[2++4] Cycloadditions of Iminium Ions − Concerted or Stepwise Mechanism of Aza Diels−Alder Reactions?

Catalyst-free inverse-electron-demand aza-Diels-Alder reaction of 4,4-dicyano-2-methylenebut-3-enoates and 1,3,5-triazinanes: access to polysubstituted tetrahydropyridines

An inverse-electron-demand aza-Diels-Alder reaction between 4,4-dicyano-2-methylenebut-3-enoates and 1,3,5-triazinanes under catalyst-free and additive-free conditions was developed, which provided a highly convenient and straightforward method to construct a series of polyfunctionalized tetrahydropyridines in high yields. This strategy features numerous advantages, including high efficiency, good functional group tolerance, broad substrate scope, and environmentally friendly conditions.

First Diels–Alder reaction of a N-nitrogen-substituted iminium ion as dienophile furnishing N-amino-substituted 1,2,5,6-tetrahydropyridines

The exploratory and preliminary work on the reaction of 1-(methoxydiphenylmethyl)-2-methyldiazene with 1,3-dienes reveals a remarkable dichotomy furnishing different heterocyclic Diels–Alder products depending on the presence or absence of water. In contrast to the acid-induced hydrolysis of the N,O-ketal starting material with 1,3-dienes furnishing 1-methyl-1,2,3,6-tetrahydropyridazines (preceding communication), the same acid-promoted reaction albeit under exclusion of water affords N-(methyleneamino)-1,2,5,6-tetrahydropyridines. The serendipitous discovery of the novel 1,2,5,6-tetrahydropyridines was corroborated by an alternative and independent synthesis providing the in situ generated formaldehyde benzophenone azine and its conjugate acid, 2-(diphenylmethylene)-1-methylenehydrazin-1-ium ion. The protonated azine serving as the putative dienophile resembles the first example of an N-nitrogen-substituted iminium ion employed as a dienophile in a bona fide normal electron-demand Diels–Alder reaction.

Graphical abstract

Understanding the Mechanism of Diels-Alder Reactions with Anionic Dienophiles: A Systematic Comparison of [ECX]- (E = P, As; X = O, S, Se) Anions

While Diels–Alder (DA) reactions involving neutral or cationic dienophiles are well-known, the characteristics of the analogous reactions with anionic dienophiles are practically unexplored. Herein we present the first comparative computational investigations on the characteristics of DA cycloadditions with anionic dienophiles on the basis of the reactions of [ECX]⁻ anions (E = P, As; X = O, S, Se) with 2H-pyran-2-one. All of these reactions were found to be both kinetically and thermodynamically feasible, enabling synthetic access toward 2-phosphaphenolate and arsaphenolate derivatives in the future. This study also reveals that the [ECO]⁻ anions show clear regioselectivity, while for [ECS]⁻ and [ECSe]⁻ anions, the two possible reaction channels have very similar energetics. Additionally, the activation barriers for the [ECO]⁻ anions are lower than those of the heavier analogues. The observed differences can be traced back to the starkly differing nucleophilic character of the pnictogen center in the anions, leading to a barrier-lowering effect in the case of the [ECO]⁻ anions. Furthermore, analysis of the geometries and electron distributions of the corresponding transition states revealed structure–property relationships, and thus a direct comparison of the cycloaddition reactivity of these anions was achieved. Along one of the two pathways, a good correlation was found between the activation barriers and suitable nucleophilicity descriptors (nucleophilic Parr function and global nucleophilicity). Additionally, the tendency of the reaction energies can be explained by the changing aromaticity of the products.

In contrast to the phosphaethynolate [PCO]⁻ anion, the cycloaddition reactivity of the heavier congeners ([ECX]⁻, where E = P, As and X = O, S, Se) is unexplored. In this computational study, the Diels−Alder reaction between the known [ECX]⁻ anions and 2-pyrone was employed to compare the reactivity patterns. The first activation barrier of these reactions correlates with the nucleophilicity of the anions, indicating a barrier-lowering effect. The feasibility of the studied reactions, leading to P and As heterocycles, was also explored.

