untitled European Journal of Chemistry 2 (3) (2011) 289‐294 European Journal of Chemistry ISSN 2153‐2249 (Print) / ISSN 2153‐2257 (Online)  2011 EURJCHEM DOI:10.5155/eurjchem.2.3.289‐294.416 European Journal of Chemistry Journal homepage: www.eurjchem.com Synthesis and properties of aromatic 1,3‐diketones and bis‐(1,3‐diketones) obtained from acetophenone and phtalic acids esters Jan Zawadiak, Marek Mrzyczek* and Tomasz Piotrowski Faculty of Chemistry, Silesian University of Technology, Krzywoustego 4, Gliwice, 44‐100, Poland *Corresponding author at: Faculty of Chemistry, Silesian University of Technology, Krzywoustego 4, Gliwice, 44‐100, Poland. Tel.: +48.32.2372971; fax: +48.32.2371032. E‐mail address: marek.mrzyczek@polsl.pl (M. Mrzyczek). ARTICLE INFORMATION ABSTRACT Received: 14 March 2011 Received in revised form: 07 April 2011 Accepted: 08 April 2011 Online: 30 September 2011 KEYWORDS Dibenzoylmethane and six aromatic 1,3‐diketones containing a dibenzoylmethane moiety were synthesized from acetophenone and the appropriate ester in crossed‐Claisen condensations. The synthesized diketones include derivatives containing carboxyl and ester groups; bis‐(1,3‐diketones) were also prepared. The absorption of UV radiation of the obtained compounds was investigated in various solvents, and their molar absorption coefficients were calculated. The ratio of tautomers and keto‐enol equilibrium constants were calculated using 1H NMR techniques. Aromatic bis‐(1,3‐diketones) demonstrated strong hyperchromic effects. The keto‐enol equilibrium of the investigated compounds is strongly shifted to the enol form, especially in non‐polar solvents. 1,3‐Diketones Tautomerism Bis‐(1,3‐diketones) UV/Vis spectroscopy Solvent effect Sunscreens 1. Introduction Aromatic 1,3‐diketones significantly absorb UV‐A radiation (λ = 320‐400 nm) and are therefore used as sunscreens in cosmetics. Aromatic 1,3‐diketones owe this feature to the formation of an intramolecular hydrogen bond in the enol tautomer (Scheme 1). Currently, 1‐(4’‐tert‐butylphenyl)‐3‐(4’’‐ methoxyphenyl)‐propane‐1,3‐dione is one of the few UV‐A sunscreens approved both in Europe and in the USA [1]. The keto‐enol equilibrium in 1,3‐diketones strongly favors the enol form, and only very polar solvents or steric hindrance cause a slight shift of the equilibrium to the keto form [2]. These compounds also form complexes with metals, a feature which makes these compounds attractive for use in many applications. For example, a complex of a 1,3‐diketone with zinc is used as a PVC stabilizer [3] and vulcanization activator [4]. Aromatic 1,3‐diketones containing a 1,3‐diphenylpropane‐ 1,3‐dione (1) moiety are synthesized by the condensation of an acetophenone derivative and a substituted ester of benzoic acid (Scheme 2). Use of phthalic acids esters instead of benzoates leads to aromatic bis‐(1,3‐diketones). Few studies concerning such compounds have been published to date, and these papers mainly describe the chelating properties of bis‐(1,3‐diketones) [5‐7]. These compounds form metal complexes, and two chelating centers can form organometallic polymers or triangles [6]. Only two publications have investigated the spectroscopic properties of aromatic