2,3-DIOXABICYCLO[2.2.2]OCT-5-ENE - A PYRAMIDALIZED OLEFIN WHOSE FACIAL SELECTIVITY DOES NOT PARALLEL ITS PYRAMIDALIZATION

The HF/3-21G-optimized geometry of 2,3-dioxabicyclo[2.2.2]oct-5-ene (1) shows that its olefinic hydrogens are bent in the syn direction (with respect to the dioxy bridge) by 2.1-degrees, thus giving rise to anti pyramidalization of the doubly-bonded carbon atoms. However, diazomethane, 2-diazopropane, 3,4-dihydroisoquinoline N-oxide, and several nitrile oxides (i.e., bulky nitrile oxides and nitrile oxides with a low dipole moment) reacted with 1 to afford the syn adduct as the dominant product. That is, the preferred approach of these 1,3-dipoles to 1 takes place from the direction opposite to that of its ground-state pyramidalization. The syn preference of these reactions is explained as a result of higher steric interactions in the anti than in the syn attack. Steric effects, which favor the syn attack, overcome electrostatic interactions and factors related to pyramidalization, which favor the anti. This conclusion is discussed on the basis of ab initio (STO-3G) and semiempirical (AM1) calculations of the syn and anti transition structures of the reactions of diazomethane and some nitrile oxides with 1. In particular, these calculations show that (i) pyramidalization of the trigonal centers of 1 is higher in anti TSs than in their syn counterparts, thus paralleling the ground-state anti pyramidalization of 1, and (ii) a more inclined trajectory of approach to 1 is followed by the 1,3-dipole in the anti than in the syn attack in order to lessen steric interactions. Transition structures also offer some explanation of the unexpected anti selectivity of the reaction of trimethylacetonitrile oxide with 1.