Ring strain energy in ether- and lactone-containing spiro compounds

Abstract

Ring strain energies (RSEs) have been calculated for oxygen-containing spiro compounds using the group equivalent reaction (GER) formalism. The RSEs for all compounds studied were calculated from the energies of fully-optimized structures at the MP2 + ZPE/cc-pVDZ level and the more computationally costly G4(MP2) method. RSEs for selected compounds were also calculated with the CBS-QB3 method, with less than 1 kcal/mol difference observed between G4(MP2) and CBS-QB3. The difference between the less costly MP2 + ZPE and G4(MP2) methods was less than 1.5 kcal/mol. The highest RSEs were found for the compounds containing two three-membered rings, and these compounds also exhibited the greatest excess strain energy (ESE) of about 12 kcal/mol. The RSEs of cyclic lactones vary with ring size differently than those of cyclic ethers. Cyclic ethers' RSEs decrease by a small amount from the three- to four-membered rings then decrease drastically as the ring increases to 5 atoms, and approaches zero for the six-membered ring, the same unexpected behavior as seen in cycloalkanes. Cyclic lactones' RSEs decrease linearly to almost zero from the three- to the five-membered ring, then increase by 1–2 kcal/mol in the six-membered ring. Lactone-containing spiro compounds exhibit regularly diminishing ESE as the size of the lactone ring increases, down to about 3 kcal/mol in the δ-lactone-containing spiro compound. Substitution of methyl groups decreases RSE in these oxygen-containing spiro compounds, while substitution of fluorine significantly increases RSE, as has been reported in other compounds. But RSE alone is shown to not correlate completely with chemical reactivity of these spiro compounds. Copyright © 2014 John Wiley & Sons, Ltd.