Synthesis of (Z)-β-(Carbonylamino)alkenylindium through Regioselective anti-Carboindation of Ynamides and Its Transformation to Multisubstituted Enamides

The regioselective anti-carboindation of ynamides by using InBr₃ and silylated nucleophiles was developed to synthesize (Z)-β-(carbonylamino)alkenylindiums. The X-ray crystallographic analysis of an alkenylindium suggested that the reaction proceeded in an anti-addition fashion. In contrast to reported syn-carbometalations of ynamides by using organometallics, a cooperation of InBr₃ and silylated nucleophiles to ynamides achieved an anti-addition, which was supported by DFT calculations. The scope of substrates included various ynamides and silylated nucleophiles, such as silyl ketene acetals and silyl ketene imines. The transformation of synthesized alkenylindiums by iodination, radical coupling, and Pd-catalyzed cross-coupling successfully afforded trisubstituted enamines with high regio- and stereoselectivities.     

Radical addition of alkyl halides to formaldehyde in the presence of cyanoborohydride as a radical mediator. A new protocol for hydroxymethylation reaction

Hydroxymethylation of alkyl halides was achieved using paraformaldehyde as a radical C1 synthon in the presence of tetrabutylammonium cyanoborohydride as a hydrogen source. The reaction proceeds via a radical chain mechanism involving an alkyl radical addition to formaldehyde to form an alkoxy radical, which abstracts hydrogen from a hydroborate anion.     

A Missing Reaction Step in Dithiobenzoate-Mediated RAFT Polymerization

The debate on the mechanism of dithiobenzoate-mediated RAFT polymerization may be overcome by taking the so-called “missing step” reaction between a highly reactive propagating radical and the three-arm star-shaped product of the combination reaction of an intermediate RAFT radical and a propagating radical into account. The “missing step” reaction transforms a propagating radical and a not overly stable three-arm star species into a resonance-stabilized RAFT intermediate radical and a stable polymer molecule. The enormous driving force behind the “missing step” reaction is estimated via DFT calculations of reaction enthalpies and reaction free enthalpies.     

Radical heterolysis reactions. Dynamics of formation, collapse, and solvation of ion pairs determined by a competition kinetic method

The kinetics of radical heterolysis reactions, including rate constants for radical cation-anion contact ion pair formation, collapse of the contact pair back to the parent radical, and separation of the contact pair to a solvent-separated ion pair or free ions were obtained in several solvents for a beta-mesyloxy radical. Rate constants were determined from indirect kinetic studies using thiophenol as both a radical trapping agent via H-atom transfer and an alkene radical cation trapping agent via electron transfer. [reaction: see text].     

丙烯与HBr/Br·发生自由基加成和取代反应的机理和竞争

含有α-H的烯烃可以与溴自由基发生自由基加成和自由基取代反应。本文以丙烯为例,论述了自由基加成和取代反应的机理和竞争关系,从而帮助读者更好地理解反应条件对优势反应及产物的影响。     

Mechanism of oxygen transfer in the epoxidation of an olefin by molecular oxygen in the presence of an aldehyde

[reaction: see text] The reaction pathway for peroxide-initiated aldehyde-mediated oxidation of olefins to epoxides by molecular oxygen has been studied. The pathways of reaction via a peroxy acid or an acyl peroxy radical have been differentiated by investigation of the reaction of 4 with oxygen to provide 6 via 8.     

Triethylborane-mediated hydrogallation and hydroindation: novel access to organogalliums and organoindiums

Hydrogallation of carbon[bond]carbon multiple bonds proceeds in the presence of triethylborane as a radical initiator. Several functionalities do not interfere with this reaction. Resulting alkenyl- and alkylgallium species can be trapped by several electrophiles. Highly regioselective radical addition of an indium hydride reagent to alkynes is also achieved. Various functionalities are tolerant under the reaction conditions. The reaction proceeds with complete anti stereoselectivity. Alkenylindiums obtained via hydroindation can be employed for the following cross-coupling reaction with aryl halides in one pot.     

Formation of the 2,4-pentadiynyl-1 radical (H2CCCCCH, X2B1) in the crossed beams reaction of dicarbon molecules with methylacetylene

The chemical dynamics to synthesize the 2,4-pentadiynyl-1 radical, HCCCCCH(2)(X(2)B(1)), via the neutral-neutral reaction of dicarbon with methylacetylene, was examined in a crossed molecular beams experiment at a collision energy of 37.6 kJ mol(-1). The laboratory angular distribution and time-of-flight spectra of the 2,4-pentadiynyl-1 radical and its fragmentation patterns were recorded at m/z = 63-60 and m/z = 51-48. Our findings suggest that the reaction dynamics are indirect and dictated by an initial attack of the dicarbon molecule to the pi electron density of the methylacetylene molecule to form cyclic collision complexes. The latter ultimately rearranged via ring opening to methyldiacetylene, CH(3)-C triple bond C-C triple bond C-H. This structure decomposed via atomic hydrogen emission to the 2,4-pentadiynyl-1 radical; here, the hydrogen atom was found to be emitted almost parallel to the total angular momentum as suggested by the experimentally observed sideways scattering. The overall reaction was strongly exoergic by 182 +/- 10 kJ mol(-1). The identification of the resonance-stabilized free 2,4-pentadiynyl-1 radical represents a solid background for the title reaction to be included into more refined reaction networks modeling the chemistry of circumstellar envelopes and also of sooting combustion flames.     

