Conceptual, Qualitative, and Quantitative Theories of 1,3-Dipolar and Diels–Alder Cycloadditions Used in Synthesis

Understanding the Electronic Effects of Lewis Acid Catalysts in Accelerating Polar Diels-Alder Reactions

The electronic effects of Lewis acid (LA) catalysts in reducing the activation energies of polar Diels-Alder (P-DA) reactions have been studied within Molecular Electron Density Theory. To this end, a quantum topological energy partitioning scheme, namely, the Relative Interacting Atomic Energy (RIAE) analysis, is applied to the transition state structures (TSs) and the ground state of the reagents of two different LA-catalyzed P-DA reactions. Analyses of the ξEtotalX total energies of the two interacting frameworks f(X) show that the electronic energy stabilization of the electrophilic frameworks, resulting from the global electron density transfer (GEDT), is the cause of an effective decrease of the activation energies. On the other hand, an in-depth analysis of the ξEintraA intra-atomic energies of the atoms belonging to the electrophilic ethylenic framework in the LA-catalyzed P-DA reactions of cyclopentadiene with acrolein indicates that the strong electronic stabilization of the carbonyl carbon, resulting from the GEDT taking place at the TSs, is the main factor responsible for the decrease of the activation energies in these LA-catalyzed P-DA reactions. Finally, the increase in GEDT at the TSs of these P-DA reactions causes an increase in the larger C-C distance, resulting from the stabilization of the electrophilic framework, thereby decreasing the suggested Pauli repulsion.

Multicomponent cyclization with azides to synthesize N-heterocycles

Heterocyclic compounds, both naturally derived and synthetically produced, constitute a wide variety of biologically active and industrially important compounds. The synthesis and application of heterocyclic compounds have garnered significant attention and experienced rapid growth in recent decades. Organic azides, due to their unique properties and distinctive reactivity, have become a convenient chemical tool for achieving a wide range of heterocycles such as triazoles and tetrazoles. Importantly, the field of multicomponent reaction (MCR) chemistry provides a convergent approach to access various N-heterocyclic scaffolds, offering novelty, diversity, and complexity. However, the exploration of MCR pathways to N-heterocyclic compounds remains incomplete. Here, we review the use of multicomponent reactions for the preparation of N-heterocycles. A wide range of reactions based on azides for the synthesis of various types of N-heterocyclic systems have been developed.

Understanding the reactivity and selectivity of Diels-Alder reactions involving furans

The reactivity and endo/exo selectivity of the Diels-Alder cycloaddition reactions involving furan and substituted furans as dienes have been computationally explored. In comparison to cyclopentadiene, it is found that furan is comparatively less reactive and also less endo-selective in the reaction with maleic anhydride as the dienophile. Despite that, both the reactivity and the selectivity can be successfully modified by the presence of substituents at either 2- or 3-positions of the heterocycle. In this sense, it is found that the presence of strong electron-donor groups significantly increases the reactivity of the system while the opposite is found in the presence of electron-withdrawing groups. The observed trends in both the reactivity and selectivity are analyzed quantitatively in detail by means of the activation strain model of reactivity in combination with the energy decomposition analysis methods.

De Novo Synthesis of Dihydrobenzofurans and Indolines and Its Application to a Modular, Asymmetric Synthesis of Beraprost

Dihydrobenzofurans and indolines are important constituents of pharmaceuticals. Herein, we describe a novel strategy for their construction in which the aromatic ring is created de novo through an inverse-electron demand Diels-Alder reaction and cheletropic extrusion sequence of a 2-halothiophene-1,1-dioxide with an enol ether/enamide, followed by aromatization. Unusually, the aromatization process proved to be highly challenging, but it was discovered that treatment of the halocyclohexadienes with a base effected an α-elimination-aromatization reaction. Mechanistic investigation of this step using deuterium-labeling studies indicated the intermediacy of a carbene which undergoes a 1,2-hydrogen shift and subsequent aromatization. The methodology was applied to a modular and stereoselective total synthesis of the antiplatelet drug beraprost in only 8 steps from a key enal-lactone. This lactone provided the core of beraprost to which both its sidechains could be appended through a 1,4-conjugate addition process (lower ω-sidechain), followed by de novo construction of beraprost's dihydrobenzofuran (upper α-sidechain) using our newly developed methodology. Additionally, we have demonstrated the breadth of our newly established protocol in the synthesis of functionalized indolines, which occurred with high levels of regiocontrol. According to density-functional theory (DFT) calculations, the high selectivity originates from attractive London dispersion interactions in the TS of the Diels-Alder reaction.

