Facile Redox‐Induced Aromatic–Antiaromatic Interconversion of a β‐Tetracyano‐21,23‐Dithiaporphyrin under Ambient Conditions

Abstract

Facile redox‐induced aromatic–antiaromatic interconversions were accomplished by using β‐tetracyano‐21,23‐dithiaporphyrin (CN4S2Por). Introduced cyano groups not only increased the reduction potential of the porphyrin core but also stabilized the antiaromatic isophlorin (CN4S2Iph) by π conjugation. The reduction of CN4S2Por with hydrazine in polar solvents quantitatively affords CN4S2Iph, even under ambient conditions. CN4S2Iph retains a nearly planar conformation and exhibits considerable antiaromaticity. Aerobic oxidation of CN4S2Iph to CN4S2Por occurs in nonpolar solvents. This study was conducted to contribute to the understanding of the structure–antiaromaticity relationship.

20π Antiaromatic Isophlorins without Metallation or Core Modification**

“…Similar solvent dependence was also reported by us for the dithia analog 2. [15] Table 1. Half-wave potentials (V vs Fc/Fc + ) of 4a, 4b and 2.…”

“…The HOMA values of [3a] 2− and [3b] 2− for pathway A were smaller than those for pathway B, whereas the HOMA values of [1] 2− for pathways A and B were comparable. [a] and harmonic oscillator model of aromaticity (HOMA) values for the crystal structures of [3a](PPh4)2, [3b](PPh4)2, and [1](PPh4)2 [15] .…”

“…This result is attributed to the considerable contribution of the nonaromatic quinoidal resonance structures for [3] 2− . [15] The anisotropy of the induced current density (AICD) [31] and the nucleus-independent chemical shift (NICS) [32,33] of [3a] 2− also confirm its antiaromatic character. The AICD plot for [3a] 2− revealed a counterclockwise ring current, as expected for antiaromatic -systems (Figure 6).…”

“…[2,[7][8][9][10][11][12][13][14] Previously, we have reported the synthesis and antiaromaticity of β-tetracyano-21,23-dithiaisophlorin 1 (CMN=2) (Scheme 1). [15] Weak reducing agents such as hydrazine were used to reduce the corresponding dithiaporphyrin 2 to its isophlorin 1, which was stable in the presence of the reducing agent even under atmospheric conditions. Moreover, the reduction of 2 to 1 was reversible.…”

“…[1](PPh4)2 mpd (Å) 0.13 0.02 0.17 The bond order of the cyano groups upon reduction of 4 to [3] 2− , evidenced by the crystal structure and IR spectra, was in agreement with that of a previously reported reduction of 2 to [1] 2− . [15] The CN bond lengths of the cyano groups in [3] 2− were longer than those in 4a [26] (Figure 5). The CN stretching peaks in the IR spectra of 4b and [3b] 2− were at 2222 and 2173 cm −1 , respectively, indicating a 49 cm −1 shift in the peak (Figure S7).…”