LUO Zai-Gang LIU Qian-Nan LI Rui XU Xue-Mei

(College of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, China)

ABSTRACT The target compound (C18H14O4) was synthesized and structurally determined by single-crystal X-ray diffraction. The crystal is of monoclinic system, space group P21/n with a = 7.9383(6), b = 19.5083(13), c = 9.4189(7) ?, β = 98.981(2)°, C18H14O4, Mr = 294.29, Dc = 1.357 g/cm3, V = 1440.75(18) ?3, Z = 4, F(000) = 616, μ(MoKa) = 0.096 mm-1, T = 293(2) K, 3320 independent reflections with 1845 observed ones (I > 2σ(I)), R = 0.0529 and wR = 0.1082 with GOF = 1.015 (R = 0.1202 and wR = 0.1278 for all data). The phenanthrene skeleton is coplanar. The overall crystal packing revealed that it has reverse staggered parallel and stratified arrangement. And a one-dimensional interaction model was formed by two kinds of π-π interactions between the two adjacent molecules at upper and lower levels. The decomposition mechanism of compound 3 is complex.

Keywords: phenanthrene, synthesis, crystal structure, thermal property;

1 INTRODUCTION

Polycyclic aromatic hydrocarbons (PAHs), such as phenanthrene derivatives, are of interest for scientists in several areas of research, from supramolecular chemistry to environmental chemistry or materials science[1-3]. Also, polyfunctionalized PAHs have attracted much attention because of their stability, solubility and optical properties[4-6]. Owing to their significant importance, numerous synthetic approaches to phenanthrene derivatives were reported[7,8]. Among them, the intermolecular [4+2] benzannulation between a functionalized biaryl derivative and an alkyne has received much attention as a simple and straightforward method for the construction of polycyclic aromatic phenanthrene units. The first example is the Pd-catalyzed [4+2] benzannulation reaction for the synthesis of 9,10-diphenylphenanthrene[9]. And then, the Pd-catalyzed decarboxylative reaction between 2-arylbenzoic acids and alkynes[10], iron-catalyzed reaction between 2-biaryl magnesium bromides and alkynes[11], iridium-catalyzed coupling of 2-arylbenzoyl chlorides with alkynes[12], rhodium- catalyzed oxidative benzannulation of (2-arylphenyl)boronic acids with alkynes[13], base-promoted homolytic aromatic substitution of 2-aminobiaryls with alkynes[14]and so on[15], have also been reported extensively. In addition, benzannu- lation of phthalic anhydrides[16]or 2-(trimethylsilyl)phenyl trifluoromethanesulfonate[17]with dimethyl but-2-ynedioate to dimethyl phenanthrene-9,10-dicarboxylate (3) was also performed using a palladium catalytic system. However, these reactions usually require harsh reaction conditions involving high pressure, strong acid or base, or the use of complicated compounds as substrates. Therefore, to develop easily operational and highly effective methods for the construction of phenanthrene derivatives is highly desirable.

On the other hand, phenylboronic acids as cheap, safe, and widely accessible starting materials are precious components in metal-catalyzed cross-coupling reactions. Herein, we report the first example for the synthesis of dimethyl phenan- threne-9,10-dicarboxylate (3) using phenylboronic acid and dimethyl but-2-ynedioate as starting materials in the presence of Ag2CO3as the catalyst and K2S2O8as the oxidant, as shown in Scheme 1. The structure was confirmedvia1H NMR,13C NMR and HRMS. Meanwhile, the crystal structure of compound 3 was determined by X-ray single-crystal diffraction analysis. Besides, thermal property of dimethyl phenanthrene-9,10-dicarboxylate (3) was also discussed in the text.

Scheme 1. Synthesis of dimethyl phenanthrene-9,10-dicarboxylate (3)

2 EXPERIMENTAL

2. 1 Instruments and reagents

The melting point was measured on a SGW X-4 monocular microscope melting point apparatus with thermometer unadjusted.1H NMR and13C NMR spectra were acquired on a 500 MHz Bruker Avance spectrometer with CDCl3as solvent. H RMS was obtained by ESI on a TOF mass analyzer. X-ray diffraction was performed using a Bruker Smart Apex CCD diffractometer. The target compound was characterized by thermogravimetric analysis (TGA) on a Perkin Elmer thermogravimetric analyzer heating from room temperature to 400 ℃ at a rate of 10 ℃/min under N2atmosphere.

Unless otherwise noted, all materials were obtained from commercial suppliers and purified by standard procedures. Column chromatography was performed with silica gel (200～300 mesh, Qingdao Haiyang Chemical Co., Ltd, China).