Is DFT Accurate Enough to Calculate Regioselectivity? The Case of 1,3-Dipolar Cycloaddition of Azide to Alkynes and Alkenes

The importance of regioselectivity in 1,3-dipolar cycloadditions (DCs) makes it surprising that no benchmarking study on this problem has appeared. We investigated whether DFT calculations are an accurate tool to predict the regioselectivity of uncatalyzed thermal azide 1,3-DCs. We considered the reaction between HN3 and 12 dipolarophiles, comprising ethynes HC≡C-R and ethenes H2 C=CH-R (R=F, OH, NH2 , Me, CN, CHO), which cover a broad range of electron demand and conjugation ability. We established benchmark data by the W3X protocol [complete-basis-set-extrapolated CCSD(T)-F12 energy with T-(T) and (Q) corrections and MP2-calculated core/valence and relativistic effects] and showed that core/valence effects and high-order excitations are important for accurate regioselectivity. Regioselectivities calculated using an extensive set of density functional approximations (DFAs) were compared with benchmark data. Range-separated and meta-GGA hybrids gave the best results. Good treatment of self-interaction and electron exchange are the key features for accurate regioselectivity. Dispersion correction slightly improves agreement with W3X results. The best DFAs provide the isomeric TS energy difference with an expected error ≈0.7 mh and errors ≈2 mh can occur. The isomer yield provided by the best DFA has an expected error of ±5 %, though errors up to 20 % are not rare. At present, an accuracy of 1-2 % is unfeasible but it seems that we are not far from achieving this goal.

Experimental and theoretical elucidation of SPAAC kinetics for strained alkyne-containing cycloparaphenylenes

Tuning strained alkyne reactivity via organic synthesis has evolved into a burgeoning field of study largely focused on cyclooctyne, wherein physical organic chemistry helps guide rational molecular design to produce molecules with intriguing properties. Concurrent research in the field of carbon nanomaterials has produced new types of strained alkyne macrocycles, such as cycloparaphenyleneacetylenes, that possess uniquely curved aromatic π systems but hover on the edge of stability. In 2018, we introduced a strained alkyne scaffold that marries the synthetic accessibility and stability of cyclooctyne with the curved π system of carbon nanomaterials. These molecules are strained alkyne-containing cycloparaphenylenes (or [n+1]CPPs), which have been shown to possess size-dependent reactivity as well as the classic characteristics of the unfunctionalized parent CPP, such as a tunable HOMO-LUMO gap and bright fluorescence for large sizes. Herein, we elaborate further on this scaffold, introducing two modifications to the original design and fully characterizing the kinetics of the strain-promoted azide-alkyne cycloaddition (SPAAC) for each [n+1]CPP with a model azide. Additionally, we explain how electronic (the incorporation of fluorine atoms) and strain (a meta linkage which heightens local strain at the alkyne) modulations affect SPAAC reactivity via the distortion-interaction computational model. Altogether, these results indicate that through a modular synthesis and rational chemical design, we have developed a new family of tunable and inherently fluorescent strained alkyne carbon nanomaterials.

A molecular electron density theory study on the Chichibabin reaction: The origin of regioselectivity

A Molecular Electron Density Theory (MEDT) study was performed on the nucleophilic amination of pyridine and benzene to elucidate the observed regioselectivity in the Chichibabin reaction. For this purpose, three possible reaction paths were considered between NaNH₂ and the 2-, 3- and 4-positions of pyridine. The reaction of NaNH₂ and benzene was also investigated. The results indicated that in each reaction, the first step involves the nucleophilic attack of the NH₂‾ ion toward the aromatic system to produce an anionic intermediate. The second step, proceeds via hydride elimination of the anionic intermediate to produce the corresponding aromatic amine. This step is more favourable both kinetically and thermodynamically for the reaction at the 2-position of pyridine relative to the 4-position. In contrast to the Parr functions analysis which indicates that the 4-position is activated more electrophilically in pyridine, the PES analysis reveals that the 2-position attack with the NH₂‾ ion is more favourable both thermodynamically and kinetically. The results for the reaction of sodium amide and benzene indicated that this reaction is not feasible due to the high activation barriers. In consistent with the experimental results, the ELF analysis indicated that in the first step of the reaction between pyridine and sodium amide, the formation of the C2-N bond takes place via a pseudoradical coupling between the C2 carbon atom of pyridine and the N atom of NaNH₂. These variations occur in the second-half of the first step of the reaction. NCI analysis on the reaction revealed that the more attractive forces between the fragments, make the transition states for the 2-position attack more stable relative to the others. Thus, the NCI analysis can satisfactorily describe the observed regioselectivity in the Chichibabin reaction.

