Synthesis and Structural Features of Quadruple Helicenes: Highly Distorted π Systems Enabled by Accumulation of Helical Repulsions

Abstract

Quadruple helicenes, bearing dithia[6]helicene and [5]helicene substructures, were prepared by a well-controlled Scholl reaction. The 4-fold helicity provides 9 stereoisomers including 4 pairs of enantiomers and 1 meso isomer. Among them, differently distorted structures of a propeller-shaped isomer (QH-A) and a saddle-shaped isomer (QH-B) were unambiguously determined by X-ray crystallography. Especially in the latter isomer, a proper accumulation of repulsions on the helical substructures twisted the naphthalene core to the limit (69.5°), the highest degree of twisting deformation per benzene unit (35.3° at the most). Photophysical and electrochemical studies showed a broadened HOMO-LUMO gap and a HOMO of QH-B lying lower compared to those of QH-A. These results together with the density functional theory (DFT) calculations have clearly demonstrated the electronic state dependency on the molecular geometry. Additionally, kinetic studies of the isomerization between these isomers using (1)H NMR, circular dichroism, and DFT calculations shed light on the interconversion pathways among the stereoisomers. The height of barriers in the inversion of a certain helical substructure may be affected by the neighboring helical substructures.

Synthetic Applications of Oxidative Aromatic Coupling—From Biphenols to Nanographenes

“…Similar to 202 , the product was also obtained as a mixture of three diastereoisomers. An analogous reaction of a thiophene‐based precursor with MoCl 5 in dichloromethane over 4 Å molecular sieves afforded the not fully cyclized tetrahelicene 204 instead of the expected double helicene . The formation of 204 was kinetically controlled; upon heating, the propeller‐like arrangement of its thiophene‐containing wings underwent quantitative isomerization to the less‐strained diastereoisomer with the wings alternately tilted “up” and “down” relative to the central naphthalene plane.…”

“…[232] Similar to 202,t he product was also obtained as am ixture of three diastereoisomers.A na nalogous reaction of at hiophene-based precursor with MoCl 5 in dichloromethane over 4 molecular sieves afforded the not fully cyclized tetrahelicene 204 instead of the expected double helicene. [233] The formation of 204 was kinetically controlled;upon heating, the propeller-like arrangement of its thiophene-containing wings underwent quantitative isomerization to the less-strained diastereoisomer with the wings alternately tilted "up" and "down" relative to the central naphthalene plane.Repeating the reaction with MoCl 5 at room temperature without the addition of molecular sieves led to the formation of the two remaining C aryl À C aryl bonds,thereby providing an analogue of 203 (double helicene) containing four chlorine atoms as aconsequence of the MoCl 5 -mediated chlorination. [234] Octa(4-tert-butylphenyl)biphenylene undergoes efficient oxidation with DDQ/MsOH to form the distorted PA H 205, which is based on a [ 7]helicene moiety with one fourmembered ring (Figure 17).…”

Racemic charge-transfer complexes of a helical polycyclic aromatic hydrocarbon molecule

“…[4][5][6] Recently, nonplanar π-conjugated molecules have attracted attention as components for the assembly of an unconventional π-electron network. [7][8][9] Two main approaches are utilised to produce nonplanar PAH molecules: endoskeletal and exoskeletal approaches. In the former approach, a nonhexagonal ring is present inside hexagonal skeletons (e.g., [n]circulenes; 10 Scheme 1a), while the latter approach exploits steric hindrance because of crowdedness (e.g., [n]helicenes; 11,12 Scheme 1b).…”

Twisted Polycyclic Arenes from Tetranaphthyldiphenylbenzenes by Controlling the Scholl Reaction with Substituents

“…Compound 2 c is a multiple helicene containing two [5]helicene and four [4]helicene moieties, which present local helical structures as a consequence of the repulsive steric interactions between terminal benzene rings in the ortho ‐fused backbone. The combination of local helicity in a multiple helicene provides molecular dynamics and defines the three‐dimensional molecular shape as well as molecular packing in the crystalline state . Here, we define the multiple helicity of 2 a – c as ( H 1, H 2)‐( h 3, h 4, h 5, h 6), where the helicity notions in the first pair of parentheses refer to the [5]helicene moieties and those in the second pair of parentheses refer to the [4]helicene moieties as depicted in Figure .…”

“…The combination of local helicity in am ultiple helicene [11] provides molecular dynamics andd efines the three-dimensional molecular shape as well as molecularp acking in the crystalline state. [12] Here,w ed efine the multiple helicity of 2a-c as (H1, H2)-(h3, h4, h5, h6), where the helicity notionsi nt he first pair of parentheses refer to the [5]helicene moieties andt hose in the second pair of parentheses refer to the [4]helicenem oieties as depicted in Figure 1. Havingabenzannulated pentacene backbones ubstituted with two phenylg roups, 3c exhibits an end-to-end twist as found from the energy-minimized model ( Figure S3 in the SupportingI nformation).…”