Visible-light-mediated catalyst-free synthesis of unnatural α-amino acids and peptide macrocycles

The visible light induced, photocatalysts or photoabsorbing EDA complexes mediated cleavage of pyridinium C-N bond were reported in the past years. Here, we report an ionic compound promote homolytic cleavage of pyridinium C-N bond by exploiting the photonic energy from visible light. This finding is successfully applied in deaminative hydroalkylation of a series of alkenes including naturally occurring dehydroalanine, which provides an efficient way to prepare β-alkyl substituted unnatural amino acids under mild and photocatalyst-free conditions. Importantly, by using this protocol, the deaminative cyclization of peptide backbone N-terminals is realized. Furthermore, the use of Et3N or PPh3 as reductants and H2O as hydrogen atom source is a practical advantage. We anticipate that our protocol will be useful in peptide synthesis and modern peptide drug discovery.

Molecular structure, enthalpies of formation and dissociation energies of the O – N bond for a number of aliphatic nitrates and nitrites

The optimal conformations of nitrates and nitrites of aliphatic alcohols C1-C4, as well as radicals formed during homolytic cleavage of O-NO2 and O-NO bonds were determined using the multistep (composite) method G4, as well as a large number of different density functional (DDF) methods and basis sets. The enthalpies of formation and dissociation energies of breaking bonds were calculated for the studied compounds. Comparison with the available experimental data shows that the best agreement with experiment is achieved when using the G4 method. In this case, the error in the enthalpies of formation does not exceed 1 kcal/mol. The paper also discusses the features of the influence of the molecular structure on the change in the series of enthalpies of formation and dissociation energies.

Clarification on the Reactivity of Diaryl Diselenides toward Hexacyclohexyldilead under Light

In this study, the reactivity of organochalcogen compounds toward a representative alkyl-lead bond compound under light was investigated in detail. Under light irradiation, the Cy-Pb bond of Cy6Pb2 (Cy = cyclohexyl) undergoes homolytic cleavage to generate a cyclohexyl radical (Cy•). This radical can be successfully captured by diphenyl diselenide, which exhibits excellent carbon-radical-capturing ability. In the case of (PhS)2 and (PhTe)2, the yields of the corresponding cyclohexyl sulfides and tellurides were lower than that of (PhSe)2. This probably occurred due to the low carbon-radical-capturing ability of (PhS)2 and the high photosensitivity of the cyclohexyl-tellurium bond.

Sodium Persulfate (Na2S2O8)

Sodium persulfate is an environmentally friendly inorganic compound commonly used as an oxidizing agent in chemistry. This reagent undergoes homolytic cleavage in solutions, affording sulfate radical anions that can present several applications. Recently, the SO4.- has been applied in diverse processes, such as the degradation of antibiotics and dyes in wastewater treatment. 1 In organic synthesis, sodium persulfate can be used as an oxidant in transition-metal catalyzed reactions or as an oxidative species in crucial steps in metal-free reactions.2,3 Na2S2O8 is an inexpensive oxidant that is stable and easy to handle, making it a good reagent choice for several strategic synthetic transformations.4 It is synthesized industrially through an electrolytic oxidation process from sodium hydrogen sulfate,5 and recent applications for this compound are presented herein.

Selective deoxygenative alkylation of alcohols via photocatalytic domino radical fragmentations

The delivery of alkyl radicals through photocatalytic deoxygenation of primary alcohols under mild conditions is a so far unmet challenge. In this report, we present a one-pot strategy for deoxygenative Giese reaction of alcohols with electron-deficient alkenes, by using xanthate salts as alcohol-activating groups for radical generation under visible-light photoredox conditions in the presence of triphenylphosphine. The convenient generation of xanthate salts and high reactivity of sequential C–S/C–O bond homolytic cleavage enable efficient deoxygenation of primary, secondary and tertiary alcohols with diverse functionality and structure to generate the corresponding alkyl radicals, including methyl radical. Moreover, chemoselective radical monodeoxygenation of diols is achieved via selective formation of xanthate salts.

Computational study on the mechanism of metal-free photochemical borylation of aryl halides

C-radical borylation is an significant approach for the construction of carbon−boron bond. Photochemical borylation of aryl halides successfully applied this strategy. However, precise mechanisms, such as the generation of aryl radicals and the role of base additive(TMDAM) and water, remain controversy in these reactions. In this study, photochemical borylation of aryl halides has been researched by density functional theory (DFT) calculations. Indeed, the homolytic cleavage of the C−X bond under irradiation with UV-light is a key step for generation of aryl radicals. Nevertheless, the generation of aryl radicals may also undergo the process of single electron transfer and the heterolytic carbon-halogen bond cleavage sequence, and the latter is favorable during the reaction.

Highly regio- and stereoselective phosphinylphosphination of terminal alkynes with tetraphenyldiphosphine monoxide under radical conditions

The homolytic cleavage of the PV(O)–PIII bond in tetraphenyldiphosphine monoxide simultaneously provides both pentavalent and trivalent phosphorus-centered radicals with different reactivities. The method using V-40 as an initiator is successfully investigated for the regio- and stereoselective phosphinylphosphination of terminal alkynes giving the corresponding trans-isomers of 1-diphenylphosphinyl-2-diphenylthiophosphinyl-1-alkenes in good yields. The protocol can be applied to a wide variety of terminal alkynes including both alkyl- and arylalkynes.

Titanium: A Unique Metal for Radical Dehydroxylative Functionalization of Alcohols

The dehydroxylative functionalization of alcohols is synthetic appealing, but it remains a long-term challenge in the synthetic community. Low-valent titanium has shown the power to produce carbon radicals from alcohols via homolytic cleavage of the C−OH bonds and thus offers the potential to overcome this problem. In this perspective manuscript, we summarized the recent advance on radical dehydroxylative transformation of alcohols either promoted or catalyzed by titanium. The limitation and outlook of the studies in this field are also provided. 1 Introduction 2 Recent Developments in Dehydroxylative Functionalization of Alcohols 3 Summary and Outlook

THERMODYNAMICALLY INVESTIGATIONS OF FREE RADICAL SCAVENGER POTENCY OF 1,2,4-TRIHYDROXYTHIOXANTHONE

The operative mechanism of the antioxidative action of 1,2,4-trihydroxythioxanthone (TX) is investigated in this contribution. Conclusions are made based on enthalpy values, as thermodynamical parameters. All calculations are done using the M06-2X/6-311++G(d,p) level of theory. To imitate polar and non-polar environments, calculations are done in water and benzene as the medium. It is found that, among three possible radicals that TX can generate, the most stable is the one obtained by homolytic cleavage of the O-H group in position 4. It was found that HAT (Hydrogen Atom Transfer) is the most plausible mechanism for that purpose in benzene. On the other hand, the most favorable mechanism in water is SPLET (Sequential Proton Loss Electron Transfer). Here is estimated the capacity of TX to deactivate hydroxyl (HO●), hydroperoxyl (HOO●) and methylperoxyl radical (CH3OO●). It is found that TX can deactivate all three free radicals following HAT and SPLET reaction mechanisms competitively, in the polar and non-polar environment. SET-PT (Single-Electron Transfer followed by Proton Transfer) is the inoperative mechanism for radicals scavenging, in the polar and non-polar environment.

Homolytic Cleavage of Diboron(4) Compounds by Diazabutadienes

Diazabutadiene derivatives have been identified as a distinct class of reagents, capable of cleaving B-B bond of the diboron(4). The cleavage is accompanied by formation of a new C=C bond...