Rationalizing the Substituent Effects in Diels-Alder Reactions of Triazolinediones with Anthracene

In this work we tackle the problem of the substituent effects in the Diels-Alder cycloadditions between triazolinediones (TADs) and anthracene. Experiments showed that aryl TADs substituted with electron-withdrawing groups (EWG) are more reactive than those substituted with electron-donating (EDG) or alkyl groups. However, the molecular origin of this preference is not yet understood. By a combination of methods including the activation strain model (ASM), energy decomposition analysis (EDA), molecular orbital (MO) theory, and conceptual density functional theory (CDFT), we disclosed the substituent effects of TADs. First, ASM/EDA analysis revealed that the reactivity of alkyl and aryl-substituted TADs is controlled by interaction energies, ΔE_int, which are ultimately defined by orbital interactions between frontier molecular orbitals. Moreover, alkyl-TADs are also controlled by the extent of strain at the transition state. The MO analysis suggested that the rate acceleration for EWG-substituted TADs is due to a more favorable orbital interaction between the HOMO of anthracene and the LUMO of the TADs, which is corroborated by calculations of charge transfer at the transition states. From CDFT, the chemical potential of anthracene is higher than those of TADs, indicating a flow of electron density from anthracene to TADs, in agreement with the results from the electrophilicity index.

Pericyclic reaction benchmarks: hierarchical computations targeting CCSDT(Q)/CBS and analysis of DFT performance

Hierarchical, convergent ab initio benchmark computations were performed followed by a systematic analysis of DFT performance for five pericyclic reactions comprising Diels-Alder, 1,3-dipolar cycloaddition, electrocyclic rearrangement, sigmatropic rearrangement, and double group transfer prototypes. Focal point analyses (FPA) extrapolating to the ab initio limit were executed via explicit quantum chemical computations with electron correlation treatments through CCSDT(Q) and correlation-consistent Gaussian basis sets up to aug'-cc-pV5Z. Optimized geometric structures and vibrational frequencies of all stationary points were obtained at the CCSD(T)/cc-pVTZ level of theory. The FPA reaction barriers and energies exhibit convergence to within a few tenths of a kcal mol-1. The FPA benchmarks were used to evaluate the performance of 60 density functionals (eight dispersion-corrected), covering the local-density approximation (LDA), generalized gradient approximations (GGAs), meta-GGAs, hybrids, meta-hybrids, double-hybrids, and range-separated hybrids. The meta-hybrid M06-2X functional provided the best overall performance [mean absolute error (MAE) of 1.1 kcal mol-1] followed closely by the double-hybrids B2K-PLYP, mPW2K-PLYP, and revDSD-PBEP86 [MAE of 1.4-1.5 kcal mol-1]. The regularly used GGA functional BP86 gave a higher MAE of 5.8 kcal mol-1, but it qualitatively described the trends in reaction barriers and energies. Importantly, we established that accurate yet efficient meta-hybrid or double-hybrid DFT potential energy surfaces can be acquired based on geometries from the computationally efficient and robust BP86/DZP level.

Exploring the maximum Fukui function sites with the frontier-controlled soft-soft reactions using 1,3-dipolar cycloaddition reactions of nitrilium betaines

According to P.K. Chattaraj. J Phys Chem A 2001, 105, 511-513 "the maximum Fukui function site is the best for the frontier-controlled soft-soft reactions whereas for the charge-controlled hard-hard interactions the preferred site is associated with the maximum net charge and not necessarily the minimum Fukui function". Taking into account these outcomes in this research is explored this reactivity scheme using in first case the reaction between fulminic acid with ethylene (reference reaction), after is varying the dipolarophile in the reaction between fulminic acid with acetylene, and finally is varying the dipole in the reaction between formonitrile imine with ethylene. These results allow study parameter such as charge transfer, polarizability, covalent character on bonding, among other; also shown the preference by the sf^- / sf⁺ interactions in the transition state on the sf^- / sf^- interactions. On the other hand, these results also were justified using net electrophilicity which is defined as the electrophilic power of a system relative to its own nucleophilic power.

