[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.
Reaction of either 2-pyrone or 1,4-oxazinone with acetylene follows the sequence of cycloadditioncycloreversion through concerted mechanism. Transition states for both cycloaddition and cycloreversion pathways have been obtained in both the cases by modelling the reactions at B3LYP/6-31g (d) level. Cycloreversion is faster than cycloaddition in the case of 2-pyrone due to the enhancement of aromaticity resulting the product as benzene. In contrast, oxazinone has rapid cycloaddition. It is ascribed to the presence of nitrogen in this system. Removal of either CO 2 or HCN is plausible in this mechanism to complete the reaction. Even though two pathways are feasible for cycloreversion, CO 2 extrusion is more preferable than HCN elimination. In these two studied molecules, there is an enhancement of aromaticity up to transition states like any other pericyclic reaction and further it diminishes during cycloaddition. Further, aromaticity is specifically augmented in cycloreversion phase during CO 2 elimination resulting to yield pyridine whereas competitive HCN elimination results in the formation of 2-pyrone which is less facile. In both the molecules the aromatic enhancement of the cycloreversion is substantiated through the study of magnetic susceptibility of the ring fragment along the reaction coordinate. Further the study also reveals the effect of halogen substituted at different carbons of 2-pyrone ring.
2-Pyrone (2P) and 1,4-oxazinone (OXZ) react with strained alkynes viz., cyclohexyne (SA1), benzyne (SA2) and pyridyne (SA3) through concerted cycloaddition-cycloreversion reaction. All the reactions have been scanned and the activation parameters, FOE, deformation energies and variation in aromaticity computed at B3LYP/6-31g(d) level.The obtained results have been compared with that of acetylene. In SA1,faster cycloadditionis explained by its strain, in addition,aromatic gain is accountable to enhance the rate of cycloaddition in SA2 and SA3.
In (Z)-1,3-pentadienes, [1,5]-H migration is suprafacially allowed while fluorine shift in this system
takes place by a Contra Hoffmann antarafacial pathway for which aromaticity is the driving force. If
aromaticity of the transition structure (TS) can drive a reaction towards a disallowed pathway as found
in the case of fluorine, the role of aromatic ring annealed to (Z)-1,3-pentadienes in determining the
reaction pathway and barrier is worth noting. Hence, the combined role of aromaticity of transition
state and the loss in aromaticity of the annealed ring has been explored during the [1,5]-X (X = H, F,
Cl, Br) shifts in aromatic (benzene/naphthalene) annealed 1,3-pentadiene system. Notable correlations
between various aromaticity index NICS(0,1) with activation barriers show that aromaticity of transition
structure in pericyclic reaction can drive the stereochemical course of a reaction. The distinct effect of
fluorine to other halogens is the antara migration while the other halogens (Cl & Br) prefer supramode.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.