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31.
Samit Mandal J Gerl H Geissel K Hauschild M Hellström Z Janas I Kojouharov Y Kopatch RC Lemmon P Mayet Z Podolyak PH Regan H Schaffner C Schlegel J Simpson HJ Wollersheim 《Pramana》2001,57(1):161-164
Feasibility of gamma-ray spectroscopy at relativistic energies with exotic heavy-ions and new generation of germanium detectors
(segmented Clover) is discussed. An experiment with such detector array and radioactive is discussed. 相似文献
32.
Radical couplings of cyanopyridine radical anions represent a valuable technology for functionalizing pyridines, which are prevalent throughout pharmaceuticals, agrochemicals, and materials. Installing the cyano group, which facilitates the necessary radical anion formation and stabilization, is challenging and limits the use of this chemistry to simple cyanopyridines. We discovered that pyridylphosphonium salts, installed directly and regioselectively from C–H precursors, are useful alternatives to cyanopyridines in radical–radical coupling reactions, expanding the scope of this reaction manifold to complex pyridines. Methods for both alkylation and amination of pyridines mediated by photoredox catalysis are described. Additionally, we demonstrate late-stage functionalization of pharmaceuticals, highlighting an advantage of pyridylphosphonium salts over cyanopyridines.Cyanopyridines form dearomatized radical anions upon single-electron reduction and participate in photoredox coupling reactions. Pyridylphosphonium salts replicate that reactivity with a broader scope and increase the utility of these processes.Modern photoredox catalysis and electrochemistry have enabled new synthetic methods that proceed via open-shell intermediates.1 Under this regime, pyridine functionalization strategies have been developed where 4-cyanopyridines undergo single-electron reduction to form dearomatized radical species that couple with other stabilized radicals (Scheme 1A).2 The cyano group is critical for efficient reactivity via pyridyl radical anions; alternatives such as 4-halopyridines more readily undergo elimination to pyridyl radicals after single-electron reduction resulting in a distinct set of coupling processes.3 We aimed to show that pyridylphosphonium salts could replicate the reactivity of cyanopyridines and allow a broader set of inputs into dearomatized pyridyl radical coupling reactions.4Open in a separate windowScheme 1Expansion of radical coupling reactions to complex pyridines.Cyanopyridines have facilitated pyridine alkylation, allylation, and alkenylation reactions providing access to valuable building blocks for medicinal and agrochemical programs.5 The cyano group is essential for these methods, but a problem arises when applying this chemistry to complex pyridines, such as those found in pharmaceutical and agrochemical candidates. These structures are often devoid of pre-installed functional groups, and it is often challenging to install a cyano group from C–H precursors regioselectively.6 We envisioned pyridylphosphonium salts, regioselectively constructed from the C–H bonds of a diverse set of pyridines, could serve as alternatives to cyanopyridines.7 Herein, we report couplings between alkyl BF3K salts and preliminary studies of carboxylic acids and amines with pyridylphosphonium salts, including late-stage functionalization of complex pyridine-containing pharmaceuticals using this strategy.Recently, we reported a radical coupling reaction between a boryl-stabilized cyanopyridyl radical and a boryl-stabilized pyridylphosphonium radical.7a The intermediate radicals arose via an unusual inner-sphere process that would be difficult to extend to other coupling reactions. A significant advance would be to show that pyridylphosphonium salts could function more generally as radical anion precursors and mimic the reactivity of cyano-pyridines. In particular, showing their viability in photoredox and electrochemical processes would translate to numerous synthetic transformations. To demonstrate this principle, we envisioned a redox-neutral alkylation reaction (Scheme 1B) via a radical coupling between radical zwitterion I, formed through single-electron reduction of a pyridylphosphonium salt (Eredp/2 = −1.51 V vs. SCE) and benzyl radical II, resulting from single-electron oxidation of a BF3K salt (Ered = +1.10 V vs. SCE for a primary benzylic salt).8 Loss of triphenylphosphine from dearomatized intermediate (III) would furnish the alkylated pyridine product. Notably, the redox events could invert, where the photocatalyst oxidizes the BF3K salt first and reduces the pyridylphosphonium salt second, broadening the scope of amenable photocatalysts.We began our investigation by examining a series of photocatalysts for the coupling reaction of phosphonium salt 1a, formed with complete regioselectivity for the 4-position from 2-phenylpyridine, and benzylic BF3K salt 2a under irradiation from a 455 nm Kessil light (Scheme 1B are potentially interchangeable.1b The Adachi-type photocatalyst 3DPAFIPN improved the yield to 77% with a further increase to 82% after increasing the reaction concentration (entries 3 and 4). Adding 2,6-lutidine, previously shown as an effective additive for photoredox cross-coupling reactions of BF3K salts by the Molander group,9 had no impact on the yield of 2-phenylpyridine salt 1a (entry 5) and the [Ir(ppy)2(dtbbpy)]PF6 catalyst was marginally less efficient under the same conditions (entry 6). We observed that 2,6-lutidine did substantially improve the yield when isomeric 3-Ph salt 1b was employed (entries 7 and 8); without 2,6-lutidine, the crude 1H NMR indicates significant amounts of decomposition occurred, including 3-phenylpyridine, and the 4- vs. 2-position product ratio was 3 : 1. This outcome suggests that protiodephosphination and non-selective Minisci-type pathways can occur under these conditions. With 2,6-lutidine, the crude reaction pathway is cleaner, and the 4- vs. 2-position ratio improved to 8 : 1. At this point, we have not established the role of 2,6-lutidine, although it is conceivable that it reacts with BF3 produced as the reaction progresses. In 2-substituted systems, steric hindrance around the pyridine N-atom of the salt would deter BF3-coordination, whereas, in 3-substituted systems, such as salt 1b, coordination is more likely and may have a deleterious effect on the reaction (vide infra). Given the structural variation of pyridines that we anticipated applying to this process and how those structures could impact boron speciation during the reaction, we elected to use 2,6-lutidine as an additive in all subsequent reactions.10Optimization of pyridine alkylation, photocatalyst data and effect of BF3·OEt2 as an additivea
Open in a separate windowaConditions: 1a (1.0 equiv.), 2a (2.0 equiv.), photocatalyst (2 mol%), additive (3.0 equiv.), rt.bYields determined by 1H NMR analysis using 1,3,5-trimethoxybenzene as internal standard.cIsolated yield on 0.50 mmol scale.dIsolated yield on 2.00 mmol scale.e3 : 1 4- vs. 2-regioisomeric ratio determined from the crude 1H NMR.f8 : 1 4- vs. 2-regioisomeric ratio determined from the crude 1H NMR.gUsed 365 nm LEDs instead of 455 nm Kessil light for 89 h.hAll redox potentials reported vs. SCE and all values compiled from previous literature reports.1iCounterion omitted in structure for simplicity.We conducted a series of further experiments to explore the effect of light and photocatalyst type on the reaction (†).11 Furthermore, a photocatalyst with a redox potential window misaligned with the redox events in Scheme 1B, [Mes-Acr]BF4, is also competent (entry 11). An energy transfer mechanism was considered based on entry 9, but the low triplet state energies for [Mes-Acr]BF4 make this pathway unlikely (12–14Employing the optimized conditions, we investigated the scope of pyridylphosphonium salts in this coupling process (