How electrophilic are 3-nitroindoles? Mechanistic investigations and application to a reagentless (4+2) cycloaddition

The electrophilicity of 4 different 3-nitroindole derivatives has been evaluated by Mayr's linear free energy relationship (log k(20 °C) = s_N(E + N)) and reveals unexpected values for aromatic compounds, in the nitrostyrene range. 3-Nitroindoles are sufficiently electrophilic to interact with a common diene namely the Danishefsky's diene at room temperature, in the absence of any activator, to furnish smoothly the dearomatized (4+2) cycloadducts in good yields.

Unveiling the Ionic Diels-Alder Reactions within the Molecular Electron Density Theory

The ionic Diels-Alder (I-DA) reactions of a series of six iminium cations with cyclopentadiene have been studied within the Molecular Electron Density Theory (MEDT). The superelectrophilic character of iminium cations, ω > 8.20 eV, accounts for the high reactivity of these species participating in I-DA reactions. The activation energies are found to be between 13 and 20 kcal·mol-1 lower in energy than those associated with the corresponding Diels-Alder (DA) reactions of neutral imines. These reactions are low endo selective as a consequence of the cationic character of the TSs, but highly regioselective. Solvents have poor effects on the relative energies, and an unappreciable effect on the geometries. In acetonitrile, the activation energies increase slightly as a consequence of the better solvation of the iminium cations than the cationic TSs. Electron localization function (ELF) topological analysis of the bonding changes along the I-DA reactions shows that they are very similar to those in polar DA reactions. The present MEDT study establishes that the global electron density transfer (GEDT) taking place at the TSs of I-DA reactions, and not steric (Pauli) repulsions such as have been recently proposed, are responsible for the features of these types of DA reactions.

How Do Aromatic Nitro Compounds React with Nucleophiles? Theoretical Description Using Aromaticity, Nucleophilicity and Electrophilicity Indices

In this study, we present a complete description of the addition of a model nucleophile to the nitroaromatic ring in positions occupied either by hydrogen (the first step of the S_NAr-H reaction) or a leaving group (S_NAr-X reaction) using theoretical parameters including aromaticity (HOMA), electrophilicity and nucleophilicity indices. It was shown both experimentally and by our calculations, including kinetic isotope effect modeling, that the addition of a nucleophile to the electron-deficient aromatic ring is the rate limiting step of both S_NAr-X and S_NAr-H reactions when the fast transformation of σ^H-adduct into the products is possible due to the specific reaction conditions, so this is the most important step of the entire reaction. The results described in this paper are helpful for better understanding of the subtle factors controlling the reaction direction and rate.

Hetero Diels-Alder Reactions with a Dicationic Urea Azine Derived Azo Dienophile and Their Use for the Synthesis of an Electron-Rich Pentacene

Herein, the first hetero Diels-Alder (DA) reactions with a stable, dicationic urea azine derived azo dienophile, synthesized by two-electron oxidation of a neutral urea azine are reported. Several charged DA products were synthesized in good yield and fully characterized. The DA adduct of anthracene is in thermal equilibrium with the reactants at room temperature, and the reaction enthalpy and entropy were determined from the temperature-dependent equilibrium constant. Furthermore, base addition to solutions of the pentacene DA product led to deprotonation, cleavage of the N-N bond, and formation of an electron-rich 6,13-bisguanidinyl-substituted pentacene. The redox and optical properties of this new pentacene derivative were studied. Furthermore, the dication resulting from its two-electron oxidation was synthesized and fully characterized. The results disclose a new elegant route to electron-rich pentacene derivatives.

