CAIXu-MinCHENXin-FeiZHANGXue-DanHUANGYuanGUWenWANGFei

(College of Chemical Engineering, Jiangsu Provincial Key Lab for the Chemistry and Utilization of Agro-Forest Biomass, Jiangsu Key Lab of Biomass-Based Green Fuels and Chemicals, Nanjing Forestry University, Nanjing 210037, China)

ABSTRACT Three title compounds (3～5) have been successfully synthesized via interactive reactions between ferrocenecarboxylic and dehydroabietic acids, which can be characterized by FT-IR, elemental analysis, ESI-MS, and NMR spectroscopy. The crystal structures of compounds 4 and 5 can be determined by single-crystal X-ray diffraction. Compound 4 is of orthorhombic system, space group P212121 with a = 7.7010(5), b = 11.7542(9), c = 28.3173(18) ?, Z = 4, V = 2563.3(3) ?3, Mr = 512.45, Dc = 1.328 g/cm3, S = 1.041, μ = 0.619 mm-1, F(000) = 1088, the final R = 0.0396 and wR = 0.0945 for 4394 observed reflections (I > 2σ(I)). Compound 5 crystallizes in orthorhombic system, space group P21212 as well, with parameters of a = 11.9875(15), b = 19.651(2), c = 7.2163(9) ?, Z = 2, V = 1699.9(3) ?3, Mr = 582.83, Dc = 1.139 g/cm3, S = 1.091, μ = 0.070 mm-1, F(000) = 636, the final R = 0.0653 and wR = 0.0719 for 1518 observed reflections (I > 2σ(I)). Additionally, electrochemical properties of compounds 3 and 4 have been investigated by the cyclic and differential pulse voltammogram techniques.

Keywords: dehydroabietic acid, anhydride, crystal structure, electrochemical properties;

1 INTRODUCTION

Dehydroabietic acid is an important natural tricyclic diterpene resin acid, which can be readily purified from commercial disproportionated rosin[1]. Compared to other resin acids, dehydroabietic acid has a phenyl ring in the tricyclic phenanthrene structure, making it more chemically stable than other resin acids that are easily to be oxidized in air due to their free double bonds. Therefore, dehy- droabietic acid is more suitable to be a synthetic precursor for synthesizing rosin derivatives. Dehydroabietic acid and its derivatives have attracted much attention due to their application prospects in the fields of cosmetics, agriculture, medicine, surfactants and fine chemistry[2,3]. However, the current research based on dehy- droabietic acid and its derivatives is mainly focused on the biological activities due to its natural proper- ties, such as antibacterial, antiviral, antitumor, anti- inflammatory, and antiulcer activities[4-9], while the development in the preparation and application of novel derivatives with other properties is slow. Therefore, one of our recent interests is to synthesize dehydroabietic acid derivatives with new properties and explore their potential applications. Ferrocene (Fc) and its derivatives have good electrochemical properties due to the unique sandwich molecular structure and electron-rich system, endowing it with good physical and chemical properties such as easy modifiability and good redox activity[10-12]. In recent years, Fc and its derivatives have been applied in many fields such as biochemistry and medicine[13-16], photoelectromag- netic functional materials[17-21], and chemical sensors[22-26]. Hence, we are focusing on the intro- duction of the redox-active Fc moiety into dehy- droabietic acid molecule in order to obtain a novel electrochemically active dehydroabietic acid-based anhydride. In this study, we report the syntheses, characterizations, and crystal structures of the title compounds. In addition, the electrochemical properties of compounds 3 and 4 are also presented.

