Ab Initio Evaluation of Intramolecular Electron Transfer Reactions in Halobenzenes and Stabilized Derivatives

Bay-Substitution of Perylene Bisimides with Bidentate Nucleophiles: The Case of Aryloxide Anions

In this study, we delve into the regioselectivity of nucleophilic reactions involving brominated perylene bisimides (PBIs) and various bidentate aryloxide anions, previously associated with an SRN1 mechanism. We present herein a new perspective, suggesting that a single-electron-transfer aromatic nucleophilic substitution (SeT-SNAr) mechanism is a more plausible scenario. Our study reveals the favorable impact of photostimulation on reaction yields, making our method a convenient approach for accessing O-arylated PBIs.

Radical-anion coupling through reagent design: hydroxylation of aryl halides

The design and development of an oxime-based hydroxylation reagent, which can chemoselectively convert aryl halides (X = F, Cl, Br, I) into phenols under operationally simple, transition-metal-free conditions is described. Key to the success of this approach was the identification of a reducing oxime anion which can interact and couple with open-shell aryl radicals. Experimental and computational studies support the proposed radical-nucleophilic substitution chain mechanism.

Light Directs Monomer Coordination in Catalyst-Free Grignard Photopolymerization

π-Conjugated polymers can serve as active layers in flexible and lightweight electronics and are conventionally synthesized by transition-metal-mediated polycondensation at elevated temperatures. We recently reported a photopolymerization of electron-deficient heteroaryl Grignard monomers that enables the catalyst-free synthesis of n-type π-conjugated polymers. Herein, we describe an experimental and computational investigation into the mechanism of this photopolymerization. Spectroscopic studies performed in situ and after quenching reveal that the propagating chain is a radical anion with halide end groups. DFT calculations for model oligomers suggest a Mg-templated S_RN1-type coupling, in which Grignard monomer coordination to the radical anion chain avoids the formation of free sp² radicals and allows C-C bond formation with very low barriers. We find that light plays an unusual role in the reaction, photoexciting the radical anion chain to shift electron density to the termini and thus enabling productive monomer binding.

Anions involved in the initiation of the thermally induced SRN1 reaction for α-arylation of ketones

In the S_RN1 reaction between acetophenone enolate and PhI with tBuOK in DMSO under microwave irradiation, three anions present in solution act as the electron donor for the ET in the initiation step of the radical chain.

Visible Light Photocatalysis for the Generation and Use of Reactive Azolyl and Polyfluoroaryl Intermediates

Photocatalysis offers several mechanistically unique pathways that are not rivaled by mainstream catalysis. Primarily, the ability to convert photochemical energy into single electron oxidation and reduction events provides a new dimension for chemists to consider when choosing how to activate a molecule or approach a complex synthesis. Since most organic molecules do not absorb light in the visible region, they are impervious to direct visible light photochemistry, which provides an opportunity for photocatalysis in which a visible light absorbing compound can serve as a mediator. In this Account, we discuss the consequences of catalyst mediated, photoinduced electron transfer to several classes of reducible arenes. While the bulk of the work discussed within this Account utilizes iridium-based photocatalysts, in principle the chemistry is not limited to this class of photocatalyst, and the principles should be more general. Instead, this Account focuses largely on the consequences of single electron transfer to poly- and perfluorinated arenes and 2-halo azoles. Electron transfer converts these stable molecules into reactive intermediates whose behavior often depends entirely on the identity of the halogen that undergoes substitution. The result is both diverse chemistry and an alternative way of thinking about the chemical reactivity of these motifs. Specifically, we discuss our efforts and those of others to develop strategies for the generation of radicals or radical anions from perfluoroarenes and azoles and the behavior of these intermediates as implied by reactions in which they participate. The divergent pathway is illustrated by 2-bromoazoles, which yield azolyl radicals and can be utilized for addition to π-bonds, while use of the 2-chloroazole substrate leads to an entirely different reaction profile. Under the appropriate reaction conditions, the reactive and transient intermediates are useful coupling partners and often provide unrivaled access to new chemical space. The odd electron species can form challenging bonds with minimal prefunctionalization of the coupling partner. For instance, some of the intermediates can be utilized for C-H functionalizations to selectively make crowded amines or to synthesize biarenes substituted at every ortho position. While photocatalysis is not the only manner of accomplishing electron transfer, the catalytic generation of the reactive species in which the concentration of the transient odd electron species is kept low, provides a synthetic handle that can be used to improve reaction outcomes. This is elegantly demonstrated in a number of examples in which redox sensitive groups located on substrates survive the reaction. In addition, the underlying basic concepts associated with radical anion fragmentation are reviewed and provide the backdrop for discussion throughout the Account.

Iterative double cyclization reaction by SRN1 mechanism. A theoretical interpretation of the regiochemical outcome of diazaheterocycles

Diazaheterocyclic compounds are prepared in good yields by double cyclization intramolecular S_RN1 reactions from diamines bearing two aryl iodide moieties.

SET-induced biaryl cross-coupling: an S(RN)1 reaction

The SET-induced biaryl cross-coupling reaction is established as the first example of a Grignard S_RN1 reaction. The reaction is examined within the mechanistic framework of dissociative electron transfer in the presence of a Lewis acid. DFT calculations show that the reaction proceeds through a radical intermediate in the form of an Mg ion-radical cage, which eludes detection in trapping experiments by reacting quickly to form an MgPh₂ radical anion intermediate. A new mechanism is proposed.

Experimental and DFT study on the indium-mediated synthesis of benzophenones via arylstannanes

A DFT analysis was performed with the aim to explaining the narrow scope of the indium-promoted reaction of aroyl chlorides with arylstannanes.

