No abstract

Cation binding to the π-electrons of benzene is known to show a periodic trend in interaction energies. In the present work, the chemistry of cation-π interaction in a benzene ring bound with tripodal Cr(CO) 3 (BC) was considered. In contradiction to the anticipated destabilisation due to competition between two Lewis acids towards a common sandwiched base, cation binding with BC showed a similar trend as that to benzene. Furthermore, it was found to activate the benzene ring by reducing the frontier orbital energy gap substantially. The NICS zz index adds sufficient evidence to the arguments. In addition, TDDFT studies indicate a bathochromic shift upon cation binding as an immediate consequence of the reduction in the energy of the Frontier orbital.

[1,5]‐H shift of (Z,Z) 1,3‐pentadienes are known to take suprafacial mode. The stereochemical fate of the reaction when halogens are involved in the [1,5]‐shift in similar (Z,Z) 1,3‐pentadienes is presented in this work. Interesting tendency of fluorine to take up the forbidden and relatively expensive antarafacial path as an economical route for [1,5]‐fluorine shift is owed to the aromaticity of the transition state. Remarkable correlations between various aromaticity indices (bond alternation coefficient [BAC], harmonic oscillator model of aromaticity [HOMA], nucleus independent chemical shift (NICS), and ellipticity [ξ]) with activation barriers pave solid foundation to the fact that aromaticity of transition states (TS) in pericyclic reaction can manoeuvre the stereochemical course of a reaction. The differential effect of fluorine to other halogens is exemplified by the antara migration while the other halogens (Cl and Br) prefer supra mode. NBO studies and the topological parameter, ellipticity used as a measure of aromatic criterion support the above fact.

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 (Δχ 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.

The competition between the two electron donors (-OCH 3 and-OSiH 3) have been studied by positioning them at geminal and vicinal locations. We have also studied the cis and trans geometrical isomerism at the vicinal location. Thermodynamic and activation parameters such as free energies have been computed to understand the potential energy surface of the reaction. Our computed activation hardness supports the proposed torquoselectivity. Further, Nucleus Independent Chemical Shift (NICS) was calculated at the ring centre of the transition state (TS) which additionally supports the torquoselectivity. The delicate preferences for the pathways have been substantiated through NBO interactions.