Scholl Reaction of Perylene-Based Polyphenylene Precursors under Different Conditions: Formation of Hexagon or Octagon?

A planar dibenzo-peri-hexacene derivative (2) was synthesized via FeCl3 -mediated Scholl reaction from a cyclopenta-fused perylene (CP) based polyphenylene precursor (1). However, an unexpected octagon-containing, negatively curved molecule (3) was obtained in nearly quantitative yield when 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and methanesulfonic acid (MeSO3 H) were used. Similar results were observed when two smaller-sized precursors containing one (4) or two CP units (5) were tested. X-ray crystallographic analysis also revealed that there is no close π-π stacking between neighboring π-conjugated skeletons. DFT calculations suggest a radical cation mechanism in the presence of FeCl3 while an arenium ion pathway for the DDQ/MeSO3 H mediated Scholl reaction, which can well explain the selective formation of hexagons and octagons under different conditions. The obtained compounds showed tunable optical and electrochemical properties.

Sulfur-Doped (Dibenzo)heptazethrene and (Dibenzo)octazethrene Diradicaloids.

We report the synthesis and characterization of sulfur-doped dibenzoheptazethrene (DBHZ-2S) and dibenzooctazethrene (DBOZ-2S) dications. Their neutral precursors 9a/9b were synthesized by acid-mediated intramolecular cyclization reaction of sulfoxides and the dications were prepared by treating 9a/9b with NOSbF6 or AgSbF6, respectively. Both dications displayed open-shell singlet ground state with a moderate diradical character (y0 = 0.44 for DBHZ-2S and 0.66 for DBOZ-2S) and a small singlet-triplet energy gap (ΔES-T = -3.9 kcal/mol for DBHZ-2S and -3.2 kcal/mol for DBOZ-2S). The open-shell singlet diradical nature of DBHZ-2S and DBOZ-2S were supported by electronic absorption spectroscopy, electron spin resonance spectroscopy, variable-temperature NMR studies, together with spin-unrestricted density functional theory calculations. However, synthesis and isolation of the sulfur-doped heptazethrene (HZ-2S) and octazethrene (OZ-2S) dications upon chemical oxidation of their corresponding neutral precursors 5a/5b were not successful owing to their high reactivity.

Nitrogen-doped heptazethrene and octazethrene diradicaloids.

We report the synthesis and electronic properties of nitrogen-doped heptazethrene (HZ) and octazethrene (OZ) diradicaloids, including the parent HZ-1N and OZ-1N, and the N-aryl-substituted dications, HZ-2N and OZ-2N. The unprotected HZ-1N and OZ-1N were generated in situ, but they are not stable enough for isolation. On the other hand, the N-aryl precursors 7a/7b were prepared by using a one-pot Buchwald-Hartwig amination followed by the intramolecular cyclization strategy. Chemical oxidation of 7a/7b gave the corresponding dications, HZ-2N/OZ-2N. HZ-2N displayed an open-shell singlet ground state with a moderate diradical character (y0 = 27.2%) and a small singlet-triplet energy gap (ΔES-T = -4.9 kcal mol-1). OZ-2N with larger diradical character (y0 = 54.7%) is more reactive and underwent the hydrogen abstraction reaction.

A Catalyst-Enabled Diastereodivergent Aza-Diels-Alder Reaction: Complementarity of N-Heterocyclic Carbenes and Chiral Amines.

Highly efficient and diastereodivergent aza-Diels-Alder reactions have been developed to access either diastereomeric series of benzofuran-fused δ-lactams and dihydropyridines in nearly perfect stereoselectivity (d.r. >20:1, >99 % ee for all examples). The complementarity of N-heterocyclic carbene and chiral amine as the catalyst was demonstrated for the first time, together with an excellent level of catalytic efficiency (1 mol % loading).