molecules and cyclic systems for chemical synthesis. Therefore, the synthetic application of these alkene-derived radical cations represents a powerful and green tool that can be used to achieve the functionalization of alkenes partially because the necessity of stoichiometric external chemical oxidants and/or hazardous reaction conditions is eliminated. This review summarizes the recent advances in the synthetic applications of the electro-/photochemical alkene-derived radical cations, emphasizing the key single-electron oxidation steps of the alkenes, the scope and limitations of the substrates, and the related reaction mechanisms. Using electrocatalysis and/or photocatalysis, single electron transfer (SET) oxidation of the CC bonds in the alkenes occurs, generating the alkene-derived radical cations, which sequentially enables the functionalization of translocated radical cations to occur in two ways: the first involves direct reaction with a nucleophile/radical or two molecules of nucleophiles to realize hydrofunctionalization, difunctionalization and cyclization; and the second involves the transformation of the alkene-derived radical cations into carbon-centered radicals using a base followed by radical coupling or oxidative nucleophilic coupling.

Iodine-Initiated Dioxygenation of Aryl Alkenes Using tert-Butylhydroperoxides and Water: A Route to Vicinal Diols and Bisperoxides

An environment-friendly and efficient dioxygenation of aryl alkenes for the construction of vicinal diols has been developed in water with iodine as the catalyst and tert-butylhydroperoxides (TBHPs) as the oxidant. The protocol was efficient, sustainable, and operationally simple. Detailed mechanistic studies indicated that one of the hydroxyl groups is derived from water and the other one is derived from TBHP. Additionally, the bisperoxides could be obtained in good yields with iodine as the catalyst, Na₂CO₃ as the additive, and propylene carbonate as the solvent, instead.

Oxoammonium Salt-Mediated Regioselective Vicinal Dioxidation of Alkenes: Relying on Transient and Persistent Nitroxides

A novel, easy-to-handle, and regioselective vicinal dioxidation of alkenes under transition metal and organic peroxide free conditions has been developed. This approach uses N-hydroxyphthalimide and its analogues as the transient nitroxyl-radical precursors and 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate (TEMPO⁺BF₄^-) as the oxidant as well as the source of persistent nitroxide. By employing this method, multifarious structurally important dioxidation products were efficiently synthesized from simple alkenes and complex bioactive molecule derivatives.

Photocatalytic Anti-Markovnikov Radical Hydro- and Aminooxygenation of Unactivated Alkenes Tuned by Ketoxime Carbonates

A tunable photocatalytic method is reported for anti-Markovnikov hydro- and aminooxygenation of unactivated alkenes using readily accessible ketoxime carbonates as the diverse functionalization reagents. Mechanistic studies reveal that this reaction is initiated through an energy-transfer-promoted N-O bond homolysis of ketoxime carbonates leading to alkoxylcarbonyloxyl and iminyl radicals under visible-light photocatalysis, followed by the addition of alkoxylcarbonyloxyl radical to alkenes. By taking advantage of the different stability of the iminyl radicals, the generated carbon radical either abstracts a hydrogen atom from the media to form the anti-Markovnikov hydrooxygenation product, or it is trapped by the persistent iminyl radical to furnish the aminooxygenation product. Notably, this is the first example of direct hydrooxygenation of unactivated olefins with anti-Markovnikov regioselectivity involving an oxygen-centered radical.

Electrophotocatalytic Acetoxyhydroxylation of Aryl Olefins

A method for the acetoxyhydroxylation of olefins with syn stereoselectivity under electrophotocatalytic conditions is described. The procedure uses a trisaminocyclopropenium (TAC) ion catalyst with visible light irradiation under a controlled electrochemical potential to convert aryl olefins to the corresponding glycol monoesters with high chemo- and diastereoselectivity. This reaction can be performed in batch or in flow, enabling multigram synthesis of the monoester products.

Electrochemically driven stereoselective approach to syn-1,2-diol derivatives from vinylarenes and DMF

We have developed an electrochemically driven strategy for the stereoselective synthesis of protected syn-1,2-diols from vinylarenes with N,N-dimethylformamide (DMF). The newly developed system obviates the need for transition metal catalysts or external oxidizing agents, thus providing an operationally simple and efficient route to an array of protected syn-1,2-diols in a single step. This reaction proceeds via an electrooxidation of olefin, followed by a nucleophilic attack of DMF. Subsequent oxidation and nucleophilic capture of the generated carbocation with a trifluoroacetate ion is proposed, which gives rise predominantly to a syn-diastereoselectivity upon the second nucleophilic attack of DMF.

Visible light mediated synthesis of 4-aryl-1,2-dihydronaphthalene derivatives via single-electron oxidation or MHAT from methylenecyclopropanes

A new synthetic strategy of a single-electron oxidation and MHAT of methylenecyclopropanes (MCPs) for the rapid construction of 4-aryl-1,2-dihydronaphthalene derivatives by merging photoredox catalysis and cobalt catalysis has been developed.

Visible light-induced aerobic dioxygenation of α,β-unsaturated amides/alkenes toward selective synthesis of β-oxy alcohols using rose bengal as a photosensitizer

The first visible light-induced aerobic dioxygenation of alkenes for the selective synthesis of β-oxy alcohols was developed using non-toxic rose bengal as a photosensitizer.

