Synthesis of Cobalt tri(2-guanidinobenzimidazole), tetrakis[(3,5-trifluoromethyl)phenyl]borate and study of the catalytic effect of the complex on electrophilic aromatic reaction of indole with β - nitrostyrene

The addition of aromatic substrates to electron-deficient alkenes, which in many respects may be considered a Friedel– Crafts type alkylation, is a key reaction in synthetic organic chemistry for the formation of new C-C bonds. Indole and many of its derivatives are relevant units in many naturally occurring compounds, because of their pharmacological and biological properties. Due to the increased nucleophilic reactivity of C3 position of indole ring, it is often used for the subsequent transformations leading to different indole alkaloids. Here I report an efficient catalyst that can catalyzes the electrophilic substitution of indole with electron deficient alkenes. First, Cobalt tris(2-guanidinobenzimidazole), tri chloride (Co(GBI)33+, 3Cl-) is prepared and then the reaction of indole with beta - nitrostyrene has studied. No product was detected, so the reaction conditions such as the solvents and the amount of catalyst were changed, but the results were the same. After that the chloride moiety of the complex were replaced with tetrakis[(3,5-trifluoromethyl)phenyl]borate (Barf-)anion and the reaction has run on different condition. Our results showed that Cobalt tri(2-guanidinobenzimidazole), tetrakis[(3,5-trifluoromethyl)phenyl]borate (Co(GBI)33+, 3BArf- ) catalyzes the reaction of indole with β - nitrostyrene and afforded the product in high yields. It is noteworthy to mention that using lipophilic anions such as BArf- [(B(3,5-C6H3(CF3)2)4-] , the catalyst could be solubilized in nonpolar solvents that do not compete with substrates for the hydrogen bonding sites.