Functionalized Contorted Polycyclic Aromatic Hydrocarbons by a One‐Step Cyclopentannulation and Regioselective Triflyloxylation

Abstract

The oxidative cyclodehydrogenation (often named the Scholl reaction) is still a powerful synthetic tool to construct even larger polycyclic aromatic hydrocarbons (PAHs) by multiple biaryl bond formations without the necessity of prior installation of reacting functional groups. Scholl‐type reactions are usually very selective although the resulting products bear sometimes some surprises, such as the formation of five‐membered instead of six‐membered rings or the unexpected migration of aryl moieties. There are a few examples, where chlorinated byproducts were found when FeCl3 was used as reagent. To our knowledge, the direct functionalization of PAHs during Scholl‐type cyclization by triflyloxylation has not been observed. Herein we describe the synthesis of functionalized PAHs by the formation of five‐membered rings and a regioselective triflyloxylation in one step. The triflyloxylated PAHs can be used as reactants for further transformation to even larger contorted PAHs.

Contorted Polycyclic Aromatic Hydrocarbons with Two Embedded Azulene Units

“…In this case it was difficult to assign the positions of the chloride substituents by NMR methods, but single‐crystal X‐ray diffraction data gave a clear answer in this respect, showing that chlorination occurred in para ‐positions of the prior existing C−C biaryl bond. This is in contrast to the position of triflyloxylation we observed for the bisnaphthyl dibenzoperylene derivative, we described before …”

“…To the best of our knowledge, this represents the largest PAH containing at least two formal azulene units. Based on our research on diarenoperylenes, we recently found that Scholl‐type oxidations of 1 a and 1 b with both FeCl 3 in nitromethane as well as with DDQ and trifluoromethanesulfonic acid gave five‐membered ring‐cyclized products 2 a and 2 b exclusively . We envisioned combining this highly selective Scholl‐type reaction in a manner that would generate an additional seven‐membered ring (Scheme ), preferably in the same step, resulting in azulene formation during Scholl‐cyclization, something that has previously only been observed on Au(111) surfaces …”

Contorted Polycyclic Aromatic Hydrocarbons with Two Embedded Azulene Units

“…To summarize,l arger contorted and soluble PA Hs with negative curvature containing two azulene cores have been synthesized by cyclopent-and cycloheptannulation in one step.A so bserved before for another system, [20] the cyclodehydrogenation reaction is accompanied by further selective functionalization of the PA Hi nt he periphery.B esides triflyloxylation, the introduction of two dichlorovinylene groups has been found, which is,tothebest of our knowledge, unprecedented for such systems.T he OTfg roups can be transformed to aryl units under transition metal catalyzed cross-coupling conditions.H erein, we attached solubilizing mesityl groups to be able to remove the tert-butyl groups in the bay regions to study the relationship between aromaticity and the degree of curvature.T he flatter, the more aromatic the PA His. All azulene PA Hs show adistinct emission close to the near-infrared (NIR) regime between 650 and 670 nm with relatively high quantum yields f between 20 and 30 %.…”

“…To the best of our knowledge,t his represents the largest PA Hc ontaining at least two formal azulene units.B ased on our research on diarenoperylenes, [19] we recently found that Scholl-type oxidations of 1a and 1b with both FeCl 3 in nitromethane as well as with DDQ and trifluoromethanesulfonic acid gave five-membered ringcyclized products 2a and 2b exclusively. [20,21] We envisioned combining this highly selective Scholl-type reaction in amanner that would generate an additional seven-membered ring (Scheme 1), [22] preferably in the same step,r esulting in azulene formation during Scholl-cyclization, something that has previously only been observed on Au(111) surfaces. [16a] Accordingly,w es ynthesized the bisnaphthyl diareno precursor 3 from the corresponding bromide 6 [19b] by Suzuki-Miyaura cross-coupling in 67 %y ield (Scheme 2).…”

“…This is in contrast to the position of triflyloxylation we observed for the bisnaphthyl dibenzoperylene derivative,w edescribed before. [20] In contrast to the case of using FeCl 3 as oxidant, with eight equivalents of DDQ in TfOH/DCM the desired cycloheptannulation in addition to the cyclopentannulation occurred, giving am ixture of azulene embedded PA Hs 8 (14 %), 9 (30 %), and 10 (9 %), which were separated by column chromatography and identified and characterized by NMR and MS.F or all those PA Hs the cyclodehydrogenation reactions are accompanied by the selective addition of further moieties.A sr eported previously by our group, [20] an additional bis(triflyloxylation) was observed for PA H 8.However, for the system described herein, the triflyloxylation is competing with the attachment of ad ichlorovinylidene (DCV) unit giving PA Hs 9 and 10 in 30 %and 9% yield. To the best of our knowledge the introduction of DCV or similar groups at aromatic backbones under oxidative conditions is unprecedented to date during PA H-cyclization.…”