Unveiling the Stereoselectivity and Regioselectivity of the [3+2] Cycloaddition Reaction between N-methyl-C-4-methylphenyl-nitrone and 2-Propynamide from a MEDT Perspective

[3+2] cycloaddition reactions play a crucial role in synthesizing complex organic molecules and have significant applications in drug discovery and materials science. In this study, the [3+2] cycloaddition (32CA) reactions of N-methyl-C-4-methyl phenyl-nitrone 1 and 2-propynamide 2, which have not been extensively studied before, were investigated using molecular electron density theory (MEDT) at the B3LYP/6-311++G(d,p) level of theory. According to an electron localization function (ELF) study, N-methyl-C-4-methyl phenyl-nitrone 1 is a zwitterionic species with no pseudoradical or carbenoid centers. Conceptual density functional theory (CDFT) indices were used to predict the global electronic flux from the strong nucleophilic N-methyl-C-4-methyl phenylnitrone 1 to the electrophilic 2-propynamide 2 functions. The 32CA reactions proceeded through two pairs of stereo- and regioisomeric reaction pathways to generate four different products: 3, 4, 5, and 6. The reaction pathways were irreversible owing to their exothermic characters: -136.48, -130.08, -130.99, and -140.81 kJ mol^-1, respectively. The enthalpy of the 32CA reaction leading to the formation of cycloadduct 6 was lower compared with the other path owing to a slight increase in its polar character, observed through the global electron density transfer (GEDT) during the transition states and along the reaction path. A bonding evolution theory (BET) analysis showed that these 32CA reactions proceed through the coupling of pseudoradical centers, and the formation of new C-C and C-O covalent bonds did not begin in the transition states.

Understanding the mechanism and regio- and stereo selectivity of [3 + 2] cycloaddition reactions between substituted azomethine ylide and 3,3,3-trifluoro-1-nitroprop-1-ene, within the molecular electron density theory

The selectivity and the nature of the molecular mechanism of the [3 + 2] cycloaddition (32CA) reaction between 2-(dimethylamino)-1H-indene-1,3(2H)-dione (AY11) and trans(E)-3,3,3-trifluoro-1-nitroprop-1-ene(FNP10) has been studied, in which the molecular electron density theory using density functional theory methods at the MPWB1K/6-31G(d) computational level was used. Analysis of the global reactivity indices permits us to characterize FNP10 as a strong electrophile and AY11 as a strong nucleophile. Four reactive pathways associated with the ortho/meta regioselective channels and endo/exo stereoselective approaches modes have been explored and characterized in the gas phase and in the benzene solvent. The analysis of the relative energies associated with the different reaction pathways indicates that the 32CA reactions of the azomethine ylide (AY) with the nitroalkene (FNP) is meta regioselective with high endo stereoselectivity. This result is in good agreement with the experimental observations. electron localization function topological analysis of the most favored reactive pathways allows for characterizing the mechanism of this 32CA reactions as a non-concerted two-stage one-step mechanism. Finally, non-covalent interactions and quantum theory of atoms in molecule analyses at the meta/endo transition state structure indicate that the presence of different several weak interactions, namely, CF and NH contributed in favoring the formation of a meta-endo cycloadduct.

Unveiling the non-polar [3+2] cycloaddition reactions of cyclic nitrones with strained alkylidene cyclopropanes within a molecular electron density theory study

The role of cyclopropane substitution on the ethylene in zw-type [3+2] cycloaddition (32CA) reactions of cyclic nitrones has been studied within Molecular Electron Density Theory (MEDT) at the ωB97X-D/6-311G(d,p) computational level. Electron Localization Function (ELF) analysis of the ethylenes shows that the presence the cyclopropane only slightly increases the electron density in the C-C bonding region. Analysis of the Conceptual DFT reactivity indices indicates that the presence of the cyclopropane does not produce any remarkable change in the reactivity of these strained ethylenes. The marginal electrophilic character of ethylene makes the zw-type 32CA reactions of non-polar character. The presence of the cyclopropane in the ethylene decreases the activation enthalpy of the 32CA reactions by only 1.7 and 2.6 kcal mol-1, and also decreases the ortho regioselectivity. The loss of the strain present in the cyclopropane is responsible for the reduction of the activation enthalpy and the increase of the reaction enthalpy in these non-polar 32CA reactions. The presence of the cyclopropane does not cause any change, neither in the transition state structure (TS) geometries nor in their electronic structure. The very low global electron density transfer (GEDT) computed at the TSs confirms the non-polar character of these 32CA reactions. The ortho regioselectivity experimentally observed in these non-polar 32CA reactions is determined by the most favorable two-center interaction between the less electronegative C1 carbon of nitrone and the non-substituted methylene C5 carbon of the ethylenes.

