Passerini-Smiles Reaction of α-Ketophosphonates: Platform for Phospha-Brook/Smiles Embedded Cascades

The Passerini-Smiles reactions of α-ketophosphonates with nitrophenols has been used as a platform to observe complex cascades involving multiple Smiles transfers coupled with phospha-Brook rearrangement. When using 4-nitrophenols a rare 1,3-Truce-Smiles rearrangement is observed leading to diarylacetamide derivatives. 2-Nitro-derivatives lead to a completely different reactivity pattern that may be explained by a nitro to nitroso conversion followed by a σ-π metathesis. All mechanistic assumptions are confirmed by DFT calculations performed on both families of adducts. The potential of this work has been further demonstrated by the use of N-aryl α-ketoamides as alternative starting materials for these cascades as well as the disclosure of new aza-Nazarov access to hydroxy-indolones.

Hydrodefluorination of functionalized fluoroaromatics with triethylphosphine: a theoretical and experimental study

A new mechanistic proposal based in theoretical and experimental evidence invoking the participation of water is presented.

Unravelling nucleophilic aromatic substitution pathways with bimetallic nucleophiles

Reaction of nucleophiles containing polar (Fe–Mg) and apolar (Mg–Mg) bonds with 2-(pentafluorophenyl)pyridine are calculated to proceed by stepwise and concerted S_NAr pathways respectively.

Facile C-F Bond Formation through a Concerted Nucleophilic Aromatic Substitution Mediated by the PhenoFluor Reagent

Late-stage fluorination reactions aim to reduce the synthetic limitations of conventional organofluorine chemistry with respect to substrate scope and functional group tolerance. C-F bond formation is commonly thermodynamically favorable but almost universally associated with high kinetic barriers. Apart from PhenoFluor chemistry, most modern aromatic fluorination methods reported to date rely on the use of transition metal catalysts, with C-F bonds often formed through reductive elimination. Reductive elimination chemistry to make C-X bonds becomes increasingly challenging when moving to higher atomic numbers in the periodic table from C-C to C-F, in part because of higher metal-X bond dissociation energies. The formation of C-C, C-N, and C-O bonds via reductive elimination has become routine in the 20th century, but it took until the 21st century to develop complexes that could afford general C-F bond formation. The availability of such complexes enabled the substrate scope of modern fluorination chemistry to exceed that of conventional fluorination. PhenoFluor chemistry departs from conventional reaction mechanisms for aromatic fluorination chemistry. Instead, we have revealed a concerted nucleophilic aromatic substitution reaction (CS_NAr) for PhenoFluor that proceeds through a single neutral four-membered transition state. Conceptually, PhenoFluor chemistry is therefore distinct from conventional S_NAr chemistry, which typically proceeds through a two-barrier process with Meisenheimer complexes as reaction intermediates. As a consequence, PhenoFluor chemistry has a larger substrate scope than conventional S_NAr chemistry and can be performed on arenes as electron-rich as anilines. Moreover, PhenoFluor chemistry is tolerant of protic functional groups, which sets it apart from modern metal-mediated processes. Primary and secondary amines, alcohols, thiols, and phenols are often not tolerated under metal-catalyzed late-stage fluorination reactions because C-N and C-O reductive elimination can have lower activation barriers than C-F reductive elimination. The mechanism by which PhenoFluor chemistry forms C-F bonds not only rationalizes the substrate scope and functional group tolerance but also informs the side-product profile. Fluorinated isomers are not observed because the four-membered transition state necessitates ipso substitution. In addition, no reduced product, e.g., H instead of F incorporation, as is often observed with metal-mediated methods, has ever been observed with PhenoFluor. PhenoFluor chemistry can be used to deoxyfluorinate both phenols and alcohols. PhenoFluor is an expensive reagent that must be used stoichiometrically and therefore cannot replace cost-efficient methods to make simple fluorinated molecules on a large scale. However, PhenoFluor is often successful when other fluorination methods fail. The synthesis of ¹⁸F-labeled molecules for positron emission tomography (PET) is one application of modern fluorination chemistry for which material throughput is not an issue because of the small quantities of PET tracers used in imaging (typically nanomoles). The high emphasis on functional group tolerance, side-product profiles, and reliability combined with less stringent cost requirements render PhenoFluor-based deoxyfluorination with ¹⁸F promising for human PET imaging.

Nucleophilic Substitution of Hydrogen Facilitated by Quinone Methide Moieties in Benzo[cd]azulen-3-ones

The built-in o- and p-QM (QM = quinone methide) moieties in benzo[cd]azulen-3-ones account for an easy switch between the bridged 10π- and 6π-aromatic systems in organic synthesis. We report conjugate additions, oxidative nucleophilic substitutions of hydrogen, and reversible Michael additions under very mild conditions. In the presence of thiol nucleophiles, the protonated σ^H-adducts could be isolated and characterized. The typical preference for either the o- or p-QM moiety led to high regioselectivity. Furthermore, the inhibitory potency of the novel benzo[cd]azulenes against the human Pim-1 kinase was evaluated.

