Site-, enantio- and stereo-selectivities of the 1,3-dipolar cycloaddition reactions of oxanorbornadiene with C,N-disubstituted nitrones and dimethyl nitrilimines: a DFT mechanistic study

Theoretical insight into the unexpected initial (3 + 2) cycloaddition reaction of mesitonitrile oxide with 1, 4-diazepine derivatives: A computational study

1,4-Diazepine as an active drug component underlies the potency of most psychotic, anticancer, anticonvulsant, and antibacterial drugs in the market and is, therefore crucial in chemotherapeutic treatment in biomedicine. Proper functionalization of this moiety can afford even more potent drugs. As a result of their therapeutic significance, this study aims at precisely giving a comprehensive computational insight into the unexpected initial reactivity of 1,4-diazepine derivatives and mesitonitrile oxide. The initial reaction between mesitonitrile oxide and 1,4-diazepine derivatives proceeds via a (3 + 2) cycloaddition reaction which leads to the formation of a cycloadduct where the mesitonitrile oxide unexpectedly adds across the imine functionality at the expense of the potential olefinic carbon-carbon double bond. Calculations at the density functional theory (DFT) M06/6-311G (d, p) level of theory indicate that the initial (3 + 2) cycloaddition reaction of mesitonitrile oxide (1,3-dipole) and 1,4-diazepine derivatives (dipolarophile) in all cases proceeds to form the cycloadduct where the 1,3-dipole adds preferentially to the imine functionality at the expense of the potential olefinic carbon-carbon double bond. In light of the parent reaction, the most kinetically favored cycloadductP3A had a rate constant of 5.1 × 10⁶ M^-1s^-1, which is about 12 manifolds faster than the next competing stereoisomer P1A with a rate constant of 4.1 × 10⁵ M^-1s^-1 and about 10²⁴ faster than the most favored cycloadduct P3B with a rate constant of 7.2 × 10^-19 M^-1s^-1 in the unfavored pathway (Path B). Irrespective of the electronic and steric nature of the electron-donating (EDG) and electron-withdrawing (EWG) substituents placed on the dipolarophile, the selectivities of the reaction were maintained. Rationalization of the potential energy surface depicts that the 1,3-dipole adds across the dipolarophile via an asynchronous concerted mechanism. Rationalization of the HOMO-LUMO energies of the mesitonitrile oxide (1,3-dipole) and the 1,4-diazepine derivatives (dipolarophile) depict that the EDG-substituted dipolarophile react as nucleophiles, whereas the dipole reacts as an electrophile. Conversely, the HOMO-LUMO interaction between the EWG-substituted dipolarophile indicates that the EWG-substituted dipolarophile react as electrophiles, whereas the dipole reacts as a nucleophile. The electrophilic parr function at various reactive sites of the dipolarophile shows that the 1,3-dipole preferentially adds across the local centers with the largest electrophilic NBO or Mulliken spin densities which is consistent with the energetic trend observed. The reactivity of the 1,4-diazepine derivatives and the mesitonitrile oxide showed poor stereoselectivity.

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.

Investigating the site-, regio-, and stereo-selectivities of the reactions between organic azide and 7-heteronorbornadiene: a DFT mechanistic study

The site-, regio-, and stereo-selectivities of the title reactions have been studied using density functional theory (DFT) at the M06/6-311G(d,p) level of theory. The effects of substituents on both the three-atom component (TAC) and norbornadiene derivatives have been investigated with a focus on the site-selectivity. The reaction of benzylazide with (1S,4R)-2-tosyl-7-oxabicyclo[2.2.1]hepta-2,5-diene and (1R,4S)-2-bromo-3-tosyl-7-oxabicyclo[2.2.1]hepta-2,5-diene proceeds via addition across the substituted olefinic bond of the two norbornadiene derivatives. Substituents on the TAC do not affect the selectivity of the reaction while substituents on the norbornadiene significantly affect the selectivity of the reaction. Benzylazide preferentially adds across the substituted olefinic bond of the norbornadiene derivative when strong electron-withdrawing group (EWGs) and electron-releasing group (ERGs) substituents are on the norbornadiene while weak ERGs and EWGs on the norbornadiene significantly decreases the site-selectivity such that addition across either double is no longer favored over the other. The formation of exo-cycloadducts is generally favored over the endo-cycloadducts. The reaction of benzylazide and norbornadiene derivatives is a highly irreversible exergonic reaction. The direction of electron density flux is dependent on the nature of the substituent on the reactants. Global reactivity indices and Parr function calculations are in good agreement with the activation barriers and the selectivity of the reactions.

