Formation Of Acyl Radicals Research Articles

Photochemical‐, Electrochemical‐, and Photoelectrochemical‐ Catalyzed Hydrogen Atom Transfer from Aldehydes to Acyl Radicals and Their Transformations

Acyl radical has assumed an eminent position in the synthetic chemistry due to its unique and often highly reactive nature. Using aldehyde as acyl radical source does not require the prefunctionalization of substrate. Furthermore, the formation of acyl radical can be achieved through a hydrogen atom transfer (HAT) process. In recent years, photochemical‐, electrochemical‐, and photoelectrochemical‐catalyzed intermolecular HAT from aldehydes have been regarded as mild and sustainable routes for the generation of acyl radicals. Herein, we discuss some of the key advancements in the past 6 years in photo‐, electro‐, and photoelectro‐catalyzed generation of acyl radicals from aldehydes and their utilization. We also highlight the mechanistic insights that have emerged from these transformations.

Visible-Light-Mediated Cross Dehydrogenative Coupling of Thiols with Aldehydes: Metal-Free Synthesis of Thioesters at Room Temperature.

Thioesters play a crucial role in biological systems and serve as important building blocks for organic synthesis. Herein, Eosin Y and TBHP mediated photochemical cross dehydrogenative coupling (PCDC) between feedstock aldehydes and thiols has been described at room temperature to synthesize thioesters. This thioesterification protocol proceeds smoothly to give the desired products in good to excellent yields by the suitable PCDC of both alkyl/aryl- aldehydes with a variety of alkyl/aryl-thiols and generates water and tBuOH as green byproducts. This method is also found to be scalable with good efficiency. Mechanistic investigations reveal that under this photochemical condition, the formation of acyl radical can be achieved from aldehyde. This acyl radical was further intercepted with an intermediate disulfide, generated in situ via the dehydrogenation of thiol to give the desired thioester. Moreover, disulfides, which are relatively easier to handle, also provided good to excellent yields in the optimized reaction condition. This protocol was further extended toward the more challenging direct transformation of alcohols to thioesters.

Frontispiece: Synthetic Approaches to Selenoesters

Selenoesters are compounds of great synthetic relevance, since they can be used in several chemical transformations and mainly due to their great capacity in the formation of acyl radicals. Therefore, the scientific community has been developing several methods for the synthesis of this class of molecules. The synthesis of these compounds is addressed from different starting materials, such as carboxylic acids derivatives (acid chlorides and anhydrides), aldehydes, selenoacetylenes and miscellaneous methods. For more information, see the Minireview by L. L. Baldassari and D. S. Lüdtke on page 8656 ff.

Synthetic Approaches to Selenoesters.

Selenoesters are compounds of great synthetic relevance since they can be used in several types of chemical transformations and mainly due to their great capacity in the formation of acyl radicals. Therefore, the scientific community has been developing several methods for the synthesis of this class of molecules. This review will address the synthesis of these compounds from different starting materials, such as carboxylic acids derivatives (acid chlorides and anhydrides), aldehydes, selenoacetylenes and miscellaneous methods.

The visible-light-induced acylation/cyclization of alkynoates with acyl oximes for the construction of 3-acylcoumarins.

A nitrogen-centered radical-mediated carbon-carbon bond cleavage strategy is described to synthesize functionalized 3-acylcoumarins. The strategy is enabled by the visible-light-induced acylation/cyclization of alkynoates with various acyl oxime compounds in acetonitrile. The difunctionalization of carbon-carbon triple bonds precedes the generation of iminyl radicals, which is followed by the formation of acyl radicals. The acyl radicals then attack the carbon-carbon triple bonds, followed by 5-exo-trig cyclization and 1,2-ester migration. This strategy has wide substrate adaptability and good substituent tolerance.

Mechanism of Metal-Free C-H Activation of Branched Aldehydes and Acylation of Alkenes Using Hypervalent Iodine Compound: A Theoretical Study.

The mechanism of the C-H activation of aldehydes and the succeeding acylation of an alkene using a hypervalent iodine reagent is investigated by theoretical calculations. In contrast to the initial proposed mechanism, the present calculations show that the hypervalent iodine is the initiator of the radical reaction. The formation of acyl radical is rate-determining, and the resulting radical acts as the chain carrier. The kinetic isotope effect (KIE) of deuterated aldehyde, as well as other experimental observations, can now be rationalized from the newly proposed mechanism.