Total facial selectivity of a d-erythrosyl aromatic imine in [4π + 2π] cycloadditions; synthesis of 2-alkylpolyol 1,2,3,4-tetrahydroquinolines

Different electron-rich dienophiles were combined with the imine obtained from 2,4-O-benzylidene-d-erythrose and p-anisidine furnishing enantiomerically pure tetrahydroquinolines, by inverse electron-demand [4π + 2π] cycloaddition.

Aminocatalytic cross-coupling approach via iminium ions to different C - C Bonds

Given the attractive ability of iminium ions to functionalize molecules directly at ostensibly unreactive positions, the reactivity of iminium ions, in which an α CH2 group is replaced by CO was explored. Background studies on the ability of such iminium cations to promote reactions via an iminium-catalyzed or iminium-equivalent pathway are apparently unavailable. Previously, tandem cross-coupling reactions were reported, in which an iminium ion undergoes nucleophilic 1,2-addition to give a putative three-component intermediate that abstracts a proton in situ and undergoes self-deamination followed by unprecedented DMSO/aerobic oxidation to generate α-ketoamides. However, later it was observed that iminium ions can generate valuable α-ketoamides through simple aerobic oxidation. In all reactions, iminium ions were generated in situ by reaction of 2-oxoaldehydes with secondary amines.

A quantum chemical topological analysis of the C-C bond formation in organic reactions involving cationic species

ELF topological analysis of the ionic Diels-Alder (I-DA) reaction between the N,N-dimethyliminium cation and cyclopentadiene (Cp) has been performed in order to characterise the C-C single bond formation. The C-C bond formation begins in the short range of 2.00-1.96 Åvia a C-to-C pseudoradical coupling between the most electrophilic center of the iminium cation and one of the two most nucleophilic centers of Cp. The electron density of the pseudoradical center generated at the most electrophilic carbon of the iminium cation comes mainly from the global charge transfer which takes place along the reaction. Analysis of the global reactivity indices indicates that the very high electrophilic character of the iminium cation is responsible for the negative activation energy found in the gas phase. On the other hand, the analysis of the radical P(k)(o) Parr functions of the iminium cation, and the nucleophilic P(k)(-) Parr functions of Cp makes the characterisation of the most favourable two-center interaction along the formation of the C-C single bond possible.

The mechanism of ionic Diels–Alder reactions. A DFT study of the oxa-Povarov reaction

Topological ELF analysis along these I-DA reactions indicates that both mechanisms present a similar C–C bond formation.

Molecular iodine induced/1,3-dipolar cycloaddition/oxidative aromatization sequence: an efficient strategy to construct 2-substituted benzo[f]isoindole-1,3-dicarboxylates

A useful method for a molecular iodine induced 1,3-dipolar cycloaddition/oxidative aromatization sequence to construct 2-substituted-benzo[f]isoindole-1,3-dicarboxylates is reported.

Nucleophilicity parameters of pyridinium ylides and their use in mechanistic analyses

Kinetics of the reactions of pyridinium, isoquinolinium, and quinolinium ylides with diarylcarbenium ions, quinone methides, and arylidene malonates (reference electrophiles) have been studied in dimethylsulfoxide solution by UV-vis spectroscopy. The second-order rate constants (log k2) were found to correlate linearly with the electrophilicities E of the reference electrophiles, as required by the linear free-energy relationship log k(20°C) = sN(N + E) (J. Am. Chem. Soc. 2001, 123, 9500), which allowed us to derive the nucleophile-specific parameters N and sN for these ylides. Pyridinium substitution is found to have a similar effect on the reactivity of carbanionic reaction centers as alkoxycarbonyl substitution. Agreement between the rate constants measured for the 1,3-dipolar cycloadditions of pyridinium, isoquinolinium, and quinolinium ylides with acceptor substituted dipolarophiles (arylidenemalononitrile and substituted chalcone) and those calculated from E, N, and sN shows that the above correlation can also be employed for predicting absolute rate constants of stepwise or highly unsymmetrical concerted cycloadditions. Deviations between calculated and experimental rate constants by a factor of 10(6) were demonstrated to indicate a change of reaction mechanism.

