The reaction was also carried out with electronically rich 1.2 and diphenylacetylene which provided 81% yield of an unequal mixture of E-Z isomers (E:Z = 1:0.45) in 10 h reaction time (Table 2, entry 2). The mixture of E-Z-isomers (2.2a and 2.2b, figure 1) isolated as pure products has a strong predominance of E-isomer over Z-isomer probably because of the C-H–O=C hydrogen bonding in E isomer which was analyzed through 1H NMR spectroscopy. In 1H NMR spectrum of E-Z isomer 2.2a and 2.2b (figure 1), integration shows that signals correspond to the E-Z isomers are in the ratio 1:0.5 and peaks at 10.83 and 9.34 ppm belong to the aldehydic (CHO) proton of hydrogen- bonded E-isomer and non-bonded Z-isomer respectively. The downfield shift of the CHO proton at 10.83 ppm in the E-isomer corresponds to the intramolecular hydrogen bonding (2.2a, figure 1). Another pair of signals at 7.83 and 7.52 ppm was assigned to the proton at the C-7 position in the E- and Z-isomers respectively. The 0.31 ppm downfield shift of the first signal with respect to the second one might be due to the H–O=C hydrogen bond in the E-isomer. Thus, it can be concluded that mixture of E and Z-isomers isolated (2.2a and 2.2b) as pure product exhibited a strong predominance of E-isomer probably because of H-bonding as compared to Z-isomer.In addition, electron deficient acrylaldehydes like (E)-3-(2-bromo-4-chlorophenyl)acrylaldehyde (1.3) and sterically hindered (E)-3-(2-bromo-6-fluorophenyl)acrylaldehyde (1.4) were involved in the carboannulation process to deliver the desired products. Reaction of 1.3 with diphenylacetylene delivered corresponding indene derivative 2.3 of 74% yield with mixture E-Z diastereomers (E:Z = 1:0.31) in 12 h reaction time (Table 2, entry 3).
For the study of a sterically hindered substrate and to analyze the effect of more stronger hydrogen bonding, reaction of fluoro-substituted (E)-3-(2-bromo-6-fluorophenyl) acrylaldehyde were carried out with diphenylacetylene under the established condition to furnish desired product 2.4 as a mixture of E-Z isomer (E:Z = 2:1) with 74% yield in a 15 h reaction time (Table 2, entry 4).
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