Configurational Stability of [5]Helicenes

Abstract

A series of [5]helicenes difunctionalized in the fjord region with either fluoro, methoxy, or methyl groups was synthesized via photochemical and benzylic coupling route. Resolution of each compound into enantiomers and determination of the Gibbs activation energies of enantiomerization (ΔG(T)) revealed high configurational stability in all three cases. The ΔG(T) values of difunctionalized [5]helicenes were compared with those of their monofunctionalized analogues and the parent [5]helicene. Within this series, an exponential correlation between the torsional twist and ΔG(T) was found. The dimethyl derivative exhibits one of the highest configurational stabilities among [n]helicenes reported to date, comparable to that of [9]helicene.

Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch

“…In addition to improving the stability of c- 1b , the methyl substituents expedite the synthesis of the [5]helicene core as well as increase its configurational stability. 16 Our present results demonstrate that 13,14-dimethylcethrene can be switched reversibly between an open (o- 1b ) and a closed (c- 1b ) form by light, and we introduce this system as a prototype of a diradicaloid photoswitch as well as the first example of a carbohelicene-based chiroptical photoswitch. 17 During the switching process, the (1) HOMO–LUMO and (2) ST gaps as well as (3) the degree of helical twist are altered simultaneously, leading to significant changes in the optical and chiroptical properties.…”

“…Compound 5 was described 16 previously and was prepared according to the published protocols. All chemicals and solvents were purchased from commercial sources and were used without further purification unless stated otherwise.…”

Hierarchy of Asymmetry in Chiral Supramolecular Polymers: Toward Functional, Helical Supramolecular Structures

“…On the other hand, helicenes with n ≥6 are configurationally stable, with Gibbs activation energies of enantiomerization (Δ G ≠ ( T )) larger than 40 kcal mol −1 . The case of [5]helicenes is very appealing because the configurational stability depends on substitution at the fjord region . We have investigated the configurational stability of self‐assembling [5]helicenes to demonstrate that this property notably increases upon supramolecular polymerization.…”

Synthesis of [7]Helicene Enantiomers and Exploratory Study of Their Conversion into Helically Chiral Iodoarenes and Iodanes

“…[47] Such a bridged [5]helicene is configurationally locked and showed no racemization even after prolongedh eatinga t8 0 8C. [47] We assume that the usual thermal instability of regular [5]helicenes, which can slowly racemize even at room temperature, [70][71][72] was the reason why the authors did not attempt to cleave off the chiral tether.H owever,s ince the racemization barriers of higher helicenes require temperatures wella bove 100 8C, [73][74][75] we thought of employing as trategy adapted from that of Marinettia nd her co-workers [47] to build helicenes that could fulfill our needs and wishes.…”