Design of a Primary-Amide-Functionalized Highly Efficient and Recyclable Hydrogen-Bond-Donating Heterogeneous Catalyst for the Friedel–Crafts Alkylation of Indoles with β‑Nitrostyrenes

A primary-amide-functionalized
metal organic framework, {[Zn2(2-BQBG)­(BDC)2]·10H2O}n (1) (in which 2-BQBG = 2,2′-(butane-1,4-diylbis­((quinolin-2-ylmethyl)­azanediyl))­diacetamide
and BDC = 1,4-benzenedicarboxylate), has been found to be a highly
efficient hydrogen-bond-donating (HBD) heterogeneous catalyst for
the Friedel–Crafts alkylation of indole with β-nitrostyrenes
under mild reaction conditions (catalyst loading: 3 mol %; reaction
conditions: 12 h and 35 °C). The catalyst can be easily separated
from the reaction mixture by simple filtration for its reuse in four
consecutive cycles with very little loss of activity. More importantly,
the one-pot room temperature synthesis of 1 from the
self-assembly of Zn­(OAc)2·2H2O and 2-BQBG
(in CH3OH) and Na2BDC (in H2O) can
be easily scaled-up for obtaining multigram quantities in few hours.
In order to showcase its versatility, the substrate scope included
a variety of substituted indoles and different β-nitrostyrene
derivatives forming the desired products in good to high yields. For
its catalytic action, a direct proof for the key step in the proposed
mechanism, based on the interaction of a primary-amide group in the
2-BQBG ligand with the nitro group of β-nitrostyrene through
hydrogen bonding, is provided from the enhancement in fluorescence
intensity of 1 upon successive addition of β-nitrostyrene.