Visible-Light Photoredox-Catalyzed Remote Difunctionalizing Carboxylation of Unactivated Alkenes with CO2

Remote difunctionalization of unactivated alkenes is challenging but a highly attractive tactic to install two functional groups across long distances. Reported herein is the first remote difunctionalization of alkenes with CO₂. This visible-light photoredox catalysis strategy provides a facile method to synthesize a series of carboxylic acids bearing valuable fluorine- or phosphorus-containing functional groups. Moreover, this versatile protocol shows mild reaction conditions, broad substrate scope, and good functional-group tolerance. Based on DFT calculations, a radical adds to an unactivated alkene to smoothly form a new carbon radical, followed by a 1,5-hydrogen atom-transfer process, the rate-limiting step, generating a more stable benzylic radical. The reduction of the benzylic radicals by an Ir^II species generates the corresponding benzylic carbanions as the key intermediates, which further undergo nucleophilic attack with CO₂ to generate carboxylates.     

Photosynthetic Fixation of CO2 in Alkenes by Heterogeneous Photoredox Catalysis with Visible Light

Light-driven fixation of CO₂ in organics has emerged as an appealing alternative for the synthesis of value-added fine chemicals. Challenges remain in the transformation of CO₂ as well as product selectivity due to its thermodynamic stability and kinetic inertness. Here we develop a boron carbonitride (BCN) with the abundant terminal B/N defects around the mesoporous walls, which essentially enhances surface active sites as well as charge transfer kinetics, boosting the overall rate of CO₂ adsorption and activation. In this protocol, anti-Markovnikov hydrocarboxylation of alkenes with CO₂ to an extended carbon chain is achieved with good functional group tolerance and specific regioselectivity under visible-light irradiation. The mechanistic studies demonstrate the formation of CO₂ radical anion intermediate on defective boron carbonitride, leading to the anti-Markovnikov carboxylation. Gram-scale reaction, late-stage carboxylation of natural products and synthesis of anti-diabetic GPR40 agonists reveal the utility of this method. This study sheds new insight on the design and application of metal-free semiconductors for the conversion of CO₂ in an atom-economic and sustainable manner.     

Copper‐Catalyzed Carbotrifluoromethylation of Unactivated Alkenes Driven by Trifluoromethylation of Alkyl Radicals

We report herein an unprecedented protocol for radical carbotrifluoromethylation of unactivated alkenes. With Cu(OTf)₂ as the catalyst, the reaction of unactivated alkenes, TMSCF₃ and activated alkyl chlorides at room temperature provides the corresponding carbotrifluoromethylation products in satisfactory yields. Directed by trifluoromethylation of alkyl radicals, the method exhibits an excellent regioselectivity that is opposite to those driven by CF₃ radical addition.     

Transition metal promoted carboxylation of unsaturated substrates with carbon dioxide

The use of CO₂ as a C1 building block for the synthesis of useful chemicals is of great significance, and has attracted increasing attention in recent years. The transition metal catalyzed or mediated addition of CO₂ to unsaturated chemical bonds has proved to be a powerful and versatile protocol for the incorporation of CO₂ into various unsaturated organic substrates such as alkynes, alkenes, allenes, aldehydes, and 1,3-dienes. The hydrogenative, alkylative and arylative carboxylation, heterocarboxylation, and carboxylative cyclization with CO₂ have led to efficient and selective formation of various functionalized carboxylic acids and derivatives. This review focuses on recent advances in this area with emphasis on conceptual reaction design.     

Efficient electrochemical dicarboxylations of arylacetylenes with carbon dioxide using nickel as the cathode

The electrochemical dicarboxylation of arylacetylenes with carbon dioxide could be smoothly achieved in an undivided cell using Ni as the cathode and Al as the anode with n-Bu₄NBr-DMF as the supporting electrolyte, at a constant current under CO₂ pressure of 3 MPa and room temperature in the absence of additional catalysts. The corresponding aryl-maleic anhydrides and 2-arylsuccinic acids were afforded in excellent total yields (82-94%). Under anhydrous conditions, an unsaturated aryl-maleic anhydride as the main product was obtained, while the presence of H₂O would lead to the formation of saturated 2-arylsuccinic acids. The results of cyclic voltammetric experiments show that a nickel cathode itself plays the catalytic role in the reduction reaction of arylacetylenes with CO₂.     

