Factors Governing Reactivity and Selectivity in Hydrogen Atom Transfer from C(sp³)–H Bonds of Nitrogen-Containing Heterocycles to the Cumyloxyl Radical

Abstract

A kinetic study of the hydrogen atom transfer (HAT) reactions from nitrogen-containing heterocycles (secondary and tertiary lactams, 2-imidazolidinones, 2-oxazolidinones, and succinimides) to the cumyloxyl radical has been carried out employing laser flash photolysis with ns time resolution. HAT occurs from the C–H bonds that are α to nitrogen, activated by hyperconjugative overlap with the N–C=O π system. In the lactam series, the second-order HAT rate constant ( k H ) was observed to decrease by a factor of ∼4 going from the five- and six-membered ring derivatives to the eight-membered ones, a behavior that was rationalized on the basis of a reduced extent of hyperconjugative activation associated to the greater flexibility of the larger rings compared to the smaller ones. In the five-membered-ring substrate series, the k H values were observed to increase by >3 orders of magnitude on going from succinimide to 2-imidazolidinones, a behavior that was explained in terms of the divergent contribution of hyperconjugative activation and deactivating electronic effects determined by ring functionalities. The results are discussed in the framework of the development of HAT-based C–H bond functionalization procedures.

Metal-Free Photoinduced Acylboration of [1.1.1]Propellane via Energy Transfer Catalysis

“…However, in the reaction, N -centered iminyl radicals did not undergo the coupling with the BCP radicals. Instead, the diphenyliminyl radicals derived from oxime esters were favorably consumed by a radical homo-coupling pathway, while the BCP radicals underwent the hydrogen atom transfer (HAT) pathway with DMA solvent under photosensitizing conditions (Figure a) . In this regard, B 2 pin 2 was deemed an essential component to operate the difunctionalization of [1.1.1]­propellane successfully.…”

Quantitative evaluation of H-donating abilities of C(sp3)–H bonds of nitrogen-containing heterocycles in hydrogen atom transfer reaction

“…According to previous research, the HAT reactions between CumOc and nitrogen-containing heterocycles predominantly occur from the endocyclic a-C(sp 3 )-H bonds that are a to nitrogen. [5][6][7][8][9][10] The reaction sites of these substrates are shown in Scheme 1, where the hydrogen atoms involved in the HAT reactions are labelled red. The results of the kinetic studies for the HAT reactions from the ɑ-C(sp 3 )-H bonds of the studied Hdonors to the CumOc radical in acetonitrile at 298 K are listed in the ESI.…”

“…The kinetics of the HAT reactions from the C(sp 3 )-H bonds of alkanes and alkane derivatives to CumOc radical in acetonitrile at 298 K were quoted from the literature. [5][6][7][8][9][10] LFP experiments were carried out with a laser kinetic spectrometer using the third harmonic (355 nm) of a Q-switched Nd:YAG laser, delivering 8 ns pulses. Argon-or nitrogen-saturated acetonitrile solution of dicumyl peroxide (1.0 M) was employed.…”

“…HAT reactions between XH with the cumyloxyl radical [PhC(CH 3 ) 2 Oc, CumOc] in acetonitrile (MeCN) at 298 K were constructed, and the kinetics of these HAT reactions were investigated. [5][6][7][8][9][10] The effect of substrate structure, the insertion of heteroatoms, the ring size, and the substituent attached to the nitrogen atom on the H-donating abilities of these substrates was discussed in detail.…”

“…1–4 In recent years, the selectivity and reactivity of these nitrogen-containing heterocycles as hydrogen atom donors (H-donor), XH, in hydrogen atom transfer reactions (HAT, X–H + Y → X + Y–H) have attracted significant attention from a large number of researchers, gradually becoming a research hotspot. 5–12 Accordingly, it is necessary to develop methods to evaluate the H-donating ability of these nitrogen-containing heterocyclic compounds as H-donors using their characteristic physical parameters. Also, it is important to quantitatively study the H-donating abilities and compare the antioxidant activities of antioxidants.…”