COMMUNICATION A Quintuple [6]Helicene with a Corannulene Core as a C5- symmetric Propeller-shaped p-System Kenta Kato,[a] Yasutomo Segawa,* [a,b] Lawrence T. Scott,[c] and Kenichiro Itami* [a,b,d] Abstract: The synthesis and structural analysis of a quintuple more helicene moieties (Figure 1a). The combination of the [6]helicene with a corannulene core is reported. The compound was stable helical chirality of [5]- or [6]helicene moieties (P or M) produces a number of stable conformations as local minima, and synthesized from corannulene in three steps including a five-fold the C2- and C3-symmetric conformations of I–III and IV–VI, intramolecular direct arylation. X-ray crystallographic analysis respectively, represent the most stable structures. However, in revealed a C5-symmetric propeller-shaped structure and one- contrast to multiple helicenes, multiple corannulenes and dimensional alignment in the solid state. The enantiomers of the corannulene-helicene hybrid p-systems have not been quintuple [6]helicene were successfully separated by HPLC, and the investigated intensively, with the eXception of corannulene trimer [8] chirality of the two fractions was identified by CD spectroscopy. A VI (Figure 1b). kinetic study yielded a racemization barrier of 34.2 kcal·mol–1, which is slightly lower than that of pristine [6]helicene. DFT calculations indicate a rapid bowl-to-bowl inversion of the corannulene moiety and a step-by-step chiral inversion pathway for the five [6]helicene moieties. Nonplanar polycyclic aromatic hydrocarbons (PAHs) have I [4]helicene × 2, [5]helicene × 2 been of great interest to organic chemists and material scientists IV on account of their structural variety and unique optical, [5]helicene × 3 electronic, and physical properties.[1] Corannulene and helicenes are representative structural motifs that can be used to create a variety of nonplanar p-systems. Corannulene, a C5-symmetric bowl-shaped PAH, is a fragment of fullerenes and exhibits distinguishing features that involve concave/convex π-surfaces, thermodynamic bowl-to-bowl inversion, unusual reactivity, and electron-accepting ability.[2] On the other hand, [n]helicenes, i.e., ortho-fused helical PAHs, where n is the number of fused benzene rings, possess characteristic chiroptical properties and [3] II V dynamic behavior owing to their helical chirality. Considering [6]helicene × 2 [5]helicene × 6 the thermostability of the helical chirality, [n]helicenes (n ≥ 5) are suitable motifs for applications in chiroptical materials.[4] Moreover, several p-extended corannulenes[5] and helicenes[6] have been synthesized, and their dynamic behavior and electronic properties have been studied extensively. Recently, a new series of nonplanar PAHs, in which at least two corannulene or helicene moieties are incorporated in a fused p-system, have received substantial attention due to their complicated structures and the characteristic dynamic behavior. III VI Multiple helicenes[6p,q,r,7] are nonplanar PAHs that contain two or [4]helicene × 6, [5]helicene × 2 corannulene trimer [5]helicene × 3 Figure 1. Multiple helicenes with C2 (I–III) and C3 symmetry (IV–VI). [a] K. Kato, Prof. Dr. Y. Segawa, Prof. Dr. K. Itami Graduate Scholl of Science Nagoya University Chikusa, Nagoya 464-8602, Japan We have already reported the synthesis and structural E-mail: [email protected] (YS), [email protected] analysis of corannulene-helicene hybrid molecules (VIII and IX (KI) in Figure 1b), and discussed the effect of the concave/conveX [9] [b] Prof. Dr. Y. Segawa, Prof. Dr. K. Itami surfaces on the dynamics of the [6]helicene moieties. To shed JST-ERATO, Itami Molecular Nanocarbon Project light on the structural properties and dynamic behavior of Nagoya University corannulene-multiple helicene hybrid systems, we herein report Chikusa, Nagoya 464-8602, Japan the synthesis of the corannulene-core quintuple [6]helicene 1 [c] Prof. Dr. L. T. Scott (Figure 2). This study comprises the first example for a PAH that Merkert Chemistry Center contains five [6]helicenes in a fused p-system. Boston College Chestunt Hill, Massachusetts 02467-3860, USA [d] Prof. Dr. K. Itami Institute of Transformative Bio-Molecules (WPI-ITbM) Nagoya University Chikusa, Nagoya 464-8602, Japan Supporting information for this article can be found under: http://dx.doi.org/10.1002/anie.201701238. COMMUNICATION a) × 1 corannulene DDQ, CF3SO3H bowl-shaped π-system × 5 1 2 1 ref 5b ref 5e [6]helicene C5-symmetric propeller-shaped quintuple [6]helicene helical π-system Figure 2. Quintuple [6]helicene 1 with a corannulene core. Quintuple [6]helicene 1 was synthesized by a five-fold Pd- catalyzed intramolecular direct arylation, and its C5-symmetric propeller-shaped structure was confirmed by X-ray 3 4 crystallography. Owing to the thermostability of the helical b) chirality of the [6]helicene moieties of 1, an enantioenriched miXture of 1 was obtained by chiral HPLC, and the circular Cl Bpin Br dichroism (CD) spectrum of each enantiomer of 1 was recorded. Cl pinB The energy barrier for the enantiomerization of 1 was Cl 7 ii) determined experimentally and further analyzed by density Bpin 1 Cl functional theory (DFT) calculations. i) Cl Previously, we have attempted to synthesize 1 via an pinB 76% 10% oxidative cyclization (Scholl reaction)[10] of pentakis(biphenyl-2- Bpin Cl yl)corannulene (2). However, under these reaction conditions, 6 both the 6-membered and the 7-membered rings were 5 generated to form warped nanographene 3.