Synthesis, Structures, and Properties of Hexapole Helicenes: Assembling Six [5]Helicene Substructures into Highly Twisted Aromatic Systems

Abstract

Hexapole helicenes 1, which contain six [5]helicene substructures, were synthesized by Pd-catalyzed [2+2+2]cycloadditions of aryne precursor 6. Among the possible 20 stereoisomers, which include ten pairs of enantiomers, HH-1 was obtained selectively. Density functional theory (DFT) calculations identified HH-1 as the second most stable isomer that quantitatively isomerizes under thermal conditions into the most stable isomer (HH-2). Both enantiomers of HH-2 can be separated by chiral HPLC. Single-crystal X-ray diffraction analyses revealed a saddle-like structure for (P,M,P,P,M,P) HH-1 and a propeller-like structure for (P,M,P,M,P,M) HH-2. Because of the helical assembly and the resulting steric repulsion, the structure of HH-1 is significantly distorted and exhibits the largest twisting angle reported so far (up to 35.7° per benzene unit). Electrochemical studies and DFT calculations indicated a narrow HOMO-LUMO gap on account of the extended π-system. Kinetic studies of the isomerization from HH-1 to HH-2 and the racemization of enantiomerically pure HH-2 were conducted based on H NMR spectroscopy, HPLC analysis, and DFT calculations.

Stereoselective Syntheses, Structures, and Properties of Extremely Distorted Chiral Nanographenes Embedding Hextuple Helicenes

“…Syntheses and conformational properties. It was envisioned that the chiral three-blade propeller-shaped hextuple helicene D 3 -2 of formula C 90 H 48 could be prepared by aY amamoto-type cyclotrimerization of 9,10-dibromo- [7]helicene 1 [9a] (Figure 1a). This new chiral PA He mbeds three homochiral [7]helicene units on its outer shell and three homochiral [5]helicene units of opposite configuration on its inner edges.…”

“…It was envisioned that the chiral three-blade propeller-shaped hextuple helicene D 3 -2 of formula C 90 H 48 could be prepared by aY amamoto-type cyclotrimerization of 9,10-dibromo- [7]helicene 1 [9a] (Figure 1a). This new chiral PA He mbeds three homochiral [7]helicene units on its outer shell and three homochiral [5]helicene units of opposite configuration on its inner edges. [ 7]Helicene itself has ah igh barrier to enantiomerization (178.8 kJ mol À1 ) [9b] precluding its inversion of configuration at as ignificant rate under the projected reaction conditions (ca.…”

“…Af ruitful approach to racemic triple helicenes and higher order congeners was based on cyclotrimerization reactions to forge ad istorted triphenylene core. [7,8] Using this approach, we report herein the diastereoselective and, when applicable, enantiospecific syntheses of two large and extremely distorted diastereomeric hextuple helicenes of formula C 90 H 48 .T he conformational behavior, the structure,t he chiroptical properties and the torsion-induced deficient local aromaticity in these molecules are discussed in detail, together with the exploration of the complexation ability toward silver(I) ions for one of them.…”

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We report am olecular design and concept using psystem elongation and steric effects from helicenes surrounding at riphenylene core towards table chiral polycyclic aromatic hydrocarbons (PAHs) with am aximal p-distortion to tackle their aromaticity,s upramolecular and molecular properties. The selective syntheses,and the structural, conformational and chiroptical properties of two diastereomeric large multihelicenes of formula C 90 H 48 having at riphenylene core and embedding three [5]helicene units on their inner edges and three [7]helicene units at their periphery are reported based on diastereoselective and, when applicable,enantiospecific Yamamoto-type cyclotrimerizations of racemic or enantiopure 9,10dibromo[7]helicene.B oth molecules have an extremely distorted triphenylene core,a nd one of them exhibits the largest torsion angle recorded so far for abenzenering (twist = 36.98 8).

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[19]Dendriphene: A 19‐Ring Dendritic Nanographene

“…Twenty years ago, some of us introduced the palladium‐catalyzed cyclotrimerization of arynes as an efficient synthetic procedure to obtain triphenylene derivatives . For years, this method allowed the preparation of large three‐fold‐symmetric PAHs such as starphenes and cloverphenes . When exploring the limits of this methodology as a bottom‐up approach to obtain nanographenes by solution chemistry, we encountered serious limitations to characterize the corresponding products.…”