Solid-phase tandem radical addition-cyclization reaction: triethylborane-induced reaction of oxime ethers anchored to polymer support

Tandem radical addition-cyclization of oxime ethers anchored to polymer support was studied. The reaction of oxime ethers with stannyl radical proceeded effectively by the use of triethylborane as a radical initiator. The alkyl radical addition-cyclization reactions of oxime ether connected with alpha,beta-unsaturated carbonyl group proceeded under iodine atom-transfer reaction conditions to give the functionalized azacycles via two carbon-carbon bonds-forming process.     

One-pot multistep reactions based on thiolactones: extending the realm of thiol-ene chemistry in polymer synthesis

The in situ generation of thiols by nucleophilic ring-opening of a thiolactone with amines, followed by a UV-initiated radical thiol-ene reaction in a one-pot fashion, has been evaluated as an accelerated and versatile protocol for the synthesis of several types of polymeric architectures. After elaboration of a model amine-thiol-ene conjugation reaction, a number of routes based on readily available thiolactone-containing structures have been developed to successfully assemble functional, linear polymers and networks via a mild and facile radical photopolymerization process.     

Radical and Electron Recycling in Catalysis

An increasing number of enzymes are being discovered that contain radicals or catalyze reactions via radical intermediates. These radical enzymes are able to open reaction pathways that two‐electron steps cannot achieve. Recently, organic chemists started to apply related radical chemistry for synthetic purposes, whereby an electron energized by light is recycled in every turnover. This Minireview compares this new type of reaction with enzymes that use recycling radicals and single electrons as cofactors.     

Enantioselective cyclization of alkene radical cations

[reaction: see text] Enantiomerically enriched beta-(diphenylphosphatoxy)nitroalkanes undergo radical ionic fragmentation, induced by tributyltin hydride and AIBN in benzene at reflux, to give alkene radical cations in contact radical ion pairs. These contact ion pairs are trapped intramolecularly by amines to give pyrrolidines and piperidines with significant enantioselectivity ( approximately 60% ee), indicative of cyclization competing effectively with equilibration within the ion pairs. Use of an intramolecular N-propargylamine as a nucleophile provides an enantiomerically enriched pyrrolizidine skeleton via a tandem polar/radical crossover sequence.     

Hydroperoxides and aryl diazonium salts as reagents for the functionalization of non-activated olefins

Hydroperoxides, olefins, and arenediazonium salts selectively combine to give azo compounds via an iron(II)-mediated three-component reaction. Starting with a fragmentation liberating acetic acid, the hydroperoxides act as radical source and the diazonium ions as nitrogen-centered radical scavengers.     

Off-pathway, oxygen-dependent thiamine radical in the Krebs cycle

The catalytic cofactor thiamine diphosphate is found in many enzymes of central metabolism and is essential in all extant forms of life. We demonstrate the presence of an oxygen-dependent free radical in the thiamine diphosphate-dependent Escherichia coli 2-oxoglutarate dehydrogenase, which is a key component of the tricarboxylic acid (Krebs) cycle. The radical was sufficiently long-lived to be trapped by freezing in liquid nitrogen, and its electronic structure was investigated by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR). Taken together, the spectroscopic results revealed a delocalized pi radical on the enamine-thiazolium intermediate within the enzyme active site. The radical is generated as an intermediate during substrate turnover by a side reaction with molecular oxygen, resulting in the continuous production of reactive oxygen species under aerobic conditions. This off-pathway reaction may account for metabolic dysfunction associated with several neurodegenerative diseases. The possibility that the on-pathway reaction may proceed via a radical mechanism is discussed.     