EPR analysis of the interacting properties and the degradation over time and irradiation of mastic resin used for painting protection

Mastic resin used as a covering film for painting protection was analyzed by electron paramagnetic resonance (EPR) spectroscopy, both as received and upon aging in sun-light. The effect of prolonged exposure to sun-light was mimicked by UV and, more so, by xenon lamp irradiation. Solid mastic presented EPR signals due to radicals trapped by PBN in solution. Data in the literature indicated the formation of acyl radicals (RCO·). These radicals preferentially dissolved in medium polarity solvents. The radical concentration in the solid mastic increased over time more than 50 times upon UV irradiation for 96 h and, even more, by xenon irradiation for 800 h. Also the PBN-trapped radicals in solution increased in concentration by irradiation. Small nitroxide radicals (TEMPO) interacted with a polar fraction of mastic dissolved in methanol, but mainly interacted with low polar mastic molecules in hydrophobic solvents. It was suggested, on the basis of both the PBN-spin trapping data and the TEMPO mobility variation in the solvents at different polarities, that terpenoid molecules partially polymerize by a radical mechanism to form low molecular weight products. A polyaromatic-radical (pyrene-TEMPO) and a biomolecule-radical (doxylcholestane) both interact weakly with mastic molecules in cyclohexane solutions. A positively charged surfactant radical (dimethylammonium-TEMPO bromide) was easily adsorbed onto the solid mastic surface suggesting that detergents are responsible for mastic degradation. In conclusion, this study provided information on the degradation mechanism of mastic resin and on its interacting ability towards external molecules and pollutants.

Formation of acyl radical in lipid peroxidation of linoleic acid by manganese-dependent peroxidase from Ceriporiopsis subvermispora and Bjerkandera adusta

Formation of acyl radical in lipid peroxidation of linoleic acid by manganese-dependent peroxidase from Ceriporiopsis subvermispora and Bjerkandera adusta

Generation of Acyl Radicals From 2-Naphthyl Thioesters

A series of S-2-naphthyl thioesters were synthesized from the corresponding carboxylic acids or acid chlorides. Irradiation of these thioesters in the presence of a hydrogen source (i.e., 1,4-cyclohexadiene) generated the corresponding aldehydes. In this fashion, primary, secondary, tertiary, and aryl carboxylic acids were converted to the aldehydes in high yields. Intramolecular radical cyclization reactions support the hypothesis that the reaction proceeds via the formation of acyl radicals. The formation of aldehydes was not perturbed by possible Norrish Type II reactions

Electron spin resonance and molecular orbital study of radicals in irradiated fluoro-derivatives

The low temperature matrix isolation technique coupled with e.s.r. spectrometry was employed to study the radicals produced by low temperature irradiation by a 60Co source of a number of fluoro and fluoro-chloro derivatives including chloroformyl- and fluoroformyl-perfluoropolyethers, trichlorofluoromethane, 1,1,2-trichloro-1,2,2-trifluoroethane, and perfluoro-1-methyldecalin (PFMD). The major radiolysis process in the acyl chloride is thought to be dissociative electron capture with formation of acyl radicals; the splitting of the C–O bonds in the chains is also inferred from the identification of RR′CF· and RCF2· species. In the acyl fluoride direct splitting of the COF group is also assumed to take place in order to explain the higher relative yield (with respect to the acyl chloride) of RCF2·. The irradiation of PFMD yields RR′CF· and RCF2· at low temperature and the 1-perfluoro-1-methyldecalin-8a-yl radical after warming above 160–180 K. This radical has a spectrum which is consistent with an average planar configuration at C-8a. By irradiation of CFCl3 and CF2ClCFCl2(thiourea clathrate)·CFCl2 and CF2ClĊFCl were obtained; the structure and conformation of these radicals have been investigated by the CNDO/2 MO method.

Mechanism for the formation of acyl radicals during the oxidation of aldehydes with manganese (III) acetate

Mechanism for the formation of acyl radicals during the oxidation of aldehydes with manganese (III) acetate

Formation of acyl radicals by reaction of manganese triacetate with aldehydes

1. Manganese triacetate reacts with aliphatic aldehydes in equimolar ratio to form acyl radicals. 2. The acyl radicals so produced can participate in reactions typical of them: recombination, decarbonylization, and addition to a double bond.