2 EXPERIMENTAL

2. 1 General procedures and materials

Iodine (I2, TCI, > 98%), triphenylphosphine (Ph3P, energy chemical, 98%), dicyclohexylcarbo- diimide (DCC, energy chemical, 98%), triethyla- mine (Et3N, TCI, > 99%), chloroform-d3(CDCl3, J&K scientific, 99.8%), thionyl chloride (SOCl2), benzene and other organic solvents were purchased from Nanjing Chemical Reagent Co., Ltd. and used without further purifications. Ferrocenecarboxylic acid (Fc-COOH, energy chemical, 98%) was purified by recrystallization from methanol and dried under vacuum at 60 °C for 24 h before use. Dehydroabietic acid was obtained via reported purification procedures of commercially dispropor- tionated rosin (Guangxi Jinxiu Songyuan forest products Co., Ltd.)[1]. Reactions were monitored by TLC silica gel 60 F254sheets from Taizhou Biochemical Plastic Factory, China and detected under UV light (254 and 365 nm). Purifications were performed by flash chromatography on silica gel (300 ～400 mesh) from Qingdao Marine Chemical Factory, China. Melting points were determined on an OptiMelt MPA100 apparatus (SRS, USA) without correction. FT-IR spectra were recorded on a Thermo Nicolet 380 FT-IR spectro- meter (Thermo Electron, USA) with KBr methods in the 4000～500 cm-1range. The ESI-MS spectrum was measured on a Mariner System 5304 mass spectrometer. NMR measurements were performed on a Bruker AVANCE-III-600 spectrometer (1H, 600.13 Hz;13C, 150.92 Hz) with CDCl3as solvent unless otherwise stated. Chemical shifts were given as parts per million (ppm) and referenced to the solvent as an internal standard.Jvalues were given in Hz. Elemental analyses (C and H) were performed on a 2400 II elemental analyzer (PE, USA). Cyclic and differential pulse voltammograms were recorded in a 0.10 M [n-Bu4N][PF6] solution (CH2Cl2as solvent) on a CHI 660E electrochemical analyzer with a glassy carbon working electrode, a platinum plate auxiliary electrode, and a Ag/AgCl reference electrode. The concentration of all the samples was 2.0 mM, and all the potential values were referenced to Fc/Fc+.

2. 2 Syntheses of compounds 3～5

As shown in Scheme 1, ferrocenecarboxylic and dehydroabietic acids were used as starting materials to synthesize compounds 3 ～5. Ferrocenecar- boxylic anhydride 3 was obtained from the reaction of ferrocenecarboxylic acid with Ph3P/I2/Et3N[27]and dehydroabietic anhydride 5 was prepared by dehydration of dehydroabietic acid with dehydra- ting agent DCC[28]. However, the two methods mentioned above are not suitable for the synthesis of mixed ferrocenecarboxylic-dehydroabietic anhy- dride 4. Compound 4 can be obtained by the reaction of dehydroabietyl chloride with ferrocene- carboxylic acid in a hexane/CH2Cl2mixture with triethylamine as a catalyst[29].

Scheme 1. Syntheses of anhydrides 3～5: (i) PPh3, I2, Et3N, DCM, 0 °C→r.t.; (ii) SOCl2, benzene, reflux; Et3N, Hexane/DCM, r.t.; (iii) DCC, DCM, r.t.

2. 2. 1 Improved synthesis of ferrocenecarboxylic anhydride 3

Compound 3 has been reported to be synthesized via adding triphosgene as an additive[30]. In order to avoid the potential risk of using triphosgene, the improved synthesis of compound 3 has been reported as follows. To a solution of iodine (76.2 mg, 0.3 mmol) in CH2Cl2(2 mL) was added triphenylphosphine (78.6 mg, 0.3 mmol) in one portion at 0oC. Ferrocenecarboxylic acid (46 mg, 0.2 mmol) was subsequently added into the above mixture, followed by the addition of triethylamine (83.2 μL, 0.6 mmol) at 0oC. The reaction mixture was allowed to warm up to room temperature and stirred until completion of the reaction (typically within 10 minutes). The crude mixture was con- centrated under reduced pressure and then purified by silica column chromatography with a petroleum ether/ethyl acetate (8.5:1.5, v/v) mixture as the eluent. The product was obtained as an orange solid. Yield: 65%, m.p. 126～128 °C. All characterization data match with those in the previously published paper[30].

