In‐Fjord Substitution in Expanded Helicenes: Effects of the Insert on the Inversion Barrier and Helical Pitch

Abstract

A series of expanded helicenes of different sizes and shapes incorporating phenyl-and biphenyl-substituents at the deepest part of their fjord have been synthesized via sequential Au-catalyzed hydroarylation of appropriately designed diynes, and their racemization barriers have been calculated employing electronic structure methods. These show that the overall profile of the inversions (energies, number of transition states and intermediates, and their relative position) is intensively affected by the interplay of steric and attractive London dispersion interactions. Hence, in-fjord substitution constitutes an additional tool to handle the mechanical properties in helicenes of uncommonly large diameter. The photochemical characterization of the newly prepared helical structures is also reported.

Expanded [23]-Helicene with Exceptional Chiroptical Properties via an Iterative Ring-Fusion Strategy

“…Yashima and co-workers have shown that fusing multiple helical anthracenes together also effectively enables the separation of enantiomers, but these multiple helicenes display inferior | g abs | of ≤0.015 . Notably, the original expanded helicenes illustrated in Figure a have larger diameters than helical anthracenes (∼13.5 vs ∼11.5 Å), suggesting that they should show further improvement in | g abs |; however, a similar noncovalent locking strategy was attempted on such an expanded helicene that failed to provide configurational stability …”

“…32 Notably, the original expanded helicenes illustrated in Figure 1a have larger diameters than helical anthracenes (∼13.5 vs ∼11.5 Å), suggesting that they should show further improvement in |g abs |; however, a similar noncovalent locking strategy was attempted on such an expanded helicene that failed to provide configurational stability. 33 Lengthening the helicene backbone also increases ΔG ‡ e , but this poses a considerable synthetic challenge. Since Newman's landmark synthesis of [6]helicene in 1956, 34 there has been significant effort devoted to this challenge.…”

Consecutively fused single‐, double‐, and triple‐expanded helicenes

“… 43–45 Since then, several expanded helicenes with unique spiral frameworks have been designed and synthesized, including expanded helicene‐based macrocycles ( II ), 46 a helical analogue of kekulene ( III ), 47 helical anthracenes ( IV ), 48,49 and in‐fjord‐substituted expanded helicenes ( V ) (Figure 1a). 50 …”

“…[43][44][45] Since then, several expanded helicenes with unique spiral frameworks have been designed and synthesized, including expanded helicene-based macrocycles (II), 46 a helical analogue of kekulene (III), 47 helical anthracenes (IV), 48,49 and in-fjord-substituted expanded helicenes (V) (Figure 1a). 50 For the synthesis of expanded helicenes with a larger helical diameter than the corresponding classical helicenes, a substantial number of aromatic rings are required to be fused by intramolecular multifold cyclizations, which often afford a complicated product mixture due to incomplete cyclizations and/or undesired side reactions, resulting in low yields after a time-consuming isolation process. 43,[45][46][47][48][49][50] Therefore, the development of a novel synthetic approach for quantitatively producing expanded helicenes through a complete annulative π-extension of aromatization precursors is highly demanded for exploring an unexplored domain of helically twisted PAHs, which will provide a novel class of expanded helicenes with specific chiroptical, optical, electrochemical, and physical properties [43][44][45][46][47][48][49][50] inaccessible from the angularly fused typical single 17,18 and multiple [19][20][21][22][23] helicenes.…”

“…On the other hand, the previously reported expanded helicenes have a certain structural flexibility because the helicene subunits are partially composed of linearly fused aromatic rings with a low helicity inversion barrier, and hence readily racemized. 43,47,50 Successful examples of the isolation of optically active expanded helicenes are, to the best of our knowledge, limited to only two precedents, II and IVa, reported by the groups of Tilley 46 and Toyota, 48 which, however, gradually racemized in solution.…”