Catalytic oxyalkylation of alkenes with alkanes and molecular oxygen via a radical process using N-hydroxyphthalimide

A novel catalytic method for the radical addition of alkanes and molecular oxygen to electron-deficient alkenes was achieved by the use of N-hydroxyphthalimide (NHPI) combined with a Co species as the catalyst. This reaction is referred to as oxyalkylation of alkenes with alkanes and O(2). For instance, the reaction of 1,3-dimethyladamantane with methyl acrylate under molecular oxygen in the presence of catalytic amounts of NHPI and Co(acac)(3) at 70 degrees C for 16 h gave oxyalkylated products in 91% yield. Other alkenes such as fumarate and acrylonitrile also serve as good acceptors of alkyl radicals and O(2) to afford the corresponding adducts in high yields. The generality of the present reaction was examined between various alkanes and alkenes under dioxygen. The behavior of Co ions during the reaction course was discussed. The present reaction involves (i) an alkyl radical generation via hydrogen abstraction of alkane by phthalimide N-oxyl generated in situ from NHPI and O(2) assisted by Co(II), (ii) the addition of the resulting alkyl radical to an electron-deficient alkene to form an adduct radical, (iii) trapping of the adduct radical by O(2) yielding a hydroperoxide, and (iv) the decomposition of the hydroperoxide by Co ions to form an adduct in which a hydroxy or a carbonyl function is incorporated.     

Unusual spin-trap chemistry for the reaction of hydroxyl radical with the carcinogen N-nitrosodimethylamine

The reaction of the potent carcinogen N-nitrosodimethylamine (NDMA) with hydroxyl radical generated via radiolysis was studied using EPR techniques. Attempts to spin trap NDMA radical intermediates with 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS) produced only unusual DBNBS radicals. One of these radicals was shown to be generated by both reaction of DBNBS with nitric oxide, and direct oxidation of DBNBS with an inorganic oxidant (^.Br^-₂). Another DBNBS radical was identified as a sulfite spin adduct resulting from the degradation of DBNBS by a NDMA reactive intermediate. In the absence of DBNBS, hydroxyl radical reaction with NDMA gave the dimethylnitroxide radical. Unexpectedly, addition of DBNBS to a solution containing dimethylnitroxide produced an EPR spectrum nearly identical to that of NDMA solutions with DBNBS added before radiolysis. A proposed mechanism accounting for these observations is presented.     

Polymerization of Vinyl Monomers with Binary Initiator System of N-Chlorosuccinimide and Lewis Acids

The polymerization of vinyl monomers with a binary system composed of N-chlorosuccinimide (NCS) and some Lewis acids was investigated by means of kinetic and spectral determinations. It was found that the NCS-ZnCl₂ system was the most effective as an initiator of radical polymerization of methyl methacrylate, and this polymerization was initiated by a radical produced via the reaction of NCS and ZnCl₂, and terminated bimolecularly. By applying a spin trapping technique to the reaction of NCS with ZnCl₂, it was shown that the initiating radical was N-succinimidyl radical which was obtained through a homolysis at the N-Cl bond.     

Electron Transfer‐Initiated Epoxidation and Isomerization Chain Reactions of β‐Caryophyllene

The abundant sesquiterpene β‐caryophyllene can be epoxidized by molecular oxygen in the absence of any catalyst. In polar aprotic solvents, the reaction proceeds smoothly with epoxide selectivities exceeding 70 %. A mechanistic study has been performed and the possible involvement of free radical, spin inversion, and electron transfer mechanisms is evaluated using experimental and computational methods. The experimental data—including a detailed reaction product analysis, studies on reaction parameters, solvent effects, additives and an electrochemical investigation—all support that the spontaneous epoxidation of β‐caryophyllene constitutes a rare case of unsensitized electron transfer from an olefin to triplet oxygen under mild conditions (80 °C, 1 bar O₂). As initiation of the oxygenation reaction, the formation of a caryophyllene‐derived radical cation via electron transfer is proposed. This radical cation reacts with triplet oxygen to a dioxetane via a chain mechanism with chain lengths exceeding 100 under optimized conditions. The dioxetane then acts as an in situ‐formed epoxidizing agent. Under nitrogen atmosphere, the presence of a one‐electron acceptor leads to the selective isomerization of β‐caryophyllene to isocaryophyllene. Observations indicate that this isomerization reaction is a novel and elegant synthetic pathway to isocaryophyllene.     

Diastereoselective synthesis of 1,2-O-isopropylidene-1,6-dioxaspiro[4,4]nonane applying the methodology of generation of radical cations under non-oxidizing conditions

We report the stereoselective synthesis of an optically pure spiroketal via an intramolecular tandem hydrogen abstraction reaction promoted by an alkoxy radical. Expanding the use of alkene radical cation under non-oxidizing conditions in the synthetic scenario.     

Mechanochemical Radical Boronation of Aryl Diazonium Salts Promoted by Sodium Chloride

Arylboronates are helpful building blocks in organic synthesis. Here, we present an efficient mechanochemical synthesis of arylboronates from arene diazonium salts. Importantly, this transformation was significantly enhanced by sodium chloride, which probably promotes the decomposition of diazonium salts via anion exchange. Chloride anions also participate in the formation of strongly reducing Cl−B anion radical intermediate that promotes radical chain reaction. The reaction proceeds more efficiently with a small amount of polar solvent as a liquid-assisted grinding additive. Quantum chemical calculations support the mechanistic proposal.