Mechanism for the SNAr reaction of atrazine with endogenous thiols: experimental and theoretical study

The mechanism for the S_NAr reaction of atrazine with endogenous thiols: a stepwise or concerted process?

Application of heterocyclic aldehydes as components in Ugi-Smiles couplings

Efficient one-pot Ugi–Smiles couplings are reported for the use of furyl-substituted aldehyde components. In the presence of these heterocyclic aldehydes, reactions tolerated variations in amine components and led to either isolated N-arylamide Ugi–Smiles adducts or N-arylepoxyisoindolines, products of tandem Ugi–Smiles Diels–Alder cyclizations, in moderate yields. A thienyl-substituted aldehyde was also a competent component for Ugi–Smiles adduct formation.

Optimized Conditions for Passerini-Smiles Reactions and Applications to Benzoxazinone Syntheses

Initial conditions disclosed for the Passerini-Smiles reaction are associated with a lack of efficiency that has prevented chemists from using it since its discovery. We wish to report herein our thorough study in the development of new experimental conditions for this coupling between electron-poor phenols, isocyanides, and carbonyl derivatives. These new conditions have been applied to several synthetic strategies towards benzoxazinones.

Concerted nucleophilic aromatic substitution with (19)F(-) and (18)F(-)

Nucleophilic aromatic substitution (SNAr) is widely used by organic chemists to functionalize aromatic molecules, and it is the most commonly used method to generate arenes that contain (18)F for use in positron-emission tomography (PET) imaging. A wide range of nucleophiles exhibit SNAr reactivity, and the operational simplicity of the reaction means that the transformation can be conducted reliably and on large scales. During SNAr, attack of a nucleophile at a carbon atom bearing a 'leaving group' leads to a negatively charged intermediate called a Meisenheimer complex. Only arenes with electron-withdrawing substituents can sufficiently stabilize the resulting build-up of negative charge during Meisenheimer complex formation, limiting the scope of SNAr reactions: the most common SNAr substrates contain strong π-acceptors in the ortho and/or para position(s). Here we present an unusual concerted nucleophilic aromatic substitution reaction (CSNAr) that is not limited to electron-poor arenes, because it does not proceed via a Meisenheimer intermediate. We show a phenol deoxyfluorination reaction for which CSNAr is favoured over a stepwise displacement. Mechanistic insights enabled us to develop a functional-group-tolerant (18)F-deoxyfluorination reaction of phenols, which can be used to synthesize (18)F-PET probes. Selective (18)F introduction, without the need for the common, but cumbersome, azeotropic drying of (18)F, can now be accomplished from phenols as starting materials, and provides access to (18)F-labelled compounds not accessible through conventional chemistry.

Influence of Solvation and Dynamics on the Mechanism and Kinetics of Nucleophilic Aromatic Substitution Reactions in Liquid Ammonia

The role of the solvent and the influence of dynamics on the kinetics and mechanism of the SNAr reaction of several halonitrobenzenes in liquid ammonia, using both static calculations and dynamic ab initio molecular dynamics simulations, are investigated. A combination of metadynamics and committor analysis methods reveals how this reaction can change from a concerted, one-step mechanism in gas phase to a stepwise pathway, involving a metastable Meisenheimer complex, in liquid ammonia. This clearly establishes, among others, the important role of the solvent and highlights the fact that accurately treating solvation is of crucial importance to correctly unravel the reaction mechanism. It is indeed shown that H-bond formation of the reacting NH3 with the solvent drastically reduces the barrier of NH3 addition. The halide elimination step, however, is greatly facilitated by proton transfer from the reacting NH3 to the solvent. Furthermore, the free energy surface strongly depends on the halide substituent and the number of electron-withdrawing nitro substituents.

Origins of the ANRORC reactivity in nitroimidazole derivatives

Highlighting the key role of proton transfer along the PES: unraveling the mechanism of ANRORC reactions in 1,4-dinitro-1H-imidazole derivatives.

Accelerating effect of triazolyl and related heteroaryl substituents on SNAr reactions: evidence of hydrogen-bond stabilized transition states

The remarkable accelerating effect of 1,2,3-triazolyl substituents on SNAr reactions has been investigated through systematic experiments and density functional theory calculations. The lone pair electrons of an ortho-triazolo substituent play a key role in lowering the activation energy for nucleophilic addition via formation of a preferential hydrogen bond with the amine nucleophile at the transition state for addition. In an extension of this finding, a series of related heteroaryl groups with similar electron pair donor properties have also been found to facilitate SNAr reactions. The experimentally determined solvent effect provides further support for this rationale, which was utilized to achieve an ortho-selective substitution on a difluoroarene substrate.

