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 (wit h respect to the dioxy bridge) by 2.1-degrees, thus giving rise to ant i pyramidalization of the doubly-bonded carbon atoms. However, diazome thane, 2-diazopropane, 3,4-dihydroisoquinoline N-oxide, and several ni trile oxides (i.e., bulky nitrile oxides and nitrile oxides with a low dipole moment) reacted with 1 to afford the syn adduct as the dominan t product. That is, the preferred approach of these 1,3-dipoles to 1 t akes place from the direction opposite to that of its ground-state pyr amidalization. The syn preference of these reactions is explained as a result of higher steric interactions in the anti than in the syn atta ck. 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 s tructures of the reactions of diazomethane and some nitrile oxides wit h 1. In particular, these calculations show that (i) pyramidalization of the trigonal centers of 1 is higher in anti TSs than in their syn c ounterparts, thus paralleling the ground-state anti pyramidalization o f 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 lesse n steric interactions. Transition structures also offer some explanati on of the unexpected anti selectivity of the reaction of trimethylacet onitrile oxide with 1.