A Theoretical Evaluation of the Behavior of Nitrosoamidine upon Reacting with Methoxy Butadiene, as Potential Heterodiene or Heterodienophile

Abstract:

The molecular mechanism of experimentally observed regio- and chemoselectivity of the cycloaddition reaction of nitrosoamidine 1 and 1-methoxy butadiene 2 has been investigated using DFT calculations at M06-2X/cc-pVDZ level. Accordingly, the possible reaction pathways and factors, which govern selectivity are investigated systematically. Analysis of the calculated results show that the most favorable cyclization reaction takes place through the [2+4] endo-proximal pathway, which is under kinetically and thermodynamically controls. Moreover, analysis of the global and local reactivity indices correctly explains the source of the experimentally observed regio- and chemoselectivity. The electron localization function (ELF) analysis of some selected points along the IRC profile of the most preferred pathway suggests that the reaction takes place via a two-stage one-step mechanism. NCI topological analysis of the possible pathways of [2+4] cycloaddition reaction of 1-E and 2-Z reveals the roles of the attractive interactions between reaction sites, the weak noncovalent interactions in the endo approaches, and the repulsive interactions in the regio- and stereoselectivity of the reaction.

Machine Learning to Predict Diels-Alder Reaction Barriers from the Reactant State Electron Density

Reaction barriers are key to our understanding of chemical reactivity and catalysis. Certain reactions are so seminal in chemistry that countless variants, with or without catalysts, have been studied, and their barriers have been computed or measured experimentally. This wealth of data represents a perfect opportunity to leverage machine learning models, which could quickly predict barriers without explicit calculations or measurement. Here, we show that the topological descriptors of the quantum mechanical charge density in the reactant state constitute a set that is both rigorous and continuous and can be used effectively for the prediction of reaction barrier energies to a high degree of accuracy. We demonstrate this on the Diels-Alder reaction, highly important in biology and medicinal chemistry, and as such, studied extensively. This reaction exhibits a range of barriers as large as 270 kJ/mol. While we trained our single-objective supervised (labeled) regression algorithms on simpler Diels-Alder reactions in solution, they predict reaction barriers also in significantly more complicated contexts, such a Diels-Alder reaction catalyzed by an artificial enzyme and its evolved variants, in agreement with experimental changes in k_cat. We expect this tool to apply broadly to a variety of reactions in solution or in the presence of a catalyst, for screening and circumventing heavily involved computations or experiments.

Exploring the chemo-, regio-, and stereoselectivities of the (3 + 2) cycloaddition reaction of 5,5-dimethyl-3-methylene-2-pyrrolidinone with C,N-diarylnitrones and nitrile oxide derivatives: a DFT study

The (3 + 2) cycloaddition (32CA) reaction is an efficient method for the synthesis of many biologically active heterocyclic compounds, but there are several regio- and stereochemical issues that must be fully understood to exploit the full utility of its synthetic power. We herein explored the chemo-, regio-, and stereoselectivities of the 32CA reaction of 5,5-dimethyl-3-methylene-2-pyrrolidinone (B1) to C,N-diarylnitrones (B2), and nitrile oxide derivatives (B3) with DFT at the M06/6-311G(d,p) level of theory. The reactions occur via an asynchronous one-step mechanism, with the chemoselective addition of the C,N-diarylnitrones, and nitrile oxide derivatives across the olefinic bond of 5,5-dimethyl-3-methylene-2-pyrrolidinone being the most preferred kinetically and thermodynamically. The regio- and stereoselectivities of the reactions are affected by the electronic and steric nature of substituents on B2 but they are not affected by the electronic and steric nature of substituents on B3. The C,N-nitrones and the nitrile oxide derivatives add across the atomic centers with the largest atomic spin densities on 5,5-dimethyl-3-methylene-2-pyrrolidinone as seen through the local electrophilic ([Formula: see text]) and nucleophilic ([Formula: see text]) Parr functions of the various reaction centers. Results from the global electron density transfer (GEDT) reveal the low polar nature of the reactions.

