Diradicals Photogeneration from Chloroaryl-Substituted Carboxylic Acids

Abstract

With the aim of generating new, thermally inaccessible diradicals, potentially able to induce a double-strand DNA cleavage, the photochemistry of a set of chloroaryl-substituted carboxylic acids in polar media was investigated. The photoheterolytic cleavage of the Ar-Cl bond occurred in each case to form the corresponding triplet phenyl cations. Under basic conditions, the photorelease of the chloride anion was accompanied by an intramolecular electron-transfer from the carboxylate group to the aromatic radical cationic site to give a diradical species. This latter intermediate could then undergo CO₂ loss in a structure-dependent fashion, according to the stability of the resulting diradical, or abstract a hydrogen atom from the medium. In aqueous environment at physiological pH (pH=7.3), both a phenyl cation and a diradical chemistry was observed. The mechanistic scenario and the role of the various intermediates (aryl cations and diradicals) involved in the process was supported by computational analysis.

Keywords: diradicals; heterolytic cleavage; intramolecular electron transfer; phenyl cations; photochemistry.

Scheme 1

Literature precedents relevant to the present investigation.

Scheme 2

Proposed approach for the formation of diradicals based on Intramolecular Electron Transfer (IET).

Figure 1

Structures of compounds 1 a–e.

Figure 2

Potential energy surfaces (PESs) of the S₀ (blue line with square symbols) and T₁ (red line with circle symbols) states for: 1 a (part a), 1 e (part b), 1′ a (part c), 1′ e (part d) at the ωB97xD/def2TZV level of theory in bulk methanol. The reported structures are the optimized geometries for the S₀ (lower part) and T₁ (upper part) states. The values reported in magenta do refer to the charge localized at the chlorine atom (green) at the equilibrium geometry and upon elongation of the C−Cl bond (evaluated on the last point of each curve). The values reported in blue (for S₀ states) and red (for T₁ states) refer to the energy change associated with the last point with respect to the minimum for each curve.

Figure 3

Spin density plots for intermediates ³ 21⁺ arising from 1 a (part a) and 1 e (part b), and intermediates ³ 22 arising from 1′ a (part c) and 1′ e (part d), as determined from ωB97xD/def2TZV calculations in bulk methanol.

Figure 4

Singlet‐triplet (S−T) gaps for intermediates 21⁺ (green bars), 22 (red bars) and 23 (blue bars) as determined from ωB97xD/def2TZV calculations in bulk methanol. A positive bar indicates a singlet ground‐state intermediate.

Scheme 3

Proposed mechanistic scenario for the photoreactivity of compounds 1 a–e.