bis‐(1,3‐diketones) [8,9]. Because these compounds contain two 1,3‐dicarbonyl moieties, they might prove to be superior UV absorbing agents, therefore research of UV absorption properties of bis‐(1,3‐diketones) was the main objective of this work. 2. Experimental 2.1. Instrumentation The melting points were determined in an open capillary tube on a Stanford Research Systems EZ‐Melt MPA120 automated melting point apparatus. NMR spectra were recorded on NMR Varian Inova 300 MHz. UV‐VIS spectra were collected on Shimadzu UV‐2101 PC double‐beam spectro‐ photometer. Infrared spectra were measured on Mettler‐ Toledo ReactIR iC10 spectrometer. 2.2. General procedure for synthesis of bis‐(1,3‐diketones) (1,4) To a three‐necked flask equipped with a mechanical stirrer, reflux condenser and drying tube, 50 mmol of ester, 100 mL of THF and 200 mmol of the base (see Table 1) was added, and the mixture was warmed to the boiling temperature. A mixture of the proper amount of acetophenone (see Table 1) and 25 mL THF was added dropwise over 30 min, and the formation of a yellow solid was observed. The reaction was rapidly stirred for 8 h. The mixture was cooled to 5 °C in an ice bath to avoid ester hydrolysis. Upon cooling, the mixture color changed to dark brown. Neutralization of the formed salt of bis‐(1,3‐diketone) was performed using two methods: Method 1: Maintaining rapid stirring, 10% HCl was added to the reaction mixture until pH = 7. During neutralization, the mixture color changed to light‐yellow. The neutralized mixture was filtered with a Büchner funnel, and the filtrate was poured into a separatory funnel. The aqueous layer was extracted with methylene chloride. Organic fractions were merged and evaporated. 290 Zawadiak et al. / European Journal of Chemistry 2 (3) (2011) 289‐294 Scheme 1 Scheme 2 From the resulting solid residue, the main products and by‐ products were separated by column chromatography (100%, CH2Cl2). After several crystallizations from methanol, pure bis‐ (1,3‐diketones) were obtained. See Table 1 for yields. Method 2: Maintaining rapid stirring, 100% acetic acid was added to the reaction mixture until pH = 7. During neutralization, the mixture color changed to light‐yellow. The gelatinous precipitate containing by‐products was centrifuged, and the solution was concentrated and placed in refrigerator for 24 h. The precipitate that formed was filtered and crystallized from DMF/MeOH (1:1; v/v) to give pure product. See Table 1 for yields. 1,4‐bis‐(3‐phenyl‐3‐oxopropionyl)benzene (1): Yellow. M.p.: 177, 197‐198 °C (Ref. [8], 176‐177 °C). IR (νmax, cm‐1): 3060 (w), 2670 (w), 1596 (m), 1529 (s), 1495 (w), 1301 (s‐m), 1237 (m), 1184 (m‐w), 1159 (w) 1129 (m‐w), 1103 (vw), 1069 (m‐w), 1028 (w), 1013 (w), 987 (w), 926 (w), 856 (m‐w), 800 (m‐w), 755 (s), 703 (w), 680 (m). 1H NMR (CDCl3, 300 MHz, δ, ppm): 16.82 (2H, br, C(OH)=CH‐CO), 8.09 (4H, s, Ar‐H), 8.02 (4H, d, J=6.9 Hz, Ar‐H), 7.59 (2H, t J=7.2 Hz, Ar‐H), 7.52 (4H, t, J=7.2 Hz, Ar‐H), 6.92 (2H, s, C(OH)=CH‐CO). 13C NMR (CDCl3, 75 MHz, δ, ppm): 187.0, 183.5 (CO), 138.7, 135.5, 132.8, 128.8, 127.4, 127.3 (Ar‐C), 93.7 (C(OH)=CH‐CO). Anal. Calcd. for C24H18O4: C, 77.82; H, 4.90. Found: C, 77.78; H 4.91%. 