Ynamides as Three-Atom Components in Cycloadditions: An Unexplored Chemical Reaction Space

While 1,3-dipolar cycloadditions have appeared to be a fertile area for research, as attested by the numerous synthetic transformations and resulting applications that have been developed during the past 60 years, the use of neutral three-atom components (TACs) in (3+2) cycloadditions remains comparatively sparse. Neutral TACs, however, have great synthetic potential given that their reaction with a π system can produce zwitterionic cycloadducts that may be manipulated for further chemistry. We report herein that ynamides, a class of carbon π systems that has seen wide interest over the last two decades, can be used as neutral TACs in thermally induced intramolecular (3+2) cycloaddition reactions with alkynes to yield a variety of functionalized pyrroles. The transformation is proposed to occur in a stepwise manner via the intermediacy of a pyrrolium ylide, from which the electron-withdrawing group on the nitrogen atom undergoes an intramolecular 1,2-shift to produce the neutral pyrrole. This work demonstrates a yet unexplored facet of ynamide reactivity with great potential in heterocyclic chemistry.

Mechanistic and Kinetic Factors of ortho-Benzyne Formation in Hexadehydro-Diels-Alder (HDDA) Reactions

With the rapid development of the hexadehydro-Diels-Alder reaction (HDDA) from its first discovery in 1997, the question of whether a concerted or stepwise mechanism better describes the thermally activated formation of ortho-benzyne from a diyne and a diynophile has been debated. Mechanistic and kinetic investigations were able to show that this is not a black or white situation, as minor changes can tip the balance. For that reason, especially, linked yne-diynes were studied to examine steric, electronic, and radical-stabilizing effects of their terminal substituents on the reaction mechanism and kinetics. Furthermore, the influence of the nature of the linker on the HDDA reaction was explored. The more recently discovered photochemical HDDA reaction also gives ortho-arynes, which display the same reactivity as the thermally generated ones, but their formation might not proceed by the same mechanism. This minireview summarizes the current state of mechanistic understanding of the HDDA reaction.

1,3-Dipolar Cycloadditions by a Unified Perspective Based on Conceptual and Thermodynamics Models of Chemical Reactivity

The main aim in the present report is to gain a deeper understanding of typical 1,3-dipolar cycloadditions by means of three chemical reactivity models in a unified perspective: conceptual density functional theory, distortion/interaction, and reaction force analysis. The focus is to explore the information provided by each reactivity model and how they complement or reinforce each other. Our results showed that the Bell-Evans-Polanyi (BEP) relationship is fulfilled, which is consistent with the Hammond-Leffler postulate. The electronic chemical potential based analysis classifies the reactions as HOMO-, HOMO/LUMO-, and LUMO-controlled reactions as the activation energy increases. It seems likely that HOMO-controlled reaction shifts into LUMO-controlled one as the transition state (TS) position does from early into late. Therefore, the transition from HOMO- (and early TS) into LUMO-controlled (and late TS) is paid by shifting the overall energy change into an endothermic direction, thus supporting the fulfillment of the BEP principle. While thermodynamic models unveil that the distortion or structural rearrangements mainly drive the activation barriers rather than interaction or electronic rearrangements in accord with the distortion/interaction and reaction force analysis, respectively. It is also found that both models are consistent when energy associated with structural and electronic reordering from reaction force analysis is respectively confronted with destabilizing (distortion and Pauli repulsion) and stabilizing (electrostatic and orbital interactions) contributions from the distortion/interaction model, which, on the other hand, increases as low activation barrier and high exothermicity are converted into the high barrier and low exothermicity along with the BEP relation. Finally, the reaction force constant reveals that all 1,3-dipolar cycloaddition reactions proceed by a synchronous single-step mechanism, unveiling that the degree of synchronicity is quite the same in all reactions, confirming the statement that BEP is fulfilled for similar reactions proceeding by a quite alike degree of synchronicity.

