Palladium(II)/Lewis Acid-Catalyzed Olefination of Arylacetamides with Dioxygen

Abstract

The present work introduces Pd(II)/LA-catalyzed (LA: Lewis acid) olefination of arylacetamides with dioxygen as the oxidant source. This protocol tolerates with different functional groups on the substrates, and the catalytic efficiency is highly Lewis acidity-dependent on added LA, that is, a stronger LA provided a better promotional effect. The 1 H NMR studies of the semireaction between the arylacetamide and the Pd(II)/Sc(III) catalyst in HOAc-d 4 disclosed the formation of a palladacycle intermediate, and the C−H activation step was reversible, which led to the formation of the deuterated arylacetamide substrate and the palladacycle intermediate. Further semireaction between the palladacycle intermediate and the olefin disclosed that it was a clean and much faster reaction than the C−H activation step, thus revealing multiple mechanistic information for Pd(II)-catalyzed C−H activation.

Observing the Agostic Hydrogen in Pd(II)-Catalyzed Aromatic C–H Activation

“…However, by treating 1a with 1 equiv of Pd­(OAc) 2 /Sc­(OTf) 3 , its 1 H NMR signals changed completely, giving three sets of aromatic protons that belong to three compounds 2 , 3 , and 4 (Figure c). As shown, compound 2 was assigned to an aromatic η 2 -complex of Pd­(II)/Sc­(III) in which the Pd­(II)/Sc­(III) core was a diacetate-bridged heterometallic unit as well as that in previous studies. ,, The presence of the Pd­(II)/Sc­(III) core in 2 , 3 , and 4 was also indicated by 19 F NMR studies that showed their identical 19 F chemical shifts to that of Pd­(OAc) 2 /Sc­(OTf) 3 in MeCN, thus indicating that they had the identically chemical environment (see Figure S60 in the Supporting Information for details). The Pd­(II)/Sc­(III) core was linked to 1a through its directing N -carbonyl group with unsymmetrical η 2 -coordination at the aromatic C 1  C N bond, which caused the chemical shift of H1 significantly upshifting from 7.26 to 7.03 ppm due to the shielding effect of Pd­(II), while H2 and H3 slightly downshifted from 6.99 and 6.84 ppm to 7.08 and 6.89 ppm, respectively, due to the corresponding deshielding effect.…”

“…In investigating Lewis acid (LA)-modulated catalytic oxidation by redox metal ions, we even unexpectedly found that the LA properties of the Cu 2+ cation play a significant role in Pd­(II)-catalyzed Wacker-type oxidations in addition to its redox properties . This finding inspired us to define Pd­(II)/LA catalysis for organic synthesis, in which the binding of LA to the Pd­(II) cation through the diacetate bridge can substantially improve its catalytic activity . Notably, Yu, Wu, and Houk also proposed an acetate-bridged heterometallic Pd­(II)/Ag­(I) structure through DFT calculations for their Pd­(II)-catalyzed C–H activation with Ag­(I) salt as the oxidant .…”

Nickel(ii)/Lewis acid catalysed olefin hydroamination and hydroarylation under mild conditions

“…Inspired by the finding of the LA promoted Wacker-type oxidations by Pd(OAc) 2 in our previous studies, which were even more efficient than Pd(II)/Cu(II) oxidation, 26 we outlined the Pd (II)/LA catalysis pathway for use in organic synthesis. This Pd (II)/LA catalysis could not only be applied in versatile Pd(II)-catalysed syntheses by aromatic C-H activation with dioxygen as the sole oxidant, [27][28][29][30][31][32][33] it could also be applied in nitrile hydration with water and alkyne oxidation with DMSO, in which Pd(II)/LA functioned as the Lewis acid catalyst. 34,35 In this Pd(II)/LA catalysis, the linkage of the LA to the Pd(II) species by a diacetate bridge sharply improved its catalytic efficiency.…”

Pd(II)/Lewis Acid Catalyzed Intramolecular Oxidative C−H Amination to Construct Carbazoles with Dioxygen

“…So far, this catalysis has been applied in a list of organic syntheses, [6] including one most recent work on oxidative C−H olefination/annulation to construct dihydrophenanthridines, [7] and expanded to Ni(II)/LA catalyzed S−P bond formation [8] . Particularly, in these Pd(II)/LA catalysis, versatile intermediates from different reactions, including the long‐missing agostic hydrogen intermediate in aromatic C−H activation, were identified through in situ NMR characterizations which provided multiple mechanistic information for Pd(II) catalysis [6i,j,9] . Remarkably, dioxygen was generally employed as the sole oxidant in those oxidation‐involved syntheses which makes them more applicable.…”