Anti-Markovnikov Radical Hydro- and Deuteroamidation of Unactivated Alkenes

Radical anti-Markovnikov hydro- and deuteroamidation of unactivated alkenes was achieved by merging photoredox and thiol catalysis. Reactions proceed by addition of the electrophilic CbzHN-radical (Cbz=carbobenzyloxy), readily generated by single-electron-transfer (SET) oxidation of an α-Cbz-amino-oxy acid to an alkene. The adduct radical is reduced by thiophenol added as an organic polarity reversal cocatalyst, which mediates the H transfer from H₂O to the alkyl radical intermediate. Accordingly, deuteroamidation of alkenes was realized with excellent D incorporation by using D₂O as the stoichiometric formal radical-reducing reagent. The reaction features low redox catalyst loading, excellent anti-Markovnikov selectivity, and the use of a large alkene excess is not required. Diverse Cbz-protected primary amines, including β-deuterated amines, can be obtained by applying this method.     

Trifluoromethylfluorosulfonylation of Unactivated Alkenes Using Readily Available Ag(O2CCF2SO2F) and N‐Fluorobenzenesulfonimide

Presented is a novel intermolecular radical trifluoromethylfluorosulfonylation of unactivated alkenes under mild reaction conditions with good functional‐group tolerance in the most atom‐economic manner by using readily available Ag(O₂CCF₂SO₂F) and N ‐fluorobenzenesulfonimide (NFSI). Both the trifluoromethyl and sulfonyl groups in the products originate from Ag(O₂CCF₂SO₂F).     

Hydrotrifluoromethylthiolation of Unactivated Alkenes and Alkynes with Trifluoromethanesulfonic Anhydride through Deoxygenative Reduction and Photoredox Radical Processes

An ongoing challenge in trifluoromethylthiolation reactions is the use of less expensive and easily available trifluoromethylthio sources. Herein, we disclose an unprecedented usage of trifluoromethanesulfonic anhydride (Tf₂O) as a radical trifluoromethylthiolating reagent. Hydrotrifluoromethylthiolation of unactivated alkenes and alkynes with Tf₂O in the presence of PMePh₂ and H₂O under visible‐light photoredox catalysis gave the addition products. The trifluoromethylthio radical (^.SCF₃) was first formed from Tf₂O through a photoredox radical processes and deoxygenative reduction of PMePh₂, and H₂O serves as the H‐atom donor for the hydrotrifluoromethylthiolation reaction. This reaction provides a new strategy for radical trifluoromethylthiolation.     

Copper‐Catalyzed Oxyboration of Unactivated Alkenes

The first regiodivergent oxyboration of unactivated terminal alkenes is reported, using copper alkoxide as a catalyst, bis(pinacolato)diboron [(Bpin)₂] as a boron source, and (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) as an oxygen source. The reaction is compatible with various functional groups. Two regioisomers are selectively produced by selecting the appropriate ligands on copper. The products may be used as a linchpin precursor for various other functionalizations, and net processes such as carbooxygenation, aminooxygenation, and dioxygenation of alkenes can be achieved after C?B bond transformations. Mechanistic studies indicate that the reaction involves the following steps: 1) Transmetalation between CuOtBu and (Bpin)₂ to generate a borylcopper species; 2) regiodivergent borylcupration of alkenes; 3) oxidation of the thus‐generated C?Cu bond to give an alkyl radical; 4) trapping of the resulting alkyl radical by TEMPO.     

Carboamination of Unactivated Alkenes through Three‐Component Radical Conjugate Addition

Two‐component Giese type radical additions are highly practical and established reactions. Herein, three‐component radical conjugate additions of unactivated alkenes to Michael acceptors are reported. Amidyl radicals, oxidatively generated from α‐amido oxy acids using redox catalysis, act as the third reaction component which add to the unactivated alkenes. The adduct radicals engage in Giese type additions to Michael acceptors to provide, after reduction, the three‐component products in an overall alkene carboamination reaction. Transformations which can be conducted under practical mild conditions feature high functional group tolerance and broad substrate scope.     