2. 2. 2 Synthesis of ferrocenecarboxylic- dehydroabietic mixed anhydride 4

To a solution of dehydroabietic acid (90 mg, 0.3 mmol) in 2 mL of benzene was added slowly 32.7 μL of SOCl2(53.55 mg, 0.45 mmol). The mixture was then refluxed for three hours. After cooling down, the solvent and excessive SOCl2were removed in vacuo to yield dehydroabietyl chloride as a light yellow oily product. Then a solution of the obtained dehydroabietyl chloride (0.3 mmol), ferrocenecarboxylic acid (57.5 mg, 0.25 mmol), and triethylamine (41.6 μL, 0.3 mmol) in a hexane/CH2Cl2(12 mL, 5:1, v/v) mixture was stirred at room temperature for three hours. When the reaction was completed, the precipitated triethylamine hydrochloride was filtered off, and the solvent was evaporated to yield an orange oily product. Then crude product was purified by neutra- lized silica column chromatography with a petro- leum ether/ethyl acetate/triethylamine (30:1:0.6, v/v/v) mixture as the eluent. Single crystals were obtained by slow evaporation in a MeOH/CH2Cl2(1:1, v/v) mixture. Yield: 60%, m.p. 143～145 °C. Anal. Calcd. (%) for C31H36FeO3: C, 72.66; H, 7.08. Found (%): C, 72.61; H, 7.04. FT-IR (KBr):νmax2954, 2928, 1782, 1724, 1497, 1452, 1375, 1271, 1221, 1163, 1045, 1021, 995, 894, 823 cm-1.1H NMR (CDCl3, 600 MHz):δ7.20 (d,J= 8.2 Hz, 1H), 7.02 (dd, 1H), 6.92 (d, 1H), 4.80 (t, 2H), 4.52 (t,J= 1.9 Hz, 2H), 4.30 (s, 5H), 3.04～2.95 (m, 2H), 2.84 (m, 1H), 2.37 (m, 1H), 2.31 (m, 1H), 2.00～1.91 (m, 2H), 1.86～1.79 (m, 3H), 1.73 (m, 1H), 1.55 (m, 1H), 1.40 (s, 3H), 1.27 (s, 3H), 1.23 (d,J= 6.9 Hz, 6H).13C NMR (CDCl3, 151 MHz):δ174.36, 168.00, 146.64, 146.04, 134.61, 127.14, 124.30, 124.19, 72.87, 70.93, 70.26, 69.43, 48.95, 44.86, 38.05, 37.09, 36.20, 33.60, 30.24, 25.33, 24.10, 21.96, 18.62, 16.62. MS (ESI):m/z[M+Na]+: 535.2.

2. 2. 3 Synthesis of dehydroabietic anhydride 5

A solution of dehydroabietic acid (60 mg, 0.2 mmol) and DCC (20.6 mg, 0.1 mmol) in CH2Cl2(2 mL) was stirred at room temperature overnight. The resulting precipitate was filtered off via a plug of Celite and the filtrate was evaporated to yield a white precipitate. It was then purified by silica column chromatography with a petroleum ether/ethyl acetate (20:1, v/v) mixture as the eluent. Single crystals were obtained by slow evaporation in a MeOH/CH2Cl2(10:1, v/v) mixture. Yield: 60%, m.p. 153～155 °C. Anal. Calcd. (%) for C40H54O3: C, 82.43; H, 9.34. Found (%): C, 82.36; H, 9.30. IR (KBr):νmax2956, 2928, 2869, 1799, 1736, 1497, 1461, 1386, 1363, 1206, 1045, 994, 821 cm-1.1H NMR (CDCl3, 600 MHz):δ7.16 (d,J= 8.1 Hz, 2H), 7.02 (dd, 2H), 6.89 (d, 2H), 2.92～2.81 (m, 6H), 2.33 (m, 2H), 2.17 (m, 2H), 1.90～1.74 (m, 10H), 1.66 (m, 2H), 1.49 (m, 2H), 1.33 (s, 6H), 1.26 (s, 6H), 1.24 (d,J= 9.1 Hz, 12H).13C NMR (CDCl3, 151 MHz):δ174.86, 146.61, 145.99, 134.72, 127.19, 124.25, 124.12, 49.24, 45.03, 38.04, 37.17, 36.12, 33.62, 30.21, 25.40, 24.10, 21.86, 18.53, 16.64. MS (ESI):m/z[M+H]+: 583.4.