1,3‐bis‐(3‐phenyl‐3‐oxopropionyl)benzene (4): White. M.p.: 159, 173 °C (Ref. [8] 153‐154 °C). IR (νmax, cm‐1): 3075 (w), 2672 (w), 1603 (s), 1525 (s), 1484 (s), 1304 (m), 1267 (m), 1241 (m), 1215 (s), 1184 (w), 1159 (w), 1099 (m), 1069 (m), 1024 (m‐w), 1002 (m‐w), 972 (w), 934 (w), 908 (w), 848 (w), 804 (w), 759 (s), 695 (m), 680 (m), 654 (w). 1H NMR (CDCl3, 300 MHz, δ, ppm): 16.89 (2H, br, C(OH)=CH‐CO), 8.15 (1H, s, Ar‐H), 8.18 (1H, d, J=7.8 Hz, Ar‐H), 8.02‐8.05 (4H, m, Ar‐H)7.49‐ 7.68(8H, m, Ar‐H), 6.95 (2H, s, C(OH)=CH‐CO). 13C NMR (CDCl3, 75 MHz, δ, ppm): 186.4, 185.1 (CO), 138.6, 136.5, 133.0, 131.0, 129.4, 129.0, 127.5, 126.0 (Ar‐C), 93.6 (C(OH)=CH‐CO). Anal. Calcd. for C24H18O4: C, 77.82; H, 4.90. Found: C, 77.75; H, 5.02%. 2.3. General procedure for synthesis of 1,3‐diketones with ester group (2,5) To the three‐necked flask equipped with a mechanical stirrer, reflux condenser and drying tube, 50 mmol of ester, 100 mL of THF and 30% solution of sodium methoxide (50 mmol) in methanol was added, and the mixture was warmed to the boiling temperature. The mixture of 33 mmol of acetophenone and 25 mL of THF was added dropwise over 30 min, and the formation of a yellow solid was observed. The reaction was rapidly stirred for 4 h. The mixture was cooled to 5 °C in an ice bath to avoid ester hydrolysis. Upon cooling, the mixture color changed to dark brown. Maintaining rapid stirring, to the reaction mixture 10% HCl was added until pH = 7 was reached. During neutralization, the mixture color changed to light‐ yellow. The organic layer was separated and evaporated, and the resulting precipitate was filtered and crystallized from methanol to give pure product. Yields: 45% (2), 42% (5). Methyl 4‐(3‐phenyl‐3‐oxopropionyl)benzoate (2): White. M.p.: 123‐124 °C (Ref. [10] 123‐125 °C). IR (νmax, cm‐1): 3057 (w), 2966 (w), 1731 (s), 1596 (s‐m), 1529 (s), 1506 (s‐m), 1443 (s), 1405 (m‐w), 1286 (s), 1230 (m), 1196 (m), 1118 (s), 1073 (m), 1061 (m), 1017 (s), 960 (m‐w), 927 (w), 867 (m), 833 (w), 804 (m), 792 (m), 751 (s), 706 (m), 680 (m). 1H NMR (CDCl3, 300 MHz, δ, ppm): 16.79 (1H, br, C(OH)=CH‐CO), 8.15 (2H, d, J=8.7 Hz, Ar‐H), 7.98‐8.07(4H, m, Ar‐H), 7.59 (1H, t J=7.5 Hz, Ar‐ H), 7.51 (2H, t J=7.35 Hz, Ar‐H), 6.90 (1H, s, C(OH)=CH‐CO) 3.97 (3H, s, ‐COOCH3). 13C NMR (CDCl3, 75 MHz, δ, ppm): 187.1, 183.5 (CO), 166.3 (COOCH3), 139.2, 135.4, 133.2, 132.8, 129.8, 128.6, 127.3, 127.0 (Ar‐C), 93.8 (C(OH)=CH‐CO), 52.4 (COOCH3). Anal. Calcd. for C17H14O4: C, 72.33; H, 5.00. Found: C, 72.30; H, 5.04%. Methyl 3‐(3‐phenyl‐3‐oxopropionyl)benzoate (5): White. M.p.: 82‐83 °C. IR (νmax, cm‐1): 3072 (w), 2956 (w), 2661 (vw), 1727 (s), 1607 (s), 1559 (s), 1480 (s), 1439 (s), 1301 (s), 1263 (s), 1222 (s), 1121 (m), 1076 (m‐w), 1065 (w), 1028 (w), 1002 (w), 983 (m‐w), 930 (w), 863 (w), 811 (w), 755 (s), 721 (m), 703 (m), 684 (m), 662 (w). 1H NMR (CDCl3, 300 MHz, δ, ppm): 16.84 (1H, br, C(OH)=CH‐CO), 8.63 (1H, s, Ar‐H), 8.22 (2H, t, J=7.2 Hz, Ar‐H), 8.02 (2H, d, J=6.8 Hz, Ar‐H), 7.48‐7.63 (4H, m, Ar‐H), 6.92 (1H, s, C(OH)=CH‐CO), 3.99 (3H, s, ‐COOCH3). 