Ethyl 4-acetyl-5-oxo-3-phenylhexanoate

The reaction of ethyl 3-bromo-3-phenylpropanoate with pentane-2,4-dione, in the presence of palladium(II) acetate and triphenylphosphine, in dimethylformamide, unexpectedly gave the title product, C16H20O4. The molecule contains one chiral C atom but the crystal is racemic. In the crystal, neighboring molecules form a chain along [100] through three weak C—H⋯O interactions. Furthermore, a double-stranded structure is formed through weak C—H⋯O interactions between two parallel chains.

The reaction of ethyl 3-bromo-3-phenylpropanoate with pentane-2,4-dione, in the presence of palladium(II) acetate and triphenylphosphine, in dimethylformamide, unexpectedly gave the title product, C 16 H 20 O 4 . The molecule contains one chiral C atom but the crystal is racemic. In the crystal, neighboring molecules form a chain along [100] through three weak C-HÁ Á ÁO interactions. Furthermore, a double-stranded structure is formed through weak C-HÁ Á ÁO interactions between two parallel chains.
We thank the National Science Foundation of China (project Nos. 21072003 and 20872002) for financial support for this work.  Palladium-catalyzed coupling reactions have become an important tool in modern organic synthesis chemistry (Hu et al. 2008). They have made a wide variety of active pharmaceutical ingredients, natural substances and other complex organic molecules economically accessible (Hu & Yu et al., 2009;Hu, Ouyang et al., 2009). The pentane-2,4-dione derivatives, which have physiological activity, are effective intermediates in the synthesis of many complex natural products (Vijaikumar & Pitchumani, 2010). We have reported some novel palladium-catalyzed intermolecular and intramolecular reactions of aryl halides with the olefins and diynes Hu, Ren et al., 2010). The reaction of ethyl 3-bromo-3-phenylpropanoate with pentane-2,4-dione, in the presence of palladium(II) acetate and triphenylphosphine, in DMF at 373 K for 22 h, gave the unexpected title product.
The molecular structure of the title compound, C 16 H 20 O 4 , reveals that all the bond lengths and angles have normal values.
As shown in Fig. 1, one chiral carbon, C7, was observed in the molecule. Due to the existence of inversion centers in the crystal packing, the C7 atoms exhibit R-conformation in the half of the molecules, and display S-conformation in the other half of the molecules. So the whole crystal is racemic (Fig. 4). In the crystal packing, the weak C-H···O interactions play important roles. Neighboring molecules form a one dimensional chain through the weak C6-H6···O1 ii , C8-H8b···O1 ii and C12-H12···O1 ii (ii: 1+x, y, z) interactions (Fig. 2). Furthermore, two neighboring chains are parallel to each other to form a double-stranded structure through the weak C14-H14C···O4 i (i: 2-x, 1-y, 1-z) interactions (Fig. 3).

Experimental
An oven-dried Schlenk flask was evacuated, filled with nitrogen, and then charged with pentane-2,4-dione (1.00 g, 10 mmol), ethyl-3-bromo-3-phenylpropanoate (2.82 g, 11 mmol), tributylamine (3 ml), PPh 3 (52.5 mg, 0.2 mmol), Pd(OAc) 2 (24 mg, 0.1 mmol), and DMF (10 ml) to give a yellow solution. The reaction mixture was heated at 373 K with stirring. The reaction mixture was cooled to room temperature after 22 h and the resulting yellow-orange mixture was diluted with Et 2 O (10 ml). The mixture was washed with H 2 O (15 ml) and the aqueous layer was extracted with Et 2 O (20 ml). The combined organic layers were dried (MgSO 4 ), filtered, and concentrated in vacuo. The crude material was purified by flash column chromatography on silica gel (petroleum ether:EtOAc, 9:1) and recrystallized from EtOAc, yield 2.27 g (82%). Colorless crystals suitable for X-ray diffraction were obtained by recrystallization from a solution of the title compound from ethyl acetate, over a period of one week.

Refinement
H atoms were positioned geometrically and refined using a riding model (including free rotation about the methyl C-C bond), with C-H = 0.93-0.97 Å and with U iso (H) = 1.2 (1.5 for methyl groups) times U eq (carrier C).    Ethyl 4-acetyl-5-oxo-3-phenylhexanoate