A thermoreversible crosslinking hot-melt adhesive: reversibility and performance

The need to improve the environmental friendliness and achieve the recycling of resins is an ongoing process for hot-melt adhesive technology. In this work, a new type of thermoreversible crosslinking hot-melt adhesive-based Diels Alder (DA) reaction was prepared. The critical idea was to efficiently initiate the esterification to yield furoic acid (FA)-grafted poly(vinyl alcohol) (PVA-g-FA), and then PVA-g-FA was mixed with N,N′-(4,4′-methylenediphenyl)dimaleimide (MDI) to finally obtain the thermoreversible crosslinking adhesive (PVA-g-FA/MDI). The experimental results indicated that the reversibility of the DA reaction between the furan rings and the maleimide groups allowed PVA-g-FA/MDI to be dynamically crosslinked. It was able to crosslink at a temperature of 80 °C and decrosslink at 120 °C. Moreover, the performances of hot-melt adhesive were investigated. The PVA-g-FA/MDI had a better peeling strength (43.33 N mm⁻¹), bond strength (11.84 MPa), and thermal conductivity (0.263 W m⁻¹ K⁻¹) than PVA resins. The light-transmittance and haze value were 52.8% and 12.24‰, respectively. The PVA-g-FA/MDI resin could be reused more than two times.

The need to improve the environmental friendliness and achieve the recycling of resins is an ongoing process for hot-melt adhesive technology.

The devil is in the details: The chemical basis and mechanistic versatility of flavoprotein monooxygenases

The ubiquitous flavoenzymes commonly catalyze redox chemistry such as the monooxygenation of organic substrates and are both widely utilized in nature (e.g., in primary and secondary metabolism) and of significant industrial interest. In this work, we highlight the structural and mechanistic characteristics of the distinct types of flavoprotein monooxygenases (FPMOs). We thereby illustrate the chemical basis of FPMO catalysis, which enables reactions such as (aromatic) hydroxylation, epoxidation, (de)halogenation, heteroatom oxygenation, Baeyer-Villiger oxidation, α-hydroxylation of ketones, or non-oxidative carbon-hetero bond cleavage. This seemingly unmatched versatility in oxygenation chemistry results from extensive fine-tuning and regiospecific functionalization of the flavin cofactor that is tightly controlled by the surrounding protein matrix. Accordingly, FPMOs steer the formation of covalent flavin-oxygen adducts for oxygen transfer in the form of the classical flavin-C4a-(hydro)peroxide or the recently discovered N5-functionalized flavins (i.e. the flavin-N5-oxide and the flavin-N5-peroxide), while in rare cases covalent oxygen adduct formation may be foregone entirely. Finally, we speculate about hitherto undiscovered flavin-mediated oxygenation reactions and compare FPMOs to cytochrome P450 monooxygenases, before addressing open questions and challenges for the future investigation of FPMOs.

Diels-Alder Reactivity of a Chiral Anthracene Template with Symmetrical and Unsymmetrical Dienophiles: A DFT Study

In this work, we used Density Functional Theory calculations to assess the factors that control the reactivity of a chiral anthracene template with three sets of dienophiles including maleic anhydrides, maleimides and acetoxy lactones in the context of Diels-Alder cycloadditions. The results obtained here (at the M06-2X/6-311++G(d,p) level of theory) suggest that the activation energies for maleic anhydrides and acetoxy lactones are dependent on the nature of the substituent in the dienophile. Among all studied substituents, only -CN reduces the energy barrier of the cycloaddition. For maleimides, the activation energies are independent of the heteroatom of the dienophile and the R group attached to it. The analysis of frontier molecular orbitals, charge transfer and the activation strain model (at the M06-2X/TZVP level based on M06-2X/6-311++G(d,p) geometries) suggest that the activation energies in maleic anhydrides are mainly controlled by the amount of charge transfer from the diene to the dienophile during cycloaddition. For maleimides, there is a dual control of interaction and strain energies on the activation energies, whereas for the acetoxy lactones the activation energies seem to be controlled by the degree of template distortion at the transition state. Finally, calculations show that considering a catalyst on the studied cycloadditions changes the reaction mechanism from concerted to stepwise and proceed with much lower activation energies.

Roles of Lewis Acid Catalysts in Diels-Alder Reactions between Cyclopentadiene and Methyl Acrylate

The Diels-Alder reaction of cyclopentadiene with methyl acrylate catalyzed by AlCl₃ has been theoretically investigated. M06-2X level DFT calculations have shown that the formation of two C-C bonds is asynchronous in the cycloaddition both in the endo path and in the exo path, thus making a good contrast to the well-known concept of [4+2] reactions based on the orbital symmetry arguments. It was found that the catalyst facilitates the cycloaddition and brings a higher endo selectivity in the highly asynchronous process, as compared with the reaction of the diene and the dienophile without the catalyst.

