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[摘要]：The facial selectivity of 2,3-dioxabicyclo[2.2.2]oct-5-ene in cycloaddition reactions is distressingly variable and akin to that of 2-oxa-3-azabicyclo[2.2.2]oct-5-ene derivatives. Osmilation takes place on the face anti to the dihetero bridge, while cycloaddition of butadiene affords exclusively the syn cycloadduct. Nitrile oxides add unselectively. B3LYP/LANL2DZ calculations of cycloadditions to 2,3-dioxabicyclo[2.2.2]oct-5-ene reproduce well the experimental selectivities and give insights into the origin of the changes. The double bonds of the olefins are moderately pyramidalized toward the anti space, implying a preferred anti addition. This natural predisposition to anti addition is tuned by the mutual interaction between the addends. On going from osmilation to nitrile oxide and butadiene cycloadditions, the steric effects between the addend and the dimethylene bridge of the olefin increase remarkably and affect the attacking angle T and the tilting angle a of the anti addition, enforcing enhanced deformations of the olefin, which almost offset its natural anti predisposition. In the case of butadiene, the residual part of the steric effect increases further the anti barrier leading to a neat reversal of selectivity. 2,3-Dioxabicyclo[2.2.1]hept-5-ene shows a larger anti pyramidalization, which is akin to that of norbornene itself and to 2-oxa-3-azabicyclo[2.2.1]hept-5-ene derivatives, and mutual steric interactions have only a subordinate influence on selectivity. |
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