1H NMR study revealed that stable E isomer (2.4b) is formed in a major amount which may be attributed to hydrogen bonding between fluorine and hydrogen atoms. While the Z isomer (2.4a) was formed in a minor amount due to sterical hindrance of CHO group and F atom (Figure 2).
Additionally, more complex substrate (E)-3-(6-bromobenzod1,3dioxol-5-yl) acrylaldehyde (1.5) was used for carboannulation with internal alkyne under Pd-catalyzed condition furnished (E)-2-(6,7-diphenyl-5H-indeno5,6-d1,3dioxol-5-ylidene)acetaldehyde 2.5 with 84% yield and E:Z = 1:0.2 isomer mixture (Table 2, entry 5).
We have proposed a mechanism of the reaction is shown in scheme 1. Initially, formation of oxidative addition product A from the reaction of C-Br bond of (E)-3-(2-bromophenyl)acrylaldehyde with Pd(0) active species. The insertion of an internal alkyne with the oxidative addition intermediate species A leads to the generation of vinyl palladium intermediate B which then undergoes carboannulation leading to the formation of intermediate C. Finally, elimination of H-Pd-Br and reductive elimination occurs to give 1H-inden-1-ylidene and regeneration of Pd(0) active species. In conclusion, we have developed new synthetic approach to excess diverse range of functionalized indene derivatives through the pentannulation process of internal alkyne and substituted (E)-3-(2-bromophenyl)acrylaldehyde under palladium catalysis. The reaction proceeded in moderate to high yield with excellent functional group tolerance. Different ratio of E and Z mixture of diastereomers were synthesized with a strong predominance of E-isomer over Z-isomer depending upon the nature of hydrogen bonding and different substituents (electron rich, electron deficient, sterically hindered). Protocol condition established for annulation involves routinely used palladium catalyst in the convenient reaction conditions.