13C NMR (CDCl3, 75 MHz, δ, ppm): 186.5, 184.7 (CO), 166.6 (COOCH3), 136.2, 135.5, 133.4, 132.9, 131.7, 131.0, 129.2, 129.0, 128.5, 127.5 (Ar‐C), 93.5 (C(OH)=CH‐CO), 52.7 (COOCH3). Anal. Calcd. for C17H14O4: C, 72.33; H, 5.00. Found: C, 72.15; H, 5.09%. Table 1. Yields and substrates ratio in bis‐(1,3‐diketone) synthesis. Product Base AcPh:Ester [mol:mol] Yield [%] 1 NaOMe 2.8 50a 1 NaH 2.8 61a 1 NaOMe 4,0 51a, 38b 1 NaH 4.0 61a, 49b 2 NaOMe 2.8 28a 2 NaH 2.8 29a 2 NaOMe 4.0 2a 2 NaH 4.0 2a 4 NaOMe 4.0 44b 4 NaH 4.0 64b a Determined by HPLC after neutralization with 10% HCl (Method 1). b Separated product after neutralization with 100% CH3COOH (Method 2). 2.4. General procedure for synthesis of 1,3‐diketones with carboxylic group (3,6) To the round bottom flask proper ester (2,5) (3,5 mmol), 15 mL of THF and a solution of LiOH.H2O (35 mmol) in 15 mL H2O was added. The mixture was stirred with magnetic stirrer for 6h at room temperature. To the separated aqueous layer, 3% HCl (52 mmol) was added. Zawadiak et al. / European Journal of Chemistry 2 (3) (2011) 289‐294 291 OOOO O O O O OO O O OO O OH -MeOH +AcPh -MeOH +AcPh +H2O -MeOH Scheme 3 The resulting white solid was filtered and crystallized from ethanol to give pure product. Yields: 67% (3), 73% (6). 4‐(3phenyl‐3‐oxopropionyl)benzoic acid (3): White. M.p.: 235 °C. IR (νmax, cm‐1): 3027 (m), 2833 (m), 2684 (m), 1682 (s), 1592 (m), 1577 (w), 1532 (m), 1510 (m), 1432 (m), 1327 (w), 1293 (s), 1230 (w), 1189 (w), 1155 (vw), 1121 (m), 1103 (w), 1073 (w), 1058 (w), 1017 (w), 982 (w), 931 (m), 871 (m), 807 (w), 781 (m), 751 (s), 714 (m), 680 (m). 1H NMR (DMSO‐d6, 300 MHz, δ, ppm): 17.04 (1H, br, C(OH)=CH‐CO), 13.34 (1H, br, ‐ COOH), 8.27 (2H, d, J=8.4 Hz, Ar‐H), 8.19 (2H, d, J=7.3 Hz, Ar‐H), 8.08 (2H, d, J=8.4 Hz, Ar‐H), 7.67 (1H, t, J=7.4 Hz, Ar‐H), 7.57 (2H, t, J=7.4Hz, Ar‐H), 7.42 (1H, s, C(OH)=CH‐CO). 13C NMR (DMSO‐d6, 75 MHz, δ, ppm): 186.7, 183.1 (CO), 166.6 (COOH), 138.1, 134.6, 134.2, 133.3, 129.6, 128.9, 127.6, 127.5 (Ar‐C), 94.1 (C(OH)=CH‐CO). Anal. Calcd. for C16H12O4: C, 71.64; H, 4.51. Found: C, 71.55; H, 4.64%. 3‐(3‐phenyl‐3‐oxopropionyl)benzoic acid (6): White. M.p.: 195‐196 °C. IR (νmax, cm‐1): 2975 (w), 2841 (m), 2672 (w), 1682 (s), 1611 (m), 1532 (m), 1491 (w), 1446 (m) , 1308 (s), 1230 (m), 1185 (w), 1099 (m‐w), 1069 (m‐w), 1028 (w), 1002 (w), 946 (m), 930 (m), 826 (w), 804 (w), 777 (vw), 748 (s), 706 (m‐ w), 680 (m), 665 (m). 1H NMR (DMSO‐d6, 300 MHz, δ, ppm): 17.14 (1H, br, C(OH)=CH‐CO), 13.32 (1H, br, ‐COOH), 8.62 (1H, s, ArH), 8.44 (1H, d, J=7.8 Hz, Ar‐H), 8.15‐8.23 (3H, m, Ar‐H), 7.62‐7.74 (2H, m, Ar‐H), 7.57 (2H, t, J=7.4 Hz, Ar‐H), 7.42 (1H, s, C(OH)=CH‐CO). 13C NMR (DMSO‐d6, 75 MHz, δ, ppm): 186.6, 184.8 (CO), 167.4 (COOH), 135.7, 135.2, 134.1, 133.9, 132.4, 132.3, 130.0, 129.6, 128.5, 128.3 (Ar‐C), 94.3 (C(OH)=CH‐CO). Anal. Calcd. for C16H12O4: C, 71.64; H, 4.51. Found: C, 71.58; H, 4.40%. 2.5. Synthesis of 1,3‐diphenylpropane‐1,3‐dione (7) To a three‐necked flask equipped with a mechanical stirrer, reflux condenser and drying tube, 60 mmol of methyl benzoate, 100 mL of THF and 30% solution of sodium methoxide (60 mmol) in methanol was added and mixture was warmed to the boiling temperature. A mixture of 50 mmol of acetophenone and 25 mL of THF was added dropwise over 30 min, and the formation of yellow solid was observed. The reaction was rapidly stirrred for 4 h. The mixture was cooled to 5 °C, and 10% HCl was added until the pH = 7 was reached. The organic layer was separated and evaporated, and the resulting precipitate was filtered and crystallized from ethanol to give pure product. Yield: 80%. 