Shape-selective isopropylation of naphthalene over mordenite catalysts: Computational analysis using MOPAC

In our experimental work on shape-selective isopropylation of naphthalene, the selectivity for 2,6-diisopropylnaphthalene (2,6-DIPN) and the ratio of 2,6-DIPN to 2,7-DIPN were increased by mordenite catalyst dealumination. Ho wever, it was not clear whether the differentiation between the two isomers was caused by their differences in molecular dimensions or in electronic p roperties. In this work we performed a computational analysis of the molecu lar dimensions and frontier electron density f(r)(E) using MOPAC program fo r naphthalene, isopropyl- and diisopropylnaphthalene. The f(r)(E) value for electrophilic substitution reaction represents the density of electrons in the highest occupied molecular orbital (HOMO). According to the frontier m olecular orbital theory, the most reactive position (the carbon atom on whi ch electrophilic attack occurs most likely) has the highest frontier electr on density. The calculation shows that 2,6-DIPN has a slightly smaller crit ical diameter. More importantly, position 6 in 2-IPN has higher f(r)(E) val ue than that of position 7 in 2-IPN. This suggests that during 2-IPN isopro pylation inside a mordenite channel, the formation of 2,6-DIPN is electroni cally more favored than that of 2,7-DIPN. (C) 1999 Elsevier Science B.V. Al l rights reserved.