Chemoselective and diastereodivergent synthesis of new spirooxindolo-pyrrolizidines and pyrrolidines stemming from unsymmetrical 1,3-bis(arylidene)tetral-2-ones: a combined experimental and theoretical study

An experimental and theoretical study of an efficient one-pot three-components cycloaddition reaction leading to pentacyclic dispiropyrrolizidin/pyrrolidinoxindoles endowed by four contiguous stereogenic centres with moderate to good yields was reported.

Construction of Saturated Oxazolo[3,2-b][1,2]oxazines via Tandem [3+2]-Cycloaddition/[1,3]-Rearrangement of Cyclic Nitronates and Ketenes

Reaction of six-membered cyclic nitronates with disubstituted ketenes affords hitherto unknown saturated oxazolo[3,2-b][1,2]oxazines possessing up to four contiguous stereogenic centers. The process involves a tandem of [3+2]-cycloaddition across the C═O bond of ketene, followed by a spontaneous [1,3]-rearrangement of transient vinylidene-substituted bicyclic nitrosoacetals. DFT calculations of the mechanism suggest that the [1,3]-O,C-shift proceeds through a recyclization of a biradical intermediate formed by an unusually mild homolytic cleavage of the N-O bond. The resulting products can be utilized as precursors of other fused 1,2-oxazines derivatives, in particular 1,2-oxazino-1,2,4-triazin-3-ones.

A molecular electron density theory study for [3+2] cycloaddition reactions of N-benzylcyclohexylnitrone with methyl-3-butenoate

The [3+2] cycloaddition (32CA) reactions of N-benzylcyclohexylnitrone 1 with methyl-3-butenoate 2 have been studied within molecular electron density theory (MEDT) at the B3LYP/6-311++G(d,p) level of theory.

Elucidating the origin of selectivity of [3 + 2]-cycloaddition reactions between thioketone and carbohydrate-derived nitrones by the DFT

The mechanism and origin of selectivity for [3 + 2]-cycloaddition (32CA) reactions between thioketone and carbohydrate-derived nitrones in THF were investigated by using the density functional theory (DFT) at the M06-2X/6-311+G(d,p)//M06-2X/6-31+G(d,p) level of theory combined with the solvation SMD model. The calculated results revealed that the 32CA reactions proceed through the asynchronous one-step manner. For the chemoselectivity in thioketone, the C=S bond as a dipolarophile attacking three-atom-component (TAC) nitrone in reactivity was more preferential than the C=O bond. The theoretical results also confirmed the stereoselectivity of two 32CA reactions of thioketone with carbohydrate-derived nitrones with the anti-form product being more favored than the syn-form product, and the predicted anti/syn product ratios are in agreement with the experimental ones in literature. Furthermore, the analysis of the conceptual density functional theory reactivity indices showed that the 32CA reactions have polar character. Weak noncovalent interaction and Parr function analyses are used to reveal the origin of the stereoselectivity. Graphical abstract [3 + 2]-cycloaddition reactions between thioketone and carbohydrate-derived nitrones.

Insights into the N-Heterocyclic Carbene (NHC)-Catalyzed Intramolecular Cyclization of Aldimines: General Mechanism and Role of Catalyst

One of the most challenging questions in the Lewis base organocatalyst field is how to predict the most electrophilic carbon for the complexation of N-heterocyclic carbene (NHC) and reactant. This study provides a valuable case for this issue. Multiple mechanisms (A, B, C, D, and E) for the intramolecular cyclization of aldimine catalyzed by NHC were investigated by using density functional theory (DFT). The computed results reveal that the NHC energetically prefers attacking the iminyl carbon (AIC mode, which is associated with mechanisms A and C) rather than attacking the olefin carbon (AOC mode, which is associated with mechanisms B and D) or attacking the carbonyl carbon (ACC mode, which is associated with mechanism E) of aldimine. The calculated results based on the different reaction models indicate that mechanism A (AIC mode), which is associated with the formation of the aza-Breslow intermediate, is the most favorable pathway. For mechanism A, there are five steps: (1) nucleophilic addition of NHC to the iminyl carbon of aldimine; (2) [1,2]-proton transfer to form an aza-Breslow intermediate; (3) intramolecular cyclization; (4) the other [1,2]-proton transfer; and (5) regeneration of NHC. The analyses of reactivity indexes have been applied to explain the chemoselectivity, and the general principles regarding the possible mechanisms would be useful for the rational design of NHC-catalyzed chemoselective reactions.

A Molecular Electron Density Theory Study of the Reactivity and Selectivities in [3 + 2] Cycloaddition Reactions of C,N-Dialkyl Nitrones with Ethylene Derivatives

The zw-type [3 + 2] cycloaddition (32CA) reactions of C,N-dialkyl nitrones with a series of ethylenes of increased electrophilic character have been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G(d,p) computational level. Both, reactivity and selectivities are rationalized depending on the polar character of the reaction. Due to the strong nucleophilic character of C,N-dialkyl nitrones, the corresponding zw-type 32CA reactions are accelerated with the increased electrophilic character of the ethylene, which also plays a crucial role in the reaction mechanism, thus determining the regio- and stereoselectivities experimentally observed. While, in the 32CA reactions with nucleophilic ethylenes, the reaction begins with the formation of the C-C single bond, determining the ortho regioselectivity, in the 32CA reactions with strong electrophilic ethylenes, the reaction begins with the formation of the C-O single bond involving the β-conjugated carbon of the ethylene, determining the meta regioselectivity. The present MEDT study also provides an explanation for the unexpected ortho regioselectivity experimentally found in the 32CA reactions involving weak electrophilic ethylenes such as ethyl acrylate and acrylonitrile.

