A Practical Indium Tribromide Catalysed Addition of Indoles to Nitroalkenes in Aqueous Media

Abstract

The 1,4-conjugate addition of indoles to nitroalkenes was efficiently carried out in aqueous media using a catalytic amount of indium tribromide (5 mol%). The reusability of the indium tribromide was tested by performing consecutive cycles with the same catalyst.

alkaloids - catalysis - indium - indoles - Michael additions

References

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<A NAME="RZ05302SS-8">8</A> Recently, Yadav et. al. reported that InCl₃ (10 mol%) was also able to promote the conjugate addition of indole 5b to 6a in 78% yield using CH₂Cl₂ as the solvent. However, only one example of conjugate addition to nitroalkenes was described, see: Yadav JS. Abraham S. Reddy BVS. Sabitha G. Synthesis  2001,  2165.  Under our conditions, InCl₃ was found less effective in comparison to InBr₃ in the addition of 5a to 6a, 82% yield

<A NAME="RZ05302SS-9">9</A>

Other common Lewis acids were found to be less effective in term of chemical yields and reaction times (i.e. AlCl₃, 68% yield, 24 h).