Quantification of the Ambident Electrophilicities of Halogen-Substituted Quinones

Kinetics and mechanisms of the reactions of <i>p</i>-quinone, 2,5-dichloro-<i>p</i>-quinone, 2,3,4,5-tetrachloro-<i>p</i>-quinone (chloranil), 2,3,4,5-tetrafluoro-<i>p</i>-quinone (fluoranil), and 3,4,5,6-tetrachloro-<i>o</i>-quinone with π-nucleophiles (siloxyalkenes, enamines) and amines have been investigated. Products arising from nucleophilic attack at all conceivable sites, that is, at C and O of the carbonyl groups (pathways <i>a</i>, <i>b</i>) as well as at halogenated and nonhalogenated conjugate positions (pathways <i>c</i>, <i>d</i>), were observed. The partial rate constants for the C-attack pathways (<i>a</i>, <i>c</i>, <i>d</i>), which are derived from the photometrically determined second-order rate constants and the product ratios followed the linear free energy relationship log <i>k</i> (20 °C) = <i>s</i>_N(<i>E</i> + <i>N</i>) (Mayr, H.; J. Am. Chem. Soc. 2001, 123, 9500−9512). It was, therefore, possible to calculate the electrophilicity parameters <i>E</i> of the different positions of the quinones from log <i>k</i> (20 °C) and the <i>N</i> and <i>s</i>_N parameters of the nucleophilic reaction partners, which have previously been derived from their reactions with benzhydrylium ions. Almost all rate constants for the C-attack pathways (<i>a</i>, <i>c</i>, <i>d</i>) were considerably larger than those calculated for the corresponding SET processes, indicating the operation of polar mechanisms. SET mechanisms may only account for the formation of the products formed via O-attack. With the <i>E</i> parameters determined in this work, it is now possible to predict rate constants for the reactions of these quinones with a large variety of nucleophiles and, thus, envisage unprecedented reactions of quinones

Manifestation of Polar Reaction Pathways of 2,3-Dichloro-5,6-dicyano‑<i>p</i>‑benzoquinone

Reactions of 2,3-dichloro-5,6-dicyano-<i>p</i>-benzoquinone (DDQ) with silyl enol ethers, silyl ketene acetals, allylsilanes, enamino esters, and diazomethanes have been studied in CH₃CN and CH₂Cl₂ solutions. The second-order rate constants for C attack at DDQ (log <i>k</i>_C) correlate linearly with the nucleophile-specific parameters <i>N</i> and <i>s</i>_N and are 2–5 orders of magnitude larger than expected for SET processes, which strongly supports the polar mechanism for C–C bond formation. The second-order rate constants for O attack agree well with the calculated rate constants for rate-determining single electron transfer (SET). As a radical clock experiment ruled out outer sphere electron transfer, an inner sphere electron transfer mechanism is suggested for O attack

Quantification of the Ambident Electrophilicities of Halogen-Substituted Quinones

Kinetics and mechanisms of the reactions of <i>p</i>-quinone, 2,5-dichloro-<i>p</i>-quinone, 2,3,4,5-tetrachloro-<i>p</i>-quinone (chloranil), 2,3,4,5-tetrafluoro-<i>p</i>-quinone (fluoranil), and 3,4,5,6-tetrachloro-<i>o</i>-quinone with π-nucleophiles (siloxyalkenes, enamines) and amines have been investigated. Products arising from nucleophilic attack at all conceivable sites, that is, at C and O of the carbonyl groups (pathways <i>a</i>, <i>b</i>) as well as at halogenated and nonhalogenated conjugate positions (pathways <i>c</i>, <i>d</i>), were observed. The partial rate constants for the C-attack pathways (<i>a</i>, <i>c</i>, <i>d</i>), which are derived from the photometrically determined second-order rate constants and the product ratios followed the linear free energy relationship log <i>k</i> (20 °C) = <i>s</i>_N(<i>E</i> + <i>N</i>) (Mayr, H.; J. Am. Chem. Soc. 2001, 123, 9500−9512). It was, therefore, possible to calculate the electrophilicity parameters <i>E</i> of the different positions of the quinones from log <i>k</i> (20 °C) and the <i>N</i> and <i>s</i>_N parameters of the nucleophilic reaction partners, which have previously been derived from their reactions with benzhydrylium ions. Almost all rate constants for the C-attack pathways (<i>a</i>, <i>c</i>, <i>d</i>) were considerably larger than those calculated for the corresponding SET processes, indicating the operation of polar mechanisms. SET mechanisms may only account for the formation of the products formed via O-attack. With the <i>E</i> parameters determined in this work, it is now possible to predict rate constants for the reactions of these quinones with a large variety of nucleophiles and, thus, envisage unprecedented reactions of quinones