Synergistic Porosity and Charge Effects in a Supramolecular Porphyrin Cage Promote Efficient Photocatalytic CO2 Reduction**

We present a supramolecular approach to catalyzing photochemical CO₂ reduction through second-sphere porosity and charge effects. An iron porphyrin box ( PB ) bearing 24 cationic groups, FePB-2(P) , was made via post-synthetic modification of an alkyne-functionalized supramolecular synthon. FePB-2(P) promotes the photochemical CO₂ reduction reaction (CO₂RR) with 97 % selectivity for CO product, achieving turnover numbers (TON) exceeding 7000 and initial turnover frequencies (TOF_max) reaching 1400 min⁻¹. The cooperativity between porosity and charge results in a 41-fold increase in activity relative to the parent Fe tetraphenylporphyrin ( FeTPP ) catalyst, which is far greater than analogs that augment catalysis through porosity ( FePB-3(N ), 4-fold increase) or charge (Fe p-tetramethylanilinium porphyrin ( Fe-p-TMA ), 6-fold increase) alone. This work establishes that synergistic pendants in the secondary coordination sphere can be leveraged as a design element to augment catalysis at primary active sites within confined spaces.     

Thermocatalytic Conversion of CO2 to Valuable Products Activated by Noble-Metal-Free Metal-Organic Frameworks

Thermocatalysis of CO₂ into high valuable products is an efficient and green method for mitigating global warming and other environmental problems, of which Noble-metal-free metal–organic frameworks (MOFs) are one of the most promising heterogeneous catalysts for CO₂ thermocatalysis, and many excellent researches have been published. Hence, this review focuses on the valuable products obtained from various CO₂ conversion reactions catalyzed by noble-metal-free MOFs, such as cyclic carbonates, oxazolidinones, carboxylic acids, N-phenylformamide, methanol, ethanol, and methane. We classified these published references according to the types of products, and analyzed the methods for improving the catalytic efficiency of MOFs in CO₂ reaction. The advantages of using noble-metal-free MOF catalysts for CO₂ conversion were also discussed along the text. This review concludes with future perspectives on the challenges to be addressed and potential research directions. We believe that this review will be helpful to readers and attract more scientists to join the topic of CO₂ conversion.     

Amidyl Radicals by Oxidation of α‐Amido‐oxy Acids: Transition‐Metal‐Free Amidofluorination of Unactivated Alkenes

A three‐component transition‐metal‐free amidofluorination of unactivated alkenes and styrenes is presented. α‐Amido‐oxy acids are introduced as efficient and easily accessible amidyl radical precursors that are oxidized by a photoexcited organic sensitizer (Mes‐Acr‐Me) to the corresponding carboxyl radical. Sequential CO₂ and aldehyde/ketone fragmentation leads to an N‐centered radical that adds to an alkene. Commercial Selectfluor is used to trap the adduct radical through fluorine‐atom transfer. The transformation features by high functional‐group tolerance, broad substrate scope, and practical mild conditions. Mechanistic studies support the radical nature of the cascade.     

Alkene Carboarylation through Catalyst-Free,Visible Light-Mediated Smiles Rearrangement

A light-mediated Truce–Smiles arylative rearrangement is described that proceeds in the absence of any photocatalyst. The protocol creates two C−C bonds from simple starting materials, with the installation of an aryl ring and a difluoroacetate moiety across unactivated alkenes. The reaction proceeds via a radical mechanism, utilizing a light-mediated reduction of ethyl bromodifluoroacetate by N,N,N′,N′-tetramethylethylenediamine (TMEDA) to set up intermolecular addition to an unactivated alkene, followed by Truce–Smiles rearrangement.     

Radical Cyclization of 1,n-Enynes and 1,n-Dienes for the Synthesis of 2-Pyrrolidone

2-Pyrrolidones have aroused enormous interest as a useful structural moiety in drug discovery; however, not only does their syntheses suffer from low selectivity and yield, but also it requires high catalyst loadings. The radical cyclization of 1,n-enynes and 1,n-dienes has demonstrated to be an attractive method for the synthesis of 2-pyrrolidones due to its mild reaction conditions, fewer steps, higher atom economy, excellent functional group compatibility, and high regioselectivity. Furthermore, radical receptors with unsaturated bonds (i. e. 1,n-enynes and 1,n-dienes) play a crucial role in realizing radical cyclization because of the ability to selectively introduce one or more radical sources. In this review, we discuss representative examples of methods involving the radical cyclization of 1,n-enynes and 1,n-dienes published in the last five years and discuss each prominent reaction design and mechanism, providing favorable tools for the synthesis of valuable 2-pyrrolidone for a variety of applications.     

Homolytic hydrogermylation of alkenes with dibutylchlorogermane

In the presence of Et₃B-dry air, dibutylchlorogermane (Bu₂GeClH) reacted smoothly with alkenes at room temperature to give hydrogermylation products in high yields. This homolytic hydrogermylation was applicable to various alkenes including electron-deficient, electron-rich, and internal alkenes. Under the same conditions, tributylgermane (Bu₃GeH) showed much lower reactivity than Bu₂GeClH. The Et₃B-initiated reaction of 1,6-dienes with Bu₂GeClH gave germylmethylated cyclopentanes.     

