Structure and Reactions of the Succinimidyl Radical:  A Density Functional Study

Abstract

Recent density functionals and a variety of basis sets were employed in the study of three electronic states of the succinimidyl radical in C 2v symmetry: 2 A 1 (σN), 2 B 1 (πN), and 2 B 2 (σO). The lowest energy σO state is a genuine (local) minimum, as demonstrated by the harmonic vibrational analysis; the stationary points corresponding to the two remaining states are higher-order saddle points on the potential energy surface. Similar results were obtained with the Møller−Plesset method, while the complete active space calculations predict that the πN state has the lowest total energy. The ring-opening reaction from the σO state is symmetry allowed and was calculated with the density functional theory to proceed via a true transition state. The β-scission process was found to exhibit a large secondary deuterium kinetic isotope effect, with k H/k D = 1.2. The infrared spectra of the open forms of the radical that result from β-scission reaction were calculated and compared to the calculated spectrum of β-bromopropanoyl isocyanate.

Electrophilic Aromatic Coupling of Hexapyrrolylbenzenes. A Mechanistic Analysis

“…Similarly to benzylic bromination, succinimide substitution could be suppressed in NBS-fueled reactions employing acid additives (e.g., p -TsOH) or solvents (lactic acid). This observation may indicate potential involvement of the succinimidyl radical in the formation of 11b – 13b .…”

“…Similarly to benzylic bromination, succinimide substitution could be suppressed in NBS-fueled reactions employing acid additives (e.g., p-TsOH) or solvents (lactic acid). This observation may indicate potential involvement of the succinimidyl radical 55 in the formation of 11b−13b. Observation of reaction rates and isolated product yields indicate that the order of reactivity of differently substituted HPBs toward complete cyclization is 1d ≫ 1a ≈ 1b ≫ 1c, reflecting the combined influence of steric and electronic factors.…”

Use of Bromine and Bromo-Organic Compounds in Organic Synthesis

“…Initially, Bloomfield proposed that the succinimidyl radical, formed via photolytic cleavage, is the chain carrier for bromination reactions. Later, Goldfinger suggested that the active species in the bromination by NBS is the bromine atom, which also forms via the Bloomfield mechanism. , Several experimental − as well as theoretical − investigations were taken up subsequently to support both the mechanisms.…”

π- vs σ-Radical States of One-Electron-Oxidized DNA/RNA Bases: A Density Functional Theory Study

“…The σ (O)-radical in the gas phase (solution) is only 0.5 (0.33) kcal/mol lower in energy than the π-radical whereas the σ (N)-centered radical is 2.05 (2.24) kcal/mol higher in energy than the π-radical (Table 1). Using the B3LYP method and different basis sets Gainsforth et al 31 also predicted the O-centered σ-radical to be the most stable whereas an ab initio calculation including electron correlation predicted the π-radical to be more stable than the σ (N)-radical by ca. 5 kcal/mol.…”