Oxidative Trifluoroacetoxylation of 1°, 2°, and 3° Benzylic C(sp³)–H Bond Donors Using N-Trifluoroacetoxyquinuclidinium Salts under Photoredox Catalysis

Abstract

N-Trifluoroacetoxyquinuclidinium trifluoroacetate was prepared in situ from quinuclidine N-oxide and (CF3CO)2O. Except for some electron-poor substrates, this reagent allows for the high-yielding oxidative trifluoroacetoxylation of 1°, 2°, and 3° benzylic C–H bonds under photocatalytic conditions. The trifluoroacetoxylation of an ibuprofen methyl ester allowed the selective functionalization of a 2° benzylic C–H bond. For alkylbenzenes, hydrogen-atom transfer from a benzylic C–H bond to a quinuclidine cation radical was proposed to be the reaction-product-determining step.

Oxidative Imidation of Benzylic and Cycloalkane C(sp3)–H Bond Donors Using N-Aroyloxyquinuclidinium Salts and Nitriles under Photoredox Catalysis

“…The deuterium kinetic isotope effect was estimated similarly by employing 1:1 mixtures of toluene and toluene- d 8 with the resulting k H / k D = 2.7 ± 0.1 (Scheme b). These observations are consistent with the reaction mechanism involving a product-determining hydrogen atom transfer from a hydrocarbon benzylic C–H bond to Q ●+ , similar to the oxidative benzylic C–H trifluoroacetoxylation using N -trifluoroacetoxyquinuclidinium salts . Benzylic radicals resulting from a hydrogen atom transfer (HAT) reaction with toluene were trapped using 4- tert -butylpyridinium triflate 11 in a form of a product 12 of Minisci-type oxidative coupling that was detected using ESI-MS­(+) technique (Scheme c).…”

“…These observations are consistent with the reaction mechanism involving a product-determining hydrogen atom transfer from a hydrocarbon benzylic C−H bond to Q •+ , similar to the oxidative benzylic C−H trifluoroacetoxylation using N-trifluoroacetoxyquinuclidinium salts. 30 Benzylic radicals resulting from a hydrogen atom transfer (HAT) reaction with toluene were trapped using 4tert-butylpyridinium triflate 11 in a form of a product 12 of Minisci-type oxidative coupling 37 that was detected using ESI-MS(+) technique (Scheme 4c). Altogether, on the basis of our observations, we propose a mechanism of the imidation reaction shown in Scheme 5 with toluene as a representative substrate.…”

“…The reaction works under blue LED light (435 nm) in the presence of a photoredox catalyst, such as [Ir­(ppy) 2 ( t Bu 2 bpy)]­(PF 6 ), 5a , and allows for a moderate- to high-yielding, up to 94%, preparation of 6 using alkylarenes 2 as limiting reagents , as well as a low-yielding, 22–26%, formation of 7 using 3 equiv of cycloalkanes 3 . Similar to oxidative benzylic C–H trifluoroacetoxylation employing N -trifluoroacetoxyquinuclidinium salts in DCM solutions, the reaction in Scheme d may involve quinuclidine cation radicals, Q ●+ , resulting from the one-electron reduction of 1 , along with corresponding carboxylate anions, with Q ●+ acting as a hydrogen atom abstractor with respect to C­(sp 3 )–H bond donors 2 – 3 . Overall in this work, we explore the synthetic applications of novel N -acyloxyquinuclidinium salts serving as stoichiometric reagents for C­(sp 3 )–H bond functionalization, thereby complementing studies by other groups employing Q ●+ as a catalyst in electrochemical and photoredox C–H bond functionalization. − …”

The Multifaceted Chemistry of [2.2]Paracyclophane‐Based Thioethers with Palladium(II) Complexes

“…The authors have cited additional references with the Supporting Information. [27][28][29][30][31][32][33][34][35]…”

Electrochemical Benzylic C(sp3)-H Imidation Enabled by Benzoic Acid Derived Radicals

“…Moreover, the current approach exhibits a good site selectivity for C(sp 3 )-H bonds, that is inaccessible in the photocatalysis counterpart. 18 ■…”