Direct N-H Activation to Generate Nitrogen Radical for Arylamine Synthesis via Quantum Dots Photocatalysis.

Represented herein is the first example of N-radical generation direct from N-H bond activation under mild and redox-neutral conditions. The in situ generated N-radical intercepts a reduced heteroarylnitrile/aryl halide for C-N bond formation under visible-light irradiation of quantum dots (QDs). A series of aryl and alkylamines with heteroarylnitriles/aryl halides exhibit high efficiency, site-selectivity and good functional-group tolerance. Moreover, consecutive C-C and C-N bond formation using benzylamines as substrates is also achieved, producing N-aryl-1,2-diamines with H2 evolution. The redox-neutral conditions, broad substrate scope, and efficiency of N-radical formation are advantageous for organic synthesis.

Direct Excitation of Aldehyde to Activate the C(sp2 )-H Bond by Cobaloxime Catalysis toward Fluorenones Synthesis with Hydrogen Evolution.

A new way to form fluorenones via the direct excitation of substrates instead of photocatalyst to activate the C(sp2 )-H bond under redox-neutral condition is reported. Our design relies on the photoexcited aromatic aldehyde intermediates that can be intercepted by cobaloxime catalyst through single electron transfer for following ß-H elimination. The generation of acyl radical and successful interception by a metal catalyst cobaloxime avoid the use of a photocatalyst and stoichiometric external oxidants, affording a series of highly substituted fluorenones, including six-membered ketones, such as xanthone and thioxanthone derivatives in good to excellent yields, and with hydrogen as the only byproduct. This catalytic system features a readily available metal catalyst, mild reaction conditions and broad substrate scope, in which sunlight reaction and scale-up experiments by continuous-flow approach make the new methodology sustainable and amenable for potentially operational procedures.

Amine-Free, Directing-Group-Free and Redox-Neutral α-Alkylation of Saturated Cyclic Ketones.

The activation of the α-C-H bond of ketones typically requires an amine and a directing group to guide the reaction selectivity in amine-catalysis carbonyl chemistry. For an α-C-H bond activation of ketone, directing groups are also required to control the reaction selectivity. Reported herein is the first α-alkylation of cyclic ketones in the absence of an amine catalyst and directing group. 1 H NMR, XPS, EPR studies and DFT calculations indicate that an α-carbon radical intermediate is formed through direct and selective activation of the inert α-C-H bond of ketones chelating on the surface of colloidal quantum dots (QDs). Such an interaction is essential for weakening the C-H bond, as exemplified, using CdSe QDs as the sole photocatalyst to execute α-C-H alkylation of cyclic ketones under visible-light irradiation. Without an amine catalyst and directing group, the high step- and atom-economy transformation under redox-neutral condition opens a new way for α-C-H functionalization of ketones in carbonyl chemistry.

Transition-Metal-Free, Site-Selective C-F Arylation of Polyfluoroarenes via Electrophotocatalysis.

Direct CAr-F arylation is effective and sustainable for synthesis of polyfluorobiaryls with different degrees of fluorination, which are important motifs in medical and material chemistry. However, with no aid of transition metals, the engagement of CAr-F bond activation has proved difficult. Herein, an unprecedented transition-metal-free strategy is reported for site-selective CAr-F arylation of polyfluoroarenes with simple (het)arenes. By merging N,N-bis(2,6-diisopropylphenyl)perylene-3,4,9,10-bis(dicarboximide)-catalyzed electrophotocatalytic reduction and anodic nitroxyl radical oxidation in an electrophotocatalytic cell, various polyfluoroaromatics (2F-6F and 8F), especially inactive partially fluorinated aromatics, undergo sacrificial-reagents-free C-F bond arylation with high regioselectivity, and the yields are comparable to those for reported transition-metal catalysis. This atom- and step-economic protocol features a paired electrocatalysis with organic mediators in both cathodic and anodic processes. The broad substrate scope and good functional-group compatibility highlight the merits of this operationally simple strategy. Moreover, the easy gram-scale synthesis and late-stage functionalization collectively advocate for the practical value, which would promote the vigorous development of fluorine chemistry.

Direct, Site-Selective and Redox-Neutral α-C-H Bond Functionalization of Tetrahydrofurans via Quantum Dots Photocatalysis.

As one of the most ubiquitous bulk reagents available, the intrinsic chemical inertness of tetrahydrofuran (THF) makes direct and site-selective C(sp3 )-H bond activation difficult, especially under redox neutral condition. Here, we demonstrate that semiconductor quantum dots (QDs) can activate α-C-H bond of THF via forming QDs/THF conjugates. Under visible light irradiation, the resultant alkoxyalkyl radical directly engages in radical cross-coupling with α-amino radical from amino C-H bonds or radical addition with alkene or phenylacetylene, respectively. In contrast to stoichiometric oxidant or hydrogen atom transfer reagents required in previous studies, the scalable benchtop approach can execute α-C-H bond activation of THF only by a QD photocatalyst under redox-neutral condition, thus providing a broad of value added chemicals starting from bulk THFs reagent.

Direct Allylic C(sp3 )-H and Vinylic C(sp2 )-H Thiolation with Hydrogen Evolution by Quantum Dots and Visible Light.

Direct allylic C-H thiolation is straightforward for allylic C(sp3 )-S bond formation. However, strong interactions between thiol and transition metal catalysts lead to deactivation of the catalytic cycle or oxidation of sulfur atom under oxidative condition. Thus, direct allylic C(sp3 )-H thiolation has proved difficult. Represented herein is an exceptional for direct, efficient, atom- and step-economic thiolation of allylic C(sp3 )-H and thiol S-H under visible light irradiation. Radical trapping experiments and electron paramagnetic resonance (EPR) spectroscopy identified the allylic radical and thiyl radical generated on the surface of photocatalyst quantum dots (QDs). The C-S bond formation does not require external oxidants and radical initiators, and hydrogen (H2 ) is produced as byproduct. When vinylic C(sp2 )-H was used instead of allylic C(sp3 )-H bond, the radical-radical cross-coupling of C(sp2 )-H and S-H was achieved with liberation of H2 . Such a unique transformation opens up a door toward direct C-H and S-H coupling for valuable organosulfur chemistry.