Design, validation, and implementation of a rapid-injection NMR system

A Rapid Injection NMR (RINMR) apparatus has been designed and constructed to allow the observation of fast chemical reactions in real time by NMR spectroscopy. The instrument was designed to allow the rapid (<2 s) injection and mixing of a metered volume of a reagent into a spinning NMR tube followed by rapid acquisition of the data resulting from the evolution of the chemical process. The various design criteria for this universal system included the ability to deliver any chemical reagent at any temperature and allow for the observation of any nucleus. The various challenges associated with the construction and implementation of this instrument are documented along with the validation of the accuracy of the apparatus with respect to volume and temperature. Finally, the ultimate validation and reproducibility of the technique is presented in the form of three case studies that used the instrument to elucidate various aspects of organic reaction mechanisms. The authors urge interested parties to not embark on the construction of their own instrument and invite those whose research problems might be amenable to this kind of analysis to contact the corresponding author for access to the apparatus described herein.

The Diels-Alder reaction of 4,6-dinitrobenzofuroxan with 1-trimethylsilyloxybuta-1,3-diene: a case example of a stepwise cycloaddition

The reaction of 4,6-dinitrobenzofuroxan (DNBF) with 1-trimethylsilyloxybuta-1,3-diene (8) is shown to afford a mixture of [2+4] diastereomeric cycloadducts (10, 11) through stepwise addition-cyclization pathways. Zwitterionic intermediate sigma-adduct 9, which is involved in the processes, has been successfully characterized by (1)H and (13)C NMR spectroscopy and UV/visible spectrophotometry in acetonitrile. A kinetic study has been carried out in this solvent that revealed that the rate of formation of 9 nicely fits the three-parameter equation log k=s(E+N) developed by Mayr to describe the feasibility of nucleophile-electrophile combinations. This significantly adds to the NMR spectroscopic evidence that the overall cycloadditions take place through a stepwise mechanism. The reaction has also been studied in dichloromethane and toluene. In these less polar solvents, the stability of 9 is not sufficient to allow direct characterization by spectroscopic methods, but a kinetic investigation supports the view that stepwise processes are still operating. An informative comparison of our reaction with previous interactions firmly identified as prototype stepwise cycloadditions is made on the basis of the global electrophilicity index, omega, defined by Parr within the density functional theory, and highlighted by Domingo et al. as a powerful tool for understanding Diels-Alder reactions.

Povarov reactions involving 3-aminocoumarins: synthesis of 1,2,3,4-tetrahydropyrido[2,3-c]coumarins and pyrido[2,3-c]coumarins

Condensation of 3-aminocoumarin (5) with 4-nitrobenzaldehyde (8) afforded a 2-azadiene (9), which reacted with various electron-rich alkenes (10 examples) in the presence of Yb(OTf)3 to afford 1,2,3,4-tetrahydropyrido[2,3-c]coumarins. Yields were generally good, but the diastereomeric ratios were highly variable. The products arose through a formal [4 + 2] cycloaddition (inverse electron demand Diels-Alder reaction) followed by tautomerization. As such, these are examples of the Povarov reaction. A range of 1,2,3,4-tetrahydropyrido[2,3-c]coumarins was then synthesized using a three-component version of this reaction, which involves in situ formation of the 2-azadiene component. Some of these products were converted into the corresponding pyrido[2,3-c]coumarins upon treatment with various oxidants, the most effective of which proved to be nitrous gases.