Bisoxazoline–copper(I)-catalyzed aziridination of diazoacetate with imines — A DFT study

Density functional theory (DFT) has been used to study bisoxazoline–copper(I)-catalyzed aziridination of diazoacetate with syn-imines or anti-imines. All the intermediates and transition states were optimized completely at the B3LYP/6-31G(d) level. Calculation results confirm that Cu(I)-catalyzed aziridination goes mainly through the catalyst–diazoacetate complex (M1), the copper(I)–carbene intermediate (M2), the copper–carbene–imine complex (M3), and the catalyst–aziridine carboxylate complex (M4). For syn-imines, the reaction mode I (C3–N5 bond attacking the Cu–C1 bond of M2) is more dominant than the reaction mode II (C3–N5 bond attacking the carbene–carbon C1 of M2), and the attack from the si-surface of M2 is prior to the re-surface. For anti-imines, the reaction modes and attacks from the si- or re-surface coexist. The reactivity of syn-imines is stronger than anti-imines. The favorable reaction channel is CA2 → M1b → TS1b → M2 → syn-TS2b → syn-M3b → syn-TS3b → syn-M4b → syn-P2. The dominant product theoretically predicted is of (S,S)-chirality. On the whole, the solvent effect decreases the free energies of the species.

Competitive reaction pathways for o-anilide aryl radicals: 1,5- or 1,6-hydrogen transfer versus nucleophilic coupling reactions. A novel rearrangement to afford an amidyl radical

The photoinduced reactions of o-iodoanilides (o-IC6H4N(Me)COR, 4a-d) with sulfur nucleophiles such as thiourea anion (1, -SCNH(NH2)), thioacetate anion (2, MeCOS-), and sulfide anion (3, S(2-)) follow different reaction channels, giving the sulfides by a radical nucleophilic substitution or the dehalogenated products by hydrogen atom transfer pathways. After an initial photoinduced electron transfer (PET) from 1 to iodide 4, the o-amide aryl radicals 12 are generated. These aryl radicals 12 afford alternative reaction pathways depending on the structure of the alpha-carbonyl moiety: (a) 12b (R = Me) adds to 1 to render the methylthio-substituted compounds by quenching the thiolate anion intermediate with MeI after irradiation; (b) 12c (R = -CH2Ph) follows a 1,5-hydrogen transfer to give a stabilized alpha-carbonyl radical (17); and (c) 12d (R = t-Bu) affords 1,6-hydrogen transfer, followed by a 1,4-aryl migration to render an amidyl radical (20), which is reduced to the N-benzyl-N,2-dimethylpropanamide (10). Together with this last rearranged product, the ipso substitution derivative was also observed. Similar results were obtained in the PET reactions of 4d (R = t-Bu) with anions 2 and 3 under entrainment conditions with the enolate anion from cyclohexenone (5) or the tert-butoxide anion (6). From this novel rearrangement, and only under reductive conditions by PET reaction with anion 5, iodide 4d (R = t-Bu) affords quantitatively the propanamide 10. The energetic of the intramolecular rearrangements followed by radicals 12b-d were rationalized by B3LYP/6-31+G* calculations.

Species with negative electron affinity and standard DFT methods. Finding the valence anions

Recently, we have shown that traditional bound-electron DFT models are reliable enough to reproduce negative electron affinities (EA) within a few meV, as long as the valence anion state is found, but they seem to fail in predicting the lowest EA when the ground anion state obtained is non-valence, which holds the extra electron in a diffuse orbital around the molecule; here we propose an alternative approach for finding the valence anion state, based on the stabilization exerted by a polar solvent; the methodology yields correct EA values (i.e. beyond the Koopman's theorem approximation) by gradually decreasing the dielectric constant of the medium.

Inter- and intramolecular-bridge-mediated electron transfer in aliphatic halides

The anionic surfaces of the 1-chloro- and 4-chlorobicyclo[2.2.1]heptan-2-one, 1-chloro- and 4-chlorobicyclo[2.2.2]octan-2-one, 1-chloro- and 5-chloroadamantan-2-one, and 2-chlorotricyclo[7.3.1.0(2,7)]tridecan-13-one were explored using DFT functionals with full geometry optimization in solution. The reductive cleavage of these compounds is controlled by the rigidity of the polycycle, its capability to form an unstrained radical, and by the relative carbonyl/C-Cl disposition on the bridge. Such control can be exerted by either a concerted-dissociative or a stepwise mechanism with radical anions as intermediates. 5-Chloroadamantan-2-one is the most suitable compound to follow the latter pathway.

A Theoretical Study of the Competitive Homolytic/Heterolytic Aniomesolytic Cleavages of C−O Alkyl Ether Bonds

Density functional theory electronic structure calculations of the homolytic/heterolytic aniomesolytic C-O fragmentations in the gas phase of a series of radical anions of substituted-phenyl benzyl ethers and substituted-benzyl phenyl ethers have been carried out. Along the series, the electron-withdrawing strength of the substituents increases. An intramolecular electron transfer from the pi system to the sigma molecular orbital of the scissile C-O bond is required to produce the fragmentation. As the electron-withdrawing strength of the substituents increases, the transition-state structures appear later with higher potential energy and Gibbs free energy barriers. The homolytic mesolytic cleavages are always thermodynamically favored versus the corresponding heterolytic mesolytic ones. The heterolytic mesolytic fragmentations in radical anions containing only weak electron-withdrawing groups are faster than the corresponding homolytic mesolytic ones. Conversely, in radical anions supporting strong electron-withdrawing groups the homolytic mesolytic fragmentations are faster in terms of potential energy barriers. However, the entropic contribution makes it comparable the homolytic and the heterolytic Gibbs free energy barriers in this case. The main factors that determine the relative rates of those kind of aniomesolytic cleavages are discussed.