The Markovnikov regioselectivity rule in the light of site activation models

Abstract

The electrophilic addition of HCl to a series of asymmetric alkenes-propene, 2-methyl-2-butene, styrene, 2-phenylpropene, and 1-cyanopropene-is used as a model system to study the regioselectivity Markovnikov rule using density functional theory reactivity descriptors. The results show that this rule may be interpreted on the basis of a site activation model that goes beyond the Li-Evans model of selectivity if both the fluctuations in global softness and Fukui functions at the active site are taken into account. A local static analysis based on the condensed Fukui function at the ground state of alkenes was also performed. For all the systems considered, the Markovnikov carbon (M) atom (i.e., the less substituted one) displays electrophilic Fukui function values that are larger than those associated with the more substituted anti-Markovnikov (AM) carbon atom at the double bond. In most cases, they are also larger than the corresponding nucleophilic Fukui function values at both carbon centers of the ethylenic functionality. Site activation at the nucleophilic and electrophilic centers of the alkenes considered was probed by changes in regional softness with reference to the transition state structures. The results are consistent with the empirical Markovnikov rule. A global analysis of involved structures in the electrophilic addition of HCl shows that while the ground state and transition state structures display relative values of the energy and molecular hardness ordered in a way that is consistent with the maximum hardness principle (MHP), the comparison between the Markovnikov and anti-Markovnikov transition state structures do not: the Markovnikov channel presents a transition state which is lower in energy and softer than the one corresponding to the anti-Markovnikov addition.