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Reactivity of Strained Compounds:  Is Ground State Destabilization the Major Cause for Rate Enhancement?

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posted on 17.01.1996, 00:00 by Ariel Sella, Harold Basch, Shmaryahu Hoz
Reaction and activation energies were computationally determined for the nucleophilic attacks of OH- on 1-cyanobicyclobutane, 2-cyanobicyclobutane, and propionitrile using ab initio methods at the RHF/6-31+G* level. In the first reaction the central bond of the bicyclobutane moiety is cleaved. In the second reaction a side bond is fissioned, and in the third reaction an unstrained reference C−C bond is cleaved. The reaction energies are −38.3, −34, and −0.1 kcal, and the activation energies are 4.4, 30.6, and 41.6 kcal, respectively. Based on these data, traditional analysis suggests that the percent of strain relieved at the transition states of the first two reactions which have nearly the same thermodynamic driving force is 97% and 32%, respectively. These values, according to the linear free energy relationship approach point to an early transition state for the first reaction and a late transition state for the side bond cleavage. Examination of the computed geometrical parameters shows the opposite trends. Detailed analysis of these results suggests that the destabilization of the ground state cannot be considered as the major cause for the rate enhancement observed for strained substrates. Rather, an early transition state, which is usually accompanied by a low activation energy, results from a better capability of the frontier orbitals of the substrate to bond the entering nucleophile. Thus, the main chain of cause and effect in rate enhancement is molecular deformation → rehybridization → lower LUMO → better bonding capabilities. In bicyclobutane the lowest σ* orbital is associated with the central bond which therefore is cleaved much faster than the side bond which in turn is more reactive than the C−C bond of propionitrile.