How persistent is cyclopropyl upon nucleophilic substitution, and is frontside displacement possible? A model study

Quantum chemical model calculations (MP2/6-31G(d,p)) demonstrate that front side nucleophilic substitution is not possible in the reaction between wate r and protonated cyclopropanol. Instead, ring opening occurs, in accordance with a well-known disrotary ring-opening mechanism. When the cyclopropane ring is embedded in a stabilizing bicyclic structure, as in protonated bicy clo[3.1.0]hexanol, the mechanistic landscape changes. In this case frontsid e nucleophilic substitution occurs, and has a potential energy barrier whic h is lower than that of the corresponding backside substitution, which impl ies that the stereochemical outcome of this gas-phase nucleophilic substitu tion reaction is uncoupled from its kinetic order. This and similar results challenge the traditional view that nucleophilic substitution reactions sh ould be categorized as being either S(N)1 or S(N)2.