Preparation and Characterization of (E)- and (Z)-2-(Biphenyl-4-yl)-1-(4-bromophenyl)-1-phenylethene Isomers

Conclusion

In summary, bromotriphenylethylene 3 was prepared as a 55:45 mixture of geometric isomers, which were separated to yield the pure (Z)- and (E)-isomers by a careful recrystal-lization process in ethyl acetate. NOESY and single-crystal X-ray crystallographic studies revealed the initially crystal-lized minor product (3a) to be the (E)-isomer and the latter-crystallized major product (3b) to be the (Z)-isomer. A single molecule of 3a was disclosed to have a triclinic crystal struc-ture. The synthesis and purification procedures described in this paper are believed to be useful for preparing pure inter-mediates of unsymmetrical stilbenes and distyrylarylenes. These results should facilitate the systematic development of functional π-conjugated hydrocarbons such as organic fluore-scent materials.

 

Experimental

Preparation of 2-(Biphenyl-4-yl)-1-(4-bromophenyl)-1-phenylethene (3). To a mixture of diethyl biphenyl-4-yl-methyl phosphonate (2, 24.89 mmol, 7.58 g) and potassium tert-butoxide (32.36 mmol, 3.63 g) in THF (50 mL) was added a solution of 4-bromobenzophenone (1, 19.14 mmol, 5.00 g) in THF (200 mL) at room temperature under an Ar atmosphere. The mixture was stirred at room temperature for 24 h. The mixture was first diluted with diethyl ether (300 mL), then washed with 1％ aqueous HCl, water, and brine, and finally dried over MgSO4 and evaporated. Purification of the crude product by column chromatography yielded 3 as a white powder (7.01 g, 89％) in a 55:45 mixture of (Z)- and (E)-stereoisomers by GC analysis. The mixture was dissolv-ed in ethyl acetate and heated to 78 °C. The solution was allowed to slowly cool to room temperature. The (E)-isomer (3a) was initially crystallized with 92％ purity. Further recrystallization of the initially crystallized product using the same procedure provided 3a (2.24 g, 32％) as pure crystals suitable for X-ray diffraction analysis: TLC Rf = 0.78 (n-hexane: Et2O = 2:1); mp 171–172 °C (uncorrected); 1H NMR (600 MHz, CDCl3) δ 0.00 (TMS), 6.97 (s, 1H), 7.07 (d, J = 8.34 Hz, 2H), 7.19 (d, J = 8.46 Hz, 2H), 7.20–7.23 (m, 2H), 7.30 (t, J = 7.37 Hz, 1H), 7.34–7.41 (m, 5H), 7.37 (d, J = 8.34 Hz, 2H), 7.43 (d, J = 8.46 Hz, 2H), 7.53 (d, J = 7.85 Hz, 2H); 13C NMR (150 MHz, CDCl3) δ 121.5, 126.6 (×2), 126.8 (×2), 127.2, 127.7, 128.0, 128.7 (×2), 128.8 (×2), 129.1 (×2), 129.9 (×2), 130.2 (×2), 131.3 (×2), 136.0, 139.5, 139.6, 140.4, 141.5, 142.3: Anal. calcd. for C26H19Br: C, 75.92; H, 4.66. Found: C, 76.02; H, 4.70.

The double recrystallization process (as outlined above) of the material remaining in the mother liquor yielded the (Z)-isomer (3b) as a very thin needle-like crystalline solid (1.00 g, 14％): TLC Rf = 0.78 (n-hexane:Et2O = 2:1); mp 152–153 °C (uncorrected); 1H NMR (600 MHz, CDCl3) δ 0.00 (TMS), 7.00 (s, 1H), 7.12 (d, J = 8.24 Hz, 2H), 7.14 (d, J = 8.34 Hz, 2H), 7.30–7.36 (m, 6H), 7.42 (t, J = 7.40 Hz, 2H), 7.43 (d, J = 8.24 Hz, 2H), 7.49 (d, J = 8.32 Hz, 2H), 7.57 (d, J = 7.27 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 121.6, 126.7 (×2), 126.8 (×2), 127.3, 127.6 (×2), 127.7, 128.2, 128.3 (×2), 128.7 (×2), 129.9 (×2), 131.9 (×2), 132.2 (×2), 136.0, 139.4, 139.6, 140.4, 141.4, 142.9.

X-ray Crystallographic Analysis of 3a.

A single crystal of 3a was mounted on a glass fiber. The diffraction data were collected on a Bruker SMART BREEZE diffractometer with sealed-tube monochromatized Mo-Kα radiation (λ = 0.7107 Å) at 200 K. The reflection data were collected as φ and ω scans. Cell parameters were determined and refined using the SAINT program. Data reduction and empirical absorption corrections were performed using SAINT software and the SADABS program, respectively.