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

Photocatalytic dual decarboxylative alkenylation mediated by triphenylphosphine and sodium iodide

An efficient photocatalytic dual decarboxylative alkenylation of α,β-unsaturated carboxylic acids and alkyl N-hydroxyphthalimide (NHP) esters mediated by triphenylphosphine and sodium iodide has been developed. This protocol proceeds under 456-nanometer irradiation by visible blue light in the absence of transition metals or organic dye based photoredox catalysts. The reaction is successfully applied to a wide range of redox-active esters derived from aliphatic carboxylic acids (1°, 2° and 3°) and α-amino acids, enabling transformations of diverse α,β-unsaturated carboxylic acids to α,β-alkylated styrenes with high efficiency and excellent selectivity under mild conditions.

Redox Potential Controlled Selective Oxidation of Styrenes for Regio- and Stereoselective Crossed Intermolecular [2 + 2] Cycloaddition via Organophotoredox Catalysis

A redox potential controlled intermolecular [2 + 2] cross-cycloaddition has been developed in the presence of a thioxanthylium photoredox catalyst. Electron-rich styrenes such as β-bromostyrene (E_p/2 = +1.61 V vs SCE) were selectively oxidized by a thioxanthylium photoredox catalyst (E_1/2 (C*/C^•-) = +1.76 V vs SCE) to styryl radical cations and reacted with styrene (E_p/2 = +1.97 V vs SCE) to furnish polysubstituted cyclobutanes in high yields. The present reaction can be successfully applied to intermolecular [2 + 2] cross-cycloaddition of β-halogenostyrenes, which cannot be effectively achieved by the hitherto reported representative organophotoredox catalysts.

Cu/Ni-Catalyzed Cyanomethylation of Alkenes with Acetonitrile for the Synthesis of β,γ-Unsaturated Nitriles

We have developed a protocol for the Cu/Ni-catalyzed cyanomethylation of alkenes with acetonitrile for the synthesis of β,γ-unsaturated nitriles. This is the first example of a direct coupling of the alkene sp² C-H bond and the acetonitrile sp³ C-H bond for the preparation of β,γ-unsaturated nitriles. Acetonitrile, an inexpensive and stable solvent, is demonstrated to be a useful cyanomethyl source. The combination of copper and nickel catalysts resulted in a high reaction efficiency.

Syn-Dihydroxylation of Alkenes Using a Sterically Demanding Cyclic Diacyl Peroxide

The syn-dihydroxylation of alkenes is a highly valuable reaction in organic synthesis. Cyclic acyl peroxides (CAPs) have emerged recently as promising candidates to replace the commonly employed toxic metals for this purpose. Here, we demonstrate that the structurally demanding cyclic peroxide spiro[bicyclo[2.2.1]heptane-2,4'-[1,2]dioxolane]-3',5'-dione (P4) can be effectively used for the syn-dihydroxylation of alkenes. Reagent P4 also shows an improved selectivity for dihydroxylation of alkenes bearing β-hydrogens as compared to other CAPs, where both diol and allyl alcohol products compete with each other. Furthermore, the use of enantiopure P4 (labeled P4') demonstrates the potential of P4' for a metal-free asymmetric syn-dihydroxylation of alkenes.

Cu-catalyzed oxygenation of alkene-tethered amides with O2 via unactivated C[double bond, length as m-dash]C bond cleavage: a direct approach to cyclic imides

An efficient aerobic unactivated C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond cleavage process was achieved, in which the succinimide or glutarimide derivatives could be prepared directly from alkenyl amides.

The transformations of unactivated alkenes through C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond double cleavage are always attractive but very challenging. We report herein a chemoselective approach to valuable cyclic imides by a novel Cu-catalyzed geminal amino-oxygenation of unactivated C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds. O₂ was successfully employed as the oxidant as well as the O-source and was incorporated into alkenyl amides via C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond cleavage for the efficient preparation of succinimide or glutarimide derivatives. Moreover, the present strategy under simple conditions can be used in the late-stage modification of biologically active compounds and the synthesis of pharmaceuticals, which demonstrated the potential application.

Transition-metal free oxidative C-H etherification of acylanilines with alcohols through a radical pathway

A transition metal free approach for the synthesis of methyl/ethyl aryl ether via oxidative C-H etherification of acylanilines with alcohols has been developed. Various acylanilines are compatible under standard conditions, giving the corresponding products in moderate to good yields. This strategy avoids transition-metal catalyst and excessive alcohol, providing a simple and reliable alternative method for the synthesis of methyl/ethyl aryl ether. Control experiments reveal that a radical mechanism is involved in this transformation.

Oxidative Deprotection of p-Methoxybenzyl Ethers via Metal-Free Photoredox Catalysis

An efficient and greener deprotection method for p-methoxybenzyl (PMB) ethers using a metal-free visible light photoredox catalyst and air and ammonium persulfate as the terminal oxidants is presented. Various functional groups and protecting groups were tolerated in the developed method to achieve good to excellent yields in short reaction times. Significantly, the developed method was compatible with PMB ethers derived from primary, secondary, and tertiary alcohols and a gram-scale reaction. Mechanistic studies support a proposed reaction mechanism that involves single electron oxidation of the PMB ether.

Pd-Catalyzed Vinylation of Aryl Halides with Inexpensive Organosilicon Reagents Under Mild Conditions

Pd-catalyzed Hiyama vinylation reaction of non-activated aryl chlorides and bromides under mild conditions was developed. The use of efficient vinyl donors and electron-rich sterically hindered phosphine ligands was critical for the success of the reaction. The products of this transformation can be used for Am/Cm separation, an important challenge in nuclear fuel reprocessing. The substituent effect on Am/Cm separating selectivity was also achieved, which could contribute to the development of new chromatographic materials for the separation of Am and Cm.