Mechanisms of Hydride Abstractions by Quinones

The kinetics of the hydride abstractions by 2,3-dichloro-5,6-dicyano-<i>p</i>-benzoquinone (DDQ) from 13 C–H hydride donors (acyclic 1,4-dienes, cyclohexa-1,4-dienes, dihydropyridines), tributylstannane, triphenylstannane, and five borane complexes (amine–boranes, carbene–boranes) have been studied photometrically in dichloromethane solution at 20 °C. Analysis of the resulting second-order rate constants by the correlation log <i>k</i>₂(20 °C) = <i>s</i>_N(<i>E</i> + <i>N</i>) (J. Am. Chem. Soc. 2001, 123, 9500) showed that the hydride abstractions from the C–H donors on one side and the Sn–H and B–H hydride donors on the other follow separate correlations, indicating different mechanisms for the two reaction series. The interpretation that the C–H donors transfer hydrogen to the carbonyl oxygen of DDQ while Sn–H and B–H hydride donors transfer hydride to a cyano-substituted carbon of DDQ is supported by quantum-chemical intrinsic reaction coordinate calculations and isotope labeling experiments of the reactions of D₈-cyclohexa-1,4-diene, Bu₃SnD, and pyridine·BD₃ with 2,5-dichloro-<i>p</i>-benzoquinone. The second-order rate constants of the reactions of tributylstannane with different quinones correlate linearly with the electrophilicity parameters <i>E</i> of the quinones, which have previously been derived from the reactions of quinones with π-nucleophiles. The fact that the reactions of Bu₃SnH with quinones and benzhydrylium ions are on the same log <i>k</i>₂ vs <i>E</i> (electrophilicity) correlation shows that both reaction series proceed by the same mechanism and illustrates the general significance of the reactivity parameters <i>E</i>, <i>N</i>, and <i>s</i>_N for predicting rates of polar organic reactions

Electron Accumulation on Naphthalene Diimide Photosensitized by [Ru(2,2′-Bipyridine)₃]²⁺

In a molecular triad comprised of a central naphthalene diimide (NDI) unit flanked by two [Ru­(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) sensitizers, NDI^2– is formed after irradiation with visible light in deaerated CH₃CN in the presence of excess triethylamine. The mechanism for this electron accumulation involves a combination of photoinduced and thermal elementary steps. In a structurally related molecular pentad with two peripheral triarylamine (TAA) electron donors attached covalently to a central [Ru­(bpy)₃]²⁺-NDI-[Ru­(bpy)₃]²⁺ core but no sacrificial reagents present, photoexcitation only leads to NDI^– (and TAA⁺), whereas NDI^2– is unattainable due to rapid electron transfer events counteracting charge accumulation. For solar energy conversion, this finding means that fully integrated systems with covalently linked photosensitizers and catalysts are not necessarily superior to multicomponent systems, because the fully integrated systems can suffer from rapid undesired electron transfer events that impede multielectron reactions on the catalyst

Direct Observation of All Open-Shell Intermediates in a Photocatalytic Cycle

Molecular photocatalysis has shown tremendous success in sustainable energy and chemical synthesis. However, visualizing the transient open-shell intermediates in photocatalysis is a significant and long-standing challenge. By employing our recently developed innovative time-resolved electron paramagnetic resonance technique, we directly observed all radicals and radical ions involved in the photocatalytic addition of pempidine to tert−butyl acrylate. The full picture of the photocatalytic cycle is vividly illustrated by the fine structures, chemical kinetics, and dynamic spin polarization of all open-shell intermediates directly observed in this prototypical system. Given the universality of this methodology, we believe it greatly empowers the research paradigm of direct observation in both photocatalysis and radical chemistry

A Tris(diisocyanide)chromium(0) Complex Is a Luminescent Analog of Fe(2,2′-Bipyridine)₃²⁺

A <i>meta</i>-terphenyl unit was substituted with an isocyanide group on each of its two terminal aryls to afford a bidentate chelating ligand (CN^tBuAr₃NC) that is able to stabilize chromium in its zerovalent oxidation state. The homoleptic Cr­(CN^tBuAr₃NC)₃ complex luminesces in solution at room temperature, and its excited-state lifetime (2.2 ns in deaerated THF at 20 °C) is nearly 2 orders of magnitude longer than the current record lifetime for isoelectronic Fe­(II) complexes, which are of significant interest as earth-abundant sensitizers in dye-sensitized solar cells. Due to its chelating ligands, Cr­(CN^tBuAr₃NC)₃ is more robust than Cr(0) complexes with carbonyl or monodentate isocyanides, manifesting in comparatively slow photodegradation. In the presence of excess anthracene in solution, efficient energy transfer and subsequent triplet–triplet annihilation upconversion is observed. With an excited-state oxidation potential of −2.43 V vs Fc⁺/Fc, the Cr(0) complex is a very strong photoreductant. The findings presented herein are relevant for replacement of precious metals in dye-sensitized solar cells and in luminescent devices by earth-abundant elements

Iron-Catalyzed Oxidation of Tertiary Amines: Synthesis of β-1,3-Dicarbonyl Aldehydes by Three-Component C–C Couplings

β-1,3-Dicarbonyl aldehydes were synthesized by iron-catalyzed oxidative reactions between 1,3-dicarbonyl compounds and two molecules of tertiary amines in the presence of <i>tert</i>-butyl hydroperoxide (TBHP). α,β-Unsaturated aldehydes generated by tertiary amine oxidation in situ act as key intermediates under mild reaction conditions

Photoinduced Selective B–H Activation of <i>nido</i>-Carboranes

The development of new synthetic methods for B–H bond activation has been an important research area in boron cluster chemistry, which may provide opportunities to broaden the application scope of boron clusters. Herein, we present a new reaction strategy for the direct site-selective B–H functionalization of nido-carboranes initiated by photoinduced cage activation via a noncovalent cage···π interaction. As a result, the nido-carborane cage radical is generated through a single electron transfer from the 3D nido-carborane cage to a 2D photocatalyst upon irradiation with green light. The resulting transient nido-carborane cage radical could be directly probed by an advanced time-resolved EPR technique. In air, the subsequent transformations of the active nido-carborane cage radical have led to efficient and selective B–N, B–S, and B–Se couplings in the presence of N-heterocycles, imines, thioethers, thioamides, and selenium ethers. This protocol also facilitates both the late-stage modification of drugs and the synthesis of nido-carborane-based drug candidates for boron neutron capture therapy (BNCT)