Unveiling the molecular mechanisms of the cycloaddition reactions of aryl hetaryl thioketones and C,N-disubstituted nitrilimines

Many synthetic routes to constructing biologically active heterocyclic compounds are made feasible through the (3 + 2) cycloaddition (32CA) reactions. Due to a large number of possible combinations of several heteroatoms from either the three-atom components (TACs) or the ethylene derivatives, the potential of the 32CA reactions in heterocyclic syntheses is versatile. Herein, the cycloaddition reaction of thiophene-2-carbothialdehyde derivatives and C,N-disubstituted nitrilimines have been studied through density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level of theory. In the present study, a one-step 32CA and two-step (4 + 3) cycloaddition (43CA) reaction mechanisms involved in TACs reactions and ethylene derivative have been investigated. In all reactions considered, the one-step 32CA cycloaddition is preferred over the two-step 43CA. The TAC chemoselectively adds across the thiocarbonyl group present in the ethylene derivative in a 32CA fashion to form the corresponding cycloadduct. Analysis of the electrophilic [Formula: see text] and nucleophilic [Formula: see text] Parr functions at the various reaction centers in the ethylene derivative show that the TAC adds across the atomic centers with the largest Parr functions, which is in total agreement with the experimental observation. The selectivities observed in the titled reactions are kinetically controlled.

Unveiling the Unexpected Reactivity of Electrophilic Diazoalkanes in [3+2] Cycloaddition Reactions within Molecular Electron Density Theory

The [3+2] cycloaddition (32CA) reactions of strongly nucleophilic norbornadiene (NBD), with simplest diazoalkane (DAA) and three DAAs of increased electrophilicity, have been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G (d,p) computational level. These pmr-type 32CA reactions follow an asynchronous one-step mechanism with activation enthalpies ranging from 17.7 to 27.9 kcal·mol−1 in acetonitrile. The high exergonic character of these reactions makes them irreversible. The presence of electron-withdrawing (EW) substituents in the DAA increases the activation enthalpies, in complete agreement with the experimental slowing-down of the reactions, but contrary to the Conceptual DFT prediction. Despite the nucleophilic and electrophilic character of the reagents, the global electron density transfer at the TSs indicates rather non-polar 32CA reactions. The present MEDT study establishes the depopulation of the N–N–C core in this series of DAAs with the increase of the EW character of the substituents present at the carbon center is responsible for the experimentally found deceleration.

Investigating the regio-, stereo-, and enantio-selectivities of the 1,3-dipolar cycloaddition reaction of C-cyclopropyl-N-phenylnitrone derivatives and benzylidenecyclopropane derivatives: A DFT study

The biomedical importance of spirocyclopropane isoxazolidine derivatives is widely known. The 1,3-dipolar cycloaddition (1,3-DC) of C-cyclopropyl-N-phenylnitrone derivative and benzylidenecyclopropane derivatives leading to the formation of 5- and 4-spirocyclopropane isoxazolidines derivatives have been studied using density functional theory (DFT) at M06-2X/6-311G (d,p) level of theory. An extensive exploration of the potential energy surface shows that the 1,3-dipole adds across the dipolarophile via an asynchronous concerted mechanism. While electron-donating groups (EDGs) on the benzylidenecyclopropane favor the formation of the 4-spirocyclopropane isomer, electron-withdrawing groups (EWGs) favor the reaction channels that furnish the 5-spirocyclopropane isoxazolidine isomer. Both EWDs and EDGs on the 1,3-dipole favor the formation of the 5-spirocyclopropane isoxazolidine isomer. Irrespective of the electronic nature of substituents on the C-cyclopropyl-N-phenylnitrone, the reaction channels that regioselectively lead to the formation of the 5-spirocyclopropane isoxazolidine isomer are favored. In all reactions considered, the channels that selectively lead to the formation of the cis-diastereoisomers proceed with lower activation barriers than the trans-diastereoisomers. In all cases, the observed selectivities in the title reaction are kinetically controlled.

Peri-, Chemo-, Regio-, Stereo- and Enantio-Selectivities of 1,3-dipolar cycloaddition reaction of C,N-Disubstituted nitrones with disubstituted 4-methylene-1,3-oxazol-5(4H)- one: A quantum mechanical study

The peri-, chemo-, regio-, stereo- and enantio-selectivities of 1,3-dipolar cycloaddition reaction of C,N-disubstituted nitrones with disubstituted 4-methylene-1,3-oxazol-5(4H)-one have been studied using density functional theory (DFT) at the M06-2X/6-311G (d,p) level of theory. The 1,3-dipole preferentially adds chemo-selectively across the olefinic bond in a (3 + 2) fashion forming the corresponding spirocycloadduct. The titled reaction occurs with poor enantio- and stereo-selectivities, but a high degree of regio-selectivity is observed for the addition of the 1,3-dipole across the dipolarophile. Electron-withdrawing groups on the dipolarophile significantly reduce the activation barriers while electron-donating groups on the dipolarophile increase the activation barriers. Analysis of the HOMO and LUMO energies of the two reacting species indicates that the 1,3-dipole reacts as a nucleophile while the dipolarophile reacts as the electrophile. Investigation of the electrophilic Parr function (P_K⁺) at the various reaction centers in the dipolarophile indicates that the 1,3-dipole selectively adds across the atomic species with the largest electrophilic Mulliken and NBO atomic spin densities which is in accordance with the energetic trends observed.