To a well-stirred mixture of trans-cinnamaldehyde (15 g, 1 mol.) in benzene (150 mL) at about 0 ^oC to -5 ^oC was added Triflic acid (2 ml, 0.2 mol.) in portions. After the addition of the reagent, the cooling bath was removed. The reaction mixture was stirred at 60 ^oC for 15 h. The resultant reaction mixture was cooled at 0 ^oC and quench with triethyl amine (approx 5-7 ml) followed by ice cold water and extracted with ethyl acetate (200 mL×2). The combined organic layer dried over sodium sulphate and concentrated under reduced pressure. The residue was flash chromatographed (the eluent was pet ether) to give 20 g (92 %) of 3-phenylindene.

This reaction has been carried out at number of time by graduate trainees. The reaction seems to be ideal for gram scale (87-92%). Triflic acid addition should not go beyond -5 ^oC to 0 ^oC (some time decomposition of reaction mixture happening). Excess amount of triflic acid addition decreasing the yield, quenching with triethyl amine gives a clean reaction mixture if we pour direct it in ice water it will form back colored emulation.

¹H NMR (200 MHz, CDCl₃): δ = 3.51 (d, J = 2.0 Hz, 2H), 6.58 (t, J = 2.0 Hz, 1H), 7.22–7.26 (m, 1H), 7.28–7.31 (m, 3H), 7.45–7.58 (m, 2H), 7.56–7.63 (m, 3H) ppm