Expanded Helicenes: A General Synthetic Strategy and Remarkable Supramolecular and Solid-State Behavior

Abstract

A divergent synthetic strategy allowed access to several members of a new class of helicenes, the "expanded helicenes", which are composed of alternating linearly and angularly fused rings. The strategy is based on a three-fold, partially intermolecular [2+2+n] (n = 1 or 2) cycloaddition with substrates containing three diyne units. Investigation of aggregation behavior, both in solution and in the solid state, revealed that one of these compounds forms an unusual homochiral, π-stacked dimer via an equilibrium that is slow on the NMR time scale. The versatility of the method was harnessed to access a selenophene-annulated expanded helicene that, in contrast to its benzannulated analogue, exhibits long-range π-stacking in the solid state. The new helicenes possess low racemization barriers, as demonstrated by dynamic H NMR spectroscopy.

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

“…Furthermore it has been used to prepare aza[ n ]helicenes such as 2 . Recently the transition‐metal‐mediated [2+2+2]‐cycloisomerization has been utilized for the synthesis of long helicenoids that have regions of linear annulation being part of the helical backbone, as in 3 , or the inclusion of 2H ‐pyranes as in oxa[19]helicenoid ( 4 ) . Using furan formation as the annulating step allows for the convenient preparation of oxa[ n ]helicenes, such as 5 and 6 .…”

“…[15] Recently the transition-metal-mediated [2+ +2+ +2]cycloisomerization has been utilized for the synthesis of long helicenoids that have regions of linear annulation being part of the helical backbone,asin3,orthe inclusion of 2H-pyranes as in oxa [19]helicenoid (4). [17][18][19] Using furan formation as the annulating step allows for the convenient preparation of oxa[n]helicenes,such as 5 and 6. [20,21] Herein we describe how an annulative p extension (APEX) benzofuran scaffolding, between 3,6-dihydroxy-carbazoles (7)a nd 2-naphthols (8), allows us to construct long heterocyclic helicenes:d iazaoxa-[7]-, diazadioxa[10]-, and diazatrioxa [13]helicenes (9-11;F igure 1b).…”

Stereocontrolled Synthesis of Chiral Heteroaromatic Propellers with Small Optical Bandgaps

“…Embedding helical motifs into polyaromatic structures [1] provides access to inherently curved and multilayered aromatic molecules with exceptional chiroptical properties and the potential to be used in organic electronics. [2][3][4][5][6] Recent activity in this field has produced adiverse range of new chiral aromatics,i ncluding helicenes fused to large polycyclic structures, [7][8][9][10][11] helically hindered PA Hs, [12,13] helical nanoribbons, [2][3][4] and multi-helicene architectures. [14][15][16][17][18][19][20][21][22][23][24][25][26][27] Inclusion of heteroatoms or nonbenzenoid rings in such systems can have ap rofound effect on their electronic and chiroptical characteristics.H owever,s uch modifications are synthetically challenging and thus rarely attempted.…”

Stereocontrolled Synthesis of Chiral Heteroaromatic Propellers with Small Optical Bandgaps

“…Embedding helical motifs into polyaromatic structures provides access to inherently curved and multilayered aromatic molecules with exceptional chiroptical properties and the potential to be used in organic electronics . Recent activity in this field has produced a diverse range of new chiral aromatics, including helicenes fused to large polycyclic structures, helically hindered PAHs, helical nanoribbons, and multi‐helicene architectures . Inclusion of heteroatoms or nonbenzenoid rings in such systems can have a profound effect on their electronic and chiroptical characteristics.…”