2. 3 X-ray structure determination

An orange single crystal of compound 4 (0.20mm × 0.15mm × 0.15mm) and a colorless single crystal of compound 5 (0.20mm × 0.20mm × 0.10mm) were selected for X-ray diffraction analysis. The data were collected on a Bruker D8 VENTURE PHOTON 100 diffractometer equipped with a graphite-monochromatic MoKαradiation (0.71073 ?) by using anω-2θscan mode in the ranges of 2.25<θ<25.99° for 4 at 283(2) K and 2.68<θ<24.99° for 5 at 296(2) K. Both structures of 4 and 5 were solved by direct methods withSHELXT[31]and refined by full-matrix least-squares method onF2withSHELXL[32]. All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were located by geometric calculations and refined by using a riding mode. For compound 4, a total of 36740 reflections were collected, of which 5050 were independent (Rint= 0.0359) and 4394 (-8≤h≤9, -14≤k≤14, -34≤l≤34) were observed withI> 2σ(I). The final refinement gaveR= 0.0396,wR= 0.0945 (w= 1/[σ2(Fo2) + (0.0506P)2+ 0.8475P], whereP= (Fo2+ 2Fc2)/3),S= 1.041, (?/σ)max= 0.000, (?ρ)max= 0.394 and (?ρ)min= -0.243 e/?3. For compound 5, a total of 7547 reflections were collected, of which 2979 were independent (Rint= 0.0625) and 1518 (-14 ≤h≤11, -23 ≤k≤20, -8 ≤l≤8) were observed. The final refinement gaveR= 0.0653,wR= 0.0719 (w= 1/[σ2(Fo2) + (0.0056P)2], whereP= (Fo2+ 2Fc2)/3),S= 1.091, (?/σ)max= 0.001, (?ρ)max= 0.167 and (?ρ)min= -0.174 e/?3. The selected bond distances and bond angles of the two com- pounds are listed in Tables 1 and 2, respectively.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) of Compound 4

O(1)-C(10)-O(3) 120.3(4) C(5)-O(1)-C(10) 119.5(3) C(10)-C(13)-C(22) 124.1(4) C(2)-C(19)-C(9) 122.2(3) C(5)-C(4)-C(14) 106.7(3) C(26)-C(24)-C(29) 108.0(4) C(3)-C(13)-C(22) 107.8(4) C(6)-C(1)-C(12) 110.2(3) C(27)-C(15)-C(28) 111.9(4) O(2)-C(5)-C(10)-O(3) -59.1(4) C(4)-C(14)-C(18)-C(16) 59.2(4) C(7)-C(11)-C(9)-C(19) -2.0(5) C(3)-C(13)-C(22)-C(21) 0.8(5) C(6)-C(9)-C(11)-C(23) -0.0(5) C(9)-C(6)-C(1)-C(12) 57.8(3)

Table 2. Selected Bond Lengths (?) and Bond Angles (°) of Compound 5

3 RESULTS AND DISCUSSION

3. 1 FT-IR spectroscopy

The FT-IR spectra of three synthesized anhy- drides exhibit weak absorption at around 2920 cm-1corresponding to the =C-H stretching of aromatic ring and medium absorption around 2900 cm-1corresponding to the -C-H stretching ofsp3carbon atoms. The absorption bands around 1580, 1497 and 1455 cm-1are due to the benzene skeleton vibration and the absorption bands around 1105 and 1000 cm-1are attributed to the C=C stretching vibration of cyclopentadiene (Cp) ring. Besides, two strong C=O stretching bands of the anhydrides at 1766 and 1710 cm-1(compound 3), 1782 and 1724 cm-1(compound 4), and 1799 and 1736 cm-1(compound 5) are due to the symmetric and asymmetric C=O stretching vibrations.

