Symmetric, Unsymmetrical, and Asymmetric [7]‐, [10]‐, and [13]Helicenes

Abstract

Fully aromatic helicenes with more than one pitch‐length are illustrious synthetic targets with potential applications in advanced optical devices and nano‐electronics. The task of extending the length of fully conjugated helicenes past one pitch length is challenging. Now, the synthesis of a series of azaoxa[7]‐, [10]‐, and [13]helicenes is described. The synthesis is based on iterative oxidative furan formation between 3,6‐dihydroxycarbazoles and/or 2‐naphthols. The flexibility of the presented method allows the convenient and scalable synthesis of symmetric, unsymmetrical, and asymmetric homo‐chiral structures. The [13]helicenes can be synthetically functionalized both at the termini and the periphery. The full range of helicenes were characterized using NMR and optical spectroscopy (UV/Vis, fluorescence, and CD) along with single‐crystal X‐ray crystallography. The enantiomers of the [13]helicenes are the longest optically pure helicenes isolated to date.

Compressing a Non‐Planar Aromatic Heterocyclic [7]Helicene to a Planar Hetero[8]Circulene

“…From the first oxidation of the series of [7]helicenes, the dehydro[7]helicenes and the [9]circulenes it is apparent that the sulfones ( 7‐SO 2 , 8‐SO 2 and 9‐SO 2 ) are more difficult to oxidize than their corresponding sulfides ( 7 , 8 and 9 ). The first oxidation potential for the [8]circulenes ( 9 and 9‐SO 2 ) are comparable to previously determined oxidation potentials for diazadioxa[8]circulenes (0.63–0.67 V), tetraoxa[8]circulenes (0.72–1.10 V), and azatrioxa[8]circulenes (0.65–0.67 V) …”

“…From the first oxidation of the series of [7]helicenes, the dehydro [7]helicenes and the [9]circulenes it is apparent that the sulfones (7-SO 2 , 8-SO 2 and 9-SO 2 ) are more difficultt oo xidize than their corresponding sulfides (7, 8 and 9). The first oxidation potentialf or the [8]circulenes (9 and 9-SO 2 )a re comparable to previously determined oxidation potentials for diazadioxa [8]circulenes( 0.63-0.67 V), [14] tetraoxa [8]circulenes( 0.72-1.10 V), [25] and azatrioxa [8]circulenes (0.65-0.67 V). [26] Both the [7]helicenes (7 and 7-SO 2 )a nd the dehydro [7]helicenes (8 and 8-SO 2 )c omprise intrinsic axial chirality and hence the ability to racemize thermally.U nsuccessful attempts of resolving 7, 7-SO 2 , 8 and 8-SO 2 using HPLC analysisw ith chiral stationary phases (columns of LUX cellulose type from Phenomenex were tested, in different solvents ystemsM eCN/ MeOH/H 2 Om ixtures with 0.1 %H COOH or TFA) indicate that the compounds racemizer apidly at room temperature or that we simply have not identifieda na ppropriate combinationo f stationary and mobile phase to achieve theseseparations.…”

“…Omission of BF 3 ⋅OEt 2 did not result in conversion, and increasing the temperature above 0 °C, resulted in a loss of regioselectivity due to formation of the 2,4’‐coupled isomer (for more details see Supporting Information). Interestingly, no formation of furan by oxidative dehydration was observed, despite this being the major product for 2,7‐di‐ tert ‐butyl analogues of hydroxycarbazole 11 . To install the annulating thiophene heterocycle, attention was turned to the thermal Newman–Kwart rearrangement reaction, which has proven useful in synthesis of thia[ n ]helicenes from biarylic 2,2’‐Newman–Kwart substrates (bis‐ O ‐aryl‐thiocarbamates) .…”

“…Interestingly,n of ormation of furan by oxidative dehydration was observed, despite this being the major product for 2,7-di-tert-butyla nalogues of hydroxycarbazole 11. [6,14] To install the annulating thiophene heterocycle, attention was turned to the thermal Newman-Kwart rearrangement reaction, which has proven usefuli ns ynthesis of thia[n]helicenes from biarylic 2,2'-Newman-Kwart substrates (bis-O-aryl-thiocarbamates). [19] Heating bis-O-arylthiocarbamate (13)a t3 00 8Cf or 25 minutes gave both the rearranged product (14)a nd the diazathia [7]helicene (15).…”

Anti‐Aromatic versus Induced Paratropicity: Synthesis and Interrogation of a Dihydro‐diazatrioxa[9]circulene with a Proton Placed Directly above the Central Ring

“…Previously we have reported on the DDQ-mediated oxidative hetero-coupling between diazadioxa[10]helicene 5 and 7-methoxy-2-naphthol (6)g iving 7 (Scheme 1). [19] In this work it is presented how,byconducting the reaction without naphthol 6,w ea ccess the intramolecular oxidative coupling product 8 in very high yield. Compound 8 contains a 7-methoxy-dihydronaphthalene orthogonally positioned above a [ 9]circulene (4)s ystem (highlighted in blue for structure 8,s ee the box in Scheme 1).…”