E. Uggerud, How persistent is cyclopropyl upon nucleophilic substitution, and is frontside displacement possible? A model study, J ORG CHEM, 66(21), 2001, pp. 7084-7089
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.