1,3‐diphenylpropane‐1,3‐dione (7): White. M.p.: 77‐78 °C. IR (νmax, cm‐1): 3131 (w), 3069 (m), 2990 (w), 2665 (w), 1600 (s), 1536 (s), 1513 (s), 1488 (s), 1469 (s), 1435 (m), 1308 (s), 1267 (s), 1230 (s), 1185 (m), 1166 (w), 1102 (m), 1065 (m), 1028 (m), 1002 (m), 976 (w), 931 (m), 897 (w), 845 (w), 815 (w), 789 (w‐m), 755 (s), 736 (s), 706 (s), 680 (s). 1H NMR (CDCl3, 300 MHz, δ, ppm): 16.90 (1H, br, C(OH)=CH‐CO) 7.97‐8.01 (4H, m, Ar‐H), 7.44‐7.58 (6H, m, Ar‐H), 6.86 (1H, s, C(OH)=CH‐CO). 13C NMR (CDCl3, 75 MHz, δ, ppm): 185.9 (CO), 135.7, 132.7, 128.9, 127.4 (Ar‐C), 93.3 (C(OH)=CH‐CO). Anal. Calcd. for C15H12O2: C, 80.34; H, 5.39. Found: C, 80.39; H, 5.42%. 2.6. Melting point During melting point determination, the transformation in the capillary was observed approximately 20 °C below the typical melting point. The substance was becoming muddy and probably partially melted. The material remained in this state without change until the appropriate melting point temperature was achieved. The existence of two melting points is probably connected with the formation of intramolecular hydrogen bonds and two possible crystalline forms, which may coexist in various ratios depending on which solvent was used for crystallization. Crystallization from DMF/MeOH leads to the formation of large crystals of product containing mainly the form with the higher melting point. 3. Results and discussion Bis‐(1,3‐diketones) were obtained by the crossed‐Claisen condensation of esters of phthalic acid isomers and aceto‐ phenone (Scheme 3) ‐ a modified procedure first reported in [8]. Reactions were conducted in tetrahydrofuran (THF) with a 30% solution of sodium methoxide as a base. Besides bis‐(1,3‐ diketones), the intermediate product with one unreacted ester group was found in the reaction mixture (Scheme 3). We investigated the influence of the quantity of acetophenone on bis‐(1,3‐diketone) yield. While an excess of acetophenone decreases the amount of intermediate product, it has little effect on the yield of bis‐(1,3‐diketone). After neutralization of the reaction mixture, we also found 1,3‐diketones with carboxylic groups that are formed via hydrolysis of the ester group of the intermediate product (Scheme 3). Thus, we obtained three diketones from dimethyl p‐phthalate: 1,4‐bis‐(3‐phenyl‐3‐oxopropionyl)benzene (1), methyl 4‐(3‐phenyl‐3‐oxopropionyl)benzoate (2), 4‐(3‐phenyl‐ 3‐oxopropionyl)benzoic acid (3), and three diketones from dimethyl m‐phthalate: 1,3‐bis‐(3‐phenyl‐3‐oxopropionyl) benzene (4), methyl 3‐(3‐phenyl‐3‐oxopropionyl)benzoate (5), 3‐(3phenyl‐3‐oxopropionyl)benzoic acid (6). For comparison, we also obtained 1,3‐diphenylpropane‐1,3‐dione (7) from methyl benzoate (Scheme 4). 