Looking at the big picture in activation strain model/energy decomposition analysis: the case of the ortho-para regioselectivity rule in Diels-Alder reactions

The regioselectivity of the Diels-Alder reaction is predicted by the ortho-para rule which has been explained from FMO theory. Using DFT calculations, the activation-strain model and energy decomposition analysis we studied the reaction of methyl acrylate with four unsymmetrical dienes. We found that if the analysis is carried out considering the TS structures, the selectivity would not be explained by the interaction energy as expected considering the FMO arguments. However, a thorough analysis along the reaction path revealed that the interaction energy is responsible for the regioselectivity. A deeper analysis with the EDA model showed that the decisive term that accounts for the HOMO-LUMO interactions favors the ortho and para paths, as predicted by FMO arguments.

AFIR explorations of transition states of extended unsaturated systems: automatic location of ambimodal transition states

Based on the artificial force induced reaction (AFIR) method, we proposed a procedure to systematically explore ambimodal transition states (TSs) that cause the dynamical bifurcation.

A molecular electron density theory study on the [3+2] cycloaddition reaction of 5,5-dimethyl-1-pyrroline N-oxide with 2-cyclopentenone

In the present work, the [3 + 2] cycloaddition reaction of 5,5-dimethyl-1-pyrroline N-oxide (Nit-5) and 2-cyclopentenone (CPN-6), experimentally reported by Tamura et al., was theoretically studied using the newly introduced molecular electron density theory (MEDT). Based on the experimental findings, this reaction takes place in an O3-C4 regio- and an exo-stereospecific fashion to give corresponding [3 + 2] exo cycloadduct as the sole product. The results of the potential energy surface analysis indicated that the experimentally reported product is more favorable both thermodynamically and kinetically relative to other possible adducts. In complete agreement with the experimental outcomes, the conceptual density functional theory reactivity indices explained the reactivity and regioselectivity of the reaction. Calculation of global electron density transfer of the energetically most preferred transition state indicated that the electron density fluxes from Nit-5 as a nucleophilic species toward CPN-6 as an electrophilic species. Analysis of the molecular electrostatic potential map of the most favorable transition state showed that approach of Nit-5 and CPN-6 locates the oppositely charged regions over each other leading to attractive forces between two reagents rationalizing the exo stereoselectivity predominance. The molecular mechanism of the reactions was specified using electron localization function analysis over some relevant points along the intrinsic reaction coordinate profile of the most favorable transition state and the results indicated that this zwitterionic-type [3 + 2] cycloaddition reaction proceeds through a two-stage one-step mechanism. In fact, while the O3-C4 single bond is initialy formed between two fragments through donation of some electron density from the O3 oxygen lone electron-pairs of Nit-5 toward the C4 carbon atom of CPN-6, the delayed C1-C5 single bond begins to form via C1- to -C5 coupling of pseudodiracal centers created on theses atoms over the course of reaction.

Experimental and Theoretical Approaches in the Study of Phenanthroline-Tetrahydroquinolines for Alzheimer's Disease

The imino-Diels-Alder reaction is one of the most common strategies in organic chemistry and is an important tool for providing a broad spectrum of biologically active heterocyclic systems. A combined theoretical and experimental study of the imino-Diels-Alder reaction is described. The new phenanthroline-tetrahydroquinolines were evaluated as cholinesterase inhibitors. Their cytotoxicity in human neuroblastoma SH-SY5Y cells was also evaluated. The theoretical results suggest that compounds formation in stages can be explained by endo cycloadducts under the established reaction conditions, thereby confirming experimental results obtained for percentage yield. These results allowed us to establish that pyridine substituent remarkably influences activation energy and reaction yield, as well as in acetylcholinesterase (AChE) activity. Among these derivatives, compounds with 4-pyridyl and 4-nitrophenyl showed favorable AChE activity and proved to be non-cytotoxic.

