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A. V. Joshi et al.
pressure to obtain 0.203 g (80% isolated yield) of crude 1,1,2,2-tetrachloro-
1,2-diphenylethane, which upon recrystallization from acetic acid gave
almost 180 mg of pure, white, crystal material, Mp 160–1628C; mass
(m/z): 283, 247, 212, 195, 178, 159, 89. Anal. calcd. for C14H10Cl4
(317.95): C, 52.49; H, 3.12; Cl, 44.32. Found: C, 52.48; H, 3.25; Cl, 44.00.
NMR (CDCl3) g 6.90–7.67 (m, Ar, 10 H).
trans-Stilbene (3) was synthesized from benzal chloride by refluxing a
mixture of 1.9 mmol of benzal chloride (305 mg) in dimethylacetamide
(DMA) solvent at 1208C oil-bath temperature in the presence of 1.9 mmol
of sodium formate (129 mg), 2.5 mmol of sodium carbonate (265 mg), and
23.5 mmol of 5% Pd/C catalyst. After 7.5 h, a 61% yield to trans-stilbene
was obtained after workup and crystallization as stated previously. Mp
122–1238C; mass (m/z) 180, 165, 152, 115, 102, 89, 76, 63, 51. Anal.
calcd. for C14H12 (180.09): C, 93.29; H, 6.71. Found: C, 93.25; H, 6.75.
1,2-Dichloro-1,2-diphenylethane (4) was prepared from benzal chloride
by heating a mixture of 1.9 mmol of benzal chloride (305 mg), 1.9 mmol of
sodium formate (129 mg), 2.5 mmol of sodium carbonate (265 mg),
23.5 mmol of 5% Pd/C, and dimethylacetamide (DMA) for 3 h at 808C.
After workup, a 42% yield to product was realized. Mp 191–1938C; mass
(m/z) 250, 215, 195, 178, 165, 152, 125, 115, 99, 89, 76, 63, 51. Anal.
calcd. for C14H10Cl2 (250.03): C, 66.95; H, 4.82; Cl, 28.23. Found: C,
67.45; H, 4.05; Cl, 28.50.
1,2-Diphenylethane (5) was synthesized from benzyl chloride by
refluxing a mixture of dimethylacetamide (DMA), 1.9 mmol of benzyl
chlorides (240 mg), 1.9 mmol of sodium formate (129 mg), 2.5 mmol of
sodium carbonate (265 mg), and 23.5 mmol of 5% Pd/C catalyst in a hot oil
bath at 1208C. After 7.5 h, a 18% yield to product was obtained. Mp 51–
528C; mass (m/z) 182, 165, 152, 115, 104, 91, 77, 65, 51. Anal. calcd. for
C14H14 (182.11): C, 92.26; H, 7.74. Found: C, 92.20; H, 7.77.
CONCLUSION
We have successfully demonstrated a simple, high-yield catalytic process for
the coupling of a-halogenated toluenes to bibenzyls and stilbenes. The trans-
formation of benzotrichloride to 1,2-diphenylethane is studied in detail and it
is realized that the reaction proceeds in four distinct steps, starting with
reductive coupling at the first step followed by reductive dehalogenation at
the second, which involves electron transfer from the metals to the substrates.
Subsequently, in the third step, the resulted dichlorostilbene undergoes hydro-
genolysis followed by hydrogenation to bibenzyl in the final step, which can
only occur in the presence of a hydrogen source such as formate salt. The
reaction is selective and catalytic in Pd catalyst. Therefore, this work will
certainly encourage chemists to eliminate toxic waste of excess metallic
reducing agents previously used for this type of reaction.