Aza‐Michael Mono‐ and Bis‐Addition of Primary and Secondary Amines Promoted by Silica‐Supported Polyphosphoric Acid, PPA/SiO₂

Abstract

The development of new protocols using non-toxic, low cost, eco-friendly, recyclable catalyst systems which give high productivity under mild reaction conditions has received much attention in organic synthesis in the last couple of years. A collection of structurally diverse β-amino carbonyl compounds was successfully synthesized by aza-Michael mono-and double-additions using silica-supported polyphosphoric acid as catalyst under solventless conditions. Highlighted advantages of this particular catalyst include ease of preparation and longterm stability under storage.

Thia-Michael Reaction under Heterogeneous Catalysis

“…In continuation of our previous studies on sustainable hetero-Michael reactions [9,10] in this research, following the 12 principles of green chemistry, a cheap, commercially available, and easily accessible heterogeneous catalyst (Amberlyst ® A21) was found to be ideal for the combination of aliphatic/aromatic thiols with common Michael acceptors (Scheme 1) to afford products (11 examples, 3a-k) in good to excellent yields. The procedure benefits from easy catalyst recovery and minimal solvent use during work-up.…”

“…All the commercially available chemicals were purchased from Aldrich and used without further purification. For the characterization of final products and monitoring of the reactions, the same procedures described in detail in our previous publication were followed in order to obtain FTIR, NMR, and MS and GC spectra [9,10]. Specifically, IR spectra were recorded on an IRAffinity-1 FTIR spectrometer (Shimadzu, Kyoto, Japan) which was previously calibrated at the absorption at 1602 cm −1 for a polystyrene strip.…”

Aza‐Michael Reaction: A Decade Later – Is the Research Over?

“…The examples of reusable, heterogeneous, inexpensive solid Brønsted acid catalysts – silica sulfuric acid (SSA) [85] or silica polyphosphoric acid (PPA‐SiO 2 ) [86] are proposed for the nucleophilic addition of aliphatic and aromatic amines to acrylates 2 or acrylonitrile 1 . It is very important that in the case of primary amines the use of these catalytic systems are favored the selective formation either mono‐ (with SSA) or dialkylated adducts (for PPA).…”