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

A quantum mechanistic investigation into the unusual reactions of nitrilimine and nitrile oxide with 2,3,4,5-tetraphenylcyclopentadienone

CONTEXT

The theoretical study investigates the [3 + 2] cycloaddition (32CA) reactions between C, N-diphenyl nitrilimine with 2,3,4,5-tetraphenylcyclopentadienone and benzonitrile oxide with 2,3,4,5-tetraphenylcyclopentadienone. Nitrilimines and nitrile oxides are dipoles used in the synthesis of several heterocyclic compounds, including spiropyrazoline oxindoles and isoxazolines. The derivatives of these compounds are found with different biological activities, such as ion channel blockers, anti-inflammatory and anticancer agents as well as antimalarial. Conceptual density functional theory (CDFT) analysis, along with the activation energies of the 32CA reaction between C, N-diphenyl nitrilimine with 2,3,4,5-tetraphenylcyclopentadienone, demonstrates concordance with the empirical findings. The 32CA reaction is found to proceed through a very polar single-step asynchronous mechanism. While deductions from the activation energies of the 32CA reaction between benzonitrile oxide and 2,3,4,5-tetraphenylcyclopentadienone are found to lead to the experimental product, the parr function analysis could not explain the observed chemo- and regioselectivity. This 32CA reaction is also found to proceed through a one-step asynchronous mechanism, though with a non-polar character. The modulation of substituents positioned at the reactive sites of the reactants is found to influence the kinetics, thermodynamics, and CDFT parameters of the two 32CA reactions, consequently impacting the observed selectivities.

METHODS

The 32CA reactions between C, N-diphenyl nitrilimine with 2,3,4,5-tetraphenylcyclopentadienone and benzonitrile oxide with 2,3,4,5-tetraphenylcyclopentadienone have been explored theoretically using the density functional theory method at the hybrid ωB97X-D coupled with the split valence triple-ξ (TZ) basis set as implemented in the Gaussian 09. Solvent effects were taken into account by full optimization of the gas phase geometries through the polarizable continuum model developed within the self-consistent reaction field.

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 peri- and stereo- selectivities of the cycloaddition reaction of 2-(2- dimethylaminovinyl)-1-benzopyran-4-one with N-phenylmaleimide (NPM) and dimethylacetylenedicarboxylate (DMAD) - A DFT study

The [4 + 2] cycloaddition reactions of 2-styrylchromones have been predominantly described as one of the efficient methods for the synthesis of xanthones-a prominent class of tricyclic molecules that occur widely in nature. These xanthones are well known for their pharmacological activities especially their role as anti-cancer agents in the medicinal world. In this study, the mechanistic insight into the unusual (peri- and stereo-) selectivities of the reaction of 2-(2-dimethylaminovinyl)-1-benzopyran-4-one (A1) with N-phenylmaleimide (NPM) and dimethylacetylenedicarboxylate (DMAD) has been studied using density functional theory (DFT) at the M06-2X/6-311G (d, p) level of theory. The reaction of A1 and NPM in dimethylformamide (DMF) is periselective towards the initial formation of a [4 + 2] cycloadduct and stereoselectively in an exo fashion with an activation energy of 6.8kcalmol^-1 and a rate constant of 6.43×10⁷s^-1 which occurs about 878 million times faster than the closest competing pathway for the initial [2 + 2] cycloaddition fashion with an activation energy of 19.0kcalmol^-1 and a rate constant of 7.32×10^-2s^-1. For the substituent effect on the reaction, the reaction selectivity is still maintained where the exo intermediate remains the most kinetically favored cycloadduct. However, the magnitude of the barriers increases slightly with a margin of about 0.1-4.8kcalmol for the electron-donating groups (EDGs) in the order; strong EDGs (OH < NH₂ < OCH₃) < weak EDGs (<Ph) and 5.3-6.4kcalmol for electron-withdrawing groups in the order; strong EWGs (CF₃) < weak EWGs (Cl < Br). On the other hand, DMAD periselectively adds across the amino substituted olefinic bond of A1 via an initial [2 + 2] stepwise cycloaddition fashion followed by an intramolecular rearrangement to form the xanthone product. The rate constant of the rate-determining step in the pathway for the formation of the kinetically favored [4 + 2] cycloadduct with an activation barrier of 17.3kcalmol^-1 is 1.29s^-1 which occurs about 373,000 times slower than the most preferred pathway affording a [2 + 2] cycloadduct with an activation barrier of 9.7kcalmol^-1 and a rate constant of 4.81×10⁵s^-1. Both reactions are normal electron-demand cycloaddition reactions and are kinetically controlled.