3. 2 NMR spectroscopy

The1H NMR spectrum of compound 3 contains three typical signals for a monosubstituted Fc (two triplets atδ4.90 and 4.55 ppm for the substituted Cp rings, and a singlet atδ4.37 ppm for the unsubstituted Cp rings). In1H NMR spectrum of 4, three shifts atδ7.20, 7.02, and 6.92 ppm can be assigned to the aromatic protons at C(19), C(2), and C(7), respectively. The two triplets atδ4.80 and 4.52 ppm, and a singlet atδ4.30 ppm can be attributed to the substituted Cp protons at C(3, 22), C(21, 25) and unsubstituted Cp protons at C(24, 26, 29-31), respectively. Two singlets atδ1.40 and 1.27 ppm can be assigned to methyl protons at C(17) and C(20), respectively. Moreover, there is a doublet atδ1.23 ppm, corresponding to the two methyl protons of the isopropyl group (C(27) and C(28)), together with a multiplet atδ2.84 ppm due to the vicinal CH proton. Similarly, in1H NMR spectrum of compound 5, two doublets atδ7.16, 6.89 ppm and a double doublet atδ7.02 ppm can be attributed to the aromatic protons at C(11), C(14) and C(12), respectively. Two singlets atδ1.33 and 1.26 ppm can be assigned to methyl protons at C(19) and C(18), respectively. Furthermore, there is a doublet atδ1.24 ppm, corresponding to the two methyl protons of the isopropyl group (C(16) and C(17)). The13C NMR spectrum of compound 3 exhibits 5 well resolved resonances for 22 carbon atoms. Among them, the carbonyl (C=O) carbon appears atδ167.73 ppm, while the carbon signals on the Cp rings are observed atδ72.78, 70.99, 70.37 and 69.72 ppm. The13C NMR spectrum of compound 4 exhibits 24 well resolved resonances for 31 carbon atoms. Among them, two peaks atδ174.36 and 168.00 ppm are confirmed to be the signals of C(5) and C(10) on the carbonyl group, respectively. Six peaks atδ146.64, 146.04, 134.61, 127.14, 124.30 and 124.19 ppm can be attributed to the carbons of benzene ring. Moreover, the carbon signals on the Cp rings are observed atδ72.87, 70.93, 70.26 and 69.43 ppm. The13C NMR spectrum of compound 5 exhibits 19 well resolved resonances for 40 carbon atoms. Among them, the peak atδ174.86 ppm is confirmed to be the signal of C(20) on the carbonyl group, while the carbon signals of C(9)-C(14) on the benzene ring are observed atδ146.61, 145.99, 134.72, 127.19, 124.25 and 124.12 ppm. The assignments of the signals in the1H and13C NMR spectra of com- pounds 3～5 are in good accordance with their structures.

3. 3 Crystal structure description

Compound 4 crystallizes in the orthorhombic space groupP212121and the structure with corres- ponding atomic numbering scheme is shown in Fig. 1. The bond lengths of both C=O and C-O are different as expected, with lengths of the former as 1.195(5) (C(5)-O(2)) and 1.191(6) ? (C(10)-O(3)), and the latter as 1.366(5) (C(5)-O(1)) and 1.417(5) (C(10)-O(1)). The bond length of C(10)-C(13) (1.450(6) ?) is shorter than that of C(5)-C(4) (1.525(5) ?) because C(10) has an aromatic conjugation with the Fc group. The two C=O bonds (C(5)-O(2) and C(10)-O(3)) in the anhydride 4 are not coplanar, with the torsion angle O(2)- C(5)-C(10)-O(3) of 59.11°. Besides, each molecule contains two Cp rings and three six-membered rings (Rings A, B and C). Different from the staggered conformation of Fc unit in some Fc-containing compounds[33], the Fc unit in compound 4 displays a conformation close to eclipse, which may be due to the influence of the adjacent anhydride bond and dehydroabietyl groups. The Cp rings are almost parallel with an interplanar angle of 1.94°. The C-C bonds in the substituted Cp ring are not all statistically equivalent. Unsurprisingly, the longest C-C bonds in the ring are adjacent to the electron- withdrawing anhydride group, C(13)-C(3) (1.426(7) ?) and C(13)-C(22) (1.426(6) ?). The C-C bonds in the unsubstituted Cp ring are statistically equivalent, averaged by about 1.420(9) ?. The cyclohexane fragment A (C(1), C(4), C(14), C(18), C(16), C(20)) has a classical chair conformation, and the aromatic ring C (C(2), C(8), C(7), C(11), C(9), C(19)) is planar, while ring B (C(1), C(6), C(9), C(11), C(23), C(12)) is characterized as a cyclohexene half-chair conformation with the out-of-plane position of C(1) and C(12) atoms to be 0.654 and 0.112 ?, respectively, relative to the other atoms of ring B. Each molecule has two methyl groups (C(17) and C(20)) attached to the cyclohe- xane ring in the axial positions because the larger ferrocenyl group should sit on the more stabilized equatorial position. Moreover, compound 4 is a chiral molecule with three chiral centers derived from the natural diterpene dehydroabietic acid, and its absolute configuration can be confirmed as 1R, 4R and 6S.