292 Zawadiak et al. / European Journal of Chemistry 2 (3) (2011) 289‐294 Scheme 4 Yields are listed in Table 1. HPLC analysis of the obtained compounds showed that the reaction provides almost complete conversion of the phthalates. The main product after hydrolysis is proper phthalic acid, which is probably formed during hydrolysis with 10% hydrochloric acid. Because of the extremely low solubility of phthalic acids and diketones with carboxylic acid groups (3,6) and the relatively low solubility of bis‐diketones (1,4) (Table 2), these products are difficult to separate. The separation process described in [8], repeated in later publications [5,7], gave contaminated product with low yields. Table 2. Solubility in g/100 g of solvent of 1, 2 and 3. Product Solvent Solubility [g/100 g of solvent] at 21 oC at boiling temp. 1 Benzene 0.22 ‐ 1 Acetone ‐ 2.40 (56°C) 1 CH2Cl2 1.17 2.27 (39°C) 1 MeOH 0.08 0.10 (65°C) 1 THF 4.42 12.68 (66°C) 1 DMF 4.20 ‐ 1 MeOH/DMF (1/1, v/v) 0.57 4.07 (81°C) 2 CH2Cl2 27.87 ‐ 2 MeOH 0.56 5.50 (65°C) 3 Acetone 0.02 ‐ 3 CH2Cl2 0.41 ‐ 3 MeOH 0.83 ‐ Therefore, we carried out several syntheses (method 2), using glacial acetic acid in the hydrolysis step, to prevent formation of diketones with carboxylic acid groups. Using this method, we obtained the by‐products as a gelatinous precipitate. After centrifugation and concentration, a relatively pure product precipitated from solution. To achieve high purity, we crystallized the products from a mixture of dimethylformamide and methanol (1:1, v/v), a protocol which required significantly less solvent and increased the crystallization yield relative to that obtained using only methanol as solvent. The use of sodium hydride instead of sodium methoxide increased the yield of bis‐(1,3‐diketones). When o‐phthalate was used as the ester in this synthesis, we did not obtain the desired products. However, we isolated a compound that was identified by 1H NMR as mixture of E and Z isomers of 3‐(2’‐oxo‐2’‐phenyl‐ethylidene)‐3H‐isobenzofurane‐ 1‐one in a ratio of 5:3. This compound probably results from intramolecular transestrification of the intermediate product (Scheme 5). Scheme 5 3.1. UV absorption We investigated the UV absorption spectra of the generated compounds in anhydrous ethanol (λ = 210‐450 nm), acetonitrile (λ = 210‐450 nm) and n‐heptane (λ = 200‐450 nm). The concentration of all solutions was 50±0.5 μmol/dm3. Because of their low solubility, the spectra of some compounds were not investigated in n‐heptane. Molar absorption coefficients and absorption maxima are given in Tables 3 and 4. The UV spectra of all obtained compounds in ethanol are given in Figures 1 and 2. Figure 1. Absorption spectra of 1, 2, 3 and 7. Ethanol, 0.5 cm, 5x10‐5 mol/dm3. Figure 2. Abosrption spectra of 4, 5, 6 and 7. Ethanol, 0.5 cm, 5x10‐5 mol/dm3. All investigated compounds absorb strongly in the range of λ1 = 280‐400 nm. In addition, two weaker absorption bands can be distinguished: λ2 = 230‐280 nm, λ3 = 200‐230 nm. However, only 7 and compounds synthesized from methyl terephthalate (1‐3) have distinct peaks in band λ2; peaks of 5 and 6 in this range are barely noticeable, covered by peaks in third band, which come probably from non‐conjugated carboxylic group. We found that in the first band, the molar absorption coefficient of 1 is over two times higher than that of 7, and the absorption maximum is strongly red‐shifted. Such strong hyperchromic and bathochromic (Δλ ≈ 33 nm) effects are probably caused by the interaction of two conjugated 1,3‐ dicarbonyl moieties. Zawadiak et al. / European Journal of Chemistry 2 (3) (2011) 289‐294 293 Table 3. Molar absorption coefficients and absorption maxima of synthesized compounds in the range of λ = 280‐400 nm. Product λmax / nm εmax / dm3 mol‐1 cm‐1 Ethanol CH3CN n‐Heptane Ethanol CH3CN n‐Heptane 1 375.3 374.5 ‐ 54019 44979 ‐ 2 350.4 347.0 342.7 21066 20373 19748 3 348.7 346.5 ‐ 25409 23636 ‐ 4 354.3 351.6 ‐ 43142 38806 ‐ 5 342.5 341.8 337.5 23789 22622 22686 6 343.1 341.2 ‐ 23636 23333 ‐ 7 342.4 342.0 335.6 25037 23741 23629 Table 4. Molar absorption coefficients and absorption maxima of synthesized compounds in the range of λ = 200‐280 nm. Product λmax / nm εmax / dm3 mol‐1 cm‐1 Ethanol CH3CN n‐Heptane Ethanol CH3CN n‐Heptane 1 258.8 256.8 ‐ 16740 14980 ‐ 2 253.2 252.2 252.2 10830 11199 10735 3 253.2 251.8 ‐ 11547 12861 ‐ 4 233.2 233.6 ‐ 16111 16410 ‐ 5 249.7 246.6 247.7 10846 11380 10850 221.6 219.6 221.0 21333 21460 21352 6 249.0 247.8 ‐ 10067 11619 ‐ 219.8 219.2 ‐ 19222 21958 ‐ 7 253.0 249.6 248.6 8979 9444 9884 Table 5. Enol percentages and keto‐enol tautomerism equilibrium constants of synthesized compounds. Product Enol form contents [%] KT DMSO‐d6 Acetone‐d6 CDCl3 C6D6 DMSO‐d6 Acetone‐d6 CDCl3 C6D6 1 95.7 97.9 98.2 >99.9 22 46 53 >999 2 95.4 98.0 >99.9 >99.9 21 49 >999 >999 3 95.1 95.3 98.3 >99.9 20 20 57 >999 4 92.7 97.6 96.8 >99.9 13 41 30 >999 5 92.7 97.0 97.1 >99.9 13 32 33 >999 6 93.1 96.7 97.8 >99.9 13 29 45 >999 7 94.0 97.7 97.8 99.9 16 42 45 999 Table 6. Chemical shifts of characteristic protons of synthesized compounds*. Product Chemical shift referenced against TMS / ppm C(OH)=CHCO C(OH)=CHCO CO‐CH2‐CO D A C B D A C B D A C B 1 17.11 17.14 16.82 16.33 7.48 7.37 6.92 6.31 4.98 4.91 4.69 ‐ 2 17.01 17.08 16.79 17.21 7.42 7.33 6.90 6.22 4.94 4.89 ‐ ‐ 3 17.04 17.10 16.74 17.20 7.42 7.35 6.92 6.20 4.94 4.90 4.68 ‐ 4 17.18 17.29 16.89 17.65 7.47 7.41 6.93 6.64 5.02 4.98 4.73 ‐ 5 17.08 17.21 16.85 17.60 7.47 7.35 6.92 6.59 5.02 4.93 4.67 ‐ 6 17.14 17.20 16.83 17.56 7.42 7.35 6.92 6.59 4.95 4.92 4.71 ‐ 7 17.21 17.27 16.90 17.74 7.36 7.27 6.86 6.61 4.89 4.84 4.63 4.10 * D ‐ DMSO‐d6, A ‐ Acetone‐d6, C ‐ CDCl3, B ‐ C6D6. While hyperchromic and bathochromic effects in this band are present with 4, they are not as strong as those observed with 1. This finding results from the lack of conjugation between the dicarbonyl groups through the aromatic ring. Intermediate products (2 and 5) and by‐products (3 and 6) do not exhibit the significant spectral effects observed with bis‐(1,3‐ diketones), and their spectra are similar to that of 7. However, we observed small bathochromic effects for 2 (Δλ ≈ 8 nm) and 3 (Δλ ≈ 6 nm), which arise from the conjugation of the 1,3‐ dicarbonyl group with ester or carboxylic acid groups. 3.2. Keto‐enol tautomerism We determined the enol percentage of each diketone by recording their 1H NMR spectra at ambient temperature in four solvents: DMSO‐d6, acetone‐d6, CDCl3 and benzene‐d6. The keto‐ enol equilibrium constant was calculated using the formula: KT = [enol] / [keto]. The concentration of all solutions was 100±10 mmol/dm3. Enol percentages and keto‐enol tautomerism constants are given in the Table 5. Chemical shifts of the characteristic protons are given in Table 6. Influence of solvent is slight; however, the equilibrium is increasingly shifted to the keto form as solvent polarity is increased. In deuterated benzene, the equilibrium is shifted to the enol form to such an extent that a peak corresponding to the characteristic protons of the keto form was not observed (except 7). 