Understanding the reactivity of captodative ethylenes in polar cycloaddition reactions. A theoretical study

The electrophilic/nucleophilic character of a series of captodative (CD) ethylenes involved in polar cycloaddition reactions has been studied using DFT methods at the B3LYP/6-31G(d) level of theory. The transition state structures for the electrophilic/nucleophilic interactions of two CD ethylenes toward a nucleophilically activated ethylene, 2-methylene-1,3-dioxolane, and an electrophilically activated ethylene, 1,1-dicyanoethyelene, have been studied, and their electronic structures have been characterized using both NBO and ELF methods. Analysis of the reactivity indexes of the CD ethylenes explains the reactivity of these species. While the electrophilicity of the molecules accounts for the reactivity toward nucleophiles, it is shown that a simple index chosen for the nucleophilicity, Nu, based on the HOMO energy is useful explaining the reactivity of these CD ethylenes toward electrophiles.

Mayr electrophilicity predicts the dual Diels–Alder and σ-adduct formation behaviour of heteroaromatic super-electrophiles

We report on the dual reactivity, i.e. anionic Meisenheimer sigma adduct formation and Diels-Alder adduct formation, of a series of heteroaromatic super-electrophiles, including 4,6-dinitro-benzofuroxan, -N-arylbenzotriazoles (4), -benzothiadiazole and -benzoselenadiazole. Measured pK(a)(H(2)O) values for sigma adduct formation provide a quantitative measure of super-electrophilic reactivity with a satisfactory correlation between the Mayr E electrophilicity parameter and pK(a)(H(2)O): E = -0.662 pK(a)(H(2)O) (or pK(R+) -3.20 (r(2) = 0.987). The most highly electrophilic, pre-eminent super-electrophile is 4,6-dinitrotetrazolopyridine (E = -4.67, pK(a)(H(2)O) = 0.4), which supercedes the reference Meisenheimer super-electrophile, 4,6-dinitrobenzofuroxan (E = -5.06, pK(a) = 3.75), having itself an E value superior by 8 orders of magnitude compared to 1,3,5-trinitrobenzene as the benchmark normal Meisenheimer electrophile (E = -13.19, pK(a)(H(2)O) = 13.43). (For relevant kinetic parameters as well as E and pK(a) values, see .) In a parallel study we have investigated Diels-Alder (normal and inverse electron demand) reactivity of this series of heteroaromatic electrophiles and have shown that Mayr E values are valid predictors of whether DA adducts will form and how rapidly. The observed order of pericyclic reactivity corresponds to E = -8.5 as the demarcation E value, in close agreement with sigma complexation; thus pointing to a common origin for the two processes, i.e. an inverse relationship between the degree of aromaticity of the carbocyclic ring and ease of sigma complexation, or DA reactivity, respectively.

Definition of a Nucleophilicity Scale

This work deals with exploring some empirical scales of nucleophilicity. We have started evaluating the experimental indices of nucleophilicity proposed by Legon and Millen on the basis of the measure of the force constants derived from vibrational frequencies using a probe dipole H-X (X = F,CN). The correlation among some theoretical parameters with this experimental scale has been evaluated. The theoretical parameters have been chosen as the minimum of the electrostatic potential V(min), the binding energy (BE) between the nucleophile and the H-X dipole, and the electrostatic potential measured at the position of the hydrogen atom V(H) when the complex nucleophile and dipole H-X is in the equilibrium geometry. All of them present good correlations with the experimental nucleophilicity scale. In addition, the BEs of the nucleophiles with two other Lewis acids (one hard, BF(3), and the other soft, BH(3)) have been evaluated. The results suggest that the Legon and Millen nucleophilicity scale and the electrostatic potential derived scales can describe in good approximation the reactivity order of the nucleophiles only when the interactions with a probe electrophile is of the hard-hard type. For a covalent interaction that is orbital controlled, a new nucleophilicity index using information of the frontier orbitals of both, the nucleophile and the electrophile has been proposed.

Mechanistic studies on the formal aza-Diels-Alder reactions of N-aryl imines: evidence for the non-concertedness under Lewis-acid catalysed conditions

The reaction of a para-methoxyaniline, ethyl glyoxalate-derived imine with a series of dienes has resulted in products, which initially suggest the operation of different modes of aza-Diels-Alder reaction. However, a more likely explanation is that a common reaction mechanism is operating, involving a step-wise Lewis-acid catalysed process, which only appears to behave similarly to alternative concerted cycloaddition reactions.