Tetrazoles via Multicomponent Reactions

Tetrazole derivatives are a prime class of heterocycles, very important to medicinal chemistry and drug design due to not only their bioisosterism to carboxylic acid and amide moieties but also to their metabolic stability and other beneficial physicochemical properties. Although more than 20 FDA-approved drugs contain 1 H- or 2 H-tetrazole substituents, their exact binding mode, structural biology, 3D conformations, and in general their chemical behavior is not fully understood. Importantly, multicomponent reaction (MCR) chemistry offers convergent access to multiple tetrazole scaffolds providing the three important elements of novelty, diversity, and complexity, yet MCR pathways to tetrazoles are far from completely explored. Here, we review the use of multicomponent reactions for the preparation of substituted tetrazole derivatives. We highlight specific applications and general trends holding therein and discuss synthetic approaches and their value by analyzing scope and limitations, and also enlighten their receptor binding mode. Finally, we estimated the prospects of further research in this field.

Drug–DNA interaction, a joint DFT-D3/MD study on safranal as an anticancer and DNA nanostructure model

In this research, using a combination of quantum mechanics and molecular dynamic (MD) simulations, the interaction of safranal (2,6,6-trimethylcyclohexa-1,3-dien-1-carboxaldehyde) as an anti-cancer drug and Dickerson B-DNA was studied. MD simulations were executed for 35 ns in water. Binding energy analysis in three definite parts of the B-DNA and comparison between different contributions of the binding energy shows that the van der Waals energy part of the interaction is impressive among the standard molecular mechanic energy terms. On the basis of Gibbs energies, it is confirmed that the most important interactions in the safranal complex are related to the A–T and C–G rich regions, which is in agreement with the experimental data. Quantum theory of atoms in molecules and natural bond orbital analyses were applied. A diminution in the electronic chemical potential of the safranal–DNA complex in comparison with the isolated DNA, 0.026 and 0.022 au for the S1 region and 0.012 and 0.017 au for the S2 region, was obtained in the gas phase and water, respectively, which increases the complex stability. An enhancement in the electrophilicity character, during the complexation process, shows the electron charge flux between the safranal and DNA, especially in water. The strengths of the CH⋯O bonds at the center of safranal–DNA interaction were also evaluated. A mean value of 0.06 au for the electron density of the bond critical point of the H⋯O in the complex confirms the H-bond formation during the complexation.

Unveiling the high reactivity of cyclohexynes in [3 + 2] cycloaddition reactions through the molecular electron density theory

[3 + 2] cycloaddition (32CA) reactions of cyclohexyne, a cyclic strained acetylene, with methyl azide and methoxycarbonyl diazomethane have been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G(d) computational level. These 32CA reactions, which take place through a one-step mechanism involving highly asynchronous transition state structures, proceed with relatively low activation enthalpies of 6.0 and 4.3 kcal mol-1, respectively, both reactions being strongly exothermic. The reactions are initiated by the creation of a pseudoradical center at one of the two acetylenic carbons of cyclohexyne with a very low energy cost, 1.0 kcal mol-1, which promotes the easy formation of the first C-N(C) single bond in the adjacent acetylenic carbon. This scenario is completely different from that of the 32CA reaction involving non-strained but-2-yne; thus, strain in cyclohexyne triggers a high reactivity as a consequence of its unusual electronic structure at the ground state. Finally, the experimental regioselectivity of the 32CA reactions involving 3-alkoxy-cyclohexyne derivatives is correctly explained within MEDT.

Site-selectivity control in hetero-Diels-Alder reactions of methylidene derivatives of lawsone through modification of the reactive carbonyl group: an experimental and theoretical study

A new perspective on the reactivity of hydroxyquinones was revealed as an acetal derivative of lawsone was synthesized, isolated, and used in tandem Knoevenagel/hetero-Diels-Alder reactions catalyzed by S-proline. The intermediate alkylidene-1,3-diones that were formed in situ reacted with electron rich alkenes to predominantly afford pyrano-1,2-naphthoquinone (β-lapachone) derivatives along with the isomeric pyrano-1,4-naphthoquinone (α-lapachone) derivatives in high to excellent total yields. Interestingly, the highly reactive arylidene-1,3-dione derivatives were found to be stable and isolable. DFT calculations suggest that these hetero-Diels-Alder reactions have a high polar character, taking place through a two-stage one-step mechanism. An analysis of the conceptual DFT indices allows explaining the remarkable site-selectivity observed.