Fig. 1. Molecular diagram of compound 4 with atomic labeling scheme

The molecular structure of compound 5 is shown in Fig. 2. It crystallizes in the orthorhombic space groupP21212. The bond lengths of C=O and C-O are different as expected, with lengths of the former to be 1.180(6) ? (C(20)-O(1)) and the latter of 1.382 ? (C(20)-O(2)). In addition, two C=O bonds (C(20)-O(1) and C(20a)-O(1a)) in the anhydride 5 are not coplanar, with the torsion angle O(1)- C(20)-C(20a)-O(1a) of -48.6(5)°. Similar to the structure of compound 4, 5 contains two chair- conformed cyclohexane rings and a phenyl ring. The aromatic ring (C(9)～C(14)) is planar. One cyclohexane ring (C(1)～C(6)) presents a classical chair conformation with two methyl groups (C(18) and C(19)) in the axial positions, while the other (C(5)～C(10)) adopts a half-chair conformation because of the fusion with phenyl ring. At the meantime, compound 5 is a chiral molecule with three chiral centers. Normally, the absolute con- figurations of dehydroabietic acid-derived com- pounds obtained under mild reaction conditions will maintain their original absolute configurations as R, R and S[34,35]. Therefore, the absolute configuration of compound 5 can be reasonably defined as 4R, 5R and 6S. The two dehydroabietyl groups in the molecule face in opposite directions due to the effect of steric hindrance.

Fig. 2. Molecular diagram of compound 5 showing atomic labeling scheme

3. 4 Electrochemical properties

The electrochemical properties of compounds 3 and 4 were investigated using cyclic (CV) and differential pulse voltammogram (DPV) techniques. The CVs and DPVs are shown in Fig. 3. As depicted in Fig. 3, the curve CV of 3 is a clear overlap of two pairs of redox peaks, which can be attributed to the redox processes of two Fc/Fc+couples in compound 3, and the curve DPV of 3 is an asymmetric wide peak, with the oxidation potential of around 0.344 V. Theoretically, only one redox couple could appear and the curve DPV of 3 will be a regular symmetric peak if the two end Fc groups in compound 3 have no internal electronic coupling. Therefore, the results of electrochemical investigation indicate that compound 3 has the potential applications as molecular wires[33,36,37]. With regard to compound 4, only one reversible one-electron redox peak appears, which can be attributed to the redox process of Fc/Fc+couples in compound 4, and the potentials of its anodic and cathodic peaks are as follows:Epa= 0.424 andEpc= 0.274 V (vs. Fc/Fc+). Moreover, there is only one symmetrical peak in the curve DPV of 4 (oxidation potential of around 0.336 V), which further proves that the redox activity of compound 4 is a one-electron reversible redox process. Compared to the oxidation potential of compound 1 at around 0.248 V in its DPV curve, the oxidation potentials of both compounds 3 and 4 shift positively with circa 0.096 and 0.088 V, respectively, due to the introduction of electron-withdrawing groups of both anhydrides. All in all, compound 4 has good redox performance, which might be potentially used as natural product-based electrochemical sensors[22-26].

Fig. 3. CVs of compounds 3 and 4 and DPVs of compounds 1, 3 and 4 recorded in a 0.10 M CH2Cl2 solution of electrolyte [n-Bu4N][PF6]. All potential values are referenced to Fc/Fc+

4 CONCLUSION

In summary, the title compounds 3～5 were synthesized based on ferrocenecarboxylic and dehydroabietic acids via using three different synthetic methods. Their structures were determined by the use of spectroscopic methods. The crystal structures of two new compounds 4 and 5 were characterized by single-crystal X-ray diffraction. Furthermore, electrochemical properties of com- pounds 3 and 4 were examined by cyclic (CV) and differential pulse voltammogram (DPV) techniques showing that the natural product-derived anhydride 4 has good redox activities.