Paraformaldehyde, phosphorus pentoxide and acetone (Panreac, Barcelona, Spain), toluene (QUINSA, D.F., Mexico), piperidine hydrochloride (Acros Organics, Geel, Belgium), monohydrated p-toluene sulfonic acid (APTS), and tetrahydrofuran (THF) (Merck Schuchardt OHG, Hohenbrunn, Germany), hydroquinone Analar (BDH Chemicals Ltd, Poole, England), and ethanol (96%) (national production) were used without further purification. n-Butyl cyanoacetate (Probus, Barcelona, Spain) was distilled previous to its use. The Fourier transform IR spectra were measured using a JASCO FT/IR-4100 spectrometer and KBr tablets. 1H-NMR spectra were obtained at 250 MHz using a Bruker 250 Avance NMR spectrometer with tetramethylsilane as reference. Microwave radiation was carried out using a microwave Milestone’s START System. The BCA synthesis occurs fundamentally in two steps (Equations (1) and (2)). To check the reproducibility of the methods, three experiments were carried out applying both heating sources in the cases of “direct” and “indirect synthesis”.
3.2. Direct Synthesis of BCA Using an Oil Bath
n-Butyl cyanoacetate (50 mL, 0.3517 mol), paraformaldehyde (11.689 g, 0.3 mol), piperidine hydrochloride (0.3 g, 0.002 mol) and toluene (37 mL, 0.3 mol) were mixed in a three-neck round-bottom flask. The mixture was heated to about 130 °C under stirring, while distilling out the water and toluene. When the distillation stopped, the mechanical stirring was removed and the rest of the water-toluene mixture extracted under reduced pressure. The reaction ended after about 75 min, when distillation stopped. The formed water was measured. Phosphorus pentoxide (1.4218 g, 0.01 mol), hydroquinone (0.4405 g, 0.004 mol) and APTS (0.4517 g, 0.002 mol) were then added to the product, which was pyrolyzed for one hour at 109 °C between 0.15–1 mm Hg. Hydroquinone (0.1767 g, 0.002 mol) and APTS (0.0961 g, 0.0005 mol) were then added to the obtained monomer, which was purified by distillation. APTS (0.02 g, 0.0001 mol) was added to the final product (36 mL, 67%). The distillation equipment used in the last two stages had been previously treated with a 20% APTS solution. Figure 1 shows all these stages.
BCA: IR (KBr): 3124, 2960–2878, 2235, 1735, 1612, 1461, 1283-1187, 1386 cm−1. 1H-NMR (acetone-d6, 250 MHz): δ 0.96 (3H, m3, J = 7.23 Hz, (CH2)3-CH3), 1.44 (2H, m6, J = 8.03 Hz, (CH2)2-CH2-CH3), 1.72 (2H, m5, J = 7.76 Hz, CH2-CH2-CH2-CH3), 4.29 (2H, m3, J = 7.23 Hz, O-CH2-(CH2)2-CH3), 6.63 (1H, s,=CH ), 7.06 (1H, s, =CH).
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