Build-up of double carbohelicenes using nitroarenes: dual role of the nitro functionality as an activating and leaving group†

The construction of double carbohelicenes is highly fascinating yet challenging work. Disclosed herein is a streamlined and simplified synthetic route to double carbohelicenes starting from nitroarenes through sequential nitro-activated ortho-C–H arylation, denitrative alkenylation and intramolecular cyclodehydrogenation. In this synthetic strategy, the nitro group plays a dual role namely as a leaving group for the denitrative alkenylation and as an activating group for ortho-C–H arylation, which is distinct from those of aryl halides in a conventional coupling reaction. In this work, the palladium-catalyzed Heck-type alkenylation of nitroarenes has been presented, in which the conventionally inert Ar–NO2 bond is cleaved. This work provides a novel synthetic strategy for polycyclic aromatic hydrocarbons (PAHs).


Introduction
Nonplanar polycyclic aromatic hydrocarbons (PAHs) have attracted increasing attention in the past decade because of their unique optoelectronic properties, fascinating supramolecular characteristics and conformational dynamics, distinct from those of planar PAHs. 1 Helicenes, helical-shaped structures with ortho-fused benzene rings, are one of the most investigated nonplanar PAHs. 2,3 In particular, considerable effort has been devoted to the helicenes with multihelicity, which can provide more diverse conformations, increased nonplanarity and multidimensional intermolecular interactions. 4 Among these multi-helicenes, double helicenes, which usually exhibit effective intermolecular p-p stacking, are regarded as promising candidates for application in organic semiconductors and chiroptical devices. 5 Although a variety of double helicenes have been reported (Scheme 1a), 5-7 the synthesis of double carbohelicenes typically suffers from inaccessible precursors and tedious synthetic routes. Therefore, the development of facile preparation of double carbohelicenes starting from easily available substrates remains challenging yet highly demanded.
Recently, the development of transition metal-catalyzed reactions using unconventional coupling partners has emerged as an attractive and challenging topic. 8 Reactions involving the cleavage of inert chemical bonds can not only replenish the carbon-carbon bond formation toolbox but also improve synthetic approaches to complex structures. Nitroarenes, which could be easily prepared by nitration of aromatics, are common and cheap chemical feedstocks. 9 Because of the strong electron-withdrawing nature of the nitro group, the ortho-position of nitroarenes easily undergoes C-H bond arylation with aryl halides via palladium catalysis. 10 We envisioned that if a denitrative Heck-type reaction could be developed, double helicenes would be easily accessed from simple nitroarenes by ortho-C-H arylation, two consecutive denitrative alkenylation and intramolecular cyclodehydrogenation, which avoids the preparation of di-or multihalogenated arenes (Scheme 1b, path II). However, because of the difficulty of oxidative addition of Ar-NO 2 to a metal center, nitroarenes are typically inert in conventional transition metalcatalyzed coupling reactions. 11c Recently, pioneered by Nakao, the palladium-catalyzed denitrative amination, arylation, alkylation, alkynylation and hydrogenation of nitroarenes have been reported. 10c,11 Herein we wish to describe a streamlined and simplied synthetic route to double carbohelicenes starting from nitroarenes based on our investigations on the palladiumcatalyzed denitrative alkenylation of nitroarenes. During the preparation of this work, Yamaguchi and co-workers reported a palladium-catalyzed Mizoroki-Heck reaction of nitroarenes and styrene derivatives. 12
With the denitrative Heck-type reaction in hand, the construction of double carbohelicences was next investigated.
The crystals of 5d were grown by slowly volatilizing a saturated dichloromethane solution and the helical structure was clearly conrmed by X-ray crystallographic analysis (Fig. 1a). 13 The geometry of helicene 5d slightly deviates from the C 2 symmetry, which is reected by the unequal torsion angles of the two [5]helicene substructures. The dihedral angles between the terminal rings A and D and rings B and C are 60.2 and 60.5 , respectively (Fig. 1b), which are larger than those of recently presented double carbo[5]helicene (47.8 ) 7b and OBOfused double [5]helicene (48 ). 5d Additionally, because of the repulsion of terminal benzene rings, the central naphthalene ring is distorted with dihedral angles of 32.0 for a-b-e-d and 31.8 for c-b-e-f, which are among the largest twisting deformations of the naphthalene ring in a nonplanar PAH. 15 The embedded naphthalene unit (C-C bond lengths: 1.39-1.46 A) possesses less aromaticity than pristine naphthalene (1.36-1.42 A) presumably owing to the twisted structure. This observation is consistent with the computational results of the nucleusindependent chemical shi (NICS) and anisotropy of the induced current density (ACID) (Fig. 1c). As shown in Fig. 1d, the HOMO and LUMO energies of 5d were estimated by molecular orbital (MO) calculation to be À5.20 and À1.55 eV, respectively, corresponding to a HOMO-LUMO gap of 3.65 eV, which is larger than that of the carbo[5]helicene (3.41 eV) and double carbo[5]helicenes reported by Miao (2.73 eV) 5a and Kamikawa (2.92 eV). 7b This result might be attributed to the decreased p-conjugation induced by the signicant geometric distortion of the central naphthalene ring. In the crystal packing of 5d, one unit cell contains four pairs of (P,P)-and (M,M)-enantiomers (Fig. 1e). The homochiral isomer layers lying on the bc plane stack alternatively along the a-axis (Fig. 1f), with a slipped face-to-face p-p distance of 3.67 A to the adjacent heterochiral layer (Fig. 1e). In addition, every molecule interacts with the neighboring homochiral molecules through edge-toface contact within the range of 2.78 to 2.99 A (Fig. 1e).
The thermal stability and isomerization process of 5d were evaluated by DFT calculations at the B3LYP/6-31G(d) level (Fig. 2). The (P,P)-and (M,M)-enantiomers possess the same thermodynamic stability, but are more stable than the (P,M)enantiomer. This result coincides with the fact that only (P,P)and (M,M)-enantiomers were observed in the single crystals of 5d, whereas no (P,M)-enantiomer was observed. The (P,P)-to-(M,M) interconversion proceeds through the transition states TS-1 and TS-2, in which the terminal benzene rings stretch outward and bend inward. The calculated activation free energy (approximately 33.0 kcal mol À1 ) was much higher than that of carbo[5]helicene (22.9 kcal mol À1 ), 16 and slightly higher than that of the [5,5]helicenes reported by Miao (approximately 28.6 kcal mol À1 ) 5a and Kamikawa (31.8 kcal mol À1 ). 7b Because the calculated activation free energy approximates to that of carbo[6]helicene (35.0 kcal mol À1 ), 16 the racemization process of 5d might be slow.

Conclusions
In conclusion, we have presented the promising potential of nitroarenes in the construction of polycyclic aromatic hydrocarbons (PAHs) based on our investigations on the palladiumcatalyzed denitrative alkenylation reaction. The nitro group can act as a leaving group for the denitrative alkenylation and an activating group for ortho-C-H arylation, which provides a rapid access to double carbohelicenes through sequential nitro-activated ortho-C-H arylation, two consecutive denitrative alkenylation and intramolecular cyclodehydrogenation. Further applications of this strategy in the construction of other twisted and p-extended aromatic systems are underway in our laboratory.

Conflicts of interest
There are no conicts to declare.   This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020, 11, 7424-7428 | 7427