[5b] Thereafter, we discovered that 1 could not be obtained from the Scholl reaction Scheme 1. a) Scholl reactions of 2, b) Synthesis of 1. Reaction conditions: i) of 2 even with an insufficient amount (4–7 equiv) of oxidant Pd2(dba)3·CHCl3, SPhos, Cs2CO3, toluene/water, 80 °C, 24 h; ii) PdCl2(PCy3)2, because formation of the 7-membered ring starts before all of DBU, DMAc, 140 °C, 3 d. Abbreviations: DDQ = 2,3-dichloro-5,6- the 6-membered rings close, which leads to the formation of 4 dicyanobenzoquinone; Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl; dba (Scheme 1a).[5e] Thus, we concluded that the Scholl reaction of 2 = dibenzylideneacetone; SPhos = 2-dicyclohexylphosphino-2',6'- should not be suitable for the synthesis of 1. dimethoxybiphenyl; DMAc = N,N-dimethylacetamide; DBU = 1,8- We envisioned that Pd-catalyzed intramolecular direct diazabicyclo[5.4.0]undec-7-ene. arylation[11,12] could be an effective alternative to synthesize 1.[11,12] Based on this strategy, we designed pentakis(2’- chlorobiphenyl-2-yl)corannulene (5) as a precursor for 1 b) (Scheme 1b). A five-fold Suzuki–Miyaura coupling of a) pentaborylcorannulene (6)[13] with 2-bromo-2’-chlorobiphenyl (7) afforded 5 in 76% yield. Then, a N,N-dimethylacetamide (DMAc) solution of 5 was stirred at 140 °C for 3 days in the presence of PdCl2(PCy3)2 and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), i.e., typical direct arylation conditions, to generate 1 in 10% yield as an orange solid. The structure of 1 was assigned on the basis of a combined 1H and 13C NMR spectroscopic analysis supported by high-resolution mass spectrometry. In the 1H NMR spectrum of 1, four signals (two doublets at 8.70 and 7.86 ppm, as well as two triplets at 7.52 and 6.79 ppm) with equal integration ratio were observed, which indicates that 1 exhibits a ten-fold symmetric structure in solution. A single-crystal X-ray diffraction analysis of racemic crystals of 1 confirmed a propeller-shaped structure and a one- Figure 3. a) ORTEP drawing of one of the two crystallographically independent molecules of 1 with thermal ellipsoids set at 50% probability, b) dimensional allignment in the solid state (Figure 3a).[14] Suitable Packing structure of 1 (red and blue: enantiomers; green: chlorine atoms of single crystals of 1 were obtained from the slow evaporation of a the chloroform). saturated chloroform solution of 1. The unit cell of these crystals contained two crystallographically independent molecules of 1, and the helical chirality of the five [6]helicene moieties in each molecule were identical (PPPPP or MMMMM) to form the The structural analysis of 1 was performed by comparing propellar-shaped structure. The enantiopairs of 1 face each the structures of corannulene and [6]helicene as reference other with their conveX sides, and seven chloroform molecules molecules. The averaged structural parameters of the experimentally observed and theoretically optimized structure of are incorporated in the cavity between the two concave faces [15] (Figure 3b). 1 together with the corresponding values of corannulene and [6]helicene[16] are summarized in Table 1. In pristine COMMUNICATION ‡ corannulene, the bonds iii and iv are symmetrically equal, and This value is slightly lower than that of [6]helicene (∆G 300 K = similarly, [6]helicene exhibits two pairs of symmetrically equal 36.2 kcal·mol–1).[20] dihedral angles (a–b–c–d/d–e–f–g and b–c–d–e/c–d–e–f). The geometric optimization of the structure of 1 was carried out by DFT calculations at the B3LYP/6-31G(d) level of theory, and the a) optimized structure reproduced the experimentally observed P P structure well, except for the slightly shallower bowl depth in the P P optimized structure. In the corannulene moiety of 1, the bonds i P P and ii are almost identical, while the bonds iii, iv, and v are P P [17] longer, and the bowl depth is shallower than those of pristine P M corannulene. The long bond v and the shallow bowl depth of 1 are consistent with previously reported values for benzannulated corannulenes.[5,18] In the [6]helicene moieties, the dihedral angle A (PPPPP) B (PPPPM) –1 9.2 kcal·mol–1 c–d–e–f is smaller, while the others are bigger than the 0.0 kcal·mol corresponding values in [6]helicene.