Highly Efficient Povidone–Phosphotungstic Acid Catalyst for the Tandem Acetalization of Aldehydes to Bis‐ and Tris(indolyl)methanes

Abstract

A novel, nonleachable hybrid of heteropoly acid and polyvinylpyrrolidone (or povidone) catalyzes the acetalization of aldehydes in methanol at room temperature followed by reaction with indole to give bis(indolyl)methanes (BIMs) and tris(indolyl)methanes (TIMs) in quantitative yields (90–97 %). The catalyst was shown by pyridine FTIR spectroscopy to possess Brønsted acidity, and the hybrid formation was confirmed by XRD and 31P NMR studies. Friedel–Crafts alkylation of indole as well as the tandem synthesis of BIMs and TIMs were established with several types of carbonyl and indole substrates to give the corresponding products quantitatively. The catalyst was recycled efficiently for three successive runs without losing its original activity.

Cascade Synthesis of 2‐Cyanoacrylamides through Deacetalization and/or Knoevenagel Condensation followed by Selective Monohydration of Acetals and Aldehydes over Solid Acid Ferrites

“…The acetals were prepared by the reported procedures . The products were purified by using recrystallization in methanol and characterized by 1 H NMR and 13 C NMR spectroscopy using D 6 ‐DMSO (0.01 % TMS) as a solvent on 200 MHz frequency of Bruker instrument.…”

“…Based on this perspective and our previous results on the tandem acetalization of aldehydes to bis‐indolylmethanes (BIMs) and tris‐indolylmethanes (TIMs) using heteropolyacid salts we report here the highly efficient single‐site Brønsted acidic cascade synthesis of 2‐cyanoacrylamides ( 1 ) involving deacetalization and/or Knoevenagel condensation followed by selective monohydration of acetals ( 5 ) and aldehydes ( 2 ) (Scheme C) over a magnetically retrievable ferrite‐based solid acid composite of Fe 3 O 4 , poly(vinylpyrrolidone) (povidone, PVP), and phosphotungstic acid (PWA), Fe 3 O 4 /PVP–PWA, by varying the ratio of PVP as 1:3:1, 1:4:1, and 1:8:1, which are subsequently named as Fe 3 O 4 /PVP–PWA (131, 141, and 181, respectively). Our catalyst, Fe 3 O 4 /PVP–PWA, offered the distinct advantage of recovery and reuse by simple magnetic separation, thus eliminating the time‐consuming and tedious filtration/separation/isolation protocols.…”

Highly efficient triphenyl(3-sulfopropyl)phosphonium functionalized phosphotungstic acid on silica as a solid acid catalyst for selective mono-allylation of acetals

“…In the continuation of our previous work on the heterogeneous povidone (PVP) based phosphotungstic acid catalyst system, 28 we report here the silica supported triphenylIJ3sulfopropyl)phosphonium salt of phosphotungstic acid (PW-Si/TPSP) as a novel heterogeneous and recyclable acid catalyst for the selective mono-allylation of acetals by the Hosomi-Sakurai reaction (Scheme 1).…”

Methylene Surrogates for the Synthesis of 3,3’‐Diindolylmethanes

“…reported a Friedel Crafts type alkylation of indoles with formaldehyde dimethyl acetal in the presence of recyclable polyvinylpyrrolidone-phosphotungstic acid (PVP-PWA) ionic liquid catalyst. [25] The treatment of 1 equivalent of formaldehyde dimethyl acetal with 2 equivalents of indole in the presence of 1 mol% of the catalyst with dichloromethane as solvent at 25°C afforded di(1H-indol-3-yl)methane in an excellent yield of 92%. The acetal can also be generated in situ with the treatment of the aldehyde with methanol in the presence of PVP-PWA catalyst.…”

“…The iminium species (23) is then attacked by indole followed by a deprotonation to give (24). The same involves the removal of N-Methyl acetamide molecule to give (25) which undergoes nucleophilic attack by indole to give the DIM derivative (Scheme 25).…”