1,1′-(2,5-Dimethylthiophene-3,4-diyl)diethanone

The title compound, C10H12O2S, crystallizes with four molecules in the asymmetric unit. The main conformational difference between these molecules is the orientation of the acetyl groups with respect to the ring. Whereas one acetyl group is only slightly twisted with respect to the thiophene ring [C—C—C—O torsion angles = 165.7 (4), −164.6 (4), 164.3 (4) and −163.6 (4)°], the other acetyl group is markly twisted out of the ring plane [C—C—C—O torsion angles = −61.2 (6), 61.3 (7), −59.7 (7) and 59.9 (6)°]. In the crystal, molecules are linked by weak C—H⋯O interactions into infinite chains along the c axis.

The title compound, C 10 H 12 O 2 S, crystallizes with four molecules in the asymmetric unit. The main conformational difference between these molecules is the orientation of the acetyl groups with respect to the ring. Whereas one acetyl group is only slightly twisted with respect to the thiophene ring [C-C-C-O torsion angles = 165.7 (4), À164.6 (4), 164.3 (4) and À163.6 (4) ], the other acetyl group is markly twisted out of the ring plane [C-C-C-O torsion angles = À61.2 (6), 61.3 (7), À59.7 (7) and 59.9 (6) ]. In the crystal, molecules are linked by weak C-HÁ Á ÁO interactions into infinite chains along the c axis.   Table 1 Hydrogen-bond geometry (Å , ).

Comment
Azomethines are an important class of compounds which have been intensively investigated owning to their strong coordination capability, antibacterial activity, antitumor property and so on. Considering this, on our way to getting novel photochromic molecules on which we focused in the past few years, we first designed and synthesized a key intermediate 1-(2,5-Dimethylthiophen-3,4-yl) diethanone, which has two carbonyl groups. Usually, azomethines are obtained by the condensation of carbonyl compounds with primary amines. Herein, the design and synthesis of this compound provides a wide space for the new azomethines of thiophene. Recently the introduction of Schiff base ligands into photochromic diarylethene system and their photochromic properties in solution has been reported (Li et al., 2011). We are trying to push forward that work through introducing the title compound to the system. Moreover, in our recent study we also found that the title compound played a good role in the synthesis of Schiff-base macrocycles. When we took different type or the length of chain diamines, we got varying size of the macrocycles and some of them had good ability of cooperation with metals.

Experimental
We used 2-methylthiophene as the starting material via, in turn, Vilsmeier, Wolff-Kishner-Huang, and Friedel-Crafts reactions and got the title compound. The synthetic processes are as follows:

5-Methylthiophene-2-carbaldehyde
To a 10 g anhydrous dimethylformamide solution of 2-methylthiophene (10 g, 0.1 mol), a (17 g, 0.11 mol) phosphorus oxychloride (POCl 3 ) was added drop by drop slowly at 0°C. After addition, the ice bath was removed and the mixture was stirred for 0.5 h at room temperature. Then, the reddish solution was heated slowly to reflux. After refluxed for 1 h and cooled to room temperature, the mixture was poured into ice water and K 2 CO 3 was added until pH=10. The mixture was extracted with diethyl ether (3×25 ml). The combined organic layers were washed with a saturated NaCl solution (2×25 ml) and H 2 O (1×25 ml), dried (MgSO 4 ), filtered and the solvents evaporated in vacuum to yield: 10.8 g, 84.2%.

2,5-Dimethylthiophene
To a 360 ml e thyl glycol solution of 5-methyl-thiophene-2-carbaldehyde (112 g, 0.9 mol), a 120 ml hydrazine hydrate (85%) was added in 1000 ml flask. The mixture was refluxed for 0.5 h, and then evaporated the excessive water and hydrazine hydrate until the oil drops showed up. After the evaporation, KOH (20 g, 0.3 mol) was added in portions to the cooled mixture. Then refluxed for 0.5 h, distilled and the mixture of oil and water was washed with a saturated NaCl solution (3× 25 ml), After being extracted, the organic phase was distilled and the fraction boiling between 134 °C and 135 °C was collected to yield 110 g, 91%.
1-(2,5-Dimethylthiophen-3,4-yl) diethanone supplementary materials sup-2 To a 200 ml dichloromethane solution of anhydrous aluminium chloride (41 g,0.3 mol), a 11 ml dichloromethane solution of acetyl chloride(16.4 g, 0.21 mol) and 15 ml dichloromethane solution of 2,5-Dimethylthiophene (23.5 g, 0.21 mol) was added dropwise in turn at 0°C. After addition, the reaction mixture was stirred for 8 h at room temperature. Then the mixture was poured into 45 ml ice-hydrochloric acid. The product was extracted with dichloromethane and the solution was dried (MgSO 4 ). After evaporation of the solvent, the pure product was obtained as a yellow solid (33.5 g, 81%) by column chromatography with petroleum/ethyl acetate(8:1) as eluent.

Refinement
H atoms were geometrically positioned with C-H = 0.96Å and U(H)=1.5U eq (C).