equiv. (Table 1, entry 12), simple un-substituted (E)-3-(2-bromophenyl) acrylaldehyde

equiv. PdCl2 as catalyst with 0.2 equiv. PPh3 ligand delivered an improved yield of 40% (Table 1, entry 8). Addition of 1 equiv. of TBAI (tetra butyl ammonium iodide) did not show any improvement in the yield (Table 1, entry 9). Increasing the reaction time to 10 h in the presence of PdCl2 catalyst, PPh3 ligand, Cs2CO3 as a base and 1,4-dioxane solvent furnished higher yield of 50% at 100 oC (Table 1, entry 10). Additional reaction parameters were also studied and it is noteworthy that significant improvement in the yield of desired indene derivative was observed at 120 oC and product 2.1 was isolated in 78% yield (Table 1, entry 12). Increasing the reaction temperature to 130 oC did not show major enhancement in the yield and decomposition of (E)-3-(2-bromophenyl)acrylaldehyde 1.1 was observed at a higher temperature (Table 1, entry 13). Reaction with Pd(PPh3)4 also did not improve the yield of the product (Table 1, entry 14). Controlled experiments were carried out; one without base and the other without the catalyst. It was observed that in the reaction without a base, the yield of the product was very low (Table 1, entry 15) whereas in the absence of catalyst there was no product formation at all (Table 1, entry 16). On the basis of these optimization efforts, the combination of 1.5 equiv. diphenylacetylene, 1 equiv. of 1.1, 0.1 equiv. PdCl2 as catalyst, 0.2 equiv. PPh3 ligand with 0.5 equiv. Cs2CO3 as base and 1,4-dioxane solvent furnished yield 78% of 2.1 at 120 oC for 10 h reaction time (Table 2, entry 1).

Thus, by using above established condition, we examined the reaction scope of various functionalized (E)-3-(2-bromophenyl) acrylaldehyde and diphenylacetylene for the synthesis of complex indene derivatives as shown in table 2. As observed from optimization (Table 1, entry 12), simple un-substituted (E)-3-(2-bromophenyl) acrylaldehyde and diphenylacetylene furnished E-Z diastereomeric mixture of the corresponding indenes; (E)-2-(2,3-diphenyl-1H-inden-1-ylidene) acetaldehyde and (Z)-2-(2,3-diphenyl-1H-inden-1-ylidene) acetaldehyde. A good yield (78%) and E and Z ratio of 1:0.33 (Table 2, entry 2.1) was also observed. 

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