Rearrangement of the 1-oxaspiro[4.5]deca-6,9-dien-8-ylium ion. Part 1. A semiempirical (AM1), density functional and ab initio molecular orbital computational study

The energetics of the ring-expanding rearrangement of the 1-oxaspiro[4.5]deca-6,9-dien-8-ylium ion 14 involving a 1,2-shift of either carbon or oxygen have been investigated through molecular orbital calculations at the semiempirical (AM1), ab initio (HF/z, HF/6-31G* and MP2/6-31G*) and density functional (B3LYP/6-31G*) levels of approximation. Our results have shown that rearrangement via migration by carbon is thermodynamically (and probably also kinetically) favoured over the alternative 1,2-shift by oxygen. The geometries and charge distributions computed for the structure 14 as well as the rearranged cations and possible transition structures are also described.