Substituent Effects on Pyrid-2-yl Ureas toward Intramolecular Hydrogen Bonding and Cytosine Complexation

Equilibria between two conformational isomers of pyrid-2-yl ureas, the (E,Z) and (Z,Z) forms, have been studied in DMF-d(7) at -70 degrees C. Most of them show a small preference for the (E,Z) form with an equilibrium constant K(i) around 1-2. However, the K(i) value for 1-methyl-2-(3-(pyrid-2-yl)ureido)pyridinium iodide (12) was found to be 14.2 +/- 1.2. That is 1 order of magnitude larger than those of the others, which indicates that the positively charged 1-methylpyridinium-2-yl substituent would facilitate the (E,Z) form formation. Pyrid-2-yl ureas bind cytosine in DMF-d(7) with binding constants K(B) ranging from 30 to 1700 M(-1). Electron withdrawing substituents, such as the 4-O(2)NC(6)H(4)- or 1-methylpyridinium-4-yl substituent, preferentially facilitate the intermolecular cytosine complexation with large binding constants.

Global and local electrophilicity patterns of diazonium ions and their reactivity toward pi-nucleophiles

The global and local electrophilicity patterns of a series of 15 diazonium ions have been evaluated using the absolute scale of electrophilicity proposed by Parr et al. (J. Am. Chem. Soc. 1999, 121, 1922). The predicted global electrophilicity pattern of the whole series of diazonium ions correctly compares with the experimental electrophilicity recently determined for these charged electrophiles. The global electrophilicity is then projected into the different potential active sites of the molecular ions using the electrophilic Fukui function. The highest regional electrophilicity power is found at the terminal nitrogen atom of the arenediazonium ions, yet the highest positive charge is located on the nitrogen atom directly attached to the aromatic ring. This result is consistent with the observed reactivity displayed by diazonium ions toward substituted alkenes, thereby suggesting that the formation of the azocarbocation intermediate is mostly orbital rather than charge controlled.

Comparison between experimental and theoretical scales of electrophilicity in benzhydryl cations

The theoretical electrophilicity pattern of a series of benzhydryl cations as described in terms of a model based on global reactivity indexes is compared with an experimental scale recently proposed by Mayr et al. [J. Am. Chem. Soc. 2001, 123, 9500]. A good correlation between both theoretical and experimental quantities is found. The effect of chemical substitution on the electrophilic power of these charged electrophiles may also be assessed as local responses at the active site, in terms of a global contribution described by changes in global electrophilicity with reference to the dianisylcarbenium ion, and a local contribution described by the variations in electrophilic Fukui function at that site.

A theoretical study of the reaction between cyclopentadiene and protonated imine derivatives: a shift from a concerted to a stepwise molecular mechanism

The reaction between cyclopentadiene and protonated pyridine-2-carboxaldehyde imine derivatives has been studied by using Hartree-Fock (HF) and B3LYP methods together with the 6-31G basis set. The molecular mechanism is stepwise along an inverted energy profile. This results from the protonation on both nitrogen atoms of the imine group and the pyridine framework. The first step corresponds to the nucleophilic attack of cyclopentadiene on the electron-poor carbon atom of the iminium cation group to give an acyclic cation intermediate, and the second step is associated with the ring closure of this intermediate via the formation of a C-N single bond yielding the final cycloadduct. Two reactive channels have been characterized corresponding to the endo and exo approach modes of the cyclopentadiene to the iminium cation. The role of the pyridium cation substituent and the nitrogen position (ortho, meta, and para) along the reaction pathway has been also considered. Solvent effects (dichloromethane) by means of a continuum model have been taken into account to model the experimental environment.