4. Conclusion We investigated the UV‐absorption and keto‐enol tautomerism of seven 1,3‐dicarbonyl compounds. The keto‐ enol equilibrium constant of all compounds suggests that the equilibrium favors the enol form. With increasing solvent polarity, the equilibrium slightly shifts to the keto form, a finding which probably results from the association of polar solvent with the keto tautomers. Keto tautomers are more polar than the pseudo‐cyclic, intramolecularly‐bonded enol tautomers. Domination of the enol tautomer results in strong absorption of radiation in the UV‐A range by all investigated compounds, regardless of the solvent used. The absorption profile of the aromatic 1,3‐diketones with carboxylic acid and ester groups is similar to that of 1,3‐diphenylpropane‐1,3‐dione (7). However, aromatic bis‐(1,3‐diketones) show strong bathochromic and hyperchromic effects which result from the presence of two 1,3‐dicarbonyl moieties. The strongest 294 Zawadiak et al. / European Journal of Chemistry 2 (3) (2011) 289‐294 hyperchromic effect is observed for 1, where the dicarbonyl groups are conjugated through the aromatic ring. This conjugation also causes the strong bathochromic shift and absorption in visible range of the radiation. This effect on the absorption profile of 1 gives rise to its yellow color, which obviates some of its applications. In the case of 4, we observed a somewhat smaller, but still very strong hyperchromic effect, with significantly smaller bathochromic effects resulting from the non‐conjugated dicarbonyl groups. Because of its better absorption properties in comparison to other 1,3‐diketones, compound 4 has a chance to become a promising future substitute for currently used sun‐protective agents. Acknowledgement This work was supported by the European Community from the European Social Fund within the RFSD2 project. References [1]. Lim, H. W.; Draelos, Z. D. In Clinical Guide to Sunscreens and Photoprotection; Informa Healthcare USA, Inc., New York, 2009. [2]. Zawadiak, J.; Mrzyczek, M. Spectrochim. Acta, Part A 2010, 75, 925‐ 929. [3]. Wypych, G. In PVC Degradation & Stabilization; 2nd Edition, ChemTec Publishing, Toronto, 2008. [4]. Przybyszewska, M.; Zaborski, M.; Jakubowski, B.; Zawadiak, J. Express Polymer Lett. 2009, 3(4), 256‐266. [5]. Clegg, J. K.; Bray, D. J.; Gloe, K.; Gloe, K.; Jolliffe, K. A.; Lawrance, G. A.; Lindoy, L. F.; Meehan, G. V.; Wenzel, M. Dalton Trans. 2008, 10, 1331‐ 1340. [6]. Soldatov, D. V.; Zanina, A. S.; Enright, G. D.; Ratcliffe, C. I.; Ripmeester, J. A. Cryst. Growth Des. 2003, 3(6), 1005‐1013. [7]. Bao‐Li, A.; Chuan‐Wei, H.; Hong‐Liang, M.; Qing‐Yi, P.; Ming‐Xin, L. J. Lumin. 2007, 127, 297‐301. [8]. Martin, D. F.; Shamma, M.; Fernelius, W. J. Am. Chem. Soc. 1958, 80(18), 4891‐4895. [9]. Xuan, W. M.; Zou, F.; Chen, L. Q.; Fang, X. M.; Lian, W.; Zhang H. Acta Phys. Chim. Sin. 2008, 24(6), 955‐960. [10]. Adam, W.; Kita, F.; Harrer, H. M.; Nau, W. M.; Zipf, R. J. Org. Chem. 1996, 61, 7056‐7065.