Back Cover: Bio-Inspired Bimetallic Cooperativity Through a Hydrogen Bonding Spacer in CO2 Reduction (Angew. Chem. Int. Ed. 8/2023)

At the core of carbon monoxide dehydrogenase (CODH) active site two metal ions together with hydrogen bonding scheme from amino acids orchestrate the interconversion between CO₂ and CO. We have designed a molecular catalyst implementing a bimetallic iron complex with an embarked second coordination sphere with multi-point hydrogen-bonding interactions. We found that, when immobilized on carbon paper electrode, the dinuclear catalyst enhances up to four fold the heterogeneous CO₂ reduction to CO in water with an improved selectivity and stability compared to the mononuclear analogue. Interestingly, quasi-identical catalytic performances are obtained when one of the two iron centers was replaced by a redox inactive Zn metal, questioning the cooperative action of the two metals. Snapshots of X-ray structures indicate that the two metalloporphyrin units tethered by a urea group is a good compromise between rigidity and flexibility to accommodate CO₂ capture, activation, and reduction.     

Pd-catalyzed halocyclizations of unactivated 1,6-diynes through a formal anti-carbopalladation/bromide radical cascade

We report a Pd-catalyzed halocyclization of unactivated 1,6-diynes with N-bromosuccinimide (NBS). This approach produces stereo-defined dibromo substituted dihydropyrans, tetrahydropyridines, and 3-methylene cyclohexenes with exocyclic double bond appendages in mostly good yields. Copper salt was found to be a useful Lewis acid in this reaction. Mechanistically, a formal anti-carbopalladation and a bromide radical promoted Pd^II-Pd^III-Pd^I-Pd^II catalytic cycles were proposed to be involved in the formation of the dibromo-substituted products. Further functionalization of the dihydropyran derivatives underwent B(C₆F₅)₃-catalyzed ring opening, and reduction afforded dibrominated 1,3-dienes with excellent stereoselectivity.     

Nickel‐Catalyzed Mizoroki–Heck‐ versus Michael‐Type Addition of Organoboronic Acids to α,β‐Unsaturated Alkenes through Fine‐Tuning of Ligands

Various arylboronic acids reacted with activated alkenes in the presence of [Ni(dppe)Br₂], ZnCl₂, and H₂O in CH₃CN at 80 °C to give the corresponding Mizoroki–Heck‐type addition products in good to excellent yields. Furthermore, 1 equivalent of the hydrogenation product of the activated alkene was also produced. By tuning the ligands of the nickel complexes and the reaction conditions, Michael‐type addition was achieved in a very selective manner. Thus, various p‐ and o‐substituted arylboronic acids or alkenylboronic acid reacted smoothly with activated alkenes in CH₃CN at 80 °C for 12 h catalyzed by Ni(acac)₂, P(o‐anisyl)₃, and K₂CO₃ to give the corresponding Michael‐type addition products in excellent yields. However, for m‐substituted arylboronic acids, the yields of Michael‐type addition products are very low. The cause of this unusual meta‐substitution effect is not clear. By altering the solvent or phosphine ligand, the product yields for m‐substituted arylboronic acids were greatly improved. In contrast to previous results in the literature, the present catalytic reactions required water for Mizoroki–Heck‐type products and dry reaction conditions for Michael‐type addition products. Possible mechanistic pathways for both addition reactions are proposed.     

Visible-Light-Induced ortho-Selective Migration on Pyridyl Ring: Trifluoromethylative Pyridylation of Unactivated Alkenes

The photocatalyzed ortho-selective migration on a pyridyl ring has been achieved for the site-selective trifluoromethylative pyridylation of unactivated alkenes. The overall process is initiated by the selective addition of a CF₃ radical to the alkene to provide a nucleophilic alkyl radical intermediate, which enables an intramolecular endo addition exclusively to the ortho-position of the pyridinium salt. Both secondary and tertiary alkyl radicals are well-suited for addition to the C2-position of pyridinium salts to ultimately provide synthetically valuable C2-fluoroalkyl functionalized pyridines. Moreover, the method was successfully applied to the reaction with P-centered radicals. The utility of this transformation was further demonstrated by the late-stage functionalization of complex bioactive molecules.