Antiaromaticity of Planar Bisdehydro[12]- and Tetrakisdehydro[16]annulenes Fused with Dithieno[3,4-b:3′,4′-d]thiophenes

Abstract

Thermally stable bisdehydro[12]- and tetrakisdehydro[16]annulenes with planar structures were successfully synthesized by fusion with two dithieno[3,4- b:3',4'- d]thiophene units. The planarized [4n]annulene cores induced substantial antiaromaticity. However, the HOMO-LUMO gap slightly increased as the π-core expanded from the planar cyclooctatetraene. This phenomenon, in contrast to that of typical π-conjugated systems, was attributed to the decrease in antiaromaticity of the [4n]annulene cores. Both TD-HF and GIAO-HF calculations supported this conclusion.

Oxidation of a Dithieno[3,4‐b:3′,4′‐d]thiophene Cyclic Dimer Containing a Planar Cyclooctatetraene Ring: Retention of High Antiaromaticity During Reactions

“…Thus, in the present study, we attempted to generate dication 2 Mes by both direct two‐electron oxidation of 2 Mes and one‐electron oxidation of 2 Mes .+ SbCl 6 − with SbF 5 in CD 2 Cl 2 at low temperatures. However, NMR signals indicative of 2 Mes 2+ were not observed, even though the previous CV measurement showed the reversible two‐steps one‐electron oxidation process . The NICS values of 2 H 2+ in the central COT ring (GIAO‐HF: NICS(0) iso =17.5/NICS(1) zz =45.8; GIAO‐B3LYP: NICS(0) iso =14.1/NICS(1) zz =36.8) were comparable to those of 2 H .…”

“…Conversely, for 2 , the unsubstituted (R=H) derivative did not dissolve in common organic solvents and further chemical reaction was difficult, whereas the TMS derivative showed a quasi‐reversible one‐electron oxidation wave in cyclic voltammetry (CV) measurements, which suggests the instability of the generated radical cation. In contrast, 2 Mes , which has the bulky protecting group, exhibited reversible two‐step one‐electron oxidations at 0.41 and 0.92 V vs Fc/Fc + , respectively . Therefore, we attempted the preparation of the radical cation and dication of 2 Mes .…”

“…Similarly, the total energy of 4 H was the least among the monoxides of 2 H at the M06‐2X/6‐311+G(3df,2p) level, which was in consistent with the experimental result shown in Scheme . As for sulfoxide 13 , the aromaticity in the corresponding thiophene ring in the precursor 2 H is considered to be reduced by the neighboring antiaromatic COT ring . When the relative energy of 13 in comparison to that of 2 H (9.9 kcal mol −1 (M06‐2X/6‐311+G(3df,2p))) was compared with that (12.4 kcal mol −1 ) of 11 , the difference (2.5 kcal mol −1 ) was considered to be the effect of the reduced aromaticity in the thiophene ring.…”

“…In contrast, the annelation with the less‐aromatic thiophene rings was shown to be effective for better retention of the antiaromaticity of the 4n π‐cores . Thus, in our previous studies, it was demonstrated that the 4n π‐systems planarized by annelation with dithieno[3,4‐ b : 3′,4′‐ d ]thiophene units retained high antiaromaticites, as indicated by the COT ring in cyclic dimer 2 (R=H) ( D 2 ), which has a NICS value of ca. 70 % of NICS value (NICS(0) iso =17.4/NICS(1) zz =44.8, GIAO‐HF/6‐311+G(d,p)//B3LYP/6‐31G(d,p)) in comparison to that of D 4h COT (NICS(0) iso =26.6/NICS(1) zz =62.5 with the identical calculation level).…”

π‐Conjugated Macrocycles Bearing Angle‐Strained Alkynes

“…The diyne‐containing macrocycle 155 was not isolated through the copper‐mediated homocoupling because the similar valence isomerization shown in Figure g proceeded smoothly to give thiophene‐fused dehydrobenzo[8]annulene 156 (Figure c) . Nishinaga and co‐workers synthesized planar dehydro[12]annulene 158 by using Sonogashira–Hagihara coupling (Figure d) . The copper‐mediated homocoupling of diyne 157 also proceeded to afford dehydro[16]annulene derivative 159 in moderate yield.…”