2. 2 Synthesis of the title compound 3

General synthetic approach is described as follows: A 25 mL Schlenk tube charged with phenylboronic acid (1, 1 mmol), dimethyl but-2-ynedioate (2, 0.5 mmol), Ag2CO3(0.03 mmol), K2S2O8(4 mmol) and PhF/H2O (3:1, 2 mL) was sealed, and then the mixture was stirred under air at 60 oC for 0.5 h. After cooling, the solvent was diluted with water (5 mL) and extracted with dichloromethane (3 × 5 mL). The combined organic layers were dried over anhydrous Na2SO4and concentrated by a rotary evaporator, and the residue was purified by column chromatography on silica gel (petroleum ether/ethyl acetate 10:1) to provide the desired product (3), obtaining 85 mg of the fine product as white powder. Yield: 58%. m. p.: 129～130 ℃.1H NMR (CDCl3, 500 MHz,δppm) 8.72 (d, 2H,J= 8.0 Hz), 8.16 (dd, 2H,J= 1.0, 8.0 Hz), 7.65～7.77 (m, 4H), 4.04 (s, 6H);13C NMR (CDCl3, 125 MHz,δppm) 168.34, 131.08, 129.90, 128.48, 127.66, 127.08, 126.90, 122.87, 52.77; HRMS (ESI)m/z: calcd. for C18H15O4[M + H]+, 295.0970; found, 295.0981.

The NMR spectra analysis of the title compound (3) is consist with the related literatures[16,17]. The solid of 3 was re-crystallized from dichloromethane/ethanol to give colorless single crystals of 3 suitable for single-crystal X-ray diffraction. The crystal of the title compound is colorless and stable in air at room temperature.

2. 3 Structure determination

A colorless block crystal of 3 was mounted on a glass fiber for X-ray crystallographic data collection which was performed on a Bruker Smart Apex CCD diffractometer equipped with graphite-monochromated MoKαradiation (λ= 0.71073 ?) at 293(2) K. The structure of 3 was solved by direct methods, and then the non-hydrogen atoms were refined anisotropically with SHELXS-97 by applying a full-matrix least-squares procedure onF2after being located from the trial structure[18]. Moreover, the hydrogen atoms were fixed geometrically at calculated distances, and allowed to ride on the parent atoms. A total of 22611 reflections were selected in the range of 6.25≤2θ≤55.07o (h: -10～10,k: -25～25,l: -12～12) by using aψ-ωscan mode, of which 3320 were independent and 1845 were observed withI> 2σ(I). The finalR= 0.0529,wR= 0.1082 withGOOF= 1.015 (w= 1/[σ2(Fo2) + (0.1615P)2+ 2.6496P], whereP= (Fo2+ 2Fc2)/3), (Δρ)max= 0.158, (Δρ)min= -0.156 e/?3and (Δ/σ)max= 0.000. Data were collected by Rapid Auto program[19,20]. The representative bond lengths and bond angles are illustrated in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

3 RESULTS AND DISCUSSION

3. 1 X-ray crystallographic analysis

The X-ray diffraction of the molecular structure of the polycyclic aromatic phenanthrene 3 is demonstrated in Fig. 1. Obviously, the three fused six-membered rings of phenyl of the phenanthrene skeleton are coplanar, and the bond lengths and bond angles are within normal limits. The skeleton of dimethyl phenanthrene-9,10-dicarboxylate (3) owns symmetric structure. And the two bulky substituents, methoxy carbonyl groups at 9- and 10-position of the molecular frame structure, are at the opposite directions in a symmetric manner. It means that the molecule has thermodynamically stable conformation (Fig. 1). However, intermolecular and intramolecular hydrogen bonds are not found.

Fig. 1. Coordination environment of compound 3

Moreover, the crystal packing of 3 illustrated in Fig. 2a reveals that the overall packing has reverse staggered parallel arrangement. Besides, two kinds ofπ-πinteractions between the two adjacent molecules at upper and lower levels can be observed, which formed a one-dimensional interaction model (Fig. 2b). As can be detected by the packing diagram with stratified arrangement (Fig. 2b), the distances between the central and lateral benzene rings of phenanthrene skeleton at the upper and lower levels are respectively 3.735 and 3.729 ?, which have a slight dif- ference.

Fig. 2. Packing diagram of compound 3

3. 2 Thermal analysis

The thermal stability of compound 3 was measured by the thermogravimetric analyses (TGA) at the heating rate of 10 ℃/min from room temperature to 400 ℃ under nitrogen protection[21]. A graphical representation of the thermoana- lytical results is shown in Fig. 3. The detailed analysis of the TG and DTG curves is presented in Table 2.

Fig. 3. TG and DTG curves of compound 3

On the basis of the TG and DTG curves, compound 3 went through two stages of mass loss. The first one occurred gradually, but the second one happened quickly and became sharp, as described by the DTGmaxat 351.2 ℃. Furthermore, in both stages, the thermal mass loss took place at the range 155.5～315.3 ℃ and from 315.4 to 375.0 ℃ associated with the mass loss of 29.88% and 68.13% totaled by 98.01%. As can been seen from Fig. 3, the second thermal mass loss stage is the basic and the decomposition mechanism of 3 is complex.

Table 2. Thermoanalytical Results of Compound 3

4 CONCLUSION

In summary, the dimethyl phenanthrene-9,10-dicarboxy- late 3 was synthesized and confirmed by1H NMR,13C NMR and H RMS. The X-ray diffraction of the molecular structure disclosed that phenanthrene skeleton is coplanar. The overall crystal packing revealed that it has reverse staggered parallel and stratified arrangement. And one-dimensional interaction model was formed by two kinds ofπ-πinteractions between the two adjacent molecules at upper and lower levels. The decomposition mechanism of compound 3 is complex. The more applications of dimethyl phenanthrene-9,10-dicarboxy- late are underway in our lab.