A faux hawk fullerene with PCBM-like properties

Reaction of C60, C6F5CF2I, and SnH(n-Bu)3 produced, among other unidentified fullerene derivatives, the two new compounds 1,9-C60(CF2C6F5)H (1) and 1,9-C60(cyclo-CF2(2-C6F4)) (2). The highest isolated yield of 1 was 35% based on C60. Depending on the reaction conditions, the relative amounts of 1 and 2 generated in situ were as high as 85% and 71%, respectively, based on HPLC peak integration and summing over all fullerene species present other than unreacted C60. Compound 1 is thermally stable in 1,2-dichlorobenzene (oDCB) at 160 °C but was rapidly converted to 2 upon addition of Sn2(n-Bu)6 at this temperature. In contrast, complete conversion of 1 to 2 occurred within minutes, or hours, at 25 °C in 90/10 (v/v) PhCN/C6D6 by addition of stoichiometric, or sub-stoichiometric, amounts of proton sponge (PS) or cobaltocene (CoCp2). DFT calculations indicate that when 1 is deprotonated, the anion C60(CF2C6F5)- can undergo facile intramolecular SNAr annulation to form 2 with concomitant loss of F-. To our knowledge this is the first observation of a fullerene-cage carbanion acting as an SNAr nucleophile towards an aromatic C-F bond. The gas-phase electron affinity (EA) of 2 was determined to be 2.805(10) eV by low-temperature PES, higher by 0.12(1) eV than the EA of C60 and higher by 0.18(1) eV than the EA of phenyl-C61-butyric acid methyl ester (PCBM). In contrast, the relative E1/2(0/-) values of 2 and C60, -0.01(1) and 0.00(1) V, respectively, are virtually the same (on this scale, and under the same conditions, the E1/2(0/-) of PCBM is -0.09 V). Time-resolved microwave conductivity charge-carrier yield × mobility values for organic photovoltaic active-layer-type blends of 2 and poly-3-hexylthiophene (P3HT) were comparable to those for equimolar blends of PCBM and P3HT. The structure of solvent-free crystals of 2 was determined by single-crystal X-ray diffraction. The number of nearest-neighbor fullerene-fullerene interactions with centroid···centroid (⊙···⊙) distances of ≤10.34 Å is significantly greater, and the average ⊙···⊙ distance is shorter, for 2 (10 nearest neighbors; ave. ⊙···⊙ distance = 10.09 Å) than for solvent-free crystals of PCBM (7 nearest neighbors; ave. ⊙···⊙ distance = 10.17 Å). Finally, the thermal stability of 2 was found to be far greater than that of PCBM.

Mechanistic insights into the ANRORC-like rearrangement between methylhydrazine and 1,2,4-oxadiazole derivatives

The first in-depth theoretical study devoted to elucidate the mechanism of the ANRORC reaction between 1,2,4-oxadiazole derivatives and methylhydrazine is described.

Predicting new Ugi-smiles couplings: a combined experimental and theoretical study

Following our previous mechanistic studies of multicomponent Ugi-type reactions, theoretical calculations have been performed to predict the efficiency of new substrates in Ugi-Smiles couplings. First, as predicted, 2,4,6-trichlorophenol experimentally gave the corresponding aryl-imidate. Theoretical predictions of nitrosophenols as good acidic partners were then successfully confirmed by experiments. In the latter case, the reaction offers a new access to benzimidazoles.

Site activation effects promoted by intramolecular hydrogen bond interactions in SNAr reactions

The nucleophilic aromatic substitution reaction of benzohydrazide derivatives towards 2-chloro-5-nitropyrimidine is used as model system to experimentally and theoretically show that intramolecular hydrogen-bond formation operates as a perturbation that elicits a dual response at the reaction center of the transition state (TS) structure.

Reactivity indices profile: a companion tool of the potential energy surface for the analysis of reaction mechanisms. Nucleophilic aromatic substitution reactions as test case

We herein report on the usefulness of the reactivity indices profiles along a reaction coordinate. The model is tested to fully describe the reaction mechanism of the title reactions. Group nucleophilicity and electrophilicity profiles help describe the bond-breaking/bond-formation processes and the intramolecular electron density reorganization. The reactivity indices' profile analysis is consistently complemented with hydrogen bonding (HB) effects along the reaction coordinate: the final outcome of the reaction is determined by the stage at which the HB complex can be formed. Transition-state structures located for six reactions studied, including the charged nucleophile thiocyanate, show that the main stabilizing interaction is that formed between the hydrogen atom of the nucleophile and the o-NO(2) group. This result discards the role of HB interaction between the nucleophile and the leaving group previously proposed in the literature.