Abstract

Nasrin Farahani¹ and Seyed Majid Musavi²

DOI : http://dx.doi.org/10.13005/ojc/330326

The energetic and activation barriers for 4 possible [2+2] and [4+2] cycloadditions of ketene and dihaloketenes with cyclopentadiene are investigated at B3LYP/6-31G* level of theory. Three solvents including DMSO, CCl₄ and water, using the CPCM model, are considered and their results are compared to those of gas phase. The entire studied cycloadditions take place via clear asynchronous TSs. For the parent ketene, all solvents stabilize the reactants within the range of 9.4-13.7 kcal/mol and reduce the thermodynamic achievability of this reaction (∆E=-18.4, -10.9, -11.6 and -12.8 kcal/mol for gas phase, CCl₄, DMSO and H₂O, respectively, for [2+2] across C=C) and the energy barriers are increased to the maximum of 6.1 kcal/mol. In contrast, for dihaloketenes all solvents make the thermodynamics less favorite by decreasing the exoergicity (within 1-5 kcal/mol), however, most of reaction pathways become kinetically favored by lowering the energy barriers (within 1-7 kcal/mol) in the presence of polar solvents. The nonpolar solvent CCl₄ behaves like that of gas phase. Moreover, the preference of two-step cycloaddition via [3+3] sigmatropic rearrangement is reduced in the presence of solvents. The ultimate effect is pertaining to dichloroketen where it prefers the routine one-step cycloaddition pathway in H₂O like that observed for the parent ketene.

Dihaloketene; Cycloaddition; Dielectric constant; Density functional theory; Dielse-Alder

[ View HTML Full Text]