Acridizinium cation (ACR) and 2,3-dimethylisoquinolinium ion (DMIQ) undergo [$2 + 4^+$] addition with dienophiles methyl vinyl ether (MVE) and propylene (PY) and the addition takes place across the diene unit containing the cationic centre and the dienophile acts as the electron donor. These reactions have two regiochemical paths and in each of them two possible stereo adducts could be formed. DFT calculations performed at B3LYP/6-31G(d) level have shown that the reactions pass through concerted mechanism and the TSs are highly asynchronous. Methoxy group in the dienophile can take up cis or trans orientation with respect to the double bond and in that trans orientation of the methoxy group is preferred. Calculations further show that syn 2 adduct is kinetically and thermodynamically more favoured in both the reactions in excellent agreement with the experimental observations. ACR is found to be more reactive than DMIQ as a diene and as a dienophile MVE is found to be more reactive than PY. Computed bond orders establish that the syn 2 transition states are the most `reactant’ like. Though the reactions have both electrostatic control and frontier orbital control the former dominates in the initial stages of the reaction.

Electrocyclic ring opening (ERO) reactions of 2-pyrone, 2-pyranol and pyran and their fluoro compounds (1-6) have been studied at MP2/6-31G(𝑑) level with special emphasis on the influence of fluorine on these pericyclic/pseudopericyclic processes. Calculations clearly predict that substitution of fluorine at C6 favour the reaction both kinetically and thermodynamically. Magnetic susceptibility anisotropy ($\Delta_{\chi\text{aniso}}$), NICS(0), NBO and bond critical property (BCP) analyses clearly illustrate the following; 2-pyrone (1) and 6-fluoro-2-pyrone (2) reactions are pseudopericyclic; 6-fluoro-2-pyranol (reaction 4) corresponds to a borderline case; 2-pyranol (3) and pyran (5) and 6-fluoro pyran (6) reactions are clearly pericyclic in character. Correspondingly pseudeopericyclic reactions show up orbital disconnections and fluorine delays the occurrence of orbital disconnections on the reaction trajectory.

Biocatalytic azidolysis of 9 unsymmetrical epoxides by halohydrin dehalogenase enzyme (HheC) in gas phase and uncatalysed azidolysis of the same epoxides in gas phase and in aqueous solution have been modelled at DFT level. Aliphatic epoxides (1-6) and aromatic epoxides (9) undergo 𝛽 cleavage while styrene oxide (7) and 𝑝-nitro styrene (8) oxide prefer 𝛼 cleavage in the gas phase. Inclusion of aqueous solvation effect via Polarizable Continuum Model (PCM) increases the activation barrier and makes the reaction endothermic due to extensive solvation of azide anion and oxido anionic products, but does not alter the regioselectivity. Halohydrin dehalogenase from Agrobacterium radiobactor AD1 catalyses (E1-E9) ring opening of all these epoxides by azide ion with 𝛽 selectivity and the reversal of selectivity in epoxide 7 and 8 is notable. These reactions follow, in both enzymatic and non-enzymatic environment, S$_N$2 mechanism. Calculations while agreeing totally with experimental results offer better insights on the factors determining the regioselectivity and particularly the role of enzyme. Active site model and crystal structure data reveal that the Tyr145 and Ser132 form weak hydrogen bonds with epoxide oxygen lone pair and form reactant enzyme complex (REC). The enzyme complex activates the epoxide ring towards azidolysis. The NBO deletion and second order perturbation analyses clearly bring out the role of catalytic duo Tyr145 and Ser132 and particularly shed light on the dominant contribution of Tyr145 in selectively activating C$_{\beta}$-O bond. The present results indicate that Arg149 or other residues in the pocket do not seem to have any significant effect on the reaction.

Imidozirconocene complex is known for its bifunctional reactivity and catalytic ability and this complex mediates ring cleavage of epoxides. Cyclooctene oxide (1) Norbornene oxide (2) and 2,5-dimethyl cyclohexene oxide (3) undergo ring cleavage in the presence of imidozirconocene complex. Epoxide 1 has accessible 𝛽-hydrogens (type I) while epoxide 2 and 3 do not have them (type II). Normally type I epoxides undergo elimination while type II epoxides prefer insertion. All the insertion reactions lead to five-membered metallacycle formation and elimination results in thermodynamically stable allyl-alkoxy product. The insertion is a two-step process following either diradical or zwitterionic pathway, while elimination is a one-step concerted reaction. DFT (density functional theory) modelling of these reactions at B3LYP/LANL2DZ level show that epoxide 1 undergoes elimination in agreement with experiment. However, calculations indicate that epoxide (2) proceeds through diradical intermediate in contrast to experimental observations. Surprisingly, epoxide (3) that has both the 𝛽 positions blocked by methyl groups undergoes elimination rather than insertion. AIM and EDA analyses offer further insights on the reaction mechanism and bifunctional reactivity of imidozirconozene complex.