Selective N‐Alkylation of Indoles with α,β‐Unsaturated Compounds Catalyzed by a Monomeric Phosphate

Abstract

Catalytic N‐alkylation of indoles is challenging because the N1 nitrogen atoms are inert toward electrophilic reagents. Herein, an organic‐solvent‐soluble alkylammonium salt of a simple monomeric phosphate ion, [PO4]3−, with a high charge density acts as an efficient homogeneous catalyst for selective N‐alkylation of indoles with α,β‐unsaturated compounds. For the reaction of indole with ethyl acrylate, the turnover number reached up to 36 and the turnover frequency was 216 h−1; these values are the highest among those reported for base‐mediated systems so far. In the presence of [PO4]3− ions, various combinations of nitrogen nucleophiles (ten examples) and α,β‐unsaturated compounds (four examples) were efficiently converted to the desired N‐alkylated products in high yields. NMR and IR spectroscopies showed formation of the indolyl anion through the activation of indole by the [PO4]3− ion, which plays an important role in the present N‐alkylation.

Molybdenum-doped α-MnO2 as an efficient reusable heterogeneous catalyst for aerobic sulfide oxygenation

“…TBA 2 MoO 4 (TBA = tetra-n-butylammonium) was prepared according to a reported procedure with modification. 15 The manganese oxide catalysts were prepared by the procedures described below.…”

Weak Nucleophiles in the Aza‐Michael Reaction

“…Tetrabutylammonium phosphate TBA 3 [PO 4 ] ⋅ 9H 2 O has been later proposed as excellent catalyst for selective N‐alkylation of indoles with electron‐poor alkenes (Scheme 26). [68] This salt exhibits higher catalytic activity that commonly used strong inorganic and organic bases such as Na 3 PO 4 , K 3 PO 4 , K 2 CO 3 , Cs 2 CO 3 , t‐BuOK, TEA, Py, DBU, and DABCO which are almost inactive under the optimal reaction conditions. Taking into account that in the presence of catalytic amount of TBAOH, TBA 2 [WO 4 ] or TBA 2 [MoO 4 ] the reaction hardly proceeded, authors have concluded that anion PO 4 3− plays an important role in the successful reaction completion.…”

Molybdate ionic liquids as halide-free catalysts for CO2 fixation into epoxides

“…S9, S30, S25, S34 and S38 †). [40][41][42] These differences in the chemical shifts are ascribed to the interaction of the monomolybdate species with the cations, which influence the electronic environment. In some cases (Fig.…”