Benzyl triphenylphosphine: Sigma Aldrich, 96%, 430056
Potassium tert-butoxide: Sigma Aldrich, ≥98%,156671
2,4,6-Trimethoxybenzaldehyde: Sigma Aldrich, 98%, 138711
Trifluoroacetic acid (SD fine Chemicals Ltd), 98%, 40508L05 

Preparation of 2,4,6-trimethoxy-1-styrylbenzene. To the stirred solution of benzyl triphenylphosphine bromide salt (1.23 g, 2.8 mmol) in anhydrous THF (10 ml) under N₂ atmosphere, was added potassium tert-butoxide (0.342 g, 3.0 mmol) in small portions at 0-5 ºC and the resulting mixture stirred at 80 ºC for 30 min. Formation of orange color was observed in the reaction.  Reaction was cooled to the room temperature and 2,4,6-trimethoxy benzaldehyde (0.5 g, 2.5 mmol) was added. The resulting reaction mixture was again refluxed at 80 ºC for 30 min. Reaction mixture was quenched with ethyl acetate (10 ml), and solvent was evaporated under vaccum, followed by partitioning between water and ethyl acetate (50 ml x 2). The organic layer was collected, dried on anhydrous sodium sulphate and evaporated on rotary evaporator to get the crude product. The crude product was purified by silica gel (#100-200) column chromatography using 20% EtOAc: hexane to get (E/Z)-1-styryl 2,4,6-trimethoxy benzene (0.667g) in 92% yield.

Procedure for synthesis of 2-phenylnaphthalene. The solution of 2,4,6-trimethoxy-1-styrylbenzene (100 mg) in 50% trifluoroacetic acid in water (1 ml) or 100% TFA (0.5 ml) was stirred at 80 ºC for 30 min. Reaction was monitored with TLC using 100% n-hexane as an eluent. Reaction mixture was then cooled to room temperature and neutralised slowly with saturated NaHCO₃ solution; and extracted with ethyl acetate (25 ml x 2). The combined organic layer was dried over anhydrous sodium sulphate and evaporated on vacuo rotavapor to get crude product. The crude product was purified by silica gel (#60-120) column chromatography using 100% n-hexane as an eluent to get pure 2-phenylnaphthalenes (35 mg) in 94% yield.

This work provides simple, efficient and metal free protocol for the synthesis of 2-phenylnaphthalenes from 1-styryl-2,4,6-trimethoxybenzenes. The reaction proceeds via TFA catalyzed C-C bond cleavage followed by intermolecular [4+2]-Diels-Alder cycloaddition of in situ formed styrenyl trifluoroacetate intermediate. The quantum chemical calculations identified the transition state for the cycloaddition reaction and helped in tracing reaction mechanism.

2-Phenylnaphthalene: white crystalline solid

M.P: 102-103 °C

¹H NMR (CDCl₃, 400 MHz): d  8.08 (s, 1H), 7.95 (m, 3H), 7.79 (t, J = 8.4 Hz, 3H), 7.55-7.49 (m, 4H), 7.42 (t, J = 6.0 Hz, 1H).

¹³C NMR (CDCl₃, 100 MHz): d 141.2, 138.7, 133.8, 132.7, 128.9, 128.5, 128.3, 127.7, 127.5, 127.4, 126.4, 126.0, 125.9, 125.7.

IR (CHCl₃): v_max 3436, 3054, 2922, 2850, 1916, 1598, 1500, 1451, 1402, 1383, 1260, 1239,1192, 1129,  1090, 1019 cm^-1.

 GC-MS: m/z (EI) 204 (M⁺, 100), 205 (M+1, 13).

Mudududdla, R.; Sharma, R.; Abbat, S.; Bharatam, P. V.; Vishwakarma, R. A.; Bharate, S. B. Synthesis of 2-phenylnaphthalenes from styryl-2-methoxybenzenes. Chem. Commun. 2014, 50, 12076-12079. http://dx.doi.org/10.1039/C4CC05151C

Bharate, S. B.; Mudududdla, R.; Sharma, R.; Vishwakarma, R. A. The first method for C-devinylation of aromatic systems. Tetrahedron Letters 2013, 54, 2913-2915. http://dx.doi.org/10.1016/j.tetlet.2013.03.064

Mudududdla, R.; Sharma, R.; Guru, S.K.; Kushwaha, M.; Gupta, A.P.; Bharate, S.S.; Aravinda, S.; Kant, R.; Bhushan, S; Vishwakarma, R.A.; Bharate, S.B. RSC Adv. 2014, 4, 14081-14088. http://dx.doi.org/10.1039/c3ra47874b