Synthesis of Terminally Fluorinated [7]Helicenes and Their Application to Photochemical Domino Reactions

Abstract

The intramolecular Diels−Alder reactions of helicenes deform their π‐conjugated screw‐shaped skeletons. In particular, terminally tetrafluorinated [7]helicene (F4‐[7]helicene) undergoes a photoinduced Diels−Alder reaction followed by a photoinduced double fluorine atom transfer. Herein, we thoroughly investigated this photochemical domino process by decreasing the level of fluorine substitution. F3‐[7]Helicenes bearing two fluorine atoms at the dienophile terminal underwent photoinduced Diels−Alder reactions, but the whole domino process became slow. F2‐[7]Helicene, which is difluorinated only at the dienophile terminal, was also photolabile. As a result, two fluorine atoms were sufficient for the photochemical domino reaction to occur. X‐ray crystallographic analysis revealed that F2‐[7]helicene was less compressed than F4‐[7]helicene, indicating that terminal polyfluorination enhanced the intramolecular arene−fluoroarene stacking interactions and thus promoted the transformations.

Effect of Terminal Fluorination on Chiroptical Properties of Carbo[5–8]helicenes: A Systematic Computational Study at the RI-ADC(2) Level

“…On the other hand, changes in ϕ 2 are very small for all the four systems in this case. It is observed that 2F7H shows similar structural features to that of experimentally reported homologous 1,2-difluoro[7]­helicene . Values of R pitch increase by 0.02 Å from 7H to 2F7H .…”

“…Keeping this in mind, several modifications have been proposed in recent times ,,, aiming at improving the g -factor and Φ FL . Typical modifications are based on either substitution of hydrogens in helicenes by other atoms/groups − or fusion of aromatic rings onto the helical backbone to produce ring-fused carbo­[ n ]­helicenes. − Recently, Kubo et al reported dimethoxy and dicyano substituted-carbo[7]­helicenes with improved g CPL values compared to the results for parents systems. While 3.5- and 6-fold increments were obtained for Φ FL and fluorescence rate constant ( K f ), respectively, for dicyanocarbo[5]­helicene, ≥4-fold increments were obtained for the dicyano-carbo[7]­helicene .…”

“…While 3.5- and 6-fold increments were obtained for Φ FL and fluorescence rate constant ( K f ), respectively, for dicyanocarbo[5]­helicene, ≥4-fold increments were obtained for the dicyano-carbo[7]­helicene . Similar to pristine carbohelicenes, Φ FL was also found to decrease for substituted-carbohelicenes with increase in chain length. , Halogenation of carbohelicene rings is another strategy to tune the chiroptical properties. , Synthesis of various fluorinated carbohelicenes are reported in the literature. ,,,− In the work of Nakai et al, a systematic improvement in CD intensities of the 1 B b band was observed along with their corresponding dissymmetry factors, after substitutions by F, Cl, and Br atoms. This was attributed to increase in | μ | and | m |, and a small angle between the two vectors.…”

Skeletal Transformation Triggered by C−F Bond Activation after Photochemical Rearrangement of Fluorinated [7]Helicenes

“…The F4 atom obviously accelerates the latter S N Ar process through its strong electron‐withdrawing inductive effect. As a control experiment, we next investigated the reactivity of 2F‐transferred compound 2 b prepared from the photochemical rearrangement of F 3 ‐[7]helicene 1 b (Scheme 2; X=H) [4b] . As with 2 a , the silica gel activated the allylic C−F1 bond of 2 b , but the product in this case was difluorinated ketone 4 b (69 %).…”

Reactivity and Mechanistic Issues in the Photocyclization of Dihalostyryl‐Naphthalenes towards Halo‐[4]helicenes: A Transposition on a Mallory Theme

“…Hypothesizing that this unprecedented elimination of HX should be halogen dependent, we have decided to extend the scope of the reaction towards difluoro and dichloro derivatives 1 b and 1 c, since the Mallory reaction is normally compatible with fluoro and chloro substituents. [1,4,25] Accordingly, starting from the phosphonium salt 3 and using 2,3-difluoro and 2,3dichlorobenzaldehyde we have obtained the two stilbenes 1 b and 1 c as a mixture of Z and E isomers (Scheme 3). Subsequent photocyclization of 1 b and 1 c provided strikingly different results compared to 1 a (Scheme 6).…”