Hydrogen Atom Transfer (HAT) Processes Promoted by the Quinolinimide-N-oxyl Radical. A Kinetic and Theoretical Study

Abstract

A kinetic study of the hydrogen atom transfer (HAT) reactions from a series of organic compounds to the quinolinimide-N-oxyl radical (QINO) was performed in CH 3 CN. The HAT rate constants are significantly higher than those observed with the phthalimide-N-oxyl radical (PINO) as a result of enthalpic and polar effects due to the presence of the Nheteroaromatic ring in QINO. The relevance of polar effects is supported by theoretical calculations conducted for the reactions of the two N-oxyl radicals with toluene, which indicate that the HAT process is characterized by a significant degree of charge transfer permitted by the π-stacking that occurs between the toluene and the N-oxyl aromatic rings in the transition state structures. An increase in the HAT reactivity of QINO was observed in the presence of 0.15 M HClO 4 and 0.15 M Mg(ClO 4 ) 2 due to the protonation or complexation with the Lewis acid of the pyridine nitrogen that leads to a further decrease in the electron density in the N-oxyl radical. These results fully support the use of N-hydroxyquinolinimide as a convenient substitute for N-hydroxyphthalimide in the catalytic aerobic oxidations of aliphatic hydrocarbons characterized by relatively high C−H bond dissociation energies.

Activation and Deactivation Strategies Promoted by Medium Effects for Selective Aliphatic C−H Bond Functionalization

“…OH discussed above . Along a similar line, the up to 20‐fold increase in the k H value for HAT from the benzylic C−H bonds of alkylaromatic substrates to the quinolinimide‐ N ‐oxyl radical (QINO) measured on going from acetonitrile to acetonitrile containing 0.15 m HClO 4 was explained accordingly in terms of QINO activation through protonation of the heteroaromatic nitrogen atom …”

“…[59] This behavior can be rationalized on the basis of an activation of the PINO radical toward HATb yp rotonation, in line with the effect of HBD solvents on HATf rom C À H bonds of aliphatic hydrocarbons to CumOC and COH discussed above. [61] When the reactions of PINO with tertiary amides were investigated, decreases in the k H values by greater than two orders of magnitude were measured in acetonitrile following addition of 0.1m HClO 4 . [61] When the reactions of PINO with tertiary amides were investigated, decreases in the k H values by greater than two orders of magnitude were measured in acetonitrile following addition of 0.1m HClO 4 .…”

“…[30,[41][42][43] Along asimilar line,the up to 20-fold increase in the k H value for HATf rom the benzylic CÀHb onds of alkylaromatic substrates to the quinolinimide-N-oxyl radical (QINO) measured on going from acetonitrile to acetonitrile containing 0.15 m HClO 4 was explained accordingly in terms of QINO activation through protonation of the heteroaromatic nitrogen atom. [61] When the reactions of PINO with tertiary amides were investigated, decreases in the k H values by greater than two orders of magnitude were measured in acetonitrile following addition of 0.1m HClO 4 . [60] HClO 4 is asufficiently strong acid to quantitatively protonate amide substrates (pK a = 2.0 and 6.0 for HClO 4 and protonated acetamide in MeCN,r espectively).…”

Practical N-Hydroxyphthalimide-Mediated Oxidation of Sulfonamides to N-Sulfonylimines

“…According to the literature, it has been demonstrated that PhI(OAc) 2 was able to initiate the generation of PINO radical from NHPI [30,31]. It is also known that the PINO radical was easy to abstract a hydrogen atom from a benzylic position of the organic substrate [39,40,41]. We designed one radical scavenging experiment (Figure 5a) to validate that the PINO radical was indeed formed in our reaction system.…”

Anwendung von Mediumeffekten in Aktivierungs‐ und Deaktivierungsstrategien zur selektiven Funktionalisierung aliphatischer C‐H‐Bindungen

“…OH übereinstimmt . Ähnlich wurde auch die bis zu 20‐fache Zunahme von k H für den HAT aus den benzylischen C‐H‐Bindungen von Alkylaromaten auf das Chinolinimid‐ N ‐oxyl‐Radikal (QINO) beim Übergang von Acetonitril zu Acetonitril mit 0.15 m HClO 4 mit der QINO‐Aktivierung über die Protonierung des heteroaromatischen Stickstoffatoms begründet …”