A molecular electron density theory study on the [3+2] cycloaddition reaction of thiocarbonyl ylides with hetaryl thioketones

In situ generated TCY 2 is trapped with THK 3 over the course of a non-polar, entirely C1–C4 regioselective, pdr-type 32CA reaction passing through TS 1 in a non-concerted two-stage one-step molecular mechanism.

Theoretical Study on DBU-Catalyzed Insertion of Isatins into Aryl Difluoronitromethyl Ketones: A Case for Predicting Chemoselectivity Using Electrophilic Parr Function

The possible mechanisms of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-catalyzed chemoselective insertion of N-methyl isatin into aryl difluoronitromethyl ketone to synthesize 3,3-disubstituted and 2,2-disubstituted oxindoles have been studied in this work. As revealed by calculated results, the reaction occurs via two competing paths, including α and β carbonyl paths, and each path contains five steps, that is, nucleophilic addition of DBU to ketone, C-C bond cleavage affording difluoromethylnitrate anion and phenylcarbonyl-DBU cation, nucleophilic addition of difluoromethylnitrate anion to carbonyl carbon of N-methyl isatin, acyl transfer process, and dissociation of DBU and product. The computational results suggest that nucleophilic additions on different carbonyl carbons of N-methyl isatin via α and β carbonyl paths would lead to different products in the third step, and β carbonyl path associated with the main product 3,3-disubstituted oxindole is more energetically favorable, which is consistent with the experimental observations. Noteworthy, electrophilic Parr function can be successfully applied for exactly predicting the activity of reaction site and reasonably explaining the chemoselectivity. In addition, the distortion/interaction and noncovalent interaction analyses show that much more hydrogen bond interactions should be responsible for the lower energy of the transition state associated with β carbonyl path. The obtained insights would be valuable for the rational design of efficient organocatalysts for this kind of reactions with high selectivities.

Cyclopentadienones via a tandem C-cyclopropylnitrone cyclization-cycloreversion sequence

Aldonitrones derived from spiro[2.4]hepta-4,6-diene-1-carbaldehyde and its benzo analog undergo a tandem uncatalyzed intramolecular cyclopropane-nitrone cyclization-5,6-dihydro-1,2-oxazine cycloreversion to give cyclopentadienones. Similarly, the NH-nitrone generated in situ from spiro[cyclopropane-1,1'-indene]carbaldehyde oxime leads to benzocyclopentadienone (1H-inden-1-one) by the same mechanism. DFT calculations are in favor of a concerted yet highly asynchronous pathway for the cyclizations. Control experiments with the dihydro and tetrahydro derivatives show that the spirocyclopentadiene unit is essential for the success of the reaction, invoking spiroconjugative effects for increased cyclopropane reactivity.

BET & ELF Quantum Topological Analysis of Neutral 2-Aza-Cope Rearrangement of γ-Alkenyl Nitrones

The 2-Aza-Cope rearrangement of γ-alkenyl nitrones is a rare example of the neutral thermal 2-aza-Cope process that usually takes place with cationic species. During the rearrangement, a redistribution of bonds and electronic density occurs in one kinetic step. However, the introduction of substituents with different steric requirements and electronic features might alter the activation energies and the synchronicity of the reaction. The electron localization function (ELF) analysis and its application to Bonding Evolution Theory (BET) analysis within the context of Molecular Electron Density Theory (MEDT) is an excellent tool to monitor the electron density along the reaction coordinate and thus investigate in detail bond breaking and formation and the corresponding energy barriers. By analyzing topological ELF calculations of seventeen 2-aza-Cope nitrone rearrangements with selected substituents, the main factors influencing the synchronicity of the process were investigated. This MEDT study results revealed that the rearrangement is a non-polar process mostly influenced by steric factors rather than by electronic ones, and confirms the pseudoradical character of the process rather than any pericyclic electron-reorganization.

Non-pericyclic cycloaddition of gem-difluorosubstituted azomethine ylides to the CO bond: computational study and synthesis of fluorinated oxazole derivatives

The cycloaddition of α,α,α-trifluoroacetophenones with gem-difluoro-substituted azomethine ylides, generated from N-benzhydrylideneamines and difluorocarbenes, occurs regioselectively to give fluorinated oxazolidines.

New mechanistic interpretations for nitrone reactivity

The reactivity of nitrones in cycloadditions and related reactions is revisited by introducing a topological perspective.