Efficient synthesis of 3,6,13,16-tetrasubstituted-tetrabenzo[a,d,j,m]coronenes by selective C–H/C–O arylations of anthraquinone derivatives

An efficient synthesis of tetrabenzo[a,d,j,m]coronene derivatives having alkyl and alkoxy substituents at the 3, 6, 13, and 16-positions was achieved based on the ruthenium-catalyzed coupling reactions of anthraquinone derivatives with arylboronates via C–H and C–O bond cleavage. The reaction sequence involving the arylation, carbonyl methylenation, and oxidative cyclization effectively provided various tetrabenzo[a,d,j,m]coronenes in short steps from readily available starting materials. Tetrabenzo[a,d,j,m]coronenes possessing two different types of substituents were obtained selectively by sequential chemoselective C–O arylation and C–H arylation. The 1H NMR spectra of the tetrabenzo[a,d,j,m]coronene product indicated its self-assembling behavior in CDCl3.


Introduction
Polycyclic aromatic hydrocarbons (PAHs) and their derivatives have attracted much attention from researchers due to their high potential as organic optoelectronic materials [1][2][3] and great efforts have been devoted to the development of their efficient synthetic methods [4]. One of the important steps in most of the PAH syntheses is the construction of carbon-carbon bonds between aryl groups to form biaryl frameworks. Traditionally, transition-metal-catalyzed cross-coupling reactions of aryl halides or pseudohalides with arylmetal reagents have been employed for the connection of two aryl units [5][6][7][8][9].
In the course of our reaction development, it was found that an introduction of two different aryl groups at the ortho-positions can be achieved by chemoselective C-O arylation of aromatic ketones possessing both C-H and C-O bonds and subsequent C-H arylation. We envisioned that the application of this strategy to the anthraquinone template would provide its derivatives possessing two different types of aryl groups in a designed manner and lead to the preparation of tetrabenzo[a,d,j,m]coronenes having two different types of substituents (Scheme 1). While 1,4,5,8-tetraarylanthraquinones prepared by the ruthenium-catalyzed C-H arylation of anthraquinone with a para-substituted arylboronate may be converted to 3,6,13,16-tetrasubstituted tetrabenzo[a,d,j,m]coronenes via carbonyl methylenation and oxidative cyclization, the two-step sequential C-O and C-H arylation of 1,4-dimethoxyanthraquinone followed by subsequent methylenation/cyclization would provide tetrabenzo[a,d,j,m]coronenes having two different types of substituents.
Here, we describe a convenient method for the synthesis of 3,6,13,16-tetrasubstituted tetrabenzo[a,d,j,m]coronenes based on a ruthenium-catalyzed C-O/C-H multiarylation of anthraquinone and its derivatives.

Results and Discussion
Our efforts toward the synthesis of tetrabenzo[a,d,j,m]coronenes started with the preparation of tetraarylanthraquinones. Previously, we reported the synthesis of anthraquinone derivatives possessing four aryl groups at the 1, 4, 5, and 8-positions through a RuH 2 (CO)(PPh 3 ) 3 -catalyzed C-H arylation of anthraquinone (1) with arylboronates 2 [16]. Using this method, we examined the synthesis of a tetraarylanthraquinone possessing four hexyloxy groups. When the reaction of 1 with 4-hexyloxyphenylboronate 2a was carried out in the presence of 20 mol % of RuH 2 (CO)(PPh 3 ) 3 (3) in refluxing pinacolone, the corresponding tetraarylation product 4aa was obtained in 57% yield (Scheme 3). Tetrakis(4-hexylphenyl)anthraquinone (4bb), the synthesis of which has been described in the above- mentioned publication [16], was also prepared using 4-hexylphenylboronate (2b) accordingly and used for further transformations. The reaction of 1 with 2b was also attempted using 10 mol % of 3, however, it only gave mono-, di-and triarylation products, and the tetraarylation product 4bb was not detected.
Next, the conversion of tetraarylanthraquinones 4 to tetrabenzo[a,d,j,m]coronenes was investigated. After examining various carbonyl methylenation methods, we found that the dimethylenation products 6 can be obtained in high yields through methylation of the carbonyl groups, followed by dehy-dration. Thus, the reaction of 4aa with methyllithium and subsequent treatment of the crude diol with NaH 2 PO 2 ·H 2 O and NaI in refluxing acetic acid gave the corresponding dimethylenation product 6aa in 96% yield (Table 1, entry 1). The oxidative cyclization of 6aa by treatment with 12 equiv of FeCl 3 at 35 °C for 0.5 h provided the desired tetrabenzo[a,d,j,m]corronene derivative 7aa in 37% yield. This method was also applicable to the conversion of tetrakis(4hexylphenyl)anthraquinone 4bb as well as the tetraarylanthraquinones containing two different aryl groups, 4ac and 4ba, leading to the corresponding tetrabenzo[a,d,j,m]corronene derivatives, 7bb, 7ac, and 7ba, respectively ( Table 1, entries 2-4).
Next, the effect of the substituents in compounds 7 on their optical properties was studied by UV-vis spectroscopy ( Figure 1). The UV-vis spectra of 7aa, 7bb, and 7ba were measured in chloroform, and the normalized UV-vis spectra are shown in Figure 1. These compounds showed similar peak patterns between 300 and 500 nm, and the π-π* transitions (p-band) for 7aa, 7bb, and 7ba were observed at 426 nm [11]. The peaks corresponding to n-π* transitions (α-band) were observed at 456-469 nm and were red-shifted with growing intensity as the number of hexyloxy groups increases.
The self-assembling property of tetrabenzo[a,d,j,m]coronene 7aa was also examined (Figure 2), as Wu and co-workers have studied the self-assembling property of the tetra-    benzo[a,d,j,m]coronene derivative possessing n-octyl groups at the 2, 7, 12, and 17-positions by 1 H NMR spectroscopy and suggested its intermolecular π-π interaction in solution [10]. When the 1 H NMR spectrum of 7aa was measured at 2 × 10 −3 M at rt, a considerable broadening of the signals in the aromatic region was observed. By increasing the temperature from rt to 60 °C, all signals in the aromatic region became sharper and low-field shifted. The dilution of the solution to 5 × 10 −4 M at rt also led to a downfield shift and sharpening of the signals. These observations suggest that 7aa assembles by intermolecular π-π interaction in CDCl 3 as was seen by Wu and co-workers for their compound possessing substituents at different positions.